Regio- and stereoselective carbon-carbon bond forming reactions catalyzed by titanium aryloxide compounds
Abstract
The titanacyclic complexes (($\rm ArO)\sb2Ti(C\sb4Et\sb4$)) (8) (ArO = 2,6-diphenylphenoxide), (($\rm ArO)\sb2Ti\{C\sb2(SiMe\sb3)\sb2C\sb6H\sb8\}$) (9) (ArO = 2,6-diisopropylphenoxide), (($\rm ArO)\sb2Ti(C\sb4Bu\sp{t}\sb2H\sb2$)) (10) (ArO = 2,6-diphenylphenoxide), (($\rm ArO)\sb2Ti\{C\sb2(Et)\sb2C\sb6H\sb8\}$) (17) (ArO = 2,6-diphenylphenoxide), and (($\rm ArO)\sb2Ti(CH\sb2CMe = CMeCH\sb2$)) (7) (ArO = 2,6-diisopropylphenoxide) have been synthesized by the room temperature sodium amalgam reduction of dichloride complexes ((ArO)TiCl$\sb2$) in the presence of alkynes (for (8), (9) and (10)), or 2,3-dimethylbutadiene (for (7)). The reactivity of these complexes in selective carbon-carbon bond forming reactions has been investigated. The catalytic cross-coupling of 2,3-dimethylbutadiene with the $\alpha$-olefins (CH$\sb2$ = CHR, R = H, Me, Et, Ph, Bu$\sp\rm\sp{n}$, SiMe$\sb3$) and isoprene with styrene have been carried out using complexes (7), (8), and (9) as catalyst precursors. The regio- and stereoselective formation of 1,4-dienes in these reactions is consistent with rate determining $\beta$-hydrogen abstraction from titanacyclohept-3-ene intermediates. The results of deuterium labeling experiments show that $\beta$-substituted titanacyclohept-3-ene intermediates undergo isomerization to $\alpha$-substituted titanacyclohept-3-ene intermediates. These intermediates form by olefin insertion into the Ti-C bond of titanacyclopent-3-ene complexes in the catalytic cycle. Plots of concentration of substrates vs time and the results of a competition experiment have shown that the rate of this insertion and the rate of $\beta$-hydrogen abstraction is strongly dependent on the type of $\alpha$-olefin and catalyst precursor used. The (2+2+2) cycloaddition of alkynes with olefins catalyzed by titanium aryloxide compounds results in the formation and isomerization of 1,3-cyclohexadienes. The mechanism of the isomerization reaction has been investigated using complex (10) in the cycloaddition of styrene with 3,3-dimethyl-1-butyne. Mechanistic results were obtained by varying reaction conditions and by stereochemical analysis of the 1,3-cyclohexadiene isomers. These results suggest a reaction pathway in which titananorbornene intermediates carry out the stereoselective isomerization of 1,3-cyclohexadienes by metal-mediated 1,5-hydrogen shifts. The scope of the titanium aryloxide catalyzed (2+2+2) cycloaddition reaction has been extended to reactions of diyne substrates with olefins catalyzed by (8) and (17) to form substituted hexahydronapthalenes. The titanacyclopentadiene complexes are efficient catalysts for the cyclotrimerization of terminal alkynes. The results of the cross-coupling reactions and the cycloaddition reactions suggest that alkyne cyclotrimerization by titanium aryloxide compounds proceeds by a concerted pathway in which titananorbornadiene complexes are intermediates.
Degree
Ph.D.
Advisors
Rothwell, Purdue University.
Subject Area
Chemistry|Organic chemistry
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