Abstract
Two P2-phosphazenyl groups were linked via a C2-symmetric binaphthyl backbone resulting in two novel chiral superbases with dimethylamino and pyrrolidino substituents. We investigated their basic properties and coordination chemistry towards a cationic alkyl aluminum fragment. The outstanding basicity of the chiral tetrasphosphazenes presented herein leads to interesting perspectives for application in asymmetric Brønsted base catalysis.
Highlights
Two P2-phosphazenyl groups were linked via a C2-symmetric binaphthyl backbone resulting in two novel chiral superbases with dimethylamino and pyrrolidino substituents
Chiral guanidines were applied to asymmetric Michael addition, silylation, nucleophilic epoxidation and TMS cyanation reactions.3a,b Taking into consideration that the pKBH+ values of organic superbases range from 25 to about 45 on the acetonitrile scale, it must be stated that chiral guanidines presented to date only populate the very low end of the basicity scale (pentamethylguanidine: pKBH+ (MeCN) = 25.0010)
Since we have thoroughly studied the basicity-enhancing effect of proton chelation,[14] we were interested to combine this proton pincer ligand concept with a chiral binaphthyl skeleton to achieve a unique interaction of two amino-substituted Schwesinger-P2 bases in a chiral environment
Summary
Two P2-phosphazenyl groups were linked via a C2-symmetric binaphthyl backbone resulting in two novel chiral superbases with dimethylamino and pyrrolidino substituents. The outstanding basicity of the chiral tetrasphosphazenes presented leads to interesting perspectives for application in asymmetric Brønsted base catalysis.
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