Diradical intermediates, and

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). 

From the photoreactions presented in this section it appears that both diradical intermediates and ionic intermediates can be involved in the photochemistry of dienones. 

The observation that the transition state volumes in many Diels-Alder reactions are product-like, has been regarded as an indication of a concerted mechanism. In order to test this hypothesis and to gain further insight into the often more complex mechanism of Diels-Alder reactions, the effect of pressure on competing [4 + 2] and [2 + 2] or [4 + 4] cycloadditions has been investigated. In competitive reactions the difference between the activation volumes, and hence the transition state volumes, is derived directly from the pressure dependence of the product ratio, [4 + 2]/[2 + 2]p = [4 + 2]/[2 + 2]p=i exp —< AF (p — 1)/RT. All [2 + 2] or [4 + 4] cycloadditions listed in Tables 3 and 4 doubtlessly occur in two steps via diradical intermediates and can therefore be used as internal standards of activation volumes expected for stepwise processes. Thus, a relatively simple measurement of the pressure dependence of the product ratio can give important information about the mechanism of Diels-Alder reactions. 

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 mixed dimerization of polyhalogenated alkenes with both activated and nonactivated alkcncs has been well documented (see Houben-Weyl, Vol. 4/4, p 206) and continues to represent a convenient preparative method for polyhalogenated cyclobutanes. Of the polyhalogenated alkenes, the fluorinated ethenes are the most reactive towards [2 + 2] cycloadditions. The method is, however, limited by the nonstereoselectivity due largely to the formation of 1,4-diradical intermediates and the requirement of high temperatures. The observation of stereochemical equilibration is seen in the cycloaddition products of tetrafluoroethene (1) with (island (Z)-but-2-ene and (Z)-l,2-[2H2]ethene where mixtures of stereoisomeric cyclobutanes are obtained.19-20... 

Difluoroallene cycloadditions occur via 1,4-diradical intermediates and are generally nonregioselective.4 This is seen in the production of both l,l-difluoro-2-methy cnecyclobutanes and (difluoromethylenc)cyclobutanes as major products in the cycloadditions of 1,1-difluoroal-iene with alkenes.14- The same is true for fluoroallcne cycloadditions except that additional isomers can occur due to EjZ isomerization of the (fluoromethylene)cyclobutanes.17... 

The evidence for such a mechanism results from both the reaction stereochemistry and also from the observation of minor diphenyltoluene by-products (Scheme 8). The major pathway is outlined using heavy arrows. This can be seen to afford the stereospecificity of equations 14a and 14b. Additionally, each of the diradical intermediates and intermediate excited states—B, C, E, F and G—undergo a minor extent of internal bond fission [i.e. Grob or 2,3- (1,4) fragmentation] to afford a diphenyltoluene with the corresponding ring skeleton. The basis for the choice of a main pathway versus the minor ones comes from the observed stereochemistry. 

The regiochemistry of the latter reaction was thought to be due to the stability of the 1,4-diradical intermediate and interaction between the electrophilic oxygen of the aldehyde and the C-2 nucleophilic carbon of the 0,A-ketene acetal. The stereochemistry of this reaction was attributed to sulfur effects controlling the approach of the electrophilic oxygen of the triplet mt aldehyde to the nucleophilic alkene. 

To understand this effect requires a discussion of the mechanism of autoinitiation. Of the mechanisms proposed, two receive most of the discussion in the literature. A mechanism proposed by Flory [15] involves a 1,4-cyclobutane diradical intermediate and has been supported experimentally by polymerization in the presence of a free radical scavenger. Several of the dimers found in thermally initiated polystyrene are also attributed to a Flory initiation mechanism. 

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. 

However, in studies conducted at about the same time on benzosilacyclobutenes, in which carbonyl compounds were inserted into the silacyclobutene ring (in yields which varied from 25-80%), the products were interpreted as arising from attack of an excited carbonyl species directly on the ring. The mechanism implies formation of a diradical intermediate, and explains the formation of minor isomeric products35 (equation 21). 

This is only found for reactions involving non-polar diradical intermediates and is not observed for polar zwitterionic intermediates. In the latter case the volume of activation of the zwitterionic intermediate can be more negative than that of a competing pericyclic reaction due to the solvent-dependent effect of electro-... 

The photoinduced cis-trans interconversions of 1,2-diarylethenes are reviewed in Chapter 3 of this Volume. It is, however, pertinent to note here the study into the geometric isomerization of cis 2,3-diphenylcyclopropane-l-earboxylic acid derivatives. Such compounds, contrary to what is known about 1,2-di-phenylcyclopropanes, are now reported to have triplet energies of approximately 311 kJ mol , to undergo the less common adiabatic isomerization to the Irons isomers, to display emission from the electronically excited 1,3-diradical intermediates, and not to undergo intersystem crossing on direct excitation. 

Fig. 7 Mechanochemical reactivity of gDFC. At high forces, cis- and trans-gOFC open to form a 1,3-diradical intermediate and A,ram respectively). Upon releasing the force (B), the inter-...

In the diradicaloid region of the surface, the following critical points have been found (a) an extended diradicaloid transition state for the formation of the first bond (b) a diradical intermediate, which exists in a trans and a cis form and (c) a second diradicaloid transition state which connects the cis form of the diradical intermediate with the product isoxazole. On the basis of these results, it seems likely that the transition state for the formation of the first bond connects the reactants with the cis form of the diradical intermediate and the trans form represents a subsidiary minimum accessible via an in-plane inversion process. 

Studies of the diradical (stabilized by a phenyl group) or zwitterionic intermediate have been undertaken for the Schmittel reaction of the en)me-allene with larger substituents (SiMes, /-Bu). It was reported that C2-C6 cyclization occurs via the diradical intermediate and aryl or bulky groups at the alkyne terminus trigger a general thermal reaction pathway for the enyne-allene. ... 

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. 

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