Silylation cyano

Conversion to a more facile, sulfur-derived, leaving group can be achieved by treatment with sodium thiosulfate or salts of thio and dithio acids (75,87). Under anhydrous conditions, boron tribromide converts the 3 -acetoxy group to a bromide whereas trimethyl silyl iodide gives good yields of the 3 -iodide (87,171,172). These 3 -halides are much more reactive, even when the carboxyl group is esterified, and can be displaced readily by cyano and by oxygen nucleophiles (127). 

The nucleophilic acylation of 2-phenylpropanal or 3-phenyI-2-butanone with cyano(trimethyl-silyloxy)phenylmethyllithium proceeds with high Cram selectivity6. The primary addition product 7, after silyl migration and loss of lithium cyanide, gives the a-silyloxy ketones 86. 

Sulfoxides containing an a-chloro group 1191 or an a-trimethylsilyl group 1193 rearrange on silylation with TMSOTf 20/triethylamine or with LDA followed by TCS 14 to the olefins 1192 and 1194 in 86 and 75% yield and HMDSO 7 [22, 23], whereas a sulfoxide with an a-cyano or a-carbomethoxy group as in 1195 reacts... 

To improve the detectability of silyl ethers, silylation reagents containing an electron-capturing group [443,449-451,468] or cyano group for thermionic detection [469] have been prepared, the 2-cyanoethyldimethylsilyl derivatives are only marginally aore sensitive (ca. 5 fold) to the thermionic detector than to the flame ionization det ftpr which Units their usefulness. The... 

Polar functional groups such as carbonyl, cyano, and sulfoxide, as well as silyl and stannyl groups, exert a strong directing effect, favoring proton removal from the geminal methyl group.171... 

The intermediate N-acylpyridinium salt is highly stabilized by the electron donating ability of the dimethylamino group. The increased stability of the N-acylpyridinium ion has been postulated to lead to increased separation of the ion pair resulting in an easier attack by the nucleophile with general base catalysis provided by the loosely bound carboxylate anion. Dialkylamino-pyridines have been shown to be excellent catalysts for acylation (of amines, alcohols, phenols, enolates), tritylation, silylation, lactonization, phosphonylation, and carbomylation and as transfer agents of cyano, arylsulfonyl, and arylsulfinyl groups (lj-3 ). 

In each of these cases the angle is wider in solution than it is in the gas phase by about 2°. We believe that this is caused by weak interactions between the cyano groups of the solvent and the silyl groups, interactions which are comparable to those seen in many silyl compounds in the crystalline phase. 

No conformational dependence (NCD). Groups of this type include monatomic substituents such as hydrogen and the halogens cylindrical substituents such as the ethynyl and cyano groups, and tetracoordinate symmetric top substituents such as the methyl, trifluoromethyl and silyl groups. 

This process affords silyl derivatives of a-cyano oximes (452) as the primary products, detected by spectroscopic methods and, in some cases, can be isolated. Their desilylation gives aminooxazoles (451). (The synthesis of aminoisoxazoles by the reactions of the cyanide anion with nitrosoalkenes was also documented (503).) The reaction shown in Scheme 3.242 has a general character and can be performed with both terminal and internal BENAs, although special procedures are required for some functionalized BENAs. 

An efficient procedure for the synthesis of 5-aminoisoxazoles starting from BENAs (Scheme 3.242) was described in Section 3.5.4.2.2.1. An analogous approach to the synthesis of aminooxazoles can be successfully used in the case of silyl derivatives of a-cyano-substituted oximes (476a,b) (Scheme 3.255). 

A similar explanation may also hold for the result of conjugate addition to y-phthalimido enoate 80 (Scheme 6.16). Thus, addition of the bulky cyano-Gilman silyl cuprate gave the syn diastereomer 81 (dr = 96 4) [32, 33]. Preference for the sterically least hindered nucleophile trajectory seems to dictate the overall stereochemical outcome (transition state 82). 

On the basis of this pioneering work, Rubenbauer and Bach developed a Bi(OTf)3-catalyzed highly diastereoselective benzylation of silyl enol ethers [65]. Various cyclic and acyclic silyl enol ethers were amenable to this protocol (Scheme 24). Various a-substituted benzyl acetates were tested with terf-butyl-substituted silyl enol ether 31a, and the use of only 1 mol% of Bi(OTf)3 was enough to obtain the desired benzylated ketones 32 in high yields and with excellent diastereoselectivities (up to 95 5). Whereas a-nitro- (30a), ot-cyano- (30b) and a-methylester-substituted (30d) benzyl acetates gave the anti diastereoisomer as the major product, the phosphonate-substituted benzyl acetate (30c) exclusively resulted in the syn isomer (Scheme 24). 

