O-Silylations

Alkyl halides and sulfonates are the most frequently used alkylating acceptor synthons. The carbonyl group is used as the classical a -synthon. O-Silylated hemithioacetals (T.H. Chan, 1976) and fomic acid orthoesters are examples for less common a -synthons. In most synthetic reactions carbon atoms with a partial positive charge (= positively polarized carbon) are involved. More reactive, "free carbocations as occurring in Friedel-Crafts type alkylations and acylations are of comparably limited synthetic value, because they tend to react non-selectively. 

Treatment of O-silyl enols with silver oxide leads to radical coupling via silver enolates. If the carbon atom bears no substituents, two such r -synthons recombine to symmetrical 1,4-dicarbonyl compounds in good vield (Y. Ito, 1975). 

Another variation of the Madelung synthesis involves use of an O-alkyl or O-silyl imidate as the C2 electrophile. The mechanistic advantage of this modification stems from avoiding competing N-deprotonation, which presumably reduces the electrophilicity of the amide group under the classical conditions. Examples of this approach to date appear to have been limited to reactants with a EW substituent at the o-alkyl group[15,16]. 

StericaHy hindered silyl ethers such as ferZ-hutyl dimethyl silyl, / fZ-butyldiphenylsilyl, and tricyclohexylsilyl have been proposed as alternatives to trityl ethers. Reaction of sucrose with 3.5 molar equivalents of ferZ-hutyl dimethyl silyl chloride produces the 6,1/6 -tri-O-silyl derivative in good yield (27). 

Alkylation of pyrazinones and quinoxalinones may be carried out under a variety of conditions and it is usually observed that while O-alkylation may occur under conditions of kinetic control, to yield the corresponding alkoxypyrazines or alkoxyquinoxalines, under thermodynamic control the A-alkylated products are formed. Alkylation using trialkyl-oxonium fluoroborate results in exclusive O-alkylation, and silylation under a variety of conditions (75MI21400) yields specifically the O-silylated products. Alkylation with methyl iodide or dimethyl sulfate invariably leads to A-methylation. 

Alkyldithio carbamates are prepared from the acid chloride (Et N, EtOAc, 0°) and amino acid, either free or as the O-silyl derivatives (70-88% yield). The N- i-propyldithio) carbamate has been used in the protection of proline during peptide synthesis. Alkyldithio carbamates can be cleaved with thiols, NaOH, Ph P/TsOH. They are stable to acid. Cleavage rates are a function of the size of the alkyl group as illustrated in the table below. 

A modified procedure, which uses trimethylsilyl chloride as an additional reagent, gives higher yields of acyloins and is named after Riihlmann. In the presence of trimethylsilyl chloride, the Zjiis-O-silylated endiol 7 is formed and can be isolated. Treatment of 7 with aqueous acid leads to the corresponding acyloin 2 ... 

Condensation with activated chloroaromatics with O-silylated bisphenols in NMP in presence of K2CO3 has also been reported by Kricheldorf and Jahnke [103]. The... 

In addition to the boron trifluoride-diethyl ether complex, chlorotrimcthylsilanc also shows a rate accelerating effect on cuprate addition reactions this effect emerges only if tetrahydrofuran is used as the reaction solvent. No significant difference in rate and diastereoselectivity is observed in diethyl ether as reaction solvent when addition of the cuprate, prepared from butyllithium and copper(I) bromide-dimethylsulfide complex, is performed in the presence or absence of chlorotrimethylsilane17. If, however, the reaction is performed in tetrahydrofuran, the reaction rate is accelerated in the presence of chlorotrimethylsilane and the diastereofacial selectivity increases to a ratio of 88 12 17. In contrast to the reaction in diethyl ether, the O-silylated product is predominantly formed in tetrahydrofuran. The alcohol product is only formed to a low extent and showed a diastereomeric ratio of 55 45, which is similar to the result obtained in the absence of chlorotrimethylsilane. This discrepancy indicates that the selective pathway leading to the O-silylated product is totally different and several times faster than the unselective pathway" which leads to the unsilylated alcohol adduct. A slight further increase in the Cram selectivity was achieved when 18-crown-6 was used in order to increase the steric bulk of the reagent. 

Diisopropylamino(dimethyl)silyl]-2-propenyl]lithium adds to aromatic and x-branched aldehydes in the presence of anhydrous zinc chloride with essentially complete anti stereoselectiv-ity3s. as expected from the chair-like pericyclic transition state formed by the ( -intermediate. The addition products are not isolated, but after O-silylation, oxidative desilylation with retention of configuration forms the rmft-diols. 

