Thermal a-elimination

Cycloalkyl(silyl)carbenes with an a-C—H bond have not yet been investigated systematically. When cyclohexyl(trimethylsilyl)carbene was generated by thermal a-elimination from cyclohexyl-bis(trimethylsilyl)methanol at 500 °C, only the (1,2) hydride shift took place, whereas cyclopentyl(trimethylsilyl)carbene, generated analogously, gave both the endocyclic alkene and the 1,3-C,H insertion product85,86 (equation 47). 

Thermal a-elimination (see a-Eliminatiori) of neopentane from the dimethylallyl complex, Cp W(NO)(CH2Bu0( -Me2C3H3), generates a reactive 16e ) -allene intermediate that activates hydrocarbon solvent C-H bonds to... 

Since photon energies are insufficient for simultaneous cleavage of two SiSi bonds to afford two silyl radicals and a silylene, Path C is an unlikely mechanism. The stepwise mechanism (Path A) could also be excluded because thermal a-elimination from pentamethyldisilanyl radical (i.e. equation 5) does not occur at room temperature. The concertedness of the photochemical silylene extrusion from a peralkyltrisilane is corroborated by the high stereoselectivity. Thus, irradiation of E-and Z-l,3-diphenyl-l,2,2,3-tetramethyl-l,2,3-trisilacycloheptane gave, respectively, E-and Z-l,2-diphenyl-l,2-dimethyl-l,2-disilacyclohexane, in a highly stereospecific manner, together with dimethylsilylene (equations 8 and 9). 

New pathways for the preparation of novel classes of stable heteroaromatic carbenes are proposed in [86] for the first time l,3-di-(l-adamantyl)benzimidazolin-2-ylidene (133) was obtained by thermal a-elimination of acetonitrile from l,3-di-(l-adamantyl)-2-cyanomethyl-2H-benzimidazoline in vacuum or in organic solvents. 

Enders et al. isolated the first triazol-5-ylidene 79 by thermal a-elimination of methanol from 5-methoxytriazoline in 1995 (see Scheme l.lOa)." 79 was stable up to a temperature of 150 °C. "C NMR spectroscopic [<5(C5)=214.6 ppm] and structural parameters such as the short Cca bene N bond lengths [1.351(3) A and 1.373(4) A] and the small N-Cearbene N bond angle [100.6(2)°] confirmed similar properties of 79 and the unsaturated imidazol-2-ylidenes 89. NHC 79 showed no tendency to dimerize to the entetraamine and thus also behaved ehemically like an unsaturated imidazol-2-ylidene. 

The nitrene can be generated by a variety of methods, the most popular being the thermal or photolytic decomposition of azidoformates. Other methods, particularly the base-catalyzed a-elimination of arylsulfonate ion from 7V-[(arylsulfonyl)oxy]urethanes, are useful as they avoid the use of the potentially explosive azido esters. 

Another important family of elimination reactions has as its common mechanistic feature cyclic TSs in which an intramolecular hydrogen transfer accompanies elimination to form a new carbon-carbon double bond. Scheme 6.20 depicts examples of these reaction types. These are thermally activated unimolecular reactions that normally do not involve acidic or basic catalysts. There is, however, a wide variation in the temperature at which elimination proceeds at a convenient rate. The cyclic TS dictates that elimination occurs with syn stereochemistry. At least in a formal sense, all the reactions can proceed by a concerted mechanism. The reactions, as a group, are often referred to as thermal syn eliminations. 

The second preparation for a free iminosilane was found by the Klinge-biel group. It is based on a fluorine/chlorine exchange of lithiated fluoro-silylamines with subsequent thermal LiCl elimination.17... 

Other indications for the generation of phosphasilenes involved compounds 520 and 6.21 Both derivatives were generated by thermally induced elimination of LiF/THF from corresponding lithium(fluorosilyl)phospha-nylides. Whereas 5 was directly observed by means of 3,P-NMR spectroscopy, surprisingly, the sterically more hindered phosphasilene 6 was not detected due to its lower thermostability. Therefore, evidence for the existence of 6 was achieved solely by trapping experiments. Due to the more crowded substitution in 5 compared with 2, the dimerization process to give 7 is relatively slow (Eq. 1) but is complete within a few hours. 

