Some Diradical Intermediates

The decomposition of diphenylenediazomethane produces the di-phenylenemethylene diradical as an intermediate. This intermediate will only be a diradical in case the two electrons follow the usual rule 

Whether or not the intermediate is a diradical, its reactions with organic bases are those appropriate for an electron deficient or electrophilic substance.80 

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. 

Although the Wolf rearrangement of diazoketones in the presence of silver benzoate and triethylamine in methanol could involve rearrangement of an intermediate diradical or carbonium ion conjugate 

Hine has explained the solvolysis of chloroform by means of a diradical (or carbonium ion conjugate base) intermediate M 

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For most Diels-Alder reactions a concerted mechanism as described above, is generally accepted. In some cases, the kinetic data may suggest the intermediacy of a diradical intermediate 18 ... 

The reaction is stereospecific for at least some aliphatic ketones but not for aromatic carbonyls.197 This result suggests that the reactive excited state is a singlet for aliphatics and a triplets for aromatics. With aromatic ketones, the regioselectivity of addition can usually be predicted on the basis of formation of the more stable of the two possible diradical intermediates obtained by bond formation between oxygen and the alkene.198... 

These results can be interpreted in terms of competition between recombination of the diradical intermediate and conformational equilibration, which would destroy the stereochemical relationships present in the azo compound. The main synthetic application of azo compound decomposition is in the synthesis of cyclopropanes and other strained-ring systems. Some of the required azo compounds can be made by 1,3-dipolar cycloadditions of diazo compounds (see Section 6.2). 

Since then, the photocycloaddition reaction has been extensively studied and has become a powerful tool for the construction of complex polycyclic molecules. High stereoselectivities are observed in some cases. The configuration of the diradical intermediate determines the stereochemistry of the adduct [33], Typical examples... 

That the situation is different for photochemical reactions is indicated by a particularly interesting recent study of some dialkylketones (239). In solution, 5-nonanone, 152, reacts photochemically to yield the cyclobutanol 153 and its isomer 154 in comparable amounts. Within the urea clathrate, however, 153 is the dominant product, with only traces of 154 being formed. The cyclobutanols analogous to 153, that is, having methyl and hydroxyl cis, also predominate in the urea-clathrate-mediated photocyclization of 2-hexanone and 2-undecanone. It might be expected that the bulky cyclobutane derivatives, which almost certainly cannot be crystallized in a urea clathrate, would also not be formed in such a clathrate. There are decomposition pathways (cleavage reaction 0 of the diradical intermediate that occur both in the clathrate and in solution. Nevertheless, the ring closure is a major pathway of reaction even in the clathrate. 

Cyclopropane derivatives, including spiropentanc, have proven to be virtually inert towards carbenes,1 For this reason, no literature report that describes cyclobutane synthesis from a C3 and a Cj building block by ring enlargement of cyclopropanes exists. However, due to the partial p character, as well as the increasing reactivity caused by its strain, the central bond of bicyclo[1.1.0]butane (l)2 has been found to react with carbenes.1 Photolysis of diazomethane in the presence of bicyclo[1.1.0]butane (1) at — 50 C provides a mixture of several compounds. The major fraction of the material (80%) was analyzed by means of NMR spectrometry and found to consist of penta-1,4-diene (2, 21%) and bicyclo[l.l.l]pentane (3, 1%), plus several other known compounds as well as some unidentified products.3 The mechanistic pathway for the formation of bicyclo[l.l.l]pentane (3) has not been addressed in detail, but it is believed that a diradical intermediate is involved, as shown below.3... 

Triazolines (116), in which there is no free hydrogen on C-4, also thermolyze to aziridines and enamines.67,454 When R is phenyl, regardless of the X substituent, aziridine is the only thermolysis product but when R is alkyl or hetaryl,453 a mixture of aziridine and enamine is obtained, the latter being the predominant product in some cases.67,453 Because enamine formation from the ring-opened diazo isomers does not seem feasible, a diradical intermediate is proposed.67... 

It has been thought for some time that the interactions of triplet ketones with olefins involve a competition between hydrogen abstraction from, energy transfer to, and cycloaddition to the double bond 126>. Cycloaddition has generally been considered to proceed via a diradical intermediate. At first, the only evidence for a diradical was that the regioselectivity of oxetane formation often — but not always — is such as would be expected from the relative stabilities of the possible diradical intermediates 126>. Moreover, n,n ketone triplets are known to act like alkoxy radicals, which add to olefins, albeit less efficiently than do most other... 

