Transition-metal catalysis of cyclocarbonylation and cycloisomerization reactions

Abstract

Chapter 1. The asymmetric Pauson-Khand cyclization of nitrogen-containing enynes using carbon monoxide and a catalytic amount of (EBTHI)TiMe2 was examined. The influence of the nitrogen substituent and the concentration of the catalyst on the enantioselectivity of this cyclization was explored. It has been found that enynes with an octyl-, benzyl-, or allyl-amino group, positioned [beta] to the alkyne and the olefin, were cyclized with a high degree of enantioselectivity. Substrates with bulky and/or electron-withdrawing nitrogen substituents were converted to products in low to moderate enantioselectivity. Chapter 2. The first example of an early transition-metal-catalyzed enyne cycloisomerization is described. Titanocene dicarbonyl was shown to catalyze the cycloisomerization of enynes and dienynes to 1,4-dienes and allylic allenes, respectively. Examination of the scope of the reaction demonstrates that only trans-substituted enynes were cyclized. The products were exclusively 1,4-dienes and the reaction was completely diastereoselective for a chiral substrate with a substituent [gamma] to the alkyne. Chapter 3. A variety of monocyclopentadienyl-titanium-aryloxide complexes were used to catalyze or mediate cyclization reactions of alkynes, dienes, and enynes. Structural analysis, by X-ray crystallography, provided steric parameters, including wedge angles, for the aryloxide ligands. A titanium metallacycle was prepared from a sterically-hindered enyne containing a trisubstituted olefin. A monocyclopentadienyl titanium (2,6-dipheyl)phenoxide was used to promote a Pauson-Khand reaction of trimethylsilyl-substituted enynes,

(cont.) generating [alpha]-silyl cyclopentenones. The 2,6-substituted aryloxide ligands allowed for modulation of steric and electronic parameters. Chapter 4. A complex formed from dicobalt octacarbonyl and a chiral aryl bisphosphite served as a catalyst for the intramolecular asymmetric Pauson-Khand reaction. Bicyclic cyclopentenones were obtained in up to 75% enantiomeric excess. For a terminal 1,6-enyne, the incremental enantiomeric excess was found to increase from 4-26% over the course of the reaction. The scope of this process was examined for a variety of 1,6- and 1,7-enynes, and a moderate degree of enantioselectivity was maintained only in the case of aryl-substituted 1,6-enynes.