Cycloadditions unimolecular reactions

The majority of radical ion reactions are bimolecular in nature, although some of these are merely variations of the unimolecular reactions discussed above, and many occur as pair reactions, albeit with a modified partner. Radical ions may react with polar or nonpolar neutral molecules, with ions, with radicals, or with radical ions of like or opposite charge (Scheme 6.3). Alkene radical ions undergo a particularly rich variety of reactions, including additions and cycloadditions. 

Diels-Alder reaction, for example, is much better suited as a unimolecular reaction than the bimolecular cycloaddition because the former allows better control of precursors, in which the structural properties are well defined (96JA8755). Fragmentation of a molecule may be initiated by various methods depending on how the required energy is supplied. 

Cycloadditions are easier to treat than unimolecular reactions they only require an evaluation of the best FO overlap (rule 4). Let us look at the cyclodimerization of butadiene. Woodward and Hoffmann suggested that the experimentally observed endo compound is due to secondary interactions (shown by the double arrows above), which increase the stabilizing the FO s interaction.23 Cisoid configurations are often adopted by the dienophile in Diels-Alder reactions,24 as first suggested by Dewar.20 For... 

Large negative values two molecules go to one or unimolecular reaction with ordered TS (cycloaddition, etc.)... 

In addition to these reactions, which formally involve unimolecular reactions, it is also been reported (84CC190) that certain ylids derived from tetrachlorothiophene will undergo cycloaddition reactions. 

Considerations of this kind, that were not emphasized in connection with the unimolecular reactions dealt with in the preceding chapter, attain crucial importance when the geometric requirements of cycloadditions and cycloreversions are compared. Like the isomerizations previously discussed, cycloreversions are unimolecular a non-totally symmetric vibrational motion that may be called for by the correspondence diagram will ordinarily be opposed by a restoring force. Cycloadditions, at least the prototypical ones, are bimolecular the two reactants can approach each other in a variety of ways, their reorientation in space costing no energy at all. It then becomes reasonable to ask how the conclusions which may be reached by the orbital symmetry analysis depend on the initial geometry assumed for the approach of the reactants towards one another. 

Species A is transformed into the higher-energy form B as a result of irradiation with Hght. The reverse reaction to reform A usually occurs as a spontaneous thermal process, but may also be Hght induced. Equation 96.1 holds only for unimolecular reactions. More recently, bimolecular photochromic systems, based on a cycloaddition or cycloreversion,have also been discovered. 

The following compounds have been obtained from thiete 1,1-dioxide Substituted cycloheptatrienes, benzyl o-toluenethiosulfinate, pyrazoles, - naphthothiete 1,1-dioxides, and 3-subst1tuted thietane 1,1-dioxides.It is a dienophile in Diels-Alder reactions and undergoes cycloadditions with enamines, dienamines, and ynamines. Thiete 1,1-dioxide is a source of the novel intermediate, vinylsulfene (CH2=CHCH=SQ2). which undergoes cyclo-additions to strained olefinic double bonds, reacts with phenol to give allyl sulfonate derivatives or cyclizes unimolecularly to give an unsaturated sultene. - Platinum and iron complexes of thiete 1,1-dioxide have been reported. 

Direct photochemical excitation of unconjugated alkenes requires light with A < 230 nm. There have been relatively few studies of direct photolysis of alkenes in solution because of the experimental difficulties imposed by this wavelength restriction. A study of Z- and -2-butene diluted with neopentane demonstrated that Z E isomerization was competitive with the photochemically allowed [2tc + 2n] cycloaddition that occurs in pure liquid alkene. The cycloaddition reaction is completely stereospecific for each isomer, which requires that the excited intermediates involved in cycloaddition must retain a geometry which is characteristic of the reactant isomer. As the ratio of neopentane to butene is increased, the amount of cycloaddition decreases relative to that of Z E isomerization. This effect presumably is the result of the veiy short lifetime of the intermediate responsible for cycloaddition. When the alkene is diluted by inert hydrocarbon, the rate of encounter with a second alkene molecule is reduced, and the unimolecular isomerization becomes the dominant reaction. 

The latter number incorporates just the chemical step(s) of formation of triazole within cucurbituril. Since the product release step apparently is at least 100-fold slower than the actual cycloaddition, the net catalytic acceleration should be adjusted downward by that amount. An instructive alternative estimation of kinetic enhancement is to compare the extrapolated limiting rate for cycloaddition within the complex (i.e. cucurbituril saturated with both reactants, k — 1.9xl0 s ) with the uncatalyzed unimolecular transformation of an appropriate bifunctional reference substrate as in Eq. (3) (k, = 2.0x 10 s ). Such a comparison of first-order rate constants shows that the latter reaction is approximately a thousandfold slower than the cucurbituril-engendered transformation. This is attributable to necessity for freezing of internal rotational degrees of freedom that exist in the model system, which are taken care of when cucurbituril aligns the reactants, and concomitantly to an additional consideration which follows. 

Whereas cycloadditions are characterized by two components coming together to form two new a-bonds, electrocyclic reactions are invariably unimolecular. They are characterized by the creation of a ring from an open-chain conjugated system, with a a-bond forming across the ends of the conjugated system, and with the conjugated system becomes shorter by one p-orbital at each end. 

Looking more at host-guest type systems, the cucurbiturils are often very efficient catalysts in cases where they simultaneously bind two guests, thus increasing effective concentration and changing a bimolecular reaction into a guas/ -unimolecular one. To take one recent example, cucurbit[6]uril (Section 6.2.4) proves to be a very efficient catalyst for the click 1,3-dipolar cycloaddition reaction of an azide with an alkyne, particularly where the substrates are suitable guests for the cavity, as in Scheme 12.20. Cucurbituril binds very effectively to ammonium ions and addition of a small amount... 

The mechanism of the decomposition reaction of 5-methoxy- 1,2,3,4-thiatriazole to dinitrogen sulfide and methoxy-nitrile was studied by the DFT method at the CCSD(T)//MP2/6-31+G level of theory <2003JOC6049>. The calculations indicated that this is a concerted retro-[2+3]-dipolar cycloaddition process with an activation energy of 28.9 kcal mol 1 and a reaction energy of 1.9 kcal mol. This unimolecular decomposition is favored due to the entropy gain (25.8 eu) involved in the overall reaction (Scheme 1 and Table 2). 

The decomposition of dichlorosilacyclopent-3-ene has been proved to be unimolecular by the study of inert dilution. The mass spectral study suggested that the decomposition was concerted, and, in the opinion of the investigators, it corresponded formally to the retro Diels-Alder reaction 13, 29). Thus the reaction in Eq. (38) was believed to proceed via a concerted 1,4-cycloaddition. 

The thermal unimolecular cyclization of 3-diazoalkenes to pyrazoles appears to be an intramolecular 1,3-dipolar cycloaddition and the first-order rate coefficients of four substituted // art5-3-diazo-l-phenylpropenes fit the Hammett equation (p = — 0.40). The small value of p, indicating a lack of sensitivity of the cyclization rate to the electronic nature of the substituents supports the belief that the reaction involves a synchronous, cyclic electron shift. Table 11 lists the measured rate coefficients. 

Theoretical studies are also done to interpret the synthesis reactions and mechanism of reactions. The regioselectivity of 1,3-dipolar cycloaddition reaction between substituted trimethylstannyl-ethynes and nitrile oxides yielding isoxazoles, was interpreted by the application of frontier electron theory <93CPB478>. By the combination of experimental and molecular orbital (ab initio) studies, a multistep mechanism is proposed for unimolecular radical chemistry of isoxazoles in the gas phase <920MS(27)317>. 

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