Pericyclic reactions irradiation

A simple example is the chlorination of methane (CH4), in which CH4 and elemental chlorine are mixed and irradiated to yield a mixture of chlorohydrocarbons, such as CH3C1 and CCI4. The energy for reaction comes from the UV photons. Diels-Alder and other pericyclic reactions also require photons of light. 

Danheiser et al. developed a new aromatic annotation methodology for the total s)mthesis of hyellazole (245) by irradiation of the heteroaryl a-diazo ketone 675 in the presence of 1-methoxypropyne (590). This reaction proceeds via the photochemical Wolff rearrangement of the heteroaryl a-diazo ketone 675 to generate a vinylketene, followed by a cascade of three pericyclic reactions. 

Many pericyclic reactions take place photochemically, that is, by irradiation with ultraviolet light. One example is the conversion of norbornadiene to quadricyclene, described in Section 13-3D. This reaction would have an unfavorable suprafacial [2 + 2] mechanism if it were attempted by simple heating. Furthermore, the thermodynamics favor ring opening rather than ring closure. However, quadricyclene can be isolated, even if it is highly strained, because to reopen the ring thermally involves the reverse of some unfavorable [2 + 2] cycloaddition mechanism. 

The best solvent from an ecological point of view is without doubt no solvent. There are many great reactions that can already be carried out in the absence of a solvent, for example numerous industrially important gas-phase reactions and many polymerizations. Diels-Alder and other pericyclic reactions are also often carried out without solvents. Reports on solvent-free reactions have, however, become increasingly frequent and specialized over the past few years. Areas of growth include reactions between solids [5], between gases and solids [6], and on supported inorganic materials [7], which in many cases are accelerated or even made possible through microwave irradiation [8]. 

Huckel was also able to show that if a cyclic conjugated tt-system is irradiated with light so that it goes into the first excited singlet or triplet electronic state, it is especially stable if the number of cyclically conjugated electrons equals [4n]. Hence, photochemically activated pericyclic reactions will proceed suprafacially via a Huckel transition state if the electron count corresponds to [An],... 

The ubiquitous and reversible formation of radical cations in photoelectrochemical transformations allows pericyclic reactions to take place upon photocatalytic activation since the barriers for pericyclic reactions are often lower in the singly oxidized product than in the neutral precursor. For example, ring openings on irradiated CdS suspensions are known in strained saturated hydrocarbons [176], and formal [2 -I- 2] cycloadditions have been described for phenyl vinyl ether [ 177] and A-vinyl carbazole [178]. The cyclization of nonconjugated dienes, such as norbomadiene, have also been reported [179]. A recent example involves a 1,3-sigmatropic shift [180]. 

As is the case for other pericyclic reactions, the selection rules for a thermal [i, ] sigmatropic reaction are reversed for the photochemical reaction. If irradiation of a 1,5-hexadiene produces the electronically excited state of one and only one of the two allyl components, then the HOMO of one component is (/f3, and the HOMO of ihe other component is suprafacial-suprafacial reaction (Figure 11.46) is forbidden (as is the antarafacial-antar-afacial pathway), but the antarafacial-suprafacial and suprafacial-antarafacial pathways are allowed (Figure 11.47). Analysis of higher sigmatropic reactions shows that the selection rules also reverse with the addition of a carbon-carbon double bond to either of the n systems. Thus, the [3,5] sigmatropic reaction is thermally allowed to be suprafacial-antarafacial or antarafacial-suprafacial and photochemically allowed to be suprafacial- suprafacial or antarafacial-antarafacial. Two of these reaction modes are illustrated in Figure 11.48. 

PROBLEM 20.28 Vitamin D3 (3) is produced in the skin as a result of UV irradiation. It was once believed that 7-dehydro-cholesterol (1) was converted directly into 3 upon photolysis. It is now recognized that there is an intermediate, previtamin D3 (2), involved in the reaction. This metabolic process formally incorporates two pericyclic reactions, 1 — 2 and 2 — 3. Identify and analyze the two reactions. 

An interesting example of pericyclic reaction is cyclization of precalciferol to steroisomeric I and II under thermal condition both of which are cis-products. Similar reaction under photochemical conditions, i.e., upon irradiation gives ergsterol(III), which is irans-product. Thus pericyclic reaction may result is different products under thermal and photochemical conditions. 

The photochemical reactions of psoralens with DNA have been described in detail [1, 4, 5], and is proposed to proceed via a stepwise mechanism. First, the molecule intercalates between the base pairs in double stranded DNA interacting with the 7i-stack of the nucleobases. After irradiation with UVA light, the photo-excited furocoumarins react with DNA and form covalent adducts through a [2 + 2] pericyclic reaction. The photoaddition takes place mostly between the psoralen 4, 5 double bond (furan ring) and the C5, C6 double bond of a pyrimidine (usually thymine). The furan-side adduct can absorb an additional UVA photon, forming an interstrand cross-linked derivative through photoaddition at the 3, 4 double bond in the pyrone ring [6-8]. 

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