In contrast to the established dogma that B(C 6 F 5 ) 3 is irreversibly poisoned by excess H 2 O/amine (or imine) bases, B(C 6 F 5 ) 3 is actually a water-tolerant catalyst for the reductive amination of primary and secondary arylamines with aldehydes and ketones in "wet solvents" at raised temperatures and using only 1.2 equiv of Me 2 PhSiH as reductant. Arylamines/ N-arylimines do not result in the irreversible deprotonation of H 2 O−B(C 6 F 5 ) 3 , allowing sufficient B(C 6 F 5 ) 3 to be evolved at raised temperatures to effect catalytic reductions. Stronger Brønsted basic amines such as t BuNH 2 (and derived imines) result in irreversible formation of [HO−B(C 6 F 5 ) 3 ] − from H 2 O−B(C 6 F 5 ) 3 , precluding the formation of B(C 6 F 5 ) 3 at raised temperatures and thus preventing any imine reduction. A substrate scope exploration using 1 mol % nonpurified B(C 6 F 5 ) 3 and "wet solvents" demonstrates that this is an operationally simple and effective methodology for the production of secondary and tertiary arylamines in high yield, with imine reduction proceeding in preference to other possible reactions catalyzed by B(C 6 F 5 ) 3 , including the dehydrosilylation of H 2 O and the reduction of carbonyl moieties (e.g., esters).
. (2015) This document is the Accepted Manuscript version of a Published Work that appeared in nal form in ACS Catalysis, copyright c American Chemical Society after peer review and technical editing by the publisher. To access the nal edited and published work see http://dx.doi.org/10.1021/acscatal.5b02106. Additional information:Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details.
Abstract1‐Boraphenalenes have been synthesized by reaction of BBr3 with 1‐(aryl‐ethynyl)naphthalenes, 1‐ethynylnaphthalene, and 1‐(pent‐1‐yn‐1‐yl)naphthalene and they can be selectively functionalized at boron or carbon to form bench‐stable products. All of these 1‐boraphenalenes have LUMOs localized on the planar C12B core that are closely comparable in character to isoelectronic phenalenyl cations. In contrast to the comparable LUMOs, the aromatic stabilization of the C5B ring in 1‐boraphenalenes is dramatically lower than the C6 rings in phenalenyl cations. This is due to the occupied orbitals of π symmetry being less delocalised in the 1‐boraphenalenes.
N−Me‐Benzothiazolium salts are introduced as a new family of Lewis acids able to activate Si−H σ bonds. These carbon‐centred Lewis acids were demonstrated to have comparable Lewis acidity towards hydride as found for the triarylboranes widely used in Si−H σ‐bond activation. However, they display low Lewis acidity towards hard Lewis bases such as Et3PO and H2O in contrast to triarylboranes. The N−Me‐benzothiazolium salts are effective catalysts for a range of hydrosilylation and dehydrosilylation reactions. Judicious selection of the C2 aryl substituent in these cations enables tuning of the steric and electronic environment around the electrophilic centre to generate more active catalysts. Finally, related benzoxazolium and benzimidazolium salts were found also to be active for Si−H bond activation and as catalysts for the hydrosilylation of imines.
A substoichiometric quantity of the Lewis acid B(C 6 F 5 ) 3 is sufficient to initiate the aldehyde−amine−alkyne reaction, in a one-pot methodology that enables the synthesis of a range of functionalized quinolines. Optimization studies revealed that key requirements for the high-yielding tricomponent reaction initiated by B(C 6 F 5 ) 3 at raised temperatures include an excess of the in situ generated imine (which acts as a hydrogen acceptor) and an alkyne substituent able to stabilize positive charge buildup during the cyclization. Mechanistic experiments revealed that under these conditions B(C 6 F 5 ) 3 is acting as a Lewis acid-assisted Brønsted acid, with H 2 O−B(C 6 F 5 ) 3 being the key species enabling catalytic quinoline formation. This was indicated by deuterium labeling studies and the observation that the cyclization of N-(3-phenylpropargyl)aniline using B(C 6 F 5 ) 3 under anhydrous conditions afforded the zwitterion [(N-H-3-B(C 6 F 5 ) 3 -4-Ph-quinolinium], which does not undergo protodeboronation to release B(C 6 F 5 ) 3 and the quinoline product under a range of conditions. Finally, a brief substrate scope exploration demonstrated that this is an operationally simple and effective methodology for the production of functionalized quinolines.
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