The vicinal functionalization of olefins: a facile route to the direct synthesis of β-chlorohydrins and β-chloroethers

Abstract

An efficient and environmentally benign protocol for the synthesis of vicinal chlorohydroxy and chloromethoxy derivatives in a highly regioselective manner from olefins using NH₄Cl as a chlorine source and oxone as an oxidant in aqueous acetone and methanol is demonstrated. This methodology offers an additive and metal chloride free approach and is endowed with simple reaction conditions, high yields a broad substrate scope and good functional group tolerance. Moreover, the aromatic substrates with a terminal double bond exhibited merely Markovnikov selectivity, while the internal alkenes show exclusive regiocontrol and low to moderate diastereoselectivity.

---

Introduction

Halogen-containing organic compounds have received considerable attention from synthetic chemists as targets of synthesis, because they can be allowed to contain a variety of functional groups containing carbon, nitrogen, oxygen, and sulphur.¹ Over 2000 naturally occurring organochlorine compounds have been discovered,² and many of these compounds exhibit interesting biological activity of various kinds.³ In many instances, the chlorine atoms within the structures are responsible for either enhancing the biological activity or chemical stability and intrinsic potency of a natural product or a synthetic compound.⁴ A few examples of naturally occurring bioactive polychlorides, such as chlorosulfolipids, are shown in Fig. 1.⁵ In recent years, the increasing interest in chlorine-containing compounds spearheaded efforts to develop synthetic methodologies for the stereoselective installation of sp³ C–Cl (sp³ carbon–chlorine) bonds.⁶

Fig. 1 Natural bioactive chlorosulfolipids (I–IV).

Chlorination constitutes one of the most important reactions in organic chemistry and has been widely studied in the synthetic field.⁷ The traditional chlorination processes uses molecular chlorine as the chlorinating agent. To avoid difficulty with the use of hazardous, toxic and corrosive gaseous chlorine, different chlorine surrogates under various conditions have been utilized.⁸ For instance, oxidative chlorination, via in situ generation of the chlorinating agent from the oxidation of the chloride ion with a suitable oxidant, has emerged as a powerful tool for the production of chlorinated synthons.⁹

The vicinal functionalization of olefins, by the selective introduction of two different functional groups in a highly regio- and stereoselective manner with the formation of two new bonds, is a powerful synthetic method which rapidly increases molecular complexity.¹⁰ Vicinal halohydrin derivatives have found widespread applications in synthetic organic chemistry. Such halo derivatives serve as extremely versatile building blocks,¹¹ valuable bioactive materials¹² and key intermediates.¹³ One appealing approach for the synthesis of vicinal chlorohydrins involves the cleavage of the epoxide ring with HCl or metal chlorides or other chlorine reagents.^11a,14 Though these procedures have their own advantages, they also suffer from some limitations which include low atom efficiency with respect to the direct conversion of alkenes to chlorohydroxy derivatives and require the prior synthesis of epoxides from their precursors. Alternatively, a few reagent systems have been reported in the literature for the direct transformation of carbon–carbon double bonds to the corresponding 1,2-difunctionalized alkane i.e., the chloroalcohol derivative. They include: chloramine T trihydrate or 1,3-dichloro-5,5-dimethylhydantoin (DCDMH) in N-tosyl-L-threonine (NTsLT)/t-BuOH/water,¹⁵ NaIO₄/NaCl or LiCl in 30% H₂SO₄/CH₃CN/H₂O,¹⁶ trichlorocyanuric acid in acetone/H₂O¹⁷ and N-chlorosuccinimide (NCS)/thiourea in THF/H₂O.¹⁸ However, most of the current methods usually have the disadvantages of using expensive reagents, acidic additives, metal chlorides or strong acids as chlorinating agents and a catalyst, producing low yields and have a limited applicability to olefinic substrates. Owing to the limitations of the above-mentioned approaches and the widespread interest in chlorine-containing compounds, still there is a need for the development of a simple, efficient and sustainable protocol for the production of chlorohydrins.

From a green chemistry point of view, avoiding toxic and/or hazardous reagents and solvents, diminishing the energy consumption, obtaining more atom efficiency and using healthy and safer reaction processes are highly desirable.¹⁹ In our ongoing interest in the development of eco-friendly halogenation protocols,^9c,20 we have previously disclosed the oxidative chlorination of aromatic rings^20a and carbonyl compounds.^20b We report herein a facile synthesis of β-chlorohydrins and β-chloroethers from olefins using NH₄Cl as a chlorine source and oxone® as an oxidant under mild conditions in environmentally preferred²¹ solvents (Scheme 1).

