Benzophenone, photocycloaddition

Intermolecular photocycloadditions of alkenes can be carried out by photosensitization with mercury or directly with short-wavelength light.179 Relatively little preparative use has been made of this reaction for simple alkenes. Dienes can be photosensitized using benzophenone, butane-2,3-dione, and acetophenone.180 The photodimerization of derivatives of cinnamic acid was among the earliest photochemical reactions to be studied.181 Good yields of dimers are obtained when irradiation is carried out in the crystalline state. In solution, cis-trans isomerization is the dominant reaction. 

The photocycloaddition of a carbonyl compound to an olefin with the formation of an oxetane 3 was first observed by Paterno and Chieffi in 1909.1 They reported obtaining a compound in good yield from the irradiation of benzophenone in a petroleum mixture rich in 2-methyl-2-butene, for which they proposed the oxetane structure. Although Paterno and Chieffi had suggested the correct structure, it was not until 1954 that Buchi and his collaborators reinvestigated this reaction and... 

There has been very little study of the photocycloaddition reaction, where the carbonyl compound was excited with light of varying wavelengths. For the simple carbonyl compounds, irradiation absorbed only by the n - it transition has been used with success. Irradiation of the more complex carbonyl compounds, for example, benzophenone, can be such that both the w and the (of the benzene chromo-... 

Table II must apply to both reactions. This has been verified by quenching the photocycloaddition of benzophenone to 2-methyl-2-butene with ferric dipivaloylmethane25 and that of benzaldehyde to cyclohexene with naphthalene.37...

One of the limitations of the photocycloaddition reaction is that the unsaturated system may itself act as a quencher. Conjugated dienes fall within this category since they quench the n,ir triplet of some carbonyl compounds. For the photoreduction of benzophenone in benzhydrol, the ratio, kqlka, for m-piperylene is 750 (Table II), which indicates that this diene is indeed an efficient quencher for the reaction. 

An interesting extension of the forementioned generalizations has recently been reported.43 When a dilute solution of 2,3-dimethyl-l,3-butadiene 11a is irradiated in the presence of a large excess of benzo-phenone, a product 16 arising from the photocycloaddition reaction is observed. This anomalous result has been explained as involving the 77,77- triplet of the diene, formed by triplet-triplet transfer from the n,n triplet of benzophenone, which then reacts with ground-state benzophenone. The critical steps are shown in structures 14-16. 

The phenylsubstituted benzophenones provide another series to illustrate the correspondence of the photoreduction and photocycloaddition reactions and, in addition, afford another example of an anomaly in the correlation of photochemical reactivity with emission spectra. These experiments are summarized in Table V. This series is of... 

The photocycloaddition of carbonyl compounds to unsymmetrical olefins (electron rich) can give two products however, usually one predominates. For example, the photocycloaddition of benzophenone to isobutylene gives a mixture of the two oxetanes 30 and 31 in the ratio of 9 1.17 This ratio is consistent with the preferential formation and/or closure of the intermediate 30a relative to 31a. The diradical 30a is more stable than 3la since a tertiary radical is more stable than a primary radical by about 8 kcal.62 Many of the examples listed in Section VII are consistent with this apparent generalization there are, however, exceptions. 

The photocycloaddition of a,a, -trifluoromethylacetophenone to isobutylene gives a mixture of 32 and 33 in the ratio of 1.5 l37 that of benzophenone to 2-methylnorbornene, a mixture (1.5 1) rich in the unexpected isomer 3437 and that of benzaldehyde to 2-methyl-2-butene 1.6 1 in favor of the expected isomer 36.25 These results indicate that the product orientation can be influenced by factors other than... 

When benzophenone is irradiated in either cis- or trans-2-butene solution, the photocycloaddition yields essentially the same mixture of both isomeric oxetanes, 40 and 41, before significant isomerization of the starting olefin occurs.37 This result strongly supports a mechanism involving the diradical 39. Since 39 was undoubtedly formed initially in the triplet state (from the n,ir triplet) spin inversion was necessary before bonding. There is no evidence pertaining to the formation of this intermediate from the n,n singlet. 

The photocycloaddition reaction is subject to the usual directive forces caused by steric interaction during the approach of most reagents. For example, the photocycloaddition of benzophenone to norbornene gives primarily the exo oxetane 47 with little or none of the endo isomer... 

The photocycloaddition of benzaldehyde to cyclohexene (p. 309) afforded a moderate yield of the oxetane along with the radical coupling products.17,37 Compare this result with the photocycloaddition of benzophenone to cyclohexene where the oxetane was a minor product.17,66 This indicates that the kaiijkabs is higher for benzaldehyde than for benzophenone. 

Some Olefins Which Do Not Undergo the Usual Photocycloaddition Reaction with Benzophenone... 

It is conceivable that a carbonyl compound with an n,n triplet energy lower than that of benzophenone could yield the photocycloaddition product in some of these cases. A reaction which may illustrate this point is the photocycloaddition of ethyl glyoxylate to styrene and 1,1-diphenylethylene.66 Unfortunately, the triplet energy of ethyl glyoxalate has not been measured however, there is adequate reason to believe it is lower than that of benzophenone (see Table VI). 

One limitation inherent in the mechanism involving attack by the oxygen of the n,ir triplet is that the predominant product will usually be that from the most stable diradical intermediate. As mentioned previously (p. 317), the photocycloaddition of benzophenone to isobutylene gives a high yield (93%) of the oxetanes with 30 predominating... 

