Heterobimetallic dinuclear lanthanide alkoxide complexes as acid–base bifunctional catalysts for synthesis of carbamates under solvent-free conditions

Abstract

Heterobimetallic dinuclear lanthanide alkoxide complexes Ln₂Na₈(OCH₂CH₂NMe₂)₁₂(OH)₂ [Ln: I (Nd), II (Sm), III (Yb) and IV (Y)] were used as efficient acid–base bifunctional catalysts for the synthesis of carbamates from dialkyl carbonates and amines as well as the N-Boc protection of amines. The cooperative catalysts showed high catalytic activity and a wide scope of substrates with good to excellent yields under solvent-free conditions. The systems have shown higher catalytic activities due to the noteworthy synergistic interactions of Lewis acid center–Brønsted basic center. The comparison of catalytic efficiency between mono- and dinuclear heterobimetallic lanthanide alkoxide analogues was also investigated.

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Introduction

Organic carbamates are crucial intermediates in the synthesis of pharmaceuticals, pesticides, herbicides, fungicides and commodity chemicals.¹ Previously, carbamates were prepared from the action of corresponding amines with phosgene or its derivatives.² These reactions have resulted in serious environment and safety problems due to the extreme toxicity of phosgene and HCl produced as the main by-product. To overcome this drawback, great efforts have been made to synthesize carbamates using other reagents for the replacement of phosgene, such as oxidative carbonylation of aromatic amines,³ catalytic reductive carbonylation of nitroaromatics,⁴ coupling reactions of amines and carbon dioxide with a different third component (such as alcohols, alkyl halides, tosylhydrazones or diaryliodonium salts),⁵ treatment of alcohols and thiols with cyanate salts,⁶ reacting azidoformates with boronic acids,⁷ rearrangement reactions,⁸ and C–H bond functionalization⁹ etc.

Compared with the conventional methods, synthesis of carbamates from amines and dialkyl carbonates is considered as one of the most promising methods for its environmental friendly and nontoxic nature, and potential industrial applications.¹⁰ Since the only by-product of this reaction is alcohol, which could be removed easily from the reaction, the usage of dialkyl carbonates is an alternative to phosgene or chloroformate. Recently, several methods for the synthesis of carbamates from dialkyl carbonates and amines have been reported.^10,11 However, these methodologies may present some disadvantages such as high cost,^11d high temperature^{10c,10d,11c,11e} or pressure,^10d,11c high toxicity,^10a,11f large excess amounts of dialkyl carbonates and narrowed scope of amines.^{10c,11c–11e}

Lanthanide complexes have shown better catalytic properties in many organic reactions.¹² Heterobimetallic lanthanide complex as an important family of lanthanide complexes has been developed over the last two decades.¹³ Shibasaki and co-workers developed ground breaking lanthanide-alkali metal heterobimetallic complexes M₃[Ln(BINOL)₃] (where Ln = rare earth metal, M = Li, Na, K, and BINOL is 1,1′-bis(2-naphthol)), which exhibited both Lewis acidity/Brønsted basicity bifunctional catalysis in a variety of stereoselective organic transformations due to the cooperativity of catalysts.¹⁴ Recent advances from their research on cooperative asymmetric catalysis were focused on dinuclear Schiff base catalysis.¹⁵ For instance, heterobimetallic Cu/Sm–Schiff base complex catalyze daza-Henry reaction with high syn-selectivity.¹⁶ Besides, the highly efficient heterogeneous Nd/Na heterobimetallic catalyst was successfully applied to asymmetric nitroaldol reaction.¹⁷

As a part of our ongoing studies on heterometallic rare earth-alkali metal catalysts,¹⁸ we previously found that heterobimetallic rare earth-alkali metal alkoxide complexes were the efficient catalysts for ring-opening polymerization of ε-caprolactone and trimethylene carbonate.^18a,18c,18d Recently, we have reported that heterobimetallic dinuclear lanthanide alkoxide complexes as acid–base bifunctional catalysts were successfully applied to the transesterification under mild conditions.^18g In this system, the dinuclear lanthanide catalysts acted as not only binary Lewis acid centers to activate esters, but also multi-Brønsted base centers to activate alcohols, which was important to achieve high activity (Fig. 1). Currently, Kazushi and Takashi group reported the transesterification of β-keto esters by tetranuclear zinc cluster catalyst. High catalyst activity depended on the balance between lewis acidity and Brønsted basicity, it is consist with the dual activation by the cooperative zinc centers.¹⁹

Fig. 1 Dual activation by the heterobimetallic dinuclear lanthanide complex as an acid–base bifunctional catalyst.

On the basis of the facts mentioned above, we attempted to perform the synthesis of carbamates through the dual activation of carbonates and amines by the heterobimetallic dinuclear lanthanide alkoxide complexes. So, we report an efficient method for the synthesis of carbamates from dialkyl carbonates and amines, as well as the N-Boc protection of amines under solvent-free conditions in this work. Meanwhile, the comparison of catalytic efficiency between readily available heterobimetallic mononuclear lanthanide alkoxide analogues LnNa₈(O^tBu)₁₀(OH) [Ln: i (Nd) and ii (Yb)] and above dinuclear catalysts Ln₂Na₈(OCH₂CH₂NMe₂)₁₂(OH)₂ [Ln: I (Nd), II (Sm), III (Yb) and IV (Y)] (Fig. 2), was also investigated.

Fig. 2 (a) Structural framework of heterobimetallic dinuclear lanthanide alkoxide complexes Ln₂Na₈(OCH₂CH₂NMe₂)₁₂(OH)₂ [Ln: I (Nd), II (Sm), III (Yb) and IV (Y)].^18a (b) Structural framework of heterobimetallic mononuclear lanthanide alkoxide analogues LnNa₈(O^tBu)₁₀(OH) [Ln: i (Nd) and ii (Yb)].^18d

Results and discussion

The synthesis of carbamate 3aa from dimethyl carbonate 1a (DMC) and benzyl amine 2a was chosen as a model system (Table 1). The optimal reaction conditions, including various catalysts, different catalyst loadings, and diverse solvents for the reaction process were investigated in detail. Four kinds of alkoxide complexes, including heterobimetallic dinuclear lanthanide alkoxide complexes Ln₂Na₈(OCH₂CH₂NMe₂)₁₂ (OH)₂ [Ln: I (Nd), II (Sm), III (Yb) and IV (Y)], monometallic lanthanide alkoxide complex Nd(OCH₂CH₂NMe₂)₃, monometallic sodium alkoxide complex NaOCH₂CH₂NMe₂ and heterobimetallic mononuclear lanthanide alkoxide analogues LnNa₈(O^tBu)₁₀(OH) [Ln: i (Nd) and ii (Yb)] were chosen to study the difference of catalytic activity between monometallic vs. heterobimetallic, and heterobimetallic mono- vs. dinuclear lanthanide catalysts. The above alkoxide complexes were synthesized according to the literature methods.¹⁸ And X-ray crystal structures of Ln₂Na₈(OCH₂CH₂NMe₂)₁₂(OH)₂ [Ln: I (Nd), II (Sm), III (Yb) and IV (Y)] and LnNa₈(O^tBu)₁₀(OH) [Ln: i (Nd) and ii (Yb)] were also reported,^18a,d whose structural frameworks are shown in Fig. 2.