Scheme 1 The vicinal functionalization of olefins using NH₄Cl and oxone®.

Oxone® (2KHSO₅·KHSO₄·K₂SO₄), a potassium triple salt containing potassium peroxy monosulfate, is a commercially available, low-cost, non-toxic and eco-friendly oxidant. Because of the discovery of multiple innovative applications, oxone® is becoming an increasingly popular reagent for several organic transformations.^22b

Results and discussion

Initially, the reaction of styrene with NH₄Cl and oxone® was selected as the model reaction and the effect of the solvent on the reaction in terms of the reaction time and yield was investigated (Table 1). A series of non-polar and polar solvents (single or their combination with water) were evaluated. The reaction in organic solvents or water alone did not provide the desired chlorohydroxylation product 2a in a high yield (Table 1, entries 1–10). By changing the reaction medium to a mixture of non-polar solvent and water, the reaction time was decreased but a low yield was observed. However, water combined with polar solvents dramatically improved the yield of the corresponding chlorohydrin (Table 1, entries 14–18). Remarkably, the best results were obtained when a 1 : 1 mixture of acetone and water was used as the solvent system (Table 1, entry 15).

Table 1 The 1,2-difunctionalization of styrene: the effect of the solvent^a,b

S. no.	Solvent	Time (h)	Yield (%)
a Reaction conditions: substrate 1a (2 mmol), NH4Cl (2.2 mmol), oxone® (2.2 mmol), solvent (10 mL), room temperature.b The products were characterized by NMR spectroscopy and the yields were based on GC.
1	DCM	24	00
2	CHCl3	24	00
3	CCl4	24	00
4	CH3CN	24	<5
5	Acetone	24	<5
6	THF	24	<5
7	DME	24	<10
8	1,4-Dioxane	24	<10
9	Methanol	3.33	00
10	H2O	1	21
11	DCM–H2O (1 : 1)	0.66	<10
12	CHCl3H2O (1 : 1)	0.66	<10
13	CCl4–H2O (1 : 1)	0.66	<10
14	CH3CN–H2O (1 : 1)	3.5	88
15	AcetoneH2O (1 : 1)	3.5	95
16	THF/H2O (1 : 1)	3.5	92
17	DME/H2O (1 : 1)	3.5	73
18	1,4-Dioxane/H2O (1 : 1)	3.5	79

---

With the optimal conditions in hand, we proceeded to explore the utility and scope of the chlorohydroxylation with other alkenes. As is evident in Table 2, this method is compatible with a wide array of terminal (aromatic and aliphatic) and 1,2-disubstituted (symmetrical and unsymmetrical) olefins. First, we tested the aromatic substrates with a terminal double bond and obtained the corresponding products 2a–k in good to excellent yields with exclusive Markovnikov regioselectivity (Table 2). The styrenyl substrates with deactivating groups on the phenyl ring, such as 4-bromostyrene (1b), 4-chlorostyrene (1c), 4-vinylbenzoic acid (1d) and 3-nitrostyrene (1e), reacted smoothly to give the preferred chlorohydroxylation products 2b, 2c, 2d and 2e in 90%, 91%, 81% and 83% yields, respectively (Table 2, entries 2–5). However, those with activating substituents on the phenyl group including 4-methylstyrene (1f), 2,4-dimethylstyrene (1g) and 4-methoxystyrene (1h) afforded the respective products 2f, 2g and 2h in 83%, 72% and 82% yields, respectively (Table 2, entries 6–8). These results indicate that, both the activating and deactivating groups were well tolerated with the present reaction conditions. A polyaromatic olefin, i.e. 2-vinylnaphthalene (1i), was also observed to be an excellent substrate for the reaction and generated the compound 2i in an 82% yield (Table 2, entry 9). In addition, the α-substituted styrene derivatives 1j and 1k were reacted in a similar manner and furnished the corresponding products 2j and 2k in 91% and 77% yields, respectively (Table 2, entries 10–11). Moreover, the aliphatic terminal alkene 1l produced the corresponding vicinal chlorohydrins in a 95% yield as a mixture of regioisomers 2l′ (75%) and 2l′′ (20%) (Table 2, entry 12).

