fumarate (59.8g, 347 mmol) at room temperature. During the reaction the temperature of the mixture is kept at 35-4OT by water cooling. After stirring for 1 h, all the components that are volatile at 1 lS"C/O.l torr are distilled off to leave 2.1 g of a black viscous residue. Fractional distillation affords 42.7 g of 96 YO pure (GC) diethyl fumarate and 17.Sg of 93.8 % pure (GC) (Zb), b. p. 63-65"C/0.001 torr, which still contains diethyl fumarate (1.6%) and six other unknown compounds (4.6 %).Received: December 10, I973 [Z 2 b IE] German version: Angew. Chem. 86. 518 (1974) [ I ] P. Diazaborolines are derived from pyrroles by formal isosteric replacement of a C=C by a BN group and are isoelectronic with the cyclopentadienide ion. They therefore warrant attention as potential complex ligands. The first representative of this class, 1,3-dimethyl-2-phenyldiazaboroline~", has been described in a recent publication. We have now found a novel elegant preparation of these heterocycles in the synthesis of 1,3-diphenyl-2,4,S-trimethyl-1,3,2-diazaboroline ( 1 ). Reduction of the boronium salt ( 2 ) with sodium amalgam in diethyl ether affords (1) in about 50% yield.

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