The bond activation chemistry of B(C6F5)3 and related electron-deficient boranes is currently experiencing a renaissance due to the fascinating development of frustrated Lewis pairs (FLPs). B(C6F5)3's ability to catalytically activate Si-H bonds through η(1) coordination opened the door to several unique reduction processes. The ground-breaking finding that the same family of fully or partially fluorinated boron Lewis acids allows for the related H-H bond activation, either alone or as a component of an FLP, brought considerable momentum into the area of transition-metal-free hydrogenation and, likewise, hydrosilylation. This review comprehensively summarises synthetic methods involving borane-catalysed Si-H and H-H bond activation. Systems corresponding to an FLP-type situation are not covered. Aside from the broad manifold of C=X bond reductions and C=X/C-X defunctionalisations, dehydrogenative (oxidative) Si-H couplings are also included.

Visible-light-induced ruthenium catalysis has enabled remote CÀHa lkylations with excellent levels of position control under exceedingly mild conditions at room temperature.T he metallaphotocatalysis occurred under exogenousphotosensitizer-free conditions and features an ample substrate scope.The robust nature of the photo-induced mild meta-C À H functionalization is reflected by the broad functional group tolerance,and the reaction can be carried out in an operationally simple manner,setting the stage for challenging secondary and tertiary meta-CÀHa lkylations by ruthenaphotoredox catalysis.The development of catalytic methods for the positionselective functionalization of C À Hb onds represents ak ey challenge in molecular synthesis. [1] Thus far, chelation assistance by directing groups has been identified as av ersatile tool for site-selective CÀHmetalations. [2] Thereby,proximityinduced C À Ha ctivations have set the stage for ap lethora of ortho-selective C À Hf unctionalizations.I ns harp contrast, strategies for the assembly of meta-substituted arenes continue to be scarce.T oo vercome the challenge of meta-CÀH functionalization, useful approaches have been devised ( Figure 1). [3] Exploiting the substratesi nherent substitution pattern has proven useful, but this approach still largely suffers from limited substrate scope. [4] Directing-group-based reactions with transient norbornene mediators, [5] hydrogenbond-forming ligands, [6] or template-based directing groups [7] have recently provided significant momentum towards metadecorated arenes.W hile representing key advances,t hese methods require multistep syntheses of ligands or templates, and often give mixtures of regioisomeric products that are difficult to separate.As au niquely versatile alternative, meta-C À Hf unctionalizations through arene s-activation [8] were realized by chelation-assisted ortho-cycloruthenation. [9][10][11][12][13] Despite indisputable progress,t he s-activation approach is limited to elevated reaction temperatures,w hich resulted in signifi-cantly reduced yields and low functional group tolerance. Conversely,r oom-temperature metal-catalyzed meta-CÀH functionalization has thus far unfortunately proven elusive.During the past decade,p hoto-induced C À Hf unctionalization has emerged as ap owerful tool for molecular synthesis, [14] both in terms of classical ortho-functionalizations or with electronically biased heteroarenes. [15] In sharp contrast, within our program on metallaphotoredox-catalyzed CÀH functionalization, [16] we have now devised an unprecedented visible-light-induced meta-C À Ha lkylation at room temperature.N otable features of our findings include 1) expedient ruthenium-catalyzed meta-C À Ha lkylations,2 )visible-lightinduced metallaphotocatalysis for remote CÀHf unctionalization, 3) exogenous-photosensitizer-free photocatalysis,a nd 4) exceedingly mild reaction conditions at room temperature. Figure 1. Strategies for meta-selective CÀHfunctionalization. a) Control by steric interactions. b) Transien...

The hydrogenation of oximes and oxime ethers is usually hampered by N-O bond cleavage, hence affording amines rather than hydroxylamines. The boron Lewis acid B(C6F5)3 is found to catalyze the chemoselective hydrogenation of oxime ethers at elevated or even room temperature under 100 bar dihydrogen pressure. The use of the triisopropylsilyl group as a protecting group allows for facile liberation of the free hydroxylamines.

The silylium ion promoted reduction of imines yielding the corresponding amines is reported. Both tert-butylferrocenylmethylsilane and triethylsilane are efficient hydride donors for the reduction of intermediate silyliminium ions, thereby regenerating the catalytically active silylium ion and closing the catalytic cycle.

Typical congeners of the boron Lewis acid tris(pentafluorophenyl)borane, B(C 6 F 5 ) 3 , are fluorinated at the aryl groups directly attached to the boron atom. The chemistry of related electron-deficient boranes with fluorination distal to the Lewis acidic center is largely unexplored. The preparation and characterization of tris(5,6,7,8tetrafluoronaphthalen-2-yl)borane are reported. It serves as a model system that provides sites for further substitution at C-1 and C-3 of the naphthalen-2-yl units. A Gutmann− Beckett analysis of its Lewis acidity revealed that, despite remote fluorination, it is as Lewis acidic as B(C 6 F 5 ) 3 . The new Lewis acid performs equally well in CO and CN reduction as well as dehydrogenative Si−O coupling involving Si−H bond activation. Adducts with water and a phosphine oxide are crystallographically characterized.