Diradical mechanism

Cis- and trans-3,5-diphenyl derivatives 1,3-Diradical mechanism 76T619... 

Thiirane 1,1-dioxides extrude sulfur dioxide readily (70S393) at temperatures usually in the range 50-100 °C, although some, such as c/s-2,3-diphenylthiirane 1,1-dioxide or 2-p-nitrophenylthiirane 1,1-dioxide, lose sulfur dioxide at room temperature. The extrusion is usually stereospeciflc (Scheme 10) and a concerted, non-linear chelotropic expulsion of sulfur dioxide or a singlet diradical mechanism in which loss of sulfur dioxide occurs faster than bond rotation may be involved. The latter mechanism is likely for episulfones with substituents which can stabilize the intermediate diradical. The Ramberg-Backlund reaction (B-77MI50600) in which a-halosulfones are converted to alkenes in the presence of base, involves formation of an episulfone from which sulfur dioxide is removed either thermally or by base (Scheme 11). A similar conversion of a,a -dihalosulfones to alkenes is effected by triphenylphosphine. Thermolysis of a-thiolactone (5) results in loss of carbon monoxide rather than sulfur (Scheme 12). 

With certain donor substituents at C-3 the experimental findings may be rationalized rather by a diradical mechanism, where formation of the new carbon-carbon single bond leads to a diradical species 6, which further reacts by bond cleavage to give the diene 2 ... 

No firm decision, between an anion diradical mechanism and a concerted S ->0 1,2-anionic shift, could be made from the available evidence106. Interestingly, the use of a stronger base such as ethylmagnesium bromide results in rearrangement to trans-1,2-diphenylcyclopropanesulfinic acid in highly stereoselective manner (equation 36)107. 

It must be emphasized once again that the rules apply only to cycloaddition reactions that take place by cyclic mechanisms, that is, where two s bonds are formed (or broken) at about the same time. The rule does not apply to cases where one bond is clearly formed (or broken) before the other. It must further be emphasized that the fact that the thermal Diels-Alder reaction (mechanism a) is allowed by the principle of conservation of orbital symmetry does not constitute proof that any given Diels-Alder reaction proceeds by this mechanism. The principle merely says the mechanism is allowed, not that it must go by this pathway. However, the principle does say that thermal 2 + 2 cycloadditions in which the molecules assume a face-to-face geometry cannot take place by a cyclic mechanism because their activation energies would be too high (however, see below). As we shall see (15-49), such reactions largely occur by two-step mechanisms. Similarly. 2 + 4 photochemical cycloadditions are also known, but the fact that they are not stereospecific indicates that they also take place by the two-step diradical mechanism (mechanism... 

The diradical mechanism b is most prominent in the reactions involving fluorinated alkenes. These reactions are generally not stereospecificand are insensitive to solvent effects. Further evidence that a diion is not involved is that head-to-head eoupling is found when an unsymmetrical molecule is dimerized. Thus dimerization of F2C=CFC1 gives 106, not 107. If one pair of electrons moved before the other, the positive end of one molecule would be expeeted to attack the negative end of the other. 

Thermal cleavage of cyclobutanesto give two alkene molecules (cyclorever-sion, the reverse of 2 -I- 2 cycloaddition) operates by the diradical mechanism, and the [ 2s -I- o2a] pathway has not been found " (the subscripts a indicate that cr bonds are involved in this reaction). 

It has been contended that another mechanism, involving single electron transfer, may be taking place in some cases Olah, G.A. Krishnamurthy, V.V. J. Am. Chem. Soc., 1982,104, 3987 Yamataka, H. Nagareda, K. Hanafusa, T. Nagase, S. Tetrahedron Lett., 1989, 30, 7187. A diradical mechanism has also been proposed for certain cases Ward Jr., W.J. McEwen, W.E. J. Org. Chem., 1990, 55, 493. 

Thermal suprafacial [1,5] migrations of carbon have been found to take place with retention,but also with inversion. A diradical mechanism has been suggested for the latter case. 

A special problem is the high yield of triplet carbonyl compounds being formed — neither the concerted nor the diradical mechanism are fully explaining this fact. Further data on the identities and yields of excited products from different dioxetanes are needed. 

The above reactions reinforce the diradical mechanism proposed for the BF3 reaction. Hexafluorobenzene and the various fluorinated ethylenes 73,74 however, react quite differently. The products in these reactions formally correspond to C-F bond insertion by an SiF2 monomer. 

Even though these products are not formed by a diradical mechanism, these polar species may still have relevance to DNA damage, because of the potential alkylating ability of their electrophilic (cationic) sites. 

The positional selectivity of the cycloadditions of 351 is of particular interest, since the [2 + 2]-cydoadditions proceed exclusively at the double bond bearing the oxygen atom, whereas the [4 + 2]-cycloadditions occur at the double bond remote of the oxygen atom with the exception of the major product resulting from 1,3-cydohexadiene (369, Scheme 6.75). Supposing diradical mechanisms, these findings require that... 

The [2 + 2]-cycloaddition of allene proceeds via a stepwise diradical mechanism rather than a concerted one-step mechanism. The allenes come together in a crossed configuration. The bond formation between the central sp carbon atoms is accompanied by a simultaneous conrotatory twisting leading to a perpendicular 2,2 -bisallyl diradical 3. Rotation about the central bond of 3 gives the planar diradical and a disrotatory closure leads to the formation of dimer 2. The stereochemistry of some of the following examples is explained by this mechanism. 

Investigations on the thermal isomerization of various disubstituted [2.2]paracyclophanes at 200 °C by Cram and Reich 111 112> showed that only the diradical mechanism (Route A) was consistent with experimental findings. Thermal isomerization starting from the pure pseudo-geminal... 

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. ... 

The cycloaddition reaction of dipoles has been known since the late eighteenth century however, before Huisgeris introduction of the concept of a 1,3-dipole, these reactions were considered to proceed via a diradical mechanism [16]. One of the earhest examples of metal-catalyzed 1,3-dipole formation involved the controlled decomposition of an a-dia-... 

The reason for this difference is that if 16 were to undergo a concerted elimination, it would have to follow the forbidden (high-energy) [2ns + 2ns] pathway. For 17, the elimination can take place by the allowed [2ns + 4rcv] pathway. Thus, these reactions are the reverse of, respectively, the [2 + 2] and [4 + 2] cycloadditions, and only the latter is an allowed concerted process. The temperature at which 16 decomposes is fairly typical for strained azo compounds, and the decomposition presumably proceeds by a noncon-certed diradical mechanism. Because a C—N bond must be broken without concomitant compensation by carbon-carbon bond formation, the activation energy is much higher than for a concerted process. 

One method which is diagnostic of the diradical intermediate is the extent of retention of cis- and trans-substitution in the product from a cis- or tram-olefin. In a concerted process both bonds are formed concurrently, thereby prohibiting isomerization, whereas in the diradical mechanism the bond can rotate. 

The toluene sensitized photorearrangement of (+ )-car-2-ene gave a racemic mixture of l,4,4-trimethylbicyclo[3.2.0]hept-2-ene (IS) in 60% yield by a diradical mechanism.74... 

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