Insertion of acetylene

Insertion of acetylene into a cobaltaxanthene yields dibenz[A,/]oxepin (41 %), see Houben-Weyl, Vol. 13/9, p 279. 

The proposed mechanism (Scheme 7-20) includes (a) oxidative addition of Y-Ge (Y = S or Se) bonds to Pd(PPh3)n, (b) insertion of acetylene into the Pd-Y bond to give 92 or insertion of acetylene into the Pd-Ge bond to form 93, (c) formation of 91 by either a G-Ge or a G-Y bond-forming reductive ehmination with regeneration of Pd(PPh,),. 

Acrylonitrile or methyl acrylate readily inserts into allylnickel bonds (example 34, Table HI). A trans double bond is formed by loss of a proton. Insertion of acetylene followed by oxidative elimination with allyl halides gives cis double bonds (example 32, Table III). Insertion of methyl propiolate, followed by proton uptake, leads to a trans double bond (example 33, Table III). Norbomene has been shown to insert stereoselectively cis.exo into an allylnickel bond (example 35, Table III). 

Besides direct carbonylation, insertion of acetylene and of other molecules and groups into C—Ni bonds is possible. A variety of linear or cyclic products results. Thiourea proved an exceedingly efficient ligand (180). 

A unique bis-silylation system, in which a bis(silyl)palladium intermediate is generated via recombination of two Si-Si bonds, has been developed.8,97 A bis(disilanyl)dithiane reacts with alkynes in the presence of a palladium/ isocyanide catalyst, giving five-membered ring bis-silylation products in high yield with elimination of hexamethyl-disilane (Scheme 14). The recombination, that is, bond metathesis, is so efficient that no product derived from direct insertion of acetylene into the Si-Si bonds of the bis(silyl)dithiane is formed at all. 

The palladium catalysed reductive insertion of acetylenes is also viable through the use of formate ions as hydride equivalent (c.f 3.24.), The ring closure of /V-acetylenic-2-iodoanilines gave the corresponding 3-alkylideneindolines in a selective manner (3.27.), demonstrating that the... 

The insertion of acetylene derivatives might also be utilised in the preparation of six membered rings. A characteristic distinction between such processes and olefin insertion is the fact, that the intermediate formed by the insertion of an acetylene into the palladium-carbon bond is unable to undergo /2-hydride elimination, therefore the concluding step of these processes is usually reductive elimination. 

The mechanism of this reaction appears similar to the allyl chloride car-bonylation discussed above, with an additional insertion of acetylene in the allylnickel intermediate before the CO insertion. A possible formulation is the following ... 

Reaction of the oct-4-yne complex with HBF4 liberates cis -oct-4-ene, and this is postulated as occurring via initial protonation of the metal (to yield [Cpf Ta(H)2(alkyne)]+), followed by insertion of acetylene into the Ta-H bond and C-H reductive elimination, cis-Oct-4-ene also results when H2 is reacted with the hydrido complex at 100°C 178). 

Under conditions similar to those for allyl halides, 1,4-dichlorobutene reacts with nickel carbonyl to give butadiene. However, a double insertion of acetylene and carbon monoxide can be successfully carried out using 4-chloro-2-buten-l-ol and generating hydrogen halide in situ with a weak acid inorganic halide combination, e.g., NaBr-H3P04 (58). 

One of the first transition metal-catalyzed ring-expansion reactions of SCBs with the formation of new C-C bonds involved the insertion of acetylenes catalyzed by Pd-complexes to furnish silacyclohexenes (Scheme 46) <1975CL891, 1991BCJ1461>. In addition to the acetylene-insertion products (silacyclohexenes), ring-opened allyl-vinylsilane products that also incorporate the acetylene moieties were observed. The ratio of the two types of the products depends heavily on the nature of acetylenic compounds. 

Acetylenes are also oligomerized to mono- or divinylacetylenes, or dienyl-acetylenes by Ni(0) (112), Rh(I) (118), or Pd(II) (114) complexes (Scheme 5). Meriwether et al. (112) proposed hydrido-a-alkynylnickel complexes as active intermediates in the catalytic linear oligomerization. Subsequent insertion of acetylene into an M-o--alkynyl bond has been assumed. 

The insertion of acetylenes into carbon-heteroatom double bonds (see 364-365) was extended to include carbene complexes and the reaction between Et2NC=CMe and [M(C(Ph)SR (CO)5](M = Cr or W, R =... 

Oligomerization of alkynes probably occurs by multiple insertions of acetylene into metal-carbon bonds. An insertion reaction of alkynes is shown as follows ... 

Insertion Reactions into Element-Halogen Bonds 11.6.2. Insertions of Acetylenes and Olefins... 

