Photochemical cycloaddition conjugated dienes

Conjugated dienes also undergo photochemical cycloaddition reactions. Related thermal cycloadditions of alkadienes have been discussed in Sections 13-3A, 21-10A, and 21-10D, but the thermal and photochemical reactions frequently give different cyclic products. Butadiene provides an excellent example of the differences ... 

Photochemical cycloaddition reactions of t-1 and several of its analogues with conjugated dienes have been reported. The reaction of 1-t with 2,5-dimethyl-2,4-hexadiene yields the isomeric [2+2] cycloadducts 49 and 50 in a 1.2 1 ratio with a... 

Ultraviolet irradiation of [Cr(CO)3(ty6-l,3,5-cycloheptatriene)] (41) in the presence of conjugated dienes causes no CO evolution. A photochemical [6 + 4] cycloaddition takes place with the dienes la-Id, li, and 2,3-dimethyl-1,3-butadiene (It) (69-71) [Eq. (21)]. Within the coordination sphere of chromium, two C—C bonds are formed between C-l and C-6... 

As already mentioned, no [6 + 4] cycloaddition is observed when tri-carbonyl-i 6-l,3,5-cycloheptatrienemolybdenum(0) is irradiated in presence of conjugated dienes. So an explanation of these experimental results has to be provided. One reason might be an unfavorable fit of the CuH14 n(CH3 ) and C, 4H18 - (CH3) hydrocarbons into the coordination sphere of molybdenum, arising from the increased Mo—C bond lengths. The other reason concerns the possible different stabilities of the intermediates of the photochemical [6 + 4] cycloaddition with molybdenum as the central metal. 

The photochemical reactivity of alkenes is also of great interest [1,2]. Studies in this area have led to an expansion of the synthetic utility of these substances. Typical photochemical reactions include cis-trans isomeriza-tions, inter- and intramolecular cycloadditions, photooxidations, and electrocyclic ring opening and closing of conjugated dienes and polyenes. Many of these photoreactions have thermal counterparts. In contrast,... 

Arene oxides show the characteristic reactions of epoxides (isomerization to ketones, reductions to alcohols, nucleophilic additions, deoxygenations) and olefins or conjugated dienes (catalytic hydrogenation, photochemical isomerization, cycloaddition, epoxidation, metal complexation). Where a spontaneous, rapid equilibration between the arene oxide and oxepin forms exists, reactivity typical of a conjugated triene is also found. 

For a review of photochemical cycloaddition reactions of conjugated dienes and polyenes, see Dilling, W. A. Chem. Rev. 1969, 69, 845. 

Dilling, W. L. "Photochemical Cycloaddition Reactions of Nonaromatic Conjugated Hydrocarbon Dienes and Polyenes." Chem. Rev., 69,845 (1969). 

The mechanism of this cycloaddition can be explained by FMO theory, in which one component acts as a HOMO and other as LUMO in a favorable low-energy TS to afford a stereoselective product. The Woodward-Hofifmann mles for electro-cyclic reactions are also applied to this cycloaddition reaction. The reaction of an alkene with a carbene is considered as a 4n electron process and of a conjugated diene with an electrophilic molecule as a 4n+2 electron process. Therefore, for thermal reaction of 4n electron process, conrotatory motion of the substituents from the termini of the n system will favor a low-energy TS to afford the product and in photochemical process, the reverse disrotatory mode of motion will be the favored path. Similarly, for a 4n + 2 electron process, disrotatory mode is a symmetry allowed process in thermal reaction and conrotatory mode for its photochemical reaction. 

Copper(i) trifluoromethanesulphonate catalyses the photochemical cycloadditions of cyclohexene and cycloheptene with conjugated dienes (e.g. butadiene) and norbornene with allyl alcohol. ... 

Cycloaddition Reactions. Furan is known to undergo various t) es of cycloaddition reactions. It functions as an ene and participates in [2 + 2] and [2 + 3] cycloadditions under photochemical and thermal conditions, respectively. With an embedded cyclic conjugated diene system in it, furan also functions as a diene that undergoes various [4 + 2] and [4 + 3] cycloaddition reactions. ... 

Evers, J.T.M. and Mackor, A., Photocatalysis II. Photochemical cycloadditions of cyclohexenes and cycloheptene with conjugated dienes catalyzed by copper(I) trifluoromethane sulphonate. Tetrahedron Lett., 2317-2320,1978. 

Bis-allylic oxidation of 23 and related cyclohexa-1,4-dienes provides a convenient and general preparation of cyclohexa-2,5-dien-l-ones (Scheme 7). These cross-conjugated die-nones are substrates for a variety of photochemical rearrangement and intramolecular cycloaddition reactions. Amide-directed hydrogenations of dienones 24a and 24b with the homogeneous iridium catalyst afford cyclohexanones 25a and 25b, containing three stereogenic centers on the six-... 

Electron transfer to or from a conjugated tr-system can also induce pericyclic reactions leading to skeletal rearrangements. A typical example is the Diels-Alder cycloaddition occurring after radical-cation formation from either the diene or the dienophile [295-297], The radical cation formation is in most cases achieved via photochemically induced electron transfer to an acceptor. The main structural aspect is that the cycloaddition product (s Scheme 9) contains a smaller n-system which is less efficient in charge stabilization than the starting material. Also, the original radical cations can enter uncontrollable polymerization reactions next to the desired cycloaddition, which feature limits the preparative scope of radical-type cycloaddition. 

The photochemistry of alkenes, dienes, and conjugated polyenes in relation to orbital symmetry relationships has been the subject of extensive experimental and theoretical studyThe analysis of concerted pericyclic reactions by the principles of orbital symmetry leads to a complementary relationship between photochemical and thermal reactions. A process that is forbidden thermally is allowed photochemically and vice versa. The complementary relationship between thermal and photochemical reactions can be illustrated by considering some of the reaction types discussed in Chapter 10 and applying orbital symmetry considerations to the photochemical mode of reaction. The case of [2Tr- -2Tr] cycloaddition of two alkenes, which was classified as a forbidden thermal reaction (see Section 10.1), can serve as an example. The correlation diagram (Figure 12.17) shows that the ground state molecules would lead to a doubly excited state of cyclobutane, and would therefore involve a prohibitive thermal activation energy. 

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