Photodimerization stereospecific

The stereospecificity of these reactions is surprising in light of the large energies absorbpd by these molecules. Indeed, the major photochemical product of these photolyses was the alternate olefin isomer (1-butene was also observed). These results indicate that free rotation about the photo-excited double bond does not occur in those molecules that dimerize. This suggests the participation of ground state complexes or excimers in the photodimerization. This view is supported by the observations that dilution of cw-2-butene with neopentane (1 1) decreased the yield of dimers and a 1 4 dilution almost completely suppressed dimerization. 

In contrast to the intramolecular photodimerizations reported above, inlermoleeular [2 + 2] photocyclodimerizations very often suffer from the fact that the reactions are neither regio- nor stereospecific, affording product mixtures, and are, therefore, less useful for synthetic purposes. 

For conceptually similar approaches, see (a) Ito, Y. Borecka, B. Trotter, J. Scheffer, J. R. Control of Solid-State Photodimerization of fraws-Cinnamic Acid by Double Salt Formation with Diamines. Tetrahedron Lett. 1995, 36, 6083-6086 (b) Amirsakis, D. G. Garcia-Garibay, M. A. Rowan, S. J. Stoddart, J. F. White, A. J. P. Williams, D. J. Host-Guest Chemistry Aids and Abets a Stereospecific Photodimerization in the... 

The direct irradiation of the parent coumarin in the presence of alkenes results only in an inefficient photodimerization and [2 + 2]-photocycloaddition. Lewis acid coordination appears to increase the singlet state lifetime, and leads to improved yields in the stereospecific [2 + 2]-photocycloaddition [95]. Alternatively, triplet sensitization can be employed to facilitate a [2 + 2]-photocycloaddition. Yields of intramolecular [2 + 2]-photocycloadditions remain, however, even with electron-rich alkenes in the medium range at best. The preference for HT addition and for formation of the exo-product is in line with mechanistic considerations discussed earlier for other triplet [2 + 2]-photocycloadditions [96, 97]. Substituted coumarins were found to react more efficiently than the parent compound, even under conditions of direct irradiation. 3-Substituted coumarins, for example, 3-methoxy-carbonylcoumarin [98], are most useful and have been exploited extensively. The reaction of 3-ethoxycarbonylcoumarin (100) with 3-methyl-l-butene yielded cleanly the cyclobutane 101 (Scheme 6.36) with a pronounced preference for the exo-product (d.r. = 91/9). Product 101 underwent a ring-opening/ring-closure sequence upon treatment with dimethylsulfoxonium methylide to generate a tetrahydrodibenzofur-an, which was further converted into the natural product ( )-linderol A (102) [99]. 

Photodimerization often involves an excimer that can be treated as a su-permolecule. (Cf. Section 6.2.3.) Then, the state correlation diagram for the singlet process (Figure 7.27a) ordinarily calls for a two-step return from S, to So along the concerted reaction path. First, an excimer intermediate E is formed. Second, a thermally activated step takes the system to the diagonally distorted pericyclic funnel P" (cf. Section 4.4.1), and the return to So that follows is essentially immediate. The reaction will be stereospecific and concerted in the sense that the new bonds form in concert. However, it will not be concerted in the other sense of the word, in that it involves an intermediate E. ... 

These concepts are in very good agreement with experimental findings. There are relatively few examples of photodimerization of simple noncon-jugated acyclic olefins because these compounds absorb at very short wavelengths. Irradiation of neat but-2-ene, however, yields tetramethylcyclobu-tane with a quantum yield d> = 0.04. For very low conversion, the observed stereochemistry of the adducts is the stereospecific one expected from Scheme 13 for a concerted [ 2s + cycloaddition. However, since the major pathway is cis-trans isomerization with a quantum yield d> = 0.5 (cf. Section 7.1.2), it has been concluded that the molecules that undergo cis-trans isomerization are not involved in photodimerization (Yamazaki et al., 1976). 

