Isomerism reactive species

As described above, high-resolution reaction time control enables switching product selectivity at will. A product derived from a reactive species that has yet to be isomerized can be obtained by setting a shorter reaction time, or a product derived from an isomerized reactive species can be obtained by setting a longer reaction time. 

Thiothionyl Fluoride and Difluorodisulfane. Thiothionyl fluoride [1686-09-9] S=SF2, and difluorodisulfane [13709-35-8] FSSF, are isomeric compounds which may be prepared as a mixture by the action of various metal fluorides on sulfur vapor or S2CI2 vapor. Chemically, the two isomers are very similar and extremely reactive. However, in the absence of catalytic agents and other reactive species, FSSF is stable for days at ordinary temperatures and S=SF2 may be heated to 250°C without significant decomposition (127). Physical properties of the two isomers are given in Table 6. The microwave spectmm of S=SF2 has been reported (130). 

There is some debate in the literature as to the actual mechanism of the Beirut reaction. It is not clear which of the electrophilic nitrogens of BFO is the site of nucleophilic attack or if the reactive species is the dinitroso compound 10. In the case of the unsubstituted benzofurazan oxide (R = H), the product is the same regardless of which nitrogen undergoes the initial condensation step. When R 7 H, the nucleophilic addition step determines the structure of the product and, in fact, isomeric mixtures of quinoxaline-1,4-dioxides are often observed. One report suggests that N-3 of the more stable tautomer is the site of nucleophilic attack in accord with observed reaction products. However, a later study concludes that the product distribution can be best rationalized by invoking the ortho-dinitrosobenzene form 10 as the reactive intermediate. 

An example of this has been reported for reactions of certain quinodimethanes.9 On its own, this reactive species dimerizes. The substance shown actually yields several dimers, but only the major one is shown in Eq. (3-82). When a dienophile like methyl methacrylate is present, a competing reaction, Eq. (3-81), occurs, where again one of several isomeric products is shown. 

The lack of any difference in the rate of isomerization between fluoro-sulfonic acid solutions of 34 which had been thoroughly degassed, and those which were saturated with oxygen, suggests that the reaction does not proceed via a triplet mechanism. In fluorosulfonic acid no unproton-ated acid is detected, ruling out the possibility of n,7r excitation. Thus, there is little doubt in this case that it is the Tr,Tr singlet state which is the reactive species. Experiments carried out with a variety of methyl-substituted protonated cydohexadienones have likewise ruled out the... 

Thermal insertion occurs at room temperature when R is XCH2CHAr-, at 40° C when R is benzyl, allyl, or crotyl (in this case two isomeric peroxides are formed), but not even at 80° C when R is a simple primary alkyl group. The insertion of O2 clearly involves prior dissociation of the Co—C bond to give more reactive species. The a-arylethyl complexes are known to decompose spontaneously into CoH and styrene derivatives (see Section B,l,f). Oxygen will presumably react with the hydride or Co(I) to give the hydroperoxide complex, which then adds to the styrene. The benzyl and allyl complexes appear to undergo homolytic fission to give Co(II) and free radicals (see Section B,l,a) in this case O2 would react first with the radicals. 

A much explored pathway to simple silenes involves the thermolysis of silacyclobutanes at 400-700°C, the original Gusel nikov-Flowers (155) route. Such temperatures are not readily conducive to the isolation and study of reactive species such as silenes except under special conditions, and flash thermolysis, or low pressure thermolysis, coupled with use of liquid nitrogen or argon traps has frequently been employed if study of the physical properties is desired. Under these high temperature conditions rearrangements of simple silenes to the isomeric silylenes have been observed which can lead to complications in the interpretation of results (53,65). Occasionally phenyl-substituted silacyclobutanes have been photolyzed at 254 nm to yield silenes (113) as has dimethylsilacyclobutane in the vapor phase (147 nm) (162). 

