Structure and properties of 1-phospha-allene (H2CCPH). 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.