Addition of PhCC n Pr to a solution of 1 at 10 °C leads to the formation of a single product at very short reaction times (t < 20 min; Figure ), but to a mixture of this compound and a second product in a ratio of ca. 3:1 after ca. 1 h, as determined by 31 P NMR measurements (signals at δ P = 60.2 and 61.5 ppm, respectively). After much longer reaction times this ratio is reversed (ca. 1:3 after 7 h). The 1 H NMR spectrum of the first-formed product (2-Pr,Ph) features a resonance for the CH 2 CH 2 CH 3 protons at 2.74 ppm, that shows no additional coupling to 31 P. This signal is characteristic of an AA'XX' spin system, which is well described for n-propyl groups featuring restricted rotation. By contrast, the corresponding signal for the minor product (2-Ph,Pr) is a doublet of triplets, the result of additional 31 P coupling to the CH 2 group. These data imply that that the regiochemistry of the insertion process is such that the major product features the alkyl group geminal to the aluminium fragment, and the phenyl group adjacent to the [CuP t Bu 3 ] unit. The initial minor product is the alternate regioisomer, in which the alkyl group is adjacent to [CuP t Bu 3 ]. Although the major product (2-Pr,Ph; Scheme 1) could not be crystallized, trapping experiments with CO (see below) confirm both the (syn) stereochemistry and the regiochemistry of the alkyne addition. Interestingly, a temporal plot shows that the concentration of 2-Pr,Ph in solution reaches a maximum after ca. 60 min, and that the growth of the second-formed product 2-Ph,Pr then occurs at its expense (Figure ). A potential explanation for this observation involves reversibility in the initial alkyne uptake step -a phenomenon which we could subsequently establish unequivocally by substitution of an inserted alkyne moiety by a different alkyne (see below).