Photoisomerizations fragmentations

Any proposed mechanism for the unprecedented transformation described by Equation 18 must account for the promotion of this photoisomerization by CO, although CO is not required by the stoichiometry. A possible initial step would be similar to that for the Ru3(CO)i2 fragmentation (Scheme 1). In this a Ru-Ru bond is broken concomitant with the movement of a CO from a terminal to a bridging site to form an unsaturated intermediate analogous to I. A speculative proposal along these lines is presented in Figure 5. The key feature of this proposal would be the formation of III with one unsaturated ruthenium, which could be captured by CO to promote the subsequent steps leading from the /1-methylidyne to the acyl... 

A number of very important synthetic methods are based on ring interconversions of six-membered heterocyclic systems. As with the transformations of five- into six-membered rings, examples are known of all of the possible atom fragments, but [5 + 1] and [4+2] processes are by far the most important. As described in Chapter 2.02, a number of [6+0] photoisomerizations of diazines have been discovered, but while these are of considerable mechanistic and theoretical importance they are of little preparative significance. 

I 1 The nitro-nitrito photoisomerization occurs in solution through an Intramolecular mechanism involving the homolytic fission of Co—NOa ifeond and then cage recombination of the two fragments by means of a Co—ONO bond. This complex exhibits CT and CTLM character at 239 and 325 nm and a ligand field band at 458 nm. 

Photoisomerization of c/.y-stilbene 191 Ionic fragmentation reaction 191 Cyclopropyl radical ring-opening 192 Ionic molecular rearrangement 193 Ene reaction 196 Thermal denitrogenation 198 Unimolecular dissociation 199 Sn2 reaction 200... 

We demonstrated how the photoisomerization hypothesis can be supported by accurate quantum chemical calculations (103). The experimental infrared and resonance Raman study of complex 5 led to the first determination of normal modes and force constants of diazene coordinated to a metal fragment. Isotope substitution yielding 15N- and 2H-isotopomers permitted the assignment of diazene normal modes in the experimental spectrum. Moreover, the spectra of these three isotopomers indicated that a laser-induced photoisomerization occurred in the Raman sample. However, a detailed assignment of the split bands was not possible in the experiment. 

The most important monomolecular photoreactions are photodissociation and photoisomerization. The former leads to a fragmentation in a molecular species by a photon of sufficient energy to rupture a bond, which may be of homo- or heterolytic character. The light-induced bond cleavage is often called photolysis, especially when flash techniques are used (flash photolysis). 

An example of the photoisomerization of a cyclopropyl- ir-methane system is depicted in equation (18). Five products are observed, the first three of which are primary products, while the last two derive from secondary photochemical fragmentation of the vinylcyclobutane. ... 

After the formaldehyde polymerization described in the beginning of this section, the next, example of a low-temperature kinetic plateau in reactions related to heavy molecular fragment transfer was observed by Buchwalter and Closs [111] in the photoisomerization of a 1,3-cyclopentadiyl (CPDY) biradical isomerized into a bicyclo-(2,10)-pentane (BCP) ... 

This part covers the phototransformations initiated by electronic excitation which do not fit in (i) - (iv). The photoisomerization of the amino acid derivative PHT=Val-OMe is one example. In this reaction, described by A. G. Griesbeck, the first event is abstraction of a hydrogen from a y-position. Neither cyclization (Yang reaction) nor fragmentation (Norrish II) follows this step. Another hydrogen (now from the 5-position) migrates and leads to the formation of a photoisomerization product which could be transformed into N,C-protected isodehydrovaline. This reaction indicates that a broad variety of transformations should be possible at the primary 1,4-biradical stage. 

Some paths are available for the synthesis of cyanocyclopropyl systems from cyanoalkenes. An interesting example of this is the photoisomerization of 2-aminopropenenitrile to yield the aziridine 201. Products of fragmentation such as HCN and acetonitrile are also obtained The A-acylimine 202 is converted on irradiation into the bicyclic products 203 and 204. The route to products involves bond formation by attack of the carbonyl oxygen atom on the styryl moiety. This affords the biradical intermediate 205, where radical stabilization by the cyano group is important. Cyclization within this yields the two bicyclic compounds. The same reaction path is followed on irradiation of202 in the crystalline phase. Here, however, this path is minor (30%) and the major path affords a (2 + 2)-cycloadduct. ... 

Synthesis of the oxazole diene fragment 1270 began with the aldehyde 1272, which had been prepared earlier via Swem oxidation of 1214." " Thus 1272 was converted to a 5 1 Z/E) mixture of stereoisomeric dienes 1273, which was photoisomerized to 1274 (Scheme 1.326). Saponification of 1274 then produced 1270. The sulfur-containing nine-membered ring lactone 1271 was prepared from... 

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