We are far from exhausting the subject of regioselectivity in dipolar cycloadditions with these few examples. Frontier orbital theory, for all its success in accounting for most of the otherwise bewildering trends in regioselectivity, is still fundamentally defective. We should keep in mind that the frontier orbitals used here must reflect some deeper forces than those that we are taking into account in this essentially superficial approach. Nevertheless, no other easily assimilated theory, whether based on polar or steric factors, or on the possibility of diradical intermediates, has had anything like such success. 

Such ESR characterization of 1,3-diradical intermediates is of considerable interest to mechanistic chemical studies. Some of the ESR results are given in Table 17. The ground-state triplet nitrene and its derivatives... 

MD trajectories have been computed for the 8 9 using an AMl-SRP PES fit to the MRCI/cc-pVDZ//CASSCF(4,4)/cc-pVDZ critical point energies." ° " Trajectories at 573 K were initiated at three TSs TS(si), TS(90,0), and TS(0,0). The product distribution from these trajectories is listed in Table 8.1. The product distribution is dependent on which TS is the origin of the trajectory. Trajectories started at TS(si) preferentially produce the Woodward-Hoffmann allowed products, while the ai product is the major one produced from the TS(0,0) trajectories. Trajectories from TS(90,0) give nearly equal distribution of the four possible products. This mode selectivity strongly discounts the presence of a diradical intermediate that has some appreciable lifetime. Such a diradical would undergo intramolecular vibrational energy redistribution, and the trajectories from the three different TSs would result in identical product distributions. 

Cyclobutanes may be converted to alkenes thermally, the reverse of the [2 + 2] cycloaddition reaction. These retroaddition or cycloreversion reactions have important synthetic applications and offer further insights into the chemical behavior of the 1,4-diradical intermediates involved they may proceed to product alkenes or collapse to starting material with loss of stereochemistry. Both observations are readily accommodated by the diradical mechanism. Generation of 1,4-tetramethylene diradicals in other ways, such as from cyclic diazo precursors, results in formation of both alkenes and cyclobutanes, with stereochemical details consistent with kinetically competitive bond rotations before the diradical gives cyclobutanes or alkenes. From the tetraalkyl-substituted systems (5) and (6), cyclobutane products are formed with very high retention stereospecificity,while the diradicals generated from the azo precursors (7) and (8) lead to alkene and cyclobutane products with some loss of stereochemical definition. ... 

The thermally induced ring expansion of 5-spirocyclopropane isoxazolidines (Brandi-Guarna reaction) has been studied extensively and proved to be a general method of synthesizing variously substituted tetrahydropyridones, indolizidinones, and quinolizidinones. The process is believed to occur through diradical intermediates and was also studied by mixed restricted/unrestricted DFT (RDFT/UDFT) calculations <2001EJ04223>. Some representative... 

The cyclization route C was thought to involve a diradical intermediate and not an intramolecular insertion mechanism, since no optical activity in the pyrrolidine formed from (+ )4-methyl -hexylazide was found. These results must be viewed with caution however, since attempts to repeat this work have been unsuccess-fuP furthermore, synthesis of pyrrolidines via route C has been shown in some cases to be a one-step process with retention of optical activity. Thus optical activity was retained in the products of the gas-phase thermolysis of l-azido-2-(2-methyl-butyl)benzene and 2-methylbutylazido formate, viz. 

The thermochemistry and photochemistry of di- and triarylcyclopropenes bearing unsaturated substituents can follow a number of different pathways. In some cases these include formal [4 -I- 2] cycloaddition, although diradical intermediates have been proposed and [2 -I- 2] addition can compete, e.g. formation of 3, 4 and 5. °... 

Seven-membered rings are formed in the thermal decomposition of some hydrocarbons containing the bicyclo[4.1.0]heptane or related structural units. Bicyclo[4.1.0]heptane itself needs a temperature of over 700 °C for ring cleavage and gives a mixture of products containing about 25% of cycloheptene. However, other compounds decompose at more moderate temperatures usually, but not always, via diradical intermediates (Table 7). 

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