Silylation reductive

The reduction of ketones or aldehydes with Pl SiFE/KF produces either the mono- or dialkoxydiphenylsilane depending on the stoichiometry of the reaction.75,319 The dendrimeric catalysts 70, 71, or 72 work with I t3Si H to give the silyl ether of acetophenone in excellent yield (Eq. 239).117 

Benzil is reductively triethylsilylated to the bis(silyl) ether in 83% yield.411 The combination of Et3SiH/ZnCl2 reductively triethylsilylates ketones in good yield.382 Excellent yields of triethylsilyl ethers from ketones are accomplished with the use of triethylsilane and catalyst 73 or 74.412 tert-liutyIditnelliyIsily 1 ethers can be synthesized by the reaction of TBSH/TBSOTf with a ketone (Eq. 240).392 

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Regioselective coniugale reduction and reductive silylation of a,p-unsaturated ketones, esters, and aldehydes using a stable copper (I) hydride cluster (Ph3P)CuH 6... 

Representative 1,2-reduction/silylation of an aldehyde, giving (2-bromoberi2yloxy)-diphenylmethylsilane [44 ... 

Reductive lithiation, 68 Reductive silylation, 26 Reformatsky reaction, 71 Riihlmann condensation, 134,139... 

The currently accepted mechanism of the alkali metal-mediated Wurtz-type condensation of dichlorosilanes is essentially that outlined in COMC II (1995) (chapter Organopolysilanes, p 98) which derived from studies by Gautier and Worsfold,42 and the groups of Matyjaszewski43 and Jones,22,44,45 a modified polymerization scheme of which is included here. The mechanism was deduced from careful observations on the progress of polymerizations in different solvents (such as those which better stabilize anions and those which do not), at different temperatures,44 with additives, and with different alkali metal reductants. Silyl anions, silyl anion radicals,42 and silyl radicals28,46,47 are believed to be involved, as shown in Scheme 3. 

Reductive silylation is preferred because it is less expensive than other methods, is adaptable to large scale work and promises to be of much synthetic value. This technique has not only improved the yields of acylain reactions but has led to the preparation of small rings also. 

Cathodic substitution stands for C,C bond or C, heteroatom bond formation with cathodically generated anions. The question of regioselectivity is encountered in the reaction of such anions with allyl halides (path a) or in the reaction of allyl anions generated in an ECE process from allyl halides (path b). Cathodic reductive silylation of an allyl halide proceeds regioselectively at the less substituted position (Fig. 15) [91]. From the reduction potentials of the halides it is proposed that the reaction follows path b. 

For example, Nakamura and Kuwajima [15] have described 1-alkoxy-l-trimethylsilyloxycyclopropanes (15) -prepared by reductive silylation of alkoxy 3-chloropropanoates-, which react with aliphatic aldehydes, but not with ketones, in the presence of one equivalent of TiCl4 to give good yields of y-lactones 17 through the acyclic derivative ethyl 4-hydroxybutanoate (16) (Scheme 5.10). With aromatic aldehydes and their acetals the reaction leads directly to acyclic 1,4-D derivatives. 

Acyloin-type reactions of esters provide the simplest route to 1-siloxy-l-alkoxycyclopropane [21,22] Eq. (6). The reaction of commercial 3-halopropionate with sodium (or lithium) in refluxing ether in the presence of Me3SiCl can easily be carried out on a one mole scale [21]. Cyclization of optically pure methyl 3-bromo-2-methylpropionate [23], available in both R and S form, gives a cyclopropane, which is enantiomerically pure at C-2, yet is a 1 1 diastereo-meric mixture with respect to its relative configuration at C-l Eq. (7). Reductive silylation of allyl 3-iodopropionate with zinc/copper couple provides a milder alternative to the alkali metal reduction [24] Eq. (8). 

Reductive silylation of a-diketones or quinones.1 a-Diketones are conveniently converted into l,2-bis(silyloxy)alkenes by reaction with ClSi(CH3)3 and zinc. Yields are increased by use of THF rather than ether as solvent and by ultrasonic irradiation. 

Reductive coupling of carbonyls to alkenes Titanium(IV) chloride-Zinc, 310 of carbonyls to pinacols Titanium(III) chloride, 302 Titanium(IV) chloride-Zinc, 310 of other substrates Samarium(II) iodide, 270 Reductive cyclization 2-(Phenylseleno)acrylonitrile, 244 Tributylgermane, 313 Tributyltin hydride, 316 Triphenyltin hydride, 335 Trityl perchlorate, 339 Reductive hydrolysis (see Hydrolysis) Reductive silylation Chlorotrimethylsilane-Zinc, 82... 

