Directed hydroarylation olefins

In parallel with the directed hydroarylation of olefins, a series of papers described the formation of ketones from heteroarenes, carbon monoxide, and an alkene. Moore first reported the reaction of CO and ethylene with pyridine at the position a to nitrogen to form a ketone (Equation 18.28). Related reactions at the less-hindered C-H bond in the 4-position of an A/-benzyl imidazole were also reported (Equation 18.29). - Reaction of CO and ethylene to form a ketone at the ortho C-H bond of a 2-arylpyridine or an N-Bu aromatic aldimine has also been reported (Equations 18.30 and 18.31). Reaction at an sp C-H bond of an N-2-pyridylpiperazine results in both alkylative carbonylation and dehydrogenation of the piperazine to form an a,p-unsaturated ketone (Equation 18.32). The proposed mechanism of the alkylative carbonylation reaction is shown in Scheme 18.6. This process is believed to occur by oxidative addition of the C-H bond, insertion of CO into the metal-heteroaryl linkage, insertion of olefin into the metal-acyl bond, and reductive elimination to form the new C-H bond in the product. 

In 1993, Murai reported the reactions of aryl ketones witti ethylene and vinylsilanes to form the product from the addition of the C-H bond ortho to the carbonyl group to the olefin (Equation 18.49). This finding led to subsequent work on the addition of the C-H bonds of a variety of aromatic groups to a series of olefins. Catalysts that react under milder conditions than the original ones and extensions of the C-H activation to intramolecular cyclizations to form heterocyclic structures have made this reaction capable of being used in the synthesis of complex molecules. For example, alkylated diterpe-noids and (+)-lithospermic acid (Equations.18.50 and 18.51) have been synthesized using directed hydroarylation. ... 

These catalytic reactions provide a unique pathway for addition of aromatic C-H bonds across C=C bonds. In contrast with Friedel-Crafts catalysts for olefin hydroarylation, the Ru-catalyzed hydrophenylation reactions of a-olefins selectively produce linear alkyl arenes rather than branched products. Although the selectivity is mild, the formation of anti-Markovnikov products is a unique feature of the Ru(II) and Ir(III) catalysts discussed herein. Typically, the preferred route for incorporation of long-chain linear alkyl groups into aromatic substrates is Friedel-Crafts acylation then Clemmensen reduction, and the catalysts described herein provide a more direct route to linear alkyl arenes. 

The hydroarylation of olefins is relatively uncommon in photochemistry, despite a high interest in this process which allows the formation of an aryl-carbon bond via the direct activation of an aromatic, with no need for leaving groups in both components of the reaction. The process follows a photo-EOCAS (Electrophile-Olefin Combination Aromatic Substitution) mechanism [32], and is initiated by a PET reaction between an electron-rich aromatic and an electron-poor olefin, as illustrated in Scheme 3.14. 

A series of arylations of olefins by C-H bond cleavage without direction by an ortho functional group has also been reported, and these reactions can be divided into two sets. In one case, the C-H bond of an arene adds across an olefin to form an alkylarene product. This reaction has been called hydroarylation. In a second case, oxidative coupling of an arene with an olefin has been reported. This reaction forms an aryl-substituted olefin as product, and has been called an oxidative arylation of olefins. The first reaction forms the same t)q)es of products that are formed from Friedel-Crafts reactions, but with selectivity controlled by the irietal catalyst. For example, the metal-catalyzed process can form products enriched in the isomer resulting from anti-Markovnikov addition, or it could form the products from Markovnikov addition with control of absolute stereochemistry. Examples of hydroarylation and oxidative arylation of olefins are shown in Equations 18.63 - and 18.64. ... 

Phosphine oxide directing groups enable both redox-neutral and oxidative C-H olefinations (Scheme 23.17) [76, 77], In combination with [Ru(p-cymene)Clj]j, cw-selective hydroarylations of alkynes can be achieved. Furthermore, in the presence of [Cp RhCy/AgSbF, Cu(OAc)j, and AgjCOj, oxidative C-H olefinations are possible with electron-poor olefins as coupling partners. The latter reactions proceed with complete selectivity for the fran -product. 

The use of nonactivated secondary alkyl halides as electrophiles in a copper-catalyzed direct alkylation of benzoxazoles has been described. Although methods exist for the direct alkylation of aromatic heterocycles, such as radical alkylations " and coupling of heterocycles with alkyl electrophiles, such methods are restricted to the incorporation of primary alkyl groups only. Secondary alkyl groups can be introduced by metal-catalyzed hydroarylation of olefins but in these cases examples are usually confined to activated alkyls such as benzyl or allyl. 

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