Grignard reagents/palladium chloride complexes

Grignard reagents convert aromatic sulfonyl chlorides or aromatic sulfonates to sulfones. Aromatic sulfonates have also been converted to sulfones with organolithium compounds.1745 Vinylic and allylic sulfones have been prepared by treatment of sulfonyl chlorides with a vinylic or allylic stannane and a palladium-complex catalyst.1746 Alkynyl sulfones can be prepared by treatment of sulfonyl chlorides with trimethylsilylalkynes, with an AICL catalyst.1747... 

Since the reaction can proceed at ambient temperature, some electrophilic functional groups can be present in the alkyl electrophiles, as exempUfied by products 1-3. Further optimizations studies on this catalyst led to the development of N-heterocychc carbene (NHC)-derived palladium catalysts for arylations of alkyl chlorides. In this system, it is assumed that the initial reaction of a precatalyst [Pd(IMes)(NQ)]2 4 and an aryl Grignard reagent should generate a coordinatively unsaturated palladium-carbene complex, which is the actual palladium-catalysis complex with high catalytic activity [9]. 

Many studies of the addition of nucleophiles to palladium-allyl complexes have been conducted. Hayashi has shown that the additions of stabilized anions, such as malonate anions or amine nucleophiles, to chiral, non-racemic allyl complexes occur with inversion of configuration.Addition of excess phosphine and either diethyl malonate or dimethylamine to a chiral, non-racemic allyl complex results in nucleophilic attack with nearly complete inversion. The reaction with sodium dimethylmalonate is shown at the right of Equation 11.40. In contrast, nonstabilized carbanions such as allyl or phenyl magnesium chloride react with the same Ti -allylpalladium complex with retention of configuration as shown at the left of Equation 11.40. The stereochemistry from reaction of the Grignard reagents likely results from nucleophilic attack at the metal, followed by reductive elimination. 

A similar positive effect is also achieved with bulky electron-rich hgands. Palladium complexes such as 237 derived from air-stable secondary phosphine chloride ligands catalyze the coupling of a range of aUcyl Grignard reagents with aryl bromides in high yields [65] (Scheme 5.57). 

The reactions discussed so far involved attack of soft nucleophiles on rr-alkene palla-dium(II) complexes. Hard carbon nucleophiles such as methyl lithium also react with these complexes.f The alkylation proceeds here by initial attack at the palladium atom. Subsequent di-insertion of the olefin into the Pd—C bond and di-/S-hydride elimination result in liberation of the alkylated alkene. The arylation of styrene by Grignard reagents can also be performed,the reaction being realized in the presence of stoichiometric or catalytic quantities of palladium chloride (Scheme 14). 

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