Diradicals trimethylenemethane

The well-established prototypes of 1,3-diradicals, trimethylenemethane (TMM, 1) and trimethylene (TM, 2) are selected to illustrate our idea. The diradical is assumed to have two singly occupied orbitals, p and q, of nearly the same energy on two radical centers (P and Q in Fig. 4). As addressed in Sect. 2, it is possible for... 

In some diradicals the two partially filled MOs are both n orbitals, and the electrons in them are, consequently, delocalized. Diradicals for which Kekul6 structures cannot be written without leaving two n electrons nonbonded are called nonKekul6 diradicals. Trimethylenemethane [C(CH2)3] and tetramethyleneethane [(H2C)2CC(CH2>2] are both diradicals of this type, and they and other nonKekul6 diradicals are discussed in Section 5. 

Figure 10 Four of the most important non-Kekul6 hydrocarbon diradicals trimethylenemethane (TMM), tetramethyleneethane (TME), m-benzoquinodimethane (MBQDM), and tetramethyleneben-zene (TMB)...

There is a degree in the continuity and discontinuity of the orbital phase [20]. 2-Oxopropane-l,3-diyl (Scheme 10) is a hetero analog of trimethylenemethane (TMM) where the orbital phase is continuous in the triplet diradical (Sect. 2.1.5) and discontinuous in the singlet diradical (Sect. 2.1.6). The n and orbitals of carbonyl bonds are lower in energy than those of C=C bonds. The lowering strengthens the interaction of the radical orbitals (a, b) with and weakens that... 

The Franck-Condon question is also an issue in diradical studies. It is particularly relevant in nonrigid systems, such as trimethylenemethane (TMM) and tetramethyleneethylene (TME), °° in which neutral and/or anion states can be planar or nonplanar. Thus, transitions can occur from planar anions to nonplanar neutral states, or from nonplanar anions to planar neutrals. In these cases, the energy differences between the planar and nonplanar states have generally been estimated by using quality electronic structure calculations. 

Presumably the reaction takes place via the monoallene 225, which can be made the sole isomerization product if the rearrangement is carried out with sodium methoxide in methanol at 65 °C (yield 60%) [76], The intended intramolecular [2 + 2] cycloaddition to the diradical 227, a hybrid of trimethylenemethane and tetramethy-leneethane, could so far not be detected. 

One such typical transformation is the thermal isomerization of the spiropentane derivative 76 into triene 80 which is assumed to occur via the diene intermediate 78 with the intermediate participation of the cyclopropyl-trimethylenemethane (TMM) 77 and the vinyl-TMM 79 diradicals (equation 29)44. It was shown by using deuterium labels that the diradical 79 forms the triene 80 by 1,6-hydrogen shift. The pathway 76 — 80 which occurs via tetramethylene-ethane diradical was recognized as a less probable route. 

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). 

The diradical species trimethylenemethane (42) was first discovered by Dowd in 1966 from cleavage of the pyrazoline, and was characterized by its ESR spectrum and the formation of methylenecyclopropane (equation 67). The study of diradicals has a long history (equation 3), " and many examples have been studied. ... 

Slipchenko, L. V. and Krylov, A. I. 2003. Electronic Structure of the Trimethylenemethane Diradical in its Ground and Electronically Excited States Bonding, Equilibrium Geometries, and Vibrational Frequencies , J. Chem. Phys., 118, 6874. 

A number of diradicals (also called biradicals) are known.186 When the unpaired electrons of a diradical are widely separated, e.g., as in CHjCHjCHjC, the species behaves spectrally like two doublets. When they are close enough for interaction or can interact through an unsaturated system (as in trimethylenemethane,187 they can have total spin numbers of + 1, 0, or -1, since each electron could be either + or -i. Spectroscopically... 

Phenylenyl monoradical (C13H9) is a strictly peri-condensed PAH6 with the methyl radical as an excised internal structure and both have e = 0. The diradical C22H12 isomer, triangulene, is a strictly peri-condensed benzenoid hydrocarbon and has trimethylenemethane diradical as an excised internal structure both triangulene and trimethylenemethane diradicals have two eigenvalues of s = 0. 

