Calculations , thermolysis

An example of the application of molecular mechanics in the investigation of chemical reactions is a study of the correlation between steric strain in a molecule and the ease of rupture of carbon-carbon bonds. For a series of hexasubstituted ethanes, it was found that there is a good correlation between the strain calculated by the molecular mechanics method and the rate of thermolysis. Some of the data are shown in Table 3.3. 

Azadienes 89, generated in situ by thermolysis of the corresponding o-aminobenzylalcohols, have been used for the derivatization of [60]-fullerene through C-N bond formation leading to tetrahydropyrido [60]-fullerenes [93]. Theoretical calculations predicted these cycloadditions to be HOMO azadiene-controlled (Equation 2.25). 

Using literature data, bystander assistance factors, B[Y], could be calculated for a number of substituents. For example, the thermolysis of certain ketone tosylhydrazone salts afforded dialkylcarbenes which gave competitive 1,2-H shifts, Eq. 23, where either Ha or Hb migrated. 

Thermolysis of (cycloheptatrienylmethyl)carbene complexes 554 [toluene, 1-2 h, 80-100°C MLn = Cr(CO)5, W(CO)5] affords an equilibrium mixture of 4,5-homotropilidenes 555 and 556. According to the NMR data and the results of AMI calculations, the formation of isomer 556 (equation 218) is strongly favored277. This course of events was called intramolecular cyclopropanation , and it was shown that the equilibrium between the 4,5-homotropilidene complexes is significantly different from that of the metal-free ligands. By reaction of the latter (555 and 556) with bis(ethylene)rhodium 1,3-pentanedionate 557, the complexes 558 and 559 of both 4,5-homotropilidenes were obtained in a 1 3 ratio. These complexes are non-fluxional and are configurationally stable at room temperature (equation 219)277. 

In connection with the captodative effect, Riichardt (Zamkanei et al., 1983) has determined the BDE of the tertiary C—H bond in [20] and compared it with the tertiary bond in isobutane. He concludes that the stabilization of 12.8 kcal mol which he derives from this comparison falls 4kcal mol short of the value of 16.5 kcal mol which he calculates for the sum of the substituent effects for phenyl (9 kcal mol ), cyano- (5.5 kcal moP ) and methoxyl (1.5kcal mol ) groups. The latter values were derived from studies on C—C BDEs. Not even additivity of the substituent effects is observed. The existence of a captodative stabilization of radical [21] is denied (see, however, the studies on the thermolysis of [24]). 

Katritzky (Katritzky et al., 1986) has recently advanced the idea that captodative-substituted radicals should be stabilized significantly by polar solvents. This hypothesis, which is qualitatively derived from the polar resonance structures for these radicals, was supported by semiempirical molecular orbital calculations. An experimental test was carried out by Beckhaus and Riichardt (1987). For the dissociation of [24] and [25] into the radicals [21] and [28], they were unable to confirm Katritzky s hypothesis. The rate of thermolysis of [24] and [25] is not affected by a change in solvent polarity. If the stabilization were of the order of Katritzky s prediction, it should, however, have become evident in the rate measurements. The experiments thus suggest that the contribution of polar resonance structures to the ground state of the radicals is not appreciable. See, however, the results obtained by Koch (1986) on the dl meso isomerization of [47]. 

An ab initio RHF/3-21 G study has shown that the decomposition of 3-hydroxy-3-methylbutan-2-one is a concerted process with hydrogen transfer and bond breaking via a five-membered cyclic transition state.AMI and PM3 methods using UHF calculations were applied to study the thermolysis of 2-cyanofuroxan. The reaction proceeds via a two-step pathway in which the second step is rate determining. The effect of solvent in the thermal decomposition reaction of fran -3,3-dimethyl-5,6-tetramethylene-l,2,4-trioxacyclohexane was studied. ... 

