Diradicals ring-opening

Thiirene intermediates in the photolysis of 1,2,3-thiadiazoIes readily undergo ring opening to a diradical which can be trapped by reaction with an alkyne (Scheme 25) (79CB1769). Significant polymerization is not observed. 

The direction of ring opening by homolytic cleavage of a cyclopropane bond is controlled by the stability of the diradical species formed. Upon heating of the mono-deuterated vinylcyclopropane 3, a mixture of the two isomeric mono-deuterated cyclopentenes 4 and 5 is formed ... 

The anticancer activity of complex natural products having a cyclodecenediyne system [for a review see <96MI93>] has prompted the synthesis of 54 (X = CH2 and OCH2) <96CC749> and 55 (R = a-OH and p-OH) <95AG(E)2393> on the basis that such compounds are expected to develop anticancer activity as the P-lactam ring opens. This is because cycloaromatization can only occur in the monocyclic enediyne and the diradical intermediate in the cyclization is thought to be the cytotoxic species. 

Goddart, J. D., Orlova, G., 1999, Density Functional Theory with Fractionally Occupied Frontier Orbitals and the Instabilities of the Kohn-Sham Solutions for Defining Diradical Transition States Ring-Opening Reactions ,... 

Thus BCP seems to follow two competitive pathways in the cycloaddition with dienes (i) a stepwise diradical process giving the [2 + 2] adduct, or (ii) a concerted pathway giving the [4 + 2] adduct. Accordingly, the proportion of the latter increases with the reactivity of diene in Diels-Alder reactions. Conversely, the reaction with 2,3-dicyanobutadiene (529), generated in situ by electrocyclic ring-opening of 1,2-dicyanoeyelobutene [142], furnishes selectively the [2 + 2] cycloadduct 530 (Table 42, entry 4) due to the presence of substituents able to stabilize the diradical intermediate [13b],... 

In the presence of proper substituents the primarily formed diradical rearranges to a more stable one, as by a ring opening reaction. In (2.7) medium ring lactones are synthesized by such a sequence 2U). 

As depicted in Scheme 11, ylides 39 derived from 4-methyl-[l,2,3]triazolo[l,5- ]pyridine react with Michael acceptors, which, upon nucleophilic attack at C3 and ring opening, lead to nucleophilic displacement of nitrogen. The intermediate diradical led to a mixture of compounds, including alkenes and a cyclobutane derivative when methyl acrylate was used, and the indolizine 40 with methyl propiolate as the electrophile <1998T9785>. Heating 4-methyl triazolopyridine with benzenesulfonyl chloride in acetone also confirmed decomposition via a radical pathway. 

Quantum-chemical calculations furnished very similar energies of 1,2-cyclobuta-diene (13) and the diradical 13-D (Scheme 6.5), with a small preference for the latter. Since the ring opening of 13 or 13-D was estimated to be highly exothermic (74.5 kcal mol-1) because of the inherent strain, the barrier for this step must be low [24]. 

One common feature does exist between the photolysis of borazine and of benzene in the absence of a second reagent that is the observation of a ring rupture in the matrix isolated species For benzene, the product is hexatriene diradical, vidiile in the case of borazine, the product has not been identified completely. IR data show it to involve a ring-opening with some loss of hydrogen, leaving B-N triple bonds and a terminal NB rnoiety ... 

The reactivity trend in terms of the ring-opening reaction for the series 43, 47, and 53 can be analyzed in terms of the geometries of imino and vinyl radicals. Most of the spin density in imino radicals is concentrated in the p-orbital on nitrogen, but vinyl radicals have a nonlinear, vp -like structure, where the spin density is in the i /j -orbital of the carbon. In the o-quinone diradicals 43,47, and 53, the / -orbitals of the nitrogens (accomodating the odd electrons) are better... 

An alternative mode of reaction available to the o-quinone diradicals 43, 47, and 53 is ring closure to benzocyclobutadienes 54, 52 and 55, respectively (Scheme 11). The computed values for the barriers for this reaction follow the opposite trend and so do the associated enthalpies. Thus, biscarbene 53 is predicted to be the most reactive towards ring closure. Furthermore, 53 is predicted to prefer this mode of reaction, in contrast to 43 and 47, for which calculations find that ring opening is more facile. The available experimental results are compatible with the computational data, as far as 43 and 47 are concerned. On the... 

Several of the phenylene-linked carbenes and nitrenes exhibit photochemical or thermal reactivity in the matrix. The photolabile p- and m-phenylene-linked species give products of rather unexpected structures via mechanisms that are not understood yet. The o-phenylene-linked species isomerize rather easily either via ring opening of the phenylene linker or by an apparent direct reaction of the two proximal diradical centers to give ring-closure products. The available data... 

Can Hyperconjugation in a 1,3-Diradical Control the Stereochemistry of Cyclopropane Ring Opening and Make a Singlet the Electronic Ground State of the Diradical ... 

Figure 22.10. Rearrangement products (13), predicted to be formed from 1,1-disilyl-cyclopropanes (11) by conrotatory ring opening, followed by a 1,2-sbift of a trimethylsilyl group in the 1,3-diradicals (12) generated. Pyrolysis of 11b should lead to a mixture of E/2) isomers (13b), but pyrolysis of 11c is predicted to form only the E stereoisomer (13c). ...

Figure 22.11. 1,3 -Diradicals (15a and 15b) formed by conrotatory and disrotatory ring opening of cis- and franj-l,2-dimethylspiropentanes (14a and 14b).

---