Catalytic asymmetric synthesis of enantioenriched β-nitronitrile bearing a C-CF₃ stereogenic center

Abstract

CF₃ substituted β-nitronitriles with an all-carbon quaternary stereogenic center have been synthesized via asymmetric cyanation reaction. In situ generated chiral Ti^IV salen complexes were used as catalysts for asymmetric addition of TMSCN to nitroolefins containing a trifluoromethylated prochiral center in the β-position, to afford the corresponding β-nitronitriles in excellent yield (up to 93%) with good enantioselectivity (up to 99%). This newly developed catalytic protocol led to the efficient synthesis of useful but challenging chiral α,α′-disubstituted β-amino acids.

---

Introduction

The catalytic enantioselective construction of an all-carbon stereocenter containing a CF₃ group is important from the perspective of producing novel bio-active motifs for their commercial exploitation. However, assembling these structural motifs in an enantioselective manner is particularly challenging and a current area of research under intense investigation. Despite the challenges, the necessity of producing such CF₃ group containing compound is driven by the fact that incorporation of trifluoromethyl group to organic molecules improve their lipophilicity, bioavailability, and metabolic stability thereby bioactivity.¹ Consequently, these facts clearly manifest its importance in pharmaceuticals, agrochemicals, and material science.² Though, great progress has been made to incorporate CF₃ group to organic scaffolds but surprisingly the synthesis of CF₃ substituted quaternary carbon stereocenter have received scant attention.³ Shibata and coworkers for the first time reported an asymmetric synthesis of an all-carbon stereocenter bearing a CF₃ group through cyanation of β-aryl-β-CF₃-disubstituted enones.⁴ Although different strategies were followed to construct CF₃-containing^5a–l and fluorinated^5m quaternary compounds however, to the best of our knowledge a direct route for the synthesis of tetrasubstituted carbon quaternary stereocenters via cyanation of CF₃-bearing simple disubstituted olefins, is not reported nevertheless there are only few reports available in literature for the synthesis of chiral β-nitro nitrile by using simple nitroalkenes.⁶ The asymmetric addition of cyanide to activated olefins like α,β-unsaturated ketones,⁷ α,β-unsaturated imides,⁸ α,β-unsaturated N-acylpyrroles,⁹ α,β-unsaturated esters,¹⁰ and nitroalkenes are scarcely studied when compared to the asymmetric cyanation of C=O¹¹ and C=N¹² double bonds. More specifically, in our continuous interest in the development of enantioselective cyanation reactions, herein we are disclosing the cyanation reaction which is synthetically different from the other existing reports. In this report, we wish to describe an efficient catalytic protocol for the enantioselective conjugate Michael addition of TMSCN to β-CF₃-β-disubstituted nitroalkenes to give CF₃-substituted β-nitro nitrile (Scheme 1).

Scheme 1 Construction of CF₃ substituted quaternary stereocentres via asymmetric cyanation reaction.

Recently, we reported asymmetric addition of cyanide to nitro olefins,^6f however in the present report, in order to incorporate CF₃ group, we prepared desired nitroolefins 1a–l (ref. 5a) for their asymmetric cyanation. Thus obtained CF₃ substituted nitro nitrile products can be readily transformed into important compound like α-CF₃ substituted β-amino acid, CF₃ substituted amino alcohol and trifluromethylated diammine.

Results and discussion

In our quest to develop a simple catalytic system for asymmetric cyanation of β,β-disubstituted nitro alkenes our first choice was the use of chiral Ti(OⁱPr)₄:salen complexes as catalysts as these have proven record in the cyanation of carbonyls and activated olefins.^11,6d Accordingly, various chiral ligands (L1–L11, 10 mol%) (Fig. 1) with Ti(OⁱPr)₄ (10 mol%) were used to in situ generate active catalyst for the cyanation of 1a used as a model substrate and TMSCN as a cyanide source in toluene at room temperature (Table 1). We observed that the yield and enantioselectivity of product 2a were greatly affected by the substituents on the salen ligands: as shown in Table 1 (entries 1–11), the best salen ligand was L3, bearing ^tBu groups at the 3′- and the 5′-positions in the salicylidene phenyl rings (90% yield, 77% ee, entry 3). In addition, dimer (L6 & L7; entries 6 & 7), polymer (L8; entry 8) and macrocyclic (L9–L11; entries 9–11) salen ligands were not so effective both in terms of reactivity and enantioselectivity. Therefore, taking L3 as best among the screened ligand, and in order to further improve the results, we varied titanium metal sources viz., Ti(OEt)₄, Ti(OBu)₄, Ti(O^tBu)₄ and TiCl₄ (entry 12–15). We observed that the chiral L3 ligand in combination with Ti(O^tBu) generates most effective catalyst to give the desired product in 95% yield and 80% ee (entry 14).

