Cleavage reactions thermolysis

Thermolysis of the t]-coordinated zirconium complex of thiophene Cp2Zr(SiMc3) (C4H3S) is a thienyl ring cleavage reaction (920M1646). 

In view of the proposed mechanism for this cleavage reaction, it was reasonable to expect that certain allylic ethers for which an elimination pathway existed (Scheme II) would also be susceptible to cleavage under thermolysis, acidolysis or a combination of both. 

The thermolysis of 1.2,6,7,8.9-hexafluoro-3.4-diazatricyclo[5.2.0.02,6]nona-3,8-diene (4) at ca. 450 C and reduced pressure gives, in a cleavage reaction, tetrafluoroethene (3) (52%) and the three possible isomers of tetrafluorobenzene in an isomerization reaction.113... 

It seems that the stabilizing effect of the double bond on the electronic charge on sulfur (in isobutenyl sulfide) overrides the normal C-S cleavage reaction and thio-phenol may be formed in an intramolecular process. In quinoline solution the thermolysis proceeds at lower temperatures to yield thiachromane and thiacouma-rane the role of the amine solvent has been discussed but lies outside the scope of this review. 

Heteroaromatic N-imines and IV-aminoazonium salts show a variety of reactivities, depending on the nature of the heteroaromatic ring and the substituents on the imino or amino nitrogen. The most important types of the reactions are (i) reactions with electrophiles at the imino or amino nitrogen, (ii) reactions with nucleophiles on the heteroaromatic ring, (iii) 1,3-dipolar cycloaddition, (in) 671-electrocyclic reaction of 1,5-dipoles (mainly thermal reaction), (n) 47t-electrocyclic reaction (mainly photochemical reaction), and (vi) N—N bond cleavage (by thermolysis, photolysis, oxidation, and reduction). 

The kinetics of thermolysis of 2,5-diaryltetrazoles (205) have been investigated by Hong and Baldwin428 445 in order to estimate the sensitivity of 1,3-dipolar cycloadditions to the electronic characteristics of the nitrilimine rather than the steric characteristics, which often tend to dominate such cycloadditions. For the species 205 the steric factors are constant, and the reverse of the cleavage reaction was considered to be analogous to the 1,3-cycloadditions with symmetrical olefins. A Hammett p value of +1.16 was obtained for the compounds 205 (X = H) and a value of —0.23 for the series 205 (Y = H). The p values... 

In contrast to the well-defined photochemical behavior of 1-azirines the thermal reactions of these compounds have been studied less thoroughly (68TL3499). The products formed on photolysis of azirines can best be rationalized in terms of an equilibration of the heterocyclic ring with a transient vinylnitrene. Thus, products formed from the thermolysis of azirines are generally consistent with C—N cleavage. For example, the vinylnitrene generated from the thermolysis of azirine (149) can be trapped with phosphines (72CCS6S). 

Attempts have been made to catalyze the arrangement of 3-oxaquadricyclane to oxepins with transition-metal complexes.1 32 1 35 When dimethyl 2,4-dimethyl-3-oxaquadricyclane-l,5-dicarboxylate is treated with bis(benzonitrile)dichloroplatinum(II) or dicarbonylrhodium chloride dimer, an oxepin with a substitution pattern different from that following thermolysis is obtained as the main product. Instead of dimethyl 2,7-dimethyloxepin-4,5-dicarboxylate, the product of the thermal isomerization, dimethyl 2,5-dimethyloxepin-3,4-dicarboxylate (12), is formed due to the cleavage of a C O bond. This transition metal catalyzed cleavage accounts also for the formation of a 6-hydroxyfulvene [(cyclopentadienylidene)methanol] derivative (10-15%) and a substituted phenol (2-6%) as minor products.135 The proportion of reaction products is dependent on solvent, catalyst, and temperature. 

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