Aldehyde complexes

The reported structural chemistry of zinc aldehyde complexes is dominated by Vahrenkamp and co-workers. A systematic investigation is reported over three papers published in 1999. All X-ray structures reported in these papers are aryl aldehydes and no aliphatic aldehyde structures with zinc were found in the CSD. Zinc is frequently exploited in preparative organic chemistry and enzyme-catalyzed transformations of organic carbonyl compounds. 

Formation of aldehyde complexes from the zinc hydroxide complexes of tris(3-cumenyl-5-methylpyrazolyl)borate] was observed. The aldehyde ligands, such as pentafluorobenzaldehyde,... 

There are a number of important N/O bidentate ligands including the chelate stabilized ketone and aldehyde complexes, and amino acid complexes. 

Chelating aldehydes such as 2-pyridine carbaldehyde and 2-dimethylamino benzaldehyde improve the stability of the aldehyde complexes via N,0 chelation. NMR studies show that the complexes are present in solution without an excess of aldehyde and can be formed in the presence of donor ligands. The X-ray structures showed longer and weaker Zn—O bonds when more than one chelating ligand was present. IR demonstrates the variation in C=0 bond strengths and how the environment of the zinc ion will influence potential catalytic activity via reaction rates or pathways. Tetrahedral chelate complexes, and octahedral bis- and tris-chelate complexes, were isolated.843... 

Amphiphilic alkylation (13,203).2 Addition of Grignard reagents to a-methyl substituted aldehydes proceeds with modest syn-selectivity (—2—3 1). In contrast, the addition to the same aldehydes complexed with MAD or MAT shows antiselectivity (equation I). Yields are also improved by complexation. 

In a similar study, Park has shown125 that cyclic dienes other than cyclo-pentadiene will also undergo a [4 + 2] cycloaddition reaction with the (trimethylsilyl)alkynylcarbene complex (184.b). When 184.b was reacted with 1,3-cyclohexadiene in THF, the diene complex 191 was isolated along with the vinylketene complex 192, which was prone to hydrolysis during column chromatography as expected.123 When 192 was stirred with silica gel and water in hexane, an almost quantitative conversion to the aldehyde complex 193 was observed. 

The major intermediate observed in solution is the alkene complex, but it interchanges rapidly with the aldehyde complex. The product formed according to this scheme is allyl alcohol, because the overall barrier 2 is lower than barrier 1 (above we named this Curtin-Hammett conditions). Barrier 2 is also the ratedetermining step in this sequence. 

Up to now, the solubility decrease of cobalt complexes with these modiflers has not been explained satisfyingly. It is assumed that the changes in the solvatization characteristics observed are caused by different interactions of the solute with the mixture of organic components and CO2 the modifier-solute (olefin/aldehyde-complex) interaction probably is stronger than the solute-scC02 interaction. Future theoretical treatment may also improve the... 

It has been shown that the lead tetraacetate-mediated 1,2-aryl shift of various meta-substituted / -cyclohexyl aryl ketones, e.g. (10), results in excellent yields of the corresponding rearranged esters (11). A unique reaction, providing 3-hydroxy-2-arylacrylic acid ethyl esters (14), has been observed between aryl aldehydes and ethyl diazoacetate in the presence of the iron Lewis acid [rj — (C5H5)Fe(CO)2(THF)BF4], It appears that the enol esters are formed by an unusual 1,2-aryl shift from a possible intermediate (13), which in turn is formed from the reaction of the iron aldehyde complex (12) with ethyl diazoacetate (see Scheme 4). 

On the other hand, carbon dioxide may be derived from the decomposition of the intermediates formed by the reactions of the acyl radical and oxygen, such as the acid radical, peroxy radical, or peracid-aldehyde complex. Therefore, they may be derived as follows ... 

Our investigation (27, 28) of the decomposition of peracrylic acid by various homogeneous metal catalysts helped to verify the above steps as the initiation reaction. But Step 1" plays no fundamental role since it occurs very slowly. The metal catalysts also accelerate the decomposition of the peracid-aldehyde complex which forms the radicals of chain branching. Step 1, and the steps just mentioned, become comprehensible in view of the initiation of the catalyzed oxidation as described above. 

TABLE 8.4 Stability and Rate Constants for the Formation of S(IV) - Aldehyde Complexes... 

T. L. Bullock, K. Breddam, S. J. Remington, Peptide aldehyde complexes with wheat serine carboxypeptidase II, implications for the catalytic mechanism and substrate specificity. /. Mol. Biol. 1996, 255, 714-725. 

Preparation, Structure, and Chemical Behavior of Aldehyde Complexes of the Catalyst (37, 41)... 

Since the aldehyde complex is a key substance for clarifying the polymerization mechanism, the structure of [Me2A OCPhNPh MeCHO] 2... 

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. 

Since the aldehyde complex has a novel type of structure and clearly plays an important role in the polymerization reaction of an aldehyde, the chemical behavior of this class of compound was examined. 

In contrast with the aliphatic aldehyde complex, the aromatic aldehyde complex reacts with A1R3 to undergo an addition reaction of A1R3 to the carbonyl group of the aldehyde, which is a well known reaction of AIR3 with an aliphatic aldehyde. This experimental result indicates that the aromatic aldehyde complex is much less stable... 

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. 

Aldehyde exchange reaction occurred readily in solution between the aldehyde complex and the added aldehyde. This exchange reaction... 

Yasuda,H., Araki,T., Tani,H. Monomer-catalyst complex in the stereospecific polymerization of aliphatic aldehyde. The structure and chemical behavior of the aldehyde complex of [R2 A10CR NPh]2. J. Organometal Chem. in press. 

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