Carbon monoxide migration

The reaction is sensitive to the presence of water, which inhibits the migration of the third alkyl group and leads to dialkyl ketones (see Chapter 12, Section II). The convenience of the hydroboration reaction combined with the use of carbon monoxide at atmospheric pressure provides the most accessible route to many trialkylcarbinols. 

As mentioned in the preceding section, the presence of water during the reaction of trialkylboranes with carbon monoxide inhibits the migration of the third alkyl group and leads to production of dialkyl ketones (i). This fact can be employed to advantage for the preparation of dialkyl ketones as shown in the scheme. 

For similar samples on alumina, these effects are not observed, as Indicated In Table I. Mo migration of aluminum or oxygen species Is observed In AES, and the capacity of the film to adsorb carbon monoxide Is not altered by changing the annealing temperature from 525 to 760 K. 

All of these results are consistent with the notion that surface migration of titanium oxide species Is an Important factor that contributes to the suppression of carbon monoxide chemisorption. The H2 chemisorption experiments on 1-2 ML of Ft, where no migration Is observed, strongly Indicate that electronic (bonding) Interactions are also occurring. Thus, for the tltanla system, both electronic Interactions and surface site blocking due to titanium oxide species must be considered In Interpreting SMSI effects. 

The carbonyl insertion step takes place by migration of the organic group from the metal to the coordinated carbon monoxide, generating an acylpalladium species. This intermediate can react with nucleophilic solvent, releasing catalytically active Pd(0). 

Coupling of organostannanes with halides in a carbon monoxide atmosphere leads to ketones by incorporation of a carbonylation step.249 The catalytic cycle is similar to that involved in the coupling of alkyl or aryl halides. These reactions involve Reactions involving a migration of one of the organic substituents to the carbonyl carbon, followed by... 

The key steps in the reaction are addition of hydridorhodium to the double bond of the alkene and migration of the alkyl group to the complexed carbon monoxide. Hydrogenolysis then leads to the aldehyde. 

The steps in the hydroformylation reaction are closely related to those that occur in the Fischer-Tropsch process, which is the reductive conversion of carbon monoxide to alkanes and occurs by a repetitive series of carbonylation, migration, and reduction... 

Several alternative procedures have been developed in which other reagents replace carbon monoxide as the migration terminus.11 The most generally applicable of these methods involves the use of cyanide ion and trifluoroacetic anhydride (TFAA). In this reaction the borane initially forms an adduct with cyanide ion. The migration is induced by N-acylation of the cyano group by TFAA. Oxidation and hydrolysis then give a ketone. 

The polymers prepared from thexylborane and diene monomers were reacted with carbon monoxide at 120°C, followed by treatment with NaOH and H202 to produce a polyalcohol (scheme 5).13 This conversion includes migration of the polymer chain and thexyl group from the boron atom to carbon, as shown in scheme 5. When this reaction was carried out under milder condition, poly(ketone) segments were included in the polymer backbone due to an incomplete migration of the thexyl group. 

In remembrance of the isolation of carbene 78 (Section m.C) it seems acceptable that 141, formed from 140 by a [l,2]-migration of the substituent, functions as a second intermediate. Carbene 141 can subsequently fragment into a nitrene 143 and carbon monoxide or open to an isocyanate 144, which alternatively may also be generated from an acid azide via acylnitrene 142. In comparison to calculated reaction pathways 75 78 and 82 81 (Schemes... 

The change in the nature of the adsorption with increasing coverage (dissociative followed by associative) has been explained by a statistical consideration of the reaction mechanism shown above120). Associative adsorption is expected to occur at vacant sites for which all adjacent olefin binding sites are occupied by earlier dissociation products (or carbon monoxide, as shown by Fig. 6b), because dissociative adsorption (formation of vinyl and hydride species, followed by hydride migration to another alkene) requires two adjacent vacant sites. 

In the case of the tantalum complexes 100, reversible hydrogen migration may occur at room temperature in the presence of carbon monoxide, or at 70°C with dihydrogen or dimethylphosphino ethane to afford complexes 106, 107, and 108, respectively.92,95 In contrast, the phosphametallacycle remains intact when 100 is treated with halogenated reagents such as CH3X (X = Cl, Br, I).92,95... 

The vinylketene complex 221.a was shown3 to undergo ligand replacement when treated with triphenylphosphine to yield the dicarbonyl(triphe-nylphosphine)vinylketene complex 234. This complex was also isolated from the reaction of vinylketone 222.a with methyllithium in the presence of triphenylphosphine, indicating that a phosphine, as well as carbon monoxide, could induce the migration of a coordinated carbonyl ligand to form a ketene carbonyl. Mitsudo, Watanabe, and Weiss have all shown that this process occurs in the reaction between phosphines and isolated rf-vinylcarbene complexes (see Sections VI,B and VI,C).48,68,106... 

The important difference between the insertion mechanism (2.2) and the migration mechanism (2.3) is the following. In the insertion mechanism carbon monoxide inserts into the metal methyl bond and the acyl bond formed takes... 

A second important migration reaction involves alkenes instead of carbon monoxide. Figure 2.6 gives a schematic representation of a hydride that migrates to a co-ordinated ethene molecule cis to the hydride. The figure shows the hydride migration, which may result in an empty space in the co-ordination sphere of the metal. This co-ordinative unsaturation can be lifted in two ways ... 

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. 

The catalyst, the way it is operated, is about 25% faster than the Monsanto rhodium catalyst. In addition, it was assumed that the oxidative addition is no longer rate-determining and that now the migration of the methyl group to the co-ordinated carbon monoxide is rate-determining (Figure 6.3). 

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