On account of their synthetic potential in organic chemistry ['], their particular ligand properties in metal complexes [21, and their unusual physicochemical properties (triboluminescence, crystal "carbodiphosphoranes" R3P=C=PR3 attract current interestr4].The aryl-substituted compounds, which are usually bright yellow (e.g. (Za)), are formed with surprising ease on deprotonation of the conjugated phosphonium salts ( I ) ; thus it is impossible to obtain the isomeric methylenebisphosphoranes (e.g. ( 2 b ) ) in this way. In order to force deprotonation in the peripheral alkyl groups we have prepared a phosphonium salt (3) which is completely alkylated at the central ' P-C-P' bridge; this would be expected to rule out formation of a carbodiphosphorane.
CH2 CH2(26)
15)However, in this case too we did not obtain the bisphosphorane ( 4 ) which appeared promising as a double ylide ligand; instead the carbodiphosphorane ( 5 ) was unexpectedly formed. In order to gain a clearer view of the reaction course, which seems to be complicated by extensive rearrangement, the monophosphonium salt (6) was also synthesized and subjected to the action of strong bases. A deep yellow ylide is formed which again does not have the structure ( 7 a ) but ( 7 b ) instead:The structures of ( 5 ) and ( 7 b ) have been confirmed by elemental analysis and 'H-, 13C-, and 31P-NMR spectra. As suggested in the formula [ ( 7 a ) + ( 7 b ) ] , the mechanism of the hitherto unobserved rearrangement is to be interpreted in terms of nucleophilic attack by the ylidic carbanion in( 7 a ) on the second P atom, whereupon the PCP bridge weakened by steric interaction opens at this P atom. A proton shift from the new PCHzP bridge to the a-C atom of the concomitantly formed isopropylidene group completes the process, which can be formulated appropriately for the formation of ( 5 ) from (3). Such prototropic processes in ylides are known and have already been carefully studiedC5I; not so the ready P-C-P cleavage. The results imply that rearrangements can also be expected in the case of analogously configurated phosphorus compounds in other systemsC6].
Procedure:( 3 ) 4 ( 5 ) : Reaction of methylenebis(dipheny1phosphane) (1.75 g, 4.25 mmol) and methyl iodide (excess, 5 ml) in a sealed tube (2d at 40°C) affords compound (3), yield 2.0g (68 %), m.p. 283"C.-'H-NMR (CD30D; 6 [ppm], J [Hz]): 6CH3P 2.8, d, 6H, J(HCP) 12.5; SCH3C 2.0, t, 6H, J(HCCP) 16.0; hC6H5 7.9, m, 20H. Methylenetrimethylphosphorane (1.44 g, 1.60 mmol) is added to (3) (0.56 g, 0.80 mmol) in ether (10 ml) and the yellow mixture stirred for two hours. After tetramethylphosphonium salt has been filtered off, the solution is concentrated, extracted with n-hexane, and crystallized. Yield 0.23g (65 %) of ( 5 ) , m.p. 103"C.-'H-NMR (C6D6): SCH3P 1.0, d, 3H, J(HCP) 2.0, m, 1 H ; SC6H5 8.0, m, 2OH. 13C-NMR (C6D6): 6CH3P 20.85, d, J(PC) 57.4; GCH3C 17.9, s; SCH 28.45, d, J(PC) 66.2; 6 c 1.04, dd, J(PC) 122 and 128. 31P-NMR (C6D6): 6P 5.03 and -17.8, AB, J(PCP) 88. ( 6 ) + ( 7 b ) : CHjCl (10ml) is cond...
