Photochemistry cyclobutenes

One important aspect of the photochemistry of aikenes 301,302) is the E—Z isomerization around the C—C double bond 303). This is also valid for cycloalkenes with the obvious exception of cyclopropenes304a,b) which exhibit a distinct photochemical behaviour, and cyclobutenes and cyclo-pentenes where the ring is to rigid to allow sufficient twisting of the double bond. 

As mentioned in the Introduction, the ring closure of s-cis butadiene to cyclobutene has been at the very center of the evolution of theoretical understanding of polyene photochemistry to its current state25,87-89,151. Early ab initio calculations recognized the crucial role of the 21Ag state in the isomerization, and successfully accounted for the disrotatory stereospecificity of the reaction in terms of a two-dimensional model in which the planarity of the carbon framework is more or less maintained throughout12,13,15. 

Dauben and coworkers reached similar conclusions on the basis of their extensive investigations of the photochemistry of 3-alkyl-6,6,9,9-tetramethyl-A3 5(10 -hexalins (122)217. Direct irradiation of these compounds leads to the formation of 129 and 130 due to secondary irradiation of the initially formed triene 128, which exists as an equilibrium mixture of conformers cZc-128 and cZz-128 (equation 48). Their results showed that as the steric bulk of the 3-substituent increases, the rate of disappearance of starting material decreases and the cyclobutene (129) bicyclo[3.1.0]hex-2-ene (130) product ratio... 

Study of the photochemistry of myrcene, 23, a triene containing both a conjugated diene system and a remote, unconjugated double bond, has been exceptionally informative. Direct excitation produces a cyclobutene derivative 24, /S-pinene, and a host of minor products287 including 25. In contrast, indirect excitation using a number of sensitizers gives only a derivative of bicyclo[2.1.1]hexane, 25.288... 

The [2 + 2]-photocycloaddition chemistry of a,(3-unsaturated lactones has been widely explored. The factors governing regio- and simple diastereoselectivity are similar to what has been discussed in enone photochemistry (substrate class Al, Section 6.2). The HT product is the predominant product in the reaction with electron-rich alkenes [84]. A stereogenic center in the y-position of ot,P-unsaturated y-lactones (butenolides) can serve as a valuable control element to achieve facial diastereoselectivity [85, 86]. The selectivity is most pronounced if the lactone is substituted in the a- and/or P-position. The readily available chiral 2(5H)-furanones 79 and 82 have been successfully employed in natural product total syntheses (Scheme 6.30). In both cases, the intermediate photocycloaddition product with 1,2-dichloroethylene was reductively converted into a cyclobutene. In the first reaction sequence, the two-step procedure resulted diastereoselectively (d.r. = 88/12) in product 80, which was separated from the minor diastereoisomer (9%). Direct excitation (Hg lamp, quartz) in acetonitrile solution was superior to sensitized irradiation (Hg lamp, Pyrex) in acetone, the former providing the photocycloaddition products in 89% yield, the latter in only 45%. Cyclobutene 80 was further converted into the monoterpenoid pheromone (+)-lineatin (81) [87]. In the second reaction... 

The photochemistry of 1,3-dienes can be highly dependent on the diene structure and reaction conditions. Important variables include the ground state conformation [22,23], the reaction concentration, the use (or not) and properties of a triplet sensitizer [14] or an electron acceptor [18], and solvent polarity. The simplest dienes also often yield the most complex chemistry. For example, 1,3-butadiene 3 undergoes unimolecular isomerization in dilute solution to give only cyclobutene 4 and bicyclobutane 5 (Sch. 2), and polymerization in concentrated solution [24]. At intermediate... 

The areas in which work on alkyne photochemistry has been most prolific are those involving cycloaddition to carbon-carbon double bonds in alkenes, aromatics and related compounds. The simplest type of reaction involves formation of a cyclobutene from an alkene and an alkyne (equation 41) . The cyclobutene product may itself be photolabile, and if radiation is used which is absorbed more strongly by the cyclobutene, the product isolated may be the 1,3-diene derived from it by electrocyclic ring opening (equation 42) . 

Monomolecular photochemistry of butadiene is rather complex. Direct irradiation in dilute solution causes double-bond cis-trans isomerization and rearrangements to cyclobutene, bicyclo[l.l.01butane, and l-methylcyclo-propene (Srinivasan, 1968 Squillacote and Semple, 1990). Matrix-isolation studies have established another efficient pathway, s-cis-s-trans isomerization (Squillacote et al., 1979 Arnold et al., 1990, 1991). 

Photoinduced [2 + 2] cycloaddition (Section 4.9) of alkenes (alkynes) to form cyclobutane (cyclobutene) derivatives is one of the best studied reactions in photochemistry.680 682 According to the Woodward Hoffmann orbital symmetry rules,336 the cycloaddition of one singlet excited (Si) and one ground-state alkene is allowed by a suprafacial suprafacial concerted stereospecific pathway (Scheme 6.45) 695 699 700 Rare concerted [4 + 2] and [4 + 4] photocycloadditions of conjugated singlet excited dienes must occur in a suprafacial antarafacial and suprafacial suprafacial manner, respectively.690 Since the suprafacial antarafacial reactant approach is geometrically difficult to achieve, [4 + 2] reactions usually proceed stepwise (involving biradical intermediates). [2 + 2] or [4 + 4] photocycloadditions can occur in either a concerted or stepwise fashion. 

Several review articles have dealt with various aspects of cycloaddition reactions. A short review has presented examples to illustrate the use to which tethered alkenes can be put in the synthesis of cyclobutenes. The photochemical (2 + 2)-cycloaddition of enones to ethene has been studied from a theoretical standpoint. A review has highlighted the use of linear templates to control photochemical reactions such as (2 + 2)-photocycloadditions. The stereochemical control of photochemical reactions in clay-intercalated compounds has been discussed. A review has highlighted the interdisciplinary nature of photochemistry. ... 

Leigh, W. J. Zheng, K. /. Am. Chem. Soc. 1991,113,4019 reported evidence for photochemical disrotatory ring opening in one bicyclic system incorporating a cyclobutene ring, but the stereospecificity in such cases seems to depend on the structural features incorporated into the reactants. For a discussion of the relationship of orbital symmetry to the photochemistry of cyclobutene, see Leigh, W. J. Can. ]. Chem. 1993, 71,147. 

Photochemistry.— The photochemical addition of dimethyl acetylene-dicarboxylate, methyl propiolate, or methyl phenylpropiolate to benzo[h]-thiophen and its 2- and 3-methyl and 2,3-dimethyl derivatives leads to cyclobutene derivatives of unexpected structures. From benzo[ >]-thiophen and dimethyl acetylenedicarboxylate (376) is obtained. Methyl propiolate adds in a direction opposite to that of methyl phenylpropiolate, suggesting that the excited state of benzo[h]thiophen is highly polarized. The mechanism of the addition is discussed, and several mechanistic alternatives are suggested. Only with diphenylacetylene, which reacts slowly, can the normal addition product (377) be obtained. The... 

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