Bis silylation with

Addition reactions of the Si-Si bonds across carbon-carbon triple bonds have been most extensively studied since the 1970s by means of palladium catalysts. In the early reports, palladium complexes bearing tertiary phosphine ligands, mostly PPh3, were exclusively employed as effective catalysts, enabling the alkyne bis-silylation with activated disilanes, i.e., disilanes with electronegative elements on the silicon atoms such as hydro [36], fluoro [37], chloro [38], and alkoxy-disilanes [39,40] and those with cyclic structure (Scheme 4) [41-44]. The bis-silylation reactions could be successfully applied to terminal alkynes and acetylenedicarboxylates to give (Z)-l,2-bis(silyl)alkenes, which are otherwise difficult to synthesize. 

Two new classes of ligands, bicyclic phosphate 47 [45] and ferf-alkyl isonitriles [46] on palladium, enable bis-silylation with hexaalkyldisilanes, which have been regarded to be much less reactive than the activated disilanes (Eq. 20). Reactions of the terminal alkynes with hexamethyldisilane in the presence of these palladium catalysts afford (Z)-l,2-bis(trimethylsilyl)alkenes 48 in high yields. 

Palladium-phosphine complexes catalyze the similar 1,4-bis-silylation with the activated disilanes. In the presence of palladium-PPh3 complexes, fluoro-[55] and chloro-disilanes [56] provide 1,4-products 59 in good yields with high stereoselectivity giving Z-alkenes (Eq. 27). In the reaction of fluorinated disilane with isoprene, a minor amount of 1 2 adduct 60, which arises from regioselective... 

With ring G in place, the construction of key intermediate 105 requires only a few functional group manipulations. To this end, benzylation of the free secondary hydroxyl group in 136, followed sequentially by hydroboration/oxidation and benzylation reactions, affords compound 137 in 75% overall yield. Acid-induced solvolysis of the benzylidene acetal in 137 in methanol furnishes a diol (138) the hydroxy groups of which can be easily differentiated. Although the action of 2.5 equivalents of tert-butyldimethylsilyl chloride on compound 138 produces a bis(silyl ether), it was found that the primary TBS ether can be cleaved selectively on treatment with a catalytic amount of CSA in MeOH at 0 °C. Finally, oxidation of the resulting primary alcohol using the Swem procedure furnishes key intermediate 105 (81 % yield from 138). 

Weiss et al. (1984) showed that A V-bis-silylated anilines react in aprotic dichloro-methane with generation of diazonium salts and formation of the non-nucleophilic hexamethyldisiloxane (Scheme 2-28). The authors indicate that the monosilylated aniline C6H5NHSi(CH3)3 reacts in many cases in an analogous way. This seems surprising, since the hydroxytrimethylsilane HOSi(CH3)3 that is formed is a potential proton donor, as it will rapidly condense to give (CH3)3SiOSi(CH3)3 + H20. 

Allyl acetates can similarly be transformed into allylsilanes by treatment with bis(silyl)cuprates (5). 

N -Fmoc serine benzyl ester 2, which could be prepared as shown or purchased commercially, was smoothly converted to the crystalHne O-methylthiomethyl (MTM) ether 3 in high yield via a Pummerer-Hke reaction using benzoyl peroxide and dimethyl sulfide in acetonitrile [39]. This common intermediate was used to synthesize both 5 and 8 [40]. Both Ogilvie [41] and Tsantrizos [42] had reported that I2 was an effective activator with similar MTM ether substrates. The H promoted nucleosidation reaction between O-MTM ether 3 and bis-silylated thymine 4 produced the nucleoamino acid 5 in 60% isolated yield (100% based on recovered 3). Hydrogenolytic deprotection of the benzyl ester with H2, Pd/C in MeOH gave the thymine-containing nucleoamino acid 6 in quantitative yield. 

In 1996, Cavell described the synthesis of neutral P(VI) compound 37 containing a divalent tridentate diphenol imine ligand and three chlorine atoms by the reaction of a bis silylated Schiff base with PCI5 to give 37 after elimination of two equivalents of Me3SiCl (Scheme 7) [51]. 

The mono-silylated or free acetamides, which are liberated during silylation with 22 a, can, furthermore, interfere with any subsequent reaction, e.g. with electrophiles. Thus in the one-pot/one-step silylation, Friedel-Crafts catalyzed, nucleoside synthesis starting from protected sugar derivatives and pyrimidine or purine bases, the mono- or bis-silylated amides such as 22 a can compete with less reactive silylated heterocycHc bases for the intermediate electrophilic sugar cation to form protected 1-acetylamino sugars in up to 49% yield [42, 47]. On silylation with trimethylsilylated urea 23 a the Hberated free urea is nearly insoluble in most solvents, for example CH2CI2, and thus rapidly precipitated [43]. 

