Carbon monoxide Lewis acid

Another formylation reaction, which is named after Gattermann, is the Gatter-mann-Koch reaction. This is the reaction of an aromatic substrate with carbon monoxide and hydrogen chloride (gas) in the presence of a Lewis acid catalyst. Similar to the Gattermann reaction, the electrophilic agent 9 is generated, which then reacts with the aromatic substrate in an electrophilic aromatic substitution reaction to yield the formylated aromatic compound 10 ... 

Borane, 1-methylbenzylaminocyanohydropyrrolyl-, 3, 84 Borane, thiocyanato-halogenohydro-, 3,88 Borane, trialkoxy-amine complexes, 3, 88 Borane, triaryl-guanidine complexes, 2,283 Borane, trifluoro-complexes Lewis acids, 3,87 van der Waals complexes, 3, 84 Borane complexes aminecarboxy-, 3,84 aminehalogeno-, 3, 84 amines, 3, 82, 101 B-N bond polarity, 3, 82 preparation, 3, 83 reactions, 3, 83 bonds B-N, 3, 88 B-O, 3, 88 B-S, 3, 88 Jt bonds, 3, 82 carbon monoxide, 3, 84 chiral boron, 3, 84 dimethyl sulfide, 3, 84 enthalpy of dissociation, 3, 82... 

A proton (H+) is an electron pair acceptor. It is therefore a Lewis acid because it can attach to ( accept") a lone pair of electrons on a Lewis base. In other words, a Bronsted acid is a supplier of one particular Lewis acid, a proton. The Lewis theory is more general than the Bronsted-Lowry theory. For instance, metal atoms and ions can act as Lewis acids, as in the formation of Ni(CO)4 from nickel atoms (the Lewis acid) and carbon monoxide (the Lewis base), but they are not Bronsted acids. Likewise, a Bronsted base is a special kind of Lewis base, one that can use a lone pair of electrons to form a coordinate covalent bond to a proton. For instance, an oxide ion is a Lewis base. It forms a coordinate covalent bond to a proton, a Lewis acid, by supplying both the electrons for the bond ... 

Carbon monoxide is a colorless, odorless, flammable, almost insoluble, very toxic gas that condenses to a colorless liquid at — 90°C. It is not very reactive, largely because its bond enthalpy (1074 kj-mol-1) is higher than that of any other molecule. However, it is a Lewis base, and the lone pair on the carbon atom forms covalent bonds with J-block atoms and ions. Carbon monoxide is also a Lewis acid, because its empty antibonding Tr-orbitals can accept electron density from a... 

Adsorption enthalpies and vibrational frequencies of small molecules adsorbed on cation sites in zeolites are often related to acidity (either Bronsted or Lewis acidity of H+ and alkali metal cations, respectively) of particular sites. It is now well accepted that the local environment of the cation (the way it is coordinated with the framework oxygen atoms) affects both, vibrational dynamics and adsorption enthalpies of adsorbed molecules. Only recently it has been demonstrated that in addition to the interaction of one end of the molecule with the cation (effect from the bottom) also the interaction of the other end of the molecule with a second cation or with the zeolite framework (effect from the top) has a substantial effect on vibrational frequencies of the adsorbed molecule [1,2]. The effect from bottom mainly reflects the coordination of the metal cation with the framework - the tighter is the cation-framework coordination the lower is the ability of that cation to bind molecules and the smaller is the effect on the vibrational frequencies of adsorbed molecules. This effect is most prominent for Li+ cations [3-6], In this contribution we focus on the discussion of the effect from top. The interaction of acetonitrile (AN) and carbon monoxide with sodium exchanged zeolites Na-A (Si/AM) andNa-FER (Si/Al= 8.5 and 27) is investigated. 

It is generally assumed that the Lewis acid in 3 decreases the charge on the metal, i.e., increases its electrophilicity. The removal of charge from the nickel allows additional electron donors to coordinate to the nickel atom, and reaction with, for example, 2 moles of carbon monoxide or 1 mole of 1,5-cyclooctadiene (COD) gives the insoluble, catalytically inactive and presumably ionic complexes 7 and 8. In contrast, 7r-allyl-nickel halides (1) add only 1 mole of carbon monoxide while they do not react with COD (52). 

The behavior of 3 toward ether or amines on the one hand and toward phosphines, carbon monoxide, and COD on the other (Scheme 2), can be qualitatively explained on the basis of the HSAB concept4 (58). The decomposition of 3 by ethers or amines is then seen as the displacement of the halide anion as a weak hard base from its acid-base complex (3). On the other hand, CO, PR3, and olefins are soft bases and do not decompose (3) instead, complexation to the nickel atom occurs. The behavior of complexes 3 and 4 toward different kinds of electron donors explains in part why they are highly active as catalysts for the oligomerization of olefins in contrast to the dimeric ir-allylnickel halides (1) which show low catalytic activity. One of the functions of the Lewis acid is to remove charge from the nickel, thereby increasing the affinity of the nickel atom for soft donors such as CO, PR3, etc., and for substrate olefin molecules. A second possibility, an increase in reactivity of the nickel-carbon and nickel-hydrogen bonds toward complexed olefins, has as yet found no direct experimental support. 

