Reactions of beryllium alkyls with carbonyl and azomethine groups: addition, reduction, complex formation, and ortho-metallation

Abstract

Reaction of beryllium alkyls with ketones and aldehydes yields compounds of the type RBeOCHR′₂ and RBeOCH₂R′ respectively, where R is Et, Buⁱ, Bu^t, and R′ is Bu^t and Ph. These alkoxides are all dimeric except BuⁱBeOCH₂Bu^t which is trimeric. t-Butylberyllium chloride gives (ClBeOCHBu^t₂)₂ and (ClBeOCH₂Bu^t,OEt₂)_1–2 respectively. The latter readily loses ether in vacuo giving benzene-insoluble ClBeOCH₂Bu^t, and reacts with NNN′N′-tetramethylethylenediamine (tmed) giving (ClBeOCH₂Bu^t)₂(tmed). These reductions proceed via a yellow or orange colour at –78°, except for benzophenone (purple), likely due to formation of a π-complex between the carbonyl group and the beryllium alkyl during or prior to hydride transfer. Dimethylberyllium adds to Bu^t₂CO, and (Me₃SiCH₂)₂Be adds to Ph₂CO giving (MeBeOCMeBu^t₂)₂ and (Me₃SiCH₂BeOCPh₂CH₂SiMe₃)₂ respectively, the latter by way of a red colour at –78°. Diphenylberyllium and (Me₃SiCH₂)₂Be give colourless co-ordination complexes with Ph₂CO, Ph₂CO → BeR₂. In contrast to the rapid reduction of aldehydes and ketones only a few azomethines and no alkyl cyanides are reduced, addition and complex formation being observed. Di-t-butylberyllium reduces PhCHNMe and PhCHCHCHNPh giving dimeric [Bu^tBeN(Me)CH₂Ph]₂ and [Bu^tBeN(Ph)CH₂CHCHPh]₂ respectively though it forms a red complex with benzylideneaniline, Bu^t₂Be,N(Ph)CHPh. Diphenylberyllium gives a tan complex with PhCHN(p-tolyl) though a colourless one with PhCHNMe. The colourless complexes Me₂Be,N(Me)CHPh. Bu^t₂Be,N(Me)CHPrⁱ, Bu^tBeCl,NCBu^t were also prepared. Diethylberyllium adds across the CN in PhCHNMe and PhCHNPh yielding [EtBeN(Me)CHEtPh₂] and [EtBeN(Ph)CHEtPh]₂. Dimethylberyllium gives the benzene-insoluble alkylation product, MeBeN(Ph)CHMePh, with the latter azomethine though the p-toluidine derivative is a dimer. Reaction of PhCHNBu^t with Et₂Be gives the yellow dimeric ortho-metallation product ethyl-2-(N-t-butylformimidoylphenyl) beryllium. The t-butyl analogue has a degree of association between 1 and 2.