Siloxy methyl

The cobalt carbonyl complex is also an effective catalyst for the siloxy-methylation of aromatic aldehydes.110 Arylethane-l,2-diol disilylethers are obtained in good yields, resulting from incorporation of one molecule of CO and two molecules of HSiR3. Good selectivity for the siloxymethylation product is observed at 0°C in hexane. At 15°C, faster reaction rates are observed, but the selectivity for the CO-incorporated product is lower. In contrast, aliphatic aldehydes react under these conditions (1 atm CO, 0°C) to give only a small amount of CO-incorporated product, with a major product resulting from hydrosilylation. 

Yamaoka Y, Yamamoto H (2010) Super silyl stereo-directing groups for complete 1,5-syn and -anti stereoselectivities in the aldol reactions of p-siloxy methyl ketones with aldehydes. J Am Chem Soc 132 5354... 

When an organofunctional group replaces a methyl group on a siloxy unit, a superscript is used to describe the unit. The most common groups that are encountered can be symbolized as alkyl (R), hydrogen (H), phenyl (Ph), hydroxyl (OH), trifluoropropyl (F), and vinyl (Vi). Thus, vinyl-endblocked-PDMS is represented by (Scheme 3), and a trimethylsiloxy-... 

Entry 6 involves a titanium enolate of an ethyl ketone. The aldehyde has no nearby stereocenters. Systems with this substitution pattern have been shown to lead to a 2,2 syn relationship between the methyl groups flanking the ketone, and in this case, the (3-siloxy substituent has little effect on the stereoselectivity. The configuration (Z) and conformation of the enolate determines the 2,3-vyn stereochemistry.113... 

The present homoallylation with siloxy- and methoxy-substituted dienes may be of great synthetic use. The product 42 is easily converted to anti-5-phenyl-5-hydroxy-3-methylpentanal (Scheme 9) hence the diene 41f may be regarded as a synthetic equivalent of a bis-homoenolate of 3-methylbutanal, being capable of introducing 1,3-anti relationship between the methyl and hydroxy groups in the product. 

CP/MAS NMR study of CH4 activation on [(=SiO)2Ta(H)J shows the formation even at 150 °C of methyUdene and methyUdyne species by an a-H elimination process on several sites that should correspond to the tris-hydride on other sites a methyl group is transferred to the surface, leading to the formation of (=Si-Me) and of [(=SiO)3Ta]. Correlation with EXAFS suggests that the tris-hydride should exist on surface sites (=20%) quite distant from siloxy bridges whereas methyl transfer to the surface should happen on the specific sites (=80%) close to the siloxy bridges. The latter, which are formally 10 electrons species, exhibit a moderate to weak activity in methane C-H activation. To the best of our knowledge, this is the first observation of methyl group transfer on a surface (Scheme 2.18). 

But there are many cases known where the (4+2) cycloaddition fails even with siloxy-activated dienes, e.g., methyl (E)-crotonate does not react with diene 1 at normal pressure and elevated temperature llO C), whereas the aprotic double Michael addition does give the desired bicyclo[2.2.2]octane in high yield. This reaction gives mainly (92%) bicyclic esters with the endo configuration. 

Silicone. Silicone elastomers have a siloxy backbone with methyl, vinyl, and phenyl groups attached. The elastomers are designated by their chemical composition, as follows methyl silicone (MQ), methyl vinyl silicone (VMQ), methyl phenyl silicone (PMQ), methyl phenyl vinyl silicone (PVMQ), and fluorovinyl methyl silicone (FVMQ). 

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). 

Scheme 5 summarizes the regiochemistry of ring opening of methyl- (6) and phenyl-substituted alkoxy-siloxy-cyclopropane (7) by LiOMe, ZnCl2, HgCl2) and TiCl4, as well as the 13C NMR chemical shifts of the respective metal methyls [27]. The NMR data correlate with the nature of the metal-carbon bond The more polarized it is, the less positive the chemical shift of the methyl group. 

