[33715-52-5] · C6H5AlCl2O · Dichloroaluminum Phenoxide · (MW 190.99)
(Lewis acid catalyst for carbocyclic ring-expansion reactions)
Physical Data: trimeric structure in benzene solution; forms a stable 1:1 complex with benzonitrile as a yellowish-white solid.
Solubility: sol benzene and methylene chloride.
Preparative Methods: the reagent should be freshly prepared in situ by the careful addition of commercially available dichloromethylaluminum to a 0 °C solution of 1 equiv of phenol in CH2Cl2, making provision for the evolution of methane.
Handling, Storage, and Precautions: moisture sensitive; should be prepared and handled under an inert atmosphere. Use in a fume hood.
Dichloroaluminum phenoxide is a powerful Lewis acid that has been shown to be superior to dichloromethylaluminum as a catalyst for the one-carbon ring enlargement of a-(trimethylsilylmethyl)cycloalkanecarbaldehydes to the corresponding a-(trimethylsilylmethyl)cycloalkanones (eq 1).2 This 1,2-migration is facilitated in part by stabilization of an incipient carbocation by the b-silicon substituent.
In a related process, treatment at 0 °C of a methylene chloride solution of 1-acyl-2-vinylcyclobutanes (eqs 2 and 3) with the reagent leads to a two-carbon ring enlargement, affording the corresponding 4-acylcyclohexenes.3 As before, the oxygenophilic character of this aluminum reagent facilitates the formation of a stabilized carbocationic intermediate.
Gary W. Morrow
The University of Dayton, OH, USA