Oxidative addition to low-valent transition

Cyclopropane oxidative addition to low-valent transition metals has been intensively investigated in the decades since the first metallacyclobutane complex was prepared by this methodology <1996CHEC-II(lb)887>. Comprehensive reviews on this topic are available <1980CGR149, 1994CRV2241>. 

An attractive pathway with a lot of potential uses the transition metal mediated reaction of organic halides with carbon monoxide. Suitable substrates are organic halides capable of oxidative addition to low-valent transition metal compounds. Insertion of carbon monoxide and reductive elimination of an acid halide will complete the catalytic cycle. In tins way it was shown tiiat allyl chloride yields butenoic acid chloride in >80% yield accor g to equation 22)P As well as palladium, rhodium and iridium also act catalytically. It is of no surprise that allylic halides, benzylic halides and aryl halides in particular are readily converted to acid halides. Simple aliphatic halid undergo the oxidative addition step more slowly and, if they cany hydrogen atoms on an sf hybridized C atom in the -position to the halogen atom, may give alkenes via 3-hydrogen elimination. Alkenes can also be converted to acid halides widi carbon monoxide in the presence of transition metal catalysts in solvents such as methylene chloride or tetrachloromethane. ... 

Similarly to cyclopropanes, four-membered carbocyclic compounds undergo oxidative addition to low-valent transition metals to form five-membered metallacycles. Rhodium(I) inserts into C-C bonds next to the carbonyl group of ketones to form a rhodacycloalkanone species [49]. The C-C bond of cyclobutanone was cleaved, even at room temperature, by oxidative addition to a rhodium(I) complex having a PBP pincer ligand [50]. In the case of cyclobutanone 70, catalytic decarbonylation was possible and afforded the alkene 71 and cyclopropane 72 (Scheme 3.40). 

Activation of Carbon—Fluorine Bonds by Oxidative Addition to Low-Valent Transition Metals... 

This reaction takes advantage of the unique reactivity of aryl halides in oxidative-addition to low-valent transition metals, providing a means of activating aryl halides toward carbon-carbon coupling, as exemplified below ... 

Although the chemistry described in the foregoing deals with the oxidative addition of low-valent transition metal complexes to terminal B—H bonds, we now describe the formal addition of the same reagents to the... 

Studies of transition-metal silyl chemistry have focused mainly on the later members of the transition series. To some degree this reflects the availability of convenient preparative routes based on oxidative additions to low-valent, electron-rich complexes (e.g. equations 1 and 2). 

Figure 27 Oxidative addition of the organochalcogen compounds to low-valent transition metal centers most often resulting in the cleavage of the chalcogen-chalcogen bond and the formation of mono- or dinuclear complexes with anionic bridging or terminal RE- (E = Se, Te) ligands.

The first successful syntheses utilizing trifluoromethyl iodide in transition metal chemistry were reported by Stone and his students. Stone reasoned that if CF3I would not react with transition metal anions to form trifluoromethyl derivatives [see Eq. (3)] then perhaps compounds containing perfluoroalkyl substituents could be generated by the oxidative addition of perfluoroalkyl halides to low valent transition metal substrates (9,10). The first reported trifluoromethyl-substi-tuted transition metal complex prepared by this route is shown in Eq. (4) (41). 

I. by Oxidative Addition of SljX, and Strained Cyclic Disilanes to Low-Valent Transition- or Inner Transition-Metal Complexes. 

Single C-0 bond in acid anhydride can be also readily cleaved by oxidative addition of the anhydride to low-valent transition metal complexes to give acyl-carboxylato-type complexes as reviewed previously (Eq. 28) [1]. 

However, a truly general method for electron-rich heterocycle arylation was not reported until 2007 [38], Electron-rich, bulky butyl-di-l-adamantylphosphine or /er/-buty I dicyclohexyl phosphine in combination with Pd(OAc)2 afforded the best results, and the former was chosen because of cost considerations. Interestingly, electron-rich AMieterocyclic carbene ligands that facilitate oxidative addition of aryl chlorides to low-valent transition metals are inefficient in heterocycle arylation. A number of structurally diverse electron-rich heterocycles are reactive (Scheme 3). Thiophene, benzothiophene, 1,2- and 1,3-oxazole derivatives, benzofuran, thiazoles, benzothiazole, 1-alkylimidazoles, 1-alky 1-1,2,4-triazoles, and caffeine can be arylated. Electron-rich, electron-poor, and heteroaryl chlorides can be used. 

Most known processes utilizing carbon monoxide are catalyzed by late transition metal complexes. This is due to the ease of oxidative addition of substrates such as organic halides to low valent transition metal complexes and reductive... 

However, others have shown that oxidative additions of much less acidic substrates, such as water, ° alcohols, unactivated anilines and azole heterocycles, " occur to low-valent transition metal complexes. For example, iridium(I) complexes containing tri-ethylphosphine as ligand undergo oxidative addition of water rapidly at room temperature (Equation 7.17). Both [Ir(PEtj)J and [Ir(PEt3)jCl] also add aniline rapidly at room temperature (Equation 7.17). The latter reaction is a step in the catalytic hydroamination of norbornene. These substrates react readily with early metal complexes, but tliese reactions do not occur by oxidative addition. ... 

The most important methods for the preparation of cyclobutadiene complexes are oxidative addition reactions of dihalogenocyclobutenes to low-valent transition metal compounds [equations (8.27) and (8.28)], as well as reactions of acetylenes with coordination compounds. 

Oxidative addition offers a direct method to cleave a covalent bond. Although a wide variety of bonds, such as C-1 and C-Br, are known to facilely undergo oxidative addition reactions to low-valent transition metal complexes, examples of oxidative addition of C—C single bonds are far more rare. The scarcity is in part associated with the thermodynamic stability of C—C bonds. Whereas oxidative addition of C-Br and C-I bonds to low-valent metals is thermodynamically favored in general, that of a C—C single bond is often thermodynamically disfavored. 

Urata H, Tanaka M, Fuchikami T (1987) Oxidative addition reaction of l,3-dialkyl-5-fluoro-6-iodouracils to low-valent transition metal complexes. Chem Lett 751-754... 

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