Transfer Hydrogenation DOI: 10.1002/anie.201409246 B(C 6 F 5 ) 3 -Catalyzed Transfer Hydrogenation of Imines and Related Heteroarenes Using Cyclohexa-1,4-dienes as a Dihydrogen Source** Indranil Chatterjee and Martin Oestreich* Abstract: The strong boron Lewis acid tris(pentafluorophen- yl)borane, B(C 6 F 5 ) 3 , is shown to abstract a hydride from suitably donor-substituted cyclohexa-1,4-dienes, eventually releasing dihydrogen. This process is coupled with the FLP- type (FLP = frustrated Lewis pair) hydrogenation of imines and nitrogen-containing heteroarenes that are catalyzed by the same Lewis acid. The net reaction is a B(C 6 F 5 ) 3 -catalyzed, i.e., transition-metal-free, transfer hydrogenation using easy-to- access cyclohexa-1,4-dienes as reducing agents. Competing reaction pathways with or without the involvement of free dihydrogen are discussed. The potent Lewis acid B(C 6 F 5 ) 3 mediates the heterolytic splitting of Si À H and H À H bonds in the presence of both weak and strong Lewis bases, provided that Lewis pair formation is either reversible [1] or frustrated. [2] The hydride is transferred to the boron atom to yield [HB(C 6 F 5 ) 3 ] À , and the silicon cation or the proton are absorbed by the Lewis base. The borohydride generated by that unique bond activation engages in various reduction processes where the silicon cation or proton lower the energy of the lowest unoccupied molecular orbital of the acceptor. In this way, several transition-metal-free hydrosilylations [1, 3–6] and hydrogena- tions [7–12] catalyzed by B(C 6 F 5 ) 3 became possible. We recently discovered that B(C 6 F 5 ) 3 is also capable of hydride abstraction from cyclohexa-1,4-dienes I that bear a silicon group in the 3-position (Scheme 1, top). [13] The C(sp 3 ) ÀH bond in I develops hydridic character through hyperconjugation with the C(sp 3 ) ÀSi bond that later stabilizes the resulting Wheland intermediate II. With [HB(C 6 F 5 ) 3 ] À as the counteranion, II collapses to liberate the free hydrosilane along with benzene at room temperature. The net reaction is catalytic in B(C 6 F 5 ) 3 and was then coupled with B(C 6 F 5 ) 3 - catalyzed alkene hydrosilylation, thereby enabling the pre- viously unprecedented ionic transfer hydrosilylation. [13] These findings led us to consider the related but more demanding B(C 6 F 5 ) 3 -catalyzed release of dihydrogen from cyclohexa-1,4- dienes III (Scheme 1, bottom). The challenge resides in the unfavorable formation of the high-energy intermediate IV . We disclose here the successful implementation of this unusual hydride abstraction [14] in the transfer hydrogenation of imines. [15] While a B(C 6 F 5 ) 3 -catalyzed transfer hydrogena- tion of imines using amines as the dihydrogen source is known (Meerwein–Ponndorf–Verley-type reduction), [16, 17] an unsa- turated hydrocarbon is used in the present approach. [18] We began our investigation by screening easily accessible cyclohexa-1,4-dienes 1 in the reduction of an aldimine (Scheme 2). The tert-butyl group at the imine nitrogen atom was shown before to be compatible with B(C 6 F 5 ) 3 -catalyzed hydrogenation. [9] We anticipated that elevated reaction tem- peratures would be necessary to facilitate hydride abstraction but cyclohexa-1,4-diene itself was reluctant to react even at 125 8C (1a, Scheme 2). We reasoned that + I and + M substituents in the appropriate positions of 1 (1,5 rather than 2,4, favoring the hydrogen atoms at the C3- over the C6- methylene group) could render the formation of the corre- sponding Wheland complexes possible at high temperature. We were then delighted to find that full conversion was seen with 1,5-dimethylcyclohexa-1,4-diene, forming m-xylene as the sole byproduct (1b, Scheme 2). The methyl groups are also capable of hyperconjugation, thereby lending further stabilization to the phenonium ion intermediate. Surprisingly, 1,5-dimethoxycyclohexa-1,4-diene was slightly less effective (1c, Scheme 2), and we attribute this to formation of a Lewis pair between B(C 6 F 5 ) 3 and the ether oxygen atoms. [14] The poor reactivity of g-terpinene with its unprofitable 1,4- substitution pattern emphasizes the requirement of elec- Scheme 1. B(C 6 F 5 ) 3 -catalyzed release of hydrosilanes (verified) and dihydrogen (planned) from cyclohexa-1,4-dienes. [*] Dr. I. Chatterjee, Prof. Dr. M. Oestreich Institut für Chemie, Technische Universität Berlin Strasse des 17. Juni 115, 10623 Berlin (Germany) E-mail: martin.oestreich@tu-berlin.de Homepage: http://www.organometallics.tu-berlin.de [**] This research was supported by the Cluster of Excellence “Unifying Concepts in Catalysis” of the Deutsche Forschungsgemeinschaft (EXC 314/2). M.O. is indebted to the Einstein Foundation (Berlin) for an endowed professorship. We thank Prof. Dr. Ernst-Ulrich Würthwein (Westfälische Wilhelms-Universität Münster) for insightful discussions. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201409246. A ngewandte Chemi e 1 Angew. Chem. Int. Ed. 2014, 53,1–5  2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim These are not the final page numbers! Ü Ü