Thermochemical parameters estimated by semiempirical AMI calculations have been found to support the proposal that isobutene formation on gas-phase thermolysis of iV-methyl-A-phenyl-fert -butylsulfenamide and morpholinyl-ferf -butylsulfenamide occurs by a unimolecular mechanism involving a four-centre cyclic transition state and co-formation of the corresponding thiohydroxylamines." ... 

Polavatapu, et al. (128) have identified a VCD band near 12(X) cm in phenylcaibinols that correlates with configuration. Wieser and co-workers (129, 130) have recently reported FTIR-VCD spectra of chiral methyloxetan molecules. In the area of theoretical calculations, Lx>we, Stephens, and Segal (131, 132) have implemented the theory of Stephens (118) in calculations on trans-1,2,-dideuteriocyclopropane, tron5-l,2-dideuteriocyclobutane, and propylene oxide. For the latter two molecules, favorable agreement with experiment was found. The first application of VCD to kinetic analysis has also been reported for (1/ , 2R)-dideuteriocyclobutane thermolysis (133). 

In parallel with the ab initio calculations, also semiempirical smdies on the thermolysis of 1,2-dioxetane were performed. Most computations have been conducted by the PM3 method because it is the best semiempirical method for describing lone electron pairs on adjacent atoms . As an illustration, only the PM3 method reveals that in the dioxetane molecule the 0-0 bond is longer and weaker compared with the C—C one, as manifested by the computed values of bond lengths [rf(0—O) = 1.600 > d(C—C) = 1.522 A] and bond orders [n(0—O) = 0.973 < w(C—C) = 0.989] . In contrast, the AMI and MNDO semiempirical methods exhibit the opposite trends, i.e. AMI gives d 0—0) = 1.334 A, d(C-C) = 1.539 A, n(O-O) = 0.995 and n(C-C) = 0.976, whereas MNDO furnishes d(0-0) = 1.316 A, d(C-C) = 1.558 A, n(O-O) = 0.996 and n(C-C) = 0.9622 f-8. Nevertheless, despite the quantitative differences in the computed bond lengths, bond orders and bond angles, both the AMI and PM3 methods disclosed qualitatively similar reaction trajectories . 

The PM3 calculations of the So and the vertical (Franck-Condon) Ti energies as a function of the 0-0 bond length [<7(0-0)] have successfully reproduced the experimental activation energy for the dioxetane thermolysis . However, an unusual shape has been found for the energy profile A flat plateau, in which the ground-state energy... 

In summary, although the computed structural details of the reaction profile depend on the method used for calculations, the general salient mechanistic conclusion is that the dioxetane thermolysis starts with the 0—0 bond rupture to generate the 0C(H2)—C(H2)0 triplet diradical, which is followed by C—C bond cleavage to afford the final ketone products one of them is formed preferentially in its triplet excited state. Since even simple 1,2-dioxetanes still present a computational challenge to resolve the controversial thermolysis mechanism, the theoretical elucidation of complex dioxetanes constitutes to date a formidable task. 

Cycloheptanes.— The C-1—C-2 bond in -y-thujaplicin is essentially single, Co"-/3-thujaplicin-amine complexes have been described, and thermodynamic data on the U -/3-thujaplicin complex have been calculated. The biomimetic cyclization of the silyl enol ether (191) to karahanaenone (192), using methyl-aluminium bis(trifluoroacetate) is almost quantitative (192) is also synthesized by thermolysis followed by desilylation of the silyl enol ether (193) which is readily available from l-bromo-2-methyl-2-vinylcyclopropane and isobutyraldehyde. Dehalogenation of 3-bromo-l-iodo-3-methylbutan-2-one with Zn-Cu couple on alumina in the presence of isoprene yields (192) and minor amounts of the isomers (194) and (195) however, dehalogenation with Fc2(CO)9 favours (195). Acetolysis of karahanaenol tosylate yields anticipated p-menthane derivatives and no filifolene. ... 

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