Table 2 The synthesis of β-chlorohydrins from olefins using NH₄Cl and oxone®^a,b,c

Entry	Olefin	Time (h)	Product	Yield (%)	dr
a Reaction conditions: substrate (2 mmol), NH4Cl (2.2 mmol), oxone® (2.2 mmol), acetone–H2O (1 : 1; 10 mL), room temperature.b The products were characterized by NMR spectroscopy and the yields were based on GC.c dr determined by crude ^1H NMR spectroscopy.d NR refers to no reaction observed.
1	1a	3.5		95	—
2	1b	4		90	—
3	1c	4		91	—
4	1d	4		81	—
5	1e	7		83	—
6	1f	3		83	—
7	1g	3		72	—
8	1h	1		82	—
9	1i	6		2	—
10	1j	1		91	—
11	1k	3.5		77	—
12	1l	0.5		95	—
13	1m	2.16		70	6.77 : 1
14	1n	1.25		88	5.28 : 1
15	1o	2		68	8 : 1
16	1p	24		22	10 : 1
17	1q	24		25	5.25 : 1
18	1r	24		NR^d	—
19	1s	24		NR^d	—
20	1t	6		63	1.42 : 1
21	1u	5		51	1.68 : 1
22	1v	3		81	3.34 : 1
23	1w	2		96	2.42 : 1
24	1x	0.33		98	11.25 : 1

---

Sequentially, we examined the 1,2-disubstituted symmetrical and unsymmetrical olefins 1m–x under the same reaction conditions and obtained the corresponding products 2m–x with exceptional regiocontrol and low to moderate diastereoselectivity. The formation of the products 2m, 2n and 2o was observed in 70% (dr 6.77 : 1), 88% (dr 5.28 : 1) and 68% (8 : 1) yields, respectively, with exclusive α-hydroxy-β-chloro selectivity (Table 2, entries 13–15). However, the deactivated olefin containing acid, ester, carbonyl and/or nitro functionality with an aryl substituent did not react completely even after 24 h and yielded the respective products with the same regioselectivity albeit in low or zero yields (Table 2, entries 16–19). The symmetrical disubstituted substrates, such as cis-stilbene (1t) and trans-stilbene (1u), generated the corresponding vicinal chlorohydroxy products 2t and 2u in 63% (dr 1.42 : 1) and 51% (1.68 : 1) yields, respectively (Table 2, entries 20–21), whereas in the case of 1,4-naphthoquinone (1y) a reaction was not observed (Scheme 2). In addition, the aromatic cyclic alkenes 1v and 1w were used in this reaction to furnish merely the Markovnikov products 2v and 2w in 81% (dr 3.34 : 1) and 96% (dr 2.42 : 1) yields, respectively (Table 2, entries 22–23). Moreover, the reaction of an aliphatic cyclic olefin, i.e. cyclohexene (1x), resulted in the formation of the product 2x in a 98% yield with 11.25 : 1 diastereoselectivity in a very short reaction time (Table 2, entry 24).

Scheme 2

Inspired by the above results obtained for the chlorohydroxylation of olefins in aqueous acetone under mild conditions, we then turned our attention to further investigate the same reaction in a nucleophilic alcoholic solvent, i.e. MeOH. In this context, a variety of olefins were subjected to the optimized conditions used for chlorohydroxylation in methanol and the corresponding β-chloroether derivatives were obtained in good to excellent yields. The representative results of the chloromethoxylation summarized in Table 3 indicate that all of the substrates 1a–x were compatible with these mild conditions. The reactions of the styrenyl substrates and polyaromatic alkene (i.e. 2-vinylnaphthalene) were completely regiospecific and provided the corresponding α-methoxy-β-chloro derivatives 3a–k in yields ranging from 64 to 91%. Whereas, the reaction of the terminal aliphatic olefin, i.e. 1-octene (1l), was non-regiospecific and afforded a mixture of regioisomers (3l′ and 3l′′) (Table 3, entries 1–12). The use of 1,2-disubstituted symmetrical and unsymmetrical olefins generated the corresponding vicinal chloromethoxylation products 3m–x with highly regio- and low to moderate diastereoselectivities (Table 3). The activated olefinic substrates 1m, 1n and 1o furnished the corresponding vicinal functionalized products 3m, 3n and 3o in 79% (dr 5.58 : 1), 83% (dr 2.45 : 1) and 61% (dr 9.16 : 1) yields, respectively (Table 3, entries 13–15). Interestingly, the deactivated compounds 1p–s proceeded efficiently to gave the relevant products 3p–s in 50% to 90% yields (Table 3, entry 16–19). The treatment of both the cis- and trans-stilbenes yielded the respective products 3t and 3u in 81% (dr 2 : 1) and 86% (dr 1.38 : 1) yields, respectively (Table 3, entries 20–21). Surprisingly, the formation of product 3y in a 57% yield was observed when the symmetrical disubstituted substrate, i.e. 1,4-naphthoquinone (1y), was used in this reaction (Scheme 2). Additionally, the aromatic and aliphatic cyclic olefins 1v–x under similar reaction conditions resulted in the formation of the desired products 3v, 3w and 3x in 85% (dr 2.86 : 1), 92% (dr 2.28 : 1) and 75% (dr 5 : 1) yields, respectively (Table 3, entries 22–24).

