MECHANISM AND STEREOCHEMISTRY OF THE REACTION OF DICHLOROPLATINUM(II)COMPLEXES WITH DIAZO-COMPOUNDS - MONO VERSUS BIS INSERTION AND COMPETITION BETWEEN CAPTURE OF CHLORIDE AND CAPTURE OF AN INTERNAL NUCLEOPHILE BY CARBENOID INTERMEDIATES IN THE 2ND INSERTION STEP

[(COD)PtCl2] (COD = 1,5-cyclooctadiene) reacts readily with RCHN2 (R = SiMe3, P(O)(OMe)2, CO2Me) to give the mono-insertion products [(COD)P t(CHClR)Cl]. With longer reaction times, the bis-insertion products [( COD)Pt(CHClR)2] (R,R/S,S isomers predominating) are formed along with other platinum-containing compounds, namely [(COD)Pt(CH2Cl)(CHClSiMe3) ] (when R = SiMe3), platinaoxaphospholanes (when R = P(O)(OMe)2), or p latinalactones (when R = CO2Me). Mixed bis-insertion products are obta ined by reacting the mono-insertion products with a second diazo compo und. When the second diazo compound is N2CHSiMe3, the simple mixed ins ertion products are accompanied by products of net hydrodesilylation; i.e., they carry a -CH2Cl ligand rather than -CHClSiMe3. [{(R,R)-Ph2PC HMeCHMePPh2}PtCl2] reacts more slowly with RCHN2 than does the COD com plex and gives mono-insertion products in which we suggest the R stere ochemistry is preferred for the newly created chiral center. A second insertion into the two monoesters with N2CHCO2Me proceeds very slowly, giving only minor amounts of simple bis-insertion products, the major products being platinalactones. The mixtures obtained from the (R)- a nd the (S)-monoester both contain all four possible isomeric platinala ctones, indicating that some inversion of the initial chiral center is taking place. Mechanisms are suggested to account for the nature and stereochemistries of the products formed and the relative rates of som e of the reactions. It appears that the results of this investigation can be rationalized by proposing the participation in these reactions of three isomeric types of square-pyramidal carbenoid intermediates, n amely (i) axial carbene, which leads to dimerization/oligomerization p roducts, (ii) in-plane carbene with its substituents lying in the main coordination plane, which leads to insertion products, and (iii) in-p lane carbene with its substituents above and below the coordination pl ane, which leads to products of cyclization and hydrodesilylation. Thi s third type of carbene is appropriately aligned to capture an alpha-C l from a neighboring CHClR moiety, thus allowing interconversion of ca rbenes and the possibility of net inversion of configuration at the al pha-carbon.