Aldehydes, metal catalysed

The transition metal catalysed addition of a hydridosilane to a multiply-bonded system is known as hydrosilylation (1). Under such conditions, alkynes undergo clear cis-addition, so providing one of the most direct routes to vinylsilanes (Chapter 3). Hydridosilanes also add to the carbonyl group of saturated aldehydes and ketones, to produce alkyl silyl ethers. Fot example, under suitable conditions, 4-t-butylcyclohexanone (2) can be reduced with a high degree of stereoselectivity. 

The ability of enzymes to achieve the selective esterification of one enantiomer of an alcohol over the other has been exploited by coupling this process with the in situ metal-catalysed racemisation of the unreactive enantiomer. Marr and co-workers have used the rhodium and iridium NHC complexes 44 and 45 to racemise the unreacted enantiomer of substrate 7 [17]. In combination with a lipase enzyme (Novozyme 435), excellent enantioselectivities were obtained in the acetylation of alcohol 7 to give the ester product 43 (Scheme 11.11). A related dynamic kinetic resolution has been reported by Corberdn and Peris [18]. hi their chemistry, the aldehyde 46 is readily racemised and the iridium NHC catalyst 35 catalyses the reversible reduction of aldehyde 46 to give an alcohol which is acylated by an enzyme to give the ester 47 in reasonable enantiomeric excess. 

Peracetic acid lowers the AIT in this case. Besides, it has been demonstrated that the AIT depends on the partial pressure of peracetic acid formed and which settled on the container s walls that contains aldehyde. Metal oxides (rust, alumina) catalyse the formation of peroxidic compounds. This explains the effect of corroded metals that is described above. It is interesting to note that ketones,... 

Glucose may auto-oxidize (like other alphahydroxy-aldehydes) and generate hydroxyl radicals in a transition-metal-catalysed reaction, and induce both fragmentation and conformational changes in glycated proteins (Hunt et al., 1990). 

More recent reports from Cordova [155] and Wang [156] have described the cyclopropanation of a, P-unsaturated aldehydes 99 with diethyl bromomalonates 100 and 2-bromo ethyl acetoacetate catalysed by a series of diaryIprolinol derivatives. Both describe 30 as being the most efficient catalyst in many cases and optimal reaction conditions are similar. Some representative examples of this cyclopropanation are shown in Scheme 40. The transformation results in the formation of two new C-C bonds, a new quaternary carbon centre and a densely functionalised product ripe for further synthetic manipulation. Triethylamine or 2,6-lutidine are required as a stoichiometric additive in order to remove the HBr produced during the reaction sequence. The use of sodium acetate (4.0 equivalents) as an additive led to subsequent stereoselective ring opening of the cyclopropane to give a,P-unsaturated aldehydes 101. It can be envisioned that these highly functionalised materials may prove useful substrates in a variety of imin-ium ion or metal catalysed transformations. 

Metal-catalysed hydrocarboxylation of olefins (Equation 3) is the poor relative of the more famous hydroformylation. It generally requires forcing reaction conditions and suffers from mediocre activities and selectivities (n/i ratio). Moreover, the same products can be made via hydroformylation and oxidation of the aldehyde product.431 Consequently, there are few industrial applications of hydrocarboxylation e.g. Ni(CO)4-catalysed production of propionic acid by hydrocarboxylation of ethylene.432,433... 

Two alternatives were proposed for the final dehydrogenation the enaminol could undergo a metal-catalysed peroxidation263 with subsequent elimination of H202 or it could add to another molecule 2-oxopropanal to form the aldehyde-ammonia with subsequent elimination of hydroxyacetone.262... 

Ionic liquids were initially developed as solvents for electrochemical applications. The electrochemical window of clean ionic liquids can be huge/11 allowing for a wide range of redox reactions/2,31 It has further been demonstrated that they are also suitable solvents for enzymatic oxidations14 71 but both topics are beyond the scope of this book. Only transformations that involve the metal-catalysed addition of oxygen to unsaturated carbon bonds as well as the oxidation of alcohols, aldehydes and ketones to their corresponding ketones, carboxylic acids and esters shall be discussed in this chapter. 