Table 1 Optimization of conditions for the reaction of 1a with 2a^a

Entry	Catalyst	Loading of catalyst (mol%)	Solvent	Yield^b (%)
a Reaction conditions: dimethyl carbonate 1a (2 mmol), benzyl amine 2a (2 mmol) and catalyst at 80 °C under an argon atmosphere for 12 h.b GC yield.c OR = OCH2CH2NMe2.
1	—	—	Toluene	8
2	I	1.0	Toluene	85
3	II	1.0	Toluene	77
4	III	1.0	Toluene	71
5	IV	1.0	Toluene	72
6	Nd(OR)3^c	2.0	Toluene	18
7	NaOR^c	8.0	Toluene	49
8	i	1.0	Toluene	66
9	ii	1.0	Toluene	54
10	I	0.5	Toluene	76
11	I	1.5	Toluene	87
12	I	2.0	Toluene	90
13	I	1.0	DMF	55
14	I	1.0	DMSO	60
15	I	1.0	Acetonitrile	61
16	I	1.0	Dioxane	58
17	I	1.0	Solvent-free	93

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As expected, the heterobimetallic dinuclear lanthanide/sodium alkoxide complexes (I–IV) could be used as catalysts for the synthesis of methyl benzylcarbamate (3aa), which was efficiently achieved with a yield ranging from 71% to 85% at 80 °C for 12 h with 1.0 mol% catalyst loading (Table 1, entries 2–5). In contrast, the desired reaction was conducted with very low reactivity and afforded only 8% yield of 3aa without a catalyst (Table 1, entry 1). Moreover, the monometallic lanthanide complex Nd(OCH₂CH₂NMe₂)₃, as well as the sodium alkoxide complex NaOCH₂CH₂NMe₂, showed much lower catalytic activity, and the corresponding 3aa was obtained with only 18% and 49% yields even by 2% and 8% catalyst loadings (Table 1, entries 6 and 7). Generally, monometallic lanthanide alkoxide complex can be regarded as a soft Lewis acid catalyst, and monometallic sodium alkoxide complex may be used as a hard Brønsted base catalyst. The experimental results indicated that heterobimetallic lanthanide/sodium alkoxide complexes can be treated as the cooperative acid–base bifunctional catalysts, which have evidently higher activity than that of monometallic ones as the simple acid or base catalyst. To further study the influence of nuclearity on the catalytic activity,²⁰ the mononuclear lanthanide/sodium alkoxide analogues with well-defined X-ray crystal structures were used as the catalysts for this reaction. The results showed mononuclear catalysts obtained relatively lower catalytic activity than corresponding dinuclear catalysts with the same rare earth metal center Nd and Yb (Table 1, entries 2, 4, 8 and 9), and yields of 3aa catalyzed by mono- and dinuclear catalysts is 66% vs. 85% and 54% vs. 71%, respectively. On the premise of having similar alkali metal alkoxide structures, the less Lewis acid center may lead to the relatively poor performance of mononuclear catalysts. Furthermore, it is found that the reactivity depends deeply on the lanthanide metal of the complex. The activity sequence observed here (Table 1, entries 2–5, 8 and 9) is consistent with the increase in ionic radius (Y ≈ Yb < Sm < Nd). The increasing of catalyst loading from 0.5% to 2.0% led to greater yield from 76% to 90% (Table 1, entries 2 and 10–12). Different solvents were screened in order to increase the conversion in the model reaction catalyzed by I at 80 °C with 1.0 mol% catalyst loading (Table 1, entries 2 and 13–16). Aromatic hydrocarbon solvent toluene was found to be a better reaction medium than polar aprotic (DMF, DMSO, CH₃CN and dioxane) solvents. Polar aprotic solvents inhibited the reaction to some degree potentially due to coordination of the solvent to lanthanide center, which diminished the Lewis acidity of the catalyst. Considering several methodologies used excess amounts of carbonates (5 or even 10 equiv.) as a solvent, our research attempts to obtain an economic and green synthesis method without excess dialkyl carbonates and other solvents. It's satisfied that the reaction could perform well without excess substrates and other solvents under the present catalyst system and the best results were that 93% yield of carbamate 3aa was achieved at 80 °C with 1.0 mol% catalyst loading for 12 h under solvent-free conditions when the reagent component ratio of 1a to 2a is 1 : 1 (Table 1, entry 17).

Under the optimized reaction conditions, a series of amines reacted with DMC and the results are presented in Table 2. From Table 2, it is obvious that transformation of various aliphatic primary and secondary amines with DMC could proceed to give the corresponding carbamates with moderate to high yields. Aliphatic primary amines such as n-butylamine (2c) and n-dodecylamine (2d) were effectively transformed with high yields up to 96% and 94% (Table 2, entries 3 and 4). 3-(Trimethoxysilyl) propan-1-amine (2e) known as silane coupling agent KH540, reacted smoothly with DMC to produce corresponding carbamate (3ae) with 90% yield (Table 2, entry 5). Meanwhile, aliphatic secondary amines 2h, 2i and 2j were efficiently catalyzed by I, the corresponding products also have good yields (Table 2, entries 8–10). It is found that both the electronic nature and steric factor had remarkable effect on the yield of this reaction. For 2-picolylamine (2b), because pyridine is a weak electron withdrawing group compared with phenyl group, the yield of 3ab is 89% (Table 2, entry 2), which is lower than that of 3aa (93%). Meanwhile, since the p–π conjugated effect of aromatic amine reduced the nucleophilicity of amine group, the reaction with aromatic amines proceeded less smoothly and lower yields was achieved under the same conditions. Aniline and substituted anilines, however, did not proceed resulted from the lower nucleophilicity. Moreover, highly selective amidation, not the possible transesterification, proceeded when DMC treated with ethanolamine (2f) (Table 2, entry 6). In addition, the double transformation of diamine with excess DMC was also successfully observed (Table 2, entry 7). Although diethyl carbonate (DEC) could react smoothly with corresponding amines compared with dimethyl carbonate (DME), the yields were relatively lower with the reaction time extending to 24 h (Table 2, entries 11–14).

