Ortho-Metallation, oxidative addition

Oxidative additions involving C-H bond breaking have recently been the topic of an extensive study, usually referred to as C-H activation the idea is that the M-H and M-hydrocarbyl bonds formed will be much more prone to functionalization than the unreactive C-H bond. Intramolecular oxidative additions of C-H bonds have been known for quite some time see Figure 2.15. This process is named orthometallation or cyclometallation. It occurs frequently in metal complexes, and is not restricted to "ortho" protons. It is referred to as cyclometallation and is often followed by elimination of HX, while the metal returns to its initial (lower) oxidation state. 

The first step of these ortho-metalation or oxidative-addition reactions which would lead to the complexes HOs(YH)(CO)iq has been seen only in the case of HjY = anilines (132, 406) where [S7] could be isolated. But similar complexes, [82] (281) and [55] (407), have also been obtained from Os3(CO)j2 and EtSH or pyridine. 

Under n-BuLi or scc-BuLi conditions, 4-pyridyl oxazoline 411 behaves differently than the 3-isomer to give mixtures of ortho metalation (412) and addition (413) products, the latter type having been trapped upon workup by Mel treatment before its oxidation (Scheme 127) (78TL227 82JOC2633). However, if metalation is carried out with MeLi at -78°C and... 

The combination of ortho metallation and meta nucleophilic acylation was used to prepare a key intermediate in a synthesis of deoxyfrenolicin (42), as outlined in Scheme 11. The complex of anisole is orf/io-metallated with n-butyllithium and quenched with chlorotrimethylsilane the resulting [(o-(tri-methylsilyl)anisole)Cr(CO)3] (43) is then metallated again, converted to the arylcuprate, and coupled with ( )-2-hexenyl bromide to give the complex of l-trimethylsilyl-2-methoxy-3-(2-hexenyl)benzene (44). Addition of the carbanion from the cyanohydrin acetal of 4-pentenal, followed by the standard iodine oxidation and subsequent hydrolysis of the cyanohydrin acetal to regenerate the carbonyl group... 

The following compounds with H—C and II—M bonds undergo oxidative addition to form metal hydrides. This is examplified by the reaction of 6, which is often called ortho-metallation, and occurs on the aromatic C—H bond at the ortho position of such donar atoms as N, S, 0 and P. Reactions of terminal alkynes and aldehydes are known to start by the oxidative addition of their C—H bonds. Some reactions of carboxylic acids and active methylene compounds are explained as starting with oxidative addition of their O—H and C—H bonds. 

The now well-known ortho-metalation reaction observed in certain low-valent transition metal complexes of arylphosphines (1, 79) provided a model for the observation of oxidative addition to terminal B—H bonds. 

In another series of experiments (59), toluene solutions of 1-(1,2-C B,oHu)P(CH ) were reacted with deuterium gas in the presence of (Ph3P)3RuHCl. Mass spectral evidence proved that up to eight deuterium atoms had been introduced into the phosphine. Since only four BH exchange sites could possibly be involved in deuterium exchange which proceeded only through ortho-metalation intermediates, an intermolecular, reversible oxidative addition of catalyst to terminal B—H bonds was indicated. Indeed, further work proved that under the same conditions 1,2-, 1,7-, and 1,12-C2BioHi2 carboranes could be totally deuterated at boron... 

The oxidative addition of C H bonds of ligands is very common and this reaction forms metal alkyl or metal aryl complexes. In osmium triarylphosphine complexes, orthomet-allation gives four-membered metaUocycles. When the ortho... 

These are reactions that incorporate metals into organic rings. The most common of these are orthometallations, oxidative additions in which the ortho position of an aromatic ling becomes attached to the metal. The first example in Figure 14-4 is an OA in which an ortho carbon and the hydrogen originally in the ortho position add to iridium. 

This ring thereby constitutes an unusual yu-ij. ij -vinyl group whereby the phosphino carbon is also metalated. Intramolecular oxidative addition of the ortho C-H bond of the phenyl substituents is a common feature in crowded iridium phosphine complexes [65, 309, 310]. 

Intramolecular oxidative-addition processes give metal atom heterocyclic compounds. When the ortho hydrogen atom of an aryl ring is transferred to the metal atom and a metal atom is inserted into a C-H bond, the reaction is also called orthometalla-tion. The more general term cyclometallation includes aryl and alkyl ring formation and is schematically described as follows " ... 

Orthometallations of arylphosphines involve oxidative additions of the ortho-C—H group, while the ortho-hydrogen atom remains with the metal. 

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