The isomerisation reaction has features that are superficially similar to the chemistry of a boryl gold compound reported recently by Yamashita et al. In this system (depending on the alkyne), a transiently formed syn insertion product undergoes isomerisation to form either the anti disubstituted alkene, or a borylalkenylgold complex in which the R group has migrated to boron and the alkyne has assimilated a B-derived aryl function. The proposed intermediate in this reaction is a gold alkynyl-borate, which can produce both products via a 1,2-shift of either an R or Ar group. The operation of a similar mechanism in the conversion of 2-Et to 3-Et (Scheme 2) is supported by the fact that the aluminium ethyl complex (NON)AlEt can be isolated from the reaction mixture as a minor product and structurally characterised (Figure ). The formation of this product demonstrates unequivocally that C-to-Al migration of an Et group is viable, with the reverse step then providing a pathway for inter-conversion between the syn and anti isomers 2-Et and 3-Et. The formation of (NON)AlEt also implies concurrent generation of a copper acetylide coproduct; while we were unable to isolate this compound, the lability of group 11 metal acetylides is well known, and the P-containing component was simply found to be 'free' t Bu 3 P. To probe regioselectivity in the insertion of alkynes into 1, unsymmetrically substituted alkynes were also investigated (Scheme 1). The products of the reaction with 1-phenyl-1-butyne (PhCCEt) were not easily amenable to crystallization, so the closely related n-propyl alkyne, 1-phenyl-1pentyne (PhCC n Pr) was studied instead. The reactivity patterns of PhCCR (R = Et, n Pr) towards 1 appear to be essentially identical.