Table 2 Synthesis of N-carbamates catalyzed by I^a

Entry	Amine	Product	Yield^b (%)
a Reaction conditions: carbonate 1 (2 mmol), amine 2 (2 mmol) and I (0.02 mmol) at 80 °C for 12 h under an argon atmosphere and solvent-free conditions.b Isolated yield.c Dimethyl carbonate 1a (4.2 mmol), amine 2 (2 mmol) and I (0.02 mmol).d Diethyl carbonate (DEC) 1b (2 mmol), amine 2 (2 mmol) and I (0.02 mmol) for 24 h.
1			93
2			89
3			96
4			94
5			90
6			91
7^c			90
8			88
9			84
10			87
11^d			76
12^d			79
13^d			72
14^d			72

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Carbamates are usually used as intermediates to protect the functionality of amines in the multistep organic synthesis.²¹ Among the protecting groups for amines, the tert-butoxycarbonyl (Boc) is one of the most used, owing to the stability of N-tert-butylcarbamates toward nucleophiles or strong basic conditions and catalytic hydrogenation.²² Various methods have been used to protect amines in their N-Boc form over the years.²³ These methods, however, have several drawbacks such as long reaction time, low yields, usage of solvents, formation of side-products and complicated reaction procedures, etc. In order to better standardize the reaction, different catalyst loadings of I were used for the N-Boc protection of aniline at room temperature under solvent-free conditions. The results are shown in Table 3. It was found that the reaction proceeded with only 54% yield until 240 min without a catalyst (Table 3, entry 1). And it is clear that the yield increased from 64% to 91% on varying the loading of catalyst from 0.1 mol% to 1.5 mol%. The best results were that 89% yield of product was achieved within 10 min at room temperature with 0.5 mol% catalyst loading under solvent-free conditions when the molar ratio of (Boc)₂O to anilineis 1 : 1 (Table 3, entry 4).

Table 3 Reaction of aniline and (Boc)₂O catalyzed by I^a

Entry	Loading of catalyst (mol%)	Time (min)	Yield^b (%)
a Reaction conditions: aniline (1 mmol), (Boc)2O (1 mmol) at room temperature under solvent-free conditions.b GC yield.
1	—	240	54
2	0.1	10	64
3	0.2	10	75
4	0.5	10	87
5	1.0	10	89
6	1.5	10	91

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After optimization of the reaction conditions, various aromatic, heteroaromatic and aliphatic amines were converted to the N-Boc derivatives under the selected conditions. Table 4 summarizes the results. Obviously, the catalyst system was very effective for the N-Boc protection of the amines. No competitive formation of N,N-di-BOC, urea and isocyanate was observed.²⁴ Various derivatives of aniline were taken under the reaction conditions and the products were obtained with good yields. As expected, electron donating groups on aryl groups gave rise to higher yields than electron withdrawing groups (Table 4, entries 1–4). Substrates having an OH group afforded the N-Boc derivatives without O-Boc formation. Protection of diamines in the presence of I was also studied. It was found that selective mono protection of diamines was achieved using an equivalent of (Boc)₂O, the mono-N-Boc-protected products were obtained with high yields (Table 4, entries 5 and 6). Moreover, the protocol could also work well with naphthylamine and heteroaromatic amines (Table 4, entries 7–9). Benzyl amine, 2-picolylamine and aliphatic secondary amines reacted more quickly than aniline derivatives due to their high nucleophilicity (Table 4, entries 10, 11 and 13–15). Furthermore, the chemoselectivity was further demonstrated in the case of ethanolamine that did not form O-Boc formation and afforded the corresponding N-Boc product with high yield (Table 4, entry 12).

Table 4 N-Boc protection of various amines catalyzed by I^a

Entry	Amine	Product	Yield^b (%)
a Reaction conditions: amine (1 mmol), (Boc)2O (1 mmol) and I (0.5 mol%) at room temperature for 10 min under solvent-free conditions.b Isolated yield.c Reaction time: 1 h.d Reaction time: 30 min.e Reaction time: 2 min.
1			87
2			93
3			91
4			96
5			95
6			93
7^c			95
8^d			92
9^d			90
10^e			99
11^e			98
12			96
13^e			98
14^e			98
15^e			98

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The bimetallic cooperative catalysts could utilize many different types of metals, such as alkali metals, transition metals and lanthanides.²⁵ The important role of lanthanide in the bimetallic catalysts, was studied by P. J. Walsh etc. groups from X-ray and NMR analysis.²⁶ Heterobimetallic dinuclear lanthanide alkoxide complexes have stronger activation than that of mononuclear analogues due to the double Lewis acid center. With consideration of these results and other mechanism studies of acid–base bifunctional catalysis in the literature,^14,27 a plausible mechanism of the reaction of carbonates and amines is illustrated in Scheme 1. Firstly, heterobimetallic dinuclear lanthanide/sodium alkoxide complex I activates both the carbonate and amine to generate the adduct A, wherein two lanthanide metal ions work as double Lewis acid centers for activating simultaneously the carbonyl and alkoxy oxygen atoms of carbonate, while the sodium alkoxide moiety functions as a Brønsted base to bring the nucleophile amine in close proximity. Then the formation of a sp³ hybridized tetrahedral intermediate (B) followed by the nucleophilic attack of the activated amine on the electrophilic carbon center of the carbonyl moiety. Subsequently, the sterically congested B collapses to regenerate the sp² hybridized carbonyl unit and the active species I, resurrected along with the release of product carbamate and alcohol.

Scheme 1 Proposed catalytic cycle for synthesis of carbamates from carbonates with amines.

Conclusions

In summary, heterobimetallic dinuclear lanthanide alkoxide complexes were used as a new class of acid–base bifunctional catalysts for synthesis of carbamates from DMC/DEC with amines as well as the N-Boc protection of various amines. The new catalysts showed high catalytic activity and a wide scope of substrates with good to excellent yields under solvent-free conditions. In this catalyst design, two lanthanide metals of dinuclear catalysts act as the binary Lewis acid centers to activate electrophiles, while alkali metal alkoxides serve as a Brønsted base to bring nucleophiles in close proximity.

Experimental

General methods

All manipulations and reactions were performed under an atmosphere of argon with standard Schlenk techniques. The carbonates and amines were obtained commercially. Heterobimetallic lanthanide/sodium alkoxide complexes Ln₂Na₈(OCH₂CH₂NMe₂)₁₂(OH)₂ [Ln: I (Nd), II (Sm), III (Yb) and IV (Y)], LnNa₈(O^tBu)₁₀(OH) [Ln: i (Nd) and ii (Yb)] and the corresponding monometallic complex Nd(OCH₂CH₂NMe₂)₃ were prepared according to the literature.^{18a,d 1}H and ¹³C NMR spectra were recorded with Bruker Avance 400 MHz spectrometer using CDCl₃ as the solvent with tetramethylsilane (TMS) as the internal standard. The GC (Shimadzu GC-2010plus) and GCMS (Shimadzu GCMS-QP2010) analyses were carried out using N₂ and He as the carrier gas, respectively. High-resolution mass spectra (HRMS) were carried out using a TOF-MS instrument with an EI or ESI source. Column chromatography was performed using silica gel (100–200 mesh). Petroleum ether (PE) used was the fraction boiling in the range 60–90 °C.

Typical procedure for the synthesis of carbamates from amines and dialkyl carbonates (3aa as an example)

A mixture of Nd₂Na₈(OCH₂CH₂NMe₂)₁₂(OH)₂ (I) (0.02 mmol), dimethyl carbonate (2 mmol) and benzyl amine (2 mmol) in a 10 mL Schlenk tube was stirred at 80 °C for the 12 h under argon. The resulting mixture was then filtered through a small plug of silica gel to remove the catalyst. The crude product was purified by silica gel column chromatography (EtOAc/PE = 1 : 4) to provide the title compound (307 mg, 93% yield) as a white solid.

