New Methods for the Synthesis of P-Chirogenic Diphosphines: An Application to the Development of an Improved Asymmetric Variation of the Rh(I)-Catalyzed [4+2] Cycloaddition

Abstract

A series of new P-chirogenic diphosphines has been prepared by efficient and complementary synthetic procedures. The utilization of a series of these bidentate ligands possessing steric linker variations and a conserved P-chirogenic domain in an asymmetric variation of the Rh(I)-catalyzed [4 + 2] enediene cycloaddition is described.

P-chirogenic ligands - dynamic resolution - catalysis - cycloadditions

References

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The stereochemical behavior (racemization) of chiral phosphorus anions has been described:

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6

The use of this substrate gave exceptionally well resolved HPLC traces so that optimization of the conditions for dynamic resolution could be readily achieved.

7

In all instances involving monohalide trapping agents, an authentic sample of racemic phosphine-borane was prepared for comparison by chiral HPLC.

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9

Reprotection of the diphosphines derived from 4b, 6c, 6d and 6e with BH₃·THF and analysis by HPLC (CHIRALPAK AD) established that no epimerization occurs during the pyrrolidine mediated deprotection reaction.

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In general, recrystallization to near optical purity could most readily be achieved by solvent diffusion in a closed system.

14

In the case of 9b, separation of the isomers 10b in an authentic racemic mixture by chiral HPLC was not achieved. For this reason, optical purity was established for the corresponding P-benzyl derivative.

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