Example 3, on the other hand, takes place by a two-step mechanism. The reductive elimination in equation 4 is essentially the reverse of reaction 1. Equation 5 shows an interesting elimination of ethane from two metal alkyl methyl substituents. Several types of insertion-elimination reactions are shown on the third page of this display.
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Equation 1 describes a sequence of double bond insertion intramolecular followed by a beta-elimination. The oxidation state of the metal remains unchanged throughout the process. The alkyl group shifts with retention of configuration. Excess CO, or addition of a phosphine ligand, forces the equilibrium toward the insertion product. Removal of a ligand tends to favor elimination, as in equation 4. Here, the alpha-elimination to a metal-carbene proceeds with oxidation of the metal. Finally, the last example illustrates an oxidative coupling reaction, similar to oxidative addition.
The effectiveness of many transition metal compounds as catalysts for reactions comes from the facility of these metals to complex reversibly with a variety of functional groups. In the case of double and triple bonds such complexation involves electron sharing between overlapping carbon-carbon pi-orbitals and certain d-orbitals of the metal.
As illustrated in the following diagram, the electron pair of the pi-orbital may be shared with an empty d-orbital to form a sigma-like bond light blue arrow.
This leaves the double bond function electron deficient, a condition that may be remedied by the sharing of a d-electron pair with the empty antibonding-pi-orbital a pi-like backbonding. A strong sigma bond is complemented by strong pi-backbonding, which essentially reduces the carbon-carbon bond order to one from two. The carbon atoms must then rehybridize and the resulting complex resembles a three-membered ring incorporating the metal atom a metallocycle. If the sigma bond is weak, backbonding will also be weak, and the structure of the double bond component of the complex will not be significantly perturbed.
An instructive example of transition metal activation of carbon-carbon double bonds is found in the homogeneous hydrogenation catalyst known as Wilkinson's catalyst. Heterogeneous reactions of this kind take place on surfaces that are often poorly characterized, so Wilkinson's discovery that a soluble rhodium complex, ClRh[P C 6 H 5 3 ] 3 , reduced alkenes to alkanes cleanly and stereospecifically provided an opportunity to study the structure and behavior of intermediate species. The information thus obtained has resulted in the following general mechanism for the hydrogenation of alkenes, and for the decarbonylation of aldehydes that takes place by action of the same catalyst.
Models of two Wilkinson species may be examined by clicking the button. Several important and general reactions are illustrated in the above mechanism. A coordinatively saturated rhodium atom would have 18 valence shell electrons. Reversible loss of one or more ligand units generates reactive 16 and 14 electron complexes which play a role in the overall transformation. Oxidative addition is a common process for converting a coordinatively unsaturated metal to a saturated complex. The addition of hydrogen in the first step is such a case, the "oxidative" adjective referring to the fact that Rh I is oxidized to Rh III in the event.
The resulting electron complex then undergoes a reductive elimination of C-H, either before or after a new ligand molecule binds to rhodium. Addition of hydrogen then regenerates the catalyst.
Similar ruthenium and iridium catalysts have been developed, and when chiral ligands are used, enantioselective addition of hydrogen is observed. The decarbonylation reaction proceeds in a similar manner, with oxidative addition of OC-H being followed by rearrangement and reductive elimination of R-H. Carbon monoxide is a common transition metal ligand, and reactions conducted in the presence of excess CO often lead to products incorporating a new carbonyl function.
In the following discussion, however, we shall focus on several new and very useful applications of these catalysts to reactions in which carbon-carbon bonds are formed.
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Nickel catalysts have been demonstrated to be effective in coupling allylic halides to each other and to various aryl, vinyl and alkyl halides. An intramolecular example of this is shown in the following example. Because nickel carbonyl is an extremely toxic gas, alternative nickel catalysts have been examined with variable results. Palladium lies just below nickel in the periodic table and, although it is expensive, a large body of useful applications of this metal have been reported.
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Some of these are outlined in the following diagram. The Trost, Heck, Negishi, Stille and Suzuki reactions have seen widespread use in the synthesis of complex molecules, and additional examples of each may be examined by clicking on one of the blue titles 2, 3, 4 or 5 in the diagram.
Clicking on an empty section of the diagram will return the viewer to the original display. The Nobel Prize in chemistry was jointly awarded to Heck, Negishi and Suzuki for their work in developing these coupling reactions. A summary of this chemistry will be displayed by Clicking Here.
In the allylic substitution examples 2 , the top equation illustrates Trost's initial discovery of allylic substitution for simple alkenes. This reaction required stoichiometric amounts of the expensive palladium reagent. To make this useful reaction catalytic in palladium Trost used substituted alkenes bearing an ester leaving group at the allylic site. Examples are given below the horizontal gray line. Equation 4 shows a different kind of palladium catalyzed reaction, in which two addition steps are followed by a reductive elimination that regenerates the catalytic species.
