Regioselectivity, photoaddition

The regioselective photoaddition of aliphatic aldehydes and ketones to pyrroles provides a route to 3-pyrrolylcarbinols and 3-vinylpyrroles in yields varying from 15 to 50% (79JOC2949) (see also Section 3.05.1.7.2). 

Kobayashi, K., Takeuchi, H., Seko, S., Kanno, Y, Kujime, H., and Suginome, H., Photoinduced molecular transformations. 142. One-step synthesis of lff-benz[fjmdole-4,9-diones and IH-indole-4,7-diones by a regioselective photoaddition of 2-amino-l,4-naphthoquinones and 2-amino-1,4-benzoquinones with alkenes, Helv. Chim. Acta, 76, 2942, 1993. 

Triplet photoaddition of simple non-cyclic monoolefins is unknown. The sensitized dimerization of ethyl vinyl ether gives exclusively head-to-head adducts, Eq. 21, and probably should not be classed as an example of simple acyclic olefin. Usually the triplets have high energies and are severly twisted. 55> Some cyclic rigid molecules, Eq. 20, that do dimerize 63> do not incorporate substituents that allow regioselectivity to be determined. Butadiene gives principally head-to-head dimerization, Eq. 19, concordant with the PMO prediction, and so does indene, Eq. 22. The anti dimer that is formed would not be expected from a singlet excimer reaction. 

Intramolecular photoaddition of tertiary amine and styrene moieties has been extensively studied by Aoyama29 and Lewis group28,30,31 (equations 4-8). Equations 4 and 5 show that if the intramolecular additions result in the formation of a five- or six-membered ring, the product yields are excellent. Highly regioselective intramolecular proton transfer is proposed to occur via least motion pathways from the lowest energy... 

Irradiation rraws-2-[3-(7V-methylamino)propyl] stilbene 89 results in the formation of 7V-methyl-l-benzyltetrahydro-2-benzazepine 90 as the only significant primary photoproduct (equation 26), which in turn undergoes secondary photochemical N-demethylation. The final mixture contains 90 (38%) and 91 (25%) at high (>95%) conversion. Intramolecular photoadditions of these (equations 24-26) secondary (aminoalkyl)-stilbenes are highly regioselective processes24. 

Only the head-to-tail adducts were obtained in the [2+2] photoaddition of 4-hydroxy-l-phenyl[l,8]naphthyridin-2(l//)-one with various alkenes in methanol (Scheme 3). The photolysis of the hypoiodites generated by the in situ reaction of the cycloadducts with excess mercury(ll) oxide-iodine reagent in benzene induced a regioselective scission of the non-ring junction bond of the alkoxyl radical to give substituted 3,9-dihydro-9-phenylyfuro[2,3- ][l,8]naphthyridin-4(2//)-one and/or 3,5-dihydro-5-phenylfuro[3,2-f][l,8]naphthyridin-4-(2//)-ones <1996T6125>. 

Ketene silyl acetals underwent photoaddition involving single electron transfer with electron-deficient alkenes and sensitizers such as phenanthrene (phen)179. Two examples of the types of products, which were formed regioselectively, are illustrated in Scheme 60. 

The proposed exciplex orientation, suggested to dictate the regioselectivity of the photoaddition, is consistent with the orientation of the dipolar attraction between the electronically n,7r excited triplet enone and the ground state alkene. Generally, electron acceptor substituents on the alkene provide preferential formation of the head-to-head (H,H) products, whereas electron donor substituents provide preferential formation of the head-to-tail (H,T) photoproducts55 (Scheme 16). 

The effect of the alkene s substituents on the regioselectivity of the photocycloaddition could probably be best presented in the photoaddition of 74 to various alkenes62 75 (Scheme 17). 

High regioselectivity was found as well in the intermolecular photoaddition of cyclo-hexenone 78 to alkenes 79. The regioselectivity was examined by several research groups63 and found to be affected by the solvent polarity, the temperature and steric effects55 (Scheme 18). 

Houck and coworkers postulate that the origin of the regioselectivity is at the biradicalforming step and directly affected by the polarity of the alkene. The /J-carbon, considered as nucleophilic, adds rapidly to the less substituted side of the electron-deficient alkene, whereas a position considered as an a-acyl radical (more electrophilic than an alkyl radical) adds rapidly to the less substituted side of electron-rich alkenes. The calculated relative energies for the addition of jtjt triplet acrolein to different substituted alkenes at the first bond-forming step (Table 3) are found to be in good agreement with experimental values determined in the photoaddition of cyclohexenone to the related alkene. 

Bcnzofuran-4,7-dioncs have been synthesized regioselectively by [3 + 2] photoaddition of 2-hydroxy-1,4-benzoquinones with a range of alkenes (equation 185)664. The reaction occurs in 30-60% yield and is a useful method for the synthesis of the benzofuran ring system, which is important in natural products like acamelin665. Substituted naphthoquinones may also be used in this reaction666,667 and this has lead to a very simple two-step synthesis of maturinone. In a similar reaction, a [3 + 2] photoaddition reaction of 2-amino-1,4-naphthoquinones with electron-rich alkenes gave 13-82% yields of 2,3-dihydro-177-bcn/ /]indole-4,9-diones in a single-step process which involved photolysis followed by oxidation (equation 186)668,669. 

Intermolecular and intramolecular photocycloaddition and photoaddition to aromatic rings in the electron-donor and electron-acceptor systems were discussed in this chapter. The highly stereoselective and regioselective photocycloaddition is a synthetically useful method for the construction of polycyclic carbon-skeleton compounds, including natural products. New aspects for the stereoselective intermolecular and intramolecular photocycloaddition reactions via exciplexes in less... 

The photoisomerisation of (E. -arylidene-p-ionones to the fused pyrans 39 is well known, but it has now been shown that [2+4] photoaddition of O2 to the pyran occurs regioselectively forming stable trioxanes <03TL1943>. 

The observed regioselectivity can be perturbed to varying degrees by choice of reaction parameters. Solvent polarity can play a role in the control of regioselectivity, as would be predicted by the polar exci-plex model. The regioselectivity of the dimerization of cyclopentenone (equation 11) produces a larger proportion of the head-to-head adduct in more polar solvents. The photoaddition of enone (11) to al-kene (12) also displays a pronounced solvent dependence (equation 12). A consequence of the solvent effect is that nonpolar solvents tend to produce products which would be predicted from the polar exci-plex model, while more polar solvents result in somewhat more of the minor product but do not cause complete reversal of the regioselectivity. 

Varying the temperature of the photoaddition reaction can also produce minor changes in the regioselectivity. Typically, lowering the temperature tends to enhance the selectivity in favor of the major regioisomer, as seen in equation (14). ... 

Preliminary investigations describing the photoadditions of ynones to alkenes have determined that these species may react through n,Tt excitation to produce aUcynic oxetanes. Irradiation of 3-butyn-2-one with isobutene gave a 46% isolated yield of (72) and (74) (14 86). 3-Octyn-2-one analogously provided a 50% isolated yield of (73) and (75) (15 85), a regioselectivity predicted on the basis of the best diradical intermediate. 

Photoaddition reactions of entrapped bolaamphiphiles should occur regioselectively if both head groups are locked on the inner and outer surface of a vesicle membrane. A detailed study of such a reaction confirmed this pre-... 

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