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Several phosphorus acids (methyl-and phenylphosphonic, diphenylphosphinic, and ethyl phosphoric acids) were found to give esters on reaction with feri-butyldimethylsilanol. The reactions occurred simply on heating the reactants in an inert solvent (hexane or toluene). The reaction is reversible, and yields of esters were improved by continuous water removal. The dibasic acids gave either mono-or disilyl esters; the latter were more easily purified and characterized. The OH groups on the surface of silica gel were also found to react with various phosphorus acids under similar conditions to give surface-bonded esters. Products were characterized by CP/MAS 3IP and 29Si NMR. In the case of phenylphosphonic acid and ethyl phosphoric acids, the products were spectrally identical with those obtained by phosphorylation of the surface OH groups with the highly reactive 3-coordinate anhydrides of these acids when generated in the presence of silica gel.In recent publications,1•2 we have described the action of alkyl metaphosphates, highly reactive and known only as transient species, on the surface OH groups of silica gel. This produces a phosphate group directly bonded to the silica surface: Three different precursors of metaphosphate have been used in these studies, illustrated in Scheme 1 for the ethyl derivative. The same phosphorylated product is formed in each reaction, as revealed by cross polarization-magic angle spinning (CP/MAS) 3IP NMR spectroscopy. The proposal that phosphorus is indeed bonded covalently to the surface is supported by the position of the31P NMR signal at -10; simple phosphates have shifts around ±1, but upfield shifts of about 10 ppm occur routinely when siloxy groups replace alkoxy or hydroxy groups (cf. (EtO)2P-(O)OH, +1, to (EtO)2P(0)OSiMe3, -91), and a similar shift can be expected for the silica phosphate. The possibility was also considered that the metaphosphate was simply hydrolyzed to ethyl phosphate by water remaining on the surface, and adsorbed

A number of bridged cyclic phosphine oxides, both saturated and unsaturated, have been reduced with silicon-based reagents. Evidence was obtained that pentacoordinate intermediates can have special importance in such reductions, since the contracted angle in the cycle is more compatible with the 90' angle offered by apical-equatorial bonding in the trigonal bipyramid. Thus, while HSiC13 and C6H5SiH3 reduce noncyclic oxides with stereochemical retention, phosphines with either retained or inverted configuration can result from angle-contracted, cyclic oxides, and in the 7-phosphanorbornene system (but not in higher homologues) the P(II1) intermediate can undergo retrocycloaddition, causing loss of the phosphorus bridge. However, when the pyridine complex of HSiC1, is used, these complications are avoided, apparently because of a change in mechanism. The bridged phosphines have been characterized by "C NMR spectroscopy, which is especially useful in revealing stereochemical features and modifications in the hybridization at phosphorus. Angle contraction in the ring diverts s-character into the exocyclic bond, causing extremely large lJpc values. Syn,anti isomers then appear to have different hybridization as judged by variations in their lJPc values. 31P NMR chemical shifts occur far downfield in 7-phosphanorbornenes, apparently as a result of U--T hyperconjugation; the anti isomer experiences a second effect, tentatively attributed to repulsion of the lone pair by interaction with the r-electrons, which superimposes shielding on 31P and causes their shifts to be significantly upfield of the syn isomers. The downfield shifting is weaker in 8-phosphabicyclo[3.2.l]octenes and absent in the [4.2.1] homologue. Saturated strained phosphines have shifts in the range of acyclic compounds. In two diphosphines, P-P coupling is present and its magnitude shown to be controlled by the orientation of the lone pair on phosphorus.The placement of phosphorus in heterocyclic frameworks can cause some important modifications in the properties associated with the particular phosphorus functionality.2 This is especially true when the creation of the cyclic structure requires strong contraction of the bond angles around the phosphorus atom, as in bridged ring systems. In working with tertiary phosphines and phosphine oxides containing this structural feature, we have encountered unique features in their reaction chemistry and NMR spectral properties. In this paper attention is focused on the stereochemical aspects of the highly important deoxygenation of phosphine oxides by silicon hydride^,^ which is generally the principal method by which phosphines with bridged rings are approached. While not a systematic study, our research has gathered enough information to show that serious departures from the mechanistic and stereochemical pathways established for simpler compounds can occur when phenylsilane and trichlorosilane are used as the reducing agents. Two configurations are usually possible for phosphines in bridged structures, and it ...

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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 sterically crowded 1-(2,4-di-tert-butyl-6-methylphenyl)-3-methylphosphole was synthesized by dehydrohalogenation of the corresponding 3,4-dibromophospholane, in order to probe the possibility that the steric congestion would cause some flattening of the phosphorus pyramid and an increase in electron delocalization. The phosphole was a recrystallizable solid with (31)P NMR delta 1.8. Semiempirical calculations indicated that the pyramidal shape was retained but was noticeably flatter than in 1-phenylphosphole. In the low energy conformation, the phosphole and phenyl ring planes are approximately orthogonal, with the 2-tert-butyl group in the less crowded position that is syn to the lone pair on phosphorus. The 6-methyl group is positioned under the phosphole ring. This conformational prediction was amply confirmed by several chemical shift and coupling effects in the (13)C NMR spectrum. The (1)H NMR spectrum displayed an unusually large four-bond coupling (6 Hz) of (31)P to the m-phenyl proton syn to the lone pair (and none to the anti-meta proton), consistent with the orthogonal conformation. The oxide of the phosphole showed more stability than that of less crowded phospholes and gave a (31)P NMR signal that was detectable over a several hour period at room temperature. The oxide proceeded to give the usual Diels-Alder dimer and also formed a cycloadduct with N-phenylmaleimide. The phosphoryl group of the latter was reduced with trichlorosilane to give the phosphine. This new 7-phosphanorbornene derivative gave the most downfield (31)P NMR shift (delta 153.3) of any member of this family, all of which are characterized by remarkable deshielding in the syn isomer.