Group 10 photoisomerization reactions

Recently, a photoisomerization reaction of azoferrocene was found to proceed in polar solvents such as benzonitrile and DMSO through both a 7t it transition of the azo-group with a UV light (365 nm) and the MLCT transition with a green light (546 nm) (Fig. 6) (Scheme 1) (153). The quantum yields of the photo-isomerization reaction at 365 nm and 546 nm were estimated to be 0.002 and 0.03, respectively. The transformation into the cis form causes the higher field shift of Cp protons in the 1H-NMR spectrum and an appearance of u(N = N) at 1552 cm-1. The cis form is greatly stabilized in polar media, and dilution of the polar solution of cis-25 with less polar solvents resulted in a prompt recovery of the trans form. 

Again two products were obtained from a reactive triplet state with a rate constant for photoisomerization of 2 x 1011. This rate, however, is over six times greater than that obtained for the unsubstituted phenyl derivative and indicates that the p-cyano-phenyl group facilitates reaction by stabilization of the intermediate biradical. 

The photochemistry of ally 1-substituted Group 14 metal compounds can be divided into two sections, namely photoisomerization reactions, and those involving some kind of intermolecular reaction. 

Vogtle and co-workers first reported a photoswitchable dendrimer [33] with six peripheral azobenzene groups, which took advantage of the efficient and fully reversible photoisomerization reaction of azobenzene-type compounds (Scheme 7). In a follow-up study [34], polypropylene imine) dendrimers bearing azobenzene moieties (p-Im-Gn, n = 1-4) on the periphery were synthesized. These dendrimers displayed similar photoisomerization properties as the azobenzene monomers. Irradiation of the all-E azobenzene dendrimers at 313 nm led to the Z-form dendrimers, while irradiation at 254 nm or heating could convert the Z-form dendrimers back to the E-form dendrimers. The observation that the... 

The effect of pressure was studied for a series of such photoisomerization reactions [123] and the volumes of activation for the photochemical reaction, AV, estimated from the plot of ln[/(l — )] versus pressure, are summarized in Table 9. In most cases, the absolute values of AV are small and are difficult to interpret. Dissociation of Cl- or Br would result in charge creation and an associated volume decrease due to the increase in electrostriction. Thus the intrinsic volume increase associated with ligand dissociation from the ES can be partially or totally compensated for by the volume collapse due to the increase in electrostriction. This is not a complication when NH3 is the leaving group, as demonstrated by the data in Table 3, and discussed in more detail in Section IV. The overall values of AV are such that they support the operation of a dissociative mechanism,... 

The excited-state deactivation pathway of PYP model chromophores is foimd to depend strongly on the substituent adjacent to the carbonyl group. The photoisomerization reaction of the deprotonated p-coumaric acid (pCA ) and of its amide analogue (pCNT) in solution does not show any spectroscopically detectable intermediate, which is quite different from PYP. On the contrary, the phenyl thioester derivative pCT exhibits a photophysical behavior in solution surprisingly close to that of the protein during its initial deactivation step. This study highlights the determining role of the thioester bond in the primary molecular events in PYP. 

The primary photoreactions in the PYP have been experimentally studied both in the protein and in solution environments. The initial structural change of PYP was directly observed by time-resolved x-ray crystallography. Ultrafast fluorescence spectroscopy was performed on the initial process of the photoreaction of PYP. It was shown that the photoisomerization reaction of the PYP chromophore is completed within 1 ps. The initial photoreaction processes were analyzed by using time-resolved spectroscopic data. Although the reaction of the PYP chromophore in solution environments has been studied by several groups, the characterization of the dynamics in these environments is not fully understood. 

Figure 24 Photoisomerization reactions of some group 10 nitro complexes (56). (A) mono nitro-to-nitrito (B) bis nitro-to-nitrito (C) bis nitro-to-nitrito and effects of metal substitution on yields.

This photoisomerization reaction is referred to as a valence isomerization. It is a reaction in which electron reshuffling occurs and the nuclei move to make or break new 7i and a bonds. A number of polymers were, therefore, prepared with the norbomandiene moieties either in the backbone or as pendant groups. Among them are polyesters that were synthesized with donor-acceptor norbomadiene residues in the main chain by polyaddition of 5-(4-methoxyphenyl)-l,4,6,7,7-pentamethyl-2,5-norbomadiene-2,3-dicarboxylic acid or 5,6-bis(4-methoxyphenyl)-7,7-dimethyl-2,5-norbomadiene-2,3-dicarboxylic acid, with bis(epoxide)s. This preparation of such polymers and the accompanying photorearrangements can be illustrated as follows ... 

