Silylations iodotrimethylsilane

Oxiranes react with iodotrimethylsilane to give silylated halo alcohols e.g. 60) which can be converted to allylic alcohols (Scheme 53) (80JOC2579, 80TL2329) cf. other syntheses of allylic alcohols (Sections 5.05.3.2.2, 5.05.3.4.3(0 and Hi)). 

The use of iodotrimethylsilane for this purpose provides an effective alternative to known methods. Thus the reaction of primary and secondary methyl ethers with iodotrimethylsilane in chloroform or acetonitrile at 25—60° for 2—64 hours affords the corresponding trimethylsilyl ethers in high yield. The alcohols may be liberated from the trimethylsilyl ethers by methanolysis. The mechanism of the ether cleavage is presumed to involve initial formation of a trimethylsilyl oxonium ion which is converted to the silyl ether by nucleophilic attack of iodide at the methyl group. tert-Butyl, trityl, and benzyl ethers of primary and secondary alcohols are rapidly converted to trimethylsilyl ethers by the action of iodotrimethylsilane, probably via heterolysis of silyl oxonium ion intermediates. The cleavage of aryl methyl ethers to aryl trimethylsilyl ethers may also be effected more slowly by reaction with iodotrimethylsilane at 25—50° in chloroform or sulfolane for 12-125 hours, with iodotrimethylsilane at 100—110° in the absence of solvent, " and with iodotrimethylsilane generated in situ from iodine and trimcthylphenylsilane at 100°. ... 

Few examples of intramolecular enol silyl ether or silyl ketene acetal cyclizations to oc,3-enones have been reported. Notable, as exemplified in Scheme 34, is the iodotrimethylsilane-mediated intramolecular cyclization of 5-(iodoacetoxy)-a,3-enones (211) to 5-lactones (214). These cyclizations proceed with in situ generated silyl ketene acetals (212) arising from iodotrimethylsilane reduction of the iodoacetoxy moiety.87... 

Methyl ethers are stable to acidic and basic conditions, and oxidising or reducing reagents. Deprotection to regenerate the alcohol is difficult (see Section 9.6.10, p. 1254) a convenient mild procedure uses iodotrimethylsilane in chloroform solution at room temperature.768 The alkyl methyl ether under these conditions gives the alkyl silyl ether and methyl iodide the former on treatment with methanol gives the deprotected alcohol. 

At the end of his review [7] dealing with the acetalization of carbonyl compounds, Sakurai reported a previously unpublished observation. In the presence of catalytic amounts of iodotrimethylsilane and one equivalent of tetramethoxysilane 38, allyl-trimethylsilane 1 underwent smooth condensation with benzaldehyde 39, leading to adduct 41 in good yield. The silyl-modified Sakurai reaction was born (Scheme 13.15). 

Enol silyl ethers (10, 97).2 Iodotrimethylsilane, generated in situ, is recommended for preparation of the enol silyl ether of acetone. The yield (60% based on chlorotrime-thylsilane) is comparable to that obtained using the more expensive trimethylsilyl triflate. 

Conjugate addition to enones. Iodotrimethylsilane can mediate conjugate addition of furanes to a,p-enones via the intermediate y-iodo enol silyl ether (9, 252-253). The adduct can be isolated as the enol silyl ether or the corresponding ketone (equation I).6... 

Little quantities of iodotrimethylsilane catalyse the rearrangement of 2-siloxy cyclopropanes to their isomeric silyl enol ethers (equation 102), whereas an equimolar amount of this silylating agent gives a highly donor substituted diene. Both product types are promising intermediates, yet their chemistry still remains to be explored. 

Ring opening of oxiranes with iodotrimethylsilane provides silylated halohydrins <84CHEC-l(7)in). An exception to this reaction mode has been reported <9iJOC4598>. Silyl enol ether-terminated oxiranes, when treated with TMS-I in the presence of hexamethyldisilazide (HMDS) at low temperatures suffer C—C cleavage and cyclization to dihydrofurans. 

With an acceptor-substituted alkene moiety tethered to the molecule, the intermediate silyl enol ether may undergo an intramolecular [2-I-2] cycloaddition.The silyl-assisted addition of hydrogen halides to cyclopropanes is not restricted to ketones with carbonyl groups as activating function or iodide as nucleophile. Esters and other acid derivatives underwent similar reactions when treated with iodotrimethylsilane alone or in the presence of an additional catalyst such as mercury(II) or zinc(II) chloride.Subsequent treatment of the y-iodo ester with potassium carbonate in tetrahydrofuran gave the respective y-butyrolactones in good yield. 

Optically active thiazolidine -oxides (259) undergo the silicon Pummerer reaction with t-butyldimethylsilyl trifluoromethanesulfonate or iodotrimethylsilane <87TL5903>. When the former is used as silylating agent, thiazolidines (260) and 4-thiazolines (261) are obtained. With iodotrimethylsilane, the iodothiazolidines (262) and the dihydro-1,4-thiazine (263) are obtained (Scheme 65). 

When exposed to iodotrimethylsilane at room temperature, 59 underwent C-O bond cleavage along with iodination to produce the silyl ester 60 (Fig. 11). In situ cyclization was carried out by treating 60 in methanol at room temperature to provide (+)-mono-morine I (62) and its C-3 epimer (63) in 42% and 40% yields from 59, respectively. An appreciable improvement of the diastereo-selectivity in the cyclization to monomorine I was obtained by the following sequence. With compound 59 in hand, the ketone 61 was prepared by Collins oxidation in 94% yield. On catalytic hydrogenation of 61, ( )-monomorine I (62) was obtained in 71% yield, along with ( )-3-epimonomorine I (63) as a minor isomer (15%). 

Cleavage of the ketene 0,5-acetals under neutral or mildly basic conditions can be executed by using iodotrimethylsilane to give the phenyl thioester or the ketene 0-silyl,5-acetal (eqs 3 and 4). In addition, amides can be prepared by sequential treatment of ketene 0,5-acetals with lithium thiomethoxide in HMPA followed by addition of an amine (eq 5). ... 

---