Retroene reactions

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. 

When running the condensation starting with 2-methyl-pentene-l and MA, they observed that a mixture of anhydrides 74a, 74b, and 74c was formed. 

In a kinetically controlled process at 140°C for 5 h only 74a and 74b are formed in a ratio of 73 27. Similar ratios have been observed with other ene 

160°C for 96 h produced an equilibrium mixture of anhydrides containing 60% 74a, 8% 74b, and 21% 74c. The olefin removed from the mixture also seemed to contain an equilibrium mixture of 2-methylpentene-l (27%) and 2-methylpentene-2 (73%). The effect of temperature on the unreacted olefin composition is shown in Table 5.7. It may be noted that in the absence of MA but using succinic anhydride, only 7% of the isomerized 2-methylpentene-1 was obtained. Thus, direct thermal isomerization of olefin plays an insignificant role. 

Eslami studied the reaction of oleic acid with MA. A maximum yield (80%) of a mixture of isomers was obtained when hydrochloric acid or phosphoric acid was employed as a catalyst. Phosphonic and phosphinic acids have also been claimed.Alkyl chlorides have been used as starting material.One may assume that olefin forms as an intermediate, however, the effect of generated HCl on the reaction is not clear. 

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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). 

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