Silylated Urethanes

Benzaldehyde can be condensed with the N-silylated urethane 671 and aUyltri-methylsilane 82 in the presence of trityl perchlorate to give, via an intermediate 0,N-acetal, the substituted urethane 672 in high yield [197]. 0,N-Acetals such as 673 condense with the enol silyl ether of acetophenone 653 in the presence of TMSOTf 20 to give the co-hydroxyurethane 674 in 94% yield [198] (Scheme 5.62). 

Si NMR has been used to study equilibria in iV-silyl-amides and N-silyl-urethanes. (135) All the N-silyl-amides investigated are found to exist in an amide-imidate equilibrium, with electron donating groups at N and lower temperatures favouring the amide form. In contrast, the silyl-urethanes are all in the amide form. 

Silylated urethane polymers allow formulators to produce fast-cure construction joint sealants with superior performance properties. The sealant is composed of silylated urethane polymer, plasticiser (e.g. diisodecyl phthalate), fillers (calcium carbonates), fumed silica, adhesion promoter, dehydrating agent, and catalyst (dibutyltin dilaurate). 

Silylated urethane polymer technology is an alternative to, and an extension of, existing conventional urethane technology. The sealants are free of residual NCO. This technology is moving from a theoretical concept to practical commercial applications. 

Feng, T.M. and Waldman, B.A. (1995) Silylated urethane polymers - enhanced properties of construction sealants, in Adhesives Age 4 reprint supplied by Osi Specialities (a Witco Co.), New York. 

The main components of adhesives are polymeric materials such as elastomers or synthetic polymers. The main component type for a certain adhesive is decided hy the designer considering the type of adherends and use conditions. Control items to he considered is as follows solubility parameters, average molecular weights, Mooney viscosity, crystallinity, and existence or nonexistence of functional groups. Materials that can be used as main components of elastomer adhesives are polychloroprene rubbers, nitrile rubbers (butadiene-acrylonitrile copolymer), SBR (styrene-butadiene copolymer), thermoplastic elastomers (SBS, SIS, SEBS, SEPS etc.), butyl rubbers, acryhc rubbers, sUicone rubbers, polysulfides, modified polysulfides, modified silicone rubbers, and silyl urethanes. 

Trimethylsilyl trichloroacetate, K2CO3, 18-crown-6, 100-150°, 1-2 h, 80-90% yield.This reagent silylates phenols, thiols, carboxylic acids, acetylenes, urethanes, and /3-keto esters, producing CO2 and chloroform as byproducts. 

Benzaldehyde dimethyl acetal 121 reacts, for example, with the silylated allylic alcohol 645, in the presence of SnCl2-MeCOCl, via an intermediate analogous to 641, to the 3-methylenetetrahydrofuran 646 and methoxytrimethylsilane 13 a [182], whereas benzaldehyde dimethyl acetal 121 reacts with the silylated homoallylalco-hol 640 in the presence of TMSOTf 20 to afford exclusively the ds 4-vinyltetrahy-drofuran 647 and 13 a [183]. A related cyclization of an a-acetoxy urethane 648 containing an allyltrimethylsilane moiety gives the 3-vinylpyrrohdine 649 in 88% yield and trimethylsilyl acetate 142 [184, 185]. Likewise, methyl 2-formylamido-2-trimethylsilyloxypropionate reacts with allyltrimethylsilane 82 or other allyltri-methylsilanes to give methyl 2-formamido-2-aUyl-propionate and some d -unsatu-rated amino acid esters and HMDSO 7 [186] (Scheme 5.56). 

The sensitivity of Si chemical shifts to structural changes and the technique of silylating compounds for more favourable analysis or synthesis have been combined by several researchers to produce a powerful structure elucidation technique for monofunctional or polyfunctional compounds. (135-141) Specifically, the trimethylsilyl derivatives of imidophosphoryl compounds, (141) sugars, (138-140) steroids, (140) amines, amides, and urethanes, (135,136) and amino-, hydroxy-, and mercaptocarboxylic acids (137) have all been studied within the past three years. 

