Chiral Starting Phosphines

Buono developed a very effective synthesis of chiral tertiary phosphine oxide starting from oxazaphospholidines. The enantiomerically pure oxazaphospholidine (Rp)-122 was prepared from PhP(NMe2)2 and (,S)-(+)-prolinol. Subsequent treatment of 122 with a variety of acids followed by hydrolysis gave both enantiomers of tert-butylphenylphosphine oxide 4c. It was found that the acid controlled the stereochemistry of the enantiomer obtained. By using acids of high acidity or Amberlyst 15 resin, (-h)-(R)-4c was obtained with good yields and enantioselec-tivities. When acids of low acidity were used, ( )-(,S)-4c was the preferred enantiomer. For example, / -toluenesulfonic acid (PTSA) afforded (R)-4c in 88% yield and 91% ee. After a recrystaUization the optically pure compound (/ )-(-t)-4c was obtained with >99% ee (Scheme 37) [67]. 

In conclusion to this section, we have found the first air-stable chiral primary phosphines (R)-5 and (5)-6, and attributed that stability to conjugation in their aryl backbones. The molecular model we have now developed suggests that despite the varied structures shown in Fig. 1.8, we may be able to rationalize and predict the sensitivity of known and unknown primary phosphines we hope that this will prompt a renaissance in the use of primary phosphines as synthetic precursors. In the following section we show that R)-5 and (5)-6 are convenient chiral starting materials. 

The same phosphine-borane used for the synthesis of BisP acted as the starting materials of the construction of MiniPHOS, the next smaller analogue to BisP (Scheme 13). The chirally induced lithium salt was treated with alkylphos-phorus dichloride, methylmagnesium bromide, and borane-THF complex to afford enantiomerically pure MiniPHOS-borane 82a. Recrystallization enabled elimination of a small amount of corresponding raeso-diastereomer formed [29]. Yields were generally low, ranging from 13 to 28%. 

The generalized application of the aminolysis of halophosphanes has been the method of choice for the preparation of a wide variety of chiral phosphinous amides by starting from enantioenriched primary amines [36]. The aminolysis reaction occurs efficiently even when the halophosphane is placed in the coordination sphere of a metal, as in the palladium and platinum complexes of the type ds-M(Ph2PCl2)2Cl (M=Pd, Pt) [37,38]. 

Chiral phosphine based transition metal complexes are nsed as a powerful tool for asymmetric synthesis (3). A fundamental mechanistic nnderstanding is required for rhodium and mthenium catalyzed reactions. The starting point of those investigations was the clear and detailed stractnral description of the isolated pre catalyst system. 

To date, the most popular and successful designs for PN-ligands have incorporated an achiral phosphine with a substituted non-aromatic heterocyde as the primary chiral unit, largely due to the ease by which the P and N modules may be altered, and the ready availability of enantiopure, amino add-derived starting materials.1281 Considerable variation of the predse steric and electronic environment imposed on a chelated metal centre has been achieved by the synthesis of a substantial library of... 

The alkylation of tertiary phosphines is, in general, compatible with elaborate structures bearing various functions or chiralities, as illustrated by the preparation of a phosphonium salt (18), intermediate in the synthesis of pseudomonic acid248 (reaction 17). For the preparation of dialkylphosphonium salts, diphenylphosphine can be directly alkylated, but it is more advantageous to use triphenylphosphine as the starting material,... 

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