Flash vacuum thermolysis precursors

The formation of 2-(t-butyl)-2-trimethylsilyl-2//-thiopyran has recently been reported from 1-substituted 1,3-butadienes with /-BuCSSiMe3 (92MI1) (see Scheme 2). An unusual product with proposed structure 47b has been reported (84JOC5143) from benzothiopyrone 46 on reaction with sec-butyl lithium (46 - 47a — 47b, Scheme 3) on the basis of spectroscopic data. 6-Methylthio-2//-thiopyran was isolated after the flash vacuum thermolysis of more complex starting precursors (93TL2605). 

The thermolytic preparation by De Shong et al. (74) of azomethine ylides from aziridines and their intermolecular reactions are the first examples of singly stabilized ylides of this type. However, the protocol has been further extended to include intramolecular processes. Aziridines tethered to both activated and unactivated alkenes were subjected to flash vacuum thermolysis generating cycloadducts in moderate-to-excellent yields. While previously singly activated alkenes had furnished low material yields via an intermolecular process, the intramolecular analogue represents a major improvement. Typically, treatment of 222 under standard conditions led to the formation of 223 in 80% yield as a single cis isomer. Similarly, the cis precursor furnished adduct 224 in 52% yield, although as a 1 1 diastereomeric mixture (Scheme 3.77). 

Both the synthesis [38] and X-ray crystal structure [39] of dilithium pentalenediide 22 -2Li+ have been reported (Scheme 7, Figure 5). Reaction of the dihydropentalene 46 with n-butyllithium yielded the crystalline dilithium pentalenediide 22 -2Li+ (Scheme 7). The more recently reported flash vacuum thermolysis of 6-norbornenylfulvene (45), initially producing 6-ethenylfulvene, which immediately cyclizes to dihydropentalene (46), allows one to prepare this immediate precursor to 22 in gram quantities [40]. A similarly convenient... 

Short-lived species may be obtained selectively in the gas phase by either UV photolysis or flash vacuum thermolysis (FVT). However, both photolysis and FVT have their limitations. In order to generate transients by photolysis in situ in the absorption cell, the precursor should have absorption bands in the UV or visible region. By FVT, transients are produced outside the absorption cell. This method is thus limited to transients that are both thermally sufficiently stable, and whose lifetimes are not shorter than a few ms. Being generated in the gas phase, such transients may be either stabilized in a matrix at low temperature and characterized by IR spectroscopy, or observed in real time by high-resolution spectroscopic measurements. The latter techniques provide information about the lifetime, the gas-phase structure, and the rotational and vibrational energy patterns of the target species. 

Flash vacuum thermolysis with Ar-matrix isolation or matrix photolysis of 2-phenylbenzo-l,3-dioxan-4-one as well as other precursors gave the ketene 6-carbonylcyclohexa-2,4-dienone 112, as detected by the IR absorption at 2134 cm Flash vacuum thermolysis above 600 °C or matrix photolysis resulted in formation of the fulvenone 114, proposed to arise by decarbonylation of 112 and Wolff rearrangement. The use of C-labeled precursor gave 114 with scrambling revealing the intermediacy of bisketene intermediates 113 in the reaction (Scheme 4.21). 

Nitriles can be used as starting material for the bisdithioesters (62 n = 4, 5, lO), whereas the derivatives with shorter chain lengths (62 n = l-3), as well as methyl dithiocinnamate (63), are accessible from /VA -disubstituted thioamides via the corresponding imidothioate iodides. Methyl di-thioacrylate (65), the simplest unsaturated dithioester, was prepared by flash vacuum thermolysis of a precursor dithioester (64), which had been obtained from the corresponding nitrile (equation 64). It can be isolated as a red monomeric compound at 77 K but dimerizes rapidly at room temperature. The p,7-unsaturated isoprene derivative (66) and the methyl 2-arylpropanedithioates (67) were prepared from phenyl isothiocyanate and used as educts for syntheses of lavandulal and ar-curcumene. Also ethyl furane 3-carbodithioate (68), as prepared from the nitrile, has found application in syntheses of the natural products egona ketone and ar-turmerone. ... 

The sesquiterpenoids (228)—(234), previously isolated from Heterotheca species, have all been synthesized.121 Further work on the photoadduct (235) derived from methyl cyclobutenecarboxylate and (—)-piperitone has demonstrated the important synthetic utility of this compound.122 Flash vacuum pyrolysis of (235) at 500 °C gives (236)—(240). Additional thermolysis experiments showed that (237) and (238) are the precursors of (236) and that this compound undergoes a further... 

The first carbene ever isolated was la, which was prepared using the most classical route to transient carbenes, namely, the decomposition of diazo compounds. The [bis(diisopropylamino)phosphino](trimethylsilyl)diazomethane precursor (la) was obtained by treatment of the lithium salt of trimethylsilyldiazomethane with 1 equiv of bis(diisopropylamino)chlorophosphine. Dinitrogen elimination occurs by photolysis (300 nm) or thermolysis (250 °C under vacuum) affording carbene la as a red oily material in 80% yield (Scheme 8.1). Carbene la is stable for weeks at room temperature and can even be purified by flash distillation under vacuum (10-2 Torr) at 75-80 °C. 

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