Allyl alcohol electrophilic attack

Polyene Cyclization. Perhaps the most synthetically useful of the carbo-cation alkylation reactions is the cyclization of polyenes having two or more double bonds positioned in such a way that successive bond-forming steps can occur. This process, called polyene cyclization, has proven to be an effective way of making polycyclic compounds containing six-membered and, in some cases, five-membered rings. The reaction proceeds through an electrophilic attack and requires that the double bonds that participate in the cyclization be properly positioned. For example, compound 1 is converted quantitatively to 2 on treatment with formic acid. The reaction is initiated by protonation and ionization of the allylic alcohol and is terminated by nucleophilic capture of the cyclized secondary carbocation. 

As a supporting evidence, it is well-known that the electron-rich 0 6-arene)Ru complex of terminal alkyne 428 rearranges easily by the treatment with NaPR, of the )/ -vinylidenc complex 429, which is a strongly electrophilic carbene complex. Attack of ROH on the carbene carbon generates the the alkoxycarbene complex 431 via 430 [166]. Formation of ketone 427 by attack of the allylic alcohol is understanable by this mechanism. Formation of Ru-vinylidene complex 429 from the terminal alkyne has been proposed as the intermediate 432 of the reaction of terminal alkyne, amine and CO2 to form the vinyl carbamate 433 [167,168]. 

The chemoselectivity of the dioxirane oxyfunctionalization usually follows the reactivity sequence heteroatom (lone-pair electrons) oxidation > JT-bond epoxida-tion > C-H insertion, as expected of an electrophilic oxidant. Because of this chemoselectivity order, heteroatoms in a substrate will be selectively oxidized in the presence of C-H bonds and even C-C double bonds. In allylic alcohols, however, C-H oxidation of the allylic C-H bond to a,/ -unsaturated ketones may compete efficaciously with epoxidation, especially when steric factors hinder the dioxirane attack on the Jt bond. To circumvent the preferred heteroatom oxidation and thereby alter the chemoselectivity order in favor of the C-H insertion, tedious protection methodology must be used. For example, amines may be protected in the form of amides [46], ammonium salts [50], or BF3 complexes [51] however, much work must still be expended on the development of effective procedures which avoid the oxidation of heteroatoms and C-C multiple bonds. 

The preparation involves (kind of) an oxymercuration (cf. Section 3.5.3) of the C=C double bond of the ethyl vinyl ether. The Hg(OAc)+ ion is the attacking electrophile as expected, but it forms an open-chain cation A as an intermediate rather than a cyclic mercurinium ion. The open-chain cation A is more stable than the mercurinium ion because it can be stabilized by way of carboxonium resonance. Next, cation A takes up the allyl alcohol, and a protonated mixed acetal B is formed. Compound B eliminates EtOH and Hg(OAc)+ in an El process, and the desired enol ether D results. The enol ether D is in equilibrium with the substrate alcohol and ethyl vinyl ether. The equilibrium constant is about 1. However, the use of a large excess of the ethyl vinyl ether shifts the equilibrium to the side of the enol ether D so that the latter can be isolated in high yield. 

Chelated complexes related to (138) may be made by closely analogous methods. If the allylic alcohol in question contains a remote, coordinated alkene, protonation can result in () -allyl)Fe(CO)3(alkene)+ cations (141). Electrophilic attack on ( -cyclooctatetraene)Fe (CO)3 gives reorganization of the carbon framework, to afford... 

---