Acetaldehyde complexes

The acetaldehyde complex takes a dimeric form in benzene solution. Three possible structures, two oxygen-bridged and one alkyl-bridged four-membered cyclic structures, are conceivable for the dimeric form of this complex. X-ray structure analysis of the single crystal of this complex (Fig. 6) demonstrated definitely the structure to be the oxygen-bridged four-membered cyclic structure [XIX]. This structure supports the assumed structure [XVIII] (46]. 

Structures of aliphatic and aromatic aldehyde complexes are concluded to be identical in principle with that of the acetaldehyde complex from the comparison of their IR and H-NMR spectra. 

The behavior of the aldehyde complex toward Lewis bases was examined. The acetaldehyde moiety in the acetaldehyde complex Me2A10CPhNPh MeCHO was not displaced by a large excess of Lewis base such as pyridine or tetrahydrofuran, but was replaced by a strong electron donor substance such as trimethylamine oxide to give a crystalline trimethylamine oxide complex, Me2A10CPhNPh ONMe3, which is identical to that obtained from trimethylamine oxide and [Me2AlOCPhNPh]2. 

The cr-dimethylacetal complex (40) is then hydrolyzed on passage through an alumina column, producing the cr-acetaldehyde complex (38) which is converted to the 7r-vinyl alcohol complex (39) by protonation. Wakatsuki, Nojakura, and Murahashi reported the synthesis of l,3-bis(7r-ethenol)2,4-dichloro- i-dichloroplatinum(II) (41) (42), however, Thyret, who recently reported the NMR evidence for the formation of tetracarbonyl(7r-ethenol)iron (42) at low temperature, could not reproduce the synthesis of (41) (43). 

Finally, carbonyls can bind two metals at once. The crystal structure of a bridging (p,), q -bound acetaldehyde complex, for example, shows the carbonyl coordinated to two molybdenum atoms (Figure 38). It appears that in this structure the carbonyl utilizes its it as well as its lone pair electrons to bond to the two metal centers. Conceptually one can think of this molybdenum complex as a bidentate Lewis acid that chelates the carbonyl group. 

The simplest example of such reactions is the decarboxylation of pyruvate. Both model and enzyme studies have shown the intermediacy of covalent complexes formed between the cofactor and the substrate. Kluger and coworkers have studied extensively the chemical and enzymatic behavior of the pyruvate and acetaldehyde complexes of ThDP (2-lactyl or LThDP, and 2-hydroxyethylThDP or HEThDP, respectively) . As Scheme 1 indicates, the coenzyme catalyzes both nonoxidative and oxidative pathways of pyruvate decarboxylation. The latter reactions are of immense consequence in human physiology. While the oxidation is a complex process, requiring an oxidizing agent (lipoic acid in the a-keto acid dehydrogenases , or flavin adenine dinucleotide, FAD or nicotinamide adenine dinucleotide , NAD " in the a-keto acid oxidases and Fe4.S4 in the pyruvate-ferredoxin oxidoreductase ) in addition to ThDP, it is generally accepted that the enamine is the substrate for the oxidation reactions. 

A number of enzymes may form abortive complexes that are nonproductive forms of the enzyme. Such complexes appear as a result of the adsorption of ligands under conditions where the enzyme may not carry out its usual chemistiy. For example, the binding of NAD " and acetaldehyde to alcohol dehydrogenase leads to the formation of an enzyme-NAD -acetaldehyde complex, wWch cannot allow for hydrogen transfer because both ligands are already in their oxidized states. 

Reaction of the alkyl MoMe(C0)3Cp with LiEt3BH leads to the acetaldehyde complex 40 via the formyl [Mo(CHO)Me(CO)2Cp] and the acyl [MoH(COMe) (CO )2Cp]. Reaction of 40 with [Mo(C0)3Cp]" or several other one electron oxidising agents provides the... 

These arguments, based only on elementary electronic considerations, are confirmed by the fact that the BF3 complex of trifluoroacetaldehyde 11 is 6.46 kcal mol less stable than the corresponding BF3-acetaldehyde complex 12 (Scheme 3.7). This means that Kcf, the equilibrium constant for the formation of complex 11 is considerably lower than that of complex 12 (/CCH3). 

The synthesis and further transformation of formyl complexes via hydrido-acyl intermediates into ti2-acetaldehyde complexes has been reported.S7 The molybdenum(II) carbonyl complex, [Mo(CO)3Me(Ti5-Cp)l, was found to react with Li[BHEt3l in THF at -66 °C to yield the formyl complex, [Mo(CHO)(CO)2Me(Ti5-Cp)]-. Slow warming allowed the spectroscopic observation of successive rearrangements of the formyl complex into hydrido-acyl and, finally, ti2-acetaldehyde derivatives. Scheme 10.9. 

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