Phenols, silyl-substituted

Structural analogues of the /]4-vinylketene E were isolated by Wulff, Rudler and Moser [15]. The enaminoketene complex 11 was obtained from an intramolecular reaction of the chromium pentacarbonyl carbene complex 10. The silyl vinylketene 13 was isolated from the reaction of the methoxy(phenyl)-carbene chromium complex 1 and a silyl-substituted phenylacetylene 12, and -in contrast to alkene carbene complex 7 - gave the benzannulation product 14 after heating to 165 °C in acetonitrile (Scheme 6). The last step of the benzannulation reaction is the tautomerisation of the /]4-cyclohexadienone F to afford the phenol product G. The existence of such an intermediate and its capacity to undergo a subsequent step was validated by Wulff, who synthesised an... 

Because steric factors strongly influence the rate of silylations, primary alcohols are normally silylated much more rapidly than secondary alcohols whereas tertiary alcohols are silylated much more slowly. The same is true for phenols - ortho-substituted phenols such as o-cresol are silylated much more slowly than unsubstituted phenols. Obviously, the same applies to cleavage of silylated alcohols or phenols on transsilylation, e.g. with excess boiling methanol (Section 2.3). 

Radical anions from aromatics which are intermediates in Birch-type reductions were prepared sonochemically. Pyridine, quinoline, and indole sonicated with lithium in THF in the presence of trimethylsilyl chloride yield the bis-TMS dihydroaromatics, which can be reoxidized, by air or benzoquinone, in a rapid and easy method to prepare silyl-substituted aromatics. The procedure was extrapolated to phenols (Eq. 6). ... 

These annulations can be extended bqrond indole syntheses. For example, the analogous reaction with ortho-iodophenols can provide an effective method to build up benzofurans (Scheme 6.50) [67]. Presumably due to the lower nudeophilicity of the phenolic oxygen, these reactions generally require higher reaction temperatures. Nevertheless, similar regioselectivity was found here as with the indole synthesis, and, in particular, 2 -silyl-substituted benzofuran is often obtained with high selectivity. This same approach can be applied to the construction of pyrroles and furans [68]. 

All these steps proceed to afford free or N -substituted crystalline cytidines 6 in high yields [11] (cf. the preparation of N (tetramethylene)cytidine 6b in 95.4% yield in Section 1.1.). This simple one-pot reaction is also very easy to perform on a technical scale, as are the subsequently discussed analogous silylation-aminations of purine nucleosides and other hydroxy-N-heterocycles (cf. Sections 4.2.4 and 4.2.5). The concept of silylation-activation while simultaneously protecting hydroxyl groups in alcohols, phenols, or phosphoric acids by silylation was subsequently rediscovered and appropriately termed transient protection [16-18]. 

Chromene acetals 39 are accessible from 2-vinyl-substituted phenols via the allylic acetals 38 through oxypalladation of benzyloxypropa- 1,2-diene and a subsequent Ru-catalysed RCM. 2-Substituted chromenes can be derived from the acetals 39 by conversion into the 1-benzopyrylium salts which are then trapped by nucleophiles (Scheme 26) <00TL5979>. In a like manner, 2-aIkoxychromans have been converted into various 2-substituted chromans by sequential treatment with SnCl4 and a silyl enol ether <00TL7203>. 

The Dotz benzannulation reaction yields either arene chromium tricarbonyl complexes or the decomplexed phenols, depending on the work-up conditions. Because of the instability of hydroxy-substituted arene chromium tricarbonyl complexes, yields of the latter tend to be low. High yields of arene complexes can, however, be obtained by in situ silylation of the crude product of the benzannulation reaction [336]. Oxidative work-up yields either decomplexed phenols or the corresponding quinones. Treatment of the benzannulation products with phosphines also leads to decomplexed phenols [272]. 

