Formation of a hydride containing amido-zincate using pinacolborane

Amido-zincates containing hydrides are underexplored yet potentially useful complexes. Attempts to access this type of zincate through combining amido-organo zincates and pinacolborane (HBPin) via a Zn-C / H-B exchange led instead to preferential formation of amide-BPin and/or [amide-BPin(Y)]- (Y = Ph, amide, H), when the amide is hexamethyldisilazide or 2,2,6,6-tetramethylpiperidide and the hydrocarbyl group was phenyl or ethyl. In contrast, the use of a dipyridylamide (dpa) based arylzincate led to Zn-C/HBPin metathesis being the major outcome. Independent synthesis and full characterisation of two (donor)Li[(dpa)ZnPh2] (donor = 3 x THF or 1 x PMDETA) complexes, 1 and 3, respectively, enabled reactivity studies confirming these species display zincate type reactivity (by comparison to the reactivity of the neutral complex Me-NdpaZnPh2, 4). This included 1 performing the rapid deprotonation of a terminal alkyne and also phenyl transfer to α,α,α-trifluoroacetophenone in contrast to neutral complex 4. Complex 1 reacted with one equivalent of HBPin to give predominantly PhBPin (90%) and a lithium amidophenylzincate containing a hydride unit, complex 7, as the second major product. Complex 7 transfers hydride to an electrophile preferentially over phenyl, indicating it can react as a hydridozincate. Attempts to react 1 with > 1 equivalent of HBPin or with catecholborane led to more complex outcomes, which included significant borane and dpaZn substituent scrambling, two examples of which were crystallographically characterised. While this work provides proof of principle for Zn-C/H-BPin exchange as a route to form an amido-zincate containing a hydride, amido-organozincates that undergo more selective Zn-C/H-BPin exchange still are required.