Biradical tetramethylene

In order to account for the large positive entropy of activation for the reaction, it is necessary to assume that there is virtually free rotation of the incipient ethylene molecules in the complex. The second possible transition complex involves the complete rupture of one carbon-carbon bond to give the tetramethylene biradical, and the reaction path may be envisaged as shown below ... 

This result suggests a stepwise mechanism. The first step is the formation of a transoid tetramethylene biradical. Then, this intermediate rotates, thereby permitting closure of the cyclobutane ring in a second step. Recent high-quality ab initio calculations [7-37] support this mechanism. The reverse of ethylene dimerization, the pyrolysis of cyclobutane, is experimentally observed [7-38]. Both quantum-chemical calculations [7-39] and thermochemical considerations [7-40] suggest that the pyrolysis proceeds through a 1,4-biradical intermediate. This shows the value of the additional information yielded by the orbital correspondence approach. 

Dervan examined the decomposition of 3,4-dideuterio-cw-tetrahydropyridazine and found that A cieavage/ cyciization f a presumed biradical intermediate of the geometry of starting material was 2.2. It was further found that rotation/ cyciization this biradical was 12 3 (Scheme 5.21). Thus, it appears that the unsubstituted tetramethylene biradical undergoes rotation ca. 5.5 times faster than cleavage and 12 times faster than cyclization. 

First, the most important step in the analysis of the above scheme requires us to characterize the structure of the intermediate since it is only when its structure is known with sufficient certainty that the predictions based on the value of the overlap determinants can be reliable. In general, the question of the structure of the intermediate can, of course, be quite complicated, but in the case of pericyclic reactions, which are of concern here, the situation is slightly more simple. This is due to the fact that the set of structures which could play the role of the eventual intermediates is restricted only to species of a biradical and/or zwitterionic nature [60,61], so that the proposal of the structure of the eventual intermediate need not be so complicated. Thus, e.g., in the case of 2j + 2g ethene dimerization, the corresponding intermediate can be naturally identifi with the tetramethylene biradical. In such a case, the whole two step reaction scheme can be desribed as follows ... 

These tendencies are almost fully developed in the bottom four plots of Figure 7 (/ = 1.7 A). Orbitals 03 and 07 (not shown) now form a new a-bond while 04 and 0g (not shown) become almost pure C 2p orbitals. It is obvious that we are already dealing with a franJ-tetramethylene biradical. The orbital changes along the cis pathway are of a very similar nature, with a cis tetramethylene biradical forming again within. l k < R < 1.8 A. 

Here we do not consider the second step of the reaction, the closure of the tetramethylene biradical to yield cyclobutane. This would require a much more detailed study of the C2H4 -I-C2H4 potential surface. 

Complexes of trimethylenemethane 143 and tetramethylene ethane 144 biradicals [224a] are typical examples of biradical coordination compounds ... 

Trimethylenemethane biradicals have been proposed as intermediates in the photodecomposition of fluorine-substituted 4-methylenepyrazolines both in the gas phase and in solution.Evidence for the intermediacy of two trimethylenemethane biradicals on direct irradiation of the bi-(l-pyrazolin-4-ylidene) (29) has also been reported. Benzophenone-sensitized irradiation of the same pyrazoline, however, takes a different course and affords the allene dimer (30), presumably via the tetramethylene-ethane biradical (31). 

Most recently, this compound has been examined by Weiner and LIF analysis of the diatomic products [126,131]. The quantum yield for SO( S ) was determined to be about 0.45 at either 193 or 248 ran. In contrast to the two smaller ring systems, the SO( S ) was foimd to be vibrationally thermalized at about 1250K with both excitation wavelengths, and the vibrational profile with 193 ran excitation was consistent only with tetramethylene as the other product after dissociation to two molecules. The thermalized nature of the SO spectra favors a stepwise process with the a-cleavage biradical 240 as an intermediate. If a stepwise process is invoked, then the tetramethylene fragment should be of a similar temperature. According to ab initio calculations by Doubleday [132], the ratio of cyclization to fragmentation should be about 0.27 at this temperature, consistent with experimental data [128] between 202 and 225 ran. 

The question whether the intermediates 6.24 and 6.26 are zwitterions or biradicals has not yet been answered. As known from the tri- and tetramethylene species (see Hoffmann, 1968 Hoffmann et al., 1970 Hiberty, 1983 Harcourt and Little, 1984 and Ejiri et al., 1992), zwitterions and biradicals are probably extremes of a structure-dependent degree of charge separation. ... 

