The ring-bridged chromocene carbonyl complex Me z Si(C sMe 4)zCr(CO) 1 is obtained by reaction of Me~ Si(C, MC 4)2 Li ~ with CrCl z. THF in the presence of CO. Reaction with CO transforms the monocarbonyl complex 1 to a dicarbonyl complex MezSi(CsMe4)(Cs(endo-3-H)(2-CHz)Me3)Cr(CO)2 2; in this reaction, a hydrogen atom is shifted from one of the cx-CH 3 groups to the adjacent l3-position of a C s ring ligand. The rate of this reaction is first order in complex and CO concentration, with an activation enthalpy of H oF = 51 ± 3 kJ mol-I and an activation entropy of S oF = 136 ± 10 J mol-I K-I. In toluene solution at 50 cC, complex 2 isomerizes to Me2Si(CsMe4)(Cs(exo-3-H)(2-CH2)Me3)Cr(CO)2 3, then to Me2Si(CsMe4)(Cs(exo-I-H)(2-CHz)Me3)Cr(CO)2 4 and further to Me2Si(CsMe4)(Cs(endo-5-HX2-CHz)Me3)Cr(CO)2 5. The kinetics of these isomerizations, which involve hydrogen migrations and ring rotation reactions, indicate monomolecular reactions. Possible reaction paths and transition states are discussed. The structures of complexes 1-5 were determined by lH NMR spectroscopy, those of 1, 2, 4 and 5 also by single-crystal X-ray diffraction.

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Nucleophilic Displacement of Homoallylic Tosylates: Influence of Steric Hindrance and Conformational Rigidity.-Three tosylates (I), (VII), and (XI) are examined in the comparative displacement study with various nucleophiles, an unhindered and flexible one, a hindered and flexible one, and a hindered and rigid one. π Bond participation is favored by hindrance and rigidity of the tosylate, and also by weak nucleophiles such as water or DMF. -(FERNANDEZ-MATEOS, A.; RENTZSCH, M.; RODRIGUEZ SANCHEZ, L.; RUBIO GONZALEZ, R.; Tetrahedron 57 (2001) 23, 4873-4879; Dep. Quim.