Another characteristic of photocycloaddition to electron-rich alkenes is the loss of any stereochemistry of the starting alkene in the oxetane structure. An example is the formation of practically the same mixture of geometric isomers of 2,2-diphenyl-3,4-dimethyloxetane from benzophenone and either cis- or trans- 2-butene (equation 103). This is understandable on the basis of the diradical intermediate having a sufficiently long lifetime for bond rotations to occur. 

In one of the earliest reports on ortho photocycloaddition, in which the reaction of benzonitrile with 2-methylbut-2-ene is described, a diradical (triplet) intermediate was proposed [73], The structure of the product corresponds to the most stable of the four possible diradical intermediates. When benzophenone was added as a sensitizer in an attempt to increase the yield of the photoadduct, only 0.05% of ortho adduct was isolated along with 54% of an oxetane formed by the addition of benzophenone to 2-methylbut-2-ene. In the absence of benzophenone, the ortho adduct was isolated in 63% yield. It is, however, thermally as well as photochemically unstable and reverts to starting materials, supposedly also via a biradical. The authors propose that benzophenone catalyzes bond cleavage of the adduct more efficiently than ortho addition and this would account for the low yield of photoadduct in the presence of benzophenone. From these experiments, no conclusion about the identity of the reactive excited state can be drawn. 

It is also possible, however, to use a more sophisticated sensitizer, for example, an acetophenone or benzophenone, in a transparent solvent. Sensitization results in a direct promotion of the substrate into the rat -triplet state, from which photocycloaddition chemistry can occur according to Scheme 6.2. 

The photocycloaddition of a variety of carbonyl compounds with methyl-substituted allenes has been reported to proceed with high quantum yields (0.59 for acetophenone/tetramethylallene) [69] to give 1 1 and 1 2 adducts [70]. The 2-alkylideneoxetanes are useful precursors for cyclobutanones, e.g., 79a,b from the benzophenone/tetramethylallene-cycloadduct 78 (Sch. 22) [71]. 

The photocycloaddition of benzophenone to furan 87a was originally described by Schenck et al. [79]. Additionally to the 1 1 adduct 88 also two regioisomeric 2 1 adducts 89a,b were isolated [80], the structure of 89a was revised by Toki and Evanega [81]. All prostereogenic carbonyl addends when photochemically added to furan showed regioselectivities >99 1 in favor of the bicyclic acetal product (Sch. 25). 

D Auria and coworkers investigated the photocycloaddition of 5-methyl-2-furyl-phenylmethanol 92 with benzophenone resulting in two adducts 93 and 94 in a 1 1 ratio while the addition to 4,4/-dimethoxy-benzophenone, benzaldehyde or 4-methoxybenzaldehyde, respectively, gave merely the adducts 93 (Sch. 27) [88]. 

The photocycloaddition of triplet benzophenone to norbornene has been originally reported by Scharf and Korte (Sch. 30) [94]. The photoproduct 101 which is formed in high cxo-selectivity could be thermally cleaved to the 5,8-unsaturated ketone 102, an application of the carbonyl-olefm-metathesis (COM) concept [95]. 

In some cases, however, a high degree of stereoselectivity could be obtained even with pure triplet excited carbonyl compounds. In these cases, e.g., the photocycloaddition of benzophenone to several methyl vinyl sulphides 113, the intermediary triplet 1,4-biradical preferentially undergoes one of two possible cyclization modes after intersystem crossing (Sch. 34) [60]. 

Adam rationalized the unprecedented experimental facts for the [2+2]-photocycloaddition of the diastereoisomeric cyclo-octenes with benzophenone in terms of a consistent mechanism [136] (i) The cis-136 displays a remarkable temperature dependence in that the trans-2 oxetane is favored with increasing temperature, (ii) For trans-136 the trans geometry is preserved in nms-cycloadduct over a broad temperature range of 180°C. (iii) The extent of trans to cis isomerization in the cycloaddition with the trans-cyclo-octene increase with temperature. 

Hydrogen bonding is a possible reason also for the diastereoselectivity of the [2+2] photocycloaddition of benzophenone also in case of acyclic chiral allylic alcohols 142 (Sch. 49). These substrates afforded only one regioisomer of the diastereomeric threo, erythro oxetanes 143 and preferentially the threo isomer [144]. The diastereoselectivity was remarkably reduced in the presence of a protic solvent (methanol as a competitive intermolecular hydrogen bonding substrate) and totally disappeared in case of the silylated substrate [145]. 

Sensitized intramolecular reaction of two 1,3-dienes (86) (Sch. 17) yields predominantly the [2+2] adduct 87, with small amounts of [4+4] adduct 89 and little, if any, [4+2] product 88 [58,59], consistent with Hammond results for intermolecular reactions of acyclic dienes (Sch. 4). Benzophenone-sensitized reaction yields a mixture of two isomers of 87. Heating this mixture to 200 °C converts both isomers of 87 to cyclo-octadiene 89 [58]. Unsensitized photoreaction of 86 in the presence of copper(I) triflate gives a significant amount of [4+2] adduct 88. Extended irradiation time converts much of 87 and 89 into 88, as well as producing secondary products [59]. Copper triflate-mediated photocycloaddition of a related tethered diene-monoalkene, gave only the [2+2] adduct [59]. 

The Paterno-BUchi Reaction. One well-known class of photocycloadditions is the Paterno-Buchi reaction in which aldehydes or ketones combine with alkenes to give oxetanes. The excited state of the ketone is 11-71, and it is the orbitals of this state which interact with the ground-state orbitals of the alkene. The orientation usually observed for C- and X-substituted alkenes is shown for benzophenone 8.15 and 2-methylpropene 8.16. 

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