Regioselective substitution reactions of a series of 2- and 3-hydroxybiaryls including BINOL have been performed via a new directed orf/io-metallation procedure.75 O-Aryl AMsopropylcarbamates, conveniently prepared from phenols and isopropyl isocyanate, have been temporarily and in situ N-protected by means of silyl inflates to form stable intermediates for low-temperature lithiation reactions using n-BuLi-TMEDA in diethyl ether. The IV,IV-dialkyl aryl O-sulfamate has been reported as a new directed metallation group.76... 

The silyl group is widely used as an oxygen protecting group, because of the ease of its removal by nucleophilic substitution by fluoride anion. The protected phenols 0-(tert-butyldimethylsilyl)-/ -(bromomethyl)phenol (45) and 0-(tert-butyldimethylsilyl)-2,6-bis(bromomethyl)phenol (46) react rapidly with fluoride anion in water to form the corresponding phenols, which then break down to the ortho-quinone methide 41 (Scheme 21A) and the substituted ortho-quinone... 

Anodic treatment of 1,2- or 1,4-dihydroxy-substituted benzenes to form the corresponding quinones or masked congeners is well known, since they represent valuable synthetic intermediates [64]. Benzoquinone ketals of electron rich arenes like 18 can be challenging since the oxidative aryl-aryl coupling reaction usually competes. When using BDD anodes the benzoquinone ketal 19 is obtained in an almost quantitative manner, demonstrating the superior properties of this electrode material. Despite the basic conditions, no deblocking of the silyl-protected phenol moiety is observed [65] (Scheme 9). 

The R1 values obtained for such phenylethynyl substituted siloxanes are higher then that reported for traditional aromatic-based systems [9] or the phenol modified ones (1.50-1.53) [10]. The synthesis of high refractive index (methyl)(diphenyle thenyl)-dichlorosilane via hydrosilylation was also described [1]. Such monomer was later hydrolyzed and condensed into silicone fluid. Similar process was also presented, applying silylative coupling process in the synthesis of an analogous (methyl)(phenylethenyl)diethoxysilane [11], so the two reactions shall be discussed in the following section. 

However, reaction of substituted silylated phenols with phosgene does not always give the chloroformate product, owing to the faster reaction represented below [1102] ... 

A transformation called metallo-Fries rearrangement was described for lithiation of 0-substituted orf/zo-nitrophenols 260 (equation 120). Analogous migrations of SiRs groups were reported for reactions of the bromine-substituted 0-silylated phenols with f-BuLi. ... 

SCHEME 76. PhI(OCOCF3)2-promoted oxidation of p-substituted phenols and silyl ethers... 

The linker was prepared starting from serine benzyl ester 68 according to Scheme 30. First, the hydroxyl function was protected as a silyl ether. The amino group was then reacted with phosgene to allow for further reaction with a substituted phenol (the educt). Finally, the benzyl ester was subjected to hydrogenolysis yielding unit 69, with the latter bearing a carboxylic acid function for attachment to the solid support to yield 66 ready for use in combinatorial synthesis. 

ZSM5 type zeolites were used as catalysts for the one-step synthesis of phenol hy benzene partial oxidation with nitrous oxide. Isomorphous substitution of Al ions by other trivalent metal ions revealed a high catalytic performance of the H-Ga-ZSM5 in a wide temperature range. Systematic variation of the partial pressures of the reactants led to satisfactory preliminary kinetic models. Deactivation could be reduced by postsynthetic catalyst silylation which is believed to block the strongest acid sites responsible for coke formation. 

Some nucleophiles add to Af-fluoropyridinium salts to give dihydropyridines in which elimination of fluoride occurs in situ to give the 2-substituted pyridine, thus avoiding the need for a dehydrogenation step. The main disadvantages of this method are that the preparation of the pyridinium salts require the use of elemental fluorine and that some carbanions give only modest yields due to competitive reactions such as C-fluorination. However, silyl enol ethers do react efficiently stabilised heteronucleophiles (phenolate, azide) can also be used. Addition to N-fluoropyridinium salts shows a strong preference for attack at an a-position. ... 

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