Sir:We wish to report the first determination of the molecular parameters of a phosphole. The synthesis of 1 -benzylphosphole (11) resembled that already employed for l-methylphosphole,2 and made use of l-benzyl-3,4-dibromophospholane oxide (I, mp 159-160', analyzing correctly) as the key intermediate. As before, some of the corresponding 3-phospholene was formed but was removed on extraction with 2 N hydrochloric I1 acid. that which proved most successful is shown below.Several methods of obtaining I were devised;
Br
Iv
C,H,CH,MgCIThe reduction of the butadiene-phosphorus tribromide cycloadduct (111) to form 1-bromo-3-phospholene [IV, 34%, bp 64-67' (27 mm), 31P nmr signal -11 1.4 ppm relative to 85 % phosphoric acid] is a new reaction of considerable synthetic value in phospholene chemistry; details on the procedure will be published elsewhere. Compound V was identified by quaternization with methyl bromide to form the same salt obtained from 1 -methyl-3-phospholene with benzyl bromide.5 I-Benzylphosphole was considerably more stable than I-methylphosphole and distilled without decomposition at 71-72' (0.2 mm); it had mp 34-34.5' and gave the correct analysis. It resembled 1 -methylphosphole in having low basicity, a uv maximum in 95 % ethanol at 286 mp (log t 3.65), and a 31P nmr signal (-7.9 ppm) Figure 1. Molecular structure of 1-benzylphosphole. Molecular dimensions are P-Cs = 1.786, P-C5 = 1.780, P-CB = 1.858, CZ-C3 = 1.343, C3-C4 =-1.438, C4-C5 = 1.343, c & 7 = 1.506, mean phenylC-C = 1.382A; CzPC5 = 90.7", CzPC6 = 106.1", C j P G = C3C4Cs = 114.1", PS6C7 = 116.4". Standard deviations of distances are &0.005 A for P-C bonds and =t0.006 A for C-C bonds, while those for angles are +0.2" for CPC angles and jz0.4" for CCC angles. 105.9", PC2C3 109.9", PCjCa = 110.1", CzC3Ca = 114.1",showing considerable deshielding relative to the corresponding 3-phospholene (V, +23.5 pprn). Its ring protons were similarly strongly deshielded (multiplet, 6 6.3-7.3 ppm, partly merged with phenyl protons), and the benzylic CH2 group (6 3.01 pprn) was not detectably coupled with phosphorus. Its mass spectrum confirmed its monomeric character (M+ at mje 174, 38.8 % of C7H7+ base peak). It is sensitive to oxygen and can be quaternized with alkyl halides. Crystals of I1 obtained from a melt pave cell dimensions a = 17.62, b = 14.60, c = 7.67 A , and belong to the orthorhombic system with eight units of CllHllP occupying general positions of space group Pbca. A total of 135 1 independent structure amplitudes were derived from visually estimated intensities recorded by equiinclination Weissenberg photography of the hk0-7 layers. The structure was solved by the heavy-atom method and refined by full-matrix least-squares calcu-lations6 to the present conventional R of 0.073. The molecular dimensions are given in the legend of Figure 1.Comparison of the phosphole dimensions with those of the heteroaromatics furan, pyrrole, and thiophene is of particuJar interest. The average P-C,,, bond length 1.783 A is significantly less7 than the su...