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Mechanism and applications of the photochemistry of Bis(η^5-cyclopentadienyl)titanacyclobutanes

Citation

Wheeler, David Roger (1990) Mechanism and applications of the photochemistry of Bis(η^5-cyclopentadienyl)titanacyclobutanes. Dissertation (Ph.D.), California Institute of Technology. https://resolver.caltech.edu/CaltechTHESIS:03182015-140227559

Abstract

The photochemically induced reductive elimination of cyclopropanes from bis(η5-cyclopentadienyl)titanacyclobutanes has been examined. Stereochemical labelling studies indicate that the cyclopropane is initially formed in a 6±1:1, ratio favoring retention of stereochemistry. The starting titanacyclobutane is isomerized during the course of the reaction. The isomerization of the starting material results from metal-carbon bond homolysis to yield a 1,4-biradical, which can either close to give the starting material or generate cyclopropane. The 1,4-biradical can be observed through a cyclopropyl carbinyl rearrangement employing 2-bis(η5- cyclopentadienyl)titana-5,5-dimethylbicyclo[2.1.0]pentane, to give the titanium alkylidene, 1-bis(η5-cyclopentadienyl)titana-3,3-dimethyl-1,4- pentadiene, which can be observed directly by NMR at low temperature.

The oxidation of titanacyclobutanes by chemical and electrochemical methods also yields cyclopropanes. Reduction of the metal center does not yield cyclopropanes. Depending on the oxidant, stereochemically labelled titanacyclobutanes yield cyclopropanes that are between 7:1 and 100:1 retention:isomerization. The fragmentation reaction resembles the photochemically induced reductive elimination. Both result from formal oxidation of a metal-carbon bond, which then results in very rapid formation of cyclopropane.

The titanocene generated photochemically reacts with a variety of substrates even at low temperature. Titanocene can be generated in a glass at 77 K. The titanocene can be trapped in noncoordinating solvents in high yield with bulky internal acetylenes to give monoacetylene adducts of titanocene. Less bulky acetylenes give the titanacyclopentadienes. The titanocene can be trapped with olefins to give less stable adducts, which appear by NMR analysis to be intermediate in structure between a titanacyclopropane and an η2 olefin adduct of titanocene. Reaction of titanocene with butadiene gives a stable product, which appears to be the s-trans butadiene adduct of titanocene. It does not isomerize on heating. Titanocene reacts with epoxides to give titanocene-µ-oxo polymer and olefin. Stereochemically labelled epoxides and episulfides yield isomerized olefin upon deoxygenation by titanocene. The observations are rationalized as a result of a 1,4-biradical formed by stepwise insertion of titanocene into a carbon-oxygen bond.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Chemistry
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemistry
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Grubbs, Robert H.
Thesis Committee:
  • Unknown, Unknown
Defense Date:12 December 1989
Record Number:CaltechTHESIS:03182015-140227559
Persistent URL:https://resolver.caltech.edu/CaltechTHESIS:03182015-140227559
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:8787
Collection:CaltechTHESIS
Deposited By: Benjamin Perez
Deposited On:18 Mar 2015 21:31
Last Modified:02 Dec 2020 02:01

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