Intermediates unsaturated coordination

In a similar [4+2] reaction of a, -unsaturated esters, the aluminum catalyst complexed with the ligand S-VAPOL resulted in autoinduction , because of cooperative interaction of the product with the catalyst to generate a more selective catalytic species (Scheme 6.48) [68]. The ee% gradually increased as the reaction time lengthened. In the proposed intermediate, penta-coordinated aluminum complex 77, the cycloadduct is recognized as a complementary ligand, leading to substantial asymmetric induction. The acrylate is activated effectively within this hybridized complex which adopts pentacoordination [87]. 

The intermediate is coordinatively unsaturated and susceptible to an electrophilic attack at the carbene anion ligand. Product formation occurs by electrophilic addition at the ligand and completion of the octahedral coordination. 

There are many discussions on the coordination numbers of the iron species in the resting and intermediate states. It is noteworthy that in many cases a six coordination is first proposed, but importance of five coordination is demonstrated. As seen in catechol dioxygenase, substrates coordinate to iron with displacing attached ligands such as histidine with forming five coordinate species. The unsaturated coordination around the iron center may be important for the oxygen activation and oxygenation. 

Recent development of the Heck reaction has also led to greater understanding of its mechanistic details. The general outlines of the mechanism of the Heck reaction have been appreciated since the 1970s and are discussed in numerous reviews [2,3]. More recently, two distinct pathways, termed the neutral and cationic pathways, have been recognized [2c-g,3,7,8,9]. The neutral pathway is followed for unsaturated halide substrates and is outlined in Scheme 8G.1 for the Heck cyclization of an aryl halide. Thus, oxidative addition of the aryl halide 1.2 to a (bisphosphine)Pd(O) (1.1) catalyst generates intermediate 13. Coordination of... 

Cycloaddition of COj with the dimethyl-substituted methylenecyclopropane 75 proceeds smoothly above 100 °C under pressure, yielding the five-membered ring lactone 76. The regiocheraistry of this reaction is different from that of above-mentioned diphenyl-substituted methylenecyclopropanes 66 and 67[61], This allylic lactone 76 is another source of trimethylenemethane when it is treated with Pd(0) catalyst coordinated by dppe in refluxing toluene to generate 77, and its reaction with aldehydes or ketones affords the 3-methylenetetrahy-drofuran derivative 78 as expected for this intermediate. Also, the lactone 76 reacts with a, /3-unsaturated carbonyl compounds. The reaction of coumarin (79) with 76 to give the chroman-2-one derivative 80 is an example[62]. 

The intermediate M(COR) is the same as that for carbon monoxide insertion. It may be a coordinatively unsaturated solvated or unsolvated a-acyl or, alternatively, a 7r-acyl. It is of interest that photolysis of MeCOMn(CO)j in an Ar matrix at 15°K produces what appears to be a trigonal bipyramidal (Cj ) MeCOMn(CO)4 209). 

The proposed mechanism, due to Chalk and Harrod, is outlined in Eqs. (113)-(116) (n = 3 or 4), though it is recognized that this scheme is an oversimplification 54,57) it is probable that, in the absence of CO pressure, the coordinatively unsaturated tricarbonyls, not tetracarbonyls, are the catalytic intermediate 54). 

For Ru(OOOl) the corresponding reaction energy scheme is shown in Figure 1.7 [4]. The relative energies of the different reaction intermediates, Cjds or CHads, may strongly depend on the type of surface and metal. When for different surfaces or metals the relative interaction with Hads increases Cads may for instance become more stable than CH. This is found for more coordinative unsaturated surfaces or more reactive metals. 

The first step consists in the attack of a proton on the W-H bond to yield a labile dihydrogen intermediate (Eq. (3)) that rapidly releases H2 to form a coordi-natively unsaturated complex (Eq. (4)). This complex adds water in the next step to form an aqua complex (Eq. (5)) that completes the reaction by substituting the coordinated water by the X anion (Eq. (6)). Steps (3)-(6) are repeated for each W-H bond and the factor of 3 in the rate constants appears as a consequence of the statistical kinetics at the three metal centers. The rate constants for both the initial attack by the acid (ki) and water attack to the coordinatively unsaturated intermediate (k2) are faster in the sulfur complex, whereas the substitution of coordinated water (k3) is faster for the selenium compound. 

In the course of the tempestuous development of organophosphorus chemistry, interest has only recently been focused on compounds of formally quinquevalent phosphorus having coordination number 3, such as 1, 2, or 3, although one of the other species of this kind has long been postulated as reactive intermediate of solvolysis of phosphorylation reactions. Definite evidence of even proof of the existence of such coordinatively unsaturated phosphorus compounds, however, has been obtained only recently in mechanistic studies, by trapping reactions with suitable cycloaddends, or actually by direct isolation. 

Transition metal centered bond activation reactions for obvious reasons require metal complexes ML, with an electron count below 18 ("electronic unsaturation") and with at least one open coordination site. Reactive 16-electron intermediates are often formed in situ by some form of (thermal, photochemical, electrochemical, etc.) ligand dissociation process, allowing a potential substrate to enter the coordination sphere and to become subject to a metal mediated transformation. The term "bond activation" as often here simply refers to an oxidative addition of a C-X bond to the metal atom as displayed for I and 2 in Scheme 1. 

More recently, Schrock has reported the formation of coordinatively unsaturated Ta and W carbyne complexes (124). Like unsaturated carbene complexes, these carbyne compounds are now established as being active intermediates in a number of catalytic reactions. The discovery of acetylene metathesis reactions catalyzed by carbyne complexes (3), for example, has generated considerable interest in this class of compound. 

However, the pathways for these reactions, particularly in the gas phase, have been only -.rtially characterized. In a wide variety of these reactions, coordinatively unsaturated, highly reactive metal carbonyls are produced [1-18]. The products of many of these photochemical reactions act as efficient catalysts. For example, Fe(C0)5 can be used to generate an efficient photocatalyst for alkene isomerization, hydrogenation, and hydrosilation reactions [19-23]. Turnover numbers as high as 3000 have been observed for Fe(C0)5 induced photocatalysis [22]. However, in many catalytically active systems, the active intermediate has not been definitively determined. Indeed, it is only recently that significant progress has been made in this area [20-23]. 

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