Carbonyl compounds atom-transfer reaction

The following mechanism of the reaction has been suggested it includes the coordination of the carbonyl compound with the aluminium atom in aluminium sopropoxide and the transfer of a hydride Ion ... 

Studies aimed at the elucidation of reaction mechanisms have been performed by many groups, notably by those of Backvall [28]. In test reactions, typically enantiopure 1-phenylethanol labeled with deuterium at the 1-position (8) is used. The compound is racemized with acetophenone (9) under the influence of the catalyst and after complete racemization of the alcohol, the deuterium content of the racemic alcohol is determined. If deuterium transfer proceeds from the a-carbon atom of the donor to the carbonyl carbon atom of the acceptor the deuterium is retained, but if it is transferred to the oxygen atom of the acceptor it is lost due to subsequent exchange with alcohols in the reaction mixture (Scheme 20.4). 

The (3-metaloxy radical was first exploited for synthetic purposes in C—H and C—C bond-forming reactions by Nugent and RajanBabu through the use of titanocene(III) chloride as an electron-transfer reagent [5]. They established that the (3-titaniumoxy radicals formed after electron transfer can be reduced by hydrogen atom donors, e. g. 1,4-cy-clohexadiene or tert-butyl thiol, that they add to a,(3-unsaturated carbonyl compounds, and that they can react intramolecularly with olefins in 5-exo cyclizations. 

Acylsilanes are a class of compounds in which a silyl group is directly bound to the carbonyl carbon, and they have received considerable research interest from the point of view of both physical organic and synthetic organic chemistry [15]. Acylsilanes have a structure quite similar to the structure of a-silyl-substituted ethers a silyl group is attached to the carbon adjacent to the oxygen atom, although the nature of the C-O bond is different. Therefore, one can expect /1-silicon effects in the electron-transfer reactions of acylsilanes. 

This observation may well explain the considerable difference between metal-olefin and metal-acetylene chemistry observed for the trinuclear metal carbonyl compounds of this group. As with iron, ruthenium and osmium have an extensive and rich chemistry, with acetylenic complexes involving in many instances polymerization reactions, and, as noted above for both ruthenium and osmium trinuclear carbonyl derivatives, olefin addition normally occurs with interaction at one olefin center. The main metal-ligand framework is often the same for both acetylene and olefin adducts, and differs in that, for the olefin complexes, two metal-hydrogen bonds are formed by transfer of hydrogen from the olefin. The steric requirements of these two edgebridging hydrogen atoms appear to be considerable and may reduce the tendency for the addition of the second olefin molecule to the metal cluster unit and hence restrict the equivalent chemistry to that observed for the acetylene derivatives. 

Diazo compounds are photoreduced to hydrazines in the presence of silanes and stannanes in a process that apparently involves initial H-atom transfer to an excited state of the diazo compound.105 LFP studies of reactions of Bu3SnH with excited states of diazo compounds 40 and 41 found fast reactions. The singlet diazo species reacted with Bu3SnH about as fast as singlet carbonyl compounds, whereas the triplet diazo species reacted with tin hydride somewhat slower than triplet excited states of carbonyl compounds.100,105... 

Probably the most familiar radical reactions leading to 1,2-D systems are the so called acyloin condensation and the different variants of the "pinacol condensation". Both types of condensation involve an electron-transfer from a metal atom to a carbonyl compound (whether an ester or an aldehyde or a ketone) to give a radical anion which either dimerises directly, if the concentration of the species is very high, or more generally it reacts with the starting neutral carbonyl compound and then a second electron is transferred from the metal to the radical dimer species (for an alternative mechanism of the acyloin condensation, see Bloomfield, 1975 [29]). 

A common photochemical reaction of carbonyl compounds is the transfer of a hydrogen atom to the carbonyl oxygen atom ... 

The Meerwein-Pondorf-Verley reaction involves transfer of a hydride from the oxygen-substituted carbon atom of an isopropoxide group to a carbonyl group, thus effecting the reduction of the carbonyl compound to an alcohol. 

The prototype of photoreduction reactions is hydrogen abstraction by carbonyl compounds in presence of suitable H-donors. Such H-atom transfer may be visualized to occur first by transfer of an electron followed by proton transfer. An electron deficient centre is the seat of reaction and the efficiency of the reaction depends on the nucleophilic nature of the donor. 

They proposed a polymerization scheme closely related to other well-known chemical reactions of metal alkoxide with carbonyl compounds (20). In Scheme 2, complex [A] is converted to [B] by hydride ion transfer from the alkoxyl group to the carbon atom of aldehyde (Meerwein-Ponndorf reduction). Addition of one molecule of monomer to the growing chain requires transfer of the alkoxide anion to the carbonyl group to form a new alkoxide [C]. Repetition of these two consecutive processes, i.e., coordination of aldehyde and transfer of the alkoxide anion, constitutes the chain propagation step. 

Free-radical autoxidation of aldehydes with 02 is facile and affords the corresponding peradds, which are used as oxidants for carbonyl compounds. The peracid can transfer an oxygen atom to a substrate such as an olefin or ketone, resulting in the formation of one equivalent of epoxide or ester and add as a co-produd in the absence of metal catalysts [59]. Kaneda and coworkers have developed several HT materials that are active for heterogeneous Baeyer-Villiger reactions with 02/aldehyde [60]. Combination with Lewis addic metals improved the reaction by allowing coordination of the peracid and the intermediate. 

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