Substitution reactions, arene --complexes

Aromatic ketones arylations, 10, 140 asymmetric hydrogenation, 10, 50 G—H bond alkylation, 10, 214 dialkylzinc additions, 9, 114-115 Aromatic ligands mercuration, 2, 430 in mercury 7t-complexes, 2, 449 /13-77-Aromatic nitriles, preparation, 6, 265 Aromatic nucleophilic substitution reactions, arene chromium tricarbonyls, 5, 234... 

Another example of an unusual reaction occurring in the gas phase is ammonia in a chromium complex ion being substituted by arenes such as benzene (4). It is important to note the uncommon oxidation state of the chromium. 

The Dotz benzannulation reaction yields either arene chromium tricarbonyl complexes or the decomplexed phenols, depending on the work-up conditions. Because of the instability of hydroxy-substituted arene chromium tricarbonyl complexes, yields of the latter tend to be low. High yields of arene complexes can, however, be obtained by in situ silylation of the crude product of the benzannulation reaction [336]. Oxidative work-up yields either decomplexed phenols or the corresponding quinones. Treatment of the benzannulation products with phosphines also leads to decomplexed phenols [272]. 

The failure of various 7r-cyclopentadienyl-metal and 7r-arene-metal complexes to undergo aromatic-type substitution reactions does not necessarily mean... 

Substitution reactions at aromatic carbon (see also Reduction reactions, Ullmann ether coupling, specific reactions such as Nitration) Arene(tricarbonyl)chromium complexes, 19... 

The known reactions of fluorinated olefins, arenes, and heterocycles with metal carbonyl anions, to afford fluorovinyl or fluoroaryl complexes resulting from net displacement of fluoride ion (see Section II1,G), prompted us to attempt such substitution reactions with OFCOT, in order to generate the required metal-substituted heptafluorocyclooctatetraenes. 

A RhCp complex (S,S)-6 (Cp =pentamethylcyclopentadienyl), which is iso-lobal with Ru(rj6-arene) complex (S,S)-5 (Scheme 13), effected the transfer hydrogenation of a cyclic imine substituted by an isopropyl group with an S/C of 200 in the presence of a 5 2 mixture of formic acid and triethylamine to give the R amine in 99% ee (Scheme 13) [31]. When the reaction was performed with an S/C of 1,000, the optical yield decreased to 93%. The methyl imine was reduced with a 91% optical yield. Reduction of a cyclic sulfonimide resulted in the R sul-tam in 81% ee. 

Three types of reaction systems have been designed and applied for the enantioposition-selective asymmetric cross-coupling reactions so far. First example is asymmetric induction of planar chirality on chromium-arene complexes [7,8]. T vo chloro-suhstituents in a tricarhonyl("n6-o-dichlorobenzene)chromium are prochiral with respect to the planar chirality of the 7t-arene-metal moiety, thus an enantioposition-selective substitution at one of the two chloro substituents takes place to give a planar chiral monosubstitution product with a minor amount of the disubstitution product. A similar methodology of monosuhstitution can be applicable to the synthesis of axially chiral biaryl molecules from an achiral ditriflate in which the two tri-fluoromethanesulfonyloxy groups are enantiotopic [9-11]. The last example is intramolecular alkylation of alkenyl triflate with one of the enantiotopic alkylboranes, which leads to a chiral cyclic system [12], The structures of the three representative substrates are illustrated in Figure 8F.1. 

Arenes are inert to nucleophilic attack and normally undergo electrophilic substitution. However, arenes coordinate to Cr(CO)6 to form the i/fi-arenechromium tricarbonyl complex 79, and facile nucleophilic attack on the arene generates the anionic jy5-cyclohexadienyl complex 80, from which substituted arene 81, or cyclohexadiene is obtained by oxidative decomplexation. In this reaction, strongly... 

The benzannulation affords arene-Cr(CO)3 complexes possessing a plane of chirality resulting from the unsymmetrical arene substitution pattern. This aspect is relevant to stereoselective synthesis, in which enantiopure arene tricarbonyl chromium complexes play a major role [56]. The benzannulation reaction avoids both harsh conditions incompatible with the retention of chiral information and the cumbersome separation of enantiomers, and is thus attractive for the diastereo- and enantioselective synthesis of arene complexes [17b, 57]. 

Fig. 12. Li near correlations of the relative reactivity of arenes in electrophilic substitution reactions (log k/kf) with the optical transition energies in the accompanying CT complexes. Data from ref. [62b],...

Complexes 53 are efficient catalysts for the homodimerization of 1-octene and styrene. Complex 53a bearing the sterically more demanding l,3-bis(2,6-diisopropylphenyl)-NHC ligand shows a higher reactivity than the mesityl-substituted 53b. These complexes also catalyze the CM of 1-octene or styrene with methyl acrylate ( 80% yield), the RCM of diethyl diallylmalonate at 40 °C ( 95% yield), and the ROMP of cyclooctene at 60 °C ( 90% yield). By GC-MS analysis the presence of free p-cymene was detected in the beginning of the reactions. From these results it may be concluded that the first step of the catalytic cycle is arene decoordination to generate a 12-electron [OsCl(= CHPh)(NHC)] + derivative as the catalytically active species [102],... 

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