Alkoxide-alkene chelates

In an extension of this chemistry the reactions of 194 with a series of aldehydes were investigated and found to direetly yield the alkoxide-alkene chelates (244—247, Scheme 14), with no evidence of iridafurans being obtained. Protonation of the alkoxide oxygen was also effected, thus affording the respective alcohol complexes (248-251, Scheme 15), the Ir-0(H)R functionalities of which are remarkably stable toward substitution by extraneous donors, a feature attributed to the chelating nature of the ligand. It is interesting to note that in this instance the trans isomers alone are obtained, with no evidence for the cis. 

A further significant feature of the ethyl-vinyl complexes Tp Ir(C2H4R)(CH = CHR)(NCR ) (R = Me (292), (473), (390), R = Bu (387)) and, in acetonitrile, 198, is their propensity for intramolecular [3 + 2] cycloaddition of the alkenyl and nitrile ligands.This affords iridapyrrole complexes 474, 475, 545, 546 and 547 (Chart 9 discussed in Section II-E) for which fully delocalised aromatic structures have been assigned on the basis of NMR spectroscopic data. Besides this study the hydrides 201 and 204 were found to react with a series of aldehydes, resulting in formation of the alkoxide-alkene chelates 244-247 (Scheme 13, Section III-B.l). 

Several strategies have recently been developed for the direct observation of non-chelated cationic zirconocenium-alkene adducts. The first is the use of a non-inserting alkoxide ligand which lowers the metal Lewis acidity. 

The alkoxide pathway occurs by initial insertion of CO into a palladium alkoxide, followed by insertion of the alkene into the bond between the metal and the alkoxycarbonyl group to form a paUadium-alkyl complex (Scheme 17.18). Protonation of this metal alkyl by alcohol would form the free organic product and regenerate the paUadium alkoxide. This mechanism has now been ruled out for the reactions of ethylene to form methyl propanoate. Although each of these steps has precedent, the absence of reduction products from the alkoxide argues against this pathway. Moreover, the alkyl generated from insertion of ethylene into the palladium-alkoxycarbonyl complex (Scheme 17.18) is chelated to the metal, and metha-nolysis of this species is slower than the steps of the alternative hydride mechanism. ... 

In contemporaneous studies by Kuwajima, the 1-silyloxy allyl anions were generated from 1-trimethylsilyl allylic alcohols such as 24. Treatment with a catalytic amount of base led to silyl enol ether 25 in excellent yield and (Z)-selectivity. The alkene geometry was proposed to arise from chelate 26, a structure consistent with Reich s results. Protonation of anionic intermediate 26 by alcohol 24, would provide product 25 and the alkoxide of 24, poised for Brook rearrangement. ... 

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