Silyl amines, reactions

Compounds containing Silicon-, Germanium-, Tii, and Lead-Nitrogen Bonds.—Molecular nitrogen is reductively silylated by MeaSiCl and lithium metal in the presence of a transition-metal catalyst to give tris(trimethyl-silyl)amine [reaction (101)]. Variable-temperature n.m.r. spectra for a number of compounds containing Si—N bonds in which potentially diastereo-... 

Thionyl imide, HNSO, is a thermally unstable gas, which polymerizes readily. It can be prepared by the reaction of thionyl chloride with ammonia in the gas phase. Organic derivatives RNSO have higher thermal stability, especially when R = Ar. The typical synthesis involves the reaction of a primary amine or, preferably, a silylated amine with thionyl chloride. A recent example is the preparation of FcNSO (Fc = ferrocenyl) shown in Eq. 9.8. In common with other thionylimines, FcNSO readily undergoes SO2 elimination in the presence of a base, e.g., KO Bu, to give the corresponding sulfur diimide FcNSNFc. 

It is also possible to prepare them from amino acids by the self-condensation reaction (3.12). The PAs (AABB) can be prepared from diamines and diacids by hydrolytic polymerization [see (3.12)]. The polyamides can also be prepared from other starting materials, such as esters, acid chlorides, isocyanates, silylated amines, and nitrils. The reactive acid chlorides are employed in the synthesis of wholly aromatic polyamides, such as poly(p-phenyleneterephthalamide) in (3.4). The molecular weight distribution (Mw/Mn) of these polymers follows the classical theory of molecular weight distribution and is nearly always in the region of 2. In some cases, such as PA-6,6, chain branching can take place and then the Mw/Mn ratio is higher. 

After silylation-amination in situ transsilylation (cf Section 2.3) of the intermediate persilylated cytidines 5 with excess boiling methanol for 3-5 h gives the desired free cytidines 6 and methoxytrimethylsilane 13a (b.p. 57°C) [13]. Thus protection of the alcohohc hydroxyl groups of the ribose moiety and silylation-activation of the 4-position in the pyrimidine moiety in persilylated uridine 3, and the concomitant amination of 3, aU in one reaction step, to 5 is followed finally by in situ transsilylation (cf. Section 2.3) with excess boihng methanol in one reaction vessel. 

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]. 

Thus removal of water from classical rather inactive fluoride reagents such as tetrabutylammonium fluoride di- or trihydrate by silylation, e.g. in THF, is a prerequisite to the generation of such reactive benzyl, allyl, or trimethylsilyl anions. The complete or partial dehydration of tetrabutylammonium fluoride di- or trihydrate is especially simple in silylation-amination, silylation-cyanation, or analogous reactions in the presence of HMDS 2 or trimethylsilyl cyanide 18, which effect the simultaneous dehydration and activation of the employed hydrated fluoride reagent (cf, also, discussion of the dehydration of such fluoride salts in Section 13.1). For discussion and preparative applications of these and other anhydrous fluoride reagents, for example tetrabutylammonium triphenyldifluorosilicate or Zn(Bp4)2, see Section 12.4. Finally, the volatile trimethylsilyl fluoride 71 (b.p. 17 °C) will react with nucleophiles such as aqueous alkali to give trimethylsilanol 4, HMDSO 7, and alkali fluoride or with alkaline methanol to afford methoxytri-methylsilane 13 a and alkali fluoride. 

As yet, a number of experiments have failed to convert ureas 205 such as N-phenylurea or imidazolin-2-one by silylation amination with excess amines R3NHR4 such as benzylamine or morpholine and excess HMDS 2 as well as equivalent amounts of NH4X (for X=C1, I) via the silylated intermediates 206 and 207 in one reaction step at 110-150°C into their corresponding guanidines 208 with formation of NH3 and HMDSO 7 [35] (Scheme 4.13). This failure is possibly due to the steric repulsion of the two neighbouring bulky trimethylsilyl groups in the assumed activated intermediate 207, which prevents the formation of 207 in the equilibrium with 206. Thus the two step Rathke-method, which demands the prior S-alkylation of 2-thioureas followed by amination with liberation of alkyl-mercaptans, will remain one of the standard syntheses of guanidines [21, 35a,b,c]. 

In the reactivity scale of Scheme 4.25 the reactivity of any of these heterocycles is substantially increased by annellation with a conjugated aromatic ring. Thus 2-quinolone is much more reactive than pyridine-2-one 245, which is the least reactive hydroxyheterocycle and requires reaction temperatures higher than 190-200 °C for silylation-amination [27]. To achieve these temperatures at normal pres-... 

Whereas silylation-amination of 2-amino-5,8-dihydroxypyrimido[4,5-d]pyridazine 269 with 3-amino-l-propanol, HMDS 2, and TsOH affords, after 24 h at 120-140 °C, the mono-8-hydroxypropylamino derivative 270 in 50% yield [79], reaction of 269 with a shght excess of ethanolamine and HMDS 2 provides, after 30 h at 120-150°C, only 20% of the bis(amino) product 271 [79]. (Scheme 4.31) A larger excess of ethanolamine and longer reaction times wiU certainly increase the yield of 271. 

A mechanistic proposal, which is based on the mthenium-catalyzed dehydration reaction reported by Nagashima and coworkers [146], is shown in Scheme 44. Reaction of a primary amine with hydrosilane in the presence of the iron catalyst affords the bis(silyl)amine a and 2 equiv. of H2. Subsequently, the isomerization of a gives the A,0-bis(silyl)imidate b and then elimination of the disiloxane from b produces the corresponding nitrile. Although the disiloxane and its monohydrolysis product were observed by and Si NMR spectroscopy and by GC-Mass-analysis, intermediates a and b were not detected. 

