Chiral metal template promoted asymmetric pyrrole Diels-Alder reaction between N-(diphenylphosphino)pyrrole and diphenylvinylphosphine

Abstract

An organoplatinum complex containing ortho-metalated (S)-(1-(dimethylamino) ethyl)-naphthalene as the chiral auxiliary has been used to promote the asymmetric [4 + 2] Diels - Alder reaction between diphenylvinylphosphine and N-(diphenylphosphino)pyrrole. The reaction was complete in 7 days at room temperature, with the formation of three isomeric chelating diphosphine exo cycloadducts in the ratio 10:3:1. The cycloadducts are thermo-dynamically unstable and undergo retro-cycloaddition reactions slowly in solution. When the major isomer of the pyrrole cycloadducts was heated with 2-(diphenylphosphino) furan, the respective furan cycloadducts were formed via the retro pyrrole Diels - Alder pathway, followed by an asymmetric furan Diels - Alder reaction. The cyclic diene N-(diphenylphos-phino)pyrrole was regenerated from this retro cycloaddition reaction. The undesired retro Diels - Alder reaction could be deterred by hydrogenation of the azanorbornene double bond. The chiral naphthylamine auxiliary could be removed chemoselectively from the metal template by treatment with concentrated hydrochloric acid to generate the corresponding dichloroplatinum(II) complex. The azabicyclic skeleton and the P-N bond remained unchanged in this acid treatment. Further treatment of the dichloro complex with aqueous cyanide liberates the chiral diphosphine cycloadduct, which could be coordinated to the Au-(I) metal ion.