Alkynes migratory insertion

The addition proceeds through (a) oxidative addition of the B-X bond to a low-va-lent metal (M=Pd, Pt) giving a ds-B-M-X complex (92), (b) migratory insertion of alkene or alkyne into the B-M bond (93 94), and finally (c) reductive elimination... 

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). 

Possible mechanisms for this reaction are shown in Scheme 8. Pathway I involves an initial cleavage of the VCP (5C to 5D) followed by migratory insertion of the alkyne (5D to 5G), whereas pathway II involves first oxidative cyclization... 

The higher catalytic activity of the cluster compound [Pd4(dppm)4(H2)](BPh4)2 [21] (20 in Scheme 4.12) in DMF with respect to less coordinating solvents (e.g., THF, acetone, acetonitrile), combined with a kinetic analysis, led to the mechanism depicted in Scheme 4.12. Initially, 20 dissociates into the less sterically demanding d9-d9 solvento-dimer 21, which is the active catalyst An alkyne molecule then inserts into the Pd-Pd bond to yield 22 and, after migratory insertion into the Pd-H bond, the d9-d9 intermediate 23 forms. Now, H2 can oxidatively add to 23 giving rise to 24 which, upon reductive elimination, results in the formation of the alkene and regenerates 21. 

Allylzirconation of alkynes with allylzirconocene chloride reagents (obtained by hydrozir-conation of allenes) takes place in the presence of a catalytic amount of methylaluminox-ane (MAO) [67,68]. MAO presumably abstracts chloride to form an allylzirconocene cation, which coordinates to the alkyne triple bond. The subsequent migratory insertion is regioselective, as it is found that the new bond is mainly formed between the a-carbon of the allylzirconium species and the internal carbon of a terminal alkyne (Scheme 8.33). 

From the energetically preferred n-alkyne complex there is an alternative pathway involving the hydride ligand (Figure 5). The first step is an easy (AE = 6.6 kcal.mol 1) migratory insertion of the C=C triple bond into the cis Ru-H bond to yield a a-vinyl complex, A, 10.4 kcal.mol 1 below the it-alkyne complex. This 14-electron o-vinyl complex has also a saw-horse... 

Conversion of a Co2(CO)6-alkyne complex into a cyclopentenone is the Pauson-Khand reaction. It proceeds by loss of CO from one Co to make a 16-electron complex, coordination and insertion of the C6=C7 K bond into the C2-Co bond to make the C2-C6 bond and a C7-Co bond, migratory insertion of CO into the C7-Co bond to make the C7-C8 bond, reductive elimination of the C1-C8 bond from Co, and decomplexation of the other Co from the C1=C2 k bond. The mechanism is discussed in the text (Section B.l.f). 

The alkyne-cobalt carbonyl complex 3 formed from the alkyne 1 and dicobalt octacarbonyl 2 should lose at least one of the GOs on the metal to provide the vacancy for the incoming olefins. Subsequently, an olefin-bound complex 5 rearranged oxidatively to yield a metallacyclic intermediate 6. Migratory insertion of GO of 6 would provide the homologated ring intermediate 7, and the following two successive reductive eliminations afford the cyclopentenone... 

Tamao and Ito proposed a mechanism for the nickel-catalyzed cyclization/hydrosilylation of 1,7-diynes initiated by oxidative addition of the silane to an Ni(0) species to form an Ni(ii) silyl hydride complex. Gomplexation of the diyne could then form the nickel(ii) diyne complex la (Scheme 1). Silylmetallation of the less-substituted G=C bond of la, followed by intramolecular / -migratory insertion of the coordinated G=G bond into the Ni-G bond of alkenyl alkyne intermediate Ila, could form dienylnickel hydride intermediate Ilia. Sequential G-H reductive elimination and Si-H oxidative addition would release the silylated dialkylidene cyclohexane and regenerate the silylnickel hydride catalyst (Scheme 1). 

In the process of olefin insertion, also known as carbometalation, the 1,2 migratory insertion of the coordinated carbon-carbon multiple bond into the metal-carbon bond results in the formation of a metal-alkyl or metal-alkenyl complex. The reaction, in which the bond order of the inserted C-C bond is decreased by one unit, proceeds stereoselectively ( -addition) and usually also regioselectively (the more bulky metal is preferentially attached to the less substituted carbon atom. The willingness of alkenes and alkynes to undergo carbometalation is usually in correlation with the ease of their coordination to the metal centre. In the process of insertion a vacant coordination site is also produced on the metal, where further reagents might be attached. Of the metals covered in this book palladium is by far the most frequently utilized in such transformations. 

Dihydroazepines have been synthesized by the first rhodium-catalyzed hetero-[5+2] cycloaddition of cyclopropylimines and alkynes (Scheme 8.62) [138]. The reaction proceeds via formation of metallacycle 147 which undergoes migratory insertion of dimethyl acetylenedicarboxylate (DMAD) to form 148. Finally, dihydroa-zepine 149 is obtained via reductive elimination. 

