Alkene complex formation

Another interesting example of dehydrative C-C coupling involves the alkylation of benzimidazole 36 with allyl alcohol 37, which is catalysed by complex 39 [15], The reaction is believed to proceed by alkene complex formation with the allyl alcohol 37 with loss of water from the NH proton of the NHC ligand and OH of the allyl alcohol to give an intermediate Ji-allyl complex. The initially formed 2-allylbenzimidazole isomerises to a mixture of the internal alkenes 38 (Scheme 11.9). 

Metal-alkene complex formation was also necessary, but again was not ratedetermining. 

Two mechanisms for the Cu-catalysed CCI4 addition to alkenes in the presence of triphenylphosphine have been proposed, based on kinetic and spectroscopic evidence. Both suggest Cu(I)-alkene complex formation. ... 

Complex a is readily converted into a Fe-y-H agnostic complex b within an early picosecond timescale and then the 7i-allyl hydride complex c is generated by hydride abstraction. The energy level of the 2-alkene isomer d, which is calculated by DPT experiments, is similar to that of the 1-alkene complex b. In the next step, Fe (CO)3(t -l-alkene)(ri -2-alkene) f, which is generated via intramolecular isomerization of the coordinated 1-alkene to 2-alkene and the coordination of another 1-alkene, is a thermodynamically favored product rather than formation of a Fe(CO)3(ri -l-alkene)2 e. Subsequently, release of the 2-aIkene from f regenerates the active species b to complete the catalytic cycle. 

Carbonylation reactions have been observed using both Pd(II)-alkene complexes and CT-bonded Pd(II) species formed by oxidative addition. Under reductive conditions, the double bond can be hydrocarbonylated, resulting in the formation of a carboxylic acid or ester.238 In nucleophilic solvents, the intermediate formed by solvopalladation is intercepted by carbonylation and addition of nucleophilic solvent. In both types of reactions, regioisomeric products are possible. 

Although there is a tendency for the alkene complexes to contain uncharged metals, a large number of complexes are known in which the metal ions are Pd2+, Fe2+, Cu+, Ag+, and Hg2 +. As we shall see, the formation of alkene complexes of these and other metals occurs as the metals catalyze certain reactions of the ligands. 

In summary, transifion-metal-catalyzed alkene-polymerization reactions highlight the metal-induced electrophilic activation of C—C n bonds to form carbo-cation-like alkene complexes. Considerations involving substituent pi-donor or pi-acceptor strength (i.e., tendency toward carbocation formation) will be useful in similarly rationalizing polymerization reactions (4.105) for more general alkenes. 

Stationary phases can be made highly selective by adding compounds to them which have affinities for certain chemical species. For example, silver nitrate, incorporated into a polar liquid preferentially retards the elution of alkenes by formation of weak -complexes. A selection of stationary phases with their maximum operating temperatures and useful applications is given in Table 4.10. 

In addition to /3-H elimination, olefin insertion, and protonolysis, the cr-metal intermediate has also proved to be capable of undergoing a reductive elimination to bring about an alkylative alkoxylation. Under Pd catalysis, the reaction of 4-alkenols with aryl halides affords aryl-substituted THF rings instead of the aryl ethers that would be produced by a simple cross-coupling mechanism (Equation (126)).452 It has been suggested that G-O bond formation occurs in this case by yy/z-insertion of a coordinated alcohol rather than anti-attack onto a 7r-alkene complex.453... 

The formation of hydride-Pt(II)-alkene complexes was thought to be the ratedetermining step, as evidenced by the isolation of a number of such species. 

The aforementioned observations have significant mechanistic implications. As illustrated in Eqs. 6.2—6.4, in the chemistry of zirconocene—alkene complexes derived from longer chain alkylmagnesium halides, several additional selectivity issues present themselves. (1) The derived transition metal—alkene complex can exist in two diastereomeric forms, exemplified in Eqs. 6.2 and 6.3 by (R)-8 anti and syn reaction through these stereoisomeric complexes can lead to the formation of different product diastereomers (compare Eqs. 6.2 and 6.3, or Eqs. 6.3 and 6.4). The data in Table 6.2 indicate that the mode of addition shown in Eq. 6.2 is preferred. (2) As illustrated in Eqs. 6.3 and 6.4, the carbomagnesation process can afford either the n-alkyl or the branched product. Alkene substrate insertion from the more substituted front of the zirconocene—alkene system affords the branched isomer (Eq. 6.3), whereas reaction from the less substituted end of the (ebthi)Zr—alkene system leads to the formation of the straight-chain product (Eq. 6.4). The results shown in Table 6.2 indicate that, depending on the reaction conditions, products derived from the two isomeric metallacyclopentane formations can be formed competitively. 

Zirconocene-catalyzed kinetic resolution of dihydrofurans is also possible, as illustrated in Scheme 6.8 [18]. Unlike their six-membered ring counterparts, both of the heterocycle enantiomers react readily, albeit through distinctly different reaction pathways, to afford — with high diastereomeric and enantiomeric purities — constitutional isomers that are readily separable (the first example of parallel kinetic resolution involving an organome-tallic agent). A plausible reason for the difference in the reactivity pattern of pyrans and furans is that, in the latter class of compounds, both olefmic carbons are adjacent to a C—O bond C—Zr bond formation can take place at either end of the C—C 7T-system. The furan substrate and the (ebthi)Zr-alkene complex (R)-3 interact such that unfavorable... 

Terminal RCH—CH2 1-Hexene C4H9CH=CH2 is isomerized by complex 1 in accordance with the factors influencing the thermodynamic stability of cis- and trans-2 -hexene [15], At the end of the reaction, the alkyne complex 1 was recovered almost quantitatively. No alkene complexes or coupling products were obtained. The corresponding zirconocene complex 2a did not show any isomerization activity. Propene CH3CH=CH2 reacts with complex 6 with substitution of the alkyne and the formation of zirconacydopentanes as coupling products, the structures of which are non-uniform [16]. 

Evans suggests that the catalyst resting state in this reaction is a 55c Cu alkene complex 58, Scheme 4 (35). Variable temperature NMR studies indicate that the catalyst complexes one equivalent of styrene which, in the presence of excess alkene, undergoes ready alkene exchange at ambient temperature but forms only a mono alkene-copper complex at -53°C. Addition of diazoester fails to provide an observable complex. These workers invoke the metallacyclobutane intermediate 60 via a formal [2 + 2] cycloaddition from copper carbenoid alkene complex 59. Formation of 60 is the stereochemistry-determining event in this reaction. The square-planar S Cu(III) intermediate 60 then undergoes a reductive elimination forming the cyclopropane product and Complex 55c-Cu, which binds another alkene molecule. 

In developing his theory of the polymerisations by ionising radiations Plesch now distinguishes between the mono-alkenes and other monomers, thus if a monomer, such as styrene, contains two donor groups, then both of these can be involved in the complex formation with the carbenium ion, but only the Jt-complex involving the double-bond can propagate. This means that Equation (40) must be replaced by... 

---