Inductive Donors

FIGURE 12.28 Resonance forms of the Wheland intermediate in electrophilic substitution of toluene at the ortho-position. 

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The CH3 group is an interesting case it is listed as both a resonance and inductive donor, but a weak one. Nevertheless it also produces a full-length gradation in... 

Carbanions are isoelectronic with ammonia and have the same structure—sp hybridization, pyramidal geometry, and sp lone pair, which we will see is associated with much of the reactivity of carbanions. The structure of [CHj]" is 4.76, and as with the carbocations, there is significant resistance to any distortion from this geometry. Carbanions are stabilized by inductive with-drawers of electrons—so [CCIJ is relatively stable. The effect of inductive donors such as alkyl groups is, predictably, exactly the opposite of their effect on carbocations—so, tertiary carbanions are the least stable, and primary carbanions are the most stable (Figure 4.17). 

Carbocations are stabiiized by inductive donors of electron density such as alkyl groups, by resonance involving r-electrons or nonbonding electrons (lone pairs) or by aromatic character. 

Carbanions are stabilized by inductive withdrawers of electron density such as halogens, but are destabilized by inductive donors such as alkyl groups. They are stabilized by resonance with i-electrons and aromatic character. 

Which product or products is/are produced, and can we account for this mechanistically The first RDS is generally irreversible and hence will also determine which product is formed, so this is the key process. The questions we must ask are whether a particular substituent donates or withdraws electron density and whether this is an inductive or a resonance process. Let s look at what happens when toluene reacts with an electrophile—remembering that the methyl group is an inductive donor of electrons, so we will expect the reaction to be more favorable than for benzene (the aromatic ring is more electron rich). If we consider the three possible resonance forms for the Wheland intermediate formed in the reaction at the orf/zo-position, shown in Figure 12.28, one of these is a tertiary carbocation, and we may expect that this will be the most important resonance form and will contribute most to the stability of the structure. 

Inductive donors are mildly activating and direct ortho, para (alkyl groups). 

It is instructive to consider Af-substituted azoles in reverse, i.e. the azole ring as the substituent linked to some other group. Hammett and Taft cr-constant values for azoles as substituents are given in Table 11. The values show that all the azoles are rather weak net resonance donors, imidazole being the strongest. They are all rather strong inductive acceptors, with pyrazole considerably weaker in this respect than imidazole or the triazoles. 

Comparison witli tlie Hajos-Parrisb asymmetric version of tlie Robinson annulation [81] iSdieme 7.25iaj) shows tlie following distinct differences between tlie two metliods. Firstly, tlie cydoalkenone in tlie CuiOTf)2/ligand 18-catalyzed procedure is tlie Midiael acceptor, whereas tlie cydoalkanone is tlie Midiad donor in tlie proline-mediated annulation. Secondly, tlie asymmetric induction occurs in tlie 1,4-addition step in tlie new metliod, in contrast to tlie asymmetric aldol-cydization in tlie Hajos-Parrisb procedure. 

Wanzlick showed that the stability of carbenes is increased by a special substitution pattern of the disubstituted carbon atom [12-16]. Substituents in the vicinal position, which provide n-donor/a-acceptor character (Scheme 2, X), stabilize the lone pair by filling the p-orbital of the carbene carbon. The negative inductive effect reduces the electrophilicity and therefore also the reactivity of the singlet carbene. 

In the N-Bz derivatives of Gly and Gly-Gly, the planar [R3Sn(IV)] moieties are bridged by -COO groups. Because of the negative inductive effect of the Ph group, which reduces the donor ability of oxygen atoms, coordination of the amide -C = O in the latter compounds could be ruled out. 

Inductive and resonance parameters, (Tr°, ct, were evaluated for the isocyanide ligand from F chemical shifts they suggest that the isocyanide is a good cr-donor and a weak ir-acceptor. 

Hj concentration as a function of time for various alcohols as electron donor. In the presence of primary and secondary alcohols the rates of Hj generation are equal although the induction periods at the start of illumination are different. In the presence of a tertiary alcohol the rate of formation is substantially lower. As mentioned in section 4.2 alcohols are not strongly adsorbed at the TiO particles and therefore do not react with the positive holes before they are trapped. The main scavenger in this system is the platinum deposit which scavenges electrons and catalyses the reduction of hydrogen ions 2 e" -I- 2 —> H2- A surprising observation was that the H2... 

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