Trimethylene diradicals

The singlet carbene adds stereospecifically, because it can correlate with the lowest singlet configuration of a trimethylene diradical, and thus also with a ground state of cyclopropane 1° ). 

The visual and conceptual impact of seeing the timed sequence of structures, a full representation of atomic-scale events as a complex chemical reaction took place, was powerful. This achievement, the product of state-of-the-art calculations applied to an ambitious objective as well as excellent presentation graphics, was not diminished through a repressed awareness that it aU depended on theory. Nothing experimentally based provided an anchor for the visually compelhng rendition of the reacting system as a cyclopropane cleaved a C C bond, formed a trimethylene diradical intermediate, and executed a net one-center epimerization before reverting to the cyclopropane structure. 

The pump-probe-detect arrangements for the femtosecond experiments was similar to those described above. When cyclobutanone was pumped with two photons of a X = 307-nm femtosecond pulse, two consecutive C—CO bond cleavages led to the formation of the trimethylene diradical, detected as an easily ionized transient at 42 amu, with buildup and decay times of 120 20 fs. The decay presumably involves isomerizations to cyclopropane and to propylene— structures not ionized by the probe pulse and thus undetected during the experiment. 

These studies raised many interesting questions and set an ambitious agenda for further work, served admirably to establish the time scales appropriate to tetramethylene and trimethylene diradicals, and confirmed them as distinct molecular species. 

Calculational estimates of the lifetimes of the trimethylene diradical " based on microcanonical variational unimolecular rate theory and direct dynamics simulations have been reported. The lifetimes derived from theory, 91 and 118 fs, are comparable to the experimental estimate, 120 20 fs. Similar lifetime estimates from theory for tetramethylene are comparable, or slightly below, the experimental value. ... 

Dynamics calculations have also provided new approaches to the stereochemical modes through which cyclopropanes and trimethylene intermediates may be related. Full quantum dynamics calculations for the trimethylene diradical based on a reduced dimensionality model that followed wave packet densities and time constants for formation of products led to the conclusion that conrotatory and disrotatory double rotations of both terminal methylene groups are favored over a single rotation of just one by a 2.2 1 ratio." °... 

Now the 1,3-cyclopentanediyl diradical is constrained to cychze in a disrotatory fashion while the trimethylene species might well close in both disrotatory and con-rotatory ways. Were all other factors constant one could infer that the geometrical restrictions imposed on the 1,3-cyclopentanediyl diradical entailed no significant deduction in rate of cyclization, and thus that conrotatory cyclization of the trimethylene diradical is not strongly preferred under the given reaction conditions and circumstances. 

Two-photon excitation at X = 307 nm provides 186 kcal/mol of energy to tetra-hydrofuran (THF) the excited molecule breaks a C—O bond with a characteristic time of 55 15 fs to form an oxytetramethylene diradical. This species, now with 114 kcal/mol of available energy, has a lifetime of 65 15 fs it decays primarily through a C—C p-cleavage reaction, giving the trimethylene diradical (42 amu, t 120 fs). The cleavage to trimethylene is the dominant reaction. ... 

An alternative p-cleavage process contributes to a lesser extent Loss of a hydrogen gives a formylalkyl radical of 71 amu, which loses ethylene to provide another formylalkyl radical of 43 amu. The transient species at 71 and 43 amu have decay times of 120 fs, similar to the time seen for the trimethylene diradical. 

Benson11 has shown that formation of the trimethylene diradical must occur as the initial step in cyclopropane isomerization and... 

Taking AH/(CH2) equal to 80 kcal. and AHf ( CH2CH2CH2 ) equal to 67 kcal.,11 18 formation of the trimethylene diradical by reaction of CH2 + H2C=CH2 is exothermic by 25.5 kcal. From the Rice-Ramsperger-Kassel relation... 

The above results, which are not strongly dependent on the particular value chosen for s, show that about 10% of the trimethylene diradicals lead to propylene, and that the limiting yield of cyclopropane is about 90%. Additional propylene may of course be formed by direct insertion into carbon-hydrogen bonds. 

Early kinetic studies on the structural isomerization of cyclopropane to propene provided estimates of activation parameters73 75 and prompted speculation that the reaction might well involve a trimethylene diradical intermediate. This possibility seemed reinforced when the thermal interconversion of the els and trans isomers of l,2-d2-cyclo-propane at 414 to 474 °C (equation 1) was reported in 195876. This structurally degenerate isomerization was found to be substantially faster than conversion to deuterium-labeled propenes—about 24 times faster at the high pressure limit76 77. 

