The complete manuscript of this communication appears in: Angew. Chem. Suppl. 1983, 710. DOI:10.1002/anie.198307100

experiments we could show that 1 -chloro-2,2-bis(trimethyl-si1yl)-1 -pho~phaethene['~] also undergoes [3 f 2]-cycloaddition with diazoalkanes. 1 also reacts with azides, nitrile imines, and nitrile oxides. Experimental 3d: 1 (1.60 g, 7.00 mmol) was treated with a solution of benzoyldiazomethane (1.00 g, 6.8 mmol) in anhydrous ether (I0 mL) at 0°C under pure nitrogen in a 25 m L flask fitted with a capillary side-arm and stopcock. The solution slowly turned dark brown. After stirring for 3 h at room temperature the solvent was removed and the viscous, dark residue dissolved in chloroform and treated with n-pentane. 3d separated from the solution as a brown powder, which could be purified by distillation from a microdistillation apparatus at torr. 3d distilled off as a bright yellow oil which rapidly crystallized; pale yellow crystals, m.p. 142°C (from ethyl acetatelpentane), yield 0.65 g (35%). Received: July 24, 1984 [Z 937 IE] German version: Angew. Chem. 96 (1984) 899 R = NMe2, does not react. The 1,2,4-diazaphospholes 3 (like azaphospholes in general) are not oxidized by atmospheric oxygen; in contrast to their isomers of the type A -Angew. Chem. Int. Ed. Engl. 23 (1984) No. I1 0 Verlag Chemie CmbH, D-6940 Weinheim. 1984 0570-0833/84/111 1-0903 $ 02.50/0

P‐Atome in der gleichen Bindungssituation wie in Phosphabenzol haben die 1H‐1,2,4‐Diazaphosphole 1. Es überrascht daher nicht, daß sich 1 auf einem zweistufigen Weg herstellen läßt, der im ersten Schritt an die klassische Phosphabenzol‐Synthese erinnert.

764 0 VCH Verlagsgereilschaji mbH. 0-6940 Wernherrn. 1985 0570-0833/85/0909-0764 $ 03.50/0 Angew. Chem. Int. Ed Engl. 24 (1985) No 9

Among the large family of azaphospholes 1,2,4-diazaphospholes feature a particularly stable ring system. 3,5-Unsubstituted and 3,5-equally substituted 1,2,4-diazaphospholes and -diazaarsoles are best made from 1,3-bis(dimethylamino)-2-phospha/arsaallyl chlorides and hydrazines. This way four examples for single crystal X-ray structure investigations were prepared: The unsubstituted 1,2,4-diazaphosphole 2 and -diazaarsole 4, the N-substituted 1 -(2-pyridyl) 1.2.4-diazaphosphole 1, and the C-substituted 3,5-di-tert-butyl 1,2,4-diazaphosphole 3. They all possess planar rings with small angles at P (86°) and As (81°). The N-unsubstituted molecules are connected to each other by NH···N hydrogen bonds forming in case of the isotypic compounds 2 and 4 a helix along a crystallographic axis and in case of compound 3 for steric reasons a dimer. The structures of the aggregates found are compared to those of pyrazoles.