Trimethylsilyl-substituted compounds

The solvolysis of 252 is one of the rare examples for a norbornyl-norpinyl rearrangement. While the ew-trimethylsilyl brosylate 251 yields mainly substitution and elimination products 253-255 with an intact norbomyl framework, 252 gives nearly 86% of norpinene 256 (equation 39). The bis-(trimethylsilyl)substituted compound 257 gives almost exclusively the norpinene derivative 258 (equation 40). While the trimethylsi-lyl group(s) in 252 and 257 exert no kinetic effect on the reaction rate, the /3-effect on the intermediate carbocations 252A and 259, respectively, determines the product distribution99. 

Enhancement was measured for a wide variety of trimethylsilyl-substituted compounds (Table I). The Si-H coupling in all cases was 7 0.5 Hz, and enhancements were substantial ( d = 4.4-8.9). This suggests that INEPT or the equivalent DEPT sequence can be applied to virtually any trimethyl-silylated compound found in organic or organometallic chemistry using a single set of standard parameters (J = 7 Hz, n = 9 INEPT x = 36 msec, A = 15 msec DEPT t = 71 msec, 6 = 0.33 radians). 

The emphasis in the approaches to boron nitride [10043-11 -5] BN, precursors has been concentrated on cycHc compounds. There have been recent reports of trimethylsilyl-substituted aminoboranes being evaluated as B—N precursors. These are linear borylamines containing up to four boron atoms. Compounds were also synthesized with free —NH2 groups amenable to condensation with either dihaloboranes or dihaloborazines (65) and offering suitable monomers for linear B—N polymer synthesis and borazine-ring-linking appHcations. 

Several 4-alkoxy-2,3-diphenyl- and one 4-methoxy-2,3-bis(trimethylsilyl)-substituted cyclopentadienones have been isolated as reasonably stable compounds, see Herndon JW, Patel PP (1997) Tetrahedron Lett 38 59... 

A further example of the diazaphosphole synthesis by [3 + 2] cycloaddition is given by the reaction of a phosphoranediyl diazomethane (26) (R = NPr j) with P-chloro-bis(trimethylsilyl)-phosphaethene. The adduct loses trimethylchlorosilane and yields a 3-phosphoranediyl-l,2,4-diazaphosphole (or 3-phosphonio-l,2,4-diazaphospholide) (27). The analogous addition to the trimethylsilyl substituted P-chloro-bis(methylene)phosphorane (28) yields a 4-methylene derivative (29) of this diazaphosphole (molecular structure in Table 1). It provides the only fully characterized example of this type up to 1995. Methyl triflate methylates the compound at N-1 and gives a phosphonio methylene diazaphosphole cation (30). 

Through the in situ deprotection of A-acyl-2-(trimethylsilyl)ethynylanilines 465 followed by palladium-catalyzed cyclization-methoxycarbonylation, stereoisomeric 4-methoxycarbonylmethylene-3,l-benzoxazine derivatives 466 and 467 were obtained (Equation 51). The (Z)-isomers 466 were consistently found to be the main product, with the exception of the -(methoxycarbonyl)phenyl-substituted compounds (R = H, = C6H4C02Me(/>)), for which a... 

In the ESR spectra of some related trimethylsilyl benzoquinone derivatives, as with the ketyls, the spin density in the quinone ring increases for trimethylsilyl substitution and decreases for alkyl substitution, consistent with the electron-accepting ability of the trimethylsilyl group. This ability is also manifested in the reduction potentials of the compounds (65). The ESR data for the trimethylsily ketyls and for the trimethylsilyl benzoquinone anion radicals are summarized in Table IX. 

A range of enolate nucleophiles have been added to trimethylsilyl-substituted cyclohexadienyl complexes (49 Scheme 18) and (51 equation 49) and related compounds. Again, high yields are obtained and stereodirection by the Fe(CO)3 group is very powerful.13... 

