Diradical mechanism, and

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. 

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). 

Three mechanisms can be proposed for the intimate reaction mechanism for c-e, analogous to the organic 2+2 cycloadditions a pericyclic (concerted) mechanism, a diradical mechanism, and a diion mechanism. In view of the polarization of the metal(+) carbon(-) bond, an ionic intermediate maybe expected. The retention of stereochemistry, if sometimes only temporary, points to a concerted mechanism. 

A similar reaction was observed when 2-methylene-l,l-diphenylcyclopropane (8, = H) was heated to 135°C. The isolated 2-methyl-3-phenylindene (9, R = R = H) was probably formed via a 2,7a-dihydro-2-methylene-3-phenyl-liy-indene and subsequent 1,5-hydrogen shift.An analogous reaction occurred when 2-ethylidene-l,l-diphenylcyclopropane (8, R = H R = Me) and 2-(l-methylethylidene)-l,l-diphenylcyclopropane (8, R = R = Me) were submitted to thermolysis. These reactions can be rationalized by both a diradical mechanism and a concerted Cope-type rearrangement, followed by a hydrogen shift to restore the aromatic system. 

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). 

Despite the body of evidence in favor of the Mayo mechanism, the formation of diphenylcyclobutanes (90, 91) must still be accounted for. It is possible that they arise via the 1,4-diradical 94 and it is also conceivable that this diradical is an intermediate in the formation of the Diels-Alder adduct 95 (Scheme 3.64) and could provide a second (minor) source of initiation. Direct initiation by diradicals is suggested in the thermal polymerization of 2,3,4,5,6-pentafluorostyrene where transfer of a fluorine atom from Diels-Alder dimer to monomer seems highly unlikely (high C-F bond strength) and for derivatives which cannot form a Diels-Alder adduct. 

Addition reactions with alkenes to form cyclopropanes are the most studied reactions of carbenes, both from the point of view of understanding mechanisms and for synthetic applications. A concerted mechanism is possible for singlet carbenes. As a result, the stereochemistry present in the alkene is retained in the cyclopropane. With triplet carbenes, an intermediate 1,3-diradical is involved. Closure to cyclopropane requires spin inversion. The rate of spin inversion is slow relative to rotation about single bonds, so mixtures of the two possible stereoisomers are obtained from either alkene stereoisomer. 

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 observation that the transition state volumes in many Diels-Alder reactions are product-like, has been regarded as an indication of a concerted mechanism. In order to test this hypothesis and to gain further insight into the often more complex mechanism of Diels-Alder reactions, the effect of pressure on competing [4 + 2] and [2 + 2] or [4 + 4] cycloadditions has been investigated. In competitive reactions the difference between the activation volumes, and hence the transition state volumes, is derived directly from the pressure dependence of the product ratio, [4 + 2]/[2 + 2]p = [4 + 2]/[2 + 2]p=i exp —< AF (p — 1)/RT. All [2 + 2] or [4 + 4] cycloadditions listed in Tables 3 and 4 doubtlessly occur in two steps via diradical intermediates and can therefore be used as internal standards of activation volumes expected for stepwise processes. Thus, a relatively simple measurement of the pressure dependence of the product ratio can give important information about the mechanism of Diels-Alder reactions. 

The primary 14C KIEs in the 1,3-dipolar addition of N-w-diphenylnitrone, PhCH=N(0)Ph, and styrene to yield 2,3,5-triphenylisoxazoline 226198 are also consistent with Huisgen s199 concerted, cyclic mechanism and inconsistent with the diradical mechanism200 (structures 227 and 228). 

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. 

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 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. 

---