Molecular structure of 1-benzylphosphole by x-ray analysis

Abstract

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...

The First Delocalized Phosphole Containing a Planar Tricoordinate Phosphorus Atom: 1-[Bis(trimethylsilyl)methyl]-3,5-bis(trimethylsilyl)-1,2,4-triphosphole

“…The most dramatic aspect of the structure is the planarity of the fivemembered 1,2,4-triphosphole ring system with a sum of angles at the tricoordinate phosphorus atom of 358.78. This value contrasts previously reported values of 342.38 for the analogous 3,5-di-tert-butyltriphosphole 1, [9] 302.78 for 1-benzyl phosphole, [20] and 331.78 for the recently described 2,4,6-tritert-butylphenyl-3-methylphosphole (4). [15] Interestingly, the shortest PÀC bond length in 3, namely P1ÀC1 (1.684 (7) ), involves the tricoordinate phosphorus atom P1 and is significantly shorter than P2ÀC2 (1.745 (7) ), P3ÀC2 (1.715 (7) ), or P3ÀC1 (1.743 (7) ), which all involve dicoordinate phosphorus atoms.…”

Phosphorus Heterocycles

“…The unstable parent system was characterized by NMR spectroscopy at low temperature in 1987 [3]. As initially shown by Quin and coworkers [4], phospholes are pyramidal at P. The reason lies in the intrinsically high inversion barrier of trivalent phosphorus, which overcomes the aromatic stabilization of the planar state. As a result, phospholes are poorly aromatic and their chemistry is widely different from that of pyrroles.…”

“…The alternation between single and double CC ring bonds is higher than in the corresponding pyrroles, thiophenes and furans. The structure of 1-benzyphosphole is given as an example [4]: ring PÀC, 1.783 A ; C¼C, 1.343 A ; CÀC, 1.438 A ; internal <CÀPÀC, 90.7 ; S < CÀPÀC, 302.7 . These data reflect the low aromaticity of the phosphole ring.…”

Das erste delokalisierte Phosphol mit planar dreifach koordiniertem Phosphor- atom: 1-[Bis(trimethylsilyl)methyl]- 3,5-bis(trimethylsilyl)-1,2,4-triphosphol

“…[28] Aus der Struktur lassen sich einige wichtige Informationen entnehmen: Der bedeutendste Aspekt ist die Planarität des fünfgliedrigen 1,2,4-Triphosphol-Ringsystems mit einer Winkelsumme am dreifach koordinierten Phosphoratom von 358.78. Dieser Wert setzt sich deutlich von den bisher bekannten Werten von 342.3 für das analoge 3,5-Di-tert-butyltriphosphol 1, [9] von 302.78 für 1-Benzylphosphol [20] und von 331.78 für das kürzlich beschriebene 2,4,6-Tri-tert-butylphenyl-3-methyl-phosphol 4 [15] ab. Interessanterweise ist an der kürzesten P-C-Bindung in 3, P1-C1 (1.684 (7) ), das dreifach koordinierte Phosphoratom P1 beteiligt.…”