David Howells

Carbonium ion rearrangements: electronic and conformational control in the migration of the electronegative group diphenylphosphinyl

In contrast to the pinacolyl derivatives (6) and (21), cis- and trans-2-diphenylphosphinyl-2-methylcyclohexyl tosylates (14a and b) give neither methyl nor diphenylphosphinyl migrations on solvolysis. Instead, two different olefins, 3-diphenylphosphinyl-3-methylcyclohexene from (14a) and the 4-diphenylphosphinyl-4-methyl isomer from (14b), are formed in high yield. The conformations of the tosylates and the structures of the olefins were elucidated by deuterium labelling and the use of the n.m.r. shift reagent Eu(dpm)3. The rates of solvolysis are slow compared with methyl or diphenylphosphinyl migration and it is suggested that electronic and not conformational factors control the corresponding acyclic rearrangements.

Rearrangements of secondary and tertiary carbonium ions by diphenylphosphinyl migration

Secondary and tertiary carbonium ions with a β-diphenylphosphinyl substituent, generated by solvolysis of sulphonates, or electrophilic attack on suitable olefins, rearrange exclusively by diphenylphosphinyl migration. The preference for this supposedly unfavourable reaction pathway is investigated.

Carbonium ion rearrangements: competitive migration of the electronically constrasted groups methyl and diphenylphosphinyl

Carbonium ion rearrangements are described in which either a methyl group or the electronegative diphenylphosphinyl group can migrate. When a very careful balance between electronic effects is made, the two groups migrate competitively with the Ph2PO migration slightly predominating. It is suggested that it is the ability of the group which stays behind to support the positive charge rather than ‘migratory aptitude’ which is important in these reactions.

Rearrangements of a cation of the neopentyl-type containing a diphenyl-phosphinyl substituent

Deamination of 2-diphenylphosphinyl-2-methylpropylamine (4) and solvolysis of the 2-diphenylphosphinyl-2-methylpropyl toluene-p-sulphonate (9) lead to diphenylphosphinyl migration and products arising from the tertiary cation (15). The deamination also gives some of a product with a cyclopropane ring, but in no case is methyl migration observed. It is suggested that a protonated cyclopropyl intermediate is formed in the deamination. Rate studies on the tosylate suggest that the diphenylphosphinyl group participates in the rate-determining step and that it is ca. 70 times less effective than a methyl group at participation in this system.

Carbonium ion rearrangements: migration of an electronegative group

2-Diphenylphosphinylethyl cations, generated in a variety of ways, rearrange by diphenylphosphinyl migration, even when the resulting cation has an identical structure to the unrearranged cation.