Ethers iodotrimethylsilane

General Discussion. 2,4-Pentadienyltrimethylsilane is used as a stable (storable) nucleophilic substitute for pentadienyllithium. This compound reacts with Lewis acid (boron trifluoride etherate, iodotrimethylsilane, titanium(IV) chloride) activated electrophiles in the same manner as allylsilanes (eq 1). Generally only e-substitution is observed (cf. Table 1). ... 

Boron tribromide is reported to be more effective than iodotrimethylsilane for cleaving aryl methyl ethers. ... 

Likewise, synthetic 2//-azepines isomerize to 3//-azepines in refluxing chloroform (2-3 h) or in tert-butyl methyl ether at room temperature.291 The isomers can be readily separated by chromatography on silica gel, as the more basic 2//-azepines30 have lower Rf values. In contrast, 7-butyl-2//-azepin-2-acetic acid (11), obtained by heating the tert-butyl ester 10 with iodotrimethylsilane, is stabilized by intramolecular hydrogen bonding and shows no tendency to rearrange to the 3//-isomer.291... 

The structurally simplest silicon reagent that has been used to reduce sulphoxides is the carbene analog, dimethylsilylene (Me2Si )29. This molecule was used as a mechanistic probe and did not appear to be useful synthetically. Other silanes that have been used to reduce sulphoxides include iodotrimethylsilane, which is selective but unstable, and chlorotrimethylsilane in the presence of sodium iodide, which is easy to use, but is unselective since it cleaves esters, lactones and ethers it also converts alcohols into iodides. To circumvent these complications, Olah30 has developed the use of methyltrichlorosilane, again in the presence of sodium iodide, in dry acetonitrile (equation 8). A standard range of sulphoxides was reduced under mild conditions, with yields between 80 and 95% and with a simple workup process. The mechanism for the reaction is probably very similar to that given in equation (6), if the tricoordinate boron atoms in this reaction scheme are replaced... 

Dialkyl and alkyl aryl ethers can be cleaved with iodotrimethylsilane ROR -bMe3SiI — Rl-bMe3SiOR. A more convenient and less exjjensive alternative, which gives the same products, is a mixture of chlorotrimethylsilane and Nal. Alkyl aryl ethers can also be cleaved with Lil to give alkyl iodides and salts of phenols in a reaction similar to 10-73. Triphenyldibromophosphorane (Ph3PBr2) cleaves dialkyl ethers to give 2mol of alkyl bromide. ... 

When an insufficient amount of iodotrimethylsilane was used by the submitters, cyclohexyl methyl ether remained at the end of the reaction and was eluted from the silica gel column before cyclohexanol. When present in the crude product, cyclohexyl iodide was also eluted from the column before cyclohexanol. 

Alkyl esters are efficiently dealkylated to trimethylsilyl esters with high concentrations of iodotrimethylsilane either in chloroform or sulfolane solutions at 25-80° or without solvent at 100-110°.Hydrolysis of the trimethylsilyl esters serves to release the carboxylic acid. Amines may be recovered from O-methyl, O-ethyl, and O-benzyl carbamates after reaction with iodotrimethylsilane in chloroform or sulfolane at 50—60° and subsequent methanolysis. The conversion of dimethyl, diethyl, and ethylene acetals and ketals to the parent aldehydes and ketones under aprotic conditions has been accomplished with this reagent. The reactions of alcohols (or the corresponding trimethylsilyl ethers) and aldehydes with iodotrimethylsilane give alkyl iodides and a-iodosilyl ethers,respectively. lodomethyl methyl ether is obtained from cleavage of dimethoxymethane with iodotrimethylsilane. 

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°. ... 

The selective and facile cleavage of the benzylic ether linkages of 1,2,3 or 4 is accomplished by treatment with iodotrimethylsilane to form the corresponding benzylic iodide. Further addition to these iodide derivatives of 1 affords dendrimers of generation 1 with phosphonium ion sites at the periphery. Such a strategy is conducted up to generation 3 with a phosphine or a phosphonium core (Scheme 3). 

The reagent is similar to iodotrimethylsilane in reactivity. It also converts alcohols into iodides, but in contrast to ISi(CH,)j, it reacts more rapidly with secondary alcohols (with inversion) than with primary ones. It also cleaves ethers, and again it cleaves secondary alkyl ethers more readily than primary alkyl ethers. 

Ether cleavage of 2,3-dimethoxyquinoxaline to give 2,3( l//,4//)-quinoxalinedione was performed by treatment with iodotrimethylsilane generated in situ from chlorotrimethylsilane and sodium iodide, albeit in 15—40% yields <1999JHC1271 >. 2,3-Dimethoxyquinoxaline reacts with 2 equiv of butyllithium to furnish 2,2-dibutyl-3-methoxy-1,2-dihydroquinoxaline (Equation 22) <1999T5389>. 

Cleavage of methyl and benzyl ethers.2 These ethers are cleaved by the combination of this silane and zinc iodide. The rate of dealkylation is enhanced by the presence of ( -C4H9)4NI. Trimethyl(methylthio)silane serves the same purpose. CH2C12 and C1CH2CH2C1 are satisfactory solvents. The cleavage probably does not involve in situ formation of iodotrimethylsilane. 

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. 

Deprotection of aryl methyl ethers may be effected under strongly acidic conditions (e.g. hydriodic acid, Section 9.6.11, p. 1253), but the milder methods employing either iodotrimethylsilane in chloroform solution at 20-50 °C for several hours,40 or boron tribromide at room temperature may be preferable.41... 

Cleavage of ethers. The actual reagent formed from these two reagents is believed to be iodotrichlorosilane and is similar to iodotrimethylsilane for cleavage of ethers but somewhat more regioselective. It also cleaves acetals and ketals quantitatively to the carbonyl compound at room temperature. 

The acetal 14, activated by the iodotrimethylsilane 17, produces the oxonium cation 16 which can be intercepted by allylsilane 1 yielding homoallylic ether 15, one equivalent of methoxytrimethylsilane 18 and the catalyst 17. 

Me3SiI, CHC13, 25-50°, 12-140 h.8 Iodotrimethylsilane in quinoline (180°, 70 min) selectively cleaves an aryl methyl group, in 72% yield, in the presence of a methylenedioxy group.9 Me3SiI cleaves esters more slowly than ethers and cleaves alkyl aryl ethers (48 h, 25°) more slowly than alkyl alkyl ethers (1.3-48 h, 25°), but benzyl, trityl, and /-butyl ethers are cleaved quite rapidly (0.1 h, 25°).7... 

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