Diradicals 5,5 -sigmatropic

For the mechanistic course of that reaction two pathways are discussed " a concerted [l,3]-sigmatropic rearrangement, and a pathway via an intermediate diradical species. Experimental findings suggest that both pathways are possible. The actual pathway followed strongly depends on substrate structure the diradical pathway appears to be the more important. 

The mechanism of the degenerate [5,5]-sigmatropic rearrangements of 5,5a,10,10a-tetrahydroheptalene and (z, z)-decatetraene-l,3,7,9 has been explained. A stepwise diradical mechanism has been predicted for both reactions. ... 

Dimerization of lff-azepines is an extensively studied phenomenon and involves a temperature dependent cycloaddition process. At low (0°C for 1 R = Me) or moderate (130 °C for 1 R = C02R or CN) temperatures a kinetically controlled, thermally allowed [6 + 4] dimerization to the exo -adduct (73) takes place, accompanied by a small amount (<10%) of symmetrical dimer (74). The latter are thermodynamically favored and become the major products (83%) when the Iff-azepines are heated briefly at 200 °C. The symmetrical dimers probably arise by a non-concerted diradical pathway since their formation from the parent azepines by a concerted [6+6]tt cycloaddition, or from dimer (73) by a 1,3-sigmatropic C-2, C-10 shift are forbidden on orbital symmetry grounds. Dimerization is subject to steric restraint and is inhibited by 2-, 4- and 7-substituents. In such cases thermolysis of the lif-azepine brings about aromatization to the correspondingly substituted JV-arylurethane (69JA3616). 

Moreover, by irradiation of 2-acetyI-2,5,6-trimethyl-3,4-aihydro-2//-pyran in pentane, a 92% conversion was obtained yielding a cis/trans mixture of 1,2-diacetyl-1,2-dimethylcyclobutane (3, 25%) and 3-methylbut-3-en-2-one (32%) as major products by 1,3-sigmatropic rearrangement or via diradical intermediates.44... 

Thus, as predicted by the orbital symmetry rules, this thermal suprafacial [1,3] sigmatropic reaction took place with complete inversion at C-7. Similar results have been obtained in a number of other cases.426 However, similar studies of the pyrolysis of the parent hydrocarbon of 103, labeled with D at C-6 and C-7, showed that while most of the product was formed with inversion at C-7, a significant fraction (11 to 29%) was formed with retention.427 Other cases of lack of complete inversion are also known.428 A diradical mechanism has been invoked to explain such cases.429 There is strong evidence for a radical mechanism for some [1,3] sigmatropic rearrangements.430 Photochemical suprafacial [1,3] migrations of carbon have been shown to proceed with retention, as predicted.431... 

Thioketenes can be formed by 1,2-sigmatropic shifts occurring through the four-electron species 89 which may be considered as a diradical, as a 1,3-dipole or as a carbene1. In... 

Fig. 7. The [1,3] sigmatropic shift of vinylcyclopropane and two transition structures involved in the reaction. Left transition structure for the concerted process right transition structure for the bond closure in the diradical pathway...

The moderate conditions required for this conversion (E = 25.8 kcalmol" ) suggest that a planar diradical is not an appropriate intermediate ". The interconversions of several vinylmethylenecyclopropanes have been studied and [l,3]sigmatropic shifts can alone account for all the products, although some [3,3]shifts are not excluded For example, the syn and anti isomers of l-allylidene-2-vinylcyclopropane (211) undergo thermal isomerization into 4-vinyl-3-methylenecyclopentene (213) and 3-methylene-1,4-cycloheptadiene (214) on heating in cyclohexane at 42-ATC (equation 142). 

A further route to the corrins (100) was found to be the photocyclization of secoconinoid complexes (101), involving an antarafacial sigmatropic 1,16-hydrogen transfer to give a formal diradical intermediate which undergoes antarafacial 1,15-(it—a) isomerization (Scheme 65). This... 

Homoazocines 49 and 50 (Section V,A) undergo thermal rearrangement to 3-azabicyclo[4.3.0]nonanes 217 and 218, respectively,405 most likely via a concerted or diradical [ 1,3]-sigmatropic vinylcyclopropane shift (Eq. 15),... 

Under thermal conditions, methylenecyclopropanes can undergo rearrangement reactions in which cyclic and exocyclic methylene groups change places. This process, which can be considered as either a concerted [1,3] sigmatropic shift of a carbon or a stepwise reaction involving a 1,3-diradical intermediate, has been studied in order to elucidate the stereochemical course of the reaction. 

The mechanism of the thermal and photochemical rearrangement is believed to proceed as shown (1 - 2), with the dilemma of concerted vs. diradical nature not definitely resolved for all cases. A comparison of energy parameters for the thermolysis of the parent 1,2-divinyl-cyclopropane and of l-(hex-l-enyl)-2-vinylcyclopropane ( - and Z-isomers, tram- and cis-cyclopropanes) has been reported. When both alkenes bear a Z-positioned substituent, the rearrangement of the divinylcyclopropane system becomes very slow and other processes, such as the [1,5] sigmatropic shift of alkyl(vinyl)cyclopropanes or the vinylcyclopropane to cyclo-pentene rearrangement, may compete (see also Section 2.4.3.). Both the mechanism and the applications of this rearrangement have been reviewed. 

---