Table 3 The synthesis of β-chloroethers from olefins using NH₄Cl and oxone®^a,b,c

Entry	Olefin	Time (h)	Product	Yield (%)	dr
a Reaction conditions: substrate (2 mmol), NH4Cl (2.2 mmol), oxone® (2.2 mmol), methanol (5 mL), room temperature.b The products were characterized by NMR spectroscopy and the yields were based on GC.c dr determined by crude ^1H NMR spectroscopy.
1	1a	3.33		90	—
2	1b	5		91	—
3	1c	3.5		91	—
4	1d	4		83	—
5	1e	9		88	—
6	1f	4		83	—
7	1g	6		64	—
8	1h	4		84	—
9	1i	10		90	—
10	1j	1.33		68	—
11	1k	6		83	—
12	1l	1.5		82	—
13	1m	6		79	5.58 : 1
14	1n	3		83	2.45 : 1
15	1o	2.25		61	9.16 : 1
16	1p	12		64	3.92 : 1
17	1q	5		90	4.62 : 1
18	1r	2.75		71	5.45 : 1
19	1s	24		50	5.25 : 1
20	1t	7		81	2 : 1
21	1u	4		86	1.38 : 1
22	1v	3		85	2.86 : 1
23	1w	3		92	2.28 : 1
24	1x	0.5		75	5 : 1

---

The above investigated results obtained for both the vicinal chlorohydroxylation and chloromethoxylation indicate that all the aromatic olefins, irrespective of the substitution on the double bond or phenyl ring, show the exclusive Markovnikov regioselectivity based on the fact that the α-position (benzylic) is more positive than the β-position due to the presence of the aromatic ring. Moreover, most of the 1,2-disubstituted alkenes exhibited predominant anti-stereoselectivity. The stereochemistry of the vicinal functionalized products is assigned based on ¹H NMR spectroscopy by comparing the chemical shifts (δ) and coupling constants (J) of the protons attached to the carbon atoms bearing –Cl and –OH or –OMe with previously reported data.

A probable reaction pathway for the formation of β-chlorohydrins and β-chloroethers in a highly regioselective manner is illustrated in Scheme 3. It is assumed that the Cl⁻ (NH₄Cl) ion is oxidized with oxone to generate the Cl⁺ (HOCl) ion in situ.^9c,22 The electrophilic addition of the Cl⁺ ion onto the olefinic double bond (A) leads to the formation of a reactive intermediate (B). The intermediate (B) reacts with the nucleophile (OH⁻ or MeO⁻) to afford the corresponding α-hydroxy or methoxy-β-chloro derivative (C).

Scheme 3 The probable reaction pathway for the formation of β-chlorohydrins and β-chloroethers.

Conclusions

In conclusion we have developed a general and green approach which utilizes environmentally friendly, cheap and stable reagents (oxone and NH₄Cl) for the vicinal functionalization of olefins. This methodology is applicable to a wide variety of olefinic substrates, which includes terminal (aromatic and aliphatic) and 1,2-disubstituted (symmetrical and unsymmetrical) alkenes, and offers an economical and attractive avenue for the production of valuable synthetic intermediates bearing regio- and stereoselectively installed functionalities, including α-hydroxy-β-chloro and α-methoxy-β-chloro derivatives, under mild conditions at room temperature.