Halogen and thiol migration, and radical bridging 29 Diradicals in the vinylcyclopropane rearrangement 30 Trimethylenemethane 30... 

Both the thermally and the photochemically initiated extrusion of nitrogen from bicyclic diazene (1) leads to the formation of a 1,3-diradical (diyl) which is structurally related to trimethylenemethane. As shown in the cascade mechanism illustrated, two forms of the diyl can be intercepted by alkenes bearing electron-withdrawing groups (diylophiles), viz. the open form of the singlet diyl and the triplet diyl which is formed via intersystem crossing from the singlet (Scheme 1). ... 

Methylenecyelopropanes readily rearrange by cleavage of the C(2)-C(3) bond, propably via a perpendicular singlet trimethylenemethane diradical (TMM) (Eq. 

To realize a new strategy like the one outlined in Eq. 75, it is necessary to find a reasonable way to create the odd-numbered carbon fragment. Faced with such a problem the view almost automatically turns to the trimethylenemethane (TMM) diradical (cf. Eq. 64), a species well studied from a physical-organic perspective 181). It provides not only the necessary three-carbon fragment but also an exocyche... 

Most often in these reactions, however, the monoadduct is isolated and used to prepare the alkylidenecyclopropane, as in the case of the 1 1 allene-2-diazopropane adduct, which on thermolysis generated the singlet trimethylenemethane diradical 59, which closed to give a mixture of l,l-dimethyl-2-methylenecyclopropane (60) and isopropylidenecyclopropane (61). Although a wide variety of alkylidenecyclopropanes have been made in this way (see Table 2), the involvement of trimethylenemethane intermediates in the deazetization step means that mixtures of isomers are normally produced. Attempted syntheses of alkylidenecyclopropanes by this route have sometimes been frustrated by tautomerism and autoxidation of the intermediate 4,5-dihydro-3i/-pyrazole or by a reversal in the regiochemistry of the diazoalkane and allene addition reaction leading to 3-alkylidene-4,5-dihydro-3//-pyrazoles. ... 

The involvement of trimethylenemethane diradicals in deazetization of diazoalkane-allene adducts or trimethylene diradicals in the deazetization of the adducts of acyclic alkenes often leads to mixture of regioisomers and stereoisomers and from the standpoint of cyclopropane syntheses, this is undesirable. Far fewer problems of this type attend deazetization of the adducts of cyclic or polycyclic alkenes and, furthermore, even a modest amount of strain in the system activates the alkene to diazoalkane addition so that there is no need for activating substituents on the double bond. Cyclopropene is highly reactive towards diazoalkanes (see also Section 1.1.5.1.5.3.1.) and cycloaddition reactions of this type provide a ready entry into the bi-cyclo[1.1.0]butane series. The addition of diphenyldiazomethane to cyclopropene gave 4,4-diphenyl-2,3-diazabicyclo[3.1.0]hex-2-ene (1), which on photolysis gave a mixture of 2,2-diphenylbicyclo[1.1.0]butane (2) and 1,1-diphenylbuta-l,3-diene (3). ... 

At 170°C and 10% conversion of the starting material, cw-l,2-dimethyl-3-methylenecyclopro-pane formed mainly ( )-l-ethylidene-2-methylcyclopropane, whereas trans-5 gave rise to the (Z)-product [(Z)-6] without any interconversion of the substrates. These data suggest that the orthogonal trimethylenemethane diradical structure is at least 12 kcal/mol more stable than the planar or bisorthogonal diradical. 

The participation of the above mentioned trimethylenemethane (TMM) diradicals in the thermal rearrangements of methylenecyclopropanes was also investigated by using systems containing an additional vinyl group which was part of rings fused with the cyclopropane (equations 32 and. The diene 91 was obtained from dichloride 89... 

The oxyallyl system, another reactive intermediate usually written with two charges 6.370 instead of as a diradical, has a similar conjugated system, except that the coefficients will be different, and the central carbon atom, although close to a node in ip2 and u>3, will not have a node exactly through it. When generated on its own, it dimerises with different regioselectivity from trimethylenemethane, giving the 1,4-dioxan 6.371.875... 

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