Oligomerization, carbonyl compounds

The precursors for >C6 acids (namely, >C6 oi-dicarbonyls) are assumed to form by aldol condensation of ot-dicarbonyl compounds with other carbonyl compounds (route 7), and termination of this oligomerization is... 

Nickel(O) complexes are extremely effective for the dimerization and oligomerization of conjugated dienes [8,9]. Two molecules of 1,3-butadiene readily undergo oxidative cyclization with a Ni(0) metal to form bis-allylnickel species. Palladium(O) complexes also form bis-allylpalladium species of structural similarity (Scheme 2). The bis-allylpalladium complexes show amphiphilic reactivity and serve as an allyl cation equivalent in the presence of appropriate nucleophiles, and also serve as an allyl anion equivalent in the presence of appropriate electrophiles. Characteristically, the bis-allylnickel species is known to date only as a nucleophile toward carbonyl compounds (Eq. 1) [10,11],... 

The additions of H20 or alcohols to the C=0 double bond of carbonyl compounds as well as the oligomerizations or polymerizations of aldehydes are reversible reactions. Therefore, the extent of product formation is subject to thermodynamic control. The equilibrium constant of the formation of the respective addition product is influenced by steric and electronic effects. 

For a given nucleophile the equilibrium lies farther on the product side the smaller the substituents R1 and R2 of the carbonyl compound are (Figure 9.1). Large substituents R1 and R2 inhibit the formation of addition products. This is because they come closer to each other in the addition product, where the bonds to R1 and R2 are closer than in the carbonyl compound, where these substituents are separated by a bond angle of about 120°. Formaldehyde is the sterically least hindered carbonyl compound. In H20 this aldehyde is present completely as dihydroxymethane, and anhydrous formaldehyde is exists completely as polymer. In contrast, acetone is so sterically hindered that it does not hydrate, oligomerize, or polymerize at all. 

Condensation reactions between carbonyl compounds and primary amines have played a central role in the synthesis of new macrocyclic ligands [28-34]. Usually, though not in all cases, such reactions are conducted in the presence of metal ions which can serve to direct the condensation preferentially to cyclic rather than oligomeric/polymeric products and to stabilize the macrocycle once formed. The relative atomic radius of the templating ion has a considerable effect on the size of the macrocycle formed. For instance, in what is now classic work, cations such as Mg(Il) (r = 0.72 A) were found to stabilize the formation of macrocycles such as 60 from 1 1 condensations [35], while larger cations such as Sr(II)... 

The carbonyl ir-bond has been found to add chemo- and regio-selectively across the alkenic ir-bond of ketenes. Thus diphenylketene readily reacts with benzoquinone to yield a stable [2 + 2] adduct (equation 1). With an excess of diphenylketene the bis-adduct is formed, which decomposes into tetraphe-nylquinodimethane and carbon dioxide (equation 2). With the less stable ketene, thermal [2 + 2] cycloadditions are observed with highly electrophilic carbonyl compounds (equation 3). With unactivated aldehydes and ketones, yields are much lower due to a faster oligomerization of the ketene reagent. However, in the presence of a Lewis acid catalyst, most aldehydes or ketones form P-lactones with ketene (equation 4). Cycloadditions with ketones usually require more active catalysts than with aldehydes. The catalyzed reaction of ketene with methyl vinyl ketone is chemoselective, yielding a 10 1 ratio of [2 + 2] versus [4 + 2] adducts (equation 5). In the absence of catalyst, methyl vinyl ketone reacts with ketene to give exclusively the [4 + 2] adduct. 

Bivalent Organo-tin and -lead Derivatives. Photolysis of dialkyltin(iv) oligomers at room temperature produces, via intermediate oligomeric diradicals, dialkyl-stannylenes, which insert into 8n—H, 8n—C, and 8n—8n bonds and react with dienes and carbonyl compounds. Dialkylstannylenes are also produced by the thermolysis of XR2Sn8nR2X (X = C1 or H) at 120—130 C. The stannylenes so... 

The proton-catalyzed hydroxyalkylation of phenolic substances with carbonyl compounds is one of the reactions in which a weak acid catalyst is required, as strong acids will lead to oligomerization. The possible reaction routes of phenol reacting with the two aldehydes studied are shown in Figure 1. Especially with formaldehyde, mono-substituted products are difficult to obtain, because of the fast consecutive dimerization when larger carbonyl compounds are applied (such as isobutanal), monomer formation might be enhanced because of steric reasons. 

Stable at room temperature for a short while. Detailed NMR experiments have demonstrated the non-aromatic property of this brr-electron system [8]. In contrast to the isomeric benzene, fulvene 1 has localized double bonds [9]. Therefore, it behaves like a diene or like a dienophile, and it readily oligomerizes at temperatures above 0°C [7]. In contrast, terminally disubstituted fulvene derivatives such as dimethylfulvene 11 which had been obtained as early as 1906 by Thiele et al. from cyclopentadiene and acetone in methanol in the presence of potassium hydroxide [10], are perfectly stable compounds. A rather efficient new synthesis of 11 and the monosubstituted fulvene derivative 12 by the reaction of cyclopentadiene with unsaturated carbonyl compounds in the presence of pyrrolidine (Scheme 2), has been described by Griesbeck [11]. Dimethylfulvene 11 in this transformation arises via Michael addition of the cyclopenta-dienide ion to the a,j9-unsaturated ketone mesityloxide and a subsequent retro-... 

The difference between the reactivity of the carbonyl groups of benzil and oligomeric polydiphenylacetylene is understandable. As a general rule the reactivity of bis-carbonyl compounds decreases in the order ... 

Allyl complexes have contributed significantly to the development of the organometallic chemistry of nickel and the applications of nickel complexes in organic synthesis, for example, nucleophilic attack on coordinated allyl ligands. In addition, allylnickel complexes have been identified as key intermediates in the oligomerization and cyclization of olefins and dienes. For example, the Ni(0)-catalyzed hydrocyanation of butadiene to adiponitrile, the main component of a major commercial process for the production of nylon, involves Ni (7r-allyl) intermediates. Moreover, the 77-rearrangements of allylnickel species have helped explain the facile isomerization of olefins in the presence of nickel complexes. The Ni-catalyzed homoallylation of carbonyl compounds with 1,3-dienes also involves Ni(7r-allyl) complexes this subject has been reviewed recently. New applications include the cleavage of G-G bonds in the deallylation of malonates, the preparation of cyclopentenones by carbonylative cycloaddi-... 

By its characteristic carbonyl IR absorption band the product of intramolecular termination, i. e. the cyclic B-ketoester was detected by Goode et al. (58), Glusker et al. (59), and Owens et al. (6o) as one of the products of the oligomerization of MMA with phenylmagnesium bromide or fluorenyllithium in toluene. The same compound was found by Lochman et al. (61) when using metalated methylisobutyrate as the initiator in a THF/pentane mixture at high initiator/monomer ratios. 

Fischer-Tropsch oligomerization of CO + hydrogen to make hydrocarbons and oxygenated compounds. Iron promoted by potassium is favored, but the original catalyst was cobalt which formed a carbonyl in process. 

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