Active hydrogen compounds with arenes

Treatment of [(arene)OsCl2]2 with lithium amide in THF results in the formation of imidoosmium complexes in high yield, which react with a with wide variety of active hydrogen compounds and polar compounds (Scheme 10.28) [253]. 

This method ensures the deposition of very reactive metal nanoparticles that require no activation steps before use. We shall review here the following examples of catalytic reactions that are of interest in line chemical synthesis (a) the hydrogenation of substituted arenes, (b) the selective hydrogenation of a, 3-unsaturated carbonyl compounds, (c) the arylation of alkenes with aryl halides (Heck reaction). The efficiency and selectivity of commercial catalysts and of differently prepared nanosized metal systems will be compared. 

Co-condensation of transition metal atoms with arenes such as benzene and toluene is well known to yield bis-arene-metal compounds. However, in many cases the yields based on the metal atoms are less than 40%. Evidence that competing reactions such as carbon-hydrogen activation can occur is provided by the isolation of non-metal-containing products such as biaryl derivatives (2JL). ... 

The generation of (difluoroiodo)arenes by anodic oxidations of aryl iodides in the presence of excess Et3N nHF is a notable recent development (Scheme 31) [96,97]. Because the fluoroiodanes are activated by hydrogen-bonding with HF, such reagent combinations are especially useful for electrophilic fluorinations of organic compounds. When oxidation potentials of the aryl iodides are lower than those of the substrates, indirect anodic fluorinations of the latter can be achieved. 

The hydrogenation of aromatics has been a topic of interest since Sabatier s first synthesis of cyclohexane from benzene with metallic nickel. The role of Nb and Ta aryloxides as catalysts for this reaction was mentioned earlier. Another system that has been studied in detail comprises allyl and hydride complexes of cobalt, e.g., (i73-C3H5)Co[P(OMe)3]3. Like the Nb and Ta compounds cobalt gives cyclohexane with cis stereoselectivity. The active species is probably the hydride, generated from (allyl)CoL3 on hydrogenolysis, which reacts with arenes in a stepwise manner. 

Mercury salts can react directly with hydrocarbons exchanging hydrogen for mercury. This reaction is an electrophilic substitution (equation 5) and hence can take place with arenes, cyclopentadienyls, terminal alkynes, and also with aliphatic hydrocarbons that contain activated carbon-hydrogen bonds (e.g. carbonyl or nitrile compounds). When the hydrocarbon contains several equivalent hydrogen atoms, polymercuration is often observed. 

On the other hand, the direct arylation of carbanionic species generated from substrates having relatively acidic hydrogens such as active methylene compounds and ketones can occur (mechanism B) [5,6]. Aryl halides are also capable of coupling directly with appropriately functionalized aromatic substrates and five-membered heteroaromatic compounds as formal carbon nucleophiles via cleavage of their unactivated C-H bonds [5,7-9]. The Fujiwra-Moritani reaction, which is the arylation of alkenes with arenes, is also useful for preparing arylalkenes without employing any halides (mechanism D) [10,11]. 

Fogassy, G., Tangier, A., Levai, A. (2003) Enantioselective hydrogenation of exocyclic alpha,beta-unsaturated ketones. Part 111. Hydrogenation with Pd in the presence of cinchonidine, y. Mol. Catal. A. Chem. 192, 189-194. Hapiat, F., Agboussou, F., Mortreux, A. (1994) Synthesis of new chiral arene Ru(ll) aminophosphine-phosphinite complexes and use in asymmetric hydrogenation of an activated keto compounds. Tetrahedron Asymm. 5, 515-518. 

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