Transition metal complexes with acetylene

All mechanisms proposed in Scheme 7 start from the common hypotheses that the coordinatively unsaturated Cr(II) site initially adsorbs one, two, or three ethylene molecules via a coordinative d-7r bond (left column in Scheme 7). Supporting considerations about the possibility of coordinating up to three ethylene molecules come from Zecchina et al. [118], who recently showed that Cr(II) is able to adsorb and trimerize acetylene, giving benzene. Concerning the oxidation state of the active chromium sites, it is important to notice that, although the Cr(II) form of the catalyst can be considered as active , in all the proposed reactions the metal formally becomes Cr(IV) as it is converted into the active site. These hypotheses are supported by studies of the interaction of molecular transition metal complexes with ethylene [119,120]. Groppo et al. [66] have recently reported that the XANES feature at 5996 eV typical of Cr(II) species is progressively eroded upon in situ ethylene polymerization. 

Reyiews.—Recent reviews on areas of acetylenic chemistry include synthetic routes to average-ring-size cycloalkynes, a study of the bonding in metal-acetylene complexes, transition-metal complexes of acetylene, intramolecular cyclization reactions with acetylenic bond participation, oligomerization of acetylenes induced by metals of the nickel triad, an article on the handling of acetylenic compounds, and a book on preparative acetylenic chemistry. ... 

The reductive cyclization of non-conjugated diynes is readily accomplished by treatment of the acetylenic substrate with stoichiometric amounts of low-valent titanium52 523 and zirconium complexes.53 533 Hence, it is interesting to note that while early transition metal complexes figure prominently as mediators of diyne reductive cyclization, to date, all catalyzed variants of this transformation employ late transition metal complexes based on nickel, palladium, platinum, and rhodium. Nevertheless, catalytic diyne reductive cyclization has received considerable attention and is a topic featured in several review articles. ... 

Although the preparation of bulk quantities of free cydo-Qg remains elusive, one of its transition metal complexes has been prepared and characterized by X-ray crystallography.11,281 Its synthesis benefited from the fact that alkynes can be protected with Co2(CO)8 to give (p-acetylene)dicobalt hexacar-bonyl complexes having dramatically reduced... 

Even more than [6 + 4] and [8 + 2] cycloaddition reactions, the [2 + 2 + 2] cycloaddition reactions require a very well preorganized orientation of the three multiple bonds with respect to each other. In most cases, this kind of cycloaddition reaction is catalyzed by transition metal complexes which preorientate and activate the reacting multiple bonds111,324. The rarity of thermal [2 + 2 + 2] cycloadditions, which are symmetry allowed and usually strongly exothermic, is due to unfavorable entropic factors. High temperatures are required to induce a reaction, as was demonstrated by Berthelot, who described the synthesis of benzene from acetylene in 1866325, and Ullman, who described the reaction between nor-bomadiene and maleic anhydride in 1958326. As a consequence of the limiting scope of this chapter, this section only describes those reactions in which two of the participating multiple bonds are within the same molecule. 

Carbonylations of olefins, acetylenes, halides, alcohols, amines, nitro compounds, etc., promoted by transition metal complexes are very important in both industrial and laboratory organic syntheses. The mechanisms of those reactions have been studied extensively, especially for those associated with commercial processes. " The research... 

The reaction of methylenecyclopropanes with transition metal complexes is well known to promote a catalytic a-ir cycloaddition reaction with unsaturated compounds, in which a trimethylenemethane complex might exist71-76. Recently, much interest has been focused on the interaction of strained silicon-carbon bonds with transition metal complexes. In particular, the reaction of siliranes with acetylene in the presence of transition metal catalysts was extensively investigated by Seyferth s and Ishikawa s groups77-79. In the course of our studies on alkylidenesilirane, we found that palladium catalyzed reaction of Z-79 and E-79 with unsaturated compounds displayed ring expansion reaction modes that depend on the (Z) and (E) regiochemistry of 79 as well as the... 

Studies on elementary reactions of acetylenes with metal complexes are now beginning to shed some light on the nature of activation caused by complexation. This activation is not a simple process. Many low-valent d8-d10 metal complexes and also some early transition metal compounds with higher oxidation state (d -d2 complexes) are capable of activating acetylenes. As already described, in the former complexes, interaction (c ) would lead to activation of an r)2-acctylcne ligand to an -acetylene having some radical as well as some anionic character ... 

