Photodecarbonylation

Iron(III) iodide [15600-49-4], Fefy, is prepared by the oxidative photodecarbonylation of diiodotetracarbonylkon(II) ki the presence of dkodine (7). The black soHd obtained is extremely hygroscopic, spariagly soluble only ki dichloromethane, and decomposes to kon(II) iodide and dkodine when exposed to donor solvents such as tetrahydrofuran, acetonitrile, water, or pyridine. It also decomposes when exposed to light. 

An example of a reaction cavity is illustrated by the van der Waals surface plots derived from the X-ray structures of di-2,6-dihydroxy-2,6-diphenylcyclohexanone 51 and its photodecarbonylation products, the cis- and tra i-2,6-diphenylcyclopen-tane-l,2-diols 52 and 53 (Scheme While the two products are formed in... 

While the examples in Scheme 7.16 hinted at the practicality of the solid state photodecarbonylation of ketones, the factors controlling this reaction remained unknown until very recently. As a starting point to understand and predict the photochemical behavior of ketones in terms of their molecular structures, we recall that most of the thermal (kinetic) energy of crystals is in the form of lattice vibrations. 

As it pertains to the solid state photodecarbonylation reaction, the model assumes that most aliphatic ketones have similar excitation energies, that reactions are more likely along the longer-lived triplet excited state, and that each reaction step must be thermoneutral or exothermic to be viable in the solid state. " Using acetone and its decarbonylation intermediates as a reference reaction (dashed lines in Fig. 7.24), we can analyze the energetic requirements to predict the effects of substituents on the stability of the radical intermediates. The a-cleavage reaction of triplet acetone generates an acetyl-methyl radical pair in a process that is 3.5 kcal/mol endothermic and the further loss of CO from acetyl radical is endothermic by 11.0... 

The possibility of predicting solid state reactivity from calculated thermochemical data was first addressed with ketodiesters 65a-e, which were substituted with methyl groups to vary the extent of the RSE in the radicals 65-BRl - 65-BR3 involved along the photodecarbonylation pathway (Scheme 7.19). " All ketones reacted in solution to give complex product mixtures from radical combination (66a-e) and disproportionation processes. Calculations revealed RSEs of 8.9 kcal/mol, 15.1 kcal/mol, and 19.8 kcal/mol for radicals 65-BRl (primary enol radical), 65-BR2 (secondary enol radical), and 65-BR3 (tertiary enol radical), respectively. In the... 

The photodecarbonylation of a series of dibenzyl ketones was studied by Robbins and Eastman/63 The results of this study are presented in Table 4.5. The data in Table 4.5 indicate that the presence of a p-methyl or a p-methoxy group has little effect on the quantum yield for this reaction. p-Cyano groups, on the other hand, essentially totally eliminated the decarbonylation. Since the reaction could also be quenched (inefficiently) by benzonitrile or biphenyl, it was concluded that the decarbonylation occurs from a short-lived triplet state. The effect of the p-cyano groups then could result from internal triplet quenching. 

Table 4.8. Some Photodecarbonylation Quantum Yields for a-Aryl Aldehydes...

The photodecarbonylation of a-aryl aldehydes has been studied as a function of structure by Kuntzel, Wolf, and Schaffner<70) ... 

Photodecarbonylation of p-tolyl benzyl ketone Photolysis of dibenzyl ketones with CuCl2 scavenger... 

In an attempt to establish the limits for ODPM reactivity of (B.y-unsaturated aldehydes, we have extended our studies to a series of aldehydes 65, (Scheme 10) which are monosubstituted at C-2. Triplet-sensitized irradiation of 65 leads to the formation of the corresponding cyclopropanecarbaldehydes 66 [59] (Scheme 10). The diphenyl-substituted aldehydes 65b and 65d yield, in addition to the ODPM products, the corresponding alkenes 67a and 67b, resulting from photodecarbonylation. The formation of these alkenes is probably due to stabilization of the radical, formed by allylic cleavage, by diphenyl conjugation. The ODPM rearrangement of aldehydes 65 is diastereoselective, yielding only one diastereoisomer of 66 (Scheme 10). 

Some phenyl esters give rise to photodecarbonylation as a side reaction [37], This is also the case for certain phenylpropionamides, which implies cleavage of the CO—C bond in the acyl radical, after the breaking of the N—CO bond [99], If CO—C cleavage occurs prior to N—CO cleavage, then an isocyanate is formed (Scheme 38) and PFR does not occur. 

An example of carbonyl compound which illustrate Photodecarbonylation and example of Abstraction of hydrogen atom to form alcohol... 

Tetracarbonylfoctahydrotriborato(l-)] manganese is an air-sensitive liquid that decomposes slowly (about 5% in 4 days) at room temperature as a neat liquid in a vacuum. It is soluble in benzene, toluene, dichloromethane, diethyl ether, and tetrahydrofuran (THE), but decomposes upon heating at reflux in these solvents (especially in THF). Gas-phase thermal decarbonylation or solution photodecarbonylation of (CO)4Mn(B3H8) yields the novel and reactive compound (CO)3Mn(B3H8), in which the octahydrotriborate(l-) ligand is tridentate.3... 

