Photochemical insertion

Pandurangi, R.S. et al. (1997a) Recent trends in the evaluation of photochemical insertion characteristics of heterobifuncdonal perfluoroaryl azide chelating agents Biochemical implications in nuclear medicine. Photochem. Photobiol. 65(2), 208-221. 

The fact that both the thermal and the photochemical insertion reactions yield the same products via formation of charge-transfer complexes leads to the conclusion that the reactive ion-radical pair in equation (52) is the common intermediate for both activation processes. Such a conclusion is further verified by the direct observation of anion-radical intermediates from the thermal reaction of TCNE and DDQ with various metal hydrides.188... 

Merlic demonstrated the direct, non-photochemical insertion of carbon monoxide from acylamino chromium carbene complexes 14 to afford a presumed chromium-complexed ketene 15 <00JA7398>. This presumed metal-complexed ketene leads to a munchnone 16 or munchnone complex which undergo dipolar cycloaddition with alkynes to yield the pyrroles 17 upon loss of carbon dioxide. 

An example is reported of photochemical insertion of oxygen into silylenamines, leading to silylperoxides31 (Scheme 17). 

Photochemical insertion into a B—H bond occurs with borazine 267). The CF3 substituted product 234b arises from radical decomposition of the HFA molecule. 

C. The optically active alkyl complex with R = CH(Ph)CH20H inserts O2 to yield the corresponding optically active peroxo derivative. By contrast, the optically active cobalt alkyl with R = CH(Me)CH2Me undergoes racemization in the course of the photochemical insertion 78a). These reactions likely proceed through homolysis of the Co—R bond. 

Use of alkanes with f,soxt C—H bonds will give three different insertion products. The ratios of which yield the relative rate of insertion. Carbena-cyclopentadiene (2 a) generated photochemically inserts in the C—H bonds of 2.3-dimethylbutane to give two products 34 and 35 in the ratio 1.22 1 26). 

Photochemical insertion of CO and S02 into the N—Cl bond in CF3-(FC0)NS02C1 gave CF3(FC0)NC(0)C1 and the unstable CF3(CFO)-NSO2CI. Several similar products were also produced.184... 

At 15K in argon matrices, gallium and indium atoms undergo photochemical insertion into water. Dimers also photoreact giving bridged species, which may then undergo a further photodecomposition (equations 1 and 2). 

The introduction of a 9(11) double bond into steroids is of practical interest, since such olefins can be converted to the 9-fluoro-l 1 -hydroxy grouping found in most useful corticosteroids. Ester 6 had a benzophenone-4-hexanoic acid tethered to the hydroxyl group of androstan-17/3-ol that curled under the steroid ring and photochemically inserted into the C-9 a-hydrogen, forming product 7 with a 9(11) double bond after lead tetraacetate cleavage of the carbinol product and hydrolytic cleavage of the tether (Scheme 6-3) [33]. A product with a 14,15 double bond was also produced. 

This work built on our earlier finding [46] that benzophenone photochemical insertion into the side chain could be achieved when the benzophenone was tethered to the j8-face of the steroid. The findings, along with X-ray crystal structure data [46], made it clear that the nominally free rotation of the steroid side chain is in fact strongly restricted by steric constraints the hydrogen on C-20 is pointed to the steroid y3-face and requires a jS-linked tether to permit its functionalization. 

Similar photochemical insertion reactions are observed for indium alkylporphy-rins In the presence of pyridine and under irradiation by visible light, carbon dioxide inserts into the carbon-indium o bond leading to stable carboxylate indium porphyrins (Scheme 10). 

Synthetic application of aluminum porphyrins afforded a series of interesting reactions. Light irradiation to TPP-Al-Et enhanced the addition reaction of ethyl group to vinyl ketone and polymerization [349] (Fig. 32). Photochemical insertion of carbon dioxide into Al-Et bond was reported in the presence of 1-methylimidazole [352]. Similar insertion of CO2 into the Al-enolate bond was also reported [353,354]. These reactions have attracted much attention in i-elation to fixation of carbon dioxide in artificial photosynthesis as described in Sec. II1.C (Fig. 33). 

Figure 33 Photochemical insertion of carbon dioxide into Al-Et bond [3S4],...

The thermal and photochemical insertions of dioxygen into the Co—C bond of alkyl cobaloximes [CoR(dmg)2L] [R=CHgC6H4X or CHgCH=CHY (thermal), R=Me, Et, MegSiCHg, PhCHgCHg, etc. (photochemical), L=pyridine or water] follow a second-order rate law (23), in all cases studied. The rates are little affected... 

Such a reaction is an important one since this intermediate has been proposed for the carbonylation of MnCH3(CO)5 to Mn(COCH3)(CO)5 under thermal condi-tions. The pathways followed in these photochemical insertion reactions have been studied both by low-temperature matrix techniques and by time-resolved infrared spectroscopy. At 12 K, photolysis of rraw - / -cpMo(CO)2MeL (L = P(OEt)3, PPh3, PBu3) results in CO loss, followed by the rapid reverse thermal carbonylation of the coordinately unsaturated intermediate ... 

R.S. Pandurangi, S.R. Karra, R.R. Kimtz, W.A. Volkert, Recent Trends in the Evaluation of Photochemical Insertion Characteristics of Heterobifunctional Perfluoroaryl Azide Chelating Agents Biochemical Implications in Nuclear Medicine, Photochem. Photohiol., 1997, 65, 208-221. 

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