MacGillivray and co-workers have recently demonstrated that the linear template approach can also be expanded to hydrogen-bond acceptor templates. In particular, co-crystallization of 2,3-bis(4-thiopyridylmethyl)naphthalene as a template with fumaric acid as the reactant produced a finite, four-component molecular assembly held together by O-H N hydrogen bonds (Fig. 2.3.10(a)) [57]. The double bonds of the stacked fumaric acid molecules were aligned suitably for a photodimerization that proceeded, upon UV-irradiation with 300 nm light, in a SCSC fashion to a maximum of 36% yield. The photoreaction provided, stereospecifically, the rctt isomer of 1,2,3,4-cyclobutane-tetracarboxylic acid (Fig. 2.3.10(b)). 

Either direct or triplet-sensitized irradiation of cyclohexene produces a stereoisomeric mixture of [2 + 2] dimers in nearly quantitative chemical yields (Scheme 6.57).557 Dimerization is interpreted in terms of initial photoinduced E Z isomerization of cyclohexene (Section 6.1.1), followed by a non-stereospecific ground-state addition of (/i)-cyclohexene to the Z-isomer. In contrast, copper(I)-catalysed photodimerization of cyclohexene produces only one major stereoisomer 129 (Scheme 6.57).706 According to... 

As mentioned above, some arylalkenes, such as stilbene, form complexes with Ag+. Such complexes are also formed between Ag+, as added silver perchlorate, and simpler alkenes. Typical of this is the interaction with 1-methylenecyclohexane when a crystalline complex is formed. Irradiation of this complex in the polycrystalline state or in solution in methanol affords isomerization to 1-methylcyclohexene by a 1,3-hydrogen migration path. Further irradiation brings about the stereospecific formation of the exo,trans,exo-dimer (24) of 1-methylcyclohexene. Less specific photodimerization is also reported for the irradiation of the /S-pinene (25)/silver perchlorate complex69 The mechanism was thought to involve a silver/cyclohexenyl radical similar in type to that observed in the y-radiolysis of silver/cycloalkene complexes70,71. 

Photochemists are well aware that solid state photodimerizations may be highly stereoselective, but they are not frequently stereospecific. Long-range molecular transports connected to phase rebuildings provide an obvious explanation. However, there are also solid state photoprocesses which yield... 

Rate data have been obtained from a detailed analysis of the previously reported 1420 maximum and fall off in the photodimerization of acenaphthylene with increasing concentrations of dissolved ethyl iodide.1420 The heavy-atom solvent dibromomethane facilitates the cross cycloaddition of acenaphthylene to trans- and cw-penta-1,3-diene by inducing intersystem crossing to the triplet state of the aromatic hydrocarbon. The intermediacy of a biradical is proposed and the stereospecificity can be understood in terms of the maintenance of the stereochemical integrity of the allylic radical units.143... 

Armisakis. D.G. Garcia-Garibay. M.A. Rowan. S.J. Stoddart. J.F. White. A.J.P. Williams. D.J. Host-guest chemistry aids and abets a stereospecific photodimerization in the solid state. Angew. Chem., Int. Ed. 2001, 40. 

The olefin adopted head-to-head geometries and participated in Cl- - -Cl forces. The C=C bonds reacted to give the head-to-head cyclobutane rc/f-l,2-bis(4-pyridyl)-3,4-bis(j9-chlorophenyl)cyclobutane (4-Cl dpcb) in quantitative yield. Recently, Ramamurthy and coworkers have employed thiourea as a template to assemble substituted stilbazoles (e.g., 4-CN, 4-F, 4-Br stilbz) to undergo stereospecific photodimerizations in the solid state (Figure 4). " Cocrystallization of 4-CN stilbz afforded infinite ID tapes... 

Cocrystalfization of 4-benzylresorcinol (4-bn-res) with l,4-bis[2-(4-pyridyl)ethenyl]benzene (1,4-bpeb) afforded a solid wherein two sets of C=C bonds were positioned for a double photodimerization (Figure 7). UV irradiation afforded the corresponding bridge-substituted [2.2]para-cyclophane (tpcp) stereospecifically and in quantitative yield. A similar approach was later reported by Brun-klaus and coworkers to direct the reactivity of a F-substituted diene in the form of p-di-[2-(4-pyridyl)ethenyl]-2-fluorobenzene (1,4-bpef). Cocrystallization of 1,4-bpef with 2,4-dihydroxybenzaldehyde produced a four-component assembly that reacted to give the corresponding F-substituted [2.2]paracyclophane in 60% yield. 

