T , dimerization

N3-coordination by monomeric and dimeric Zn2+-cyclen derivatives has also been shown to both inhibit the photo [2+2] cycloaddition of thymidilyl(3 -5 )thymidine, d(TpT), and to accelerate the photosplitting process of the [T-cis,syn-T] dimer (78% after 1 h of UV exposure) (38). 

The substituent effect of 10,10 -positions of bis[2-(9-anthryl)ethyl] glutarates 353a-e, which have more flexible chains, was investigated by irradiation with 350-nm light in dilute (10-4 M) deoxyganated benzene [349], The quantum yields for the dimerization (dimer) falls with the steric bulk of the substituents. 

The model hamiltonian takes the following form (using dimensionless units for r ). For a stacked A-T dimer, our model Hamiltonian takes the form ... 

Fig. 12.5 Comparison between the T-T dimer and Br-dU as kinetic electron traps.

Fig. 4.6.8. Chemical assay for investigation of reductive ET in DNA. T-T dimer (TAT) splitting is the chemical result of photoinduced ET from a distant flavin derivative as the charge donor D.

The reaction mechanism for dimerization of the silirene intermediate has been a controversial subject for many years 3, 4, 7, 45, 49, 58, 86). It is now generally accepted that in the case of dialkylsilylene, (T-dimerization is probably the true mechanism. Dichlorosilylene is expected to behave similarly. 

Pollock R, Clackson T. Dimerizer-regulated gene expression. 

Pollock R, Clackson T. Dimerizer-regulated gene expression. Curr. Opin. Biotechnol. 2002 13 459-467. 

ESR parameters for triplet carbenes15 and nitrenes16 have been summarized, and it has been shown that phenylnitrene is produced predominantly (87-88%) in the singlet state by direct photolysis of phenyl azide in low-temperature matrices.17 The first spectroscopic observation of a singlet nitrene has been reported nanosecond-laser photolysis of 1-azidopyrene gives the S0 nitrene (Amax 450 nm) which has a lifetime of 22 nsec at room temperature (in benzene) and 34 nsec at 77 K in rigid solution. At room temperature it decays to the triplet ground state (Tj, Amax 415 nm) with a rate constant of about 4.4 x 107 sec. Tt is formed directly by biacetyl sensitized photolysis of the azide. The lifetime of the excited triplet (T2) was about 7 nsec. T dimerizes to azopyrene.18... 

An analogous photo-isomer of A -benzyloxycarbonylmethyl-2(l//)-pyridone has been prepared and various transformations of its cyclobutene ring have been explored. The photodimerization of A -methyl-2(l/ )-pyridone in water gives exclusively the products of [4 + 4] addition the yields of trans-anti- (31), trans-syn-, cis-anti-, and cw-jjfn-dimers were 51, 0.6, 11.2, and 6.8%, respectively. In non-aqueous solvents (ethanol or benzene), only a t/-dimers were formed. The thermal cycloaddition of dimethyl acetylenedicarboxylate to 2(l/f)-pyridones also occurs across positions 3 and 6. Reaction is favoured where steric buttressing of the methyl groups in the starting pyridone occurs for example, the cyclo-adduct (32) is... 

Powell et al. (113) have utilized NMR to aid in the detection of a tri-haptoidimexy-pentahaptoijaonovaeT) equilibrium for a series of hepta-2,6-dienylpalladium acetate complexes in chloroform solution. The chemical shifts for C(l) and C(2) in IV (W = H, X = O2CCF3) are compatible with a coordinated olefin, but for IV (W = Cl, X = O2CCF3) these shifts are almost identical to those of III (W = Cl, X = hfacac) in which the olefin C(l) = C(2) is not coordinated to palladium. This evidence together with molecular weight data led the authors to conclude that IV existed in an /t (dimer)-A (monomer) equilibrium the position of equilibrium being dependent on both the olefinic substituent W and the carboxylate substituent R. 

The syn and a/t//-dimers of perfluorotetramethylcyclobutadiene, (130) and (131), when heated to 300 "C, isomerize to the thermally very stable, but photochemically labile, perfluoro-octamethylcyclo-octatetraene. Irradiation of the cyclo-octatetraene yields initially a complex mixture which includes the syn- and r/-dimers, but the final products of irradiation in cyclohexane solution are its saturated valence-bond isomers perfluoro-octamethylcubane (132) and cuneane (133) (see p. 8). The cuneane requires 1 h for thermal reversion to the cyclo-octatetraene at 300 °C, but the cubane is very stable, requiring 48 h for the same conversion. 

Thus the (M + 1) ions formed by protonation of the h-t dimers are less stable than those derived from the corresponding h-h dimers. Since both isomers should be protonated with approximately equal facility, this difference in behavior of the (M -h I) ions must be related to the fact that the h-h dimers are 1,4-diketones, while the h-t dimers are 1,5-diketones. A postulated explanation is that protonation of an h-h dimer such as (I) gives an (M -h l)" ion which can have the three resonance forms ... 

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