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Synthesis and reactivity of sterically congested metallacyclobutenes and metal-mediated carbon-sulfur bond activation

  • Author(s): Vélez, Carmen L.
  • et al.

The first sterically hindered cobalt metallacyclobutene complex, ([Eta]⁵-C₅H₅)(PPh₃)Co[Kappa]²-(C¹,C³)- C(SO₂Tol)=C(Si/iPr₃)CH(CO₂Et)]. This complex was obtained from the reaction of cobalt-alkyne complex ([Eta]⁵- C₅H₅)(PPh₃)Co(i/Pr3SiC̲=CSO₂Tol) and ethyl diazoacetate. The first experimental evidence of interconversion between a cobaltacyclobutene to [Eta]³-vinylcarbene complex, ([Eta]⁵-C₅H₅)Co[[Eta]⁵-CHCO₂Et)(CSi/iPR₃)(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 ([Eta]⁵- C₅H₅)(PPh₃)₂ under very mild conditions. Reaction with maleic anhydride led to formation of an air-stable metallacyclohexene, Co([Eta]⁵-C₅H₅)[Kappa]³-(C,O.C)- CH(SO₂Tol)=C(Si/iPr₃)CH(CO₂Et)CH(C(=0)OC(=O)CH)] in 78% yield. The treatment of cobalt metallacyclobutene with ([Eta]⁵-C₅H₅)(PPh₃)₂ afforded a dicobalt complex consistent with involvement of an intermediate like an ([Eta]⁵-C₅H₅)-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 ([Eta]⁵- C₅H₅); 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, ([Eta]⁵- C₅H₅)Co(S(O)C₆H₄CH₃)(C̲=CSi(Pr)₃PPh₃, 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 ([Eta]⁵-C₅H₅)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 ([Eta]⁵-C₅H₅)Co(PPh₃)₂ led to the new cobalt carbene complexes ([Eta]⁵-C₅H₅)Co(Im/iPr₂)(PPh₃) and ([Eta]⁵-C₅H₅)Co(Im/iPr₂)(CO) Computational and experimental studies were performed to identify the contributions from both the steric and electronic effects to the structures of both structures

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