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Photochemical Isomerizations of 5-Alkylidene-4,5-dihydro-3H-1,2,4(λ3)-diazaphospholes (4-Phosphapyrazolines): (5→4) Ring Contraction Generates Azomethineimine Dipoles

✍ Scribed by Berthold Manz; Jochen Kerth; Gerhard Maas

Publisher: John Wiley and Sons
Year: 1998
Tongue: English
Weight: 284 KB
Volume: 4
Category: Article
ISSN: 0947-6539
DOI: 10.1002/(sici)1521-3765(19980515)4:5<903::aid-chem903>3.0.co;2-j

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✦ Synopsis

UV irradiation of the title compounds 1 a ± d results not in extrusion of molecular nitrogen, but in a skeletal rearrangement generating amino(imidoyl)phosphanes 2 a ± c from 1 a ± c and 2-hydrazinobenzo[b]phosphole 4 from 1 d. The first step in these isomerizations is an unprecedented (5 34) ring contraction of 1 to form the semicyclic azomethineimine dipoles 10 incorporating a 1,2,3-diazaphosphetidine ring. Dipoles 10 a ± c, but not 10 d, can be observed by NMR spectroscopy after brief irradiation of the precursors. Furthermore, they could be trapped by [32] cycloaddition reaction with dimethyl acetylenedicarboxylate to give the bicyclic 1,2,3-diazaphosphetidines 12 a ± c, and, in the case of 10 a and 10 c, also the dihydrophospholes 13 a,c resulting from a second [32] cycloaddition.

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