Homoallylic iodide

Previtera, L. Monaco, P. Mangoni, L. Cyclo-propylcarbinyl compounds from homoallylic iodides. Tetrahedron Lett. 1984, 25, 1293-1294. 

Reaction with cyclopropyl alcohols and ketones. The primary and secondary cyclopropyl alcohols 1 and 2 are converted by reaction with P2I4 or PI, mainly into the corresponding iodides. In contrast, the tertiary a-cyclopropyl alcohol 3 is converted into the homoallylic iodide 4 under very mild conditions. a-Cyclopropyl aldehydes and ketones are also cleaved by P2I4 to -y-iodo carbonyl compounds in high yield, particularly when the solvent is acetone. 

Using an alternative method, homoallylic iodides are efficiently transformed into cyclopropylmethyl acetates using silver acetate in anhydrous benzene (Table 1). y-Disubstituted or conjugated homoallylic iodides are particularly reactive and rearrange quantitatively. y-Monosub-stituted iodides afford mixtures of cyclopropylmethyl and homoallyl acetates in a 1 1 ratio, whereas the absence of a y-alkyl or aryl group in the homoallylic iodides leads to elimination, only. 

The striking influence of y-substitution on product formation is readily explained in terms of the relative charge densities at the cyclopropylmethyl and homoallylmethyl centers in the incipient nonclassical carbocation intermediates. Starting from y-di- and monosubstituted homoallylic iodides such carbocations do have considerable cyclopropyl carbocationic character, which, as a consequence, leads to cyclopropylmethyl derivatives on substitution. In the nonsubstituted case, the intermediate carbocation has predominant homoallylic character which under these conditions results in elimination. 

It is noteworthy that diethyl 1-(ethoxycarbonyl)methylphosphonate can be first metallated, then alkylated by an homoallyl iodide to give 4,5-unsaturated phosphonate, and further converted into epoxides by treatment with MCPBA in CH2CI2 in 93% yield. ... 

Table 1. Reaction of Homoallylic Iodides with Silver Acetate in Anhydrous Benzene6...

The coupling of homoallylic iodide with a,) -unsaturated esters and nitriles is valuable in obtaining simple addition products with ethyl acrylate and acrylonitrile. While tributyltin hydride gives the sequential cyclization products 46 and 48 as the major products [17], the BER-Ni2B system yields the simple addition compounds 45... 

Homologation of allylic alcohols. The Simmons-Smith reaction of certain allylic alcohols becomes a minor pathway when they are treated with Et3Al/Et2A10Et while using CH2l2-Et2AlCl as reagents. Homoallylic iodides are obtained. 

Homoallylic iodides and bromides. Homoallylic iodides or bromides (2) can be obtained by reaction of secondary and tertiary cyclopropylcarbinols (1) with magnesium iodide or bromide in refluxing ether (3-90 hr.) ... 

The Z,Z-l,5-dienoic acid (77) has been constructed with 99.5% stereoselectivity from the homoallylic iodide (75). A limitation of the scheme appears to be that the Grignard reagent of (75) is formed in only 65—70% yield, Wurtz-coupled products being the major impurity. However, the constituent of the codling moth Aspeyresia pomonella (76) has been obtained in overall 37% yield in only four steps from prop-l-yne by this procedure (Scheme 16). ... 

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