As has been demonstrated, 2-(trimethylsilyl)ethoxymethyl (SEM) esters are selectively removed from amino acids and peptide derivatives in the presence of the most common N- and O-protecting groups applied in peptide chemistry including the urethane-type Boc, Z, Fmoc, and Troc as well as Bzl, tBu, and TBDMS ethers.The SEM ester is removed by acidolysis or with a fluoride ion source, e.g. TBAF in THF or HMPA or with aqueous HF in MeCN (—10°C).f l Deprotection with magnesium bromide in EtjO represents an even milder alternative that allows increased selectivity toward fluoride-labile silyl ethers or Fmoc groups. The SEM esters are prepared in 60-80% yield by stirring a mixture of 0.25 M N-protected amino acids in DMF with 0.8 equivalents of SEM-Cl and 1.1 equivalents of lithium carbonate at room temperature for 16 hours. 

A(-silyl-substituted ureas (silylureas) and urethanes (silylurethanes)... 

TiCLt, Zn, CH2CI2, 0°C to rt, 82-87% yield.The method was shown to be compatible with esters, aldehydes, ketones, silyl ethers, and urethanes. 

Synthesis of isocyanates,12 Thermal cleavage of urethanes gives low yields of isocyanates because of the high temperatures necessary (200-300°). However, if the urethane is N-silylated, cleavage occurs at much lower temperatures. Thus the urethane (1) is treated with trimethylchlorosilane and triethylamine in refluxing... 

Non-symmetrical secondary allyl acetates and urethanes react in a regiospecific manner with lithium(dimethylphenylsilyl)cuprate to afford allylsilanes. Thus, -pent-3-en-2-ylacetate (140) is converted into -2-dimethylphenylsilylpent-3-ene (141) (yield 70%), by the silylating agent in THF/ether/pentane at 0°C (equation 66)90. 

Finally, the strategy should be of interest because it only requires two reactions steps. This is possible because ketimine isolation is not required, and while rarely discussed can be time consuming, may provide mediocre yield, and unnecessarily lengthens the synthesis of amines. Furthermore, all the reagents are already in use by the pharmaceutical industry, a broad range of ketone substrates are suitable (even aliphatic ketones), either enantiomeric form of the a-chiral amine product can be produced, and the process has been demonstrated on a 20 g scale. The method is compatible with acetonides, ethers, silyl ethers, bulky esters, secondary amides, tertiary amides, carbamates, urethanes, etc. With these beneficial qualities noted, the method suffers when non-branched 2-alkanones are used (product des <75%). In these cases, HCl salt formation allows further enrichment via crystallization, alternatively stoichiometric Yb(OAc)3 can be used during the reductive amination to allow enhanced de. Both of these solutions require additional processing time and/or cost and require consideration before scale-up. 

The best conditions for converting 57 to the target oxazole 58 involved temporary TMS protection of the urethane followed by KHMDS and iodine (see Scheme 1.18). Temporary protection of the urethane as a silyl carbamate was critical to the success of this process. Attempts to employ a second Boc-protecting group or bulkier silyl-protecting groups failed. 

Therefore, we developed a carbamate group, which is cleavable under mild, alkaline conditions. The N-[2-(ferf-butyldiphenylsilyloxy)-ethyl)]-N-isopropyl-carbamoyl group (Cbse) [135,137] in tertiary esters 232 can be removed by de-silylation and subsequent neighboring group participation in the hydroxyalkyl-urethane 233 to give the alcohols 234. Some examples of compounds synthesized by this method are collected [Eq. (63)] [137]. 

Figure 3.8 Some possible routes for chemical modiflcation of NCC (clockwise from right) (a) sulfonation, (b) oxidation by TEMPO, (c) ester linkages via acid chlorides, (d) cationization via epoxides, (e) esterlinkages via acid anhydrides, (f) urethane linkages via isoc3 nates, (g) silylation [115].

Acylation —esters Trimethylsilation—silyl esters Isocyanates—urethanes... 

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