Free carboxylic acids such as benzoic acid, phenylacetic acid, or 4-hydroxyben-zoic acid 297 are converted on heating with HMDS 2 or OMCTS 52, via their N,0-bis(silylated) amides such as 22a, into nitriles such as 298 [99, 100] (Scheme 4.38). 

With alkyl-, allyl-, benzyl-, or aryl-Grignard or lithium reagents 72 0,N-acetals 473 give the N,N-bis-silylated primary amines 474 in high yields these are converted by methanol into the free primary amines 44 and MeOSiMc3 13a [60] (Scheme 5.22). 

It is interesting to note that condensation of the N,N-bis(silylated) enamine 538 with a variety of chalcones such as benzalacetophenone 735 proceeds, via 539 and subsequent cyclization and oxidation, to pyridines such as 540 [106, 108] whereas persilylated co-amino ketones such as the 2-substituted pyridine 541 cyclize, via 542, in 29% yield, to the pyrrole 543 [109] (Scheme 5.36). 

Silylation of hydroxylamine or N-alkyl or N-ethoxycarbonyUiydroxylamines is usually accomphshed, in 52-84% yield, by silylation with TCS 14/NEt3 [63, 161, 162]. Whereas the reaction of N,0-bis(trimethylsilyl)methylhydroxylamine 952 with aldehydes such as benzaldehyde, or with ketones, with to adducts such as 953, has already been mentioned at the beginning of Section 7.3 thermal and other reactions of N,0-bis(trimethylsilyl)hydroxylamine 1141 or N-substituted N,0-bis(trimethylsi-lyl)hydroxylamines 1121, 1128, 1131 are discussed in this section. 

The N-bis-silylated o-phenylenediamine 1511 reacts with DMF at 120°C to give benzimidazole, in 97% yield, and dimethylamine and hexamethyldisiloxane 7, whereas reaction of benzaldehyde with 1511 gives only 29% 2-phenylbenzimida-zole 1513, because the intermediate benzimidazoline 1512 is only rather slowly dehydrogenated to 1513 [52]. Heating of N,N -bis(trimethylsilyl)ethylenediamine 1514 with DMF affords imidazoline 1515 and dimethylamine and HMDSO 7 ]52] (Scheme 9.32). The lactam 1516 cycHzes analogously with SiCU 57/triethylamine in 63% yield to give 1517 ]53]. 

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. 

We also reported that CpFe(CO)2Me acts as a precursor for the Si-O-Si bond formation reaction from hydrosUane and DMF (Scheme 51)[ 166,167]. In this reaction, tertiary silanes and bis(silyl) compounds are converted into the corresponding disUox-anes and the polymers with (-R-Si-O- i)n backbone, respectively. 

The reaction of CpFe(CO)2Me with R3SiH gives the bis(silyl)hydride complex 21. Photoreaction of 21 in DMF afforded the corresponding disiloxane (Scheme 52). We believe that the oxygen in the disiloxane is derived from DMF, because NMes is concomitantly formed in this reaction. It is considered that the silyl species a, which is prepared via reductive elimination of RsSiH from 21 in situ, is the active species within the catalytic cycle. Therefore, the generation of a bis(silyl)hydride species is the dormant step. We are currently studying the details of the reaction mechanism. 

Mono- and bis(silyl)platinum(II) complexes are believed to play important catalytic roles in hydrosilylation, dehydrocoupling, and double silylation reactions with disilanes and hydrosilanes. A stable, mono(silyl)platinum(II) complex has been prepared by the oxidative addition reaction of the sterically hindered, primary arylsilane 2,6-Mes2C6H3SiH3 (Mes = 2,4,6-trimethylbenzene) to the platinum(O) species [Pt(PPr3)3] in hexane solution at room temperature.133 The colorless product m-[PLl 1(2,6-Mes2C6II3(11 )2Si)(PPr3)2] (21) was isolated as the OPPr3 adduct, and its... 

Thermolysis of 12 with frans-cinnamaldehyde afforded the insertion compound 19, formed through the di-insertion of two carbonyl ligands into the C—Si bond of 12. The reaction of 12 with fumaronitrile yielded the cyclization product 20. X-ray study revealed 20 to be a cyclization product which contains two types of disilyl moieties, imino and N,N-bis(silyl)amino, which are connected by a five-membered ring. 

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