Fig. 1 Ligands used in the model reaction.

Table 1 Ligand screening in the model reaction^a

Entry	Ligand (10 mol%)	Metal source (10 mol%)	Time (h)	Yield^b (%)	ee^c (%)
a Enantioselective hydrocyanation reaction of 1a (0.1 mmol) was carried out with Ti:salen catalyst in toluene (1 mL) using TMSCN (0.20 mmol) as a source of cyanide.b Isolated yield.c ee were determined by chiral HPLC using OD-H column.
1	L1	Ti(O^iPr)4	08	91	54
2	L2	Ti(O^iPr)4	08	92	40
3	L3	Ti(O^iPr)4	08	90	77
4	L4	Ti(O^iPr)4	24	Trace	—
5	L5	Ti(O^iPr)4	12	82	67
6	L6	Ti(O^iPr)4	10	88	59
7	L7	Ti(O^iPr)4	10	88	74
8	L8	Ti(O^iPr)4	12	85	42
9	L9	Ti(O^iPr)4	12	82	40
10	L10	Ti(O^iPr)4	12	80	15
11	L11	Ti(O^iPr)4	12	80	25
12	L3	Ti(OEt)4	08	90	74
13	L3	Ti(OBu)4	08	92	77
14	L3	Ti(O^tBu)4	08	95	80
15	L3	TiCl4	14	80	75

---

Having established, the combination of L3:Ti(O^tBu)₄ as effective catalyst, we experimented on optimization of solvent, temperature and catalyst loading. First we screened solvents viz., toluene, benzene, CH₂Cl₂, CHCl₃, DCE, THF, and dioxane to carry out the asymmetric cyanation reaction under the above optimized condition (Table 2, entries 1–7), however toluene remained the solvent of choice (entry 1). Temperature had significant effect on enantioselectivity of product (entries 1, 8–10). Lowering the temperature from room temperature to 0 °C showed beneficial effect on the product ee (from rt 80% to 89%) though the reaction took longer time (24 h, entry 9) to reach completion. A further lowering of the temperature (−10 °C) did not have any positive impact on the product ee at the same time reaction became very sluggish (48 h, entry 10). Therefore, 0 °C (entry 9) was taken as optimum for further studies on catalyst loading varied through 2, 5, 7, 10 and 15 mol% (entries 11–13, 9 and 14 respectively). However, 10 mol% catalyst loading (entry 9) was found to be optimum.

Table 2 Optimization of the reaction conditions^a

Entry	Catalyst loading (mol%)	Solvent	Temp. (°C)	Time (h)	Yield^b (%)	ee^c (%)
a Enantioselective hydrocyanation reaction of 1a (0.1 mmol) was carried out with L3:Ti(O^tBu)4 catalyst using TMSCN (0.20 mmol) as a source of cyanide.b Isolated yield.c ee were determined by chiral HPLC using OD-H column.
1	10	Toluene	RT	08	95	80
2	10	Benzene	RT	08	91	78
3	10	CH2Cl2	RT	08	92	52
4	10	CHCl3	RT	10	86	54
5	10	THF	RT	16	73	58
6	10	1,4-Dioxane	RT	20	70	59
7	10	DCE	RT	20	76	48
8	10	Toluene	10	16	93	85
9	10	Toluene	0	24	92	89
10	10	Toluene	−10	48	76	89
11	2	Toluene	0	48	Trace	—
12	5	Toluene	0	48	54	89
13	7	Toluene	0	36	70	89
14	15	Toluene	0	16	93	86

---

To further improve the results we considered using different like N-oxides, phenols, molecular sieve (4 Å) but the results (Table S1†) were rather not very encouraging (see ESI†).

