Millet CRP, Pahl J, Noone E, Yuan K, Nichol GS, Uzelac M, Ingleson MJ. Synthesis of Electrophiles Derived from Dimeric Aminoboranes and Assessing Their Utility in the Borylation of π Nucleophiles.
Organometallics 2022;
41:2638-2647. [PMID:
36185396 PMCID:
PMC9516688 DOI:
10.1021/acs.organomet.2c00393]
[Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/02/2022] [Indexed: 11/30/2022]
Abstract
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Dimeric aminoboranes,
[H2BNR2]2 (R = Me or CH2CH2) containing B2N2 cores, can
be activated by I2, HNTf2 (NTf2 =
[N(SO2CF3)2]), or [Ph3C][B(C6F5)4] to form isolable H2B(μ-NR2)2BHX (for X = I or NTf2). For X = [B(C6F5)4]− further reactivity, presumably
between [H2B(μ-NMe2)2BH][B(C6F5)4] and aminoborane, forms a B3N3-based monocation containing a three-center two
electron B-(μ-H)-B moiety. The structures of H2B(μ-NMe2)2BH(I) and [(μ-NMe2)BH(NTf2)]2 indicated a sterically crowded environment
around boron, and this leads to the less common O-bound mode of NTf2 binding. While the iodide congener reacted very slowly with
alkynes, the NTf2 analogues were more reactive, with hydroboration
of internal alkynes forming (vinyl)2BNR2 species
and R2NBH(NTf2) as the major products. Further
studies indicated that the B2N2 core is maintained
during the first hydroboration, and that it is during subsequent steps
that B2N2 dissociation occurs. In the mono-boron
systems, for example, iPr2NBH(NTf2), NTf2 is N-bound; thus, they have less steric
crowding around boron relative to the B2N2 systems.
Notably, the monoboron systems are much less reactive in alkyne hydroboration
than the B2N2-based bis-boranes, despite the
former being three coordinate at boron while the latter are four coordinate
at boron. Finally, these B2N2 electrophiles
are much more prone to dissociate into mono-borane species than pyrazabole
[H2B(μ-N2C3H3)]2 analogues, making them less useful for the directed diborylation
of a single substrate.
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