The stereomutation barriers of diastereomeric spirophosphoranes, [o-O(CF3)(CH3)C*C&r][o-O(CF3)2C&]P*X (X=OMe, SMe) were determined to be high enough to freeze the mutation at ambient temperatures [AGf(298K)=25.0,30.4 kcal mol-t; respectively]. The nucleophilic substitution reaction of SMe compounds with alkyllithium reagents resu1te.d in inversion of configuration, whereas that of OMe compounds gave various ratios of inversion and retention products depending on the stereochemistry of the diastereomeric reactant phosphoranes and solvent. However, the use of OCH2CHzNMe2 as substituent lead to almost exclusive formation of the retention product regardless of these conditions. @ 1997 Elsevier Science Ltd. Although it is established that hexacoordinate phosphorus is involved as either an intermediate or transition state in nucleophilic substitution reactions upon pentacoordinate phosphorus,1 very little stereochemical investigations of the process have been made. tG The stereochemistry of the substitution reaction of fluxional phosphorane 1 has been reported to depend upon the incoming nucleophile,3 thus implying the presence of hexacoordinate intermediates. The addition to phosphoranes such as 24 and 35
has been reported to give OXZ~.V and cis adducts, respectively, as the kinetic products, although pentacoordinate species were not spontaneously formed. The latter stands as an interesting case since the apparent reaction involves nucleophilic attack to the rear of a carbon substituent instead of an electronically preferred oxygen subs&tent. However, this reaction leaves uncertainty since the same product could also be obtained from a configuration in which a phenyl group and an alkoxy group are exchanged by stereomutation.
Recently, 4 has been isolated as a stable compound, which supports the point mentioned above.6 In a sterically restricted case, the ring closure of a P-oxidophosphorane to give 5 showed the capability of hexacoordinate phosphorus formation anti to a carbon atom.798 We have found by using stereochemically rigid spirophosphoranes that rear attack to carbon is in fact feasible even in the presence of equatorial chalcogen substituents and that the stereochemistry can be diversely altered by changing the coordinating ability of the leaving group to a lithium cation.