Ir-Allylpalladium complexes stereochemistry

An elegant example of a stereo relay in a nonrigid system employed a chiral vinyl lactone 430 For the relay to be completely successful (i) the ionization of the vinyl lactone must occur from one conformation, (ii) the intermediate ir-allylpalladium complex must retain its stereochemistry, and (iii) the nucleophile must attack regioselectively at the carbon of the allyl system distal to the incipient carboxylate. This distal transfer of chirality was achieved as shown in equation (346). This same process has been utilized in a synthesis of an optically active vitamin E side chain from D-glucose.431... 

Nucleophilic addition to ir-allylpalladium complexes is known to take place widi a range of nucleophiles. and die mechanism and steieochonistry of diese reactions have been thoroughly investigated over die last few years. - For example, reaction with acetate anion occuis in the presence of benzoqui-none at room temperature in acetic acid solution by initial cis attack at the metal atom and subsequent migration. Alternatively, in die presence of chloride ions, a trans attack takes place to give the pr uct of opposite stereochemistry (Scheme 10). Intramolecular versions of the reaction are known. ... 

Formation of a Tr-allylpalladium complex 29 takes place by the oxidative addition of allylic compounds, typically allylic esters, to Pd(0). The rr-allylpal-ladium complex is a resonance form of ir-allylpalladium and a coordinated tt-bond. TT-Allylpalladium complex formation involves inversion of stereochemistry, and the attack of the soft carbon nucleophile on the 7r-allylpalladium complex is also inversion, resulting in overall retention of the stereochemistry. On the other hand, the attack of hard carbon nucleophiles is retention, and hence Overall inversion takes place by the reaction of the hard carbon nucleophiles. 

In considering the diastereoselectivity associated with reactions of 7r-allylpalladium complexes, the stereochemical nature of two processes must be evaluated. The first is the formation of the ir-allylpalla-dium complex, and the second is the allylic functionalization of a nucleophile reacting with the allyl complex. As previously described (Section 3.3.2.1.1) the generation of ir-allyl complexes from alkenes by their reaction with stoichiometric amounts of palladium proceeds so as to leave the substituents on the allyl termini preferentially in the syn rather than the anti position and the palladium situated on the steri-cally less congested face of the allyl ligand, regardless of the stereochemistry of the starting alkene (12). 

The loss of stereospecificity in the addition of bis(sulfone) anions to cyclohexenyl allylic acetates was attributed to a scrambling of the stereochemistry of the starting acetate. The ability of Pd° catalyst to effect this epimerization was confirmed in the absence of added nucleophile. This epimerization was attributed to the ability of the acetate to return to add to the ir-allyl complex via attack at the metal center (equation 177).167 This suggestion was confirmed by treatment of a preformed allylpalladium acetate dimer with CO, which resulted in cis migration of the acetate from Pd to the allyl ligand (equation 178).164... 

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