RESUMEN
A copper-catalyzed alkyne-selective hydroboration of 1,3-enynes is disclosed, providing access to the previously elusive 2-boryl-1,3-dienes. Using CuOAc, Xantphos, and HBpin, Bpin was installed on the internal carbon of a series of symmetric and nonsymmetric 1,3-enynes, affording products with excellent Z:E selectivity. The utility of the 2-boryl-1,3-diene products was demonstrated by transformation to useful functional groups.
RESUMEN
The reduction of nitriles to primary amines is a useful transformation in organic synthesis, however, it often relies upon stoichiometric reagents or transition-metal catalysis. Herein, a borane-catalysed hydroboration of nitriles to give primary amines is reported. Good yields (48-95%) and chemoselectivity (e.g., ester, nitro, sulfone) were observed. DFT calculations and mechanistic studies support the proposal of a double B-N/B-H transborylation mechanism.
RESUMEN
Simple nucleophiles with structural similarities to known hydroboration catalysts can readily mediate the formation of BH3 and borohydride species from pinacolborane (HBpin). Alkyne and alkene hydroboration reactions were successfully mediated by nucleophiles through BH3 generation, with BH3-catalyzed hydroboration found to dominate catalysis. NMR spectroscopy and kinetic analyses showed that the nucleophiles NaOtBu, Na[N(SiMe3)2], nBu2Mg, and nBuLi only promoted the formation of BH3 and were not "true" hydroboration catalysts.
RESUMEN
The mechanism of R2BH-catalyzed hydroboration of alkynes by 1,3,2-dioxaborolanes has been investigated by in situ 19F NMR spectroscopy, kinetic simulation, isotope entrainment, single-turnover labeling (10B/2H), and density functional theory (DFT) calculations. For the Cy2BH-catalyzed hydroboration 4-fluorophenylacetylene by pinacolborane, the resting state is the anti-Markovnikov addition product ArCH = CHBCy2. Irreversible and turnover-rate limiting reaction with pinacolborane (k ≈ 7 × 10-3 M-1 s-1) regenerates Cy2BH and releases E-Ar-CHâCHBpin. Two irreversible events proceed in concert with turnover. The first is a Markovnikov hydroboration leading to regioisomeric Ar-C(Bpin)âCH2. This is unreactive to pinacolborane at ambient temperature, resulting in catalyst inhibition every â¼102 turnovers. The second is hydroboration of the alkenylboronate to give ArCH2CH(BCy2)Bpin, again leading to catalyst inhibition. 9-BBN behaves analogously to Cy2BH, but with higher anti-Markovnikov selectivity, a lower barrier to secondary hydroboration, and overall lower efficiency. The key process for turnover is B-H/C-B metathesis, proceeding by stereospecific transfer of the E-alkenyl group within a transient, µ-B-H-B bridged, 2-electron-3-center bonded B-C-B intermediate.