Metal‐Free Hydrogenation Catalyzed by an Air‐Stable Borane: Use of Solvent as a Frustrated Lewis Base

Abstract

In recent years ‘frustrated Lewis pairs’ (FLPs) have been shown to be effective metal-free catalysts for the hydrogenation of many unsaturated substrates. Even so, limited functional-group tolerance restricts the range of solvents in which FLP-mediated reactions can be performed, with all FLP-mediated hydrogenations reported to date carried out in non-donor hydrocarbon or chlorinated solvents. Herein we report that the bulky Lewis acids B(C6Cl5)x(C6F5)3−x (x=0–3) are capable of heterolytic H2 activation in the strong-donor solvent THF, in the absence of any additional Lewis base. This allows metal-free catalytic hydrogenations to be performed in donor solvent media under mild conditions; these systems are particularly effective for the hydrogenation of weakly basic substrates, including the first examples of metal-free catalytic hydrogenation of furan heterocycles. The air-stability of the most effective borane, B(C6Cl5)(C6F5)2, makes this a practically simple reaction method.

Extending the Scope of the B(C6F5)3‐Catalyzed CN Bond Reduction: Hydrogenation of Oxime Ethers and Hydrazones

“…However, our protocol is limited to ketone‐derived oxime ethers. Being aware that ethereal solvents are able to function as Lewis‐base partners to promote the FLP‐type reduction of imines at lower reaction temperature and dihydrogen pressure (Scheme , top),8–10 we hoped that employing ethers as (co‐)solvents might also be beneficial in the hydrogenation of aldehyde‐derived oxime ethers. Moreover, we disclose herein the extension of this hydrogenation method to hydrazones, a hitherto neglected substrate class in catalytic metal‐free reductions (Scheme , bottom) 11…”

“…[7] However, our protocol is limited to ketone-derived oxime ethers. Being aware that ethereal solvents are able to function as Lewis-basep artnerst op romote the FLP-type reduction of imines at lower reactiont emperature and dihydrogen pressure (Scheme 1, top), [8][9][10] we hoped that employinge thers as (co-)solvents mighta lso be beneficial in the hydrogenation of aldehyde-derived oxime ethers.M oreover,w ed iscloseh erein the extension of this hydrogenation method to hydrazones, ah itherto neglected substrate class in catalytic metal-free reductions( Scheme 1, bottom). [11] We began with benzaldehyde-derived O-silylated oxime ether 1 to test whether ethereal solvents permitt he B(C 6 F 5 ) 3catalyzed hydrogenation of aldoximee thers.W eh ad previously seen no reactivity of 1 in toluene and we attributet his to its lack of basicity at the nitrogena tom,t hat is, its inability to participate in the FLP-type heterolytic dihydrogen cleavage.…”

Conformational studies on heteroleptic trifluoromethyl-substituted phenylboranes

“…t Bu 3 P) and rather unhindered acids, such as (C 6 F 5 ) 3 B (Stephan & Erker, 2015), whereas combinations of smaller bases with sterically encumbered acids (Lu et al, 2011;Scott et al, 2014) are much rarer. t Bu 3 P) and rather unhindered acids, such as (C 6 F 5 ) 3 B (Stephan & Erker, 2015), whereas combinations of smaller bases with sterically encumbered acids (Lu et al, 2011;Scott et al, 2014) are much rarer.…”

Chemistry of Intermolecular Frustrated Lewis Pairs in Motion: Emerging Perspectives and Prospects

“…A few notable applications that showcase the versatility of FLPs are: (i) reactions with H 2 , CO,10 NO,11 CO 2 , N 2 O,12 SO 2 ,[13] olefins,14 dienes,15 and alkynes;16 (ii) stoichiometric and catalytic transfer hydrogenation reactions; [17, 18] and (iii) reduction of CO 2 19. A very promising direction of FLP chemistry is the use of non‐chlorinated, non‐hydrocarbon solvents (e.g., the distinctively donating solvents THF, with air‐stable borane LAs,20 and ether, in a recent example of the catalytic ketone hydrogenation by metal‐free FLPs21).…”