Allene from pyrolysis

The formation of propyne and allene by pyrolysis of cyclopropene arises from opposite [1,2]H shifts in diradicals 191 or 192 The substantially larger activation energy (by some 24.5 kJ mol" ) for formation of allene reflects differences in the transition state structures for the two processes. Thus, the propyne-forming reaction requires the migrating hydrogen atom to span a single bond (see 194), whilst in the allene-forming process a double bond is involved and a more strained situation ensues (see 195). The formation of but-2-yne from 3-methylcyclopropene is similarly rationalized but the... 

A mixture of o-menthanes resulted from reduction of o-cymene with Ca(NH3)6,297 and a neat route to the o-menthane skeleton involved photochemical addition of allene to 3-methylcyclohex-2-en-l-one and opening of the cyclobutyl ring with BF3.298 O-Menthadi- and tri-enes resulted from pyrolysis of verbenene.299 Acid-and base-promoted isomerizations of o-menthadienes300 and dehydration of cis- and trans- o-menthan-8-ols with a variety of reagents have been recorded.301 Optically active 2-methyl-4-isopropenylcyclohexanone (which is a useful precursor for ra-menthane derivatives) has been prepared by pyrolysis of chiral 2,2,5-trimethyl-bicyclo[3.1.1 ]heptan-2-one.302... 

Beginning with allene (1) itself, its thermal dimerization to 1,2-bismethylenecy-clobutanes (126) constitutes one of the oldest allene reactions known. Although the yield is only moderate (in the region of 30%, depending on the pyrolysis conditions), the dimerization generates a very useful diene from a readily available starting material (Scheme 5.43) [117]. 

The comparatively low yield of the initial dimerization of allene is also caused by further addition of 1 to 126 and other allene oligomers produced subsequently in the pyrolysis. A reinvestigation of the reaction has revealed that not only are new tetramers such as 287 and 288 formed in the reaction, but also numerous hexamers such as 289-292, the latter certainly not giving an indication that it originates from 1 [119]. Since some of these products still contain conjugated diene subunits - see, e.g., 291 - further growth appears likely tert-butylallene behaves similarly [120]. 

The analogous transformation of 125, also realized by flash vacuum pyrolysis, gave rise to allenic oximes 126 [165], which are not directly accessible by the classical route starting from allenyl ketones and hydroxylamine (see Section 7.3.2) [122], Because compounds 125 are prepared from allenyl ketones and furan by [4 + 2]-cycloaddition followed by treatment with hydroxylamine, the retro-Diels-Alder reaction 125 —> 126 is in principle the removal of a protecting group (see also Scheme 7.46). 

Particularly good yields of the cydoadduct 329 are obtained if R1 = R2 = H is valid for the allenyl ketone 328 [165]. The Diels-Alder products 329 can undergo many chemical transformations, for example to the oximes 330, which yield the modified allenes 331 after a subsequent flash vacuum pyrolysis. The oximes 331 generated by retro-Diels-Alder reaction are not available from ketones 328 and hydroxylamine hydrochloride directly [122] (see also Scheme 7.19). 

The kinetics and mechanism of pyrrole pyrolysis were investigated by ab initio quantum-chemical calculations. It was revealed that pyrrole undergoes tautomerization to form 2H- and 37/-pyrroles prior to any thermal decomposition. It has been shown that the major product, HCN, arises from a hydrogen migration in pyrrole to form a cyclic carbene with the NH bond intact. Ring scission of the carbene leads to an allenic imine of HCN and propyne which is the lowest energy pathway. The 277-pyrrole... 

The total contents of saponifiable and free acids in small biological samples were determined by MacGee and Allen [137]. A sample of tissue (3—5 mg) or serum (5—100 /xl) was hydrolysed with a lye, acidified and extracted with /7-hexane. Free acids were extracted from the /7-hexane into a small volume (5—10 ql) of trimethyl-(a,a,a-trifluoro-/ 7-tolyl)ammonium hydroxide. The quaternary salt was injected together with methyl propionate, and methyl esters produced by the pyrolysis were subjected to GC analysis (injection port temperature 240°C, column temperature 180°C, 10% EGSS-X). Comparison of this method with other esterification methods in Table 5.9 shows that it offers at least equally good results. 

The kinetics and mechanisms of the gas phase pyrolysis of cyclopropene in the temperature range 193-243°C has been examined experimentally and theoretically The major and minor products of reaction are propyne and allene respectively propyne results from a unimolecular isomerization with an activation energy of 147.3 kJ mol Moreover, cyclopropene is most likely the important intermediate in the thermal isomerization of allene to propyne These observations are accommodated by the reactions of equation 65. The activation energy for the conversion of allene into cyclopropene is 269 kJmol" and that for the reverse process is 182 kJmol ... 

