Bulky alkyls and amides of main group 5 elements II. Rotational barriers in phosphorus compounds containing bulky groups

The group [(CH₃)₃Si]₂CH is effectively bulkier than the (CH₃)₃C group in inhibiting rotation about the PC bond in R₂PCI compounds. In phosphonamidous chlorides, RP(CI)NR₂′, increasing the bulk of the groups at nitrogen, e.g., from CH₃ to (CH₃)₂CH to (CH₃)₃Si, increases the rotational barriers about the PN bond; however, changing R from (CH₃)₃C to [(CH₃)₃Si]₂CH increases the barrier, which is a reversal of the trend earlier observed. It is suggested that these very bulky silylated groups represent limiting cases of steric interference.