Photocatalytic Carbonylations

The carbonylation of hydrocarbons serves as an important catalytic methodology both for extending the carbon backbones of organic compounds and for adding oxygen functional groups such as alcohols, aldehydes or carboxylic acids. Hydroformylation of alkenes (eq. 23) is carried out in various thermal catalysis schemes [87] and photocatalytic examples have been described (see below) [88,89], 

Photocatalytic activation of C-H bonds of alkanes and aromatic hydrocarbons including photocatalytic carbonylation [4,90-94] has received considerable attention in recent years, although the more extensively studied systems have been those based on cyclopentadienyl or phosphine complexes, especially those of rhodium and iridium. Described in this section are the more limited examples of photocatalytic carbonylation based on metal carbonyls. 

Photocatalysis of benzene carbonylation (eq. 24) has been reported for the complexes Ru(CO)3(PPh3)2 and Ru(CO)4PPh3 in benzene under CO (500-800 torr) using a pyrex filtered 200 W Hg/Xe lamp [96]. 

The amount of benzaldehyde formed is small and apparently limited by the unfavorable thermodynamics (AG298 = + 1-7 kcal mol ) and back decarbonylation of benzaldehyde. The reversibility was demonstrated by effecting the photolytic exchange of CO into benzaldehyde. 

The disubstituted complex is converted by photolysis under the reaction conditions to Ru(CO)4PPh3 and this appears to be the precursor for the photocatalytic process. The unsubstituted cluster Ru3(CO)i2 proved be ineffective as a photocatalyst for ethene hydroformylation, although under the photolysis conditions the cluster should fragment to the unsubstituted mononuclear species Ru(CO)5 [17]. 

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Scheme 5. Palladium-catalyzed photocatalytic carbonylative coupling with boranes [73],...

Photocatalytic carbonylative coupling with 9-alkyl-9-borabicyclo[3.3.1]-nonanes (9-R-9-BBN), however, made it possible to transform alkyl halides to ketones [72]. lodoalkenes or iodoalkynes are thus cyclized to five-membered rings [73]. The oxidative addition of iodoalkyl to palladium(O) proceeds via radicals allowing the ring closure to take place prior to the dual coupling with CO and the alkylboranes. 

Reaction of Ruthenium Carbonyls with Alkyl Radicals Boese and Goldman reported that in the presence of aryl ketones, d8 metal carbonyls such as Ru(CO)3(dmpe) mediate photocatalytic carbonylation of alkanes via a free radical mechanism.161 The activity was proposed to be initiated by the addition of an alkyl radical to the metal carbonyl and the formation of a metal-acyl radical intermediate. The transition states and the products of the reaction between alkyl radicals and ruthenium carbonyls were studied utilizing the B3LYP level of theory.162 The methyl addition to a carbonyl of Ru(CO)5 or Ru(CO)3(dmpe) was computed to be about 6 kcal/mol more exothermic than addition to free CO. 

Equation 25 illustrates another photocatalytic carbonylation [97] which has been demonstrated for the mononuclear carbonyl Pt(CO)2L2 (L = AsPh3 or PPh3) and the polynuclear species Mn2(CO)io, Co2(CO)g, Fe3(CO)i2,... 

To this category belong, e.g., homogeneous photocatalytic systems based on soluble metal complexes or organic dyes as photocatalysts. Instructive examples are photoreactions assisted by heteropolyacids (HPAs), transition meal complexes with carbonyl, phosphine or some other ligands, and metal porphyrins. 

As shown in Fig. 13, a variety of metal carbonyls upon sonication will catalyze the isomerization of 1-pentene to cis- and tram-2-pentene (186). Initial turnover rates are about 1-100 mol 1-pentene isomerized/mol of precatalyst/hour, and represent rate enhancements of 102 5 over thermal controls (174). The relative sonocatalytic and photocatalytic activities of these carbonyls are in general accord. An exception is Ru3(CO)12, which is... 

Rhodium and cobalt carbonyls have long been known as thermally active hydroformylation catalysts. With thermal activation alone, however, they require higher temperatures and pressures than in the photocatalytic reaction. Iron carbonyl, on the other hand, is a poor hydroformylation catalyst at all temperatures under thermal activation. When irradiated under synthesis gas at 100 atm, the iron carbonyl catalyzes the hydroformylation of terminal olefins even at room temperatures, as was first discovered by P. Krusic. ESR studies suggested the formation of HFe9(C0) radicals as the active catalyst, /25, 26/. Our own results support this idea, 111,28/. Light is necessary to start the hydroformylation of 1-octene with the iron carbonyl catalyst. Once initiated, the reaction proceeds even in the... 

