Carbanion-Accelerated Claisen Rearrangements. 8. Phosphonamide Anion-Stabilizing Groups

The utility of various phosphonamide groups has been examined in the context of the carbanion-accelerated Claisen rearrangement (CACR). An extensive survey has identified the N,N′-dibenzyl-1,3,2-diazaphospholidine group 11 to be optimal in the ease of construction of the CACR precursors and the facility and stereoselectivity of the rearrangement. Using n-butyllithium as the base, the phosphonamides rearranged readily at -20 °C with complete regioselectivity and in good yield (74-79%). The phosphonates also showed a high level of diastereoselectivity (>95% de) but the yield from the (Z)-2-butenyl precursor (anti product) was only 45%. A chiral N,N′-dibenzyl-1,3,2-diazaphospholidine 12 derived from trans-1,2-cyclohexanediamine was examined. Although the CACR proceeded very cleanly (71-85%) and with high internal selectivity (94% de), the relative asymmetric induction was poor (16-20% de). This was also the case for a chiral N,N′-dibenzyl-1,3,2-diazaphosphorinane 15 derived from (R,R)-1, 3-diphenyl-1,3-propanediamine and N,N′-dibenzyl-1,3,2-diazaphosphepine 16 derived from 6,6′-dimethyl-2,2′-diaminobiphenyl. The characteristic features of the CACR were compared with the aryl sulfone and phosphonate versions.