Reaction of an “Invisible” Frustrated N/B Lewis Pair with Dihydrogen

Abstract

D-(+)-Camphor forms the enamine 2 with piperidine. Compound 2 adds HB(C(6)F(5))(2) at the enamine carbon atom C3 to form a Lewis acid/Lewis base adduct (exo-/endo-isomers of 3). Exposure of 3 to dihydrogen (2.5 bar, room temperature) leads to heterolytic splitting of H(2) to form the H(+)/H(-) addition products (4, two diastereoisomers) of the "invisible" frustrated Lewis pairs (5, two diastereoisomers) that were apparently generated in situ by enamine hydroboration under equilibrium conditions.

Frustrated Lewis Pair Chemistry of Chiral (+)‐Camphor‐Based Aminoboranes

“…Similar behavior was observed by Erker and co-workers for the piperidine analogue 6 b (Scheme 3). [26] The zwitterionic diastereomers 8 a and 8' a had spectral features that were almost the same as those observed for the 8 b/8' b pair. The 1 H NMR spectrum of 8 a in [D 6 ]benzene featured two main series of signals, corresponding to the two formed stereoA C H T U N G T R E N N U N G isomers 8 a and 8' a.…”

“…The diastereomeric pair 8 b and 8' b splits hydrogen through intermediate formation of 1,2-aminoboranes 9 b and 9' b, leading to the hydrogenated ammonium-borohydride diastereomers 10 b and 10' b (Scheme 4). [26] Surprisingly, the dimethyliminium-hydroborates 8 a and 8' a reacted with hydrogen in a different way to the recently described piperidinium analogues 8 b and 8' b.…”

“…Whereas piperidine-derived iminium-hydroborates 8 b and 8' b gave the corresponding ammonium-hydroborate products 10 b and 10' b (Scheme 4), their dimethyliminium analogues 8 a and 8' a transformed into amino-borane adducts 11 a and 11' a. As shown by Erker et al, [26] the former reaction proceeded through an intermediate rearrangement of and 9' b that were responsible for hydrogen splitting (Scheme 4). In our case, it seems that splitting of hydrogen takes place through the 6 a-HBA C H T U N G T R E N N U N G (C 6 F 5 ) 2 N-B adduct.…”

“…Formation of the zwitterionic ammonium-hydroborates 10 b and 10' b. [26] www.chemeurj.org structures, adduct 11' a was synthesized in an alternative way. Enamine 6 a was hydrogenated by heating with formic acid.…”

“…[25] Inspired by these thoughts, we studied reactions between HBA C H T U N G T R E N N U N G (C 6 F 5 ) 2 and chiral enamines containing a (1R)-(+)-camphor backbone. During the preparation of the present work, Erker and co-workers published parallel studies of a piperidine-substituted (1R)-(+)-camphor-1,2-aminobo-A C H T U N G T R E N N U N G rane, [26] which was found to be in equilibrium between the 1,2-aminoborane and the zwitterionic iminium-hydroborate forms. Our experiments confirm this.…”