Having established the reaction parameters for the use of the L3:Ti(O^tBu)4 catalyst in the asymmetric cyanation reaction with the substrate 1a and TMSCN at 0 °C, we next extended this protocol to a range of trifluoromethylated nitroalkenes (1a–l) to check the general applicability of our system and the results are summarized in Table 3. Both electron donating and withdrawing substituents at different positions on the aryl ring of the trifluoromethylated nitroalkenes (entries 2–11) afforded the products in high yields and enantioselectivities. The results of these studies do not indicate any pronounced effect of electronic properties of the substrates used herein.

Table 3 Substrate scope in the synthesis of β-nitro nitrile using L3:Ti(O^tBu)₄ catalyst^a

Entry	R	Time (h)	Yield^b (%)	ee^c (%)
a Enantioselective hydrocyanation reaction of 1a–l (0.1 mmol) was carried out with L3:Ti(O^tBu)4 catalyst (10 mol%) in toluene (1 mL) using TMSCN (0.20 mmol) as a source of cyanide.b Isolated yield.c ee were determined by chiral HPLC using OD-H column.d ee were determined after crystallization by chiral HPLC using OD-H column.e The absolute configuration was determined by single-crystal X-ray structure.f The reaction of 1d was carried out at room temperature for 60 h.g Reaction of 1a carried out in 10 mmol scale.
1	C6H5 (1a)	24	91	89 (99)^d (S)^e
2	3-MeC6H4 (1b)	24	87	83
3	4-MeC6H4 (1c)	24	92	88
4^f	2-MeOC6H4 (1d)	60	52	75
5	4-MeOC6H4 (1e)	24	93	88
6	4-ClC6H4 (1f)	24	86	82
7	4-FC6H4 (1g)	24	88	83
8	3-FC6H4 (1h)	24	87	79
9	2-FC6H4 (1i)	24	80	99
10	3,4-FC6H3 (1j)	24	88	74
11	3-CF3C6H4 (1k)	24	85	80
12	C6H5CH2 (1l)	36	82	53
13^g	C6H5 (1a)	24	91	89

---

Moreover, benzyl substituted nitroalkene (1l) gave product with 53% ee though in good yield (82%; entry 12). Since these products are new we have characterized them with fully, however for determining absolute configuration single-crystal X-ray structure of product 2a was determined, which was found to be S with (+) rotation. For rest of the compounds the rotation was found to be (+) but in the absence of single crystals of these compounds absolute conference was not determined. However, based on the absolute configuration of 2a, a working model for asymmetric induction is proposed (Fig. 2) where the nitro group of alkene weakly coordinates to Ti(IV) and due to the presence of ^tBu-groups in the catalyst only Re-face is open to attack of cyanide at the β-position of the nitroalkene.

Fig. 2 Proposed model for asymmetric induction.

As a showcase we have subjected the nitrile product 2a to reduction, hydrolysis and Boc-protection^6a,6d to produce enantioenriched N-Boc protected trifluoromethylated β-amino acid 5a where no racemization of the intermediates was noticed under the reaction conditions used for this conversion (Scheme 2). Analogous β-amino acids are very important molecules in pharmaceuticals.¹³

Scheme 2 Synthetic transformations of product 2a.

Experimental section

Different aldehydes and reagents were used as received. All the solvents used in the present study were dried by known purification technique.¹⁴ NMR spectra were obtained with a Bruker F113V spectrometer (500 MHz/200 MHz) and are referenced internally with TMS. Splitting patterns were reported as s, singlet; d, doublet; dd, doublet of doublet; t, triplet; m, multiplet. Enantiomeric excess (ee) were determined by HPLC using Daicel Chiralcel OD-H chiral columns with 2-propanol/hexane as eluent. Optical rotations were determined by automatic polarimeter. For the product purification flash chromatography was performed using silica gel 100–200 mesh. Different ligands L1–L4,^15a–d L6–L8,^15e–g and L9–L11 (ref. 15h) were synthesized according to the previously reported methods. Trifluoromethylated nitroalkenes (1a–l) were prepared according to the reported literature.^5a

