Photo-decarbonylation

It has been predicted from an RRKM calculation that 3-oxetanone would photolyze readily, forming formaldehyde and ketene (75JPC1990). This is in accord with the observation that 2,2,4,4-tetramethyl-3-oxetanone is transformed in high yield to dimethylketene and acetone on irradiation with 3130 A light in polar solvents, such as acetone and ethanol. In nonpolar solvents, however, photo-decarbonylation to tetramethyloxirane becomes a competing process (equation 5) (66JA1242). 

Resendiz, M. and Garcia-Garibay, M.A. (2005) Hammet analysis of photo-decarbonylation in crystalline 1,3-diarylacetones. Organic Letters, 7, 371-374. 

Cyclopropene and its deuterium-labelled derivatives can be obtained by the photo-decarbonylation of the corresponding furan at 254 nm but the method is of strictly limited value because of the photolability of many cyclopropenes (Section IV.B.2). West and his coworkers have shown that aryltrihalo- and diaryldihalocyclopropenes are available from classical Friedel-Crafts aromatic substitution reactions employing the cyclopropenyl cation as electrophile. Thus tetrachlorocyclopropene is converted to its derived cation which is then allowed to react with an aromatic compound. The exothermic reaction provides monoarylcyclopropene at low ( 0°C) temperature and the diaryl derivative at higher ( > 50° C) temperature (equation 26). In this way 2-phenyl-1,3,3-trichlorocyclopropene can be obtained in 58 % yield and the p-fluorophenyl analogue in... 

The 4,5-corane (84) is obtained in SOX yield on photo-decarbonylation of the pentacyclic ketone (85). Photochemical decomposition of the carbonate (86), by the loss of carbon dioxide, affords a mixture of products containing oxirane. styrene oxide, bibenzyl and phenylacetaldehyde. Triplet sensitized irradiation yields products solely from benzyl radicals. - An earlier study of the irradiation (at 254 nm) of the carbonate (87) reported that benzaldehyde, phenyl carbene, and carbon dioxide were produced. A reinvestigation of the irradiation of this compound (at 254 nm in acetonitrile) has provided evidence that the cis- and trans-stilbene oxides (88) and (89) are formed as well as deoxybenzoin and smaller amounts of diphenylacetaldehyde and bibenzyl. When methanol is used as the solvent the same products are produced accompanied by benzylmethyl ether, 1,2-diphenylethanol, and 2,2-diphenylethanol. These authors suggest that the oxiranes (88) and (89) are formed by way of... 

Photo-decarbonylation of the cyclohexanone (32a) is efficient with a quantum yield of 0.9. The reaction yields the two products (33) and (34) in a ratio of 1 2. The cyclopentanone (32b) also decarbonylates photochemically but is less efilcient with a quantum yield of 0.5. A laser flash study has been carried out on these systems and has identified the biradicals produced by the Norrish type I process. The lifetime of the biradicals (35a) and (35b) are O.Ojxs and 0.5 xs respectively. In a related study the photodecarbonylation of cIs- and frans-2,6-diphenylcyclo-hexanone has been shown to yield a mixture of cis- and 2-diphenyl-cyclopentane and cIs- and... 

When the photo-decarbonylation of metal carbonyl is applied to the preparation of catalytically active species on solid surface, the following advantages can be offered for the catalyst preparation. 

Photo- and thermal decarbonylation of cyclic unsaturated ketones leads to the formation of cyclic 1,3-dienes. Such decarbonylations are commonly observed in 7-ketonorbomenes and related bridged bicyclic systems to give cyclohexadienes (equation 84)132. 

Cymantrene (6) demonstrates a considerable amount of organic chemistry just as does ferrocene. No organic photochemistry has been found in cy-mantrenyl systems, however, due to the customary complication of photo-chemically induced decarbonylation. E. O. Fischer55 has investigated this in some detail and three representative reactions are listed here. 

For samples photolyzed on ZSM-5 zeolite, the product distributions of 31 and 32 are dramatically different from those photolyzed in homogeneous solutions. First, the rearrangement products were totally suppressed. Second, diphenylethane 39 resulted from coupling of benzyl radical was not found. Only phenol 38 and toluene were detected. In contrast, photolyses of 33 and 34 on ZSM-5 follow strikingly different pathways. Both photo-Fries rearrangement 36 and 37 and decarbonylation products 35 and 39 were formed. These results can be understood from consideration of size- and shape-selective sorption combined with restriction on the mobility of the substrates and reaction intermediates imposed by the pentasil pore system. 

