Insertion involving alkynes

Imines derived from o-iodoaniline and arenecarboxaldehydes react with internal arylalkynes and catalytic Pd(0) to afford isoindolo[2,l-a]indoles by a process that involves alkyne insertion, addition across the C=N double bond and substitution of the aromatic ring (Scheme 11).12 This process exhibits very... 

For the diboration reactions of alkynes catalyzed by Pt(0) complexes, the reaction mechanism involves the oxidative addition of diborane to the Pt(0) center, followed by the insertion of alkyne into the Pt-B bond and reductive... 

In an alternative strategy functionalized phenols, such as iodophenol, were involved in palladium-catalyzed carbonylation of alkynes or allenes, producing coumarin or chromone derivatives (Scheme 23) [130-133]. After oxidative addition of the iodoarene to the Pd(0) catalyst the order of insertion of either CO or the unsaturated substrate mainly depends on the nature of the substrate. In fact, Alper et al. reported that CO insertion occurs prior to allene insertion leading to methylene- or vinyl-benzopyranone derivatives [130]. On the contrary, insertion of alkynes precedes insertion of CO, affording couma-rine derivatives, as reported by Larock et al. According to the authors, this unusual selectivity can be explained by the inability of the acyl palladium species to further react with the alkyne, hence the decarbonylation step occurs preferentially [131-133]. 

This mechanism is supported by the transformation of preformed metallacyclo-pentadienes with alkynes,73,76-78 and labeling experiments79 that excluded the involvement of cyclobutadiene intermediates. It also accounts for the observation that terminal alkynes yield 1,2,4- (and 1,3,5-) trisubstituted benzene derivatives as the main product but not 1,2,3 derivatives. In contrast with this picture in cyclotrimerization with PdCl2-based catalysts, stepwise linear insertion of alkynes takes place without the involvement of palladacyclopentadiene.80... 

In a thermochemical study of the reaction (73), it is concluded that >(Pt—H) = D(Pt—Cl) and that the reaction occurs primarily because of the formation of a strong H—Cl bond rather than because of the ease of rupture of the Pt—H bond.171 Pressure effects on the insertion of alkynes into frans-PtHCl(PEt3)2 causes a large rate acceleration. The influence is large when ionic intermediates are involved, and in some cases a change of reaction mechanism is claimed.172... 

In this case a reaction sequence involving reductive elimination of H2 from Complex XV followed by insertion of alkyne into the remaining Pt-H bond, addition of alkyne to the Pt-Pt bond of the proposed diplatinum(I) intermediate, and then abstraction of chloride from the solvent is probable. Work is in progress to isolate and characterize proposed intermediates in this sequence, as well as to study the reactions in nonchlorinated solvents. 

A new and related carboannulation reaction is possible, that involves insertion of alkynes to aryl or alkenyl palladium intermediates, and subsequent cyclization onto an aromatic ring already present in the substrate [252], The cyclization of the enol triflate 509 to give 510 is an example. The reaction was applied to the synthesis of the indolocarbazole derivative 512 from the iodobisindole 511 [253],... 

The nickel-catalyzed carbonylation of allyl halides in the presence of alkynes and water produces 2,5-dienoic acids in good yields under very mild conditions (equation 25). This remarkable four-component reaction probably involves oxidative addition of the allyl chloride to the catalyst, followed by successive insertions of alkyne and CO, and finally hydrolysis. The carbon-carbon double bond derived from alkyne insertion is thus conjugated with the carbonyl group and generally has the (Z)-configuration. 

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. 

An overview of recent work from the Takats laboratory deals with divalent Ln(Tp Bu Me) complexes and preparation, molecular structure, NMR characteristics and reactivity of [Yb( x-H)(TptBu,Me)]2. Reactions of this compound with Lewis bases and Lewis acids, such as the perfluoroaryl boranes, a-bond metathesis reactions generally involving acidic H-X bonds, but also an example of C-Si bond cleavage, and insertion chemistry, primarily involving alkynes have been described. The molecular structures of representative complexes have been determined by X-ray crystallography.110... 