Numerous unsuccessful attempts to synthesize cyclopropanethione have been reported. Thermal or photochemical generation of the C3H4S species from different sources always leads to allene episulfide. Some representative experiments include (a) in vacuo pyrolysis of the sodium salt of 2,2,4,4-tetramethylthietanone tosylhydrazone (4) into the stable tetramethylallene episulfide (S), (b) pyrolytic extrusion of nitrogen from perfluorinated thiadiazoline 6, (c) in vacuo pyrolysis of spiro compound 8 into methylenethiirane (3), (d) the flash vacuum pyrolysis-microwave spectroscopic approach applied to spiro compounds 9 and 10, (e) pyrolysis of anthracene adduct 11 and tosylhydrazide salt 12, (f) thermolytic nitrogen extrusion from pyrazoline-4-thione 13, thermolysis of tetramethylallene episulfide (5) or pyrazoline 13 in dig-lyme solution, and photolytic nitrogen extrusion from pyrazoline 13, ° (g) thionation of methylenecyclopropanone 15, and (h) reaction of donor-acceptor substituted allenes 18 with elemental sulfur. ... 

There is no information in the literature concerning the existence or formation of 1,2,3-pentatriene. Since the amoimt of this imusual compound formed in the present recoil C systems is extremely limited, conventional methods for the confirmation of its identity are mostly inapplicable. However, some additional information has been obtained during a pyrolysis experiment. This compound, after individual trapping and separation from other products, was observed to imdergo a thermal conversion to CH=CCH==CHCH3, l-pentyne-3-ene. This is consistent with the expectation that allenic types of compounds will isomerize to acetylenic types of compounds upon pyrolysis, which lends further support to the conclusion that the unusual compound, 1,2,3-pentatriene, has been synthesized in this recoil C system (57). 

Extensive mechanistic studies of this cyclization reaction were carried out by Myers et al. and extended with theoretical work by Squire s et al. It is known that, in contrast to the Bergman cyclization of the ene-diyne (Chapter 4.2), this transformation proceeds as an exothermic process determined by the increased stability of a benzyl radical versus a phenyl radical. The barrier for cyclization from substrate to a diradical product is low and can further be reduced by an appropriate substitution at the allenic terminus of the substrate. The dichotomous (polar and free radical) reactivity is observed on pyrolysis in the presence of polar reactants. Both radical and polar products arise from a common intermediate, which is described as a polar diradical, a linear combination of limiting structure 7 and zwitterion 11. According to Squires, polar diradical singlet species are involved. Based on computational studies supported by experimental product distribution studies, it has been proposed that both the diradical 7 and... 

A thermal rearrangement of 7-oxabenzonorbornadienes mounted on a [3]polynorbornane bis-imide molecular rack has been shown to yield an oxepine using flash vacuum pyrolysis (13TL5335). Substituted oxepin-3(2Ji)-ones 67 have been shown to result from a tandem conjugate addition-alkylation-isomerization process between 1,2-allenic ketones 65 and ethyl 4-chloroacetoacetate 66 (13RA4156). 

Form Supplied in colorless liquid not commercially available. Analysis of Reagent Purity IR (tf) 2955,2900,1935,1250,1210, 1055,840,800,750, and 690 cm H NMR (60 MHz, CDCI3) 50.15 (s, 9H),4.27 (d, 2H, /= 7.7),4.88 (dd, IH,7= 6.6,7.7). Preparative Method two methods have been reported for the preparation of (trimethylsilyl)allene [(TMS)allene] (1). Reductive deoxygenation of the tosylhydrazone derivative affords the title compound in 51% yield. The tosylhydrazone is readily prepared from the corresponding aldehyde, which in turn is accessed by formylation of (trimethylsilyl)ethynylmagnesium bromide with DMF (eq 1). (TMS)allene has also been prepared by flash vacuum pyrolysis of methyl (trimethylsilyl)-propargyl ether, which is obtained from silylation of methyl propargyl ether (eq 2)7... 

Liu and coworkers have recently published the synthesis of (3) and (4), as well as several other fluorinated pyrrole analogues (not shown), by aminofluorination of allenes the synthetic approach is shown in Scheme 3 [15], This strategy takes advantage of a selective, silver-catalyzed intramolecular fluorination reaction, but the approach works best with substrates that possess electron-withdrawing R -groups on the 3-position. Because of this limitation, the yield of (3) is significantly higher than (4), 80 % versus 28 %, respectively. 4-Fluoro-pyrrole-2-carboxylic acid (5a, Fig. 2), synthesized from a fluorinated proline, was explored as a potential intermediate on the route to (3), but only extensive decomposition products were observed when (5a) was subject to flash pyrolysis [16]. 

The known tetrakis(trifluoromethyl)allene has been observed as a byproduct in the preparation of bis(trifluoromethyl)thioketen via pyrolysis of its dimer" at atmospheric pressure and l,l-dimethyI-3,3-bis(trifluoromethyl)-allene, apparently a new compound, has been synthesised from bis(tri-fluoromethyl)keten (see Scheme 15)." The new allenes CFa CFa-CClrCiCMea and CF2CbCF2-CCl C CMe2 have been obtained by dehydrochlorination of adducts prepared from isobutene (see Table 3, p. 12). 

Synthesis of pyrroles from ketones (through ketoximes) and propyne-allene mixture, a large-scale side product of hydrocarbons pyrolysis, essentially expands the preparative possibilities of the Trofimov reaction. 5-Methyl-substituted pyrroles, having diverse substituents in the positions 2 and 3, become readily available for the first time. 

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