Chiral carbene-borane adducts: precursors for borenium catalysts for asymmetric FLP hydrogenations.

Jolie Lam,Patrick Eisenberger,Paul D Newman,Rebecca L Melen,Brian P Bestvater,Douglas W Stephan,Jeffrey M Farrell,Benjamin A R Günther,Cathleen M Crudden

doi:10.1039/c6dt02202b

Abstract

The carbene derived from (1R,3S)-camphoric acid was used to prepare the borane adduct with Piers' borane 7. Subsequent hydride abstraction gave the borenium cation 8. Adducts with 9-BBN and the corresponding (1R,3S)-camphoric acid-derived carbene bearing increasingly sterically demanding N-substituents (R = Me 9, Et 10, i-Pr 11) and the corresponding borenium cations 12-14 were also prepared. These cations were not active as catalysts in hydrogenation, although 9-11 were shown to undergo carbene ring expansion reactions at 50 °C to give species 15-17. The IBOX-carbene precursors 18 and 19 derived from amino alcohols (S)-valinol and (S)-tert-leucinol (R = i-Pr, t-Bu) were used to prepare borane adducts 20-23. Reaction of the carbenes 1,3-dimethylimidazol-2-ylidene (IMe), 1,3-di-iso-propylimidazol-2-ylidene (IPr) 1-benzyl-3-methylimidazol-2-ylidene (IBnMe), 1-methyl-3-phenylimidazol-2-ylidene (IPhMe) and 1-tert-butyl-3-methylimidazol-2-ylidene (ItBuMe) with diisopinocampheylborane (Ipc2BH) gave chiral adducts: (IMe)(Ipc2BH) 24, (IPr)(Ipc2BH) 25, (IBnMe)(Ipc2BH) 26, (IPhMe)(Ipc2BH) 27, and (ItBuMe)(Ipc2BH) 28. Triazolylidene-type adducts including the (10)-phenyl-9-borabicyclo [3.3.2]decane adduct of 1,3,4-triphenyl-1H-1,2,3-triazolium, rac-29 and the 9-BBN derivative of (S)-2-amino-2'-methoxy-1,1'-binaphthalene-1,2,3-triazolium 34a/b were also prepared. In catalytic studies of these systems, while several species were competent catalysts for imine reduction, in general, low enantioselectivities, ranging from 1-20% ee, were obtained. The implications for chiral borenium cation catalyst design are considered.

Highlights

Following the advent of the concept of “frustrated Lewis pairs” (FLPs) a decade ago,1,2 the initial application that emerged from the metal-free activation of H2 was the use of these in hydrogenation catalysis,3–7 resulting in a burgeoning and exciting area of new developments.8–13 The scope of substrates that can be reduced by an FLP protocol has broadened dramatically from the original application to imines, protected nitriles and Analogous to the development of homogeneous transitionmetal based catalysts, following the discovery of FLP catalysts, attention has turned to the development of stereoselective systems
We describe the synthesis of chiral carbene adducts of borenium ions and preliminary evaluation of these systems in hydrogenation catalysis
For the synthesis of our first, camphor-derived borenium ion, the commercially available, enantiomerically pure (1R,3S)-camphoric acid was converted to the corresponding diamine 2 in an acid-mediated reaction with sodium azide to give the product in a yield of 58% (Scheme 2)

Summary

Introduction

Following the advent of the concept of “frustrated Lewis pairs” (FLPs) a decade ago,1,2 the initial application that emerged from the metal-free activation of H2 was the use of these in hydrogenation catalysis,3–7 resulting in a burgeoning and exciting area of new developments.8–13 The scope of substrates that can be reduced by an FLP protocol has broadened dramatically from the original application to imines, protected nitriles and Analogous to the development of homogeneous transitionmetal based catalysts, following the discovery of FLP catalysts, attention has turned to the development of stereoselective systems. 13C{1H} NMR (benzene-d6): 15.4, 17.0, 22.2, 23.2, 25.5, 28.7, 31.0, 31.4, 34.6, 35.6, 38.8, 39.0, 40.5, 41.5, 69.0, 70.2 HRMS(ES+): C19H36BN2, [M+] m/z (calc.): 302.3008, m/z (obs.): 302.3009.

Results

Conclusion