The application of organometallic compounds in medicine, pharmacy, agriculture and industry requires the accurate determination of these metals as part of their application. Most % complexes characterised by direct carbon-to-carbon metal bonding may be classified as organometallic and the nature and characteristics of the n ligands are similar to those in the coordination metal-ligand complexes. The -complex metals are the least satisfactorily described by crystal field theory (CFT) or valence bond theory (VBT). They are better treated by molecular orbital theory (MOT) and ligand field theory (LFT). There are several uses of metal 7i-complexes and metal catalysed reactions that proceed via substrate metal rc-complex intermediate. Examples of these are the polymerisation of ethylene and the hydration of olefins to form aldehydes as in the Wacker process of air oxidation of ethylene to produce acetaldehyde. 

Creighton and co-workers have extended a preliminary study on model dehydrogenase reactions. The ZnCl2-catalysed reduction of l,10-phenanthrolme-2-carbox-aldehyde by iV-propyl-l,4-dihydronicotinamide in acetonitrile was investigated, as well as the metal-catalysed borohydride reduction of 2- and 4-pyridinecarbaldehyde. The results support the view that the zinc in alcohol dehydrogenase serves to polarize the carbonyl group of the substrate aldehyde and to facilitate deprotonation of the alcohol. 

In summary, the uncatalysed oxidation of hydrocarbons at temperatures of up to 120°C leads to alkylhydroperoxides, ROOH, dialkylperoxides, ROOR, alcohols, ROH, aldehydes, RCHO and ketones, RR C=0. In addition, cleavage of a dihydroperoxide II of Reaction (4.4) leads to diketones, RCO(CH2)jCOR keto-aldehydes, RCO(CH2)jCHO, hydroxy ketones, RCH(OH)-(CH2)jCOR and so forth. Under metal-catalysed conditions or at higher temperatures, considered in Sections 4.2.2 and 4.2.3, degradation leads to a complex mixture of final products. 

This represents the first metal-catalysed addition of trimethylenemethane to a carbon-nitrogen double bond. It has been demonstrated that trimethylenemethane can be added to aldehydes to give methylenetetrahydrofurans provided that a tin cocatalyst is added to the reaction mixture (Scheme 48.)70 Trimethylenemethane... 

The scope of this chapter does not allow nor attempt a comprehensive account of all developed processes to date. A detailed summary, in particular of aldol, Mannich, or ot-functionalisation reactions, can be found in excellent reviews written on the topic." Barbas and List reported an asymmetric, direct, intermolecular aldol reaction of acetones and aldehydes (Scheme 5.4), presumably via enamine formation of proline and acetone. As compared to its metal-catalysed alternatives, no preformation of the respective enolate is required, a mode of action that mimics metal-free aldolase enzymes. ... 

While several metal-catalysed approaches to solve the task of generating fl tz-l,2-diols have been developed, this method remains remarkable, since it represents the first small molecule-catalysed, catalytic version of this transformation. The starting materials do not need activation or protection, and the reaction can be performed under standard conditions without further precautions. The selectivity of the reaction potentially results from a hydro Q acetone enamine-initiated transition state (5). More recently, Enders et al. applied a related method to the organocatalytic synthesis of sialic acid precursors (Scheme 5.6). Protected pyruvic aldehyde (6) was reacted with several aldehydes, forming the desired aldols (7) in moderate yields, but good selectivities (31-51%, 90-92% de, 73-99% ee). The conditions were optimised to limit detrimental side reaetions sueh as Mannich elimination or formation of the aeetal self-aldolisation produet. While generally easily applicable and robust, the method laeks effieiency in one key parameter, its reaction time. This limitation, combined with moderate yields unfortunately prevents a scale-up to or beyond the pilot plant. 

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