Scheme 1. Proposed mechanism of migratory insertion/addition of the coordinated alkyne ligand to the /i-CSiMe3 ligand generating the W2(/r-CRCR CSiMe3)moiety. The steric preference for R = H relative to alkyl or aryl is implied in the proposed transition state, B.

Migratory insertion is the principal way of building up the chain of a ligand before elimination. The group to be inserted must be unsaturated in order to accommodate the additional bonds and common examples include carbon monoxide, alkenes, and alkynes producing metal-acyl, metal-alkyl, and metal-alkenyl complexes, respectively. In each case the insertion is driven by additional external ligands, which may be an increased pressure of carbon monoxide in the case of carbonylation or simply excess phosphine for alkene and alkyne insertions. In principle, the chain extension process can be repeated indefinitely to produce polymers by Ziegler-Natta polymerization, which is described in Chapter 52. 

Electrophilic attack at carbyne complexes may ultimately place the electrophile on either the metal or the (former) carbyne carbon, the two possibilities being related in principle by a-elimination/migratory insertion processes (Figure 5.39). The reactions of the osmium carbyne complex are suggestive of an analogy with alkynes. Each of these reactions (hydro-halogenation, chlorination, chalcogen addition, metal complexation see below) have parallels in the chemistry of alkynes. 

Chalk and Harrod provided the first mechanistic explanation for the transition metal catalyzed hydrosilation as early as in 1965. Their mechanism was derived from studies with Speier s catalyst and provided a general scheme, which could be used also for other transition metals. The catalytic cycle consists of an initial oxidative addition (see Oxidative Addition) of the Si-H bond, followed by coordination of the unsaturated molecule, a subsequent migratory insertion (see Insertion) into the metal-hydride bond and eventually a reductive elimination (see Reductive Elimination) (Scheme 3 lower cycle). The scheme provides an explanation for the observed Z-geometry in the hydrosilation of alkynes, which is a consequence of the syn-addition mechanism. The observation of silated alkenes as by-products in the hydrosilation of alkenes along with the lack of well-established stoichiometric examples of reductive elimination of aUcylsilanes from alkyl silyl metal complexes... 

Alkyl iridium compounds are also accessible via insertion (see Migratory Insertion) of alkenes into Ir H bonds. Analogously, alkenyl iridium compounds may be formed via insertion of alkynes into Ir-H bonds. These types of reactions have been studied to shed tight on the mechanism of alkene and alkyne hydrogenation processes. For example, HIr(CO)(PPh3)2 (65) will react with ethylene and higher olefins to produce the alkyl iridium compounds (equation 17). 

Acylzirconocene chlorides are easily accessible in a one-pot procedure through the hydrozirconation see Hydrozirconation) of alkene or alkyne derivatives with the Schwartz s reagent and subsequent migratory insertion see Migratory Insertion) of carbon monoxide into the alkyl- or alkenyl zirconium bond. The stability of the acylzirconocene chlorides is remarkable at room temperature, and consequently allows many applications in organic synthesis. 

Little work has been reported on insertion of alkynes into Al-H bonds. ( 4119)2 AlH reacts readily with internal alkynes to give uniquely the cis addition product, as expected for a migratory insertion involving concerted addition via a four-center transition state". On the other hand, addition of LiAlH4 to internal alkynes results in trans addition by attack on the triple bond by hydride ion. 

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. 

We can also think of 1,2-insertion as migration of the hydride to the (3-position on the alkene or alkyne, much the same as we saw for 1,1-CO migratory insertion. 

Migration of more complex alkyl groups was recently reported [70]. Reversible migratory insertion/(3-carbon elimination occurs between the coordinated alkyne and the bound alkyl group of alkyl-niobium(alkyne) complex 52. 

Electron poor alkynes are readily trapped by migratory insertion of the highly fluorinated phenyl ligand but competitive formation of 4-electron donor alkyne complexes is observed for electron rich alkynes [48]. If a CO ligand is removed by photolysis, migratory insertion is rapid at room temperature and detailed kinetic studies of the thermal reaction [49] have been reported. The above... 

Alkyne polymerization in organic media has been reviewed [131]. A large variety of catalysts has been reported to polymerize alkynes in organic media. Similar to the polymerization of olefins, early transition metal as well as late transition metal catalysts are effective for this polymerization. Depending on the nature of the metal, two different mechanisms of polymerization have been suggested polymerization via a metal alkyl intermediate, or via a metal carbene (Scheme 7.9). With metal alkyl complexes, polymerization proceeds via migratory insertion of the alkyne into the metal-carbon bond [path (a) in Scheme 7.9] whereas with metal carbenes the mechanism is equivalent to that of metathesis [path (b)]. 

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