Picard has reported more direct approaches to acyl silanes and to their silyl enol ethers by reductive silylation of substituted benzoates and of w./i-dihalo-a, /Tunsaturated acyl chlorides, respectively, using a similar reagent mixture of trimethylchlorosilane, magnesium and IIMPA96. 

Reductive silylation of carbonyl compounds.2 This reagent in the presence of a threefold excess of chlorotrimethylsilane reduces saturated and a,/3-unsaturated aldehydes and ketones, even highly enolizable ones, to trimethylsilyl ethers. Indeed this reaction is usually superior, in respect to yield, to reduction with 1 alone. Examples ... 

Reductive silylation of p-quinones. This reagent in combination with a catalytic amount of iodine converts p-quinones into l,4-bis(trimethylsilyloxy)arenes in almost quantitative yield.1... 

The observations101 that l,2-bis(TMS)-l,2-dihydro-naphthalene (141)101,102 reacts with further chlorotrimethylsilane (142) to afford tris- and tetrakis(TMS)-tetrahyd-ronaphthalenes (143,144a, b) and rearrangements101 were an early example of the so-called reductive silylation (Scheme 19). 

The same mechanism is an explanation for the following results where l-(tri-methylsiloxy)naphthalene (145) and l,5-bis(trimethylsiloxy)naphthalene (146) can be converted via reductive silylation into 1-TMS-naphthalene (147) and 1,5-bis(TMS)-naphthalene (148) respectively103,104 (Scheme 20). 

The reductive silylation under these conditions renders possible an access to useful heterocyclic intermediates which undergo various electrophilic displacements (Scheme 21)105 of the TMS moiety. In the first step phenazine (149) is converted to the corresponding 5,10-bis(TMS)-5,10-dihydrophenazine (150) that can be acylated in excellent yields to the desired 5,10-diacetyl-5,10-dihydro-phenazine (151). 

Ruhlmann and co-workers found194-196 that carboxylic acid esters react under dimerisation to form acyloins if they are treated with sodium/TMS-Cl (142) in inert solvents. This Bouveault-Blanc-like synthesis has many parallels to the reductive silyl-ation (vide supra). 

Reductive Silylation of Imines, Nitriles and Cyano Derivatives... 

The reductive silylation of Schiff bases is reported to yield (trimethylsilyl)benzylani-line (ASMA), after hydrolysis of the reaction medium. The reaction is highly sensitive to temperature.174 An anion-radical mechanism was postulated. Similar results are obtained from diversely N-substituted benzaldimines.175... 

The reductive silylation of /V-phen yI pi vaI i m i doyI chloride has been studied in HMPA as the solvent and (l-trimethylsilyl)f-amylaniline is obtained in 65% yield along with some of the non-silylated aniline (15%).177... 

Reductive Silylation of Benzamides Using Silyllithium Reagent... 

Reductive Silylation of Amides Using Trichlorosilane/Tertiary Amine Mixture... 

A trichlorosilane/tertiary amine mixture has been shown to reduce the carbonyl moiety of aromatic amides to give ASMAs. This constitutes an alternative to the reductive silylation described in Section III.B.2.f.194 This work has been reviewed195 and extended to aliphatic amides to give RSMA. 

Polytrimethylsilylated piperidines have been obtained through the reductive silylation of quinoline (Section III.B.2.d). Among these compounds are SMA derivatives that are readily oxidized in the presence of air and hydrolyzed into pyridine derivatives. Trimethylsilyl groups on the non-aromatic ring were found to be in an all-fraws relationship.179... 

Nickel-catalyzed transformations of SCBs have been studied by Oshima and co-workers <20060L483>. Nickel-catalyzed ring opening of SCBs with aldehydes affords the corresponding alkoxyallylsilanes (Scheme 55). This transformation represents a hydrosilane-free reductive silylation of aldehydes. A wide range of aldehydes (aliphatic, aromatic, electron-rich, and electron-deficient) can be converted to akoxyallylsilanes. 

Reductive silylation of pteridine (1) with potassium and trimethylsilyl chloride in THF leads in low yield to 5,8-bis-trimethylsilyl-5,8-dihydropteridine (75), which was characterized by NMR and is a highly reactive substance due to its conjugated 8 -electron system (Equation (5)) <89JOM(362)37>. 

In the reaction of CbSi" with acid chlorides, two trichlorosilyl groups are introduced (equation 14) this probably proceeds via a two-step mechanism. The first step involves reductive silylation and loss of the carbonyl group the second involves a halide displacement as above. These trichlorosilyl compounds may be further treated with Grignard... 

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