Aromatic N-substituentS

Several compounds of the type 74 and 75 with aliphatic and aromatic N-substituents show double-bond rotation with barriers in the range of 80 to 38 kJ mol" while NMe2 torsional barriers are found13,121 in the range of 73.7 to 38 kJ mol-1. When R = Me, all barriers are lower than when R = H. When R1 (R2) is aromatic, a decrease in the C—N and an increase in the C=C barrier is observed. 

Theoretical calculations have shown that aromatic N substituents on NHC ligands can exhibit a through space interaction with the metal via a donation from the C l m of the N substituent. This interaction can either be direct or transmitted by an additional group bonded to the metal. The effects on Ru-catalysed olefin metathesis and Pd-catalysed cross-coupling have been computed and were consistent with experimental observations. 

In the first mechanism (equation 74) the nucleophile function attacks the aromatic ring in an ipso-type displacement involving a Meisenheimer complex intermediate243,244, and leads to the rearranged product after expulsion of sulfinate anion (X-). This mechanism should be favoured by the presence of an electron-withdrawing substituent in conjugation with the anion. The second mechanism (equation 75) involves a direct displacement of sulfinate anion (X ) by Y-, without involvement of the aromatic n electrons. 

The carbenes derived from six- [20-22] and seven-membered heterocycles [22-24, 70] of type 14 can be obtained both with an aliphatic or an aromatic backbone. The characteristic NMR resonances for the C2 carbon atoms in six-membered carbene heterocycles (8 236-248 ppm) [20, 21] have been recorded in the same range as the Ccarbene resonances for saturated imidazolidin-2-ylidenes of type 7. Dimerization to enetetramines occurs for some carbenes derived from a six-membered heterocycle [21] and this behavior is independent from the nature of the backbone (saturated or unsaturated), but depends strongly on the steric bulk of the N,N -substituents. 

The intrinsic stability of the aromatic n system has two major consequences for the course of reactions involving it directly. First, the aromatic ring is less susceptible to electrophilic, nucleophilic, and free-radical attack compared to molecules containing acyclic conjugated n systems. Thus, reaction conditions are usually more severe than would normally be required for parallel reactions of simple olefins. Second, there is a propensity to eject a substituent from the tetrahedral center of the intermediate in such a way as to reestablish the neutral (An + 2)-electron system. Thus, the reaction is two step, an endothermic first step resulting in a four-coordinate carbon atom and an exothermic second step, mechanistically the reverse of the first, in which a group is ejected. The dominant course is therefore a substitution reaction rather than an addition. 

To test the necessity of aromatic n electrons in the epimerization reaction, indolo[2,3-a]quinolizidines with different substituents at C-10 were prepared [14]. Subjecting these compounds to epimerization conditions (TFA, 90°C, 60 min) gave the following results, Table 1. 

The majority of studies has involved aromatic N—O ligands, a selection of which is given by (9)-(13) together with abbreviations which will be used in referring to them. All of these offer scope for inclusion of substituents with a range of electronic characteristics. 

Heteronins 24-27 have been the subject of extensive spectroscopic and chemical studies, and it has been established that certain members, such as oxonin (24a) and those azonines incorporating electron-withdrawing N-substituents (24b and 25) possess heavily localized polyenic frames, whereas others, such as parent azonine (26a) and its anion (26b-f), are extensively delocalized and decidedly aromatic.30 A third subgroup consisting of the TV-alkyl azonines (27) was said to be nondescript . In specific terms, the polyenic members of the family were shown to be (i) atropic by 4H NMR, (ii) -localized and heavily buckled by UV,30 13C NMR,34 and an X-ray determination on urea 25c,35 (iii) thermally labile, readily isomerizing to the c -bicyclo[4.3.0]triene frame... 

Similar to the success achieved with TADDOLs (see Chapter 8) [17], many chiral catalysts and chiral ligands were synthesized from tartaric add, which provides a proper C2-symmetric framework and structural diversity. TaDiAS 1 has remarkable structural diversity because a wide variety of catalysts can be easily synthesized by changing the acetal moieties (R1 and R2), aromatic parts (Ar), and counter anions (X ), making it possible to fine-tune, three-dimensionally, the catalyst (vide infra). Other candidates, such as 2, 3, and 4 (Figure 6.3), produced unsatisfactory results (<10% ee) during preliminary catalyst screening of the N-substituents [4a]. Thus,... 

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