Mechanism of the Reaction between Benzylmagnesium Chloride and Carbonyl Compounds. A Detailed Study with Formaldehyde

The reaction of benzylmagnesium chloride in THF with monomeric formaldehyde has been studied in detail. A mechanism is presented which accounts for the formation of the products, 2-phenylethanol, o-tolylcarbinol, and o-(2-hydroxyethyl)benzyl alcohol. A change in technique for Grignard titration and formaldehyde addition results in a much-improved mass balance than achieved previously. The decreased yield of the major product, o-tolylcarbinol, with increased reaction time is explained in terms of a hitherto unsuspected equilibrium influenced by tbe polymerization of monomeric formaldehyde. An intermediate organometallic species which could lead to some of the diol product, o-(2-hydroxyethyl)benzyl alcohol can be trapped as the trimethylsilyl derivative, but the quantity is insufficient to account for the amount of diol when an excess of formaldehyde is employed. An ene or Prins reaction is invoked for the formation of most of the diol. Deuterium tracer studies suggest competition between a proton abstraction pathway leading to the trimethylsilyl-trapped organometallic species and a base-catalyzed, stereospecific 1,3 hydrogen shift.