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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