Pd(0)-catalyzed insertion of acetylenes into germanium-tin bonds have been reported by Piers (Scheme 11.34) [54]. Because germyl groups attached to sp carbon can be readily transformed into iodide with retention of the stereochemistry, this reaction is a novel method for the preparation of stereochemicaUy defined tet-ra-substituted alkenes. 

The insertion of acetylene into the Pd-CH3 bond of the complex PdCl(NH3)(CH3) has been studied by de Vaal and Dedieu [63] by using the valence double-zeta basis sets. The geometries of the prereaction complex [PdCl(NH3)(CH3)(C2H2)], the transition state, and product have been optimized at the SCF level, and their energetics has been improved at the CASSCF and Cl level. It has been shown that acetylene is quite weakly bound (5.8 kcal/mol) in the square-planar Pd(II) complexes because of weak 7T back-donation from Pd to the tt orbital of C2H2. The insertion barrier calculated relative to the acetylene complex is 20.5, 22.6, and 17.1 kcal/ mol at the SCF, CASSCF, and Cl levels of theory, respectively, and the transition state corresponding to this barrier displays monohapto coordination of acetylene. The entire insertion reaction is calculated to be exothermic by 26.0, 19.3, and 22.4 kcal/mol at the SCF, CASSCF, and Cl levels, respectively, relative to the acetylene complex. 

Nakamura et al. [65] have applied the RHF and MP2 methods in conjunction with split-valence basis sets to study the insertion of acetylene into the M-CH3 bond of the complexes LiMe, CuMe, and Me2Cu. As shown in Fig. 21, the reactions of LiMe and CuMe with parent acetylene have been found to proceed via (a) and tt complex, 37, which is, by 11 kcal/ mol, stable relative to reactants at the HF level for both complexes, and (b) the transition state, 38, which is calculated to be 20 and 46 kcal/mol relative to the IT complex at the HF level, respectively, to the product, 39, which is 38 and 35 kcal/mol lower than that of the reactants, respectively. Thus, both reactions are much more exothermic than the addition to ethylene [66]. However, the activation energies of insertion of acetylene into the M-Me bond obtained in this paper is unrealistically high. This has been explained by the deficiency of the monomeric, ligand-free model used in the study. The qualitatively similar results have been found also for CuMe, except that the reaction proceeds through the tt complex, which does not exist in... 

As shown in Table 3, cis- and trans-coni nU of poly acetylene prepared by the Ziegler-Natta catalysts depends strongly upon the polymerization temperature ss x ere are two possible explanations for this observation One is that the fundamental mechanism is the formation of cis double bonds by the cis insertion of acetylene monomer into the Ti—C bond of the catalyst. This fits the orbital interaction consideration for the role of the catalyst by Fukui and Inagaki, according to which the initially formed configuration of the double bond is cis as a result of the favorable orbital interaction between the inserting acetylene monomer and the active site of the catalyst. Because the cis double bond is thermodynami-... 

The insertion of acetylene into a Pt-B(OH)2 bond has a lower barrier than insertion of ethylene (see Table 6.11) [179]. This is important in some Pt-catalyzed diboration processes that are less efficient for alkenes (where insertion is rate determining and slow) but work well for alkynes. 

The mechanism of the processes where the alkene or alkyne is functionalized by two ER groups necessarily involves the insertion of the substrate into one M-ER bond. The actual mechanism depends on the specific reaction, type of substrate, and catalyst. Eor diboration or disilation of alkenes, theoretical studies have found this step rate determining when the catalyst is a Pt complex [176,179]. The insertion of acetylene into Pt-BR2 bonds is faster than the insertion of ethylene, and this step is not rate determiiung for diboration of alkynes [178,179]. As was pointed out before (see Section 6.4.1 (a)), insertion of an alkyne into the Pd-SnRs bond is preferred over insertion into the Pd-SiRa in the silylstannation of alkynes [177]. 

Equation 2-57 describes the insertion of acetylene into a Pt-H bond to give a vinylplatinum complex. 

The chloroallylation of alkynes with allyl chlorides affords 1-chloro-1,4-dienes 358 using PdCl2 as a catalyst. The reaction can be understood by chloropalladation of alkyne to generate 356, followed by insertion of the double bond of allyl chloride to give 357. The last step is the well-established dechloropalladation to afford the chlorodiene 358 as the Pd(II)-generation step. In this case, no jS-H elimination occurs. Interaction of Pd-Cl plays a role [144]. Pd(OAc)2-catalyzed reaction of acetylene and allyl chloride in the presence of LiCl affords an E, Z-mixture of the l-chloro-l,3,6-heptariene (360). In this case, first insertion of acetylene occurs to give 359, and then the insertion of allyl chloride follows [145]. 

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