Hydrocarbon metal-carbon multiple bond

The fourth chapter gives a comprehensive review about catalyzed hydroamina-tions of carbon carbon multiple bond systems from the beginning of this century to the state-of-the-art today. As was mentioned above, the direct - and whenever possible stereoselective - addition of amines to unsaturated hydrocarbons is one of the shortest routes to produce (chiral) amines. Provided that a catalyst of sufficient activity and stabihty can be found, this heterofunctionalization reaction could compete with classical substitution chemistry and is of high industrial interest. As the authors J. J. Bmnet and D. Neibecker show in their contribution, almost any transition metal salt has been subjected to this reaction and numerous reaction conditions were tested. However, although considerable progress has been made and enantios-electivites of 95% could be reached, all catalytic systems known to date suffer from low activity (TOP < 500 h ) or/and low stability. The most effective systems are represented by some iridium phosphine or cyclopentadienyl samarium complexes. 

The addition of metal-hydrogen bonds across carbon-carbon multiple bonds, called hydrometallations, are very important, versatile transformations in organic synthesis. First, they allow the synthesis of new organometallic compounds. The products thus formed may be further transformed into other valuable compounds. The two most important reactions, hydroboration and hydrosilylation, will be treated here in detail, whereas other hydrometallation reactions (hydroalanation, hydro-zirconation) will be discussed only briefly. Hydrostannation, a very important transformation of substituted unsaturated compounds, has no significance in the chemistry of hydrocarbons possessing nonactivated multiple bonds. 

In the first of these reactions (Equation 11-2), a hydrocarbon is produced by the cleavage of a borane, R3B, with aqueous acid, or better, with anhydrous propanoic acid, CH3CH2C02H. The overall sequence of hydroboration-acid hydrolysis achieves the reduction of a carbon-carbon multiple bond without using hydrogen and a metal catalyst or diimide (Table 11-3) ... 

The formal addition of a C-H bond at activated methylenes and methynes (pronucleophiles) to activated alkenes in the presence of a base is well known as the Michael reaction (Scheme 1, Type A) [1]. In modem organic syntheses, the use of transition metal (TM) catalysts enables the C-H addition of activated methylenes and methynes to activated alkenes perfectly under neutral conditions (Scheme 1, Type B) [2]. In general, the nonfunctionalized carbon-carbon multiple bonds (for example, EWG2 = H in Scheme 1) are unreactive toward carbon nucleophiles because of their electron rich Jt-orbitals. The pioneering efforts by various research groups resulted in the development of transition metal-catalyzed addition of a C-H bond at active alkanes to such unactivated C-C multiple bonds. This reaction consists of the formal addition of a C-H bond across the C-C multiple bonds and is called a hydrocarbonation reaction. As a milestone in this hydro-carbonation area, early in the 1970s, Takahashi et al. reported the Pd-catalyzed addition of the C-H bond of pronucleophiles to 1,3-dienes [3], The first Pd-catalyzed reaction of activated methylenes with unsubstituted allenes was apparently reported by Coulson [4]. The synthetic applications of this reaction were very limited. In the last decade, the Pd-catalyzed addition of C-H bonds to various unacti-... 

Nitriles. Nitriles can be prepared by a number of methods, including ( /) the reaction of alkyl haHdes with alkaH metal cyanides, (2) addition of hydrogen cyanide to a carbon—carbon, carbon—oxygen, or carbon—nitrogen multiple bond, (2) reaction of hydrogen cyanide with a carboxyHc acid over a dehydration catalyst, and (4) ammoxidation of hydrocarbons containing an activated methyl group. For reviews on the preparation of nitriles see references 14 and 15. 

Metals which with adsorbed CO prefer to form metal-carbon bonds on the summits are Pt and Ir (Cu ) metals which promote binding in the valley are Pd > Ni > Rh, Re. Metals promoting multiple metal-carbon bonds (with hydrocarbons) are Ni, Ru, Rh Pt and Pd are much worse in this respect. Let us extrapolate and assume that what holds for CO also holds for hydrocarbon molecules, and that the characterization of the multiple-bond formation propensity is valid also at higher temperatures than were established experimentally by exchange reactions. Then we can attempt to rationalize the available information on the formation and the role of various hydrocarbon complexes. 

The recent advances in the understanding of the reaction of aromatic hydrocarbons with alkali metals have provided an explanation for many apparently unconnected experimental observations, such as those surveyed in 1942 by Campbell and Campbell 41) in a review article on the reduction and hydrogenation of molecules with multiple carbon-carbon bonds. A general approach to the course of chemical reduction, electrochemical reduction, and catalytic hydrogenation of hydrocarbons with conjugated double bonds has been given by Hoijtink 42-44). On the basis of the general reduction scheme... 

A large molecule, e.g., a hydrocarbon, may attach to a metal surface with several bonds. This is schematically illustrated in Figure 5.21. In this figure dissociative adsorption of pentane occurs by attachment of two of its carbon atoms to the metal surface. This mode of adsorption becomes suppressed when sulfur or carbon atoms are coadsorbed. The S or C atoms decrease the probability for a surface to have neighboring-reactive sites. As a result, the probability for pentane to form multiple bonds with the metal surface becomes suppressed. It will only adsorb with one of its carbon atoms attached to the surface. As a result of this site blocking effect, the heat of adsorption of pentane decreases and the probability for dissociative adsorption decreases as well. 

---