Trimethylene biradical

A direct observation of a triplet state of substituted trimethylene biradical has recently been claimed.84) This is in good agreement with the prediction 104,105) that the lowest excited state of cyclopropane will have an energy minimum at a geometry in which one C—C bond is broken. 

The rate constants were determined at a series of pressures in the fall-off region, and the fall-off curve was very similar to that obtained for the structural isomerization to propylene. The similarity of the two sets of data suggests that both reactions may proceed through similar reaction paths. One obvious possibility is that once again the trimethylene biradical is formed, which can undergo internal rotation followed by recyclization. An alternative transition state has been suggested which involves, as an activated complex, a much expanded cyclopropane ring in which hindered internal rotation occurs (see also Smith, 1958). 

Ab initio MRCI calculations showed that the barrier from trimethylene to propene is 7.9kcalmol 1 higher than that from trimethylene to cyclopropane.11 Thus, cyclopropane stereomutation may occur through trimethylene as an intermediate (Chart 3). Trimethylene biradical may cyclize by double rotation of the two C C bonds in conrotatory or disrotatory fashion or successive single rotation. The calculations showed that the PES at the... 

The reaction of trimethylene biradical was successfully treated by means of dynamics simulations by two groups with different PESs as described above.11 15 The success led one of the groups to extend the study to analyze the collisional and frictional effects in the trimethylene decomposition in an argon bath.16 A mixed QM/MM direct dynamics trajectory method was used with argon as buffer medium. Trimethylene intramolecular potential was treated by AM1-SRP fitted to CASSCF as before, and intermolecular forces were determined from Lennard-Jones 12-6 potential energy functions. 

As a final suggestion for future research, cyclobutanones have also provided the organic photochemist with the opportunity of investigating the existence of unusual and reactive intermediates oxacarbenes, trimethylene biradicals, trimethylenemethane biradicals, acyl alkyl biradicals, and ketenes. Evidence for the intervention of oxacarbenes in the ring-expansion reaction is quite compelling however, their unusual behavior relative to "typical" carbenes (e.g., failure to form cyclopropane adducts with some olefinic substrates) makes them prime subjects for further study and characterization. Unlike oxacarbenes, the existence of acyl alkyl biradicals (e.g., [30]) is tenuous at best. Ideally,... 

The gas-phase decomposition of cyclopropane, C, to propylene P, has been proposed as proceeding through a trimethylene biradical, T. 

Rupture of a carbon-carbon bond in the cyclopropane ring to yield the trimethylene biradical, followed in a distinctly separate stage by hydrogen migration, viz. 

It is interesting to note that if a similar phenomenon were to apply to cyclopropane stereomutation, one would expect inversion of configuration at both ends of the breaking C-C bond giving a stereochemistry in accord with the observations for cyclopropane-1,2-d2 (Section III. A. 3). Stabilization of the trimethylene biradical might place it in a potential... 

Thermal deazetization of pyrazolines results in the formation of cyclopropanes and alkenes, illustrated in Figure 44 for the parent compound (18). This reaction is of interest in that one could imagine that it would involve the same trimethylene biradical (19) proposed to be an intermediate in cyclopropane stereomutation (Section III.A). Supporting this notion is the observation that the parent pyrazoline gives 89% cyclopropane and 11 % propylene at 250° C. If one took this product ratio as a reflection of the branching ratio from a common trimethylene intermediate, it should then be possible to compare these figures with the relative rates of stereomutation and propylene formation from cyclopropane-d2 . Interestingly, they are identical. 

Figure 4.25. Sum-over-atoms factor in the spin-orbit coupling vector // a) in orthogonally twisted ethylene and b) in (0, 90°) twisted trimethylene biradical, using Equation (4.12) and (4.13) most localized orbitals x - Xh and nonvanishing atomic vectorial contributions from Xh (white through-space, black through-bond).