Synthesis of ligand L5

(i) Synthesis of 4-(adamantan-1-yl)-2-(tert-butyl)phenol (A1). To a solution of 2-tert-butylphenol (10 mmol) and 1-admantanol (10 mmol) were dissolved in DCM (20 mL). Con. H₂SO₄ (0.6 mL) was added dropwise slowly to reaction mixture at 0 °C over 10 minutes. The reaction mixture was stirred two hours at room temperature and neutralized by adding 5% aqueous sodium hydroxide solution. The resulting mixture was extracted with DCM (40 mL × 3) and organic phase washed with brine and dried over anhydrous sodium sulphate. After removing solvent, crude product was purified by silica gel chromatography using EtOAc/hexane to afford the desired product (yield 42%).

(ii) Synthesis of 5-(adamantan-1-yl)-3-(tert-butyl)-2-hydroxybenzaldehyde (A2). A mixture of 2,6-lutidine (2.0 mmol), SnCl₄ (1.0 mmol) and 4-(adamantan-1-yl)-2-(tert-butyl)phenol (3.5 mmol) in 15 mL of dry toluene was stirred at room temperature under inert atmosphere. After 1 h paraformaldehyde (1.3 mmol) was added to reaction mixture and heated under reflux for 8 h and the reaction progress was checked on TLC. Once the reaction was completed, the reaction mixture was allowed to cool to 25 °C and mixture of water and diethyl ether (40 mL) each was added to it. The resulting emulsion was filtered through a pad of Celite and the layers were separated. The organic phase was washed with water, brine, and dried over anhydrous Na₂SO₄, and then concentrated on rota-evaporater. The crude product was purified by silica gel chromatography using EtOAc/hexane as eluent to afford the desired product (yield 88%).

(iii) (R,R)-(−)N,N-Bis(5-adamantyl-3-tert-butylsalicylidene)-1,2-cyclohexanediamine (L5). 5-(Adamantan-1-yl)-3-(tert-butyl)-2-hydroxybenzaldehyde (1 mmol) was dissolved in 3 mL of ethanol/DCM (9 : 1). The solution of (1R,2R)-(−)-cyclohexanediamine (0.5 mmol) in ethanol (0.5 mL) was added slowly to reaction mixture, and stirred at room temperature for 6–8 h. After completion of reaction, the resulting solution was concentrated on rota-evaporater and yellow solid washed with cold ethanol to give desired compound (yield 87%).

Typical procedure for asymmetric cyanation of trifluoromethylated nitro olefins

Ti(O^tBu)₄ (10 mol%, 0.01 mmol) was added to stirring solution of ligand L3 (10 mol%, 0.01 mmol) in freshly dry toluene (1 mL) under inert atmosphere at room temperature. After stirring for 45 min, substrate (0.1 mmol) was added and resulting solution was cooled to 0 °C. To cooled solution TMSCN (0.2 mmol) was added slowly drop wise over 20 min. The reaction was monitored by TLC, after the completion of the reaction, aqueous NaHCO₃ was added to quench the reaction and extracted with DCM (15 mL × 3). The organic layer was washed with brine, dried over anhydrous Na₂SO₄ and solvent was removed on rotavapor. The residue was purified by silica gel column chromatography using EtOAc/hexane (10 : 90) to afford the desired product. The purified new products are characterized by ¹H NMR, ¹³C NMR and ¹⁹F NMR and LCMS, HRMS.

Conclusions

We have developed an enantioselective hydrocyanation reaction of β-trifluoromethyl-β-disubstituted nitroalkenes using TMSCN catalyzed by a Ti(IV) salen complex. In the present protocol we have disclosed the first and efficient enantioselective transformation to synthesis β-nitronitrile containing all-carbon quaternary stereogenic centers bearing a trifluoromethyl group with very good enantioselectivity (up to 99%). The product β-nitronitrile was conveniently transformed to the corresponding trifluoromethylated β-amino acids.