The intramolecular insertion of alkynes into a Pd-C bond has been observed and kinetically studied. The reaction involves a pre-equilibrium substitution of a phosphine ligand by the alkyne moiety, followed by rate determining insertion in a four-coordinate intermediate (Scheme 6.42). The longer the spacer chain (n) the more favorable ligand substitution K = 2.0(9) for = 2 versus K = 4.40(2) for n = 3) although the opposite is observed for the insertion step ( 2 = 7(3) for n = 2 versus k2 = 0.301(2) for n = 3). It seems that the short chain alkyne intermediate (n = 2) is strained enough to deviate from the usual perpendicular arrangement and adopts a conformation that places the alkyne closer to coplanarity and to the insertion transition state [133]. 

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. 

In connection with the synthesis of pyrrolo[l,2-a]indoles, intramolecular carbene insertion reactions have been developed. Although strictly involving interactions between two substituents, they will be considered here because of their chemical similarity. Thermal decomposition of the sodium salt of the tosylhydrazone (219) gives intramolecular cycloaddition followed by loss of nitrogen to yield the fused indole (220) (Equation (66)) <89JCS(Pi)2449>. This technique can be extended to include oxime ethers (221) in place of alkenes and thus provide formation of an aziridine (222) insertion into alkynes could not be achieved (Equation (67)) <9iJCS(Pi)i72i>. 

Although the majority of processes of this type involves alkyne insertion into the reactive C-CN species, examples of allene insertions have been demonstrated with cyanoformate and with alkynyl cyanide substrates (Scheme 3-106). ... 

An example of an alkyne insertion involving an early metal complex is shown in Equation 9.72. The insertion of dimethylacetylene into the permethylscandocene-alkyl complex occurs in a manner similar to that for the insertion of olefins into d metal-alkyl complexes. This reaction gave the product of a cis addition. The competition experiment shows that there is no measurable isotope effect of the a-hydrogen, implying that the modified Green-Rooney mechanism is not followed in this case. 

As noted in the introduction to this section, the insertions of alkynes and alkenes into metal-boron bonds appear to be involved in catalytic additions of borane reagents to alkenes and alkynes. One direct observation of the insertion of an alkyne into a metal-boron bond has been reported, and this reaction is shown in Equation 9.91. In another case, the reaction of an olefin with a metal-boryl complex generated products consistent with the... 

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). 

Electron-rich arenes and heteroarenes are known to react with alkynes in the presence of Pd or Pt catalysts in acidic solvents to give the corresponding alkenylated products. The reaction mechanism through C-H metallation and alkyne insertion, which is similar to path B in Scheme 18.82, was previously proposed [4]. However, mechanistic investigations conducted later suggested that this reaction may be initiated by the activation of alkynes rather than arenes by a catalyst, followed by electrophilic attack of formed cationic alkenyl species on arenes [87]. The observed trans-C-H bond addition across alkynes can be explained by this mechanism. These kinds of reactions as well as those involving alkyne activation by the Lewis acid catalysts have been summarized in comprehensive review articles [88]. 

A quite different process, called Reppe carbonylation, has been used to convert acetylene to acrylic acid esters. The catalysts are carbonyls of iron, cobalt or nickel and the hydrogen source is a hydrogen halide, HX. The process is thought to involve oxidative addition of HX to the metal carbonyl, followed by coordination and insertion of alkyne into the M—H bond and insertion of CO into the M—C bond. The resulting acyl complex is cleaved by alcohol to produce the ester and the metal hydride catalyst. De Angelis et al. have reported a theoretical analysis of the Ni(CO)4 system. 

The reaction involves the formation of Ph2P(0)-PdL -0P(0)Ph2 (3) by the reaction of PdMe2L with Ph2P(0)H and Ph2P(0)0H, and the subsequent insertion of alkynes into the Ph2P(0)—Pd bond of 3. Some other related reports are known, which include the Pt-catalyzed hydrophosphination of acrylonitrile via carbon-carbon double bond insertion into a Pt—P bond. ... 

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. 

Gomplexes 36 and 37 (R = Bu ) are active catalysts for the insertion of R-G=G-Ph into biphenylene to form 9,10-disubstituted phenanthrenes (Scheme 4). Significantly, no cyclotrimerization of the alkyne was observed as long as biphenylene was present, but in its absence hexaphenylbenzene was produced from PhG=GPh at a slow rate (ca. 0.4 turnover/day). On the other hand, the GF3 derivative did not promote the formation of phenanthrenes, leading instead to the formation of cyclic trimers (Scheme 5). The proposed mechanism for the insertion of alkynes into biphenylene involves the dissociation of the Ni-NMe2 moiety to allow the coordination of the incoming biphenylene... 

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