Hydrocarbon reaction with metal complexes

A number of transition metals are now known147-156 to form stable dioxygen complexes, and many of these reactions are reversible. In the case of cobalt, numerous complexes have been shown to combine oxygen reversibly.157 158 Since cobalt compounds are also the most common catalysts for autoxidations, cobalt-oxygen complexes have often been implicated in chain initiation of liquid phase autoxidations. However, there is no unequivocal evidence for chain initiation of autoxidations via an oxygen activation mechanism. Theories are based on kinetic evidence alone, and many authors have failed to appreciate that conventional procedures for purifying substrate do not remove the last traces of alkyl hydroperoxides from many hydrocarbons. It is usually these trace amounts of alkyl hydroperoxide that are responsible for chain initiation during catalytic reaction with metal complexes. 

Chapter I discusses some general questions relevant to the chemistry of alkanes and especially their reactions with metal compounds. Transformations of saturated hydrocarbons in the absence of metal derivatives and in the presence of solid metal and metal oxide surfaces are described in Chapters n and 111 (Figure 1). Since these reactions are not the main topic of the monograph their consideration here is far from comprehensiveness but the knowledge of such processes is very important for understanding the peculiarities and mechanisms of the reactions with metal complexes. Chapters IV-X are the main chapters of this book because they describe the activation of hydrocarbons in the presence of... 

A60. J. P. Candlin, K. A. Taylor, and D. T. Thompson, "Reactions of Transition-Metal Complexes. Elsevier, Amsterdam, 1968. A review of types of reactions of metal complexes (e.g., substitution, combination, redox) reactions with various reagents (e.g., hydrocarbons, halides, carbon monoxide, and isonitrile) and preparation of new stabilised organic systems (e.g., metallocenes, carbenes). Intended for research workers, consequently written at a fairly high level, with emphasis on organometallics. A61. H. J. Keller, NMR-Untersuchungen an Komplexverbindungen. Springer, Berlin, 1970. Expansion of review article 37.1. 

An important type of SO2 interaction with metal complexes results in conversion of metal-carbon bonds to O- or S-sulfinates. This type of reaction is usually referred to as an insertion , although (as will be discussed below) the mechanism almost certainly does not involve conventional insertion into the M-C bond. The area has been investigated intensively, and we refer the reader to excellent reviews of the topic. We shall limit ourselves to a few illustrative examples with d- and f-element complexes and also mention some interesting cycloaddition and rearrangement reactions of transition metal unsaturated hydrocarbon complexes with SO2. 

The C—H bond can be activated by a metal complex, particularly when the complex plays the role of catalyst or photocatalyst. The reactions of hydrocarbons with metal complexes occur at low temperatures and can be selective. There are different pathways for C—H bond activation (i) by low-valence metal complexes, (ii) by high-valent metal-oxo compounds, (iii) by molecular oxygen and oxygen atom donors, (iv) by biological oxidation, or (v) by photocatalytic enhancement (21). 

Reactions of metal complexes with saturated hydrocarbons are very important not only from the standpoint of applications but are also extremely interesting for theoretical chemistry. So it is not surprising that many papers devoted to the theoretical aspects of C-H bond activation and especially oxidative addition of C-H compounds to metal complexes (as well as ions and atoms), have been published in recent decades. Some of their results are summarized in books [ 1 ] and reviews [2]. 

HYDROCARBON REACTIONS WITH HIGH-VALENT METAL COMPLEXES... 

Reactions of hydrocarbons with metal complexes exhibit some common features. Abstraction of a hydrogen atom is a key step in most of the hydrocarbon oxidations by metal 0x0 complexes [62]. 

Chapter VIII. Hydrocarbon Reactions with High-Valent Metal Complexes... 

Acetylenes can react with metal complexes in a number of ways. Internal acetylenes usually coordinate to metal centers in a q -fashion, in whieh the C=C triple bond donates electrons to the metal center, and both carbon atoms form metal-carbon bonds. Terminal acetylenes can also coordinate to metal centers in this fashion, but ean react in other ways as well. In particular, because the acetylenic hydrogen atom is relatively acidic for a hydrocarbon, the C—H bond of terminal acetylenes ean also add oxidatively to metal centers to form metal acetyhde complexes. In some cases, however, a third kind of reaction is seen, in which the terminal aeetylene rearranges to a vinylidene complex, M=C=CHR. 

