Biphospholes derivatives

Scheme 1.62 Test reaction with 2,2 -biphosphole-derived diphosphine sulfide.

Bromo-capped biphosphole (8b) is also the precursor of cychc derivatives, as illustrated by the synthesis of the fully unsaturated macrocycle (11), via a Wittig reaction involving the 5,5 -bis(carboxaldehyde) (10) (Scheme 2) [22]. An X-ray diffraction study revealed that macrocycle (11) is distorted, with an flZZ-trans-disposition of the four P-phenyl substituants. 

Almost no attention has been paid to diphosphine sulfides employed as chiral ligands for palladium-catalysed nucleophilic substitution reactions. In this context, enantiomerically pure diphosphine sulfides derived from 2,2 -biphosphole, which combined axial chirality and phosphorus chiralities, were synthesised, in 2008, by Gouygou et al. through a four-step synthetic sequence. Among various palladium catalytic systems derived from this type of ligands and evaluated for the test reaction, that depicted in Scheme 1.62... 

Diastereomeric mixtures (three isomers) of 2,2 -biphospholes 162-164 were synthesized by asymmetric alkylation of 2,2 -biphospholyl anion 161 with enantiomerically pure diol ditosylates. The generated 2,2 -biphospholes were converted into the more stable disulfide derivatives 58, 165, and 166 <2005OM5549, 2003CC1154>. Dianion 161 was generated in two steps from l-phenyl-2,3-dimethylphosphole 159 by pyrolysis and subsequent treatment of the formed phosphole tetramer 160 with sodium naphthalene. Structures of the diastereomers of disufides 58 and 165 were established by the X-ray crystallographic data. 

The biphosphole (370) has been obtained in enantiomerically pure form by spontaneous resolution in the crystallisation of a racemic mixture, without the use of chiral auxiliaries. A new approach to -functionalised phospholes is afforded by metallation at a methyl group of l-phenyl-3,4-dimethylphosphole(in which both phosphorus and the diene unit are protected by coordination to an iron carbonyl acceptor), followed by treatment with electrophiles, to give C-substituted products, e.g., (371). Copper(II) oxidation of the intermediate lith-iomethyl derivative leads to the formation of bridged systems, e.g., (372). ... 

The reaction has been extended to the synthesis of a 2,2 -biphosphole <86iOM(3l6)27l>. The biphospholene derivative (249) is directly available from the reductive dimerization of phospholes <84JA425>. Addition of bromine gave the salt (250) which on dehydrohalogenation with a-picoline gave the biphosphole (251) (Scheme 71). 

Rob6 E, Perlikowska W, Lemoine C, Diab L, Vincendeau S, Mikolajczyk M, Daran J-C, Gouygou M (2008) Diphosphine sulfides derived from 2,2 -biphosphole novel chiral S,S ligands for palladium-catalyzed asymmetric allylic substitution. Dalton Trans 2894-2898... 

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