O-C-Silylation

OL-Silyl ketones, Reaction of a primary a-bromo ketone with LiN[Si(CH3)3]2 (1) followed by chlorotrimethylsilane or chlorotriethylsilane results in a trialkyl-silyloxyvinyl bromide (a), which is not isolated but treated with BuLi (2 equiv.) to effect O-C silyl migration (equation I). 

Acrylic acid a-anion equivalent. O —> C Silyl migration occurs on treatment of the title compound with r-BuLi in THF at —78°. The ensuing lithium enolate can be alkylated. Quenching with aldehydes generates Baylis-Hillman adducts. 

Enol silyl ethers of acylsilanes. On treatment with butyllithium and trimethyl-silyl chloride, these compounds undergo enol silylation, tellurium-lithium exchange, and O — C silyl migration. The lithium enolates are further silylated. 

These can be prepared in good yield by reaction of either the mono-anions of trimethylsilyl carboxylates or, preferably, the dianions of carboxylic acids with TMSCI. Acetic and propionic acids give mixtures of O- and C-silylated products. 

Acetates themselves normally give a mixture of O- and C-silylated products. However, using TBDMSC1 in the presence of HMPA (CAUTION —CANCER SUSPECT AGENT), pure ketene acetals of the type CH2=C(OR)OTBDMS can be obtained (6). 

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... 

Conversion of sulfones such as 1955 into their a-sulfonyl anions by treatment with n-BuIi at -78°C in THF then addition of bis(trimethylsilyl)peroxide (BTSP) 1949 afford, via intermediates such as 1956, aldehydes or ketones such as cyclohexanone and HMDSO 7 [146]. This reaction has subsequently been applied to the synthesis of aldehydes [147]. After hthiation with -BuLi thioethers such as phenyl benzyl sulfide 1957 react with BTSP 1949 to give mixtures of the O-silyl 1958 and C-silyl 1959 products [148]. On treatment with -BuLi at -30°C the a,a-bis-(trimethylsilyl)dimethylsulfide 1960 is, hkewise, converted into its anion, which reacts with 1949 to give the a-trimethylsilyloxy sulfide 1961 and MesSiOLi 98 [149] (Scheme 12.41). 

The insertion of the oxiranes into a P-O-C- bond of the mixed ester takes place in the reaction of the O-alkyl O-silyl phosphonates 3 with oxiranes in contrast to the reaction above. The formed 0-(2-siloxyethyl) O-alkyl phosphonates 11 show the typical PH-reactivity with the protected HO-group for further reactions. 

The isomerization of an O-silyl ketene acetal to a C-silyl ester is catalyzed by a cationic zirconocene—alkoxide complex [92], This catalysis was observed as a side reaction in the zirconocene-catalyzed Mukaiyama aldol reactions and has not yet found synthetic use. The solvent-free bis(triflate) [Cp2Zr(OTf)2] also catalyzes the reaction in nitromethane (no reaction in dichloromethane), but in this case there may be competitive catalysis by TMSOTf (cf. the above discussion of the catalysis of the Mukaiyama aldol reaction) [91] (Scheme 8.51). 

Fig. 7. Plots of the peak potentials (Ep, ) and decomposition potentials (Ed, ) for the oxidation of silyl substituted ethers in which rotation around Si-C bond is restricted vs. torsion angle of Si-C-O C [13]...

Problem 17.41 Acetone reacts with lda in thf and then with trimethylsilyl chloride, (CHj),SiCI, at -78 C, to give an enolsilane. (a) Give equations for the reactions, (b) Why does O- rather than C-silylation occur ... 

Now we will dwell on the problem of influence of chemical nature of substituents on the EPR spectra parameters of the silyl-type radicals. Table 7.6 illustrates the effect of chemical nature of substituents in radicals Sia C-O-C Sip/Ys/s (ASiaOSi/j = 180°, Y = H, F, OH, the cis- and transconfigurations of OH groups have been considered) on constants aiso(29Sia) and a so(29Si ). Their structures are presented in Figure 7.7. 

The selective C-silylation of some zinc enolates on treatment with chloro-trimethylsilane deserves mentioning because it is in clear contrast to the preponderant O-alkylation of other metal enolate reagents. Although this transformation is essentially confined to alkyl bromozincacetate and halozinc-... 

Since Scheme 4 implies formation of a-carbonyl radicals after deprotonation of enol radical cations, the same oxidation chemistry should potentially be accessible from various enol derivatives as enolates, silyl enol ethers and enol esters (Scheme 5). On the other hand, enol ether radical cations do not fit in this systematization since they are attacked by nucleophiles at the double bond faster than providing a-carbonyl radical intermediates through O-C bond cleavage (Sect. 4.3). 

Nelsen and coworkers [562] detected conformational equilibria in eq, eq- and ax, eq-N, A -disubstituted cyclic hydrazines from their oxidation potentials. The anodic oxidation reactions of trans- and cw-l,3-diisopropy-l-2,4-bis(diisopropylamino)-cyclodiphospha(III)azanes are quite different [563] The trans isomer is reversibly oxidized at 0.53 V (SCE) forming a stable cation radical the c/5-isomer undergoes a completely irreversible oxidation at a more positive potential because an unstable radical cation is formed. Evans and coworkers studied structural changes associated with electron transfer reactions of W(// -C5(CH3)5)(CH3)4 and related compounds [564,565]. Yoshida and coworkers found a linear correlation on plotting the oxidation potentials of a-silylated ethers, where the rotation around the C-0 bond is restricted, against the HOMO energy-torsion angle (Si-C-O-C) curve estimated by MO calculation [566]. 

Experimentally, 1,2-sllyl migrations with formation of double bonds between the migration origin and the cationic or neutral target have been found to take place intramolecularly, e.g., [1], o X -C- o X -C [2], o -C-+o -Si [3], o X -Sl-+o l -Sl [4], and intermolecularly, e.g., [5]. While in the latter three cases the silyl group shifted... 

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