The photocatalytic hydrogenation of alkenes and dienes by Group 6 metal carbonyls has been investigated in LNG solvents [15]. Photolysis of trans-[M(C0)4(C2H4)2] (M = Cr, Mo, W) in liquid xenon doped with H2 leads to formation of mer-[M(CO)3(C2H4)2(q -H2)] and ds-[M(CO)4(C2H4)(q -H2)]. The q -H2 complexes for M = Cr and Mo are much less stable than those for M = W. The evidence supported -coordination of H2 rather than oxidative addition to give dihy-... 

A few stable C02 complexes resulting from a formal oxidation of metal carbonyl complex have been described. Although not strictly true C02 complexes, these are important intermediates in the photocatalytic and electrocatalytic reductions of C02 to formate and CO, and also in biologic systems. A few examples of such species are described below. 

Under similar conditions, the photocatalytic oxidation of 2-or 3-methyl-l-butene and of 2-methyl-2-butene over Ti(>2 yielded carbonyl compounds as partial oxidation products (18). However, the selectivity to a particular aldehyde or ketone was reduced by cleavages not only of the double bond but also of the Cg-Cy bond. 

The pyrimidine nucleobases have the highest quantum yields for photoreactivity, with thymine uracil > cytosine. The purine nucleobases have much lower quantum yields for photochemistry, but can be quite reactive in the presence of oxygen. As can be seen from Figure 9-3, thymine forms primarily cyclobutyl photodimers (ToT) via a [2ir + 2tt cycloaddition, with the cis-syn photodimer most prevalent in DNA. This is the lesion which is found most often in DNA and has been directly-linked to the suntan response in humans [65]. A [2Tr + 2Tr] cycloaddition reaction between the double bond in thymine and the carbonyl or the imino of an adjacent pyrimidine nucleobase can eventually yield the pyrimidine pyrimidinone [6 1]-photoproduct via spontaneous rearrangement of the initially formed oxetane or azetidine. This photoproduct has a much lower quantum yield than the photodimer in both dinucleoside monophosphates and in DNA. Finally, thymine can also form the photohydrate via photocatalytic addition of water across the C5 = C6 bond. 

Rhenium(I) diimine carbonyl complexes have been well investigated because of their functionalities, such as the intense emission properties, capabilities as a building block for multinu-clear complexes, and photocatalytic activities. 

Even the ease of retrieval or possible photocatalytic uses or such in alkene synthesis from the carbonyl compounds principal in biochemistry (McMurry reaction) apparently cannot compensate for this as Ti is unable to bind the primary substrate. Eor Al, Zr or Ti abundance cannot replace catalytic versatility with respect to various functions, that is... 

Many fewer photocatalytic organic reductions have been reported. Reductions of organic substrates are less thoroughly studied, largely because of the early emphasis on the use of organic compounds as oxidizable source for the production of hydrogen gas. Nonetheless, some examples do exist, such as the hydrogenation of olefins, vinyl ethers, and a, S-unsaturated enones and alkynes [166, 167]. Similarly, other multiple bonds can be reduced, e.g., the N=N double bond of diaryl azo compounds [168] or carbonyl C=0 bonds [169, 170]. 

The enhanced reactivity of oxiranes due to geometric strain is manifested in their acid- and base-catalyzed reactions, in their rearrangements in the presence of metals and metal compounds, and in their thermal and photocatalytic transformations. These reactions permit the development of new and varied methods of synthesis and the preparation of derivatives that in many cases are difficult to obtain by other routes. The main products of the isomerization reaction are unsaturated alcohols and carbonyl compounds. Great progress has been made in this area of organic chemistry during the past 20 years (The reader is referred to some relevant... 

Dimeric iron or manganese carbonyl complexes such as [CpFe(C02)]2 and Mn2(CO)io, respectively, upon irradiation photocatalytically cleave carbon-halogen bonds. This leads to carbon-centered radicals which can be reduced to hydro-... 

The mechanism of photocatalytic hydrogenation has been studied (by IR) with norbomadiene (nbd) and Group 6 metal carbonyls with respect of the role of H2... 

A similarly efficient, selective and environmentally benign photocatalytic system has been developed for the oxidation by oxygen of activated benzylic and allylic alcohols into their corresponding carbonyls in moderate to excellent yields (Scheme 2.32). The process did not require a transition metal to occur. 

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