Experimental section

The typical procedure for the vicinal functionalization of olefins

Oxone® (2.2 mmol) was slowly added to a well stirred solution of NH₄Cl (2.2 mmol) and olefin (2 mmol) in acetone–H₂O (1 : 1; 10 mL) or methanol (5 mL) and the reaction mixture was allowed to stir at room temperature until the olefin completely disappeared (monitored by TLC, eluent: n-hexane–ethyl acetate). The organic layer was separated and the aqueous phase was extracted with ethyl acetate (2 × 15 mL). The combined organic layers were dried over anhydrous Na₂SO₄, filtered and concentrated. The residue was purified by column chromatography on silica gel (100–200 mesh) using n-hexane–ethyl acetate as the eluent to give the desired products.

Acknowledgements

We thank the CSIR Network project CSC-0125 for financial support. P.S. and K.S. acknowledge the financial support from UGC, India in the form of fellowship. M.A.K., M.M.R. and M.N. acknowledge the financial support from CSIR, India in the form of fellowships.

Notes and references

1. M. B. Smith and J. March, Advanced Organic Chemistry: Reactions, Mechanism and Structure, Wiley-VCH, 2001.
2. (a) N. Winterton, Green Chem., 2000, 2, 173; (b) G. W. Gribble, Prog. Chem. Org. Nat. Prod., 1996, 68, 1; (c) G. W. Gribble, Prog. Chem. Org. Nat. Prod., 2010, 91, 1 and references cited therein.
3. G. W. Gribble, J. Chem. Educ., 2004, 81, 1441.
4. (a) K. Naumann, J. Prakt. Chem., 1999, 341, 417; (b) P. Metrangolo and G. Resnati, Science, 2008, 321, 918.
5. For pioneering studies on this class of molecules, see: (a) J. Elovson and P. R. Vagelos, Biochemistry, 1970, 9, 3110; (b) J. A. Hansen, Physiol. Plant., 1973, 29, 234; (c) E. L. Mercer and C. L. Davies, Phytochemistry, 1979, 18, 457.
6. (a) C. Nilewski and E. M. Carreira, Eur. J. Org. Chem., 2012, 1685; (b) T. Kawahara and T. Okino, Stud. Nat. Prod. Chem., 2012, 36, 219; (c) T. Umezawa and F. Matsuda, J. Synth. Org. Chem. Jpn., 2011, 69, 1122; (d) D. K. Bedke and C. D. Vanderwal, Nat. Prod. Rep., 2011, 28, 15; (e) T. Yoshimitsu, N. Fukumoto and T. Tanaka, J. Org. Chem., 2009, 74, 696; (f) C. Nilewski, R. W. Geisser and E. M. Carreira, Nature, 2009, 457, 573.
7. (a) F. Ullmann, in Ullmann's Encyclopedia of Industrial Chemistry, ed. W. Gerhartz, Y. S. Yamamoto, F. T. Campbell, R. Pfefferkorn and J. F. Rounsaville, Wiley-VCH, Weinheim, 6th edn, 2002; (b) R. C. Larock, Comprehensive Organic Transformations, Wiley-VCH, New York, 2nd edn, 1999; (c) J. Fauvarque, Pure Appl. Chem., 1996, 68, 1713.
8. (a) A. Podgoršek, M. Jurisch, S. Stavber, M. Zupan, J. Iskra and J. A. Gladysz, J. Org. Chem., 2009, 74, 3133; (b) W. M. Golebiewski and M. Gucma, Synthesis, 2007, 3599; (c) L. Yang, Z. Lub and S. S. Stahl, Chem. Commun., 2009, 6460; (d) A. Podgoršek and J. Iskra, Molecules, 2010, 15, 2857; (e) V. V. Zhdankin and P. J. Stang, Chem. Rev., 2008, 108, 5299 and references cited therein.
9. (a) R. K. Dieter, L. E. Nice and E. V. Sadanandan, Tetrahedron Lett., 1996, 37, 2377 ; For reviews, see: ; (b) A. Podgorsek, M. Zupan and J. Iskra, Angew. Chem., Int. Ed., 2009, 48, 8424; (c) K. V. V. Krishna Mohan and N. Narender, Synthesis, 2012, 44, 15.
