Cyclobutane dimers intermolecular

Addition.—Intermolecular [ 2 + w2] cycloadditions in nitrogen-containing heterocycles continue to arouse interest, particularly in systems which are related to constituents of nucleic acids. Cyclobutane dimers are readily obtained on direct irradiation of furo[3,2-Z>]pyridin-2(4 )-ones and the pyrrolo[3,2-Z>]-pyridin-2-one (151), although it is not possible on the basis of the evidence available to distinguish between the head-to-head (152) and the head-to-tail (153) structures.118 Cyclobutane dimers have also been obtained from alkyl 4-stilbazole salts on irradiation in solution and in the solid state.118 Examples of analogous intramolecular [ 2 + 2] cycloadditions have also been reported.120-121 The [ 2 + 2] cycloaddition of quinol-2-one to furan has been described,122 but... 

The cis-dimer of dibenzo[a,d]cycloheptenone (80) has been prepared by intramolecular photocyclization of the ether (81). Oxidative cleavage of the initial product (82) gives the cis-dimer (83). Intermolecular cyclization of (80) gives the trans-cyclobutane dimer (84). 

In the crystal of 1,4-dicinnamoylbenzene (1,4-DCB) (see Fig. 12), the distances between the intermolecular photoadductive carbons are 3.973 and 4.086 A for one cyclobutane ring, and 3.903 and 3.955 A for the other. The two topochemical pathways may occur competitively in a single crystal of 1,4-DCB at the initial stage of reaction. Then, both intramolecular photodimerization and intermolecular photopolymerization of the diolefinic mono-cyclobutane intermediate occur competitively to give tricyclic dimer 21,22,23,24-tetraphenyl-l,4,ll,14-tetraoxo-2(13),12(13-diethanol, [4.4] para-cyclophane or oligomers (Hasegawa et al., (1985). On photoirridation at room temperature the 1,4-DCB crystal gives >90% of the tricylic... 

The same type of bis-functionalization has been reported for the palladium-catalyzed borylstannylative carbocy-cyclization of 1,6-, 1,5-, 1,7-diynes, bis-propargylamine, and ether.377 It should be noted that even 1,2-dialkylidene cyclobutane can be obtained in reasonable yield. Ito has proposed the related silaborative reaction involving nickel(O) catalysis.378 This reaction has been performed in an intra- and intermolecular fashion. The intramolecular reaction allows the formation of cyclic dienes and the intermolecular process proceeds through a dimerization of alkynes to give acyclic dienes. 

The second example is an intermolecular crystal-state reaction. Cross-conjugated 1,5-disubstituted 1,4-dien-3-ones in solution undergo both cis-trans photoisomerization and photodimerization, yielding complex mixtures of products, including die all-trans-substituted cyclobutane 2 in the case of 1,5-diphenyl-1,4-pentadien-3-one. In contrast, dienones such as 3a in whose crystals adjacent molecules lie parallel and strongly overlapped react in the solid to give 3b as the sole photoproduct. This isomerically pure tricyclic diketone results, formally, from an eight-center dimerization. It is not formed in the reaction in solution, and could be prepared by other methods only with considerable difficulty (4). 

The concept of the conservation of orbital symmetry can be extended to intermolecular cycloaddition reactions which occur in a concerted manner. The simplest case is the dimerization of ethylene molecules to give cyclobutane, the 2n + 2je cycloaddition. The proper geometry for the concerted action would be for the two ethylene molecules to orient one over the other. Two planes of symmetry are thereby set up -perpendicular to the molecular plane bisecting the bond axes oy-parallel to the molecular plane lying in between the two molecules (Figure 8.10). 

This idea was realized using crown ether styryl dyes (CESD) lc,d, 4c (Scheme 1,4). The compounds lc,d, 4c having betaine structures form supramolecular dimers with a crossed arrangement of molecules (a h-head-to-tail) in the presence of ions, due to the intermolecular interaction between the sulfo group of one of the molecules and a ion located in the crown-ether cavity of the other molecule [20,21], It was shown that photoirradiation of solutions of dimer results in stereospecific PCA giving only one of the 11 possible derivatives of cyclobutane, which is expected in conformity with the concerted superficial (s,s) addition of the reactants (Scheme 5) [22,23], It is noteworthy... 

A recent example of intermolecular photocycloaddition between acephenanthrylene demonstrates the role of 7i-stacking [26], The singlet excited state gives only the cis dimer that the authors suggest arises from an excimer state. The triplet excited state results in formation of both cis and trans cyclobutanes. Interestingly, in the presence of oxygen the cis dimer is the only product due to quenching of the triplet excited state. 

