Iron, acyl complexes regioselectivity

Reaction of Z-a./j-unsaturated iron-acyl complexes with bases under conditions similar to those above results in exclusive 1,4-addition, rather than deprotonation, to form the extended enolate species. However, it has been demonstrated that in the presence of the highly donating solvent hexamethylphosphoramide, y-deprotonation of the -complex 6 occurs. Subsequent reaction with electrophiles provides a-alkylated products such as 736 this procedure, demonstrated only in this case, in principle allows access to the a-alkylatcd products from both Z- and it-isomers of a,/j-unsaturated iron-acyl complexes. The hexamethylphosphoramide presumably coordinates to the base and thus prevents precoordination of the base to the acyl carbonyl oxygen, which has been suggested to direct the regioselective 1,4-addition of nucleophiles to -complexes as shown (see Section 1.1.1.3.4.1.2.). These results are also consistent with preference for the cisoid conformations depicted. 

The preparation of bis (r 3-aiiyi)iron(II) complexes of the type [Fe(ri3-C3H5)2(PMe3)2] has been reported and their reactions with 1,3-dienes to form a mixture of products were described. The reaction of allyl halides with salts of [Fe(CO)3(NO)]- afforded [PeCli -allyl)(CO)2(NO)] complexes, which on treatment with l,2-bis(diphenylphosphino)ethane gave P,Y-unsaturated acyl iron complexes in good yields via regioselective carbonylation with retention of configuration of the allylic double bond. The same [Fe(Ti3-allyl)(CO)2(NO)] complexes were... 

A few further general examples of zinc catalytic activity or reactivity include the following. Other zinc-containing systems include a zinc phenoxide/nickel(0) catalytic system that can be used to carry out the chemo- and regioselective cyclotrimerization of monoynes.934 Zinc homoenolates have been used as novel nucleophiles in acylation and addition reactions and shown to have general utility.935,936 Iron/zinc species have been used in the oxidation of hydrocarbons, and the selectivity and conditions examined.362 There are implications for the mechanism of metal-catalyzed iodosylbenzene reactions with olefins from the observation that zinc triflate and a dizinc complex catalyze these reactions.937... 

Anionic acyl-iron complex (8) may be intercepted by ethylene, which inserts into the acyl-iron bond to produce a S-metalloketone that rearranges to an a-metalloketone (Scheme 4). Protonation affords the ethyl ketone. Intramolecular (see Intramolecular) versions of this process are known as well, and provide an approach to cychc ketones. That the cyclization is not always regioselective (see Regioselectivity) is shown in an example used in the total synthesis of ( )-aphidicolin (Scheme 5). 4i... 

It was determined that carbon nucleophiles derived from carbon acids with p/fa > 22 or so are sufficiently reactive to combine with the diene ligand rapidly at —78°C to produce an anionic intermediate (Scheme 25). With a few exceptions, the regioselectivity favors formation of the homoallyl anionic complex from addition at C-2, by kinetic control. This intermediate can be quenched with protons to give the terminal alkene, or can react with excess CO to produce an acyl iron intermediate. Following the recipes of Collman s reaction, the acyl iron intermediate can lead to methyl ketones, aldehydes, or carboxylic acids. The processes are illustrated with the 1,3-cyclohexadiene complex (Scheme 25). ... 

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