The Norrish Type I reaction usually leads to decarbonylation. This is the case with dicyclopropyl ketone on irradiation at 193 nm. Decarbonylation, however, is a second step and this is preceded by ring opening of the cyclopropyl moieties to diallyl ketone. Calculations have shown that decarbonylation of cyclobutanone occurs from the nji triplet state. The resultant triplet trimethylene biradical undergoes ISC to the ground state before formation of cyclopropane. On the other hand, the cycloelimination reaction to yield ketene and ethene arises from the singlet excited state.Irradiation of cyclopentanone in aqueous and frozen aqueous solutions has been examined and the influence of applied magnetic fields assessed. Photodecarbonylation in the crystalline phase of the ketone (3) at 310 nm takes place stereospecifically with the formation of the cyclopentane derivative (4). The latter can be readily transformed into racemic herbertenolide (5). ... 

Cyclopropane has long been known to undergo a thermally induced structural rearrangement to propylene. The first-order rate constant at high pressures is ogk= 15.1 — 65 200/23RT The trimethylene biradical was invoked as an intermediate in the reaction. Subsequent observations that cis- and trans-1,2-dideuteriocyclopropane interconvert roughly 20 times faster than the structural isomerization with log k = 16.41 — 65 100/23RT can be understood in terms of reversible formation of the trimethylene species (Scheme 4.10). 

Conformation-different trimethylene biradicals were proposed with the biradical from the syn,anti isomer giving more vicinal hydrogen shifted material than that from the anti, anti isomer, which gives mostly intramolecular 2 + 2 product. Both also gave tetralin and cw-/3-methylstyrene. 

The key feature of this surface is the clear prediction that the heats of formation of the two transition states lie above the heat of formation of the trimethylene biradical. This... 

We have described several examples of the direct observation of triplet biradicals. However, the fleeting nature of singlet biradicals has made it very difficult to obtain direct information on their structure and reactivity. With the advent of femtosecond lasers, however, this has become possible. For example, Zewail and co-workers directly characterized simple tetramethylene and trimethylene biradicals. The approach involved a molecular beam of either cyclopentanone or cyclobutanone with crossed laser beams. Photolysis led to extrusion of CO and formation of the biradical. For tetramethylene, a clear biradical structure is seen, with a lifetime of 700 fs. However, for... 

Consider the dimethyl diazene results of Eq. 11.84. Are these results consistent with the proposal of a preferential conrota-tory motion of the trimethylene biradical Can you propose another explanation ... 

Photolysis of thietan dioxides is a preparatively useful synthesis of cyclopropanes, particularly since stabilization of an cf-carbanion by a sulphonyl group allows wide latitude for manipulation. Thietan dioxide (101) gives (102) in 95% yield upon photolysis. Trimethylene sulphone gives trimethylene biradicals... 

It is to be noted that a correct relationship between the energies of the structures XXXI and XXXI can be achieved only on condition that the electron correlation is accounted for. This is necessary in view of the fact that the structure XXXII represents a trimethylene biradical with a conformation close to the (EE) type (see Sect. 1.3). In the zone of the XXXII structure, the PES is... 

Coupling. Application to Singlet-Triplet Interaction in the Trimethylene Biradical, (b) T. R. Furlani, Ph. D. dissertation. State University of New York, Buffallo, 1984. 

Interest in the decomposition of cyclopropanes and the role of the trimethylene biradical in the decay mechanism has spanned more than three decades and has been fueled by two experiments that give apparently irreconcilable results. Experiments of S,S-trans-cyclopropane-l,2-d2 at 695 K indicate that isomerization via double-terminal rotation (i.e., con- and disro-tation of the terminal methylene groups) is at least 6 times more prevalent than isomerization via single-terminal rotation. Similar experiments with chiral... 

A kinetic scheme that includes the role of the trimethylene biradical intermediate is required to obtain the relative rates of single- and doubleterminal rotations from the experimental observables (i.e., the rate loss of optical activity and the rate of trans cis isomerization). Understanding the dynamics of the biradical is thus of pivotal importance. Doubleday has determined an accurate PES for trimethylene from a high level CASSCF ab initio calculation. Trimethylene has a very shallow potential energy minimum... 

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