A common photoisomerization reaction is intramolcculaffearrangemcnt that takes place in the nitro groups in poly(P-nitrostyrene) and its dimeric model compound upon irradiation with UV light... 

The alkyl turn isomerization is very favorable in the crystalline-state reaction. To explain the movement of the 4-cb group, the reaction cavity for 4-cb in the crystal structure before irradiation is shown in Fig. 6.22a. The reaction cavity after irradiation, which includes both of 4-cb and 1-cb, is shown in Fig. 6.22b. Since the two cavities are very similar, it is reasonable to assume that the reaction cavity does not change to a great extent during the photoisomerization. 

Furthermore, it was made clear that the concept of reaction cavity is applicable to the usual solid-state reactions. In almost all the solid-state photoisomerization from the 2-cyanoethyl group to 1-cyanoethyl group, the reaction cavity is a good... 

An efficient synthetic route to (10Z)- and (10 )-19-lluoro-la,25-dihydroxy vitamin D3 has been developed (488). The key feature of this pathway is the introduction of a 19-fluoromethylene group to a (5 )-19-nor-10-oxo-vitamin D derivative. The 10-oxo compound 445 has been obtained via a 1,3-dipolar cycloaddition reaction of (5 )-la,25-dihydroxyvitamin D with in situ generated nitrile oxide, followed by ring cleavage of the formed isoxazoline moiety with molybdenum hexacarbonyl. Conversion of the keto group of (5 )-19-nor-10-oxo-vitamin D to the E and Z fluoromethylene group has been achieved via a two-step sequence, involving a reaction of lithiofluoromethyl phenyl sulfone, followed by the reductive de-sulfonylation of the u-lluoro-j3-hydroxysulfone. The dye-sensitized photoisomerization of the (5 )-19-fluorovitamin D affords the desired (5Z)-19-fluorovitamin D derivatives, (10Z)- and (10 )-19-fluoro-la,25-dihydroxy-vitamin D3. 

A detailed analysis of photoisomerization of the nitrone group in the nitroxyl radical 4-phenyl-2,2,5,5-tetramethyl-3-imidazoline-3-oxide-l-oxyl, based on double electron-nuclear resonance methods, has shown that in the absence of oxygen the photochemical reaction occurs without affecting the radical center. The... 

Thus, the solid-state photoisomerization of 10 proceeds via a topotactic reaction mechanism, while some (Z,Z)-muconic esters (e.g., 13 and 15) can pho-toisomerize to the corresponding ( , )-muconic esters (14 and 16) without any change in the space group, i.e., proceed via topochemical EZ isomerization (Scheme 13, Fig. 15) [115]. 

The Photoactive Yellow Protein (PYP) is thought to be the photoreceptor responsible for the negative phototaxis of the bacterium Halorhodospira halophila [1]. Its chromophore, the deprotonated 4-hydroxycinnamic (or p-coumaric) acid, is covalently linked to the side chain of the Cys69 residue by a thioester bond. Trans-cis photoisomerization of the chromophore was proved to occur during the early steps of the PYP photocycle. Nevertheless, the reaction pathway leading to the cis isomer is still discussed (for a review, see ref. [2]). Time-resolved spectroscopy showed that it involves subpicosecond and picosecond components [3-7], some of which could correspond to a flipping motion of the chromophore carbonyl group [8,9]. 

This chapter is concerned with the influence of mechanical stress upon the chemical processes in solids. The most important properties to consider are elasticity and plasticity. We wish, for example, to understand how reaction kinetics and transport in crystalline systems respond to homogeneous or inhomogeneous elastic and plastic deformations [A.P. Chupakhin, et al. (1987)]. An example of such a process influenced by stress is the photoisomerization of a [Co(NH3)5N02]C12 crystal set under a (uniaxial) chemical load [E.V. Boldyreva, A. A. Sidelnikov (1987)]. The kinetics of the isomerization of the N02 group is noticeably different when the crystal is not stressed. An example of the influence of an inhomogeneous stress field on transport is the redistribution of solute atoms or point defects around dislocations created by plastic deformation. 

For compounds such as 63, a more complex rearrangement replaces the decarbonyla-tion reaction [65]. This photoisomerization yields lactone 64 by a-cleavage at C-2, C-3 position (Norrish I) and hydrogen transfer from C-l to C-3 followed by a stereoselective nucleophilic attack at the carbonyl group by the terminal carbon of the electron-rich double bond and final ring closure (Scheme 34). 

---