Insertion, migratory involving alkynes

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... 

Research on the mechanism of the Pd(II)-catalyzed ene reaction points to a hydropalladation cycle shown in Scheme 12.27, which first involves compl-exation of Pd to both n ligands of the substrate to yield 82 (step a). Migratory 1,2-insertion of the alkyne into the Pd-H bond provides 83 (step b), and then 1,2-insertion of the rf-coordinated double bond occurs to yield 84 (step c). Finally, step d is -elimination, which yields two possible regioisomers (85 and 86). 

Intramolecular Pd(0)-catalysed cyanoacylation of C=C bond in (40), involving the C—CN bond activation, has been developed as a new route to butenolides (41). DMF as a solvent, high temperatures, and short reaction times (attained by microwave irradiation) proved to minimize the competing decarbonylation. The reaction presumably proceeds with a migratory insertion of the alkyne, which is believed to be the product-determining step. ° ... 

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). 

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... 

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. 

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. 

Since most of the facile and general hydro- and carbometallation reactions involve syn-addition, the preparation of trans-a.jS-substituted alkenylmetals via 5yn-addition of alkynes would require carbometallation of terminal alkynes placing the metal in the internal position. Although such reactions exemplified by carbopalladationf are known, they are still more exceptional than normal. From the perspective of the current discussion, more commonly used are (i) some anfi-hydrometallation reactions of proximally heterofunctional internal alkynes and (ii) the hydroboration-migratory insertion tandem process of 1-haloaIkynes. Whereas the H migration produces (Z)-/3-substituted alkenylboranes (Sect. D.iii), the corresponding C migration provides trans-a,/3-substituted alkenyhnetals. (See Table 15.)... 

Pathways to form trans addition products by Qialk-Harrod and modified Qialk-Harrod mechanisms are also shown in Scheme 16.10. The formation of trans addition products is rationalized by the dynamics of ri -vinyl complexes or by a zwitterionic intermediate." AVinyl complexes are known to exchange stereochemistry and are thought to do so by one of the mechanisms shown at the bottom left of tire two cycles in Scheme 16.10. One pathway involves rotation of the C-C bond upon reopening of the -r)--vinyl complex and a second involves formation of a zwitterionic intermediate. The steric interactions in the initial cis T -vinyl intermediate make it less stable than the trans T -vinyl complex. Such trans insertions of alkynes were discussed in detail in Chapter 9 (migratory insertions). 

The mechanisms of these reactions are varied, but can still be categorized. The hydrocyanations, hydrosilylations, and many of the hydroborations, occur with late-metal catalysts. These reactions occur by oxidative addition of the H-X bond, followed by migratory insertion of the olefin into the M-H or M-X bond, and reductive elimination to form the final product. Hydrocyanation occurs by insertion of the unsaturated reagent into the M-H bond, while hydrosilylation and hydroboration have been shown to occur by insertion of the olefin into the M-H bond in some cases and into the M-X bond in others. HydrosUy-lations and hydroborations of alkenes and alkynes catalyzed by (P transition metal complexes and by lanthanides follow a different pathway because these complexes caimot undergo oxidative addition. The mechanism of the reactions catalyzed by these complexes involves u-bond metatheses. 

Schrock suggested that metallabenzenes might be involved as intermediates in the reactions of metal-lacyclobutadienes with alkynes. For example, treatment of metallabutadiene A (Scheme 26) with 2-bu-tyne leads to the formation of the cyclopentadienyl— metal complex E. perhaps through the intermediacy of metallabenzene C. In this case, alkyne insertion into a metallacyclobutadiene M—C bond is proposed, followed by carbene migratory insertion. An alternative mechanism involves the formation of a Dewar metallabenzene intermediate (D, Scheme 26). followed by reductive elimination. 

The formation of 2-alkenyl-substituted furans was observed in the palladium-catalyzed cross-coupling reactions between benzyl, aryl, or allyl bromides and conjugated ene-yne-ketones. This reaction involved oxidative addition, alkyne activation-cyclization, palladium carbene migratory insertion, P-hydride elimination, and catalyst regeneration (13JA13502). 

The stereochemistry for migratory insertion of alkynes has led to effective methods for creating each of the different stereoisomers of deuterated terminal alkenes. Eq. 12.51 shows a sequence of reactions involving f-butylacetylene. Due to the 100% stereoselective syn addition of the Zr-D bond, only one product is obtained. 

Stepwise double alkyne to vinylidene tautomerization is the key step responsible for the formation of the 77 -butadienyl iridium(in) complex [Ir K -0,C -O=G(Me)CH=CPh (77 -PhCH=CHC=CHPh)(PPh3)2]SbF6 579 346 proposed mechanism, which is illustrated in Scheme 82, involves an alkyne to vinylidene rearrangement (I —> II) followed by a hydride insertion (II — III), a second alkyne to vinylidene rearrangement (III — V), and a migratory insertion of the vinyl to the vinylidene (V — VI) resulting in the G-C bond formation. 

The cyclization did not work well with 4- or 6-carbon terminal aUcynols or with compounds containing nonterminal alkynes. The proposed mechanism involved initial oxidative addition of the OH group to the rhodium center with loss of CO and coordination of the pendant acetylene. Migratory insertion in a 5-exo-dig mode produces the coordinated cyclic vinyl ether, which could add an alcohol to the vinyl group and reductive elimination of the organic product regenerates the reactive metal complex. Alternatively, reductive elimination from the metal vinyl ether would produce a vinyl ether, which would be trapped by the alcoholic solvent (Scheme 14). 

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