Aromaticity and Activation Strain Anallysis of [3+2] Cyclloaddition Reactions between Group 14 Heteroallenes and Trip lle Bonds

We have computationally explored the trend in reactivity of [3 + 2] cycloaddition reactions between H2E=C=PH and HO CH as the terminal position position in the phosphaallene is varied along E = C, Si, Ge, Sn, Pb. The reaction barrier drops significantly from E = C (nearly 50 kcal/mol) to E = Si-Pb (ca. 20 kcal/mol). Activation strain analyses tie this trend to a reduction in activation strain in the heavier phosphaallene analogues which, in contrast to the parent compound H2C=C=PH, do already possess the bent geometry required in the TS.