Silicon bound to a phenyl group can also influence the bond system by additional (p- -d) back donation from carbon to silicon. In agreement with this model, p-trimethylsilyl-substituted benzoic acid shows a greater acidity than expected from inductive effects. Furthermore, p-trimethylsilyl phenol exhibits a greater acidity than phenol itself, and p-trimethylsilyl aniline shows a decreased basicity as compared with that of the nonsubstituted compound. This behaviour can be described by the following resonance structures [Eqs. (4) and (5)] ... 

Table 5 shows that alkyl groups such as methyl or f-butyl at the ring periphery cathod-ically shift the oxidation potential of the substituted ferrocene by ca 50 mV per alkyl group relative to the parent compound. This reflects the steady increase of electron density at the iron centre by successive ring substitution with electron-donating groups31. On the other hand, anodic shifts of the oxidation potential are observed for the trimethylsilyl substituted ferrocenes 17, 20 and 21 compared with those of the parent compounds 22... 

There are some reports in the literature concerning the cleavage of bonds between silicon and an sp2-hybridized carbon in silylcyclopentadienyl compounds of type 4 (Section II.B). For example, equation 4697 shows hydrolytic fission in the reaction of a tris(trimethylsilyl) substituted ferrocene derivative with aqueous tetrabutylammonium fluoride. 

Enantioselective enzymatic ester hydrolyses of prochiral trimethylsilyl-substituted diesters of the malonate type have been applied for the synthesis of the related optically active monoesters68. As an example of this particular type of biotransformation, the enantioselective conversion of the diester 82 is illustrated in Scheme 17. Hydrolysis of compound 82 in phosphate buffer, catalyzed by porcine liver esterase (PLE E.C. 3.1.1.1) or horse liver acetonic powder (HLAP), gave the optically active monoester 83 (absolute configuration not reported) in 86% and 49% yield, respectively. The enantiomeric purities... 

The behavior of the trimethylsilyl-substituted cyclopentadienylthallium(I) compound Llllk in solution differs from that of the so far described thallium complexes. The air-stable species LHIk is fairly soluble even in aromatic solvents and behaves as a covalent monomer. Furthermore, spin-spin coupling between the 205T1 nucleus and the ring proton nuclei and the13C and H nuclei of the trimethylsilyl groups has been observed (175). A halfsandwich structure in solution and in the gas phase has been suggested for this 7i complex. 

The y-effect of silicon is implicated in the silicon-promoted Nef reaction.133 The Nef reaction of 5-nitrobicyclo[2.2.1]hept-2-ene 156 yields none of the expected ketone 157 (Scheme 24), whereas Nef reaction of the corresponding trimethylsilyl-substituted nitro compound 158 results in the formation of high yields of the ketone 159. The silicon is believed to promote the breakdown of the hydrated intermediate 160 by stabilizing the y-carbenium ion 161 (Scheme 25). 

Dialkoxymethyllithiums 43 3649 and 434649 651, 2-lithio-l,3-dioxolane 435649 and 2-lithio-1,3-dioxane 436649 are formyl anion equivalents, which have been prepared either by reductive lithiation of 2-(phenylsulfanyl) substituted precursors at —95 °C649 or by trans-metallation of 2-(tri-n-butylstannyl) substituted compounds at —110649 or —78°C651. The starting acyclic phenylsulfanyl precursors can be prepared from the corresponding orthoformates by reaction with (phenylsulfanyl)trimethylsilane and trimethylsilyl triflate as catalyst (for compounds 433 and 434). The cyclic derivatives (435, 436) were prepared from l,2-bis(l,3-dioxolan-2-yloxy)ethane and propane, in the same way649. 

For example, in attempts to realize benzannelation reactions, alkyloxy aryl carbene complexes of manganese failed to react with alkynes even in refluxing toluene, and the starting compounds could be recovered [4]. The documented low reactivity of the Mn as opposed to Cr and Mo carbene complexes may in part explain why the electrophilic carbene C-atom and the nucleophilic diazo C-atom tolerate each other in the same molecule. Besides, the bulky substituents at the silicon atom protect it fi"om being attacked by nucleophiles leading to desilylation as reported for trimethylsilyl substituted Cr carbene complexes [5]. 

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