Some reactions of norcaranylidene with polar solvents

Abstract

<p>As part of our efforts to understand the chemistry of cyclopropylidenes in solution, the reactions of norcaranylidene in alcohols and nitriles have been studied;In alcohols (methanol or t-butanol), the partitioning of norcaranylidene between intramolecular insertion and intermolecular reaction with alcohols is a function of alcohol concentration. In methanol, an ylide intermediate is transformed into the product ether in a stepwise manner. However, in t-butanol, the corresponding ylide intermediate releases the t-butanol molecule to give the intramolecular insertion product, in competition with protonation to form the ether product;Methanol was found to react with norcaranylidene 2.5 times faster than t-butanol, irrespective of the total alcohol concentrations. A reasonable interpretation of the activation parameters is that norcaranylidene is solvated preferentially by t-butanol monomers at "high" alcohol concentration, but is solvated essentially equally by either methanol or t-butanol at "low" alcohol concentration. Thus at either "low" or "high" alcohol concentrations, the "starting carbene" is a single, albeit differently, solvated species;When nitriles are present in methanol, the nitrile ylide intermediate from the reaction norcaranylidene with nitriles reacts with methanol to yield a mixture of epimeric methanol insertion products. In acrylonitrile, the ylide is also captured by another molecule of acrylonitrile to give a 3 + 2 cycloadduct. And dipolarophiles also capture the diazo-precursor to norcaranylidene in acrylonitrile and methacrylonitrile;Pyrolysis of anti-7-bromo-syn-7-trimethylstannylbicyclo 4.1.0 heptane also generates norcaranylidene. This allows the high temperature study of the kinetic deuterium isotope effects for the reaction of norcaranylidene with methanol and t-butanol. Mechanisms to account for the isotope effects at low (diazo-precursor) and high (tin bromide precursor) temperatures are discussed. Two changes in mechanism are seen as the temperature is changed from -78(DEGREES) to 160(DEGREES)C.</p>