R)- and (,S )-1.1,2-Triphenyl-l,2-ethancdiol which are reliable and useful chiral auxiliary groups (see Section 1.3.4.2.2.3.) also perform ami-sclcctive aldol additions with remarkable induced stereoselectivity72. The (/7)-diastercomer, readily available from (7 )-methyl mandelate (2-hy-droxy-2-phcnylaeetate) and phenylmagnesium bromide in a 71 % yield, is esterified to give the chiral propanoate which is converted into the O-silyl protected ester by deprotonation, silylation, and subsequent hydrolysis. When the protected ester is deprotonated with lithium cyclohexyliso-propylamide, transmetalated by the addition of dichloro(dicyclopentadienyl)zirconium, and finally reacted with aldehydes, predominantly twm -diastereomers 15 result. For different aldehydes, the ratio of 15 to the total amount of the syn-diastereomers is between 88 12 and 98 2 while the chemical yields are 71 -90%. Furthermore, high induced stereoselectivity is obtained the diastereomeric ratios of ami-15/anti-16 arc between 95 5 and >98 2. 

A mixture of 40 mmol or the a-nitro alcohol 1, 48 mmol of (terf-butyldimethyl)chlorosilane or chlorotri-mcthylsilanc, 6.8 g (100 mmol) of I /(-imidazole and 10 mL of DMF is stirred at 20 C for 12 h. Water is added and the mixtnre is extracted with hexane. The combined extract is washed wilh water and sat. aq NaCl and dried over MgS04. Exaporation gives the O-silyl nitro alcohol as an oil which is distilled (bulb-to-bulb) yield 45 - 97%. If unreacled starting material is present in the distillate, it can be removed by chromatography over silica gel (30 g/ l g of product) with l,t,0/pcntanc 1 9 as eluant. 

On the other hand, it has recently been reported (7) that phenyl acetate forms the derived ketene trimethylsilyl acetal by exclusive O-silylation. 

Silyl enol ethers, 23, 77, 99-117,128 Silyl enolates, 77 Silyl peroxides, 57 Silyl triflate, 94 Silyl vinyl lithium, 11 (E)-l -Silylalk-1 -enes, 8 Silylalumimum, 8 Silylation, 94 reductive, 26 a-C-Silylation, 113 O-Silylation.99,100 / -SilyIketone, 54 non-cydic, 55 Silylmagnesium, 8 Silyloxydienes, 112 Sodium hexamethyldisilazide, 89 Sodium thiosulphate pentahydrate, 59 Stannylation, see Hydrostannylation Stannylethene, 11 (Z)-Stilbene, 70 (E)-Stilbene oxide, 70 /3-Styryltrimethylsilane, 141 Swern oxidation. 84,88... 

The reactivity of epoxides can be modified by various proximal functionality. For example, 2,3-epoxy sulfides 118 are converted to the corresponding TMS-thiiranium species 119 upon treatment with TMS triflate. This intermediate reacts with O-silyl amides regiospecifically to form l-substituted-3-hydroxy-2-thioethers (e.g., 120). Simple primary amines undergo polyalkylation, but imines can be used as an indirect amine equivalent <96TET3609>. 

The unsaturated oximes 224 (see Table 21) were readily prepared by AT-alkyl-ation of allyl amines with a-bromoketones or O-silyl-a-bromoaldoximes. Heating the oximes 224 in toluene under an argon atm at 110 -180 °C smoothly led to isoxazolidines 225 in good yields with cis ring junction stereochemistry. Even when three stereocenters were generated, as in 225 g-1, a single stereoisomer... 

Most of the other silylation-activation-substitution reactions reported in this review are mechanistically related. Several new reactions (such as those discussed in Sections 7.1, 7.2, and 7.4) have been discovered by following these hnes of thinking about activation of functional groups by O-silylation and subsequent or concomitant reaction with nucleophiles giving the expected products and hexamethyldisiloxane 7. It can thus be expected that current and new silylation-activation reactions will be more commonly used in preparative chemistry in the future. 

Reaction of tert-butyldimethylsilanol 85 a or tert-butyldiphenylsilanol 85 b, which are obtained on cleavage of O-silyl compounds, with SOCI2 in CHCI3, affords the desired re-usable chlorosilanes 86a and 86b in 39 and 81% yield, respectively [109] (Scheme 2.16). 

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). 

Succinimide is readily silylated by HMDS 2 to the N-silylated product 201, which seems, however, to be in equilibrium with the O-silylated derivative 202 a (cf the closely related reactive center in persilylated uridine 3) and reacts after 6-10 days at 24 °C with one equivalent of primary or secondary amines such as morpholine to give the crystalline colorless cyclic acylamidine 203 and HMDSO 7, even in the absence of any protective gas [33] (Scheme 4.12). The reaction is much faster on heating to 120 °C under argon. At these temperatures 201 and 202 a, and possibly also the acylamidine 203, are apparently partially O-silylated by HMDS 2 to the very sensitive 2,5-bis(trimethylsilyloxy)pyrrole 202b or to 2-tri-... 