Typical procedure for the N-Boc protection of amines (4k as an example)

A typical procedure for the N-Boc protection of amines from (Boc)₂O and aniline is as follows: in a 10 mL Schlenk tube with Nd₂Na₈(OCH₂CH₂NMe₂)₁₂(OH)₂ (I) (0.005 mmol), (Boc)₂O (1 mmol) and aniline (1 mmol), the mixture was then stirred at room temperature for 10 min. The resulting mixture was then filtered through a small plug of silica gel to remove the catalyst. The crude product was purified by silica gel column chromatography (EtOAc/PE = 1 : 6) to provide the title compound (168 mg, 87% yield) as a white solid.

Analysis data for products

Methyl benzylcarbamate (3aa)²⁸. White solid (307 mg, 93%); mp 59–61 °C. ¹H NMR (400 MHz, CDCl₃) δ 7.36–7.30 (m, 2H), 7.30–7.24 (m, 3H), 5.05 (br, s, 1H), 4.36 (s, 2H), 3.69 (s, 3H). ¹³C NMR (101 MHz, CDCl₃) δ 157.2, 138.6, 128.7, 127.5, 127.4, 52.2, 45.1. GC-MS m/z 165.080.

Methyl (pyridin-2-ylmethyl)carbamate (3ab). Light-green solid (295 mg, 89%); mp 99–100 °C. ¹H NMR (400 MHz, d₆-DMSO) δ 8.48 (d, J = 4.6 Hz, 1H), 7.78–7.74 (m, 2H), 7.29–7.24 (m, 2H), 4.28 (d, 2H), 3.56 (s, 3H). ¹³C NMR (101 MHz, d6-DMSO) δ 159.1, 157.2, 149.0, 136.8, 122.3, 120.8, 51.7, 46.0. GC-MS m/z 167.070. HRMS (EI): calcd for C₈H₁₀N₂O₂ 166.0742; found 166.0740.

Methyl butylcarbamate (3ac)²⁹. Colorless oil (252 mg, 96%); ¹H NMR (400 MHz, CDCl₃) δ 4.78 (br, s, 1H), 3.67 (s, 3H), 3.17 (m, 2H), 1.57–1.40 (m, 2H), 1.34 (m, 7.3 Hz, 2H), 0.92 (t, J = 7.2 Hz, 3H). ¹³C NMR (101 MHz, CDCl₃) δ 157.1, 51.8, 40.7, 32.0, 19.8, 13.6. GC-MS m/z 131.090.

Methyl dodecylcarbamate (3ad)³⁰. White solid (457 mg, 94%); mp 45–47 °C. ¹H NMR (400 MHz, CDCl₃) δ 4.69 (br, s, 1H), 3.66 (s, 3H), 3.15 (m, 2H), 1.51–1.45 (m, J = 6.1 Hz, 2H), 1.31–1.22 (m, 18H), 0.88 (t, J = 6.6 Hz, 3H). ¹³C NMR (101 MHz, CDCl₃) δ 157.1, 51.9, 41.1, 31.9, 30.0, 29.6, 29.6, 29.6, 29.5, 29.3, 29.3, 26.7, 22.7, 14.1. GC-MS m/z 243.220.

Methyl (3-(trimethoxysilyl)propyl)carbamate (3ae). Colorless oil (427 mg, 90%); ¹H NMR (400 MHz, CDCl₃) δ 4.98 (br, s, 1H), 3.66 (s, 3H), 3.57 (m, 9H), 3.16 (m, 2H), 1.61 (m, 2H), 0.76–0.51 (m, 2H). ¹³C NMR (101 MHz, CDCl₃) δ 157.2, 51.8, 50.4, 50.2, 43.3, 23.1, 7.5, 6.2. GC-MS m/z 237.100. HRMS (EI): calcd for C₈H₁₉NO₅Si 237.1032. Found 237.1034.

Methyl (2-hydroxyethyl)carbamate (3af). Colorless oil (217 mg, 91%); ¹H NMR (400 MHz, CDCl₃) δ 5.71 (br, s, 1H), 3.67 (m, 5H), 3.63 (s, 1H), 3.31 (m, 2H). ¹³C NMR (101 MHz, CDCl₃) δ 157.9, 61.9, 52.2, 43.4. GC-MS m/z 119.060. HRMS (EI): calcd for C₄H₉NO₃ 119.0582. Found 119.0586.

Dimethyl hexane-1,6-diyldicarbamate (3ag). White solid (418 mg, 90%); mp 111–113 °C. ¹H NMR (400 MHz, CDCl₃) δ 4.72 (br, s, 2H), 3.66 (s, 6H), 3.17 (m, 4H), 1.58–1.38 (m, 1H), 1.38–1.26 (m, 4H). ¹³C NMR (101 MHz, CDCl₃) δ 157.1, 52.0, 40.9, 29.9, 26.2. GC-MS m/z 232.140. HRMS (EI): calcd for C₁₀H₂₀N₂O₄ 232.1423; found 232.1426.

Methyl piperidine-1-carboxylate (3ah)³¹. Colorless oil (252 mg, 88%); ¹H NMR (400 MHz, CDCl₃) δ 3.68 (s, 3H), 3.42 (m, 4H), 1.56 (m, 6H). ¹³C NMR (101 MHz, CDCl₃) δ 156.0, 52.3, 44.7, 25.6, 24.3. GC-MS m/z 143.010.

Methyl morpholine-4-carboxylate (3ai)³¹. Colorless oil (244 mg, 84%); ¹H NMR (400 MHz, CDCl₃) δ 3.70 (s, 3H), 3.64 (m, 4H), 3.46 (m, 4H). ¹³C NMR (101 MHz, CDCl₃) δ 155.9, 66.5, 52.6, 44.0. GC-MS m/z 145.080.

Methyl pyrrolidine-1-carboxylate (3aj)³². Colorless oil (224 mg, 87%); ¹H NMR (400 MHz, CDCl₃) δ 3.69 (s, 3H), 3.51–3.21 (m, 4H), 1.85 (m, 4H). ¹³C NMR (101 MHz, CDCl₃) δ 155.6, 52.1, 46.1, 45.6, 25.7, 24.9. GC-MS m/z 129.080.

Ethyl benzylcarbamate (3ba)³³. White solid (272 mg, 76%); mp 46–49 °C. ¹H NMR (400 MHz, CDCl₃) δ 7.36–7.30 (m, 2H), 7.30–7.21 (m, 3H), 5.04 (br, s, 1H), 4.35 (s, 2H), 4.14 (q, J = 7.0 Hz, 2H), 1.24 (t, J = 7.1 Hz, 3H). ¹³C NMR (101 MHz, CDCl₃) δ 156.7, 138.6, 128.6, 127.5, 127.4, 61.0, 45.1, 14.6. GC-MS m/z 179.090.

