Carbon exchange with carbonyls

Studies aimed at the elucidation of reaction mechanisms have been performed by many groups, notably by those of Backvall [28]. In test reactions, typically enantiopure 1-phenylethanol labeled with deuterium at the 1-position (8) is used. The compound is racemized with acetophenone (9) under the influence of the catalyst and after complete racemization of the alcohol, the deuterium content of the racemic alcohol is determined. If deuterium transfer proceeds from the a-carbon atom of the donor to the carbonyl carbon atom of the acceptor the deuterium is retained, but if it is transferred to the oxygen atom of the acceptor it is lost due to subsequent exchange with alcohols in the reaction mixture (Scheme 20.4). 

The mechanism is illustrated in the simple case of the self-condensation of an aldehyde in the presence of base. Here the nucleophilic, mesomerically stabilised a-carbanion (the enolate ion) formed by the action of base, attacks the electrophilic carbonyl carbon of a second aldehyde molecule to form, after proton exchange with the solvent, the / -hydroxyaldehyde. 

When, in a sugar, a hydroxyl group adjacent to a carbonyl group is inverted, as by D-en/f/iro-pentulose 5-phosphate 3-epimerase,48 complete exchange with solvent of the hydrogen on the epimerized carbon atom is usually observed, indicating an enediol mechanism. 

The possibility of a dissociative mechanism is indicated by the observation 90, 91) that all eight carbonyl groups in Co2(CO)b exchange with carbon monoxide at the same rate. The rate law for this exchange... 

Studies of the kinetics of the exchange of carbon monoxide with the terminal carbonyl groups of Co2(CO)g(PhC2Ph) have been reported 90, 121). All six carbonyl groups of Co2(CO)g(PhC2Ph) exchange at the... 

It should be noted that cyclobutadiene always replaces carbon monoxide in reactions with metal carbonyl derivatives. Yields of product parallel the known rate of exchange of CO in the starting carbonyl 184). Highest yields of ligand transfer products are attained with nickel and cobalt carbonyls which are known to very rapidly exchange their CO groups by a D-type mechanism 185-188). Lowest yields have been reported with Mo and W complexes, the carbonyls of which exchange with CO very slowly 188). 

Kinetic studies on the exchange of radioactive carbon monoxide with nickel carbonyl have shown that the first step in the dissociation may be represented by... 

Mn(OAc)3 is usually used to remove H- from the a-carbon atoms of carbonyl compounds. The Mn(III) exchanges with H+ of the enol, and Mn-O cr-bond homolysis generates Mn(II) and the enoxy radical. 

Br/Mg exchange converts 1,2-dibromocyclopentene into the p-bromoalkenyl-magnesium chloride (LiCl complex), which reacts normally with carbonyl compounds. It is possible to peform a copper-mediated coupling at the p-carbon site while retaining the C-MgCl unit. ... 

Alkali metal-exchanged zeolites have been used to prepare activated alkenes of interest as prepolymers by Knoevenagel condensation of malononitrile with ketones having different positive charge density on the carbon of the carbonyl group benzophenone, cyclohexanone, and p-aminoacetophenone [85]. The reactivity depends both on ketone structure (the order of reactivity was benzophenone > cyclohexanone > p-aminoacetophenone) and on the catalyst used. For instance, when malononitrile is condensed with cyclohexanone in the presence of a CsY zeolite conversion was very low. CsX zeolite, however, which has a substantial number of basic sites with pX in the range 9 < P a < 10.7 and some with 10.7 < < 13.3 could be used to perform... 

With carbonyl complexes, alkyl exchange often takes place with carbon monoxide insertion. 

Carbamoyl complexes from metal carbonyls and amines 5.8.2.12.4 Carbanions reactions with alkene complexes 5.8.2.3,4 metal carbonyls 5.8.2.S.5 Carbene complexes by alkene metathesis 5.8.2.3.11 formation 5.8.2.8.5 Carbides alkali metal formation 5.10.2.1 bonding 5.10.2 formation 5.10.2 industrial uses 5.10.2 interstitial formation 5.10.2 Carbometallacycle formation 5.S.2.2.2 Carbometallacycles from n-allyl complexes 5.S.2.3.9 Carbon reaction with alkali metals 5.10.2.1 Carbon dioxide complexes formation 5.8.2.14.1 Carbon monoxide displacement by alkenes 5.8.2.3.1 Carbonyl complexes by ligand exchange 5.8.2.12.2 from carbon monoxide 5.8.2.12.1, 5.8.2.12.2... 

Explain why the aldehydic hydrogen (the one attached to the carbonyl carbon) is not exchanged with deuterium. 

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