Silyl enol ethers halogenation

Instead of direct halogenation of ketones, reactions with more reactive derivatives such as silyl enol ethers and enamines have advantages in certain cases. 

Halogen shifts have been found for tungsten, with assumed formation of iodovinylidenes in reactions of 1-iodo-l-alkynes with W(CO)5(thf) en route to cyclization of 2-(iodoethynyl)styrenes to naphthalenes and of iodo-alkynyl silyl enol ethers [147], while more substantial confirmation is found in Mn =C=C(I)CH (OR)2 (CO)2Cp [R = Me, Et (OR)2 = 0(CH2)30], of which the XRD structure of Mn =C=C(I)CH(OMe)2 (CO)2Cp was determined [148]. 

Silyl enol ethers hydrolyse by a slightly different mechanism, though the first step is the same— protonation at carbon using the lone pair on oxygen. We have already seen how easy it is to attack silicon with nucleophiles, especially those with oxygen or a halogen as the nucleophilic atom. This tips the balance towards attack by water at silicon for the next step. 

Unfortunately, the usual routes to a-PhS ketones 94, from the parent ketone33 6i, 62) by sulphenylation or halogenation, themselves require a solution of the specific enol problem This can be found if silyl enol ethers are combined with PhSCl63). 

Fig. 4. Asymmetric halogenation of silyl enol ethers derived from camphor-10-sulfonic acid esters.

Total syntheses of racemic muscarine and allomuscarine were achieved by Pirrung and DeAmicis 11), who used a new approach to formation of the stereoselectively substituted tetrahydrofuran nucleus (Scheme 8). The key step in this procedure was the stereospecific photochemical ring expansion of a cyclobutanone derivative. The cycloaddition of silyl enol ether with methylchloroketene generated by zinc dechlorination of 2,2-dichloropropionyl chloride followed by reductive removal of the halogen produced the methylsiloxycyclobutanone intermediate. Irradiation of the... 

Several synthetic methods employ the 3-halo indolenine intermediates generated by halogenations as intermediates. Danishefsky and coworkers found that the chlor-oindolenine from the methyl ester of W, W -dibenzyltryptophan can be converted to 2-substituted products using a variety of nucleophiles, including allylic boranes and staimanes, enamines and ester silyl enol ethers [68]. 

There is also a problem with i-PrCl it is a secondary hahde and chloride is the worst leaving group among the halogens Cl, Br, I—it is prone to ehmination rather than substitution reactions. To make the required product, an aza-enolate (p. 593 in the textbook) or a silyl enol ether (p. 595 in the textbook) would be a better bet. 

Reactions of silyl enol ethers with halogen and sulfur electrophiles... 

TBDMS triflate also catalyzes the ene reaction of sulfur dioxide with silyl enol ethers. The silyl sulfinate intermediates can be halogenated to the corresponding sulfonyl halide, and subsequently trapped with an amine or alcohol to give sulfonamides or sulfonate esters, respectively (eq 51). ... 

Reaction with Electrophiles. The electrophilic substitution of a-silyl ketones gives the same products as those derived from the corresponding silyl enol ether. Thus reaction with bromine or thionyl chloride gives the a-bromo (or chloro) ketone, with the halogen replacing the trimethylsilyl group (eq... 

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