The dimerization of 28 has also been studied by Prasad, who used Raman spectroscopy to monitor both changes in intermolecular vibrations and lattice phonon modes [73]. The Raman spectrum shows the disappearance of alkene stretches at 997, 1180, 1593, and 1625 cm-1 as expected, and the appearance of cyclobutane modes at 878,979, and 1001 cm-1. Phonon modes broadened as the reaction progressed, and bands around 15-40 cm-1 showed a shift in frequency. Between about 50 and 66% conversion it was difficult to define distinct bands, but after that point product bands grew in distinctly. This amalgamation behavior is good evidence for a homogeneous reaction mechanism. 

Figure 16-14. Computed energy levels for the ground state (S0) and the lowest triplet excited state (T) of the cytosine dimer in its triplet locally excited state 3(LE), step-wise intermediate 3(SWI), and ground-state cyclobutane cytosine (CBC) dimer. The main intermolecular geometric parameters (in A) for 3(SWI) are included. At the 3(SWI) optimized structure a singlet-triplet crossing, (T1/S0)x, takes place...

Intermolecular photoadditions of a,/3-unsaturated carbonyl compounds can take place either at the CC or at the CO double bond. Photodimerizations with formation of a cyclobutane ring are quite common. In cases such as cyclopentenone (81), head-to-head as well as head-to-tail dimers are produced. 

Stilbene and its derivatives have often been used in photochemical [2-1-2] cycloadditions. The intermolecular dimerization presented by H. Meier of 2,3-fcij(2-phenylethenyl)-naphthalene directly leads to cyclophane via two [2-1-2] cycloadditions in one step. The yield is surprisingly very high and comparable with those of intramolecular reactions presented by W. H. Laarhoven. Various examples of vinylstilbenes are photolyzed to form cyclobutanes via a "crossed" addition. Even [2a-H2jt] cycloadditions and rearrangements involving H-transfer are utilized to construct unusual bicyclic or tricyclic compounds. 

Cycloadditions only proceeding after electron transfer activation via the radical cation of one partner are illustrated by the final examples. According to K. Mizono various bis-enolethers tethered by long chains (polyether or alkyl) can be cyclisized to bicyclic cyclobutanes using electron transfer sensitizer like dicyanonaphthalene or dicyano-anthracene. Note that this type of dimerization starting from enol ethers are not possible under triplet sensitization or by direct irradiation. Only the intramolecular cyclization ci the silane-bridged 2>. s-styrene can be carried out under direct photolysis. E. Steckhan made use of this procedure to perform an intermolecular [4+2] cycloaddition of indole to a chiral 1,3-cyclohexadiene. He has used successfully the sensitizer triphenylpyrylium salt in many examples. Here, the reaction follows a general course which has been developed Bauld and which may be called "hole catalyzed Diels-Alder reaction". 

Inokuma et al have reported a further example of additions with crown ether derivatives. In this instance the additions are intermolecular and involve the dimerization of the vinylbenzene derivative (123) to afford the two adducts (124) and (125). The ion-complexing capabilities of the adducts were assessed. A layered ternary solid is formed between 1,2-dihydroxybenzene and trans- -(2-pyridyl)-2-(4-pyridyl)ethylene. Within this, the stilbene is held in a head-to-tail arrangement. Irradiation brings about the formation of a cyclobutane identified as (i )-c/ y,/ra y,/ra -l,3-bis(2-pyridyl)-2,4-bis(4-pyridyl)cyclobutane. An extension of this work to the use of 5-methoxyresorcinol as the template has demonstrated that quantitative yields of ladderanes (126) can be obtained by irradiation of the solid-state units represented as (127). The diazastilbene derivative (128) readily forms complexes with the tetra-acid (129). This acts as a supramolecular template and holds the ethene systems close enough for photochemical dimerization. ... 

As in the solution irradiated with light both cis- and trans-isomers of a 2cMg are present, the formation of various isomers of cyclobutane derivative is probable. The NMR and data, however, show undoubtedly that in the course of photolysis a single-type isomer is formed. It is obvious that it can be produced solely from two cis-2c molecules in the course of cycloadding by "head-to-tail" type. Thus, the existence of photocycloaddition and its high stereoselectivity give further evidence for the production of (cis-2c)Mg dimers which form the intermolecular coordination bonds in acetonitrile solution under the high concentrations of dimers. 

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