1lambda(3),3lambda(3)-Diphosphacyclobutadienes have been specifically generated for the first time through hexachloroethane-induced extrusion of the organophosphorus ligands from dimeric zirconocene-phosphaalkyne complexes. The species were chemically characterized indirectly by reactions with appropriate trapping reagents. Thus, the use of bis(acceptor)-substituted alkynes gave rise to 2,5-diphosphabenzvalenes, whereas reactions with N-methylmaleinimide and donor -substituted alkynes such as bis(diethylamino)acetylene resulted in the fort-nation of 1,3-diphospha-Dewar-benzenes. Representatives of both classes of compounds were analyzed by X-ray crystallography. Kinetically stabilized phosphaalkynes reacted similarly to furnish tetraphosphabishomoprismanes through a sequence of [4 + 2] and [2 + 2 + 2] cycloaddition processes. (C) 2002 Elsevier Science B.V. All rights reserved.
Homo-Diels-Alder reactions take place under very mild conditions when the 1,3,5-triphospha-Dewar-benzene derivative 4 is allowed to react with electron-deficient alkynes, such as dimethyl acetylenedicarboxylate or methyl propyonate, or with tert-butylphosphaacetylene. The resultant [2 + 2 + 2]-cycloadducts 5, 6, and 8 can be isolated in excellent yields and are formed regioselectively. Moreover, the first example of a [4 + 2 + 2]-cycloaddition has been realized by the reaction of 4 with tri-tert-butylazacyclobutadiene 10 which yields the two exo-cycloadducts 11 and 12. The cycloadduct 12 is unstable under the conditions of column chromatography on silica gel and undergoes rearrangement to the isomer 13.