Electrophiles metal carbon-hydrogen

Intramolecular carbon-hydrogen insertion reactions have well known to be elTectively promoted by dirhodium(ll) catalysts [19-23]. Insertion into the y-position to form five-membered ring compounds is virtually exclusive, and in competitive experiments the expected reactivity for electrophilic carbene insertion (3°>2° 1°) is observed [49], as is heteroatom activation [50]. A recent theoretical treatment [51] confirmed the mechanistic proposal (Scheme 15.4) that C-C and C-H bond formation with the carbene carbon proceeds in a concerted fashion as the ligated metal dissociates [52]. Chemoselectivity is dependent on the catalyst ligands [53]. 

For instance, if the metal is lost by Sn2 attack on coordinated carbon, this constitutes R loss, and alkyl migration to an electrophilic centre such as coordinated CO may resemble R loss. R- loss may take place by simple homolysis, or by alkyl group transfer. Moreover, as Yamamoto has pointed out an electroneutral metal-carbon bond lengthening may be a prelude to more complex processes such as 0-elimination, or may lead to internal hydrogen abstraction rather than to actual free ligand release. 

In a series of late transition metal catalyzed processes the first step in the catalytic cycle is the coordination of the reagent to the metal atom, which is in a positive oxidation state, followed by its covalent attachment through the concomitant breaking of an unsaturated carbon-carbon bond or a carbon-hydrogen bond. These processes usually require a highly electrophilic metal centre and are frequently carried out in an intramolecular fashion. The carbometalation processes that follow a similar course, but take place only at a later stage in the catalytic cycle, will be discussed later. 

As mentioned above, the electrophilic metal carbene complexes are stabilised by the presence of heteroatoms or phenyl rings at the divalent carbon atom, while hydrogen or alkyl groups stabilise the nucleophilic complexes. Therefore, there is a distinction between carbenoids and alkylidenes when designing carbene ligands corresponding to the former or the latter class. 

Since the observation that Rh(II) carboxylates are superior catalysts for the generation of transient electrophilic metal carbenoids from a-diazocar-bonyls compounds, intramolecular carbenoid insertion reactions have assumed strategic importance for C-C bond construction in organic synthesis [1]. Rhodium(ll) compounds catalyze the remote functionalization of carbon-hydrogen bonds to form carbon-carbon bonds with good yield and selectivity. These reactions have been particularly useful in the intramolecular sense to produce preferentially five-membered rings. 

The electron-deficient sulfonyl nitrene (88) can insert into electron-rich carbon-hydrogen bonds, abstract hydrogen atoms, and add to double bonds and aromatic rings. These reactions may be initiated by acids, heat, light and transition metals. The reactions are illustrated by heating methanesulfonyl azide (89) with bezene (23) (Scheme 59). Here, the electrophilic sulfonyl nitrene (90) adds to the electron-rich aromatic double bond, but the kinetically favoured azepine(91) rearranges to give the thermodynamically favoured N-phenyl sulfonamide (92) (Scheme 59). 

Aoyama Y, Yoshida T, Sakurai K-i, Ogoshi H (1983) Activation of arene carbon-hydrogen bonds. Direct electrophilic aromatic metalation with a rhodium-porphyrin complex. J Chem Soc Chem Commun 478-479... 

Lithio-heterocycles have proved to be the most useful organometallic derivatives they react with the whole range of electrophiles in a manner exactly comparable to that of aryllithiums and can often be prepared by direct metallation (C-hydrogen deprotonation), as well as by halogen exchange between a halo-heterocycle and an alkyllithium. As well as reaction with carbon electrophiles, Uthiated species are often the most convenient source of heterocyclic derivatives of less electropositive metals, such as zinc, boron, silicon and tin, as will be seen in the following sections. 

This chapter presents developments in the activation and functionalization of carbon-hydrogen bonds that have been discovered since 1993. Major breakthroughs in hydrocarbon activation appeared in the early 1980s, and in the following decade, an explosion of discoveries was seen in new examples of metal complexes that could activate C-H bonds. Mechanisms for cleavage included oxidative addition, electrophilic cleavage, radical H-abstraction, and metal atom reactions, and several texts are available that summarize the first decade of this work. " ... 

Carbon-Hydrogen Bond Cleavage by Electrophilic Metals... 

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