10. (a) H. C. Kolb, M. S. VanNieuwenhze and K. B. Sharpless, Chem. Rev., 1994, 94, 2483; (b) Z. P. Demko, M. Bartsch and K. B. Sharpless, Org. Lett., 2000, 2, 2221; (c) D. Kalyani and M. S. Sanford, J. Am. Chem. Soc., 2008, 130, 2150; (d) C. F. Rosewall, P. A. Sibbald, D. V. Liskin and F. E. Michael, J. Am. Chem. Soc., 2009, 131, 9488; (e) M. C. Paderes and S. R. Chemler, Org. Lett., 2009, 11, 1915; (f) K. B. Urkalan and M. S. Sigman, Angew. Chem., Int. Ed., 2009, 48, 3146; (g) F. Cardona and A. Goti, Nat. Chem., 2009, 1, 269.
11. (a) J. G. Smith and M. Fieser, Fieser and Fieser's Reagent for Organic Synthesis, John Wiley and Sons, New York, 1990, vol. 1–12; (b) A. Ros, A. Magriz, H. Dietrich, R. Fernández, E. Alvarez and J. M. Lassaletta, Org. Lett., 2006, 8, 127; (c) A. Solladié-Cavallo, P. Lupattelli and C. Bonini, J. Org. Chem., 2005, 70, 1605.
12. N. Shakya, N. C. Srivastav, N. Desroches, B. Agrawal, D. Y. Kunimoto and R. Kumar, J. Med. Chem., 2010, 53, 4130.
13. (a) M. B. Smith and J. March, Advanced Organic Chemistry, Wiley-Interscience, New York, NY, 5th edn, 2001, p. 478 and references cited therein; (b) Y. Akiyama, T. Fukuhara and S. Hara, Synlett, 2003, 1530; (c) D. Dolence and M. Harej, J. Org. Chem., 2002, 67, 312; (d) K. G. Watson, Y. M. Fung, M. Gredley, G. J. Bird, W. R. Jackson, H. Gountzos and B. R. Matthews, J. Chem. Soc., Chem. Commun., 1990, 1018.
14. (a) C. Bonini and G. Righi, Synthesis, 1994, 225; (b) B. C. Ranu and S. Banerjee, J. Org. Chem., 2005, 70, 4517; (c) B. Das, M. Krishnaiah and K. Venkateswarlu, Chem. Lett., 2007, 36, 82; (d) M. A. Reddy, K. Surendra, K. Bhanumathi and K. R. Rao, Tetrahedron, 2002, 58, 6003; (e) C. E. Garrett and G. C. Fu, J. Org. Chem., 1997, 62, 4534.
15. J. Zhang, J. Wang, Z. Qiu and Y. Wang, Tetrahedron, 2011, 67, 6859.
16. G. K. Dewkar, S. V. Narina and A. Sudalai, Org. Lett., 2003, 5, 4501.
17. G. F. Mendonça, A. M. Sanseverino and M. C. S. Mattos, Synthesis, 2003, 45.
18. P. A. Bentley, Y. Mei and J. Du, Tetrahedron Lett., 2008, 49, 1425.
19. R. A. Sheldon, Chem. Soc. Rev., 2012, 41, 1437.
20. (a) N. Narender, K. V. V. Krishna Mohan, P. Srinivasu, S. J. Kulkarni and K. V. Raghavan, Indian J. Chem., Sect. B: Org. Chem. Incl. Med. Chem., 2004, 43, 1335; (b) P. Swamy, M. A. Kumar, M. M. Reddy and N. Narender, Chem. Lett., 2012, 41, 432; (c) M. M. Reddy, M. A. Kumar, P. Swamy and N. Narender, Tetrahedron Lett., 2011, 52, 6554; (d) M. A. Kumar, C. N. Rohitha, M. M. Reddy, P. Swamy and N. Narender, Tetrahedron Lett., 2012, 53, 191; (e) M. A. Kumar, C. N. Rohitha and N. Narender, Tetrahedron Lett., 2012, 53, 1401; (f) M. Naresh, M. A. Kumar, M. M. Reddy, P. Swamy, J. B. Nanubolu and N. Narender, Synthesis, 2013, 45, 1497.
21. K. Alfonsi, J. Collberg, P. J. Dunn, T. Fevig, S. Jennings, T. A. Johnson, H. P. Kleine, C. Knight, M. A. Nagy, D. A. Perry and M. Stefaniak, Green Chem., 2008, 10, 31.
22. (a) R. E. Montgomery, J. Am. Chem. Soc., 1974, 96, 7820; (b) H. Hussain, I. R. Green and I. Ahmed, Chem. Rev., 2013, 113, 3329.

---

Footnote

† Electronic supplementary information (ESI) available: characterization data and copies of ¹H and ¹³C NMR spectra for all the products. See DOI: 10.1039/c4ra01641f

---