More forcing conditions (heating to 45 °C for 18 h) result in conversion of 2-Pr,Ph/2-Ph,Pr to a single species, characterized by a 31 P resonance at 57.0 ppm. In this case, X-ray quality crystals of the product could be grown from hexane, and show a dimetallated alkene, 3-Pr,Ph in which the Al and Cu fragments are situated anti to one other, with the n Pr group geminal to Al (Scheme 1 and Figure ). This regiochemical assignment is consistent with 2D NMR data for this compound in solution (Figure ). The 1 H NMR spectrum also bears a strong resemblance to that of 2-Pr,Ph, for example in featuring a resonance without 31 P coupling (now at 3.11 ppm), arising from the CH 2 CH 2 CH 3 protons. This observation provides further evidence that both 2-Pr,Ph and 3-Pr,Ph feature the same n Pr/Ph regiochemistry. Moreover, the structural validation of the anti (aluminylalkenyl)copper fragment within the thermodynamic product 3-Pr,Ph supports the assignment of 3-Et as featuring the analogous anti alignment of the two ethyl fragments. As such, for both alkyne substrates, it is evident that the syn isomer is selectively formed initially (under kinetic control), and then reacts onwards to give exclusively the anti (aluminylalkenyl)copper species under more forcing (thermodynamic) conditions (i.e. mild heating and longer reaction times). Interestingly, exchange reactions show that the inserted alkyne fragment can be substituted for another, consistent with chemical reversibility in the insertion process. Accordingly, addition of one equivalent of PhCC n Pr to a solution of 2-Et leads to the disappearance of the 31 P NMR signal at 55.0 ppm associated with 2-Et, with accompanying growth of two signals associated with the major (60.2 ppm) and minor products (61.5 ppm) of PhCC n Pr insertion (in the same ratio after 60 min as seen in the reaction of 1 itself with 1-phenyl-1-pentyne). Consistently, one equivalent of free 3-hexyne is observed in the 1 H NMR spectrum, showing that it has been released from 2-Et.