Insertion, migratory involving alkenes

In hydroboration, a boron hydride (R2BH) adds across an alkene (R CH=CH2) to give R CH2CH2BR2. The 16-electron, d° Zr(IV) complex Cp2Zr(H)Cl, popularly known as Schwartz reagent, undergoes a closely related reaction. The mechanism involves coordination of an alkene to the electrophilic Zr center followed by migratory insertion of the alkene into the Zr-H bond. The reaction proceeds for alkynes also, by exactly the same mechanism. 

The Mizoroki-Heck reaction is a metal catalysed transformation that involves the reaction of a non-functionalised olefin with an aryl or alkenyl group to yield a more substituted aUcene [11,12]. The reaction mechanism is described as a sequence of oxidative addition of the catalytic active species to an aryl halide, coordination of the alkene and migratory insertion, P-hydride elimination, and final reductive elimination of the hydride, facilitated by a base, to regenerate the active species and complete the catalytic cycle (Scheme 6.5). 

The gas-phase reaction of cationic zirconocene species, ZrMeCp2, with alkenes and alkynes was reported to involve two major reaction sequences, which are the migratory insertion of these unsaturated hydrocarbons into the Zr-Me bond (Eq. 3) and the activation of the C-H bond via er-bonds metathesis rather than /J-hydrogen shift/alkene elimination (Eq. 4) [130,131]. The insertion in the gas-phase closely parallels the solution chemistry of Zr(R)Cp2 and other isoelec-tronic complexes. Thus, the results derived from calculations based on this gas-phase reactivity should be correlated directly to the solution reactivity (vide infra). 

The accepted mechanism for hydrogenation of alkenes by Wilkinson s catalyst involves the addition of dihydrogen prior to coordination of the alkene, followed by migratory insertion [31]. The new demonstrations of the existence of solvate dihydride complexes inevitably raise the question as to whether the same mechanism can apply in rhodium enantioselective hydrogenation. The evidence in support of this possibility is analyzed in more detail later. 

An informative set of calculations was carried out by Brandt et al, coupled to experimental studies that demonstrated first-order dependence of the turnover rate on both catalyst and H2, and zero-order dependence on alkene (a-methyl-(E)-stilbene) concentration [71]. The incentive for this investigation was the absence of any characterized advanced intermediates on the catalytic pathway. As a result of the computation, a catalytic cycle (for ethene) was proposed in which H2 addition to iridium was followed by alkene coordination and migratory insertion. The critical difference in this study was the proposal that a second molecule of H2 is involved that facilitates formation of the Ir alkylhydride intermediate. In addition, the reductive elimination of R-H and re-addition of H2 are concerted. This postulate was subsequently challenged. For hydrogenation of styrene by the standard Pfaltz catalyst, ES-MS analysis of the intermediates formed at different stages in the catalytic cycle revealed only Ir(I) and Ir(III) species, supporting a cycle (at least under low-pressure conditions in the gas... 

Insertion and -elimination. A catalytic cycle that involves only one type of elementary reaction must be a very facile process. Isomerisation is such a process since only migratory insertion and its counterpart P-elimination are required. Hence the metal complex can be optimised to do exactly this reaction as fast as possible. The actual situation is slightly more complex due to the necessity of vacant sites, which have to be created for alkene complexation and for P-elimination. 

Two possible scenarios exist which are consistent with this rate equation. Rate determining alkene coordination by 3c and 3t followed by rapid alkene insertion into the Rh-H bond is one possibility. An equally valid alternative explanation for the observed kinetics involves rate determining migratory... 

The n complex 1 has not been isolated or observed directly, but its involvement is strongly supported by indirect evidence. In the second step the alkene inserts into the cobalt-hydrogen bond to yield an alkylcobalt complex (2), which is transformed via the migratory insertion of CO into a coordinatively unsaturated acylcobalt complex (3). 

As discussed earlier, the generally accepted mechanism for the Heck reaction involves the steps of oxidative addition, coordination of the alkene, migratory insertion, and P-hydride elimination [2,3], With the intramolecular Heck reaction emerging as an important synthetic reaction over the past decade, the individual steps of this mechanism have come under closer scrutiny, and attention is beginning to be directed at determining the identity of the enantioselective step [41],... 

Excess hydridocobaltcarbonyl reduces a, -unsaturated ketones and aldehydes in moderate yield and good regioselectivity. The reaction involves complexation of the double bond to cobalt, followed by migratory insertion of hydride into the enone, forming an oxaallyl cobalt complex. Poor chemoselectivity is one of the major drawbacks of this reaction, as simple alkenes are rapidly hydroformylated to the corresponding aldehyde under the reaction conditions (25 °C, 1 atm of CO). 

Pd complex-catalyzed copolymerization of alkene and CO affords the polyketones via alternating insertion of the two monomers [166, 167]. The polymer growth involves migratory insertion of CO into the metal-carbon bond as a crucial step, which is unique to the late transition metal complexes such as Ni, Pd, Rh, and Co. The copolymerization of allenes and methylenecydopropanes with CO has attracted much less attention than the alkene-CO copolymerization, although it would provide further functionalized polyketones due to the dual functionality of the dienes and the derivatives. 

The stereochemical outcome was in agreement with a mechanism for the palladium-catalyzed cyclization/carboalkoxylation of a substituted alkene (Scheme 47) that involves outer-sphere attack of the indole on the palladium-olefin complex I which, coupled with loss of HCI, would form the alkylpalladium intermediate II. 1,1-Migratory insertion of CO into the Pd-C bond of II with retention of stereochemistry would form the acyl-palladium complex III, which could undergo methanolysis to release c/.v-product and form a palladium(0) complex. Oxidation with Cu(II) would then regenerate the active Pd(II) catalyst. 

In contrast to the above results, cis addition of nitrogen and metal atoms to alkenes was suggested in Th-catalyzed intramolecular cyclizafion of a,co-aminoalkenes [4b]. Some aminometallation reactions of alkenes or alkynes using aromatic amines also proceeded via cis addition [43,44]. Addition products in Scheme 8.25 and Eq. 8.9 were characterized by X-ray structure determination. The reaction may have proceeded via migratory insertion involving a metal-anilido intermediate, which was an actual starting material in Eq. 8.9. Notice that in this case the alkyne underwent migratory insertion into Pd-N bond, rather than to Pd-C bond. 

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