Transition metal complexes intermediates

The chemistry of the selective monoepoxidation of 1,3-dienes is presented. By using transition-metal complexes and a terminal oxidant as e. g. hypochlorite it is possible to perform both regioselective and enantioselective expoxidation of a selected double-bond of the 1,3-diene. This procedure allows c. g. one to perfom regioselective epoxidation of the less-substituted double bond of the 1,3-diene, and, furthemore, to avoid the polymerization of the 1,3-diene which is in contrast to conventional oxidation reagents. The scope of this reaction will be discussed and attempts to understand the oxygen-transfer from an oxo-transition-metal complex intermediate to only one of the double bonds of the 1,3-diene will also be discussed. 

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

Investigations of silicon-metal systems are of fundamental interest, since stable coordination compounds with low valent silicon are still rare [64], and furthermore, silicon transition-metal complexes have a high potential for technical applications. For instance, coordination compounds of Ti, Zr, and Hf are effective catalysts for the polymerization of silanes to oligomeric chain-silanes. The mechanism of this polymerization reaction has not yet been fully elucidated, but silylene complexes as intermediates have been the subject of discussion. Polysilanes find wide use in important applications, e.g., as preceramics [65-67] or as photoresists [68-83],... 

As already mentioned, complexes of chromium(iii), cobalt(iii), rhodium(iii) and iridium(iii) are particularly inert, with substitution reactions often taking many hours or days under relatively forcing conditions. The majority of kinetic studies on the reactions of transition-metal complexes have been performed on complexes of these metal ions. This is for two reasons. Firstly, the rates of reactions are comparable to those in organic chemistry, and the techniques which have been developed for the investigation of such reactions are readily available and appropriate. The time scales of minutes to days are compatible with relatively slow spectroscopic techniques. The second reason is associated with the kinetic inertness of the products. If the products are non-labile, valuable stereochemical information about the course of the substitution reaction may be obtained. Much is known about the stereochemistry of ligand substitution reactions of cobalt(iii) complexes, from which certain inferences about the nature of the intermediates or transition states involved may be drawn. This is also the case for substitution reactions of square-planar complexes of platinum(ii), where study has led to the development of rules to predict the stereochemical course of reactions at this centre. 

The racemization mechanism of sec-alcohols has been widely studied [16,17]. Metal complexes of the main groups of the periodic table react through a direct transfer of hydrogen (concerted process), such as aluminum complexes in Meerwein-Ponn-dorf-Verley-Oppenauer reaction. However, racemization catalyzed by transition metal complexes occurs via hydrogen transfer processes through metal hydrides or metal dihydrides intermediates (Figure 4.5) [18]. 

Two significant communications indicate the considerable potential of transition metal complexes as multifunctional homogeneous catalysts in the silane field (5, 53). Here the same catalyst activates silanes toward different substrates and it is probable that all proceed via a common metal hydrido intermediate. Both Co2(CO)8 and (Ph3P)3CoHX [X = H2, N2, or (H)Si(OEt)j] catalyze 0-silylation and hydrosilylation the hydrogen on Si may be replaced by R O, R COO, R CONH, or R3SiO [e.g., Eqs. (117)-(120)], and excellent yields of silylated product result. Phenolic groups do... 

Transition metals have been used to trap and stabilize many different types of reactive intermediates, such as carbenes. Reactive silicon intermediates have only recently yielded to this approach. In the case of alkenes, for instance, transition metal complexes are generally made by exposing the alkene to a transition metal bearing suitable leaving groups (e.g., carbonyl). Unlike carbon-based intermediates, however, silicon-based analogs have been very difficult to prepare until recently. Unless... 

Silene-transition metal complexes were proposed by Pannell121 for some iron and tungsten systems, and such species were observed spectroscopically by Wrighton.122,123 Thus intermediates such as 33 have been proposed in the preparation of carbosilane polymers from hydrosilanes,124 both as intermediates in the isotope scrambling observed to occur in similar ruthenium hydride systems125 126 and in the 5N2 addition of alkyllithium species to chlorovinylsilanes.47... 