Ethyl dodecylcarbamate (3bd). White solid (406 mg, 79%); mp 35–36 °C. ¹H NMR (400 MHz, CDCl₃) δ 4.62 (br, s, 1H), 4.11 (dd, J = 13.4, 6.5 Hz, 2H), 3.15 (m, 2H), 1.53–1.42 (m, 2H), 1.33–1.19 (m, 21H), 0.88 (t, J = 6.3 Hz, 3H).¹³C NMR (101 MHz, CDCl₃) δ 156.7, 60.6, 41.1, 31.9, 30.0, 29.6, 29.6, 29.6, 29.5, 29.3, 29.3, 26.7, 22.7, 14.6, 14.1. GC-MS m/z 257.240. HRMS (EI): calcd for C₁₅H₃₁NO₂ 257.2355; found 257.2357.

Ethyl (2-hydroxyethyl)carbamate (3bf)³⁴. Colorless oil (192 mg, 72%); ¹H NMR (400 MHz, CDCl₃) δ 5.43 (br, s, 1H), 4.12 (q, J = 6.7 Hz, 2H), 3.76–3.61 (m, 2H), 3.32 (br, s, 1H), 3.15 (m, 2H), 1.25 (t, J = 6.9 Hz, 3H). ¹³C NMR (101 MHz, CDCl₃) δ 157.5, 61.9, 61.0, 43.3, 14.5. GC-MS m/z 133.070.

Ethyl piperidine-1-carboxylate (3bh)³⁵. Colorless oil (226 mg, 72%); ¹H NMR (400 MHz, CDCl₃) δ 4.08 (q, J = 7.1 Hz, 2H), 3.36 (m, 4H), 1.53–1.47 (m, 6H), 1.21 (t, J = 7.1 Hz, 3H). ¹³C NMR (101 MHz, CDCl₃) δ 155.6, 61.0, 44.7, 25.7, 24.4, 14.7. GC-MS m/z 157.100.

tert-Butyl benzylcarbamate (4a)³⁶. White solid (205 mg, 99%); mp 56–58 °C. ¹H NMR (400 MHz, CDCl₃) δ 7.35–7.30 (m, 2H), 7.25 (t, J = 5.0 Hz, 3H), 4.88 (br, s, 1H), 4.31 (s, 2H), 1.46 (s, 9H). ¹³C NMR (101 MHz, CDCl₃) δ 155.9, 146.7, 138.9, 128.6, 127.5, 127.3, 79.5, 44.7, 28.4.

tert-Butyl (pyridin-2-ylmethyl)carbamate (4b)³⁶. Colorless oil (204 mg, 98%); ¹H NMR (400 MHz, CDCl₃) δ 8.52 (s, 1H), 7.65 (d, J = 5.8 Hz, 1H), 7.28 (d, J = 6.5 Hz, 1H), 7.17 (d, J = 4.1 Hz, 1H), 5.89 (br, s, 1H), 4.45 (s, 2H), 1.46 (s, 9H). ¹³C NMR (101 MHz, CDCl₃) δ 157.7, 156.1, 149.0, 136.7, 122.1, 121.6, 79.3, 45.7, 28.3.

tert-Butyl (2-hydroxyethyl)carbamate (4f)³⁷. Colorless oil (155 mg, 96%); ¹H NMR (400 MHz, CDCl₃) δ 5.23 (br, s, 1H), 3.66 (m, 2H), 3.26 (m, 2H), 2.51–2.98 (br, s, 1H), 1.43 (s, 9H). ¹³C NMR (101 MHz, CDCl₃) δ 156.8, 79.6, 62.1, 43.0, 28.4.

tert-Butyl piperidine-1-carboxylate (4h)³⁸. Colorless oil (181 mg, 98%); ¹H NMR (400 MHz, CDCl₃) δ 3.35 (m, 4H), 1.63–1.48 (m, 6H), 1.45 (s, 9H). ¹³C NMR (101 MHz, CDCl₃) δ 154.9, 79.0, 44.3, 28.4, 25.7, 24.5.

tert-Butyl morpholine-4-carboxylate (4g)³⁹. White solid (183 mg, 98%); mp 57–60 °C. ¹H NMR (400 MHz, CDCl₃) δ 3.64 (m, 4H), 3.43–3.40 (m, 4H), 1.47 (s, 9H). ¹³C NMR (101 MHz, CDCl₃) δ 154.8, 79.9, 66.7, 43.5, 28.4.

tert-Butyl phenylcarbamate (4k)⁴⁰. White solid (168 mg, 87%); mp 132–134 °C. ¹H NMR (400 MHz, CDCl₃) δ 7.35 (d, J = 7.9 Hz, 2H), 7.32–7.22 (m, 2H), 7.03 (t, J = 7.3 Hz, 1H), 6.46 (br, s, 1H), 1.52 (s, 9H). ¹³C NMR (101 MHz, CDCl₃) δ 152.7, 138.3, 129.0, 123.0, 118.5, 80.5, 28.3.

tert-Butyl p-tolylcarbamate (4l)⁴¹. White solid (193 mg, 93%); mp 84–86 °C. ¹H NMR (400 MHz, CDCl₃) δ 7.25 (d, J = 5.4 Hz, 2H), 7.08 (d, J = 8.0 Hz, 2H), 6.42 (br, s, 1H), 2.29 (s, 3H), 1.51 (s, 9H). ¹³C NMR (101 MHz, CDCl₃) δ 152.9, 135.7, 132.5, 129.5, 118.7, 80.3, 28.4, 20.7.

tert-Butyl (4-bromophenyl)carbamate (4m)⁴². White solid (248 mg, 91%); mp 102–103 °C. ¹H NMR (400 MHz, CDCl₃) δ 7.39 (d, J = 8.5 Hz, 2H), 7.25 (d, J = 7.0 Hz, 2H), 6.47 (br, s, 1H), 1.51 (s, 9H). ¹³C NMR (101 MHz, CDCl₃) δ 152.5, 137.5, 131.9, 120.0, 115.4, 80.9, 28.3.

tert-Butyl (4-hydroxyphenyl)carbamate (4n)⁴³. White solid (201 mg, 96%); mp 144–145 °C. ¹H NMR (400 MHz, CDCl₃) δ 7.19 (d, J = 7.9 Hz, 2H), 6.75 (d, J = 8.5 Hz, 2H), 6.33 (br, s, 1H), 4.93 (br, s, 1H), 1.51 (s, 9H). ¹³C NMR (101 MHz, CDCl₃) δ 153.6, 152.1, 130.8, 121.7, 115.8, 80.5, 28.4.

tert-Butyl (4-aminophenyl)carbamate (4o)⁴⁴. White solid (198 mg, 95%); mp 114–115 °C. ¹H NMR (400 MHz, CDCl₃) δ 7.13 (d, J = 6.9 Hz, 2H), 6.64 (d, J = 8.4 Hz, 2H), 6.25 (br, s, 1H), 3.53 (br, s, 2H), 1.50 (s, 9H). ¹³C NMR (101 MHz, CDCl₃) δ 153.3, 142.4, 129.7, 120.9, 115.6, 80.0, 28.4.

