Carbonyl directing effect

The ort/to-directing effect of the carbonyl group in the regioselective lithiation of 2( 1/f) quinolinone has been studied by Avendano <9581362>. 

Polar functional groups such as carbonyl, cyano, and sulfoxide, as well as silyl and stannyl groups, exert a strong directing effect, favoring proton removal from the geminal methyl group.171... 

The postulated mechanism involves a directing effect of the carbonyl group to the metal center, ideally positioning this metal for insertion into the ortho-G-H bond. The resulting ruthenium hydride undergoes hydridometallation of the olefin followed by reductive elimination to give the new C-C bond. 

Rhodium carbonyl catalysts effect hydrocarbonylation under very mild reaction conditions at higher rates and selectivity. Rhodium introduced as Rh4(CO)i2, for example, probably is converted to RhH(CO) , but there is no direct evidence for such species. These rhodium carbonyls are not particularly attractive since they produce mostly branched aldehydes. 

The carbonyl oxygens are responsible for the stereo-directing effects of peroxy acids on the epoxidation of allylic and homoallylic carbamoyloxyalkenes (188 —> 189)298. For other mechanistic investigations of the peroxy acid epoxidations of alkenes, see elsewhere299-301. 

Over the years several different mechanisms for the singlet oxygen ene reaction have been considered as depicted in Sch. 2. Biradical (mechanism B in Sch. 2) [10,11] and zwitterionic (mechanism C in Sch. 2) mechanisms have been considered as very unlikely for a variety of reasons including the facts that Markovnikov directing effects are not observed [4], radical scavengers have no influence on the reaction, only minor solvent effects are observed [12-14], and cis-trans isomerizations of the substrates do not occur [6], A dioxetane intermediate (mechanism D in Sch. 2) was considered very early [6] but this suggestion was dismissed when isolated dioxetanes were shown to cleave to the carbonyl compounds rather than rearrange to the allylic... 

An interesting case in the /3-lactam series gave selective anti-Markovnikov addition, attributed to the directing effect of the lactam carbonyl group (equation 29). ... 

Axial allylic carbonyl groups have only a slight directing effect in peracid epoxidations, the effect of equatorial allylic carboxyl groups is even smaller68,69. 

When there is no C=0 present at Cn in the l,4-diene-3-one precursor, as in (134), the more nucleophilic 4,5-double bond is epoxidized with DMD affording (135) exclusively (Equation (9)) <94JOC4304>. The observed regioselectivities have been ascribed to the directing effect of the Cn-carbonyl favoring the approach of DMD from the a-face of the dienoic A-ring. 

Another often used chiral ligand is l,F-binaphthyl-2,2 -diyl-bis(phosphine), or binap.It can be built into ruthenium catalysts, which display high activities and enantio-selectivities for the hydrogenation of alkenes, amines, allylalcohols, and unsaturated carbonyls. A directing effect is observed for an a-hydroxy group in these cases. 

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