The release of N2 occurs within function 3. It involves the dissociation of NO (via a dinitrosyl-adsorbed intermediate), followed by subsequent formation of N2 and scavenging of the adsorbed oxygen species left from NO dissociation. The removal of adsorbed oxygen is due to the total oxidation of an activated reductant (CxHyOz). This reaction corresponds to a supported homogeneous catalytic process involving a surface transition metal complex. The corresponding catalytic sequence of elementary steps occurs in the coordinative sphere of the metal cation. 

It has been widely accepted that the carbene-transfer reaction using a diazo compound and a transition metal complex proceeds via the corresponding metal carbenoid species. Nishiyama et al. characterized spectroscopically the structure of the carbenoid intermediate that underwent the desired cyclopropanation with high enantio- and diastereoselectivity, derived from (91).254,255 They also isolated a stable dicarbonylcarbene complex and demonstrated by X-ray analysis that the carbene moiety of the complex was almost parallel in the Cl—Ru—Cl plane and perpendicular to the pybox plane (vide infra).255 These results suggest that the rate-determining step of metal-catalyzed cyclopropanation is not carbenoid formation, but the carbene-transfer reaction.254... 

Redox reactions are considered as being able to provide versatile and efficient methods for bringing about ring transformations. Transition metal complexes in particular are able to induce or catalyze oxidative or reductive transformations of small ring compounds. Organometallics, such as metal-lacycles derived by the insertion of metal atoms into rings, are involved as key intermediates in many cases, allowing subsequent functionalization or carbon-carbon bond formation. 

The isomerization of allylic alcohols provides an enol (or enolate) intermediate, which tautomerizes to afford the saturated carbonyl compound (Equation (8)). The isomerization of allylic alcohols to saturated carbonyl compounds is a useful synthetic process with high atom economy, which eliminates conventional two-step sequential oxidation and reduction.25,26 A catalytic one-step transformation, which is equivalent to an internal reduction/oxidation process, is a conceptually attractive strategy due to easy access to allylic alcohols.27-29 A variety of transition metal complexes have been employed for the isomerization of allylic alcohols, as shown below. 

Certain transition metal complexes catalyze the decomposition of diazo compounds. The metal-bonded carbene intermediates behave differently from the free species generated via photolysis or thermolysis of the corresponding carbene precursor. The first catalytic asymmetric cyclopropanation reaction was reported in 1966 when Nozaki et al.93 showed that the cyclopropane compound trans- 182 was obtained as the major product from the cyclopropanation of styrene with diazoacetate with an ee value of 6% (Scheme 5-56). This reaction was effected by a copper(II) complex 181 that bears a salicyladimine ligand. 

Among the main goals of electrochemical research are the design, characterization and understanding of electrocatalytic systems, (1-2) both in solution and on electrode surfaces. (3.) Of particular importance are the nature and structure of reactive intermediates involved in the electrocatalytic reactions.(A) The nature of an electrocatalytic system can be quite varied and can include activation of the electrode surface by specific pretreatments (5-9) to generate active sites, deposition or adsorption of metallic adlayers (10-111 or transition metal complexes. (12-161 In addition the electrode can act as a simple electron shuttle to an active species in solution such as a metallo-porphyrin or phthalocyanine. 

However, for Group Ylb transition metal complexes (M = Cr, Mo, and W) in basic solution, their studies led them to propose a different type of catalytic cycle involving the decomposition of formate intermediate.25,33,54 Below (Scheme 18a) is an example for Mo, but one can readily replace Mo with Cr or W. 

Reviews on the activation of dioxygen by transition-metal complexes have appeared recently 9497 ). Details of the underlying reaction mechanisms could in some cases be resolved from kinetic studies employing rapid-scan and low-temperature kinetic techniques in order to detect possible reaction intermediates and to analyze complex reaction sequences. In many cases, however, detailed mechanistic insight was not available, and high-pressure experiments coupled to the construction of volume profiles were performed in efforts to fulfill this need. 

Alkynes react with the bulky germanium hydride (MejSdjGeH to selectively yield (Z)-alkenes (Equation (105)).67 The hydrogermylation of alkynols or alkynes can be catalyzed by a rhodium complex (Equation (106), Table 18) and some of the intermediates were identified (Scheme 16).132 Similar rhodium species react with alkynes to yield alkenyl complexes,133 and other transition metal complexes have been employed as hydrogermylation catalysts including those containing palladium.134,135... 

---