Acknowledgements

CSIR–CSMCRI communication no. 111/2015. Authors are thankful to CSIR for SRF, CSIR Network Project on Catalysis and DST, New Delhi for Project (SR/S1/IC-24/2013) on Catalysis for financial assistance. Ajay Jakhar is also thankful to AcSIR for Ph.D. registration and “Analytical Discipline and Centralized Instrumental Facilities” for providing instrumentation facilities.

Notes and references

1. (a) H. Hasegawa, M. Muraoka, K. Matsui and A. Kojima, Bioorg. Med. Chem. Lett., 2003, 13, 3471; (b) P. R. Daga and R. J. Doerksen, J. Comput. Chem., 2008, 29, 1945; (c) W. K. Hagmann, J. Med. Chem., 2008, 51, 4359; (d) R. Filler and R. Saha, Future Med. Chem., 2009, 1, 777; (e) H. Xie, Y. Zhang, S. Zhang, X. Chen and W. Wang, Angew. Chem., Int. Ed., 2011, 50, 11773.
2. (a) J. T. Welch and S. Eswarakrishman, Fluorine in Bioorganic Chemistry, Wiley, New York, 1991; (b) B. E. Smart, J. Fluorine Chem., 2001, 109, 3; (c) T. Mita, T. Kikuchi, T. Mizukoshi, M. Yaosaka and M. Komoda, WO2005085216, 2005; (d) K. Mìller, C. Faeh and F. Diederich, Science, 2007, 317, 1881; (e) D. O'Hagan, Chem. Soc. Rev., 2008, 37, 308; (f) G. Chaume, N. Lensen, C. Caupéne and T. Brigaud, Eur. J. Org. Chem., 2009, 5717; (g) S. Lectard, Y. Hamashima and M. Sodeoka, Adv. Synth. Catal., 2010, 352, 2708; (h) Y. Ozoe, M. Asahi, F. Ozoe, K. Nakahira and T. Mita, Biochem. Biophys. Res. Commun., 2010, 391, 744; (i) J. Nie, H.-C. Guo, D. Cahard and J.-A. Ma, Chem. Rev., 2011, 111, 455.
3. For selected reviews on trifluoromethylation of organic compounds, see: (a) S. Purser, P. R. Moore, S. Swallow and V. Gouverneur, Chem. Soc. Rev., 2008, 37, 320; (b) O. A. Tomashenko and V. V. Grushin, Chem. Rev., 2011, 111, 4475; (c) Y. Ye and M. S. Sanford, Synlett, 2012, 23, 2005; (d) T. Furuya, A. S. Kamlet and T. Ritter, Nature, 2011, 473, 470; (e) X.-F. Wu, H. Neumann and M. Beller, Chem.–Asian J., 2012, 7, 1744; (f) A. Studer, Angew. Chem., Int. Ed., 2012, 51, 8950; (g) Y.-Y. Huang, X. Yang, Z. Chen, F. Verpoort and N. Shibata, Chem.–Eur. J., 2015, 21, 8664.
4. H. Kawai, S. Okusu, E. Tokunaga, H. Sato, M. Shiro and N. Shibata, Angew. Chem., Int. Ed., 2012, 51, 4959.
5. (a) J.-R. Gao, H. Wu, B. Xiang, W.-B. Yu, L. Han and Y.-X. Jia, J. Am. Chem. Soc., 2013, 135, 2983; (b) Q. Chen, G. Wang, X. Jiang, Z. Xu, L. Lin and R. Wang, Org. Lett., 2014, 16, 1394; (c) P. Kwiatkowski, A. Cholewiak and A. Kasztelan, Org. Lett., 2014, 16, 5930; (d) C.-H. Ma, T.-R. Kang, L. He and Q.-Z. Liu, Eur. J. Org. Chem., 2014, 3981; (e) H. Wu, R.-R. Liu and Y.-X. Jia, Synlett, 2014, 25, 457; (f) W. Chen, Z. Jing, K. F. Chin, B. Qiao, Y. Zhao, L. Yan, C.