Kinetic work on the isomeric 1,2-diphenylcyclopropanes (Scheme 2) made evident a substantial reduction in Ed and thus implied a stabilization of trimethylene diradical transition structure(s) by phenyl substituents142. In further work with 0.2 M (-)-l,2-diphenylcyclopropane in 1 -butanol, Crawford and Lynch143 uncovered a direct route from one trans antipode to the other at 220.7 °C the measured ratio of rate constants /trac(for loss of optical activity) to kK (for trans to cis geometrical isomerization) was found to be 1.49 0.05 and since krdC is (2k12 + 2/c,). and klc is 2/c,(Scheme 2), the implication is that one-center epimerizations (2kt) are favored over the two-center epimerization process (ka) by... 

Thus a substantial body of experimental evidence shows that 1,2-disubstituted cyclopropanes, including vinylcyclopropanes, react thermally to give isomeric cyclopropanes through both one-center and two-center epimerizations, with (kt + k2) kl2 ratios from 1.4 to 4. Rate constants for both (, + k2) and kl2 events respond to the capacity of substituents to stabilize adjacent radicals in a regular fashion consistent with trimethylene diradical transition structures. Rate constants for vinylcyclopropane structural isomerizations do as well, thus reinforcing the notion that these reactions are nonconcerted diradical mediated reactions. 

In addition some p.opane and n-hexane are formed. These are also believed to originate from the trimethylene diradical. A direct comparison of these results with the results reported by other workers is not possible since the only other study in the presence of a foreign gas involved water vapor at a pressure of 70 mm. (5). In the latter instance the ketone pressure was 136 mm., which is 5 to 10 times greater than the pressure used in ref. (17). The presence of water vapor does not seem to have affected the yields of propylene and of cyclopropane. Since the diradical mecha-... 

Show how the pericyclic theory predicts that the closure of the trimethylene diradical (3) to cyclopropane should be conrotatory. 

Generation of Tetra- and Trimethylene Diradicals from Cyclic... 

The Bond-Forming Initiation Theory gives a good interpretation of the observed spontaneous polymerizations of captodative monomers. The tetramethylene diradicals already implicated as initiators in the thermal (spontaneous) polymerizations of vinyl monomers can be particularly stabilized by captodative substituents. For comparison, and to initiate the polymerization of third monomers, captodative cyclobutanes and cyclopropanes are particularly appropriate precursors for generating tetra- and trimethylene diradicals. In particular the extensive work of Viehe [3,45,46] showed that thermolysis of captodative substituted cyclopropanes leads to trimethylene captodative diradicals at reasonable temperatures. Their initiating abilities for polymerization have not yet been determined. 

Horsley et a/.164 have determined a reaction pathway for the geometrical isomerization of cyclopropane by a calculation and search of the full 21-dimensional hypersurface The calculations were performed using a minimal basis set of STO s in a modified SCF calculation using the restricted Hartree-Fock formalism suggested by Nesbet.165 In the region of the trimethylene diradical, a 3 x 3 configuration interaction calculation was included. 

The photolysis of pyrazolines has attracted much attention since in this reaction 1,3-diradicals are produced, which were supposed to be related to the intermediate trimethylene diradical which would be a model for an addition of singlet... 

After a brief historical recapitulation, the substantial body of experimental and theoretical work on these thermal epimerization reactions reported over the past 40 years is summarized. Of primary concern here are examples of stereomutations involving monocyclic, stereochemically unconstricted and minimally substituted molecules. Experimental studies of more heavily substituted cyclopropanes attempts to generate trimethylene diradical intermediates from pyrazolines " and the fascinating and still incompletely understood thermal chemistry of bicyclo[2.1.0]pentanes, 2-methylenebi-cyclop.l.OJpentanes", bicyclo[3.1.0]hex-2-enes and related reactions such as the pyrolysis of cyclopropane at 1200 °C to give products such as cyclopentadiene and toluene are neglected, in spite of obvious mechanistic interrelationships. 

Most of the radical stabilization energy parameters utilized in Figure 2 (R = H or D, -2.65 Me or Et, - 0.5 CN, 6.6 vinyl or propen-2-yl, 13.2 (E )-propenyl, 15.4) are SE values calculated by Leroy and coworkers " those for phenyl (9.4) and cinnamyl (19.1) were set empirically. These particular parameters are serviceable and reasonable, but cannot be considered preferred . Other plausible radical stabilization energy values lead to very similar correlations which demonstrate, as Figure 2 demonstrates, that the rate constants for two-center epimerizations of cyclopropanes and vinylcyclopropanes are sensitive to the radical stabilizing capacities of substituent groups in a simple additive fashion. Whatever 1,3-disubstituted trimethylene diradical intermediates or transition structutes may be involved respond kinetically to substituents as though each locus of radical character were thermochemically independent. 

---