INSERTION REACTIONS OF SQUARE-PLANAR DIORGANOPLATINUM .2. STEREOSELECTIVE CARBONYLATION OF TRANS-PT(R)(R')(PPH(3))(2) LEADING TO CIS-PT(R)(COR')(PPH(3))(2) AND ISOMERS OF PT(PPH(3))(CO)(COR')(R)

Synthesis of square-planar trans dialkyl and alkyl aryl complexes of p latinum(II), trans-Pt(R)(R')(PPh(3))(2) (R = Me R' = Et (2a); R = Me R ' = Ph (2b); R = R' = Et (2c); R = Et R'= Ph (2d)) was established emp loying transmetalation of trans-Pt(R)(I)(PPh(3))(2) with Grignard reag ents R'MgX. Complexes 2a-2c underwent stereoselective carbonylation to produce cis-Pt(R)(COR')(PPh(3))(2) (R = Me R' = Et(3a); R = Me R' = P h(3b); R = R' = Et (3c)). These facile transformations were exclusive and quantitative. The reactivity of carbonylation followed the order E t > Ph > Me. A similar reaction of 2d with CO yielded EtC(O)Ph instead . The cis acyl alkyl or acyl aryl complexes 3a-3e suffered substitutio n of CO for a PPh(3), leading to two isomers SP-4-3- and SP-4-4-Pt(PPh (3))(CO)(COR')(R) (R = Me R' = Et (4a, 4a'); R = Me R' = Ph (4b, 4b'); R = R' = Et (4c, 4c')), respectively. The reverse reactions of 4 to 3 readily occurred when external PPh, was provided. The stereoselectivi ty of carbonylation of trans-Pt(R)(R')(PPh3)2 is explained by a mechan ism in which reversible displacement of a PPh, by CO in the reactant p recedes to form a four-coordinate dialkyl carbonyl intermediate Pt(PPh (3))(CO)(R)(R'). The ensuing alkyl (or aryl) migration from metal to t he carbonyl carbon achieves the cis acyl alkyl (or aryl) configuration . Recoordination of a PPh(3) completes the reaction. X-ray structures of 2b, 2c, 3b and 4b as single crystals are provided.