-H. Tan and Z. Jiang, Adv. Synth. Catal., 2014, 392, 1256; (g) F.-L. Liu, J.-R. Chen, B. Feng, X.-Q. Hu, L.-H. Ye, L.-Q. Lu and W.-J. Xiao, Org. Biomol. Chem., 2014, 12, 105; (h) J. Chen, S. Meng, L. Wang, H. Tang and Y. Huang, Chem. Sci., 2015, 6, 4184; (i) L. Wang, J. Chen and Y. Huang, Angew. Chem., Int. Ed., 2015, 54, 15414; (j) D. Du, Y. Jiang, Q. Xu, X.-Y. Tang and M. Shi, ChemCatChem, 2015, 7, 1366; (k) Y. Zhu, X. Li, Q. Chen, J. Su, F. Jia, S. Qiu, M. Ma, Q. Sun, W. Yan, K. Wang and R. Wang, Org. Lett., 2015, 17, 3826; (l) H. Wu, R.-R. Liu, C. Shen, M.-D. Zhang, J. Gao and Y.-X. Jia, Org. Chem. Front., 2015, 2, 124; (m) D. Yan, Q. Li and C. Wang, Chin. J. Chem., 2012, 30, 2714.
6. (a) J. C. Anderson, A. J. Blake, M. Mills and P. D. Ratcliffe, Org. Lett., 2008, 10, 4141; (b) L. Bernardi, F. Fini, M. Fochi and A. Ricci, Synlett, 2008, 12, 1857; (c) P. Bernal, R. Fernández and J. M. Lassaletta, Chem.–Eur. J., 2010, 16, 7714; (d) L. Lin, W. Yin, X. Fu, J. Zhang, X. Ma and R. Wang, Org. Biomol. Chem., 2012, 10, 83; (e) M. North and J. M. Watson, ChemCatChem, 2013, 5, 2405; (f) A. Jakhar, A. Sadhukhan, N. H. Khan, S. Saravanan, R. I. Kureshy, S. H. R. Abdi and H. C. Bajaj, ChemCatChem, 2014, 6, 2656.
7. (a) H. Iida, T. Moromizato, H. Hamana and K. Matsumoto, Tetrahedron Lett., 2007, 48, 2037; (b) Y. Tanaka, M. Kanai and M. Shibasaki, J. Am. Chem. Soc., 2008, 130, 6072; (c) J. Yang, Y. Wang, W. Shaoxiang and F.-X. Chen, Synlett, 2009, 3365; (d) Y. Tanaka, M. Kanai and M. Shibasaki, J. Am. Chem. Soc., 2010, 132, 8862; (e) N. Kurono, N. Nii, Y. Sakaguchi, M. Uemura and T. Ohkuma, Angew. Chem., Int. Ed, 2011, 50, 5541; (f) B. A. Provencher, K. J. Bartelson, Y. Liu, B. M. Foxman and L. Deng, Angew. Chem., Int. Ed., 2011, 50, 10565; (g) H. Kawai, S. Okusu, E. Tokunaga, H. Sato, M. Shiro and N. Shibata, Angew. Chem., Int. Ed., 2012, 51, 4959; (h) S. Lin, Y. Wei and F. Liang, Chem. Commun., 2012, 48, 9879; (i) S. Ramesh and A. Lalitha, Acta Chim. Slov., 2013, 60, 689; (j) C. Dong, T. Song, X.-F. Bai, Y.-M. Cui, Z. Xu and L.-W. Xu, Catal. Sci. Technol., 2015, 5, 4755; (k) X.-P. Zeng, Z.-Y. Cao, X. Wang, L. Chen, F. Zhou, F. Zhu, C.-H. Wang and J. Zhou, J. Am. Chem. Soc., 2016, 138, 416.
8. (a) G. M. Sammis and E. N. Jacobsen, J. Am. Chem. Soc., 2003, 125, 4442 (J. Am. Chem. Soc., 2004, 126, 9928); (b) C. Mazet and E. N. Jacobsen, Angew. Chem., Int. Ed., 2008, 47, 1762; (c) N. Madhavana and M. Weck, Adv. Synth. Catal., 2008, 350, 419.
9. (a) T. Mita, K. Sasaki, M. Kanai and M. Shibasaki, J. Am. Chem. Soc., 2005, 127, 514; (b) I. Fujimori, T. Mita, K. Maki, M. Shiro, A. Sato, S. Furusho, M. Kanaia and M. Shibasaki, Tetrahedron, 2007, 63, 5820.