tert-Butyl (2-aminophenyl)carbamate (4p)⁴⁵. White solid (193 mg, 93%); mp 111–114 °C. ¹H NMR (400 MHz, CDCl₃) δ 7.28 (d, J = 6.8 Hz, 1H), 7.01 (t, J = 7.6 Hz, 1H), 6.80 (t, J = 6.7 Hz, 2H), 6.22 (br, s, 1H), 3.01–4.45 (br, s, 2H) 1.51 (s, 9H). ¹³C NMR (101 MHz, CDCl₃) δ 153.9, 139.2, 126.2, 125.1, 124.8, 120.1, 118.0, 80.7, 28.3.

tert-Butyl naphthalen-1-ylcarbamate (4q)⁴⁶. Light-yellow solid (231 mg, 95%); mp 96–98 °C. ¹H NMR (400 MHz, CDCl₃) δ 7.86 (t, J = 8.4 Hz, 3H), 7.62 (d, J = 8.2 Hz, 1H), 7.55–7.42 (m, 3H), 6.86 (br, s, 1H), 1.56 (s, 9H). ¹³C NMR (101 MHz, CDCl₃) δ 153.5, 146.8, 134.1, 132.9, 128.8, 126.0, 125.9, 124.5, 120.4, 118.6, 80.7, 28.4.

tert-Butyl pyridin-2-ylcarbamate (4r)⁴⁷. White solid (178 mg, 92%); mp 91–94 °C. ¹H NMR (400 MHz, CDCl₃) δ 11.23 (br, s, 1H), 7.91 (d, J = 8.1 Hz, 1H), 7.79 (d, J = 7.9 Hz, 1H), 7.39 (m, 1H), 7.30–7.26 (m, 1H), 1.59 (s, 9H). ¹³C NMR (101 MHz, CDCl₃) δ 161.4, 152.9, 152.6, 147.5, 138.3, 121.0, 80.9, 28.4.

tert-Butyl benzo[d]thiazol-2-ylcarbamate (4s)⁴¹. White solid (225 mg, 90%); mp 240–242 °C. ¹H NMR (400 MHz, CDCl₃) δ 7.91 (d, J = 8.1 Hz, 1H), 7.79 (d, J = 7.9 Hz, 1H), 7.39 (t, J = 7.7 Hz, 1H), 7.28 (t, J = 5.7 Hz, 1H), 1.59 (s, 9H). ¹³C NMR (101 MHz, CDCl₃) δ 161.6, 152.8, 148.7, 131.5, 125.7, 123.4, 121.0, 120.9, 83.3, 28.4.

tert-Butyldibutylcarbamate (4t)⁴⁸. Colorless oil (224 mg, 98%); ¹H NMR (400 MHz, CDCl₃) δ 3.15 (m, 4H), 1.54–1.48 (m, 4H), 1.45 (s, 9H), 1.35–1.20 (m, 4H), 0.90 (m, 6H). ¹³C NMR (101 MHz, CDCl₃) δ 155.7, 78.9, 46.8, 30.9, 30.5, 28.5, 20.1, 13.9.

Acknowledgements

We acknowledge financial support by the National Natural Science Foundation of China (No. 20902001, 21102001), the Natural Science Foundation of Education Department of Anhui Province (No. KJ2014A013, KJ2015A047), Anhui Provincial Natural Science Foundation (1608085MB39), and the 211 Project of Anhui University.

