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)Pt(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 platinalactones (when R = CO2Me). Mixed bis-insertion products are obtained by reacting the mono-insertion products with a second diazo compound. When the second diazo compound is N2CHSiMe3, the simple mixed insertion products are accompanied by products of net hydrodesilylation; i.e., they carry a -CH2Cl ligand rather than -CHClSiMe3. [{(R,R)-Ph2PCHMeCHMePPh2}PtCl2] reacts more slowly with RCHN2 than does the COD complex and gives mono-insertion products in which we suggest the R stereochemistry 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)- and the (S)-monoester both contain all four possible isomeric platinalactones, 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 some 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, namely (i) axial carbene, which leads to dimerization/oligomerization products, (ii) in-plane carbene with its substituents lying in the main coordination plane, which leads to insertion products, and (iii) in-plane carbene with its substituents above and below the coordination plane, which leads to products of cyclization and hydrodesilylation. This third type of carbene is appropriately aligned to capture an alpha-Cl from a neighboring CHClR moiety, thus allowing interconversion of carbenes and the possibility of net inversion of configuration at the alpha-carbon.