Lewis bases having two or more lone-pair-bearing atoms can exhibit a form of isomerism in H-bonding. This can be illustrated most simply with carbon monoxide, which has sigma-type lone pairs both on C and on O, and thus is expected to form n—>-cr H-bonds of either OC HA or CO HA type with a hydride-bearing Lewis acid. For example, complexation of CO with HF gives rise to distinct OC- -HF or CO- -HF complexes.39 The two possible isomeric forms are shown in Fig. 5.3, both bound by rather typical H-bond energies ... 

Carbon monoxide rapidly inserts into the carbon—zirconium bond of alkyl- and alkenyl-zirconocene chlorides at low temperature with retention of configuration at carbon to give acylzirconocene chlorides 17 (Scheme 3.5). Acylzirconocene chlorides have found utility in synthesis, as described elsewhere in this volume [17]. Lewis acid catalyzed additions to enones, aldehydes, and imines, yielding a-keto allylic alcohols, a-hydroxy ketones, and a-amino ketones, respectively [18], and palladium-catalyzed addition to alkyl/aryl halides and a,[5-ynones [19] are examples. The acyl complex 18 formed by the insertion of carbon monoxide into dialkyl, alkylaryl, or diaryl zirconocenes may rearrange to a r 2-ketone complex 19 either thermally (particularly when R1 = R2 = Ph) or on addition of a Lewis acid [5,20,21]. The rearrangement proceeds through the less stable... 

The experimental evidence shows that the carbon of terminal carbonyl groups is positively polarized (or polarizable) and, contrary to the behaviour of free carbon monoxide, is easily attacked by strong nucleophiles (OH-, OR-) a behaviour which is general in the chemistry of metal carbonyls. Moreover, the negative polarisation (or polarizability) of oxygen atoms of carbonyl groups, particularly bridging carbonyls, is illustrated by the facile formation of adducts with Lewis acids as shown inEq. (16) 7. ... 

Co-ordinated carbon monoxide is activated towards nucleophilic attack. Through o-donation and Tt-back donation into the antibonding CO K orbitals the carbon atom has obtained a positive character. This makes the carbon atom not only more susceptible towards a migrating anion at the metal centre, but also for a nucleophile attacking from outside the co-ordination sphere. In this instance it is more difficult to differentiate between the two pathways. There are examples showing that the electrophilicity of the carbon atom can be further increased by the action of Lewis acids complexing to the oxygen atom of the co-ordinated CO. 

Low-valent, 18-electron carbene complexes (Fischer-type) are electronically and coordinatively saturated and must usually be activated to become efficient metathesis catalysts. This activation can be effected by thermal or photolytic [781,782] removal of one ligand, e.g. carbon monoxide, or by treatment with a Lewis acid (Table 3.15) [783]. 

Carbon monoxide, hydrogen cyanide, and nitriles also react with aromatic compounds in the presence of strong acids or Lewis acid catalysts to introduce formyl or acyl substituents. The active electrophiles are believed to be dications resulting from diprotonation of CO, HCN, or the nitrile.58 The general outlines of the mechanisms of... 

To understand how to control process conditions to give methyl, 4-pentadienoate, the reaction mechanism must be examined. (See Equation 2.). -palladium hydride elimination from 4 gives rise to trans and cis-methyl penta-, 4-dienoate which is the desired monocarbonylation intermediate for sebacic acid. The desired mono-carbonylation reaction is promoted by low carbon monoxide pressure ( 1000 psig) while high pressure (1800 psig) gives excellent 1,4-dicarbonylation product yield. The mono-carbonylation reaction is also facilitated by using a Lewis Acid as a co-catalyst and iodide as the preferred palladium counter-ion (Table III.). Chloride is the preferred palladium counter-ion for 1,4-dicarbonylation. 

Accordingly the best results in the methanol homologation are obtained by a compromise between the concentration of the protonic and Lewis acid components of the catalytic system a.id the hydrogen and carbon monoxide partial pressures (Table ll)... 

Deprotonation of complex 1 with butyllithium at — 78 °C generates the enolate species 2 (described in Section 1.1.1.3.4.1.1.), which reacts with electrophiles while in the anti conformation (acyl oxygen anti to carbon monoxide oxygen). Enolate 2 is inert to 1,2-epoxypropane (3a) at — 78 °C, but in the presence of a Lewis acid, rapid reaction ensues leading to preferred alkylation of the least hindered site of the epoxide13. Reaction of the enolate 2, derived from the racemic complex 1, with racemic monosubstituted epoxides results in preferential formation of one of two possible diastereomers this can be termed a double enantiomer-differentiating reaction. 

There is much evidence for silylenes reacting as Lewis bases, but complexes of silylenes acting as a Lewis acid are now well established (Fig. 14.3, Table 14.2). These complexes are also described as silaylides, R2Si —X+. Formation of silylene complexes with Lewis bases is conhrmed by a strong blue shift of the n-p transition. Matrix isolated dimesitylsilylene reacts with carbon monoxide to form the complex shown in Eq. The complex absorbs at 354 nm. 

D.F. (1980) Activation of coordinated carbon monoxide toward alkyl and aryl migration (CO insertion) by molecular Lewis acids and X-ray structure of the reactive intermediate Mn(C(OAlBrBr2) CH3)(CO)4. J. Am. Chem. Soc., 102, 5093-5100. 

The only carbon nucleophile whose coordination to silylenes was investigated theoretically is carbon monoxide. The main interest centers on the question whether this nucleophile forms a Lewis acid-Lewis base complex... 

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