Carbonyl compounds decarbonylation

Various organotin reagents react with acyl and aroyl halides under mild conditions without decarbonylation to give carbonyl compounds[390,39l]. Alkyl- or alkenyltin reagents react with acyl and aroyl chlorides to give ketones[548.733,734]. One example is the preparation of the a,/3-dnsaturated 7-keto esters 860 and 861, carried out under a CO atmosphere[735]. The reaction has been applied intramolecularly to the synthesis of the macrocyclic keto... 

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. 

Reduction of unsaturated carbonyl compounds to the saturated carbonyl is achieved readily and in high yield. Over palladium the reduction will come to a near halt except under vigorous conditions (73). If an aryl carbonyl compound, or a vinylogous aryl carbonyl, such as in cinnamaldehyde is employed, some reduction of the carbonyl may occur as well. Carbonyl reduction can be diminished or stopped completely by addition of small amounts of potassium acetate (i5) to palladium catalysts. Other effective inhibitors are ferrous salts, such asferroussulfate, at a level of about one atom of iron per atom of palladium. The ferrous salt can be simply added to the hydrogenation solution (94). Homogeneous catalysts are not very effective in hydrogenation of unsaturated aldehydes because of the tendencies of these catalysts to promote decarbonylation. 

Organomercurial carbonylation. Use of Co2(CO)g as a stoichiometric and as a catalytic reagent Organic synthesis reactions using palladium compounds Decarbonylation reactions using transition metal compounds... 

Increasing use is being made of pyran syntheses based upon [4 + 2] cycloadditions of carbonyl compounds. The appropriate unsaturated aldehyde with ethyl vinyl ether yields 53 with peracids this affords an epoxide that undergoes ring contraction to the aldehyde 54 (Scheme 23) and rhodium catalyzed decarbonylation affords the required 3-alkylfuran with the optical center intact.116 Acetoxybutadiene derivatives add active carbonyl compounds giving pyrans that contract under the influence of acids to give... 

Carbonyl compounds of metals of group 9 have been extensively used in the preparation of supported catalysts, mainly for two reasons the metals are active in several reactions of industrial interest and there are stable carbonyls of easy decarbonylation of the three metals of this group. Besides the stable neutral homo-nuclear carbonyl compounds CojjCOjg, Co4(CO)i2, Rh4(CO)i2, Rh,s(CO)iis and Ir4(CO)i2, other heteronuclear carbonyl species, mainly containing Co-Rh, Co-Ru and Rh-Fe, have been used in the preparation of catalysts. Both metals, Co and... 

On an alumina support, independently of the cobalt carbonyl precursor used, complex cobalt sub-carbonyls compounds, [Co(CO)4] and hydrogencarbonate species formed [143, 149]. However, the reactivity of the alumina surface depends on the degree of hydroxylation highly hydroxylated alumina is more reactive against Co2(CO)g and facilitates decarbonylation, whereas dehydroxylated alumina favors the formation of high nuclearity species like [Co6(CO),5] , which would need higher temperatures than the initial Co2(CO)8 to be decarbonylated [149]. 

CO into a metal-hydrogen bond, apparently analogous to the common insertion of CO into a metal-alkyl bond (6). Step (c) is the reductive elimination of an acyl group and a hydride, observed in catalytic decarbonylation of aldehydes (7,8). Steps (d-f) correspond to catalytic hydrogenation of an organic carbonyl compound to an alcohol that can be achieved by several mononuclear complexes (9JO). Schemes similar to this one have been proposed for the mechanism of CO reduction by heterogeneous catalysts, the latter considered to consist of effectively separate, one-metal atom centers (11,12). As noted earlier, however, this may not be a reasonable model. 

Upon acylation of some benzyl carbonyl compounds (25, R = H, Me 51, R = OH) dibenzo[a,tropylium salts 65 have been isolated in low yields (5-15 %) along with the major products, 2-benzopyrylium salts. Veratryl acetone 25 (R = Me) as well as homoveratric aldehyde 25 (R = H) (or carboxonium ions 31 which are formed from them) may undergo an oxidative a-cleavage, resulting in the benzyl cation 64. The formation of the same cation from homoveratric acid 51 is the result of decarbonylation of the acylium ion 63. Further interaction of the benzyl cation 64 with the substrate, followed by cyclization and oxidation, results in the polycyclic tropylium salts 65 (82ZOR589). 

Palladium chloride and metallic palladium are useful for carbonylating olefinic and acetylenic compounds. Further, palladium is active for decarbonylation of aldehydes and acyl halides. Homogeneous decarbonylation of aldehydes and acyl halides and carbonylation of alkyl halides were carried out smoothly using rhodium complexes. An acyl-rhodium complex, thought to be an intermediate in decarbonylation, was isolated by the oxidative addition of acyl halide to chlorotris(triphenylphosphine)rhodium. The mechanisms of these carbonylation and decarbonylation reactions are discussed. 

By using this property, decarbonylation of carbonyl compounds was investigated. Smooth decarbonylation of aldehydes was observed even at room temperature the reaction can be expressed by the following equation (24). Some results are given in Table I. 

Tsuji, J., Ohno, K. Organic syntheses by means of noble metal compounds. XXXIV. Carbonylation and decarbonylation reactions catalyzed by palladium. J. Am. Chem. Soc. 1968, 90, 94-98. 

Reactions of RhCl(PPh3)3 with carbonyl compounds lead to stoichiometric decar-bonylation to give the corresponding hydrocarbons and RhCl(CO)(PPh3)2 (eq (37)) [45]. Catalytic decarbonylation is possible at high temperature (eq 38) [45]. 

Other reactions. Blum et a/.15 report that the organometallic complex causes decarbonylation of carbonyl compounds and desulfonation of aromatic sulfonyl chlorides. They noted also that 20-g. samples of ethylbenzene, fluorene, and ace-naphthene on being heated with 100 mg. of rhodium complex for 5 hrs. at 130° are oxidized to the extent of 10-60% to acetophenone, fluorenone, and acenaphthenone. 

A distinctive feature of the Mn2(CO)5(PP)2 series of compounds (166) is their ability to reversibly add CO when heated in solution under a CO atmosphere. The IR spectrum of the product, for example, Mn2(CO)6(dppm)2, suggests the presence of two terminal carbonyls on each manganese as well as two C-bonded edge-bridging carbonyl ligands. This compound decarbonylates to the starting Mn2(CO)5(dppm)2 when it is allowed to stand in air for a few weeks. Systematic studies of the de-carbonylation of Mn2(CO)6(PP)2 complexes demonstrate that the tendency... 

The dominant photochemical reaction of ketones in the gas phase is cleavage of one of the carbonyl substituents, which is followed by decarbonylation and subsequent reactions of the free radicals that are formed. The initial cleavage occurs within 100 fs of excitation. There is an activation barrier for decarbonylation (see Table 11.2), so decarbonylation can be relatively slow with excitation at 270nm. At shorter wavelengths, there may be sufficient excess energy for rapid decarbonylation. This reaction is referred to as the Type-I or a-cleavage reaction of carbonyl compounds. 

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