N4F,2P2RuCl6H24, Ruthenium(II), tetrakis-(acetonitrile)(r)J-, 1,5-cyclooctadiene)-bis[hexafluorophosphate( 1 —)], 26 72 NjFeOGoHjft, Iron, carbonyltetrakis(2-iso-cyano-1,3-dimethylbenzene)-, 26 57 N4Li2Si2C2ftH5ft, Lithium, p.-[(a,cx, l,2-T) a,a, 1, 2-ti- 1,2-phenylenebis[(trimethyl-silyl)methylene]]bis(Af, V,lV, Ar-tetra-methyl-l,2-ethanediamine)di-,... 

This reaction is also a transfer dehydrogenative reaction, as two reactant hydrogen atoms are not incorporated into the enol silyl ether product but instead serve to hydrogenate another molecule of starting alkene. For example, in the reaction of vinylcyclohexane, ethylcyclohexane is obtained in equal amounts to the silylated product. Both iridium complexes effectively catalyze the reaction. Various silanes can be used, including di-ethylmethyl-, triethyl-, and dimethylphenylsilane. The reaction is successful for a range of terminal alkenes, even those bearing cyano, acetal, and epoxide functionalities. The E isomer of the product is predominantly formed. 

Considerable use has also been made of allyl carbonates as substrates for the allylation of Pd enolates.9 The reaction of Pd° complexes with allyl enol carbonates119,120 proceeds by initial oxidative addition into the allylic C—O bond of the carbonate followed by decarboxylation, yielding an allylpalladium enolate, which subsequently produces Pd° and the allylated ketone (equation 22). In like fashion, except now in an intermolecular sense, allyl carbonates have been found to allylate enol silyl ethers (equation 23),121 enol acetates (with MeOSnBu3 as cocatalyst) (equation 24),122 ketene silyl acetals (equation 25)123 and anions a to nitro, cyano, sulfonyl and keto groups.115,124 In these cases, the alkoxy moiety liberated from the carbonate on decarboxylation serves as the key reagent in generating the Pd enolate. 

Modification of these procedures by using only catalytic amounts of the cyano compound dicyanonaphthalene (DCN) allowed the cyclization of silyl enol ethers to bicyclic compounds in good yield (equation 38)178. 

In relation to the reactions of CT excited states of aryldisilanes, photochemistry of LE and ICT states of the rigid, p-cyano-substituted styryldisilane 7 was investigated by Steinmetz and coworkers154. Although no 1,3-silyl shift is observed in this sytem, 7 affords a major product attributable to addition of alcohol across the Si—Si bond in the CT state. The roles of LE and CT states in the formation of additional minor products of silylene extrusion and homolytic Si—Si cleavage have also been elucidated. 

In another attempt, we reached the tetracyclic structure 42 via two consecutive [4 + 2] cycloaddition reactions. Reaction of 3-cyano-4-benzopyrone 31 with Danishefsky s diene 38 in toluene at 300 °C for 96 h provided the desired cycloadduct 39 in 80% yield with an endo exo ratio of 1 2 [Scheme 8]. Hydrolysis of the silyl enol ether in 39 using TMSBr in CH3CN at room temperature proved to be feasible but slow, and afforded the enone 40 in 90% yield. Reaction of diene 41 with enone 40 in the presence of 2.5 equiv of BF3-Et20 yielded tetracycle 42 in 25% yield with an endo exo ratio of 1 1 after 120 h at room temperature.49... 

Allyl cyanides can be added across alkynes in the presence of a nickel catalyst prepared from (COD)2Ni and (4-CF3CeH4)3P in situ to give functionalized di- or tri-substituted acrylonitriles in a highly stereoselective manner, presumably via n-allylnickel intermediates. a-Siloxyallyl cyanides also react at the y -position of a cyano group with both internal and terminal alkynes to give silyl enol ethers, which can be converted into the corresponding aldehydes or ketones upon hydrolysis.70... 

A /3-hydroxyketone (542) forms with 543 the corresponding l,3-dioxa-2-sila-4-cyanocyclohexane (544) and the unsaturated isomer 545. With methanol, de-cyano-silylation is obtained. The /3-diketone 546 forms 544, too in that case it is better cleaved by AgF/THF or only methanol. 

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

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