N-Silyl group

Lithium salts of amino-siloxanes react with fluorosilanes to give in the absence of strong steric or electronic restraints, the isomeric silylamino-l,3-disiloxane. A l,3-(0—>N)-silyl group migration has occurred. The analogous raction with difluorosilanes leads to both isomers, and with trifluorosilanes no isomerization or silyl group migration is observed. 

The lithium derivatives of aminosiloxanes are usually isolated as 1,3-disilazan-1-olates (type A), i.e. a l,3-(0-N) silyl group migration has occurred. This... 

Usually the attacking silyl group is bonded to the O atom, which means a l,3-(0-N)-silyl group migration occurs (Scheme 3, Fig. 9). This reeirrangement can be prevented by bulky substituents. 

This procedure is representative of a new general method for the preparation of noncyclic acyloins by thiazol ium-catalyzed dimerization of aldehydes in the presence of weak bases (Table I). The advantages of this method over the classical reductive coupling of esters or the modern variation in which the intermediate enediolate is trapped by silylation, are the simplicity of the procedure, the inexpensive materials used, and the purity of the products obtained. For volatile aldehydes such as acetaldehyde and propionaldehyde the reaction Is conducted without solvent in a small, heated autoclave. With the exception of furoin the preparation of benzoins from aromatic aldehydes is best carried out with a different thiazolium catalyst bearing an N-methyl or N-ethyl substituent, instead of the N-benzyl group. Benzoins have usually been prepared by cyanide-catalyzed condensation of aromatic and heterocyclic aldehydes.Unsymnetrical acyloins may be obtained by thiazol1um-catalyzed cross-condensation of two different aldehydes. -1 The thiazolium ion-catalyzed cyclization of 1,5-dialdehydes to cyclic acyloins has been reported. 

The role of silyl groups in condensation polymerization is different from that in GTP. The use of silylated monomers in condensation polymerization was studied first by Klebe [90-92] in 1964. N-trimethylsilyl-substi-... 

CH3Ot. 6H4)3P=CHCH2SiCH3(Ct,H3)2 this /(-silyl phosphorane is a syn-thon for a vinylmetal reagent. Vinylation occurs with concomitant migration of the silyl group. b n.r. = not reported. 

The reactivity of N-silyl phosphinous amides versus some of the functional groups in the scheme above has no precedent in nonsilylated analogs. 

Monomers contained terminal phenylacetylenes protected with trimethyl-silyl groups and aryl iodides masked as N,hT-dialkyltriazenes. A typical PAM synthesis involved three basic transformations (Scheme 12) ... 

Alternative silylating reagents such as N,0-bis(trimethylsilyl)acetamide 22a (BSA) [39-43], N,0-bis(trimethylsilyl)trifluoracetamide 22b (BSTFA) [44], or N,N-bis(trimethylsilyl)formamide 22c (BSF) [41, 46], in which the N- and O-trimethyl-silyl groups are in equilibrium [45] (Scheme 2.4), are much more powerful silylating reagents [40, 45] but are more expensive than FIMDS 2, because they are usually prepared by heating formamides or acetamides with TCS 14/triethylamine... 

It should be noted here that the lithium salt of hexamethyldisilazane li-HMDS 492 (and Na-HMDS-(486) and K-HMDS in Sections 5.1.2 and 5.1.3), which is readily obtained on treatment of a solution of HMDS 2 in hexane or THF with butyUithium at -78 °C, is not only a very useful and selective strong base, e.g. for Wittig reactions, but can also add to carbonyl groups to yield the silylated Schiff bases or nitriles (cf. Sections 4.7 and 5.1.3) or to nitriles to afford N-silylated ami-dines. Alkylation of the Li-HMDS 492, e.g. with allyl bromide, affords, furthermore, N,N-bis(trimethylsilylated) primary amines such as 43 [64]. The combina-... 

The ready exchange of silyl groups is apparent from the reaction of equimolecu-lar amounts of trimethylsilyl formate 148 with N-triethylsilyhnethylamine 163 for 1 h at room temperature (Scheme 4.4) whereupon two layers separate the upper layer consists of HMDSO 7 and l,l,l-trimethyl-3,3,3-triethyldisiloxane 64 and the lower layer contains N-methylformamide 164 in almost quantitative yield [5]. 

The carbonyl group of methyl benzoate condenses with Na-HMDS 486 to give methoxytrimethylsilane 13a and 51% yield of N,0-bis(trimethylsilyl)benzamide 296 [99], which is also accessible by silylation of benzamide with TCS 14/triethyla-mine. Benzamide or N-silylated benzamide, however, are converted by Na-HMDS 486 in benzene and subsequent quenching with MesSiCl 14 into 34% N,0-bis(trimethylsilyl)benzamide 296, 24% crystalline N-silylated benzamidine 524, and HMDSO 7 [99] (Scheme 5.32). 

As with aldol and Mukaiyama addition reactions, the Mannich reaction is subject to enantioselective catalysis.192 A catalyst consisting of Ag+ and the chiral imino aryl phosphine 22 achieves high levels of enantioselectivity with a range of N-(2-methoxyphenyljimines.193 The 2-methoxyphenyl group is evidently involved in an interaction with the catalyst and enhances enantioselectivity relative to other A-aryl substituents. The isopropanol serves as a proton source and as the ultimate acceptor of the trimethyl silyl group. 

The first reported synthesis of hydroxyurea (24) consists of the condensation of hy-droxylamine with potassium cyanate (Scheme 7.14) [87]. Condensation of hydroxy-lamine with ethyl carbamate also gives pure hydroxyurea in good yield after recrystallization (Scheme 7.14) [88]. Nitrogen-15 labeled hydroxyurea provides a useful tool for studying the NO-producing reactions of hydroxyurea and can be prepared by the condensation of N-15 labeled hydroxylamine with either potassium cyanate or trimethylsilyl isocyanate followed by silyl group removal (Scheme 7.14) [89, 90]. Addition of hydroxylamine to alkyl or aryl isocyanates yields alkyl or aryl N-hydroxyureas (Scheme 7.14) [91, 92]. The condensation of amines with aromatic N-hydroxy carbamates also produces N-substituted N-hydroxyureas (Scheme 7.14) [93]. 

The /5-effect of silyl groups on -systems depends on the geometry of the molecule, because the orbital interaction between the C-Si a orbital and the n orbital reaches its maximum when they are in the same plane. For example, the... 

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