A STUDY OF THE THERMAL ISOMERIZATION OF ORGANOBORANES
Abstract
Thermal isomerization reaction of organoboranes was discovered in 1957. By 1959, the importance of this reaction in organic synthesis was recognized. However, lack of control on the migrational selectivity of the boron atom and inconveniently slow isomerization rates in the absence of any hydride excess, have seriously diminished the synthetic potential of this reaction in the past. The use of a 20% excess of hydride almost completely solved the latter problem, but it obscured the understanding of the nature of the exact species undergoing the thermal isomerization. The fundamental objective of the present research was, therefore, to resolve the above problems hampering the potential of the thermal isomerization reaction and to demonstrate its true synthetic capabilities. Our preliminary study on the thermal isomerization of structurally defined trialkylboranes has shown that increasing the steric crowding in the initial trialkylborane undergoing the thermal isomerization remarkably accelerates the rate and significantly improves the equilibrium boron distribution in favor of the thermodynamically preferred isomer. The generality of the steric acceleration of the rate of isomerization was then established by a systematic study of the thermal isomerizations of a wide variety of structurally defined organoboranes. In these studies we have developed several new monofunctional hydroborating agents and found some of them to be highly useful from the standpoint of the thermal isomerization reaction. We then established the synthetic potential of a selected set of monofunctional hydroborating agents in the hydroboration-isomerization of a variety of structurally representative olefins. Here we demonstrated exceptionally selective boron migrations in olefinic systems where the thermal isomerization was previously reported to be virtually unselective. Finally we synthesized 1-triacontanol, a plant growth stimulant of immense current interest, in excellent yield utilizing the thermal isomerization reaction in order to demonstrate the applicability of this reaction in organic synthesis.
Degree
Ph.D.
Subject Area
Organic chemistry
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