Carbonyl group directing effect

Naturally, the ideal source of starting materials for homoleptic metal isocyanide compounds is via metal carbonyl complexes, but previously only with the two carbonyls Ni(CO)4 (24) and Co2(CO)g (25) has direct substitution of all carbonyl groups been effected. Recently, however, remarkable discoveries by Coville and co-workers (26-31) on the transition-metal-catalyzed substitution of carbonyl groups in monomeric and cluster compounds have shown that Fe(CNR)s, Mo(CNR)6, and Ir4(CO)5(CNR)7 (32) can be prepared in high yield by stepwise substitution from the parent carbonyl. 

Finally, the inductive and resonance effects in compounds having the general structure C6Hs-Y=Z (with Z more electronegative than Y) are both electron withdrawing in other words, the two effects reinforce each other. This is true for benzaldehyde (C HsCHO) and all other compounds that contain a carbonyl group directly bonded to the benzene ring. 

Because of carbonium ion generation, aromatic aldehydes and ketones can usually be reduced more easily than the corresponding aliphatic compounds. However, a modified Clemmensen reduction is an effective method to reduce isolated aliphatic carbonyl groups directly to methylene groups, and typical examples are shown in equations (4)-(6)." ... 

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. 

Noto There is experimental evidence that the first two steps of this mechanism (protonation and nucleophilic attack) more likel/ occur either simultaneously or In the reverse order of what is shown above. Most nitrogen nucleophiles are sufficiently nucleophilic to attack a carbonyl group directly, before protonation occurs. Nevertheless, the first two steps of the mechanism above have been drawn in the order shown (which only rarely occurs), because this sequence enables a more effective comparison of all acid-catalyzed mechanisms in this chapter. Be sure to look In your lecture notes to seethe order of events that your instructor used for the first two steps of the mechanism. 

Anisotropy is observed when a substituent group bonds a carbonyl group directly to the benzene ring (Fig. 7.61). Once again, the ring protons fall into two groups, with the ortho protons downfield from the metalpara protons. Benzaldehyde (Fig. 7.62) and acetophenone both show this effect in their NMR spectra. A similar effect is sometimes observed when a carbon-carbon double bond is attached to the ring. The 300-MHz spectrum of benzaldehyde (Fig. 7.62b) is a nearly first-order spectrum (probably a deceptively simple AA BB C spectrum) and shows a doublet (He, 2 H), a triplet (Hb, 1 H), and a triplet (Ha, 2 H). 

Since the NO2 group is bonded to a benzene ring that is bonded directly to the carbonyl group, inductive effects and resonance effects stabilize the conjugate base. For example, a resonance structure can be drawn that places a (+) charge close to the 000 group. 

These substances, as well as the parent compound, are p-keto esters and undergo hydrol3rtio cleavage in two directions. One type of cleavage, ketonlc hydrolysis, is effected by the action of dilute caustic alkali in the cold, followed by acidification and boiling the free acetoacetic acid produced has a carboxyl and carbonyl group on the same carbon atom and therefore readily undergoes decarboxylation to yield a ketone, for example ... 

To understand the effect of a carbonyl group attached directly to the ring consider Its polarization The electrons m the carbon-oxygen double bond are drawn toward oxy gen and away from carbon leaving the carbon attached to the nng with a partial posi tive charge Using benzaldehyde as an example... 

Substitution reactions by the ionization mechanism proceed very slowly on a-halo derivatives of ketones, aldehydes, acids, esters, nitriles, and related compounds. As discussed on p. 284, such substituents destabilize a carbocation intermediate. Substitution by the direct displacement mechanism, however, proceed especially readily in these systems. Table S.IS indicates some representative relative rate accelerations. Steric effects be responsible for part of the observed acceleration, since an sfp- caibon, such as in a carbonyl group, will provide less steric resistance to tiie incoming nucleophile than an alkyl group. The major effect is believed to be electronic. The adjacent n-LUMO of the carbonyl group can interact with the electnai density that is built up at the pentacoordinate carbon. This can be described in resonance terminology as a contribution flom an enolate-like stmeture to tiie transition state. In MO terminology,.the low-lying LUMO has a... 

Significant stimulated emission is only found for the pristine side of the sample. From these results it was concluded that the photoinduccd absorption that suppresses die stimulated emission is directly or indirectly caused by the presence of oxygen-related defects. It was shown earlier that the effect of photooxidation on the emission properties of PPV can be explained by the formation of carbonyl-groups that act as sLrong electron acceptors leading to an efficient dissociation of the plioh excited slate 29). It can be concluded that the dissociated pair near the defcci leads to the strong photoinduccd absorption. The observation that... 

When the carbonyl groups are present, the transition state for syn attack is sta-bihzed by interactions between the in-phase combination of the NN lone pairs and the antisymmetric n orbital of the CO-X-CO bridge (100). Although the secondary effect (SOI) operates only during syn approach and contributes added stabilization to this transition state, the primary orbital interaction (see 103) between the HOMO of the cyclohexadiene moiety of 100 and the n orbital of the dienophile (NN, Fig. 16) is differentiated with respect to the direction of attack, i.e., syn or anti, of triazolinedione (NN, Fig. 16). 

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... 

Another example is presented in Eq. 8.5, in which the nitro group is more effective in controlling the direction of addition than the carbonyl group.12... 

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