Cycloaddition and Cycloreversion

The corresponding excited state reactions are no less ambiguous goes up 

Advisedly, Woodward and Hoffmann do not rely heavily on frontier electron arguments for their conclusion that the [4 - - 2 ] pathway is allowed in the ground-state and forbidden in the excited state for both the cycloaddition and its cy- 

A ground-state reaction is allowed whenever the electron configuration of the product s ground-state has the same symmetry as that of the reactant with respect to the symmetry element(s) retained along the pathway. Similarly, if the lowest excited states of reactant and product have the same symmetry properties, the photochemical reaction is allowed] otherwise, it is forbidden. 

The correlation of two ground-states of reactant and product and the non-correlation of their excited states along the [ 4 -h7r2s] pathway, which preserves the mirror plane, m, can be summarized compactly as follows  

The electron configuration, of a particular state is a list of its occupied orbitals, a superscript 2 indicating occupancy by two electrons of opposite spin. The two ground-state configurations correlate because each has three doubly occupied orbitals, two of them symmetric and one antisymmetric with respect to m. The correlation or non-correlation of the states of reactant and product does not depend on the energetic ordering of their occupied orbitals, but simply on whether the number of doubly-occupied orbitals with each symmetry label is the same in both. It follows that the initial slope of the HOMO is irrelevant to this criterion of allowedness . 

---

In a sequence of cycloaddition and cycloreversion, 3-phenyl-l, 2,4-triazine 1-oxides react with benzyne, generated from 2-aminobenzoic acid (see Houben-Weyl, Vol. 5/2 b, p 622 ff), to give 2-phenyl-l, 3-benzoxazepines in moderate yield.419... 

A complex sequence of pericyclic reactions, intramolecular and intermolecular cycloadditions and cycloreversions, was studied in an attempt to readily achieve bicyclic five-membered heterocycles, the methyl 4,6-dihydrothieno- and methyl-... 

If the cycloaddition and cycloreversion steps occurred under the same conditions, an equilibrium would establish and a mixture of reactant and product olefins be obtained, which is a severe limitation to its synthetic use. In many cases, however, the two steps can very well be separated, with the cycloreversion under totally different conditions often showing pronounced regioselectivity, e.g. for thermodynamic reasons (product vs. reactant stability), and this type of olefin metathesis has been successfully applied to organic synthesis. In fact, this aspect of the synthetic application of four-membered ring compounds has recently aroused considerable attention, as it leads the way to their transformation into other useful intermediates. For example aza[18]annulene (371) could be synthesized utilizing a sequence of [2 + 2] cycloaddition and cycloreversion. (369), one of the dimers obtained from cyclooctatetraene upon heating to 100 °C, was transformed by carbethoxycarbene addition to two tetracyclic carboxylates, which subsequently lead to the isomeric azides (368) and (370). Upon direct photolysis of these, (371) was obtained in 25 and 28% yield, respectively 127). Aza[14]annulene could be synthesized in a similar fashion I28). 

According to the model for [2+2] cycloaddition shown in Fig. 2, it should be possible to reach the pericyclic intermediate upon irradiation of the cycloadduct. If a common intermediate is attained from the cycloaddition and cycloreversion processes, then the sum of the quantum yields for the two processes should equal unity. This has, in fact, been observed to be the case for several exciplex and anthracene excimer systems (49b,52). Stereospecific cycloreversion of stilbene dimers 11 and 12 to t-1 has been observed to occur upon 254 nm... 

The consistently low yields in the cycloaddition of aryl azides to certain enamines have been attributed to an unfavorable equilibrium between cycloaddition and cycloreversion reactions.232 The reversibility of the cycloaddition has been demonstrated on the basis of spectroscopic (Section III) and chemical evidence (Scheme 55). The conditions for cycloaddition and cycloreversion have been examined, using MO calculations.72... 

Tandem intramolecular 1,3-dipolar cycloadditions and cycloreversion, phosphinimine alkylidenemalonate cyclization, and retro-malonate additions have been reviewed.52 The origins of the stereoselection in the 1,3-dipolar cycloadditions to chiral alkenes53 and the 3 + 2-cycloadditions of fullerene, Cea, have been reviewed.54 The selectivity of the double 3 + 2-cycloaddition of tethered double vinyl carbene species in die presence of C6o varies witii the nature of the tether.55... 

