Formation and Electrochemical Properties of a 1,4-Diphosphafulvene Including Formal Dimerization of Phosphaallene Shigekazu Ito, Satoshi Sekiguchi, and Masaaki Yoshifuji* Department of Chemistry, Graduate School of Science, Tohoku University, Aoba, Sendai 980-8578, Japan yoshifj@mail.tains.tohoku.ac.jp Received February 13, 2004 Reactions of sterically protected 2-bromo-3-phenyl-1-phosphapropenes with bases such as tert- butyllithium and potassium tert-butoxide successfully afforded a bulky 1,4-diphosphafulvene (Mes* ) 2,4,6-tBu 3 C 6 H 2 ) through a novel and formal dimerization pathway of 1-phosphaallene, together with small amounts of 3-phenyl-1-phosphaallene and 3,4-diphosphanylidenecyclobutene. The structure of the 1,4-diphosphafulvene was confirmed by X-ray crystallography indicating somewhat flattened phosphorus atoms due to the bulky Mes* groups. The electrochemical properties of the 1,4-diphosphafulvene were investigated to show promising suitability as an electron donor, and indeed, it afforded a charge-transfer complex with TCNQ. Preparation and structural elucidation of an alkoxy-functionalized 1-phosphaallene was also performed, and the effect of the alkoxy group on the 1-phosphaallene moiety was characterized. Introduction Phosphaallene [sPdCdC<] is a phosphorus congener of allene 1 and is expected to be a good starting material for novel organophosphorus compounds as the utility of the cumulene skeletons has been well established in organic synthesis. 2 Recently, we have reported a syn- thetic procedure for stable phosphaallenes 3 by utilizing a bulky 1-bromo-2-(2,4,6-tri-tert-butylphenyl)-2-phospha- ethenyllithium (a phosphanylidene carbenoid) 1, 4 to- gether with two examples of topochemical dimerization reactions of kinetically stabilized phosphaallenes afford- ing 1,3-bis(phosphanylidene)cyclobutane and 2,4-dim- ethylene-1,3-diphosphacyclobutane upon heating in the solid state. 5 These results suggest the utility of low- coordinated phosphorus compounds 1 as synthetic materi- als for unique organic compounds, leading to further progress in the well-controlled chemical reactions. We have been studying the preparation and properties of several alkoxy-functionalized phosphaallenes utilizing 1 to clarify the effects of heteroatoms on the phosphaal- lenes and, furthermore, exploring a simple and straight- forward synthetic procedure for phosphaallenes. In the course of our study on phosphaallenes, we have found the formation of a bulky 1,4-diphosphafulvene (ben- zylidene-2,3-dihydro-1H-[1,3]diphosphole) 3 as a formal dimerization product of the 1-phosphaallene derivatives. Fulvenes containing heteroatoms have played an impor- tant role in chemistry and material science, as indicated by the research on tetrathiafulvalene (TTF). 6 We report here the formation of 1,4-diphosphafulvene 3 from the precursors of 3-phenyl-1-phosphaallenes and bases. The structural elucidation and electrochemical properties of 3 together with formation of a charge-transfer complex are described. Additionally, the preparation and charac- terization of 3-methoxy-1-phosphaallene are also men- tioned. Results and Discussion 2,2-Dibromo-1-(2,4,6-tri-tert-butylphenyl)-1-phospha- ethene [Mes*PdCBr 2 ] 7 was lithiated to generate the corresponding 1-bromo-2-phosphaethenyllithium 1, 4 and the reaction mixture was treated with benzaldehyde followed by quenching with iodomethane. After the usual workup procedure, the corresponding 2-bromo-3-meth- oxy-3-phenyl-1-phosphapropene 2a was obtained and characterized. Subsequently, the reaction of 2a with an equivalent amount of tert-butyllithium afforded 3 (31% isolated yield) as yellow crystals, together with 1-phos- phaallene 4 8 in a low yield (2%) and 3,4-diphospha- nylidenecyclobutene 5 (6%). 9 In the 31 P NMR spectrum of 3, two nonequivalent signals were observed as an AB (1) (a) Multiple Bonds and Low Coordination in Phosphorus Chem- istry; Regitz, M., Scherer, O. J., Eds.; Thieme: Stuttgart, 1990. (b) Dillon, K. 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Pays-Bas 1994, 113, 278. 10.1021/jo0497407 CCC: $27.50 © 2004 American Chemical Society J. Org. Chem. 2004, 69, 4181-4184 4181 Published on Web 05/11/2004