Ag(I) ions have been used to direct [2-1-2] photodimerizations in solids. Argentophilic forces (i.e., Ag- Ag interactions) were exploited by MacGilhvray and coworkers to assemble and stack a stilbazole (4-stilbz) for reaction Reaction of Ag(I) trifluoroacetate with 4-stilbz produced a disilver complex that organized two pairs of 4-stilbz. The Ag- Ag interaction displayed a metal-metal separation of 3.41 A while the C=C bonds were crisscrossed and separated by 3.82 A (Scheme 6a). The corresponding head-to-head cyclobutane was generated quantitatively in a SCSC reaction. The formation of the photoproduct was ascribed to pedal-like rotation of the C=C bonds in the solid. A similar complex was subsequently used to achieve the first photodimerization of terminal olefins in a solid. Reaction of Ag(l) chlorate with 4-vinylpyridine (4-vp) afforded a disilver complex (Scheme 6b) that generated c/x-l,2-bis(4-pyridyl)cyclobutane stereospecifically and in quantitative yield. 

Photodimerizations with charged compounds have been studied in several styryl dyes wherein alkylation of an V-heterocycle (e.g., pyridine, benzothiazole) generates a salt." Gromov and coworkers have also shown that when a styryl dye and a crown ether (e.g., 18-C-6 Scheme 9) are covalently connected, stereospecific [2- -2] reactions are facilitated by inclusion of the cation within the ethereal host." In addition, Gromov also showed that counteranion... 

Open-chain alkenes undergo dimerizations in stereospecific [nl + tt ] paths in low yields on irradiation. For example, photodimerization of Z-2-butene 26 gives two products in which cw-geometry of methyl groups is retained. Similarly, the cycloadducts from irradiation of -2-butene 27 retains the trans-geovasXry of methyl groups [29]. Irradiation of Z-2-butene also gives cycloadduct with -stil-bene in a similar manner [29]. 

The reaction mechanism of photochemical cycloadditions is not always well known. A concerted pericyclic process is likely when the reaction is highly stereospecific, while a two-step mechanism involving a diradical or diion intermediate is more probable when a mixture of stereoisomers is obtained. This is not a rule and a recent example is the [2 - - 2] photodimerization reaction of diethyl l,2-benzoxaphosphorine-6-bromo-3-carboxylate(an analog to coumarin) performed in solvents of different polarities (H2O, MeOH, PhH, CH3C02H). In water, the reaction is highly stereoselective in favor of a centrosymmetric anti-head-to-tail stereoisomer. Theoretical data, however, indicate that the process is not pericyclic, but that the reaction proceeds through a diradical or dipolar intermediate. 

Light specificity. l,l-Dimethyl-2,5-diphenyl-l-silacyclopentadiene irradiated 2 hrs. under N2 with Pyrex-filtered light of a 450 w. high-pressure Hg-arc lamp in tetra-hydrofuran -> dimer (almost 100%) irradiated 2 hrs. under Ng with a 160 w. low-pressure Hg-arc lamp in the same solvent monomer (Y almost 100%). Y. Nakadaira and H. Sakurai, Tetrah. Let. 1971, 1183 dimerization s. a. D. G. Farnum and A. J. Mostashari, Org. Photochem. Synth. 1, 79, 103 (1971) also isomer in acetic acid s. D. J. Trecker, A. A. Griswold, and O. L. Chapman, ibid. 1, 62 stereospecific photodimerization in the presence of CuBr s. D. J. Trecker and R. S. Foote, s. ibid. 1, 81. 

Moorthy, J.N. and Venkatesan, K., Stereospecific photodimerization of coumarins in crystalline inclusion complexes. Molecular and crystal structure of 1 2 complex of (s,s)-(-)-l,6-bis(o-chlo-rophenyl)- ,6-diphenyl-hexa-2,4-diyne-l,6-diol and coumarin,/. Org. Chem., 56,6957,1991. Tanaka, K. and Toda, E, Selective photodimerizations of coumarin in crystalline inclusion compounds, /. Chem. Soc., Perkin Trans. 1, 943, 1992. 

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