Appreciable solvent effects are also obtained in photochemically induced [2 -I- 2]-cycloadditions and cycloreversions [390-392], Examples are given in Eqs. (5-143) [390] and (5-144) [680],... 

The formation of the 3-aza-l,2,4,6-tetraphosphaquadicyclanes 62a-62e was rationalized by a sequence of [2+2] cycloadditions, insertion reactions, reductive eliminations, [4+2] cycloaddition, and cycloreversion processes (Scheme 25) <1998AGE1233, 2001ZN951, 2000CEJ4558>. 

Diradical intermediates may occur in other types of cycloadditions and cycloreversions. Kinetic data for the thermal transformation of ci5,ci5-l,5-cyclooctadiene to butadiene and 4-vinylcyclohexene are consistent with a diradical intermediate the same intermediate may be involved in the reaction of butadiene leading to [2 + 2] adducts and to the Diels-Alder product 4-vinylcyclohexene. - That a Diels-Alder product may arise from a stepwise path is not as unimaginable today as may have been the case just a few years ago. In more elaborate contexts as well, regiochemistry may be successfully rationalized through estimations of the relative stabilities of diradical intermediates. ... 

As the focus of this chapter is on the synthetic utility of the rDA reaction, an overview of mechanism is beyond the scope of this review however, the subject has beoi reviewed previously. Structural and medium effects on the rate of the rDA reaction are of prime importance to their synthetic utility, and therefore warrant discussion here. A study of steric effects cm the rate of cycloreversicHi was the focus of early work by Bachmann and later by Vaughan. The effect of both diene and dioiophile substituticHi on Ae rate of the rDA reaction in anthracene cycloadducts has been reported in a study employing 45 different adducts. If both cycloaddition and cycloreversion processes are fast on the time scde of a given experiment, reversibility in the DA reaction is observed. Reversible cycloaddition reactions involving anthracenes, furans, fulvenes and cyclopentadienes are known. Herndon has shown that the well-known exception to the endo rule in tiie DA reaction of furan with maleic anhydride (equation 2) occurs not because exo addition is faster than endo addition (it is not), but because cycloreversion of the endo adduct is about 10 000 times faster than that of the exo adduct. ... 

Metallacycloalkanes are proven key intermediates in metal-catalyzed cycloadditions and cycloreversions of alkenes. The relationship of some iron metallacyclopentane derivatives with bis(olefin) complexes has been investigated theoretically. Scheme 1 shows a general route from bisalkene... 

Triazolium salts 164 react with ynamine to give products that are determined by the substitution pattern of the salts and the reaction conditions [3 + 2]-cycloaddition and cycloreversion, as well as [4 + 2]-cycloaddi-tion, are observed (83CB186). The diphenyl compound 164a yields an un-... 

A complex sequence of pericyclic reactions, intramolecular and intermolecular cycloadditions and cycloreversions, was studied in an attempt to readily achieve bicyclic five-membered heterocycles, the methyl 4,6-dihydrothieno- and methyl-4, 6-dihydrofuro[3,4-b]-furan-3-carboxylates 146 and 147. The results give further evidence of the potential of intramolecular Diels-Alder based multiple processes [129], 2-Substituted furans and thiophenes 148 and 149, heated in the presence of 3,6-di(pyridin-2 -yl)-,y-tetrazine, underwent intramolecular and intermolecular cycloadditions. The cycloadducts underwent double cycloreversion reactions with the loss of a nitrogen and dipyridyldiazine as illustrated in Scheme 2.55. The electron-deficient dipyridyltetrazine reacts with the isolated, electron-rich olefinic bond rather than with the bond conjugated with the methylcarboxylate. 

The 1,3,5/l -diazaphosphinines react with alkynes in a cascade of (4 + 2)-cycloaddition and cycloreversion reactions, giving the substituted 2 -phosphinines (190) (Scheme 37) <91AG(E)106,92TH625-01 >. With diynes the cycloadditions yield oligomeric phosphinines, and finally polymers with P-phosphinine units <96TH 625-01 >. 

Fig. 26.4 A catalyic cycle for ring-closure metathesis (RCM) showing the Chauvin mechanism which involves [2 + 2]-cycloadditions and cycloreversions.

---