The construction of the tricyclo[5.2.0.02,6]nonane (26, n = 1) and tricyclo[6.2.0.02,7]decane (26, n = 2) frameworks involved the [2 4- 2] cycloaddition of readily accessible31,32 l,2-bis(trimethyl-siloxy)cyclobutene and cnone 25, n = 1 or 2, respectively.33 The yields (75-80%) were good for adducts 26a, c, e, and g. Lower yields (40-50%) were observed for adducts 26b and 26f, while adduct 26d was only isolated in a trace amount. The most interesting and important reaction, related to the total synthesis of eudesmane sesquiterpenes, was the photochemical reaction of (-)-piperitone (25g) with l,2-bis(trimethylsiloxy)cyclobutene, which gave c/.v,(5wf/u W-2/j,7/i-dimethyl-4/ -isopropyl-l f ,8Jf -bis(triniethylsiloxy)tricyclo[6.2.0.0z 7]dec-3-one (26g) with the relative cis configuration of the methyl (R2) and isopropyl (R3) groups.33,34 Some of the other photochemical [2 + 2] cycloaddition reactions utilizing l,2-bis(trimethyl-siloxy)cyclobutene are shown by the formation of 2735,36 and 28.37... 

Methyl y-oxocarboxytates.2 Silyl enol ethers react with methyl diazoacelate in the presence of a copper salt to form siloxy-substituted cyclopropancs (1) in useful yields. Cleavage of the siloxy group with fluoride ion (4, 477 478) affords y-oxo esters (2) in good yields. 

These reagents are also useful for the preparation of 1,2-diols. Upon exposure to Lewis acids such as boron trifluoride etherate (BFa-OEta), the a-alkoxy and oc-siloxyallyl stannanes undergo a stereospecific, intermolecular 1,3-isomerization to give y-alkoxy- and y-siloxy allylic stannanes.3. .7 When tert-butyldimethylsilyl trifluoromethanesulfonate is substituted for chloromethyl methyl ether in the above procedure, the isomeric -y-siloxy allylic stannane can be obtained directly with no loss of enantioselectivity.6 These stannanes can then be added to various aldehydes to give monoprotected 1,2-diols with high diastereoselectivity.8... 

Cycloaddition of 1-Dimethylamino-3-siloxy-1,3-butadiene with Methyl Acrylate Application to the Synthesis of 4-Hydroxyethyl-2-cyclohexen-1-one. 

Ring-opening silylformylation has been observed by Murai and coworkers in reactions of cyclic ethers. When the cobalt complex Co2(CO)8 is used as the catalyst in reactions of epoxides, an excess of substrate is required to prevent further reaction of the product siloxy aldehyde.1192 Further investigation led to the discovery of [RhCl(CO)2]2/l-methyl-pyrazole as an effective catalyst combination for the reaction of oxi-ranes119b and oxetanes.J19c For example, oxetane undergoes silylformylation to give 4-(dimethylphenylsiloxy)butanal in 81% yield [Eq. (45)]. 

The quantitative analysis was performed with the technique of gas chromato-graphy/mass spectroscopy. A radioactive sample of Cisobitan , which was labelled with 14C at the geminal methyl groups and which was hexadeuterated in the 4,4-dimethyl-siloxy position, was used for this investigation. Silylation of the metabolic products was carried out with hexamethyldisilazane. 

Various [(heteroarylamino)methyl]siloxanes, their hydrochlorides and methiodides have been prepared (see some representative molecules below) and their antimicrobial activity studies. The nature of the heterocycle and number of siloxy groups affect their antitumoral activity.496 They have also been tested for neurotropic and antimicrobial activity. 

FIGURE 43. Assignment of the two lines in the 29Si NMR spectmm of methyl 3o, 12a-bis(trimethyl-siloxy)-5/i-cholenate, 37, by selective INEPT. Top trace 29Si INEPT spectmm two middle traces selective INEPT spectra measured with selective excitation of H lines indicated by arrows in the bottom trace with partially assigned ll NMR spectmm (25 mg of the sample in 0.7 ml of CDCI3, H frequency 200 MHz, 29Si frequency 39.7 MHz, 5 mm broad-band probe, selective pulse by DANTE train, r = 70 ms, A = 149 ms). Reproduced with permission of Collection of Czechoslovak Chemical Communications from Reference 304... 

The prostaglandin approach above (Eq. 29) also shows that the reaction with the carbenoid is compatible with further functions and even a second olefinic unit. However, this second double bond is left unattacked only because of its deactivation by the allylic siloxy group. Competition experiments have demonstrated that simple olefins like styrene or cyclohexene react with methyl diazoacetate under copper-catalysis in rates comparable to those of silyl enol ethers57). 

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