Enone + alkene photocycloaddition mechanism

This chapter summarizes and discusses the recent advances in the organic photochemistry of C=C double bonds. Special attention is focused on the photocycloaddition of alkenes to cyclic enones, including the mechanism, regio- and stereoselectivity and synthetic applications of the reaction. 

A very remote secondary H/D isotope effect has been measured for the 2 + 2-cycloaddition of TCNE to 2,7-dimethylocta-2,fran -4,6-triene. The reaction of nitric oxide with iV-benzylidene-4-methoxyaniline to produce 4-methoxybenzenediazonium nitrate and benzaldehyde is thought to proceed via a 2 + 2-cycloaddition between nitric oxide and the imine double bond. A novel mechanism for the stepwise dimerization of the parent silaethylene to 1,3-disilacyclobutane involves a low-barrier [1,2]-sigmatropic shift. Density functional, correlated ab initio calculations, and frontier MO analysis support a concerted 2 + 2-pathway for the addition of SO3 to alkenes. " The enone cycloaddition reactions of dienones and quinones have been reviewed. The 2 + 2-photocycloadditions of homochiral 2(5H)-furanones to vinylene carbonate are highly diastereoisomeric. ... 

Schuster s and Weedon s results support Bauslaugh s proposed mechanism that emphasizes the effect of the ratio between cyclization and the alternative fragmentation pathway of the diradical intermediate, on the regioselectivity in the intermolecular photocycloadditions, and propose not to consider the oriented 7r-complex (exciplex) as an intermediate in the mechanistic pathway of the [2 + 2] photocycloaddition of enones to alkenes. 

Orfanopoulos et al. studied the photochemical reaction of alkenes, aryalkenes, dienes dienones, and acyclic enones with [60]fullerene to obtain various substituted cyclobutylfullerenes [240,241,243,247], For example, the photocycloaddition of cis- and Irans-1 -(p-mcthoxyphenyl)-1 -propenc 68 to C6o gives only the trans [2 + 2] adducts (Scheme 27), thus the reaction is stereospecific for the most thermodynamically stable cycloadduct. A possible mechanism includes the formation of a common dipolar or biradical intermediate between 3C o and the arylalkene. Subsequent fast rotation of the aryl moiety around the former double bond leads exclusively to the trans-69 [2 + 2] adduct. Irradiation of this product, yielded 90% trans-68,10% cis-68 and cycloreversion products. Thus, a concerted mechanism can be excluded because the photocycloreversion is expected to give the trans-68 as the only product. These results can be explained by the formation of a common dipolar or diradical intermediate. Similarly, cycloreversion products from C6o and tetraalkoxyethylene... 

The mechanism of the [2 + 2] photocycloaddition of alkenes with cyclenones is a multistep process that involves the addition of alkenes to the 3 7t,tt triplet state of a cyclic enone and the formation of 1,4-biradical intermediates [35]. Except for the reaction of 1-alkenes, up to four new stereogenic centers are formed during the cycloaddition (Scheme 10). 

The mechanism of the enone-aikene [2+2] photocycloaddition presumably follows the scheme below. Upon irradiation 1) a triplet exciplex is irreversibly formed from the triplet enone and ground state alkene 2) the triplet exciplex collapses to one or more 1,4-biradicals. 3) the biradicals either cyclize to the cyclobutane or revert to starting materials and 4) the biradical reversion decreases the overall efficiency of the process. 

Cyclic enones, such as substituted cyclohex-2-enones or cyclohexa-2,5-diones, also undergo sigmatropic photorearrangement to form bicyclo[3.1.0]hexanones (lumiketones) or bicyclo[3.1.0]hex-3-en-2-ones, respectively, for which both concerted and stepwise (biradical) reaction mechanisms have been proposed.640,641,770 For example, a [l,2]-shift concurrently with the ring contraction (termed the type A reaction) is observed upon irradiation of the methylphenyl derivative 159 in polar solvents, whereas phenyl migration (termed the type B reaction) predominates in nonpolar solvents (Scheme 6.70).771,772 The reactions are believed to proceed via both the n,n and n,Tt triplet ketone states. In the presence of alkenes, cyclic enones may readily undergo a competitive photocycloaddition reaction (Section 6.1.5). 

These two mechanisms, the reversible biradical intermediate and the intermediate exciplex [25] have both been useful for analysis of the regioselectivity and stereoselectivity observed in [2 -I- 2] photocycloaddition between enones and alkenes. Hoffman et al. [26], for example, describes his stereoselective photocycloadditions as arising from a biradical from the triplet excited enone which may or may not involve exciplex formation.. Ground state trans-cycloalkenones have also been proposed as the reactive intermediates which lead to [2 + 2] cycloadducts [27]. The distance between the reacting partners is clearly an issue. If the n systems are not sufficiently close, then photocycloaddition will not occur [28]. 

The introduction of stereogenic centers in the starting material has been more fruitful, especially in the 12- -2 photocycloaddition of conjugated enones to alkenes, and several good reviews discussing the mechanism and synthetic applications of this method have appeared recently [65]. 

The mechanism of the 2 + 2 photocycloaddition of enones with alkenes, which can be viewed as the Bauslaugh-Schuster-Weedon biradical mechanism, is represented in Scheme 10 [66],... 

Scheme 10. Mechanism of [2 -F 2] photocycloaddition of enones with alkenes.

As previously established, the mechanism of the [2-1-2]-photocycloaddition of cyclopentenones with alkenes can be summarized as shown in Scheme 1. The interaction of the alkene with the enone, excited... 

Biradical reversion appears to be a much more important process in enone additions to electron-deficient alkenes. Thus, Schuster and co-workers found that, in the reaction of 3-methylcyclohexenone (3-MCH) with Z- and E-l,2-dicyanoethene (maleo- and fumaronitrile), isomerization of the alkenes accompanies formation of cycloadducts. Based upon quantum yields for aU processes and the rate constants for quenching of the enone triplet by these alkenes, Schuster et al. concluded that alkene isomerization occurred by reversion of 1,4-biradical intermediates (i.e., a Schenck-type mechanism) rather than by triplet-triplet energy transfer from the enone to the alkenes, although the latter was a distinct possibility due to the relatively low triplet energies of these particular alkenes.The full significance of biradical reversion as a critical factor in affecting the course of enone photocycloadditions is discussed below. 

These data were accepted for a long time as proper measures of alkene reactivity in photocycloadditions and were critical elements in the formulation of Corey s famous exciplex mechanism to explain enone photocycloadditions.However, as is now well known, product ratios in photochemical processes reflect relative quantum efficiencies for disappearance of starting materials and/or formation of products and rarely reflect relative rates of reaction of the photoexcited state, particularly when it is a triplet state. ° ... 

The Gorey-de Mayo exciplex mechanism " for photocycloaddition of enones to alkenes has provided a stimulus for workers in this field and undoubtedly has had heuristic value in accounting for experimental observations in a vast number of reactions. Nonetheless, the experimental findings discussed above make it clear that this mechanism for enone [2 2]-photocycloadditions has been discredited and should be abandoned. 

Thus, the role of exciplexes as intermediates in [2+2]-photocycloadditions of enones to alkenes in general remains highly doubtful. The mechanism for [2+2]-photocycloaddition shown in Scheme 6, "... 

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