This process was first demonstrated over fifty years ago by the German chemist W. Germany had few petroleum resources and lots of coal, from which acetylene can be made, so this offered an alternative source of liquid hydrocarbon fuel for their war machine. The Reppe work is outlined in equation 1 in the following diagram. The best nickel catalyst was Ni CO 4 , a toxic gas, and recent work by K. Vollhardt Berkeley has shown that dicarbonylcyclopentadienylcobalt provides a safer and more selective catalyst for this very useful synthesis reaction 2.
If a nitrile function is substituted for the third alkyne component, substituted pyridines are formed. A cobalt carbonyl complex also serves to catalyze a useful synthesis of cyclopentenones, known as the Pauson-Khand reaction reaction 3. To view examples of these reactions applied to synthetic targets Click on the Diagram. As for many other carbon-carbon bond forming reactions, intramolecular applications are especially useful for constructing polycyclic structures, and are often effected in sequence with other cyclizations.
In the second example, a double bond replaces one of the triple bonds in the cyclotrimerization reaction. The product in such cases is usually a Co I complex of a 1,3-cyclohexadiene, which may be oxidatively decomposed to the diene itself. The third example is from Schreiber's synthesis of epoxydictymene,a diterpene. Here, a hexacarbonyldicobalt complex of an alkyne serves as a neighboring group during the acid-catalyzed opening of the cyclic acetal, and then functions as a reactant in the Pauson-Khand reaction.
Transition metal complexes incorporating a formal metal-carbon double bond are termed alkylidene or carbene complexes. Two general classes are recognized, Schrock alkylidenes and Fischer carbenes. General structures for these reactive compounds are shown below, together with some of their characteristics. The preparation of typical alkylidene complexes of both types are provided in the following diagram, beginning with Schrock alkylidenes. The properties and chemical behavior of Schrock and Fischer alkylidenes are substantially different, reflecting the metal-carbene bonding.
As noted in the following diagram, the metal of a Schrock alkylidene is electrophilic, and is stabilized by electron donating ligands as well as backbonding from an occupied p-orbital of the carbon atom. This results in a strong metal-to-carbon double bond. The overall bonding in the complex leaves the carbon atom nucleophilic. Fischer carbenes have nearly opposite properties.
The metal is electron rich, in part because of coordinate donation of an electron pair from the carbene carbon atom and CO ligands. Competition for the d-electrons of the metal takes place between electron withdrawing ligands, such as CO, and the empty p-orbital on carbon. This in turn, is stabilized by overlap with electron pairs on the heteroatom substituents note the resonance structures. Many of the chemical properties of Fischer carbenes are ester-like. Nucleophilic substitution takes place by way of a tetrahedral intermediate.
Also, alpha-hydrogen atoms, e. The alkylidene complexes described above undergo many interesting and synthetically useful reactions.
Since the carbene moiety of a Fischer carbene has many of the characteristics of a carbonyl group, it is instructive to begin our survey with some ester-like transformations. The following diagram incorporates four pages of reactions that may be viewed by repetitive clicking on the diagram itself. The first page displays a few carbonyl-like reactions, which mimic organolithium addition to ketones eq.
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Equation 5 shows a sequence involving epoxide alkylation, followed by an intramolecular trans-esterification analog. The second page of reactions begins with an equation illustrating the reductive and oxidative removal of the metal component of the carbene. In most cases the carbene is a stronger activating group than the corresponding ester.
Fischer Carbene Reactions. The third and fourth pages of reactions are unique to transition metal complexes of the Fischer and Schrock type. Metallocyclobutanes are believed to be key intermediates in these transformations, as shown. Reversible loss of CO ligands frees coordination sites and facilitates the oxidative addition of alkenes or alkynes. Since the cyclopropanation reaction on page three requires an electron withdrawing group on the alkene reactant, a bonding interaction of the electrophilic beta-carbon with the nucleophilic metal is expected. If the resulting metallocyclobutane is saturated, a reductive elimination gives a cyclopropane product.
However, metallocyclobutenes derived from alkyne reactants undergo electrocyclic ring opening to a new alkylidene species. This in turn may react with other suitably located double bonds. The unsaturated carbene supplies three carbon atoms and a carbon monoxide unit to the product phenol, and the alkyne reactant provides the remaining carbons. Oxidative addition produces a metallocyclobutene in the initial step, which is then followed by ring-opening, insertion and ring closure steps.
The aromatic product is released from the stable cobalt tricarbonyl complex by treatment with mild oxidizing agents. Most of the Fischer carbenes used in the reactions listed above are zero valent metal complexes. Schrock alkylidenes generally incorporate metals having higher oxidation states, and because of the electrophilic nature of the metal their reactivity is significantly different. The following diagram shows two typical transformations of these alkylidene complexes. Olefination of carbonyl functions may be effected by Tebbe's reagent, which is the source of a Ti IV methylene complex.
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