Because both 1-methylenespiropentane (89) and 1-cyclo-propylidenecyclopropane (95) include methylenecyclopropane moiety in their structures, the methylenecyclopropane-type reversible interconversion between 89 and 95 is expected to occur upon pyrolysis, involving a trimethylenemethane biradical intermediate. However, such a rearrangement formally does not take place, though 95 rearranges to 89. Instead, on pyrolysis at 320 C, 89 rearranges to dimethylenecyclobutanes (91 and 93) through the tetramethylene-ethane biradical 90 and the vinylic-allylic biradical 92, respectively. Presumably, biradical 94 formed by C-2-C-5 bond fission... 

Finally, similar results were obtained using In bulk, 5 is a very slow initiator even at 1 1 ratio at room temperature. This is due to very slow tetramethylene formation. It is expected that 5 will form a tetramethylene intermediate with more biradical character owing to the ester substituents, but it seems that this is not the case. No copolymer was formed, because of the presence of the chloride ion which would be eliminated relatively rapidly from the tetramethylene. 

The experimental evidence cited above indicates that this does not occur. As suggested at the right of Fig. 6.6, the in-plane glide (6i) begins well before the intended HOMO-LUMO crossing, which is avoided because all four MOs have the same irrep (o ) in The HOMO and LUMO of the extended tran-soid zwitterion are qualitatively similar to those in the biradical illustrated in Fig. 6.4, but are more widely separated in energy As a result, the orbital approximation - and the symmetry analysis based upon it - is no less reliable than for the biradical mechanism. The zwitterionic mechanism can be accomodated by Fig. 6.5, with self-evident modifications arising from the polar nature of the tetramethylene intermediate. ... 

In smaller ring systems, other interesting processes become feasible. Eqs. 11.77 and 11.78 show the biradicals formed from C-C bond homolysis in cyclopropane and cyclobutane, respectively. Cyclopropane homolysis produces a 1,3-biradical, and the parent system is called trimethylene. Cyclobutane homolysis produces a 1,4-biradical, and the parent system is called tetramethylene. In both of these biradicals, the two radical centers are close enough to experience some interaction. As such, it is best to think of these systems not as two radicals, but as a single, unique reactive intermediate, a biradical, just as we consider a car-bene to be a single reactive intermediate rather than two separate radicals. 

An important aspect of tetramethylene chemistry is that similar results are seen whether the biradical is prepared by cyclobutane thermolysis, by ethylene dimerization, or by diazene photolysis. Seeing the same product ratios from different modes of preparation is one of the most stringent tests for the existence of a common reactive intermediate. 

The stereochemical isomerization implies a competition between ring closure and bond rotation in the biradical reactive intermediate. As with tetramethylene, a competition is supported by studies of appropriately substituted diazenes which, on thermolysis, lose N2 and presumably produce the biradical (Eq. 11.84). However, in an early indication that things are not as simple as with tetramethylene, the diazene experiments show a "cross-over" effect. The cis diazene preferably produces trans cyclopropane, and vice versa. 

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... 

Returning to the initiation process H K Hall and his collaborators have been investigating the spontaneous copolymerization of vinyl monomers containing donor and acceptor groups They have presented strong evidence for the existence of a tetramethylene intermediate 30 (Eq 7) which is a resonance hybrid of a zwitterion and a spin paired biradical, the terminal carbon atoms of which sense each other by through bond coupling. ... 

This intermediate has been identified in many small molecules reactions such as cycloaddition The tetramethylene 30, depending upon conditions, may cyclize or react with more monom to form either a copolymer or a homopolymer. It has been proposed that a largely zwitterionic produces cationic homopolymers whereas a largely biradical tetramethylene produces an alternating copolymer ... 

The dimerization of a quinodimethane to the biradical is analogous to the tetramethylene and may represent the initiation step in j quinodimethane homopolymerization. ... 

In the case of copolymerizations of a quinodimethane and a vinyl monomer the reaction may proceed analogously. For example in Table I the experiments resulting in alternating copolymer may be explained by formation of a largely biradical analog of the tetramethylene (a bis phenylene tetramethylene). This scheme is presented in Fig, 4,... 

On the other hand, the ab initio calculations [7-10] with the STO-3G, 4-31G basis sets and Cl point to the presence of biradical-type intermediates V, i.e., gauche- and trans(VI)-tetramethylenes. 

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