Cyclization, radicals calculations

Fig. 4 The crossings of in-plane and out-of-plane frontier MOs in the radical-anionic Bergman and C1-C5 cyclizations, respectively calculated at the B3LYP/6-31G level. Adapted from reference26.

Activation parameters. Calculate AW and AS for the cyclization of the 5-hexenyl radical, whose rate is given in Eq. (5-39). 

Another major influence on the direction of cyclization is the presence of substituents. Attack at a less hindered position is favored by both steric effects and the stabilizing effect that most substituents have on a radical center. These have been examined by DFT (UB3LYP/6-31+G ) calculations, and the results for 5-hexenyl radicals are shown in Figure 10.14. For the unsubstituted system, the 5-exo chair TS is favored over the 6-endo chair by 2.7kcal/mol. A 5-methyl substituent disfavors the 5-exo relative to the 6-endo mode by 0.7kcal/mol, whereas a 6-methyl substituent increases the preference for the 5-exo TS to 3.3 kcal/mol.322... 

Models accounting for the observed selectivities can be obtained from simple analysis of transition structures, according to the work of Spellmeyer and Houk [37]. These authors have calculated the transition states for cyclizations of radicals to be those shown in Scheme 12.19, but with a hydrogen atom being replaced by the CH2OTiCp2Cl group. 

The electrocyclic reactions of n systems containing an impaired electron are difficult to interpret using the above simple theories. The symmetry of the HOMO of the radical system corresponds to that of the corresponding anion. Thus the allyl radical would be expected to cyclize in the same manner as the alkyl anion i.e., in a conrotatory manner. In fact the interconversion takes place in a disrotatory manner. Theoretical calculations based on Huckets theory also give ambiguous or incorrect predictions. And therefore more sophisticated calculations are required to obtain reliable results. 

The transition structure trans-42 to fra s-43 TiCp2Cl was also calculated and a barrier of 11.7 kcal mol 1 was obtained for the cyclization. This value is similar to the activation energy for the formation of cis-43 TiCp2Cl, although the distance between the carbon-centered radical and oxygen atom is shorter (2.04 A). 

Table 6.23 Calculated barriers (kJ/mol) for the cyclization of the but-3-enyl radical. ...

Table 6.23 presents calculated barriers for the cyclization of the but-3-enyl radical [i.e. the reverse of reaction (7.2)]. This reaction is an example of an intramolecular radical addition. A number of the features observed in the barriers for the intermolecular radical additions (e.g. methyl radical addition to ethylene, Table 6.14) are also seen here. 

The X-ray crystal structure of the hexafluoroantimonate salt of 1,4-diithin radical cation stabilized by bicyclo[2.2.2]octane annelation revealed a planar ring and was in agreement with theoretical calculations. Tertiary aminium radical cations underwent facile 5-exo-cyclization to give distonic 2-substituted pyrrolidinium radical cations. 

CH4, and H2 while minor products such as HCCCN, H2C=CHCN, C2H4 and C4H2 were also detected. Ab initio chemical calculations revealed that the pyrolysis of acetonitrile is initiated by CH bond fission, forming a cyanomethyl radical. Products such as HCCN and H2C=CHCN have been shown to arise from the decomposition of succinonitrile, that forms by the recombination of two cyanomethyl radicals. The kinetics of thermocyclization of 2,3-diethynylquinoxaline (109) (Bergman cyclization) have been studied in various solvents. Non-polar solvents give shorter half-lives and better yields. The cyclization rates observed were found to be solvent dependent. ... 

The asymmetric rearrangement of peroxy radical (5) has recently been used as the key step in the asymmetric synthesis of Plakorin (Scheme l).17 The thermal isomerization of buta-1,2- to buta-1,3-diene has been studied using ab initio calculations and the mechanism concluded to proceed stepwise via radical intermediates.18 The competition between cyclopropyl formation and the homoallyl-homoallyl radical rearrangement has been studied in the radical (6) and found to give the 3-exo cyclization product (7) and the rearranged product (8) in a 1 5 ratio, respectively, under the conditions shown (Scheme 2).19... 

The transition-state geometries for the cyclization of a range of electrophilic radicals have been obtained by MNDO semiempirical calculations.42 The regioselectivities observed were rationalized using a frontier orbital approach. Semiempirical calculations were not suited for describing the cyclization of the pent-4-en-l-oxyl radical.43 Instead, results indicated that ab initio (UHF/6-31G ) and (UBP/DZVP) were more reliable. 

---