Structure and properties of 1-phospha-allene (H2CCPH). α-Carbon versus phosphorus protonation?

Abstract

Although 1 -phospha-allenes (CCP–) are generally only isolable when bulky groups are present, the first experimental data are now becoming available. Ab initio calculations are reported for this novel cumulene system (H₂CCPH) and four possible protonated analogues. The phospha-allene is configurationally stable with a large rotational barrier. The contribution of structures such as (1b) are minimal; in fact the CC and CP bonds are largely as in the uncumulated systems, in line with the failure to observe i.r. absorptions in the 1 600–2 300 cm^–1 region. The α-carbon is the favoured site of protonation; this behaviour is contrasted with other cumulenes such as ketenimines or phosphaketenes. The proton affinity of 1-phospha-allene is calculated to be 193 ± 5 kcal mol^–1 at MP4/6-31G**//HF/4- 31G. The predicted reactivity of the phospha-allene in cycloadditions and with reagents HX is commented on.