[2+1] Cycloadditions of Terminal Alkynes to Norbornene Derivatives Catalyzed by Palladium Complexes with Phosphinous Acid Ligands

Abstract

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

Anionic phosph(in)ito (“phosphoryl”) ligands: Non-classical “actor” phosphane-type ligands in coordination chemistry

“…This review points out the ability of O−anionic P-metalated phosph(in)ito ligands to interconvert to the corresponding phosphinous acids upon protonation/deprotonation processes. This feature is quite unique and may explain the exceptional behavior of some phosphinous acid/phosph(in)ito catalysts in optimized pH conditions [224][225][226][227][228][229]. The application of phosph(in)ito transition metal complexes in catalysis is still underexplored and the promising results obtained so far pave the way for future investigations in challenging organic chemical transformations.…”

Metal Vinylidenes as Catalytic Species in Organic Reactions

“…Norbornadiene (or norbornene) and phenylacetylene has been reported to undergo a [2+1] cycloaddition reaction, catalyzed by palladium diacetate and phosphinous acid ligand 231 (Scheme 84). [97] The proposed mechanism, outlined in Scheme 85, suggests vinylidene formation (II, supported by labeling experiments), followed by coordination of norbornadiene to give III. A [2+2] cycloaddition, followed by reductive elimination, then gives the product and regenerates the catalyst.…”

Transition metals in organic synthesis: Highlights for the year 2005

“…A palladium-catalyzed intermolecular alkylidenecyclopropanations of bridged bicyclic alkenes and terminal alkynes was reported (Eq. (236)) [1275]. An interesting rhodium-catalyzed dimerization of bisallenes to give steroid ring system was described (Eq.…”