Reactions retroene reaction

Matrix IR spectra of various silenes are important analytical features and allow detection of these intermediates in very complex reaction mixtures. Thus, the vibrational frequencies of Me2Si=CH2 were used in the study of the pyrolysis mechanism of allyltrimethylsilane [120] (Mal tsev et al., 1983). It was found that two pathways occur simultaneously for this reaction (Scheme 6). On the one hand, thermal destruction of the silane [120] results in formation of propylene and silene [117] (retroene reaction) on the other hand, homolytic cleavage of the Si—C bond leads to the generation of free allyl and trimethylsilyl radicals. While both the silene [117] and allyl radical [115] were stabilized and detected in the argon matrix, the radical SiMc3 was unstable under the pyrolysis conditions and decomposed to form low-molecular products. 

The stoichiometric interaction of an enyne and [RuCl(PCy3)(pcymene)]B(Ar )4 XVIIIa containing a bulky non-coordinating anion B(ArF)4 showed by NMR at —30 ° C the formation of the alkenyl alkylidene ruthenium complex and acrolein. This formation could be understood by the initial formation of a vinylidene intermediate and transfer of a hydride from the oxygen a-carbon atom to the electrophilic vinylidene carbon, as a retroene reaction step (Scheme 8.13) [54]. 

Barton and Burns106 obtained l-methyl-lH-l-silaindene (98) in moderate yield by FVP of 2-allyl-2-methyl-2-silaindane. The formation of this product was explained by the rearrangement of an initially formed l//-2-silaindene via a retroene reaction (Scheme 19). 

Retroenic reactions with participation of heterocycles 93S659. 

The preparation of this product is through the ene synthesis. Retroene reactions occur simultaneously by the reverse pathway and are discussed (1). 

The retroene reaction is one of the primary processes in the low-pressure pyrolysis of 4-dimethylsilyl-l-butene (equation 64)222. The products contain some of the expected dimer and much polymer, and radical processes are clearly important as well. 

Isomerization of alRenes. Exocyclic alkenes are rearranged in high yield to the more stable endocyclic isomers ((3-pinene a-pinene methylenecyclopentane 1-methylcyclopentene) in SO2. A reversible ene reaction followed by a retroene reaction has been suggested as a possible mechanism. ... 

The reverse of the ene reaction is the retroene reaction. Equation 11.40 shows the conversion of ds-l-methyl-2-vinylcyclopropane (90) to a s-1,4-hexadiene (91) by such a process. This reaction can also be described as a homo-[l,5] sigmatropic shift. 

The retroene reaction is, as the name implies, the reverse of an ene reaction. Show how the following reaction conforms to this name. Explain the cis stereochemistry in the product. 

It was obvious that 37 and the enyne generated a enyne metathesis catalytic species. Stoichiometric reactions in the presence of B(Ar )4 anion revealed by NMR the production of acrolein and the new alkenylcarbene 39, assumed to be generated by retroene reaction from the intermediate 38 (Scheme 26). 

The mechanistic study of this reaction has also shown that in the case of a substituted allyl group an exocyclic [l,3]-silatropic rearrangement is in competition Ea = 173-176 kJmoD ) with the endocyclic retroene reaction . Thus the yield of silole reaches at most about 40%. However, this method can be used for the synthesis of C-methylated siloles having a Si—H bond (see Section n.B.2.e). 

Semiempirical AMI calculations on the transition state for the retroene-type elimination of propene from allylamines reveal that the most favoured geometry resembles a half-chair or a flattened boat.66 In keeping with experimental observations, it is predicted that electron-donor substituents on nitrogen should promote the reaction since the negative charge on this atom decreases in the polar transition state. 

Usually the reaction occurs either on heating the vinylcyclopropane in the liquid phase in a sealed tube at 300°C for 30 min by passing it through a conditioned hot tube at 330°C with a contact time of 4s " or by flash thermolysis at 600°C for 10 However, careful choice of the pyrolytic conditions may be necessary. Thus, as shown in equation 130, thermolysis in lead conditioned Vycor glass at 580°C or in Pyrex glass at 400°C, leads either to the expected bicyclic ketone or to a retroene product. ... 

Thermolysis of A(-(2-propynyl)dihydroisoindole at 500 °C provides another synthesis of 2H-isoindole <84AG(E)517, 87CZ349). The product can be obtained from the condensed (— 196°C) pyr-olyzate and used as a solution. The reaction presumably is a retroene process (Scheme 73). 

Table 1.4 lists calculation data on activation and thermodynamic parameters as well as kinetic isotopic effects for three reactions of the retroene type. In the first stage, a search for the transition state structure was conducted and its compatibility with the demands of the Murrell-Laidler theorem verified. Afterwards the vibration frequencies of the reactants and the transition state structure were calculated whose values were used in the corresponding equations. Underestimation of the kinetic isotopic effect in the last two reactions is related to underestimation of the role of the tunnel mechanism (see Sect. 1.5). An exact reproduction of the values of kinetic isotopic effects is a more reliable check on the accuracy of the calculated transition state structures than that of the values of activation entropies. This is explained by the fact that the calculated values of normal vibration frequencies, corresponding to the negative force constants, are directly included into Eqs. (1.24)-(1.26) that determine the magnitude of the kinetic isotopic effect. 

As in the Diels-Alder reaction where the retroprocess contributed to the understanding of the reaction, attempts have been made to use the retroene to this advantage. Agami et have observed that the retroene process does not necessarily produce the same starting olefin. 

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