Allene ketene cycloadditions are of greater synthetic utility than cither mixed allene dimerization or mixed ketene dimerization. In this class of reaction the ketene is the more reactive species and homodimerization of ketene can be minimized by use of excess allene. Such cycloadditions always result in 2-alkylidenecyclobutanones with the sp carbons of both moieties forming the initial bond. In substituted allenes and ketenes, mixtures of stereoisomers of 2-alkylidenecyclobutanones are obtained with very little stereoselectivity, the stereoisomers arise from cisUrcins isomerism in the cyclobutane ring and EjZ isomerism of the exocyclic double bond. In unsymmetrically substituted allenes some regiochemical preference for ketene cycloaddition is observed. Examples of dimethylketene allene cycloadditions are summarized in Table 1,2... 

The generation of these reactive species was supported by trapping experiments and the dimerization of the silene to E/Z isomeric 1,3-disilacyclobutanes 264. The coproduct, 1,2-dimesityltetramethyldisilane 265, of the Me2S extrusion is also photolabile. Silene... 

The fundamental step in acid-catalyzed hydrocarbon conversion processes is the formation of the intermediate carbocations. Whereas all studies involving isomerization, cracking, and alkylation reactions under acidic conditions (Scheme 5.1) agree that a trivalent carbocation (carbenium ion) is the key intermediate, the mode of their formation of this reactive species from the neutral hydrocarbon remained controversial for many years. 

An overview of the reactions over zeolites and related materials employed in the fields of refining, petrochemistry, and commodity chemicals reviewed the role of carbocations in these reactions.15 An overview appeared of the discovery of reactive intermediates, including carbocations, and associated concepts in physical organic chemistry.16 The mechanisms of action of two families of carcinogens of botanical origin were reviewed.17 The flavanoids are converted to DNA-reactive species via an o-quinone, with subsequent isomerization to a quinone methide. Alkenylbenzenes such as safrole are activated to a-sulfatoxy esters, whose SnI ionization produces benzylic cations that alkylate DNA. A number of substrates (trifluoroacetates, mesylates, and triflates) known to undergo the SnI reaction in typical solvolysis solvents were studied in ionic liquids several lines of evidence indicate that they also react here via ionization to give carbocationic intermediates.18... 

In conclusion, the results of our study indicate that the principal features of the formation of poly (2,6-dioxo-l,4-piperidinediyl) trimethylene by thermal polymerization of / -carboxymethyl caprolactam consists in an initial isomerization of the caprolactam derivative to a reactive species and subsequent polymerization of the latter by condensation. The reactive intermediate is in all probability either or both the 3-(3-aminopropyl)-glutaranhydride or its linear dimer. Both the conversion of the lactam by isomerization and the polycondensation follow second-order kinetics. 

In lithium alkyl-initiated polymerizations only chain initiation and propagation steps need be considered in hydrocarbon solvents. Both reactions are strongly influenced by extensive association of all lithium compounds. The reactive species in chain propagation is the small amount of dissociated material which probably exists as an ion pair. Association phenomena disappear on adding small amounts of polar additives, and the aggregates are replaced by solvated ion pairs. In polar solvents of relatively high dielectric constant (e.g. tetrahydrofuran), some dissociation of the ion pairs to free ions occurs, and both species contribute to the propagation step. The polymerizations are often complicated in tetrahydrofuran by two side reactions, namely carbanion isomerization and reaction with the solvent. 

The high stereospecificity of the episulfide-producing reaction, by analogy with methylene chemistry, comes as no surprise if the reactive species are indeed singlet-state sulfur atoms. It was therefore expected that addition of CO2 would decrease the stercospecificity of the process. According to the data given in Table XI, however, CO2 has no effect on the isomeric distribution of episulfide and it appears that triplet sulfur addition is just as stereospecific a process as that of singlet-state atoms. This unexpected behavior was further confirmed by experiments in which... 

A similar amination is known in the sulfur series (equation 81). Here, the reactive species is generated in situ from (216). Deprotonation and reprotonation isomerize the vinylsulfimide (216) to an allylsulfimide which gives the amino compound (217). (217) exhibits the expected (c/. Scheme 25) ( )-configuration of the C=C bond. 

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