Silyl isocyanate

Silyl isocyanates can readily be prepared by treatment of chlorosilanes (bromo- or iodosilanes would also work but are less readily available) with metal cyanates. With simple halosilanes the product is always the isocyanate isomer although in the case of highly sterically hindered organosilanes, normal cyanates (which thermally isomerize to isocyanates) may be formed using silver cyanate. Thus Si(NCO)4 is formed from the reaction between SiCU and KOCN in liquid SO2, AgOCN in CeHe, or Pb(OCN)2 in Similarly, Si(NCS)4 is formed in... 

Silyl tethered amines are used frequently in amide-forming coupling reactions [e.g.. Fig. 16(c)] (169, 170). Isocyanate compounds are also commercially available, and have been used in the immobilization of enzymes within the sol-gel pores (171). These isocyanates can also be used to prepare silylated phen ligands that have been used in our own studies (172). Silylated isocyanates have... 

CO2 insertion into the N-Si bond of silylamido Ugands can play a key role in a few metathetical reactions of carbon dioxide with bis-silyl-amido complexes M[N (SiMe3)2] (Na, n = 2 Pr, Nd, n = 3 Ge, Sn, n = 2 Ti(IV), Zr(TV)) [82,83]. These metathetical reactions result in the oxo-transfer from CO2 to a silyl group of the bis-silylamido ligand and provide an entry into silyl isocyanates or l,3-bis(silyl) carbodiimides (Scheme 4.25). 

The highly ring strained cyclopropanone (which is conveniently stored and used as a mixed ketal) also undergoes Schmidt chemistry to afford M-substituted jS-lactams along with ethyl carbamates in about a 1 1 ratio (Scheme 7.9). Presumably, the p-laclam is formed via the typical ring expansion mechanism described above. Ethyl carbamates are more mechanistically intriguing, requiring the formal loss of ethylene and N2 followed by recombination of a silylated isocyanate with ethanol. 

Double. silylation of the a-diketone 120 with (Me3Si)2 is catalyzed by PdCl2(Me3P)2 to give the l,2-bis(siloxy)ethylene 121 in a good yield[ll7], Hydro.silylation of phenyl isocyanate (122) catalyzed by PdCN affords the /V-... 

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. 

Polyimides syntheses starting from dianhydrides (or derivatives such as bis acid-esters) and diamines (or derivatives like silyl- substituted diamine or isocyanate)... 

Silylation of amino acids such as r-leucine 180 with TCS 14 gives rise to the O-silylated ammonium salt 181, which reacts selectively with triphosgene and triethylamine to afford the isocyanate 182. Subsequent reaction of 182 with primary amines such as free r-leucine 180 or secondary amines such as N-BOC-pi-perazine 184 affords the ureas 183 and 185 in 49% or 77% overall yield, respectively [10] (Scheme 4.7). 

The silylated glycine derivative 1450 cyclizes spontaneously on heating to 85-140 °C to give the hydantoin derivative 1451 in 94% yield [20, 21]. The silylated hy-dantoin 1453 is obtained by reacting silylated N-carboxymethylglycine 1452 with trimethylsilyl isocyanate 327 and subsequent heating to 140°C [22] (Scheme 9.12). 

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]. 

Catalytic amounts of 35 (1 mol%) also promoted the reaction of aromatic aldehydes with silyl ethers [94], vinylogous silicon enolates [95] and even with isocyanates in the presence of stoichiometric amount of SiCl [98]. The products were isolated in high yield and enantioselectivity. 

Anhydride 150 and trimethylsilyl azide in dioxane yielded an acyl azide, which by thermolysis, and Curtius degradation spontaneously furnished the isocyanate 151 (72CB3958 74CB3533). Anhydride 152 was formed by silyl ester cleavage on hydrolysis [73MI2 90JCS(P1)375]. 

The silyl ether derivative of the alcohol is used in Eq. 2-195 since the corresponding alcohol OCN—R—CONH—R —OH cannot be isolated because of the high degree of reactivity of isocyanate and alcohol groups toward each other. 

Silyl-substituted diazoketones 29 cycloadd with aryl isocyanates to form 1,2,3-triazoles 194 (252) (Scheme 8.44). This reaction, which resembles the formation of 5-hydroxy-l,2,3-triazoles 190 in Scheme 8.43, has no analogy with other diazocarbonyl compounds. The beneficial effect of the silyl group in 29 can be seen from the fact that related diazomethyl-ketones do not react with phenyl isocyanate at 70 °C (252). Although the exact mechanistic details are unknown, one can speculate that the 2-siloxy-1-diazo-1-alkene isomer 30 [rather than 29 (see Section 8.1)] is involved in the cycloaddition step. With acyl isocyanates, diazoketones 29 cycloadd to give 5-acylamino-l,2,3-thiadiazoles 195 by addition across the C=S bond (252), in analogy with the behavior of diazomethyl-ketones and diazoacetates (5). 

Preparation of silyl-substituted semicarbazides from the reaction of trimethylsilyl isocyanate and hydrazine or derivatives [92]. 

The dominance of an alkoxy group over a silyl group is manifested in the generation of amides from reaction of oc-silyl enol ethers with chlorosulfonyl isocyanate, followed by hydrolysis. Simple vinylsilanes and alkynylsilanes undergo carbamidation [34],... 

Several groups have screened a variety of transition metal complexes for activity in the double silylation system, but only compounds of nickel, palladium, and platinum appear to be viable catalysts. The key factor appears to be the involvement of a M(0) species, although certain M(II) complexes can also be used, presumably with in situ reduction to M(0). Generalizations regarding the activity of the different transition metal complexes are difficult, as many variables exist in each system. However, the most active complexes seem to combine palladium metal centers with dba, small basic phosphine, or isocyanate ligands. 

Prior to 1991, no high-yield double silylation had been reported using simple peralkyldisilanes such as hexamethyldisilane. In that year, Ito and Tanaka independently reported that palladium systems, with isocyanate and P(OCH2)3CEt ligands, respectively, promote insertions into the Si-Si bond of unactivated disilanes. 

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