Photochemical carbonylation

With mononuclear transition metal carbonyls, photochemically induced substitutions of one or two carbonyl ligands by a diene can be expected to be the only reactions. Dinuclear transition metal carbonyls or carbonyl... 

With that background, we are now ready to discuss the first class of carbonyl photochemical reactions. Eq. 16.55 shows the a cleavage or Norrish type I photoreaction. After excitation, the reaction involves reformation of the carbonyl double bond with concomitant cleavage of a C-C bond a to the carbonyl and expulsion of a carbon radical. Also formed is an acyl radical that, based on the bond dissociation energies of Table 2.2, is a fairly stable radical. 

Wong S K, Hutchinson D A and Wan J K S 1973 Chemically induced dynamic electron polarization. II. A general theory for radicals produced by photochemical reactions of excited triplet carbonyl compounded. Chem. Phys. 58 985-9... 

Hydantoins can react with electrophiles at both nitrogen atoms and at C-5. The electrophilic carbonyl groups can be attacked by nucleophiles, leading to hydrolysis of the ring or to partial or total reduction of the carbonyl system. Other reactions are possible, including photochemical cleavage of the ring. 

A-Substituted pyrroles, furans and dialkylthiophenes undergo photosensitized [2 + 2] cycloaddition reactions with carbonyl compounds to give oxetanes. This is illustrated by the addition of furan and benzophenone to give the oxetane (138). The photochemical reaction of pyrroles with aliphatic aldehydes and ketones results in the regiospecific formation of 3-(l-hydroxyalkyl)pyrroles (e.g. 139). The intermediate oxetane undergoes rearrangement under the reaction conditions (79JOC2949). 

For isoxazoles the first step is the fission of the weak N—O bond to give the diradical (51) which is in equilibrium with the vinylnitrene (52). Recyclization now gives the substituted 2//-azirine (53) which via the carbonyl-stabilized nitrile ylide (54) can give the oxazole (55). In some cases the 2H-azirine, which is formed both photochemically and thermally, has been isolated in other cases it is transformed quickly into the oxazole (79AHC(2.5)U7). 

The photochemical addition of azirines to the carbonyl group of aldehydes, ketones, and esters is also completely regiospecific (77H(6)143). Besides the formation of the isomeric oxazolines (50) from (39) and ethyl cyanoformate, there is also formed the imidazole (51) from addition to C=N in the expected regioselective manner. Thioesters lead to thiazolines (52), while isocyanates and ketenes produce heterocycles (53). 

There are at least two mechanisms available for aziridine cis-trans isomerism. The first is base-catalyzed and proceeds via an intermediate carbanion (235). The second mechanism can be either thermally or photochemically initiated and proceeds by way of an intermediate azomethine ylide. The absence of a catalytic effect and interception of the 1,3-dipole intermediate provide support for this route. A variety of aziridinyl ketones have been found to undergo equilibration when subjected to base-catalyzed conditions (65JA1050). In most of these cases the cis isomer is more stable than the trans. Base-catalyzed isotope exchange has also been observed in at least one molecule which lacks a stabilizing carbonyl group (72TL3591). 

From N-oxides of aromatic bases oxaziridines were obtained only at very low temperatures, but oxaziridines were often postulated as intermediates in the photoconversion of such N-oxides (Section 5.08.3.1.2). Isolation of the more stable photoisomers of nitrones also causes some problems due to their thermal and photochemical instability leading to acid amides, e.g. (69TL2281), or, by fragmentation, to carbonyl compounds and products of stabilization of nitrenes, e.g. from (260) (69ZN(B)477). 

Azirine, trans-2-methyl-3-phenyl-racemization, 7, 33, 34 1-Azirine, 2-phenyl-reactions, 7, 69 with carbon disulfide, S, 153 1-Azirine, 3-vinyl-rearrangements, 7, 67 Azirines, 7, 47-93 cycloaddition reactions, 7, 26 fused ring derivatives, 7, 47-93 imidazole synthesis from, 5, 487-488 photochemical addition reactions to carbonyl compounds, 7, 56 photolysis, 5, 780, 7, 28 protonated... 

Paal-Knorr synthesis, 4, 118, 329 Pariser-Parr-Pople approach, 4, 157 PE spectroscopy, 4, 24, 188-189 photoaddition reactions with aliphatic aldehydes and ketones, 4, 232 photochemical reactions, 4, 67, 201-205 with aliphatic carbonyl compounds, 4, 268 with dimethyl acetylenedicarboxylate, 4, 268 Piloty synthesis, 4, 345 Piloty-Robinson synthesis, 4, 110-111 polymers, 273-274, 295, 301, 302 applications, 4, 376 polymethylation, 4, 224 N-protected, 4, 238 palladation, 4, 83 protonation, 4, 46, 47, 206 pyridazine synthesis from, 3, 52 pyridine complexes NMR, 4, 165... 

