Secondary phosphine oxides (SPOs) have been widely used for the synthesis of tertiary phosphine oxides and have found applications as Wittig-Horner reagents [1][2] and, later, as effective ligands for transition-metal complexes.[3] Recently, Li et al. showed that SPOs form air-stable palladium complexes, such as POPd1, when they are mixed with PdCl 2 (MeCN) 2 and then treated with Et 3 N.[4] These complexes proved efficient as catalysts in several cross-coupling reactions [4][5] as well as in asymmetric allylic alkylations.[6] Recent reports showed also that these new ligands are suitable for other types of catalyzed reactions such as the hydrolysis of nitriles [7] and the asymmetric hydrogenation of imines [8] and alkenes.[9] Our continued interest in the metal-catalyzed cycloaddition reactions between alkynes and norbornadiene [10] prompted us to investigate the catalytic behavior of palladium(ii) complexes coordinated by SPOs.First, we developed an easier way to synthesize the palladium catalyst 1. Upon treatment of Pd(OAc) 2 with tertbutyl(phenyl)phosphane oxide (L 1), dihydrogen di-macetatotetrakis(tert-butylphenylphosphinito -k -P)dipalladate (1) was quantitatively obtained without further treatment (Scheme 1).[11] Second, as a model we examined the reaction of phenylethyne (3 a) with norbornadiene (2) in the presence of 2.5 mol % of 1 in toluene at 50 8C for 24 h. Unexpectedly, the palladium(ii) complex 1 coordinated by L 1 favored the formation of benzylidenecyclopropane (4 a) as a single diastereomer in 17 % yield (Scheme 2) and contaminated by an unidentified byproduct (5 %). Surprisingly, a similar reaction using the known chloro-bridged analogue 5[6a-12] as catalyst did not work. Furthermore, in the reaction catalyzed by 5, the addition of 10 mol % of AgOAc (4 equiv relative to