10. (a) J. Wang, W. Li, Y. Liu, Y. Chu, L. Lin, X. Liu and X. Feng, Org. Lett., 2010, 12, 1280; (b) H. Mukherjee and C. A. Martinez, ACS Catal., 2011, 1, 1010; (c) Y. Liu, S. Shirakawa and K. Maruoka, Org. Lett., 2013, 15, 1230.
11. (a) N. H. Khan, R. I. Kureshy, S. H. R. Abdi, S. Agrawal and R. V. Jasra, Coord. Chem. Rev., 2008, 252, 593; (b) N. H. Khan, R. I. Kureshy, S. H. R. Abdi, H. C. Bajaj and A. Sadhukhan, “Asymmetric Cyanation Reactions” Elsevier Publication in Comprehensive Inorganic Chemistry II, Elsevier, Oxford, 2013, vol. 6, p. 413; (c) M. North, D. L. Usanov and C. Young, Chem. Rev., 2008, 108, 5146; (d) W. Wang, X. Liu, L. Lin and X. Feng, Eur. J. Org. Chem., 2010, 4751; (e) Z. Zhang, Z. Wang, R. Zhang and K. Ding, Angew. Chem., Int. Ed., 2010, 49, 6746; (f) H. Pellissier, Adv. Synth. Catal., 2015, 357, 857; (g) N. Kurono and T. Ohkuma, ACS Catal., 2016, 6, 989.
12. (a) S. J. Zuend, M. P. Coughlin, M. P. Lalonde and E. N. Jacobsen, Nature, 2009, 461, 968; (b) J. Wang, X. Liu and X. Feng, Chem. Rev., 2011, 111, 6947; (c) X. H. Cai and B. Xie, ARKIVOC, 2014, 1, 205; (d) S. Saravanan, N. H. Khan, A. Jakhar, A. Ansari, R. I. Kureshy, S. H. R. Abdi and G. Kumar, RSC Adv., 2015, 5, 99951.
13. (a) J. X. Qiu, E. J. Petersson, E. E. Matthews and A. Schepartz, J. Am. Chem. Soc., 2006, 128, 11338; (b) B. Weiner, W. Szymaski, D. B. Janssen, A. J. Minnaarda and B. L. Feringa, Chem. Soc. Rev., 2010, 39, 1656; (c) A. Ishii, M. Yasumoto, T. Yamazaki, K. Mogi and T. Masuda, US Pat., 20110152536 A1, 23 Jun 2011.
14. D. D. Perrin, W. L. F. Armarego and D. R. Perrin, Purification of Laboratory Chemicals, Pergamon, New York, 1981.
15. (a) C. M. Mascarenhas, S. P. Miller, P. S. White and J. P. Morken, Angew. Chem., Int. Ed., 2001, 40, 601; (b) X. Yao, M. Qiu, W. Lu, H. Chen and Z. Zheng, Tetrahedron: Asymmetry, 2001, 12, 197; (c) J. F. Larrow and E. N. Jacobsen, J. Org. Chem., 1994, 59, 1939; (d) S.-K. Chen, D. Peng, H. Zhou, L.-W. Wang, F.-X. Chen and X.-M. Feng, Eur. J. Org. Chem., 2007, 639; (e) R. I. Kureshy, N. H. Khan, S. H. R. Abdi, S. T. Patel and R. V. Jasra, Tetrahedron Lett., 2001, 42, 2915; (f) R. I. Kureshy, N. H. Khan, S. H. R. Abdi, S. Singh, I. Ahmad, R. V. Jasra and A. P. Vyas, J. Catal., 2004, 224, 229; (g) R. I. Kureshy, M. Kumar, S. Agrawal, N. H. Khan, B. Dangi, S. H. R. Abdi and H. C. Bajaj, Chirality, 2011, 23, 76; (h) S. Saravanan, N. H. Khan, P. K. Bera, R. I. Kureshy, S. H. R. Abdi, P. Kumari and H. C. Bajaj, ChemCatChem, 2013, 5, 1374.

---

Footnote

† Electronic supplementary information (ESI) available. CCDC 1412289. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra00093b

---