Alcohols, carbonylation mechanism

When dicobalt octacarbonyl, [Co(CO)4]2, is the catalyst, the species that actually adds to the double bond is tricarbonylhydrocobalt, HCo(CO)3. Carbonylation, RCo(CO)3- -CO—>RCo(CO)4, takes place, followed by a rearrangement and a reduction of the C—Co bond, similar to steps 4 and 5 of the nickel carbonyl mechanism shown in 15-30. The reducing agent in the reduction step is tetra-carbonylhydrocobalt HCo(CO)4, ° or, under some conditions, H2. When HCo(CO)4 was the agent used to hydroformylate styrene, the observation of CIDNP indicated that the mechanism is different, and involves free radicals. Alcohols can be obtained by allowing the reduction to continue after all the carbon monoxide is... 

Other mechanisms for the synthesis of alkylformates, not via formic acid esterification, are possible. Hydrogenation of C02 to CO, followed by catalytic car-bonylation of alcohol, would produce alkyl formate. This mechanism seems more likely for the anionic metal carbonyls because they are known catalysts for alcohol carbonylation. However, Darensbourg and colleagues [64, 74, 85] showed... 

The conversion of 7 to 8 is a simple hydrolysis of an acetal. Acetals are functionally equivalent to alcohols + carbonyls and can be interconverted with them under acidic conditions. Several reasonable mechanisms can be drawn for this transformation, but all must proceed via S l substitutions. 

A Zn2+ at the active site polarizes the carbonyl oxygen of acetaldehyde, allowing transfer of a hydride ion (red) from the reduced cofactor NADH. The reduced intermediate acquires a proton from the medium (blue) to form ethanol. Alcohol Dehydrogenase Mechanism... 

Termination of the autoxidation chain process occurs as peroxyl radicals couple to yield non-radical products. This reaction takes place through an unstable tetroxide intermediate. Primary and secondary tetroxides decompose rapidly by the Russell termination mechanism to yield three non-radical products via a six-membered cyclic transition state (Fig. 95). The decomposition yields the corresponding alcohol, carbonyl compound, and molecular oxygen (often in the higher energy singlet oxygen state) three... 

Reactions in which hydride leaves are less common but can occur if other reactions are precluded and the hydride is transferred directly to an electrophile. One example occurs when an aldehyde without any hydrogens on its a-carbon is treated with NaOH or KOH. (If the aldehyde has hydrogens on its a-carbon, the aldol condensation is faster and occurs instead.) In this reaction, called the Cannizzaro reaction, two molecules of aldehyde react. One is oxidized to a carboxylate anion and the other is reduced to a primary alcohol. The mechanism for this reaction is shown in Figure 20.5. The reaction begins in the same manner as the reactions described in Chapter 18 a hydroxide ion nucleophile attacks the carbonyl carbon of the aldehyde to form an anion. The reaction now begins to resemble the reactions in Chapter 19. 

In the reverse case, for the dimethylation with formaldehyde for example (R2 = R3 = H), the dehydration is impossible. Thus, in this last case, the only possible pathway is the hydrogenolysis of the hemiaminal D. The competitive hydrogenation of carbonyl must be considered carefully because it influences the choice of the reaction conditions. We assume that with amides, anilines and alcohols similar mechanisms should be possible. 

Two possible mechanism pathways may be involved for this reaction, either a common pathway for all reactants passing through the alkene formation followed by a standard hydroxycarbonylation of the substrate with CO formed in situ by decomposition from formic acid, or through oxidative insertion of iridium to an iodoakyl intermediate, corresponding to the carbonylation mechanism of an alcohol. These two possibilities are depicted in Scheme 4. 

The other major mechanism results from an initial attack at the carbonyl to form a tetrahedral intermediate. Collapse of the tetrahedral intermediate results in expulsion of the enolate. The resulting acyl cyanide is rapidly hydrolyzed to give cyanide, carbon dioxide, and an alcohol. This mechanism predominates when cyanide salts are used and when less substitution is present at the central carbon of the malonate derivative.6... 

Zeise s salt, 353, 403 Zinc, 925-1022 metallocnzymcs, 1001, 1002 biomimetic modelling, 1021 X-ray crystallography, 1002 zinc-carbonyl mechanism, 1003 zinc-hydroxide mechanism. 1003 Zinc complexes acetate, 969 alcohols, 964 amides, 944 amine oxides, 964 amines, 933 amino acids, 938... 

Reduction of an Ester (Section 18.10/ Reduction of an ester by lithium aluminum hydride gives two alcohols. The mechanism involves initial nucleophilic attack by a hydride ion onto the carbonyl carbon to give a tetrahedral addition intermediate, which collapses through the loss of alkoxide to give an aldehyde, which reacts with a second hydride to give the product alcohol. 

Catalytic oxidation of alcohols by cis-dioxomolybdenum(VI) complexes, involving oxygen atom transfer from sulfoxides has been reported [1397]. Catalytic amounts of cis-dioxomolybdenum(Vl) complexes in association with sulfoxides can be used to oxidize alcohols to carbonyl compounds. For primary alcohols, the oxidation to the aldehyde is selective, and no further oxidation to the carboxyUc add is observed. The oxidation is most effective for benzylic and allylic alcohols. The mechanism... 

Carbonylation of Alcohols and Esters. The mechanism of ttie Rh/I" catalysed alcohol carbonylation has been studied in detail. Rates decrease sharply from methanol to n-propanol. Formation of isobutyric acid as a by-product points to a p-H elimination-reinsertion sequence.This sequence has also been demonstrated for ethanol carbonylation by selective C labelling (eqn.l8). The reductive carbonylation of methanol in the presence of Col2 and PPhg generates acetaldehyde, ethanol and methyl acetate. Only diphenylether and alkanes as solvents did not decompose under the reaction conditions (17CPC,... 

Cyclopropylamines and cyclopropanols of type 209 can be prepared using alkylmagnesium halides (Kulinkovich reaction). This reaction requires the presence of titanium alcoholates. Its mechanism includes the formation of a dialkoxytitaniumcyclopropane 210, which adds to a carbonyl compound or nitrile (Scheme 2-82). The use of chiral titanium alcoholates affords enantioenriched products such as 211 with 78% ee using a TADDOL derivative (Scheme 2-83). ... 

The o-keto ester 513 is formed from a bulky secondary alcohol using tricy-clohexylphosphine or triarylphosphine, but the selectivity is low[367-369]. Alkenyl bromides are less reactive than aryl halides for double carbonyla-tion[367], a-Keto amides are obtained from aryl and alkenyl bromides, but a-keto esters are not obtained by their carbonylation in alcohol[370]. A mechanism for the double carbonylation was proposed[371,372],... 

Under CO pressure in alcohol, the reaction of alkenes and CCI4 proceeds to give branched esters. No carbonylation of CCI4 itself to give triichloroacetate under similar conditions is observed. The ester formation is e.xplained by a free radical mechanism. The carbonylation of l-octene and CCI4 in ethanol affords ethyl 2-(2,2,2-trichloroethyl)decanoate (924) as a main product and the simple addition product 925(774]. ... 

The mechanisms of the Fischer esterification and the reactions of alcohols with acyl chlorides and acid anhydrides will be discussed m detail m Chapters 19 and 20 after some fundamental principles of carbonyl group reactivity have been developed For the present it is sufficient to point out that most of the reactions that convert alcohols to esters leave the C—O bond of the alcohol intact... 

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