Ti-F bond and generate a Ti-H species when 99 was treated with phenylsilane. The chirality transfer may take place through imine insertion into the Ti-H bond, similar to that in the catalytic hydrogenation process.1000 The reaction can be carried out by the subsequent addition of imines. The corresponding silylated amines can be obtained and further converted to enantiomerically enriched amines upon acid treatment. For example, in the presence of 99, N-methylimine 100 undergoes complete hydrosilylation within 12 hours at room temperature, with 97% ee and up to 5000 turnovers.103... 

Photoinduced electron transfer promoted cyclization reactions of a-silyl-methyl amines have been described by two groups. The group of Pandey cyclized amines of type 135 obtaining pyrrolidines and piperidines 139 in high yields [148]. The cyclization of the a-silylated amine 140 leads to a 1 1 mixture of the isomers 141 and 142 [149]. The absence of diastereoselectivity in comparison to analogous 3-substituted-5-hexenyl radical carbocyclization stereochemistry [9] supports the notion that a reaction pathway via a free radical is unlikely in this photocyclization. The proposed mechanism involves delocalized a-silylmethyl amine radical cations as reactive intermediates. For stereochemical purposes, Pandey has investigated the cyclization reaction of 143, yielding... 

Repetition of the reaction with DEAD as the dipolarophile furnished the desired cycloadduct 55 in 48% yield, but with 25% yield of the enamide 56 also being isolated. This was rationalized by invoking decomposition of the ylide precursor 57 to the (trimethylsilylmethyl)silyl amine 58, which undergoes subsequent addition to the highly reactive acetylene (Scheme 3.14). 

Transition metal-catalyzed hydrosilylation, an established route for transformations of unsaturated carbon substrates, is also well known for reactions of nitriles,183 imines,184 and oximes.185 Work in this area includes the Co2(CO)8-catalyzed addition of trialkylsilanes to aromatic, aliphatic, and a,/3-unsaturated nitriles, giving /V,/V-bis(silyl)amines and/or -enamines in fair to good yields [Eqs. (73) and (74)].186... 

Acid fluorides can be used for a elean preparation of amides.9 96 The use of silylated amines is an elegant alternative, sinee liberated fluoride progressively desilylates them to more nucleophilic species. The reaction has to be started with a catalytic amount of fluoride (TBAF).92 Stannyl amines97 or phosphoryl amines (phosphoramides)98 have been used in similar reactions. 

In the latter reaction the proton typically combines with a suitable leaving group or is picked up by excess amine. The silylated amine is also sometimes used as its lithium salt, e.g. LiN(R)SiMe3. Examples of the use of this technique are shown in equations (47),75,76 (48),77 (49)77 and (50).75,79... 

The cyclization of allyl silyl amine 697 by hydrosilylation led to silaazetidine 698, which was subjected to flash vacuum thermolysis at 700-900°C at 10-4 hPa313. The silanimines 699 and 700 themselves were too reactive to be observed by high resolution mass spectrometry of the reaction mixture, but their cyclic dimers, the cyclodisilazane 701 and 702 and a trapping product with t-BuOH 703, were definitely confirmed... 

The Pandey group has developed a silver fluoride-promoted desily lation of tertiary bis(silyl)amines as an interesting alternative method to access azomethine ylides (Scheme 2.10).18 Notably, this method allows the generation of nonstabilized azomethine ylides under essentially neutral conditions. The starting materials are prepared by a three-step process, sometimes coupled into a single operation. For example, Boc-protected pyrrolidine 36 can be sequentially deprotonated and silylated twice in a one-pot reaction (Scheme 2.10). Removal of the Boc group and alkylation of the free amine leads to bis(silyl)amine 37. When this compound is treated with 2 equiv of silver fluoride in the presence of phenyl vinyl sulfone, rapid formation of products 39 as single stereoisomers results. 

Another approach involves the use of an electron-poor olefin acting both as an absorbing electron acceptor and as a radical trap. In this case, a PET reaction between a cyclohexenone derivative and a silylated amine led to a radical ion pair. Desilylation of the silyl amine radical cation intermediate in polar protic solvent (e.g., MeOH) and subsequent aminoalkyl radical attack onto the enone radical anion yielded the alkylated cyclohexanones [23]. 

Hasegawa, E., Xu, W., Mariano, P.S., Yoon, U.-C., and Kim, J.-U. (1988) Electron-transfer-induced photoadditions of the silyl amine EtjNCHjTMS to a,p-unsaturated cyclohexenones. Dual reaction pathways based on ion-pair-selective cation-radical chemistry. Journal of the American Chemical Society, 110, 8099-8111. 

Anionic 1,2-silyl migrations from N to C were found in the reactions of a-silyl amines 179 with s-BuLi (equation 107)241,242. 

Another member of this class of substances can be synthesized via the reaction of lithium pyrrolide and N,N-bis[fluoro(diisopropyl)silyl]amine. The monosubstitution product 255 is formed (Equation 61). 

Silyl amines usually react readily with water or alcohols to give compounds containing Si R groups (R = H, Me, Et, etc.) and with HCl to give a chlorosilane. The poor donor properties of (HaSi)3N mean that it does not form adducts as readily as McaN, no adduct is formed with MeaB, and only weak adducts are formed with BFa or BCI3. Reactions of (HaS aN are summarized in Scheme 53. ... 

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