Notes and references

1. (a) T. T. Wu, J. Huang, N. D. Arrington and G. M. Dill, J. Agric. Food Chem., 1987, 35, 817–823; (b) A. J. Wills, Y. Krishnan-Ghosh and S. Balasubramanian, J. Org. Chem., 2002, 67, 6646–6652; (c) S. Ray and D. Chaturvedi, Drugs Future, 2004, 29, 343–357; (d) T. Goto, Y. Ito, S. Yamada, H. Matsumoto, H. Oka and H. Nagase, Anal. Chim. Acta, 2006, 555, 225–232; (e) J. Song, H. W. Shih and L. Deng, Org. Lett., 2007, 9, 603–606; (f) E. M. Dangerfield, M. S. Timmer and B. L. Stocker, Org. Lett., 2009, 11, 535–538; (g) V. Janganati, N. R. Penthala, N. R. Madadi, Z. Chen and P. A. Crooks, Bioorg. Med. Chem. Lett., 2014, 24, 3499–3502; (h) I. B. Dhouib, A. Annabi, M. Jallouli, S. Elfazaa and M. M. Lasram, J. Appl. Biomed., 2016, 14, 85–90.
2. (a) H. Eckert and B. Forster, Angew. Chem., Int. Ed., 1987, 26, 894–895; (b) J. S. Nowick, N. A. Powell, T. M. Nguyen and G. Noronha, J. Org. Chem., 1992, 57, 7364–7366; (c) P. Majer and R. S. Randad, J. Org. Chem., 1994, 59, 1937–1938.
3. (a) R. N. Salvatore, J. A. Ledger and K. W. Jung, Tetrahedron Lett., 2001, 42, 6023–6025; (b) F. Shi, J. J. Peng and Y. Q. Deng, J. Catal., 2003, 219, 372–375.
4. (a) A. M. Tafesh and J. Weiguny, Chem. Rev., 1996, 96, 2035–2052; (b) F. Ragaini, C. Cognolato, M. Gasperini and S. Cenini, Angew. Chem., Int. Ed., 2003, 42, 2886–2889; (c) F. Shi, Y. D. He, D. M. Li, Y. B. Ma, Q. H. Zhang and Y. Q. Deng, J. Mol. Catal. A: Chem., 2006, 244, 64–67.
5. (a) M. Yoshida, N. Hara and S. Okuyama, Chem. Commun., 2000, 151–152; (b) R. N. Salvatore, S. I. Shin, A. S. Nagle and K. W. Jung, J. Org. Chem., 2001, 66, 1035–1037; (c) D. L. Kong, L. N. He and J. Q. Wang, Synlett, 2010, 1276–1280; (d) H. L. An, L. L. Zhang and X. Q. Zhao, Chin. J. Chem. Eng., 2014, 22, 607–610; (e) W. F. Xiong, C. R. Qi, H. T. He and H. F. Jiang, Angew. Chem., Int. Ed., 2015, 54, 3084–3087; (f) W. F. Xiong, C. R. Qi, Y. B. Peng and H. F. Jiang, Chem.–Eur. J., 2015, 21, 14314–14318.
6. A. R. Sardarian and I. D. Inaloo, RSC Adv., 2015, 5, 76626–76641.
7. S. Y. Moon, U. B. Kim and W. S. Kim, J. Org. Chem., 2015, 80, 1856–1865.
8. (a) P. Gogoi and D. Konwar, Tetrahedron Lett., 2007, 48, 531–533; (b) A. R. Modarresi-Alam, F. Khamooshi, M. Nasrollahzadeh and H. A. Amirazizi, Tetrahedron, 2007, 63, 8723–8726.
9. (a) P. K. Chikkade, Y. Kuninobu and M. Kanai, Chem. Sci., 2015, 6, 3195–3200; (b) R. K. Sharma, S. Dutta and S. Sharma, Dalton Trans., 2015, 44, 1303–1316.
10. (a) T. Baba, M. Fujiwara, A. Oosaku, A. Kobayashi, R. G. Deleon and Y. Ono, Appl. Catal., A, 2002, 227, 1–6; (b) M. Curini, F. Epifano, F. Maltese and O. Rosati, Tetrahedron Lett., 2002, 43, 4895–4897; (c) F. Li, W. B. Li, J. Li, W. Xue, Y. J. Wang and X. Q. Zhao, Appl. Catal., A, 2014, 475, 355–362; (d) V. Mishra, J. K. Cho, S. H. Shin, Y. W. Suh and Y. J. Kim, Appl. Catal., A, 2014, 487, 82–90.
11. (a) S. P. Gupte, A. B. Shivarkar and R. V. Chaudhari, Chem. Commun., 2001, 2620–2621; (b) S. Carloni, D. E. De Vos, P. A. Jacobs, R. Maggi, G. Sartori and R. Sartorio, J. Catal., 2002, 205, 199–204; (c) M. Selva, P. Tundo and A. Perosa, Tetrahedron Lett., 2002, 43, 1217–1219; (d) T. Sima, S. Guo, F. Shi and Y. Deng, Tetrahedron Lett., 2002, 43, 8145–8147; (e) M. Selva, P. Tundo, A. Perosa and F. Dall'Acqua, J. Org. Chem., 2005, 70, 2771–2777; (f) C. Han and J. A. Porco Jr, Org. Lett., 2007, 9, 1517–1520; (g) H. Zhou, F. Shi, X. Tian, Q. Zhang and Y. Deng, J. Mol. Catal. A: Chem., 2007, 271, 89–92; (h) N. Lucas, A. P. Amrute, K. Palraj, G. V. Shanbhag, A. Vinu and S. B. Halligudi, J. Mol. Catal. A: Chem., 2008, 295, 29–33; (i) B. Wang, J. He and R. C. Sun, Chin. Chem. Lett., 2010, 21, 794–797; (j) S. Kumar and S. L. Jain, New J. Chem., 2013, 37, 2935–2938.
12. (a) G. K. Prakash, T. Mathew and G. A. Olah, Acc. Chem. Res., 2012, 45, 565–577; (b) K. Shen, X. H. Liu, L. L. Lin and X. M. Feng, Chem. Sci., 2012, 3, 327–334; (c) D. J. Averill and M. J. Allen, Catal. Sci. Technol., 2014, 4, 4129–4137; (d) J. Chu, X. Han, C. E. Kefalidis, J. Zhou, L. Maron, X. Leng and Y. Chen, J. Am. Chem. Soc., 2014, 136, 10894–10897; (e) X. Gu, X. Zhu, Y. Wei, S. Wang, S. Zhou, G. Zhang and X. Mu, Organometallics, 2014, 33, 2372–2379; (f) J. Qin, P. Wang, Q. Li, Y. Zhang, D. Yuan and Y. Yao, Chem. Commun., 2014, 50, 10952–10955; (g) J. R. Robinson, J. Yadav, X. Y. Fan, G. R. Stanton, E. J. Schelter, M. A. Pericas and P. J. Walsh, Adv. Synth. Catal., 2014, 356, 1243–1254; (h) X. Wang, S. Y. Li, Y. M. Pan, H. S. Wang, H. Liang, Z. F. Chen and X. H. Qin, Org. Lett., 2014, 16, 580–583; (i) P.-H. Wei, L. Xu, L.-C. Song, W.-X. Zhang and Z. Xi, Organometallics, 2014, 33, 2784–2789; (j) B. Wu, J. C. Gallucci, J. R. Parquette and T. V. RajanBabu, Chem. Sci., 2014, 5, 1102–1117; (k) Q. F. Xing, T. Zhao, S. Z. Du, W. H. Yang, T. L. Liang, C. Redshaw and W. H. Sun, Organometallics, 2014, 33, 1382–1388.
13. (a) M. Shibasaki and N. Yoshikawa, Chem. Rev., 2002, 102, 2187–2210; (b) M. Shibasaki, M. Kanai, S. Matsunaga and N. Kumagai, Acc. Chem. Res., 2009, 42, 1117–1127; (c) M. Shibasaki and S. Matsunaga, J. Synth. Org. Chem., Jpn., 2010, 68, 1142–1149; (d) N. Kumagai and M. Shibasaki, Angew. Chem., Int. Ed., 2013, 52, 223–234.
14. (a) H. Sasai, T. Arai, Y. Satow, K. N. Houk and M. Shibasaki, J. Am. Chem. Soc., 1995, 117, 6194–6198; (b) M. Shibasaki, H. Sasai and T. Arai, Angew. Chem., Int. Ed., 1997, 36, 1236–1256; (c) Y. M. A. Yamada, N. Yoshikawa, H. Sasai and M. Shibasaki, Angew. Chem., Int. Ed., 1997, 36, 1871–1873; (d) E. Emori, T. Arai, H. Sasai and M. Shibasaki, J. Am. Chem. Soc., 1998, 120, 4043–4044; (e) K.-I. Yamada, S. J. Harwood, H. Gröger and M. Shibasaki, Angew. Chem., Int. Ed., 1999, 38, 3504–3506.
15. S. Matsunaga and M. Shibasaki, Chem. Commun., 2014, 50, 1044–1057.
