| The first sterically hindered cobalt metallacyclobutene complex, ( &eegr;5-C5H5)(PPh3)Co[kappa 2-(C1,C3)-C(SO 2Tol)=C(SiiPr3)CH(CO2Et)]. This complex was obtained from the reaction of cobalt-alkyne complex ( &eegr;5-C5H5)(PPh3)Co( iPr3SiC≡CSO2Tol) and ethyl diazoacetate. The first experimental evidence of interconversion between a cobaltacyclobutene to &eegr;3-vinylcarbene complex, &eegr;5-(C5H5)Co[&eegr; 3-(CHCO2Et)(CSiiPr 3)(CSO2Tol)], was studied. Equilibria of this type have been widely speculated on the literature, but have never been experimentally observed prior to our work.;Metallacyclobutene underwent reaction with cyclic activated alkenes, diazocarbonyls, and (&eegr;5-C5H 5)Co(PPh3)2 under very mild conditions. Reaction with maleic anhydride led to formation of an air-stable metallacyclohexene, Co(&eegr;5-C5H5)[kappa 3-(C,O,C)-CH(SO 2Tol)=C(SiiPr3)CH(CO2Et)CH(C(=O)OC(=O)CH)], in 78% yield. The treatment of cobalt metallacyclobutene with (&eegr; 5-C5H5)Co(PPh3) 2 afforded a dicobalt complex consistent with involvement of an intermediate like an &eegr;3-vinylcarbene complex. Lastly, 1,3-dienes were formed when cobalt-alkyne complex or cobalt metallacycle underwent reaction with ethyl diazoacetate, thereby demonstrating the formal [1+2+1] addition of two carbenes across an unsymmetrically substituted alkyne.;In order to stabilize suspected intermediates and elucidate the mechanism of carbon-sulfur bond activation, three experimental strategies were utilized. First, several Rh, Ni, Ir, and Co complexes were studied in place of ( &eegr;5-C5H5)Co(PPh3) 2; however, no observation of the insertion chemistry, and instead a number of undesired products were formed including metallacyclopentadienes. In the second strategy, the TMS group was substituted by a TIPS (triisopropylsilyl group) in order to increase the steric bulk of the alkyne substituents and avoid the formation of metallacyclopentadienes. This approach led to the formation of a new cobaltosulfoxide complex, (&eegr; 5-C5H5)Co(S(O)C6H4CH 3)(C≡CSi(iPr)3)PPh3, in 28% isolated yield. Unfortunately the metallosulfoxide transformed into a thiolato-bridged cobalt dimer, O=PPh3, and 1,4-(triisopropylsilyl)butadiyne.;The third strategy was to stabilize the cobaltosulfoxide product by substituting a PPh3 ligand in (&eegr;5-C 5H5)Co(PPh3)2 with an N-Heterocyclic carbene (NHC) ligand. This provides increased stability toward oxidative decomposition. Although the carbon-sulfur bond activation failed with this type of compound, ligand substitution of one of the phosphines of precursor (&eegr;5-C5H5)Co(PPh 3)2 led to the new cobalt carbene complexes (&eegr; 5-C5H5)Co(ImiPr 2)(PPh3) and (&eegr;5-C 5H5)Co(ImiPr2)(CO). Computational and experimental studies were performed to identify the contributions from both the steric and electronic effects to the structures of both structures. |
