Methylene diradical

Diazomethane when heated with copper powder gives nitrogen and an insoluble polymethylene, indicating that one of its reactions is the decomposition into methylene radicals. The methylene radical can also be formed in the gas phase and detected by a mirror experiment.81 The pyrolysis of ketene in the gas phase gives carbon monoxide and methylene radical. The methylene radical both reacts with itself to give ethylene and removes tellurium mirrors, forming tellurform-aldehyde.82 Thus the methylene diradical(P) behaves as expected. 

Alternatively, if the decomposition of the complex leads to the methylene diradical, then homolytic cleavage of the B—R bond would be expected. 

Bicyclo[4.4.1]undec-l(ll)-ene (28), however, exhibits different behavior, for it dimerizes in 95% yield to form structure (29). Whether this process involves hydrogen atom transfer from a tetra-methylene diradical, or an unusual pericyclic reaction reminiscent of an ene reaction, remains unclear. 

Stereochemical studies add great support to the proposition that [2 + 2] cycload tions involve tetra-methylene diradical intermediates, for these additions e place with partial or complete loss of cis/trans stereochemical relationships present in alkene reactants, llie dimerization of acrylonitrile, for example, studied with ci5-l,2-dideuteriocyanoethylene (42), gives six distinct [2 + 2] adducts the distribution of deuterium labels in the df-l,2-dicyanocyclobutane products are given in structures (43), (44) and (45) an analogous set of three trans stereoisomers is formed. Recovered starting material shows partial loss of stereochemical integrity. 

As a starting point for the mechanistic discussion, it is useful to review the structural features of the excited states. The first singlet excited state of butadiene. Si, can be approximated as the i >2->--LUMO tt-tt transition. The lack of fluorescence from this excited state indicates that a very facile path exists for nonradiative energy transfer. The S2 state has doubly excited character and relaxes to a structure with ionic character that can rotate at the pyramidal carbon but not at the allyl fragment. The minimum energy of the Tj state corresponds to the allyl-methylene diradical with a nearly 90° twist and slight pyramidalization at the methylene carbon. In substituted systems, one or the other of the zwitterion structures (e.g., allyl cation versus allyl anion) may be favored. ... 

A variation of this type of reaction involves interaction of two alkyne groups via radical formation. The mechanism proposed by Garratt involves initial base-catalysed isomerization to the bis allene, which then undergoes an intramolecular allene dimerization to give the bis methylene diradical (67). 

While direct irradiation of , E-2,4-hexadiene 5 gives only , Z-2,4-hexadiene from singlet excited state, triplet-sensitized reaction gives both E,Z- and Z,Z-2,4-hexadienes. The singlet state reaction proceeds with just one terminal double bond rotation involving allyUc methylene or cyclopropane methylene diradical with just one double bond rotation, whereas the triplet excited state reaction proceeds with double double bond rotation [11]. 

This reaction can proceed by 1,1-proton abstraction to form a carbene radical anion, but can also occur by l,n-abstraction to form the negative ion of a diradical. Thus, reaction of O with methylene chloride results in the formation of CCI2 (Eq. S.Sa), reaction with ethylene gives vinylidene radical anion, H2CC (Eq. 5.8b), and the reaction with acetonitrile gives the radical anion of cyanomethylene, HCCN (Eq. 5.8c) Investigations of these ions have been used to determine the thermochemical properties of dichlorocarbene, CCI2, vinylidene, and cyanomethylene. ... 

The substitution of the exo-methylene hydrogen atoms of MCP with halogens seems to favor the [2 + 2] cycloaddition reaction by stabilizing the intermediate diradical. Indeed, chloromethylenecyclopropane (96) reacts with acrylonitrile (519) to give a diastereomeric mixture of spirohexanes in good yield (Table 41, entry 2) [27], but was unreactive towards styrene and ds-stilbene. Anyway, it reacted with dienes (2,3-dimethylbutadiene, cyclopentadiene, cyc-lohexadiene, furan) exclusively in a [4 + 2] fashion (see Sect. 2.1.1) [27], while its... 

In this state the addition of methylene occurs in the triplet state and starting from each a mixture of cis and trans products is obtained. This is because in the triplet state, the two electrons have parallel spins and carbene behaves as diradical. To explain non stereospecificity it is assumed that rotation about the single bond occurs more rapidly than spin inversion. The entire mechanism can be written in the following manner ... 

Much experimental and theoretical work has been performed with the two allenes 1,2,6-heptatriene (32) and 1,2,6,7-octatetraene (34). Thermal isomerization of 32 leads to 3-methylene-l,5-hexadiene (346), a process that at first sight looks like a typical Cope rearrangement. However, trapping experiments with either oxygen or sulfur dioxide have shown that at least half of the rearrangement passes through the diradical 345 (Scheme 5.52) [144],... 

Quantum-chemical calculations of the transition state of the enantiomerization of 6, that is, for the interconversion of (M)-6 and (P)-6, support the diradical 6-D [13, 18] (Scheme 6.4). Most probably, its singlet state is a few kcal mol-1 more stable than the triplet state. The zwitterions 6-Zi and 6-Z2 are excited states [1], However, if a methylene group directly attached to the allene system in 6 is replaced by an appropriate heteroatom, either 6-Za or 6-Z2 may be strongly stabilised (see Section 6.3.5). 

The cyclic diradical, 2-methylene-1,4-cyclohexadiyl (18), can be formed from the hepta-1,2,6-triene 1722,23. Thermolysis of 17 gives 3-methylene-l,5-hexadiene 19 as a Cope rearrangement product, while the same treatment (155 °C, benzene) in the presence of SO2 leads to sulfones 20 and 21 instead of 19 (equation 6). It was shown that sulfone 20 is obtained by reaction of SO2 with the rearrangement product 19, while sulfone 21 originates directly from the diradical 18. 

Diradicals represent the most clear-cut application of the SF approach because in these systems the non-d namical correlation derives from a single HOMO-LUMO pair (e.g., n and n in twisted ethylene). In this section we present results for methylene and trimethylenemethane (TMM). 

Four low-lying states of methylene are diradical-type states (//) deriving from the distribution of two electrons in the two nearly degenerate orbitals, 3ai (/[/ hybrid) and lb (out-of-plain p-orbital). The ground state of methylene is triplet ... 

The origin of the difference lies in the fact that triplet carbenes are biradicals (or diradicals) and exhibit chemistry similar to that exhibited by radicals, while singlet carbenes incorporate both nucleophilic and electrophilic sites, e.g., for singlet and triplet methylene. 

Dimethylene-2,3-dihydrofuran derivatives, which are produced by fluoride-induced 1,4-conjugative elimination of trimethylsilyl acetate from the [(trimethylsilyl)methyl]-3-furan precursor 207, undergo subsequent [4-1-4] dimerization reactions to produce cycloocta[l,2-3 6,5-. ]difuran derivatives as a mixture of isomers (Equation 137) <1995JA841 >. A methyl substituent at the 3-methylene position was found to retard the rate of dimerization, an observation which is consistent with the proposed two-step mechanism involving the initial formation of a diradical intermediate in the rate-determining step (Table 16). 

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