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(C 6 Cl 5 ) x (C 6 F 5 ) 3Àx (x = 0-3) are capable of heterolytic H 2 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(C 6 Cl 5 )(C 6 F 5 ) 2 , makes this a practically simple reaction method.

Enantioselective Direct Mannich‐Type Reactions Catalyzed by Frustrated Lewis Acid/Brønsted Base Complexes

“…[1][2][3][4] Cooperative catalysts consisting of "soft" organometallic Lewis acids and stronger Brønsted bases such as ShibasakisC u(Biphep)/Bartons base catalysts C3 and Tr ostsZ n(ProPhenol) complex C4 have been shown to overcome mutual quenching and promote Mannich-type reactions with less acidic 7-azaindolineamides [2e,5] and ketones, [2d, 6] respectively.D espite such notable progress,t here exist few organic cooperative acid/ base catalysts capable of promoting the direct Mannich(-type) reaction of mono-carbonyl pronucleophiles. [9,10] These unquenched acid/base complexes are capable of activating otherwise unreactive small molecules such as H 2 and CO 2 . [9,10] These unquenched acid/base complexes are capable of activating otherwise unreactive small molecules such as H 2 and CO 2 .…”

“…[9,10] These unquenched acid/base complexes are capable of activating otherwise unreactive small molecules such as H 2 and CO 2 . [9][10][11][12] Our laboratory has described the development of strongly acidic B(C 6 F 5 ) 3 and chiral diamine catalysts for direct enantioselective CÀNb ond forming reactions between ketones and dialkyl azodicarboxylates. [9][10][11][12] Our laboratory has described the development of strongly acidic B(C 6 F 5 ) 3 and chiral diamine catalysts for direct enantioselective CÀNb ond forming reactions between ketones and dialkyl azodicarboxylates.…”

“…[4b, 7, 8] The" frustrated" Lewis pairs (FLPs) pioneered by Stephan and Erker consist of potent acidic and basic fragments that are not able to associate easily because of steric and electronic factors. [9,10] These unquenched acid/base complexes are capable of activating otherwise unreactive small molecules such as H 2 and CO 2 . [9] Although an assortment of enantioenriched FLPs have been prepared, enantioselective FLP-catalyzed transformations have been confined to hydro-genation and hydrosilylation processes until recently.…”

“…[9] Although an assortment of enantioenriched FLPs have been prepared, enantioselective FLP-catalyzed transformations have been confined to hydro-genation and hydrosilylation processes until recently. [9][10][11][12] Our laboratory has described the development of strongly acidic B(C 6 F 5 ) 3 and chiral diamine catalysts for direct enantioselective CÀNb ond forming reactions between ketones and dialkyl azodicarboxylates. [13] However, there is no precedent for diastereo-and enantioselective C À Cb ond forming reactions catalyzed by an unquenched pair of chiral organoborane and achiral amine catalysts.…”

Borane catalysed ring opening and closing cascades of furans leading to silicon functionalized synthetic intermediates

“…On the other hand, Ashley et al . reported that hydrogenation of furan derivatives under the frustrated Lewis pairs catalysis using (B(C 6 Cl 5 )(C 6 F 5 ) 2 ) delivers reduced tetrahydrofuran compounds23. Despite of such progresses on furan transformation reactions, there have been no developments thus far for the selective conversion of furans to ring-opened products bearing a sp 3 C−Si bond at ambient conditions.…”