ASYMMETRIC SYNTHESIS OF PHOSPHINES AND ARSINES PROMOTED BY CHIRAL PALLADIUM COMPLEXES

Abstract

The chiral palladium(II) compound bis(?-chlor )bis[1-[1-( dimethylamino)ethyl]-2- naphthalenyl-C,N]palladium(II) dichloromethane solvate (S)-90 has been successfully used to promote enantiospecific [ 4+2] Diels-Alder reactions between vinylphosphines and 1-phenyl-3,4-dimethylphosphole. Diphenylvinylphosphine (a), diphenyl-1- propenyl-(E)-phosphine (b) and diphenyl-1-propenyl-(Z)-phosphine (c) were used in the studies. Enantiomerically pure forms of diphosphibe ligands 5-phosphino-7-phosphabicyclo[2.2. 1]hept-2-enes, (S)-86a-c were generated as ligands on the novel palladium(II) complexes (Rp,Sc)-95a-c and (Rp,Sc)-98a-b. For the Diels-Alder reaction involving diphenyl-1-propenyl-(Z)-phosphine (c), the complex (Rp,Sc)-95c [?]D +60. 7 (CHCl3) was generated exclusively. Reactions involving diphenylvinylphosphine (a) and diphenyl-1-propenyl-(E)-phosphine (b), however, produced two isomers in unequal amounts. The major products (Rp,Sc)-95a [?]D +56.3 (CHCI3) and (Rp,Sc)-95b [?]D +40.0 (CHCl3) were isolated. The complexes were fully characterized by 1H NMR, 31P NMR and elemental analyses. The structure of (Rp,Sc)-95b was determined by X-ray crystallography. The diphenylphosphine moiety of the bidentate phosphine ligand is trans to the nitrogen atom of the orthometalated 1-[ 1-( dimethylamino )ethyl]naphthalene ring. The complex contains six asymmetric centers. The absolute configuration of the carbon stereocenter in the organometallic ring is S. The absolute configuration of the phosphorus stereogenic center is R. The donor abilities of the two phosphorus centers are similar. The free ligands (S)-86a [?]D -62.5 (CH2Cl2), (S)-86b [?]D -14.4 (CH2CI2) and (S)- 86c [?]D -56.0 (CH2CI2) were released from the palladium(II) centers stereoselectively with retention of configuration. The ligands are unstable in solution. They were recoordinated to metal centers immediately upon liberation. The 31P{1H} NMR studies of a series of recomplexation reactions involving both enantiomeric forms of 90 and their cationic derivative 91 confirmed the enantiomeric purities of the ligands. The structures of the minor product (Rp,Sc)-98a-b of the cycloadditions were also determined from the same set of experiments. They are geometric isomers of (Rp,Sc)-95a-b. The phenylphospha moiety is trans to the NMe2 group of the organometallic ring. Steric interactions between the prochiral NMe2 and the phenyl group(s) borne by the phosphorus atoms provide the main force directing the stereo chemical course of the asymmetric cycloadditions. Steric factors also led to the preferential formation of (Rp,Sc)-95a-c over (Rp,Sc)-98a-b. Analogous [4+2] cycloaddition between diphenylvinylarsine and 1-phenyl-3,4- dimethylphosphole was carried out. The absolute configuration of the novel As-P mixed donor ligand produced was postulated based on the results obtained for the diphosphines.