Transition metal complexes were known to facilitate the addition of silylene to acetylenes from a variety of different sources.60,61,90,91 These conditions, however, often required heating, and the initially formed silacyclopropene often incorporated a second molecule of the acetylene to afford a silole.92,93 With their discovery of low-temperature silver-mediated di-ferf-butylsilylene transfer conditions from cyclohexene silacyclopropane 58 to olefins, Woerpel and coworkers set out to investigate the... 

Additions of aromatic C-H bond to olefins and acetylenes result in the formation of aryl-alkyl and aryl-alkenyl bonds. This type of addition reaction is not applicable to aryl-aryl bond formation. Catellani and Chiusoli [52] reported the first example of this type of arylation in 1985. To date, several arylation reactions of aromatic rings have been developed. In almost all cases, C-H bond cleavage proceeds through electrophilic substitution with transition-metal complexes [53]. 

The NiY zeolite was also shown to be active for the cyclotrimerization of propyne with 1,2,4-trimethylbenzene being the main product. The activities of the above-mentioned transition metal ions for acetylene trimerization are not so surprising since simple salts and complexes of these metals have been known for some time to catalyze this reaction (161, 162). However, the tetramer, cyclooctatetraene, is the principal product in homogeneous catalysis, particularly when simple salts such as nickel formate and acetate are used as catalysts (161). The predominance of the trimer product, benzene, for the zeolite Y catalysts might be indicative of a stereoselective effect on product distribution, possibly due to the spatial restrictions imposed on the reaction transition-state complex inside the zeolite cages. 

The cyclotrimerization of alkynes to give benzene derivatives is perhaps the most general reaction of these compounds in the presence of transition metal complexes. Practically any mono- or di-substituted alkyne, in addition to acetylene itself, may be cyclotrimerized. In addition, cocycloadditions involving more than one different alkyne are possible with some degree of selectivity, and intramolecular versions of the reaction have seen sophisticated development. 

Intermolecular oxidative addition of H—C usually involves activated H—C bonds. The weak acid HCN reacts with transition-metal complexes e.g., HCN and NiL lead to the hydride complexes HNi(CN)Lj (L = various phosphorus ligands). The versatile complex IrCl(CO)(PPh3)j adds HCN cleanly in CH Clj at RT to form HIr(CN)(Cl(PPhj)2. The zero-valent complexes Pt(PPhj) or Pt(PPh3)3 also add HCN to yield HPt(CN)(PPh3)j. Reactions of HMNp(dmpe)j (M = Fe, Ru, Os Np = 2-naphthyl dmpe = Me PCH CH PMej) with HCN and terminal acetylenes give HMR(dmpe)2 that contain new M—C bonds (R = — CN, — CjR ) . 

To obtain vinylsilanes from alkynes, transition metal complexes of Group VIII combined with a main group metal chloride are particularly effective." In the presence of a heterogeneous catalyst like Pd/y-alumina, Rh/carbon and polymer bound Pt, trichlorosilane gives trichlorovinylsilane with atmospheric pressure of acetylene." Platinum supported on sulfur-containing silica gel is a practical catalyst for 1,2-dihydrosilylation, as exemplified in equation (10). ... 

The reactions of acetylenes with metal carbonyls can be briefly mentioned at this point. Although a number of acetylene-transition metal complexes is known, in the vast majority of reactions involving metal carbonyls and acetylenes, the latter are converted into other ligands (often dienes) which in turn undergo complex formation. Reactions of this sort also often lead to new uncoordinated organic compounds and have been of distinct importance in organic synthesis. 

According to the first information on the catalysis of hydrosilylation by orga-noactinide complexes Cp 2A Me2 (where A = Th, U), they are efficient for hydrosilylation of terminal alkynes [82]. All catalytic and kinetic examinations of catalysis by early (d°) transition metal complexes (also by metal complexes with non-Cp ligands, e.g., [83]) support the generally accepted mechanism involving rapid olefin (acetylene) insertion into an M-H bond followed by a... 

It is appropriate at this point to indicate our personal motivation for carrying out structural studies, the types of compounds we study, and the experimental conditions we employ. In a very general sense we are interested in the bonding of small molecules and ions, e.g., 02, N2, NO, N2 R+, olefins, and acetylenes, to transition-metal complexes. Because of our interest in bonding, we seek the best solutions we can attain. Rapid, qualitative answers to conformational problems are not our interest. Since those transition-metal systems that bind small molecules generally have the metal in a low oxidation state, and since a low oxidation state is usually stabilized by ligands of the type PR3 (R = alkyl or aryl), solution to our problems involves typically the determination of a large number of structural parameters. With only a few exceptions the intensity data are obtained at room temperature on a Picker FACS-1 computer-controlled diffractometer. Usually the ratio of observations to variables is at least 10, and it is often 20 to 30. 

---