This femtosecond study confirmed the involvement of the oxytetramethylene diradical as a reactive intermediate, and found that the trimethylene formed from it had the same hfetime as the trimethylene generated through the photodecarbonyl-ation of cyclobutanone. For tetrahydropyran, the oxypentamethylene drradical (86 amu) is formed readUy and the 85 amu transient, from the p-cleavage of a C H bond, is the dominant fragmentation product. 

Initial thermal sigmatropic 1,5-carbon shifts are believed to account for the product formation from spiro[2.4]hepta-4,6-diene (l) 200 and spiro[cyclopropyl-l, 2 -2 //-indene] (6),201 obtained by photodecarbonylation of dione 5. In the first case, a 1,5-hydrogen shift and an electrocyclic ring opening completes the formation of 3,4-dimethylenecyclopent-l-ene (4),200 and in the second case, the initially formed benzoannulated bicyclo[3.2.0]hepta-l,3-diene 7 dimerizes to 8 as mixture of two stereoisomers.201... 

Most photodecarbonylation reactions of cyclic ketones, especially in the vapor phase, have been postulated to proceed from various vibrational levels of excited singlet states.321 However, the elimination reaction leading to unsaturated aldehydes has now been shown to occur largely via excited triplet states. In solution, where the lowest vibrational levels of the excited states are rapidly reached, to-alkenals are the major products observed in both photolysis and radiolysis of cyclopentanone and cyclohexanone. The reaction is quenched by oxygen and dienes,322-324 as well as by the alkenal produced in the reaction.325 The reaction is also sensitized by benzene triplets.322,323 With cyclopentanone, quenching by 1M piperylene occurs some 20 times as fast... 

The formation of some Ru(III) complexes upon photodecarbonylation followed by an internal electron transfer can be expressed as follows... 

In the 1,6-anhydro series [63], 4-ketuloses are photodecarbonylated to yield the same kind of compounds as the preceding 2-ketuloses (Scheme 33). 

The photodecarbonylation reaction of thiophenones 118a-c leading to the formation of alkyl-substituted thietes 119a-c in high yields turned out to be a useful method for their synthesis (Table 12) <1997HCA510>. 

K.K., Veerman, M., Mortko, C.J., and Garcia-Garibay, M.A. (2008) Solid state photodecarbonylation of diphenyl-cyclopropenone a quantum chain process made possible by ultrafast energy transfer. Journal of the American Chemical Society, 130, 1140-1141. 

Synthetic applications of the reaction are somewhat limited as the highly reactive biradicals and radical pairs tend to undergo reactions that compete with C—C bond formation. As in previous cases, the reaction may have synthetic value for the synthesis of strained structures involving small rings. For example, the preparation of the simplest [2]-ladderane 55 by photodecarbonylation of bicyclo[3.2.0]heptan-3-one 54 gave the bicyclic structure in 5% yield with a ring-opened 1,5-heptadiene being the dominant product (Scheme 2.14) [41]. 

The utility of the solid-state photodecarbonylation of crystalline ketones was recently demonstrated in the total syntheses of two natural products, where the key step is the solid-state reaction. The first example involves the synthesis of the sesquiterpene ( )-herbertenolide [80] by the solid-state decarbonylation of cyclohexanone 189, followed by cyclization of the photoproduct 190 (Scheme 2.46). With precursor 189 obtained by simple methods and a solid-state reaction carried out to 76% conversion, herbertenolide was obtained in good overall yield in a record number of steps from commercial starting materials. With a similar synthetic strategy, samples of the natural product (i)-a-cuparenone were obtained in about 60% overall yield from 191 by a very succinct procedure that included four simple steps and a solid-state reaction at — 20 °C [81]. 

The enantioselective syntheses of (R)-oc-cuparenone and (S)-a-cuparenone, both of which are natural products from different sources, were also completed using the solid-state photodecarbonylation of diasteromerically pure difluorodioxaborinane ketones 192 and 194 (Scheme 2.47). The latter were prepared in two steps from 191, and irradiated as nanocrystalline suspensions to optimize the chemical yields of the transformation. The photoreaction of the optically pure ketones was 100% stereoselective with an isolated yield of 80%. The two natural products were obtained by simple acid removal of the chiral auxiliary. 

As is true for all chemical processes, the photodecarbonylation of ketones is limited by structural effects known to result in fast competing pathways, and by structural attributes that give rise to quenching interactions. Whilst one might have predicted, based on the RS E values of the a-substituents, that the three ketones in Scheme 2.48 should have reacted in the solid state by loss of CO and radical combination, the... 

Stereospedfic photodecarbonylation of ris- and trans-2,7-dimethyl-3,5 -cycloheptadienone observation of an apparent symmetry-forbidden concerted cheletropic fragmentation. Journal of the American Chemical Society, 105, 2900-2901. 

Ramnauth, J. and Lee-Ruff, E. (1997) Photodecarbonylation of chiral cyclobutanones. Canadian Journal of Chemistry, 75, 518-522. 

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