Reactions of metal complexes whose ligands (typically 0-, N-, P-, S-, and As- donors) contain hydrocarbon groups that are subject to metallation with the formation of metal-carbon o-bonds are called cyclometallation reactions,53,54 Scheme 12.11 ... 

To begin to develop functionalization catalysts based on these more electron-rich systems we have been investigating the development of catalytic cycles based on the reaction of O-donor metal-alkoxo complexes with CH bonds as shown in Fig. 7.41 B). This reaction is intriguing because as shown, the reaction leads to the simultaneous CH activation of the hydrocarbon as well as the formation cf a desired oxy-junetionalized product, ROH, in one step. We recently reported [24] the first intermolecular example of such a transformation with trans-(acac-0,0)2lr(0CH3)(CH30H), MeOH-fr-OMe and the corresponding pyridine complex, Py-Ir-OMe. There is no precedent for this type of CH activation reaction with alkoxo complexes [31] and such complexes typically decomposition by facile 8-hydride elimination reactions or formation of inert dinuclear complexes. 

The strained-ring compound 1,1-dimethyl-l-silacyclobutane (which may be regarded as an olefin of organosilicon chemistry) reacts with diiron nonacarbonyl in benzene at 6°-20°C as shown in Eq. (100) (89). (There is here some analogy with the reactions of transition metal complexes with strained hydrocarbons, which often produce valence tautomerization.) The... 

The discussion of the activation of bonds containing a group 15 element is continued in chapter five. D.K. Wicht and D.S. Glueck discuss the addition of phosphines, R2P-H, phosphites, (R0)2P(=0)H, and phosphine oxides R2P(=0)H to unsaturated substrates. Although the addition of P-H bonds can be sometimes achieved directly, the transition metal-catalyzed reaction is usually faster and may proceed with a different stereochemistry. As in hydrosilylations, palladium and platinum complexes are frequently employed as catalyst precursors for P-H additions to unsaturated hydrocarbons, but (chiral) lanthanide complexes were used with great success for the (enantioselective) addition to heteropolar double bond systems, such as aldehydes and imines whereby pharmaceutically valuable a-hydroxy or a-amino phosphonates were obtained efficiently. 

A few further general examples of zinc catalytic activity or reactivity include the following. Other zinc-containing systems include a zinc phenoxide/nickel(0) catalytic system that can be used to carry out the chemo- and regioselective cyclotrimerization of monoynes.934 Zinc homoenolates have been used as novel nucleophiles in acylation and addition reactions and shown to have general utility.935,936 Iron/zinc species have been used in the oxidation of hydrocarbons, and the selectivity and conditions examined.362 There are implications for the mechanism of metal-catalyzed iodosylbenzene reactions with olefins from the observation that zinc triflate and a dizinc complex catalyze these reactions.937... 

Considerable interest in the subject of C-H bond activation at transition-metal centers has developed in the past several years (2), stimulated by the observation that even saturated hydrocarbons can react with little or no activation energy under appropriate conditions. Interestingly, gas phase studies of the reactions of saturated hydrocarbons at transition-metal centers were reported as early as 1973 (3). More recently, ion cyclotron resonance and ion beam experiments have provided many examples of the activation of both C-H and C-C bonds of alkanes by transition-metal ions in the gas phase (4). These gas phase studies have provided a plethora of highly speculative reaction mechanisms. Conventional mechanistic probes, such as isotopic labeling, have served mainly to indicate the complexity of "simple" processes such as the dehydrogenation of alkanes (5). More sophisticated techniques, such as multiphoton infrared laser activation (6) and the determination of kinetic energy release distributions (7), have revealed important features of the potential energy surfaces associated with the reactions of small molecules at transition metal centers. 

Metal vapor chemistry showed that the lanthanides had quite an extensive chemistry with unsaturated hydrocarbons. Some of the early surveys of metal vapor reactions with unsaturated hydrocarbons included some lanthanide metals and showed that reactivity was present for these metals (14-18). Subsequent synthetic studies in which the products were isolated and characterized led to some of the most unusual organolanthanide complexes currently known (19-28). 

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