Reaction of all of the radical pairs within their initial cages does not preclude one or both of the radicals from undergoing a structural change (N. B., step [6] in Scheme 13.1). In the case of photo-Fries reactions, the most commonly encountered structural change is loss of carbon monoxide (CO) from the acyl radical, leading to formation of an alkyl radical. The rates of decarbonylation of acyl radicals have been measured for a wide variety of acyl structures as a function of the medium viscosity... 

The corresponding esters are much less informative because the centers of chirality in their acyl radicals are structurally protected from racemization like that experienced by translational or rotational motions of prochiral alkyl radicals. In addition, the decarbonylated radicals derived from them are formed long after their acyl precursors have moved to orientations with respect to their aryloxy partners that result in a loss of the memory of their host stereochemistry within a cage see above. Thus, of the Claisen-like photoproducts from irradiation of (7 )-lb, only the BzON (i.e., 3b) retains a measurable amount of optical activity even in the solid phases of long -aIkane. However, in polyethylene hlms, all of the Claisen products from irradiation of (7 )-lb—2-BN, 4-BN, and 3b—exhibit signihcant ee values. In the same media, the photo-Fries products from lb retain virtually all of the enantiomeric purity of the... 

Berge reported that ethylhexyldimethyl-PABA is demethylated via a photo oxidation/decarbonylation sequence (23,24). The photoproducts produced are N-mono-methyl substituted or possess a free amino-group. These products have UV absorption spectra very similar to that of the starting dimethylamino compound. Therefore, this transformation cannot be detected, utilizing a simple spectral measurement (12,13). This is an example of a nonreversible photodegradation, which, in the spectral-only measurement, produces a result, which falsely indicates that the test substance/product has a higher photostability than is actually the case. 

Beckman rearrangement of the oxime of 7-ketodehydroabietate with phosphoric acid gives the expected lactam (50) and a decarbonylated amine (51). The dihalocarbene adduct of methyl abietate is formed " across the 7(8) double bond. The photo-oxidation of methyl isopimarate, which has a A -double bond, gives 7a-hydroxysandaracopimaric acid methyl ester. However, in the pimaric acid series both the 7a- and 14a-allylic alcohols are formed. 

The standard method of synthesis involves the reaction of suitable transition metal halides with Fc-Li [49, 113, 171 — 180, 189, 190], although HgFcj has also been used as ferrocenyl transfer reagent [181 — 183]. A different approach to ferrocenyl complexes proceeds via thermal or photo-induced decarbonylation of the corresponding ferrocenoyl compounds [178,184]. All three routes have been applied to prepare the half-sandwich/sandwich complex CpFe(CO)2-Fc, which contains two different iron centers (Scheme 5-24). It is surprising that Fc-SnMej has rarely been used [185] for ferrocenyl transfer. 

Neither 1,3-shifts nor 1,5-shifts have been observed for these compounds although such products could conceivably be undetected intermediates in the decarbonylation reaction." The cyclopentenones or cyclopropanones which would arise from these shifts are, of course, photo-chemically labile and are potential intermediates prior to decarbonylation or they could be blind alley , thermally reversible, intermediates. However, no evidence for their formation has been found. 

As a minor pathway, L3 units are formed competitively to photo-Fries rearrangements some radicals formed in CO-O bond scissions may decarbonylate or decarboxylate before further radical recombination or hydrogen abstraction. This leads to the formation of hydroxy- and dihydroxybiphenyl units, aromatic ether structures, and phenol as end groups that further photolyzed into a mixture of species (in a convoluted absorption) that produces the yellowing of the PC film without any defined structure. 

Decarbonylations can occur by either an ionic or a radical mechanism. The former mechanism is involved in reactions catalysed by acids, Lewis acids, or bases, and the latter in chain reactions induced thermally, photo-lytically, or by some other radical-forming process. 

The cyclopropenone (130) undergoes decarbonylation on flash photolysis in water.The resultant ynamine (131) is accompanied by the enamine (132). The enamine is formed from the carbene (133) and its trapping with water. The ynamine (131) is unstable and transforms into the ketenimine (134). The photo-... 

The direct irradiation of the bicycloenones (62) results in a 1,3-acyl shift followed by decarbonylation. The photochemical and thermal reactivity of so-called o- and -acylcyclohexadienones has been studied in a further attempt to examine the mechanism of the photo-Fries process. 

This ruthenium complex is one of the most efficient of known homogeneous catalysts for hydrogenation, but has not found wide use because of the difficult preparation. Recently Geoffrey and Bradley found that it can be prepared efficiently by photo-induced decarbonylation of the relatively stable RuHCI(CO)-[P(C6H5)3]3 in benzene. The solutions can be used directly or the complex can be isolated in slightiy impure form. 

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