The photochemical reactions of organic compounds attracted great interest in the 1960s. As a result, many useful and fascinating reactions were uncovered, and photochemistry is now an important synthetic tool in organic chemistry. A firm basis for mechanistic description of many photochemical reactions has been developed. Some of the more general types of photochemical reactions will be discussed in this chapter. In Section 13.2, the relationship of photochemical reactions to the principles of orbital symmetry will be considered. In later sections, characteristic photochemical reactions of alkenes, dienes, carbonyl compounds, and aromatic rings will be introduced. 

The photochemistry of carbonyl compounds has been extensively studied, both in solution and in the gas phase. It is not surprising that there are major differences between the photochemical reactions in the two phases. In the gas phase, the energy transferred by excitation cannot be lost rapidly by collision, whereas in the liquid phase the excess energy is rapidly transferred to the solvent or to other components of the solution. Solution photochemistry will be emphasized here, since both mechanistic study and preparative applications of organic reactions usually involve solution processes. 

The dominant photochemical reaction of ketones in the gas phase is cleavage of one of the carbonyl substituents, which is followed by decaibonylation and sidsoetprait reactions of the alkyl free radicals that result ... 

As is clear from the preceding examples, there are a variety of overall reactions that can be initiated by photolysis of ketones. The course of photochemical reactions of ketones is veiy dependent on the structure of the reactant. Despite the variety of overall processes that can be observed, the number of individual steps involved is limited. For ketones, the most important are inter- and intramolecular hydrogen abstraction, cleavage a to the carbonyl group, and substituent migration to the -carbon atom of a,/S-unsaturated ketones. Reexamination of the mechanisms illustrated in this section will reveal that most of the reactions of carbonyl compounds that have been described involve combinations of these fundamental processes. The final products usually result from rebonding of reactive intermediates generated by these steps. 

Irradiation at 20 C of perfluoro ethers containing a carbonyl group causes almost quantitative decarbonylation [179. Azindine undergoes photochemical addition with methyl trifluoroacetate [180 (equation 47). 

By a photochemically induced elimination of CO, a chromium carbene complex with a free coordination site is generated. That species can coordinate to an alkyne, to give the alkyne-chromium carbonyl complex 4. The next step is likely to be a cycloaddition reaction leading to a four-membered ring compound 5. A subsequent electrocyclic ring opening and the insertion of CO leads to the vinylketene complex 6 ... 

Carbonyl compounds can undergo various photochemical reactions among the most important are two types of reactions that are named after Norrish. The term Norrish type I fragmentation refers to a photochemical reaction of a carbonyl compound 1 where a bond between carbonyl group and an a-carbon is cleaved homolytically. The resulting radical species 2 and 3 can further react by decarbonylation, disproportionation or recombination, to yield a variety of products. 

The fragmentation/cyclization ratio is determined by the relative orientation of the respective molecular orbitals, and thus by the conformation of diradical species 2. The quantum yield with respect to formation of the above products is generally low the photochemically initiated 1,5-hydrogen shift from the y-carbon to the carbonyl oxygen is a reversible process, and may as well proceed back to the starting material. This has been shown to be the case with optically active ketones 7, containing a chiral y-carbon center an optically active ketone 7 racemizes upon irradiation to a mixture of 7 and 9 ... 

The photochemical cycloaddition of a carbonyl compound 1 to an alkene 2 to yield an oxetane 3, is called the Patemo-Buchi reaction - This reaction belongs to the more general class of photochemical [2 + 2]-cycloadditions, and is just as these, according to the Woodward-Hofmann rules, photochemically a symmetry-allowed process, and thermally a symmetry-forbidden process. 

The primary photochemical of transition metal carbonyls [Me(CO)n] involves the scission of carbon monoxide (CO) and the formation of coordinated unsaturated species ... 

Bamford and coworkers [19] have shown that a prolonged aftereffect can be obtained with Mn-carbonyl in the presence of certain additives, notably cyclohexane and acetylacetone (S), It was suggested that the photochemical reaction between Mn2(CO)io and (S) produces the active species (Z), which generates free radicals by interaction with halide and Z probably formed from Mn (CO)6 species ... 

Strohmeier and Griibel [21] have reported that some vanadyl carbonyls of CpV(CO)4 (Cp = cyclopentadie-nyl) can photochemically induce the polymerization of vinyl chloride in the presence of CCI4. 

The chlorinated surface of glass then acted as a coinitiator, and when photochemically treated with metal carbonyl in the presence of MMA monomer, a thin layer of 0.4 /Am thickness of poly(methyl methacrylate) was obtained. 

UV irradiation on a polymer surface produces chemical modification as well as wettability and bondability improvement. It causes chain scission and oxidation on polymer surfaces. -iven in the presence of an inert gas [45]. Carbonyls are found to be introduced onto polyethylenes on UV irradiation. Sivram et al. [46] have used photochemical treatments for surface modification of polymers. They have generated surfaces of vaying surface energies by simple organic reactions. 

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