In order to overcome the difficulties of thermal instability, the phosphasilene derivatives 15, which bear a silyl or germyl group attached to phosphorus, were synthesized. Indeed, they proved to have stronger Si=P bonds (stable up to 100°C), thus allowing studies of their structures and reactivity.l0b 14 Phosphasilenes 15a-15i were synthesized from the corresponding Iithium(fluorosilyl)phosphanides 16a-16i by the thermally induced elimination of LiF (see Scheme 4).10b It has been shown that excellent steric protection of the highly reactive Si=P bond in 15 is provided by the 2,4,6-triisopropylphenyl (Is = isityl) substituent attached to the low-coordinate silicon center. The appropriate precursors 16a-16i were synthesized in a multiple-step procedure, starting from 17 (Scheme 4).10b U... 

Furthermore, as described by Mori and coworkers, the domino aldol/cyclization reaction of the 3-keto sulfoxide 2-422 with succindialdehyde (2-423) in the presence of piperidine at r.t. afforded the chromone 2-424 which, on heating to 140 °C, underwent a thermal syn-elimination of methanesulfenic acid to provide 2-426 in 22 % overall yield (Scheme 2.100) [227]. This approach was then used for the synthesis of the natural products coniochaetones A (2-425) and B (2-427) [228]. 

Finally, Scharf and Wolters report a method said to be superior to both the Paal-Knorr synthesis (starting materials more easily accessible) and the Feist-Benary synthesis (freer choice for 3-substituent). Thermal rearrangement-elimination by alkylated dioxolanes at 230 C gives alkyl substituted furans. Yields can be nearly quantitive since the only serious by-products also give the furans under proton-catalyzed thermolysis (Scheme 25).124 Photochemical methods are outlined in Section VII. 

Later we found that diperester 5 is a much better precursor for the preparation of 2.26 But the easiest way of generating 2 is the thermal HC1 elimination from 3-chlorocyclopropene (6).27,28 This reaction is astonishing and is probably due to the resonance stabilization of 2 in the singlet ground state, as the thermal elimination of HC1 (in the absence of a base) does not work with ordinary halides. It should be added that 2 has been detected in interstellar clouds and it is claimed that 2 may be one of the most abundant hydrocarbons in interstellar space. 

Organotellurium tri- and dihaUdes undergo a-elimination by oxidative, photolytic or thermal routes, giving the corresponding halides with a selective transference of the halogen at the position where the tellurium moiety was originally attached (/pxo-substitution). 

As a consequence of the conformational mobility of the thiepane 1-oxide ring (115) it was possible to form the necessary planar five-membered cyclic transition state for a thermal E elimination reaction (equation 19). The acyclic sulfenic acid intermediate was not isolated but rearranged to c/s-2-methylthiane 1-oxide by an intramolecular cyclization mechanism (75TL2235). 

Alkyl sulfoxides undergo thermal (3-elimination to yield alkenes. This strategy for the preparation of alkenes has also been applied to solid-phase synthesis, but only substrates with a high tendency to undergo elimination (e.g., y-oxo sulfoxides) could be thermally released from the support (Entry 5, Table 3.43). Unactivated sulfoxides could not be cleaved, not even under forcing conditions (199°C [767]). 

The elimination of hydrogen fluoride has also been found as a side reaction in the thermally induced elimination of fluorine (see Section 3.2.1.). 

Thermally induced elimination of fluorine occurs if polyfluorocycloalkanes, -cyclohexadienes or some heterocyclic perfluoro compounds arc passed over a nickel or iron gauze at 500-700cC (examples are summarized in Table 7). Within a certain time the catalytic activity of the metal decreases and it has to be regenerated by passing hydrogen over it at 300-600JC. The yields... 

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. 

Suitable candidates for a-elimination reactions are silylmethyl halides (— base-induced elimination of H-Hal), silylmethyl dihalides (— halide/metal exchange followed by elimination of a metal halide) and stable carbenoid-type compounds such as (a-halo-a-silylalkyl)mercury compounds (— thermal elimination of mercury(II) halide). Bis(phenylthio)(trimethylsilyl)methyl lithium (— elimination of LiSPh) represents a borderline case (see Section III.E.8). 

An anomalous X-ray scattering study of (+(-tubocurarine dibromide (73 4MeOH) confirmed the structure and absolute configuration earlier assigned. The N-N distance is 10.66 A (47,48). A purified sample of (+(-tubocurarine chloride (73a), prepared by selective quatemization of (+(-tubocurine (74) (Section V,B,1), gave the anomalous MS behavior previously ascribed to thermal disproportionation or the presence of impurities. In particular, the MS has mle 594 [17%, (M - 15)+, loss of Me] but also 608 (3.7%) and 622 (0.5%). The higher peaks seem to be due to a thermal Hofmann elimination followed by recombination with methyl radicals. Similar behavior is observed with the tertiary bases (—)-curine (75) and (+(-tubocurine (74), and may be general for alkaloids of this skeletal type (49). 

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