3-Silyl elimination reactions

Simple cyclobutanes do not readily undergo such reactions, but cyclobutenes do. Ben-zocyclobutene derivatives tend to open to give extremely reactive dienes, namely ortho-c]uin(xlimethanes (examples of syntheses see on p. 280, 281, and 297). Benzocyclobutenes and related compounds are obtained by high-temperature elimination reactions of bicyclic benzene derivatives such as 3-isochromanone (C.W. Spangler, 1973, 1976, 1977), or more conveniently in the laboratory, by Diels-Alder reactions (R.P. Thummel, 1974) or by cycliza-tions of silylated acetylenes with 1,5-hexadiynes in the presence of (cyclopentadienyl)dicarbo-nylcobalt (W.G, Aalbersberg, 1975 R.P. Thummel, 1980). 

Salt Elimination Reaction of an Alkali Metal Silyl with a Transition Metal Halide... 

This section deals with reactions that correspond to Pathway C, defined earlier (p. 64), that lead to formation of alkenes. The reactions discussed include those of phosphorus-stabilized nucleophiles (Wittig and related reactions), a a-silyl (Peterson reaction) and a-sulfonyl (Julia olefination) with aldehydes and ketones. These important rections can be used to convert a carbonyl group to an alkene by reaction with a carbon nucleophile. In each case, the addition step is followed by an elimination. 

These precursors are generally prepared by alkane elimination (Equation (7a)) or—especially useful with bulkier substituents—the coupling of metal chlorides with lithium pnictides or silyl arsines (Equation (7b)) or salt elimination or silyl halide elimination reactions (Equation (7c)) ... 

Evidently, this approach is not limited to the formation of nitronates, nitroso acetals or enoximes. The rearrangements of these compounds by elimination reactions, the trapping of intermediates and finally their reactions with various reagents are of equal importance. It should be emphasized that silylation of AN as a process in organic chemistry is characterized by an unrivalled completeness and diversity of transformations. Hence, the silylation can be considered as a separate field of application of AN in organic synthesis. 

Elimination Reactions with Silyl Nitronates and Ene Nitroso Acetals... 

Silylation of AN and cyclic nitronates affords SENA and BENA or cyclic N-siloxy-ene nitroso acetals as the major primary products (see Sections 3.2.1.3 and 3.5.1). All these relatively unstable derivatives can undergo various elimination reactions, which will be considered in separately. 

Elimination Reactions with Silyl Nitronates Most elimination reactions of SENA involve cleavage of the weak N-0 bond or cleavage of the O-Si bond. In the latter case, the reactions could occur with the participation of hyper-valent silicon in the transition state, that requires the presence of an external nucleophile. 

Triorganovinylsilanes can be used as silylating reagents. In this case, the vinyl moiety functions as a hydrogen acceptor. The Ru3(CO)i2-catalyzed reaction of 3-acetylthiophene with trimethylvinylsilane affords 3-acetyl-2-(tri-methylsilyl)thiophene in 64% yield, as shown in Scheme 15.148 This reaction involves a /3-silyl elimination, yielding a metal species. 

These P elimination reactions have been used in an olefine synthesis called the Peterson olefination reaction which is analogous (and sometimes superior) to the Wittig reaction. The Peterson olefination reaction involves the addition of an a-silyl carbanion to an aldehyde or ketone to give P-hydroxysilane, followed by P-elimination to give the olefine. 

Peterson Olefination Reaction (Peterson Elimination, Silyl-Wittig Reaction) The Reaction ... 

The most common routes to carbenes27 are also the major ones to generate silylcar-benes, namely dediazoniation of aliphatic diazo compounds and a-elimination reactions (Scheme 1). The extrusion of N2 from silyl-substituted diazo compounds can be achieved by UV-irradiation or thermally. The thermal decomposition, however, is of less importance since these diazo compounds are thermally much more stable than their nonsilylated counterparts, so that thermal impact may stimulate noncarbene pathways. 

Using the metalloradical reactivity of the Rh(II)OEP (OEP = 2,3,7,8,12,13,17,18-octaethylphorphynato) dimer, the preparation of silyl rhodium complexes was achieved by the hydrogen elimination reaction with silanes I R SiH (R = R = Et, Ph R = Me, R = Ph, OEt). The Rh—Si bond length of 2.32(1) A, found when R = Et, is comparable to those in other Rh(III) complexes (Table 11). The crystal packing indicates that all the ethyl groups on the porphyrin periphery are directed toward the silyl group. Consequently, the aromatic part of one complex molecule is in contact with the aromatic part of the next molecule and the aliphatic part is in contact with the aliphatic part of the next molecule204. 

Recently, a cyclization-elimination route to carbohydrate-based oxepines is proposed (Scheme 11). After silyl protection and hydroboration/oxidation, starting hept-l-enitols 31 give the protected heptan-l-itols 32. Swern oxidation of the latter followed by sequential acetal formation/cyclization provide methyl 2-deoxyseptanosides 33 that undergo elimination reactions to give the carbohydrate-based oxepines 34 <2005JOC3312>. 

Vinylsilanes are proper compounds which have won great appreciation as facile intermediates for many sorts of application. They are prepared either by addition reactions of acetylenes (vide supra) or by elimination reactions of saturated organosilanes. The main reactions, carried out with vinylsilanes, are addition reactions to obtain saturated organosilanes or electrophilic substitutions of the silyl group under Friedel-Crafts conditions where a rapid cleavage of the silyl moiety occurs (Scheme 8). 

The reaction illustrated in equation 24 involves a series of 1,3-alkyl, silyl and germyl migrations between the metal-silylene and germylene transients formed subsequent to an a-elimination reaction initiated by the elimination of CO109. 

In one group of reductive-elimination reactions, an HSiR3 molecule can be displaced from a silyl-substituted transition-metal hydride ML (H)(SiR3) by more efficient 7r-bonding ligands such as CO, PR3, C2H4, acetylenes or N2 which favor a lower oxidation state of M ... 

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