16. (a) S. Handa, V. Gnanadesikan, S. Matsunaga and M. Shibasaki, J. Am. Chem. Soc., 2007, 129, 4900–4901; (b) S. Handa, V. Gnanadesikan, S. Matsunaga and M. Shibasaki, J. Am. Chem. Soc., 2010, 132, 4925–4934.
17. (a) T. Nitabaru, A. Nojiri, M. Kobayashi, N. Kumagai and M. Shibasaki, J. Am. Chem. Soc., 2009, 131, 13860–13869; (b) D. Sureshkumar, K. Hashimoto, N. Kumagai and M. Shibasaki, J. Org. Chem., 2013, 78, 11494–11500.
18. (a) H. Sheng, F. Xu, Y. Yao, Y. Zhang and Q. Shen, Inorg. Chem., 2007, 46, 7722–7724; (b) H. T. Sheng, L. Y. Zhou, Y. Zhang, Y. M. Yao and Q. Shen, J. Polym. Sci., Part A: Polym. Chem., 2007, 45, 1210–1218; (c) H.-T. Sheng, J.-M. Li, Y. Zhang, Y.-M. Yao and Q. Shen, Polyhedron, 2008, 27, 1665–1672; (d) H. T. Sheng, J. M. Li, Y. Zhang, Y. M. Yao and Q. Shen, J. Appl. Polym. Sci., 2009, 112, 454–460; (e) H. T. Sheng, Y. Feng, Y. Zhang and Q. Shen, Eur. J. Inorg. Chem., 2010, 5579–5586; (f) H. Sheng, J. Shi, Y. Feng, H. Wang, Y. Jiao, H. Sheng, Y. Zhang and Q. Shen, Dalton Trans., 2012, 41, 9232–9240; (g) R. Zeng, H. Sheng, Y. Zhang, Y. Feng, Z. Chen, J. Wang, M. Chen, M. Zhu and Q. Guo, J. Org. Chem., 2014, 79, 9246–9252.
19. K. Agura, Y. Hayashi, K. Mashima and T. Ohshima, Chem.–Asian J., 2016, 11, 1548–1554.
20. (a) L. Li, M. V. Metz, H. Li, M. C. Chen, T. J. Marks, L. Liable-Sands and A. L. Rheingold, J. Am. Chem. Soc., 2002, 124, 12725–12741; (b) M. Yamamura, H. Miyazaki, M. Iida, S. Akine and T. Nabeshima, Inorg. Chem., 2011, 50, 5315–5317; (c) A. Abhervé, J. M. Clemente-Juan, M. Clemente-León, E. Coronado, J. Boonmak and S. Youngme, New J. Chem., 2014, 38, 2105–2113; (d) D. C. Charalampou, N. Kourkoumelis, S. Karanestora, L. P. Hadjiarapoglou, V. Dokorou, S. Skoulika, A. Owczarzak, M. Kubicki and S. K. Hadjikakou, Inorg. Chem., 2014, 53, 8322–8333.
21. T. W. Greene and P. G. M. Wuts, in Protective group inOrganic Synthesis, Wiley-VCH, New York, 4th edn, 2007.
22. G. Sartori, R. Ballini, F. Bigi, G. Bosica, R. Maggi and P. Righi, Chem. Rev., 2004, 104, 199–250.
23. (a) G. V. M. Sharma, J. Janardhan Reddy, P. Sree Lakshmi and P. Radha Krishna, Tetrahedron Lett., 2004, 45, 6963–6965; (b) N. Suryakiran, P. Prabhakar, T. S. Reddy, K. Rajesh and Y. Venkateswarlu, Tetrahedron Lett., 2006, 47, 8039–8042; (c) R. Varala, S. Nuvula and S. R. Adapa, J. Org. Chem., 2006, 71, 8283–8286; (d) K. S. Kumar, J. Iqbal and M. Pal, Tetrahedron Lett., 2009, 50, 6244–6246; (e) F. Jahani, M. Tajbakhsh, H. Golchoubian and S. Khaksar, Tetrahedron Lett., 2011, 52, 1260–1264; (f) N. G. Khaligh, RSC Adv., 2012, 2, 12364–12370; (g) S. V. Atghia and S. SarviBeigbaghlou, J. Org. Chem., 2013, 745–746, 42–49; (h) A. Chinnappan, D. La and H. Kim, RSC Adv., 2013, 3, 13324–13328; (i) S. Karimian and H. Tajik, Chin. Chem. Lett., 2014, 25, 218–220.
24. Y. Basel and A. Hassner, J. Org. Chem., 2000, 65, 6368–6380.
25. J. Park and S. Hong, Chem. Soc. Rev., 2012, 41, 6931–6943.
26. (a) H. C. Aspinall, J. F. Bickley, J. L. M. Dwyer, N. Greeves, R. V. Kelly and A. Steiner, Organometallics, 2000, 19, 5416–5423; (b) L. Di Bari, M. Lelli, G. Pintacuda, G. Pescitelli, F. Marchetti and P. Salvadori, J. Am. Chem. Soc., 2003, 125, 5549–5558; (c) A. J. Wooten, P. J. Carroll and P. J. Walsh, Angew. Chem., Int. Ed., 2006, 45, 2549–2552; (d) A. J. Wooten, P. J. Carroll and P. J. Walsh, J. Am. Chem. Soc., 2008, 130, 7407–7419.
27. K. Ishihara, A. Sakakura and M. Hatano, Synlett, 2007, 686–703.
28. Z.-H. Guan, H. Lei, M. Chen, Z.-H. Ren, Y. Bai and Y.-Y. Wang, Adv. Synth. Catal., 2012, 354, 489–496.
29. Z. D. Crane, P. J. Nichols, T. Sammakia and P. J. Stengel, J. Org. Chem., 2011, 76, 277–280.
30. X. G. Guo, J. P. Shang, J. A. Li, L. G. Wang, Y. B. Ma, F. Shi and Y. Q. Deng, Synth. Commun., 2011, 41, 1102–1111.
31. M. Distaso and E. Quaranta, Tetrahedron, 2004, 60, 1531–1539.
32. M. Vargas-Sanchez, S. Lakhdar, F. Couty and G. Evano, Org. Lett., 2006, 8, 5501–5504.
33. J. E. Taylor, M. D. Jones, J. M. Williams and S. D. Bull, J. Org. Chem., 2012, 77, 2808–2818.
34. F. Szurdoki, A. Székács, H. M. Le and B. D. Hammock, J. Agric. Food Chem., 2002, 50, 29–40.
35. J. F. King, Z. R. Guo and D. F. Klassen, J. Org. Chem., 1994, 59, 1095–1101.
36. S. Okumura, Y. Takeda, K. Kiyokawa and S. Minakata, Chem. Commun., 2013, 49, 9266–9268.
37. P. Gopinath, S. Nilaya and K. M. Muraleedharan, Org. Lett., 2011, 13, 1932–1935.
38. G. Barker, P. O'Brien and K. R. Campos, Org. Lett., 2010, 12, 4176–4179.
39. S. K. Verma, R. Ghorpade, A. Pratap and M. P. Kaushik, Tetrahedron Lett., 2012, 53, 2373–2376.
40. G. A. Molander and I. Shin, Org. Lett., 2013, 15, 2534–2537.
41. C. Stock and R. Bruckner, Adv. Synth. Catal., 2012, 354, 2309–2330.
42. F. Ma, X. Xie, L. Zhang, Z. Peng, L. Ding, L. Fu and Z. Zhang, J. Org. Chem., 2012, 77, 5279–5285.
43. F. Jahani, M. Tajbakhsh, H. Golchoubian and S. Khaksar, Tetrahedron Lett., 2011, 52, 1260–1264.
44. M. W. Jones, R. A. Strickland, F. F. Schumacher, S. Caddick, J. R. Baker, M. I. Gibson and D. M. Haddleton, J. Am. Chem. Soc., 2012, 134, 1847–1852.
45. F. Troisi, A. Russo, C. Gaeta, G. Bifulco and P. Neri, Tetrahedron Lett., 2007, 48, 7986–7989.
46. C. T. Chen, J. H. Kuo, V. D. Pawar, Y. S. Munot, S. S. Weng, C. H. Ku and C. Y. Liu, J. Org. Chem., 2005, 70, 1188–1197.
47. T. Vilaivan, Tetrahedron Lett., 2006, 47, 6739–6742.
48. E. Alonso, D. J. Ramon and M. Yus, Tetrahedron, 1997, 53, 14355–14368.

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Footnotes

† Electronic supplementary information (ESI) available: Copies of ¹H and ¹³C NMR spectra for the products. See DOI: 10.1039/c6ra15160d

‡ Both authors contributed equally to this work.

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