Aldehydes, catalytic agents

The periodicity of the cool flames and their merging into the region of explosion with increasing pressure are features on which there has been much speculation but as yet little clear-cut evidence. Chamberlain and Walsh have proposed that the catalytic agents responsible for the cool flames are hydroxy alkyl peroxides arising from the condensation of peroxides and aldehydes on surfaces. Frank-Kamenetskii,- on the other hand, has made the rather intriguing proposal that the mechanism itself is responsible for the periodicity.This requires that peroxides and aldehydes catalyze each others production and disappearance in a set of second-order processes such as... 

A special example of the Cannizzaro reaction results in the direct formation of an ester from two molecules of an aldehyde. The catalytic agent used here is an acid (aluminum alkoxide) according to the Lewis theory. This acid coordinates with a molecule of the aldehyde, acting as a base, and produces an acidic intermediate, as follows ... 

Self-Condensation. Aldehydes (aldol condensations). The synthetic importance of these condensations is known to all organic chemists. A wide variety of compounds can be prepared through the condensation of two carbonyl compounds. The catalysts used are bases such as acetates, carbonates, pyridine, and amines. These catalytic agents are bases according to both the Lewis and the Br0nsted theories. The mechanism of the reaction may be represented as follows ... 

For most laboratory scale reductions of aldehydes and ketones catalytic hydro genation has been replaced by methods based on metal hydride reducing agents The two most common reagents are sodium borohydride and lithium aluminum hydride... 

Reduction to alcohols (Section 15.2) Aldehydes are reduced to primary alcohols, and ketones are reduced to secondary alcohols by a variety of reducing agents. Catalytic hydrogenation over a metal catalyst and reduction with sodium borohydride or lithium aluminum hydride are general methods. 

Although the titanium-based methods are typically stoichiometric, catalytic turnover was achieved in one isolated example with trialkoxysilane reducing agents with titanocene catalysts (Scheme 28) [74], This example (as part of a broader study of enal cyclizations [74,75]) was indeed the first process to demonstrate catalysis in a silane-based aldehyde/alkyne reductive coupling and provided important guidance in the development of the nickel-catalyzed processes that are generally more tolerant of functionality and broader in scope. 

The use of trimethylsilyl-based electrophilic catalysts with organosilicon hydrides also promotes the conversion of aldehydes into ethers and avoids the need to employ the potentially hazardous trityl perchlorate salt.314,334,338 One reagent pair that is particularly effective in the reductive conversion of aldehydes into symmetrical ethers is a catalytic amount of trimethylsilyl triflate combined with either trimethylsilane, triethylsilane, PMHS,334 or 1,1,3,3-tetramethyldisiloxane (TMDO, 64) as the reducing agent (Eq. 179).314 Either... 

Acyloxy-l-cyanoalkanes [45, 46], which can be used as precursors for ketones [47], a-hydroxy ketones [48] and 1,4-dicarbonyl compounds [47], are prepared in one pot from the appropriate aldehyde, sodium or potassium cyanide, and the acylating agent under phase-transfer catalytic conditions [47-49]. Attempts to synthesize chiral cyanhydrins using chiral phase-transfer catalysts have been unsuccessful (see Section 12.3). 

The reactions of aldehydes at 313 K [69] or 323 K [70] in CoAlPO-5 in the presence of oxygen results in formation of an oxidant capable of converting olefins to epoxides and ketones to lactones (Fig. 23). This reaction is a zeolite-catalyzed variant of metal [71-73] and non-metal-catalyzed oxidations [73,74], which utilize a sacrificial aldehyde. Jarboe and Beak [75] have suggested that these reactions proceed via the intermediacy of an acyl radical that is converted either to an acyl peroxy radical or peroxy acid which acts as the oxygen-transfer agent. Although the detailed intrazeolite mechanism has not been elucidated a similar type IIaRH reaction is likely to be operative in the interior of the redox catalysts. The catalytically active sites have been demonstrated to be framework-substituted Co° or Mn ions [70]. In addition, a sufficient pore size to allow access to these centers by the aldehyde is required for oxidation [70]. 

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