The condensation of arylsulfonyl acetonitriles 369a-c with 22a proceeds via addition of the in-situ formed anion 370 to the arylsulfonyl acetonitriles 369 to afford the dimers 371, in 69-94% yield, and hexamethyldisiloxane 7 [136]. Furthermore, y9-dicarbonyl compounds such as ethyl acetoacetate 372 a or ethyl benzoyl-acetate 372b are O-silylated by 22 a or 22 c to rather stable alkyl 3-O-trimethylsilyl-oxycrotonoate 373a and alkyl 3-0-trimethylsilyloxy-3-phenyl acrylate 373b [130]. Aliphatic nitro compounds such as nitromethane are O-trimethylsilylated and further transformed into oligomers [132] (cf Section 7.6) and are thus unsuitable reactants for silylation-C-substitutions (Scheme 4.50). 

AU these results indicate that silylated amides and, in particular, silylated lactams such as 388 will react with methyl or ethyl cyanoacetate or malonate and malodinitrile in the presence of HMDS 2 (to convert the leaving group MeaSiOH 4 into HMDSO 7) via the O-silylated forms such as 384b or 389 to give similar products such as 385 and HMDSO 7 (Scheme 4.54). 

Of special preparative interest is 2,3-bis(trimethylsilyloxy)trimethylsilylpropane 408, readily obtained from allyltrimethylsilane 82 by hydroxylation and subsequent O-silylation, as a new means of protecting carbonyl groups. The silylated glycol... 

Additions of aryl- or alkyllithium reagents to N-silylated formamides 508 give the imines 509 in 55-80% yield [90, 91] some of these imines can subsequently be converted into the corresponding //-lactams by reaction with enolates of alkyl butyrates [92]. Conversion of N-silylated butyrolactam 388 into cyclic Schiff bases such as 390, by reaction with methyl- or butyllithium, via O-silylated butyrolactam 389, is discussed in Section 4.8 (Scheme 5.28). 

The diacetal 629, prepared from the carbonyl compound and O-silylated allylic alcohols in the presence of TMSOTf 20, reacts with ( )-l-trimethylsilyl-2,4-penta-diene 630, in the presence of TMSOTf 20 in CH2CI2 at -78°C, to afford 60% 631 this undergoes Diels-Alder-cyclization at 170 °C in toluene to give a substituted... 

Treatment of ninhydrin 431 with excess allyltrimethylsilane 82 and triflic acid in acetonitrile affords dehydrated ninhydrin 660 in 55% yield and the O-silylated product 661 in 18% yield [43] (Scheme 5.59). 

Nitrobenzene reacts with the O-trimethylsilyl ketene acetal 663 in the presence of tris(dimethylamino)sulfur(trimefhylsilyl)difluoride (Me2N)3S(Me3SiF2) (TASF) to give the O-silylated adduct 1007 a, which can be oxidized in situ, e. g. by bromine, to give the 4-substituted nitrobenzene 1008 in an overall yield of 79% [87] (Scheme 7.28). With less hindered ketene-acetals, however, mixtures of ortho- and para-substituted nitrobenzenes are obtained. Yet, on reaction of 4-fluoronitroben-zene with the cyclic O-trimethylsilyl ketene acetal 1009 the ortho-substitution product 1010 is obtained in 79% yield [87]. 

Aliphatic nitro compounds 1035 with an a-hydrogen atom are readily O-silylated by N-trimethylsilyl-N,N -diphenylurea 23b to give a mixture of 1036 or 1037 [99]. This silylation works in particularly well if Ri and or R2 are activating nitro or car-... 

On treatment of trialkylsilyl nitronates 1043 with MeLi, LiBr, or BuLi in THF the resulting nitrile oxide intermediates 1044 afford, in dilute THF solution (R=Me) the ketoximes 1045 in ca 50-60% yield, whereas in concentrated THF solution the O-silylated hydroxamic acids 1046 are obtained as major products [144] (Scheme 7.35). Analogously, the silyl nitronate 1047 reacts with the 2,3,4,6-tetra-O-acetyl-/ -D-glucopyranosyl thiol/triethylamine mixture to afford, via the thiohydroxi-mate 1048, in high yield, a mixture of oximes 1049 which are intermediates in the synthesis of glucosinolate [145] (Scheme 7.35). 

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