Alkyne intermediate

Treatment of 10-bromodibenz[fe,/]oxepin with potassium rm-butoxide at 100 C in dioxane results in elimination of hydrogen bromide to give 10,11-didehydrodibenz[6,/ oxepin. This highly strained alkyne intermediate can be trapped with dienophiles, for example, with 2,5-dimethylfuran to give 4.160... 

Fuchigami and co-workers utilized the regioselective anodic fluorination of aryl propargyl sulfides 301 for the preparation of fluorinated alkyne intermediates, which were subsequently isomerized by sodium ethoxide to furnish 1-fluoro-l-arylthioallenes 302 (Scheme 8.80) [166]. 

These results led to the proposal of the following mechanism. Decomplex-ation of the central C2 fragment allows coordination of the alkyne (intermediate 119), which then inserts to form the metallacycle 120. Deinsertion (reductive eliminate of the cobalt moiety allows ring closure to give the cyclohexadienone complex 121, which upon decomplexation yields the desired phenol. The regiochemistry of the alkyne insertion determines the ratio of 116 117 (for simplicity, only the sequence leading to 116 has been shown). 

On the basis of the above-mentioned calculations it seems that coordination chemistry is a viable alternative to stabilize this heterocumulene. However, the experimental access to metal complexes containing the tricarbon monoxide ligand remains a challenge. Thus, to date, the coordination chemistry of C3O is confined to [Cr(=C=C=C=0)(C0)s] (89), obtained by treatment of [n-Bu4N] [CrI(CO)5] with the silver acetylide derived of sodium propiolate in the presence of Ag" (Scheme 28) [105]. Reaction of the presumed Tt-alkyne intermediate complex 88 with thiophosgene generates the heterocumulene 89. Neither structural nor reactivity studies were undertaken with this complex. 

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). 

The oxapropatrienylidene complex (54) was obtained from [CrI(CO)5] and AgC=CCOzNa in the presence of AgBF4, probably via the tp-alkyne intermediate (53) (17) ... 

This is further indicated in the reactions of 3-butyn-l-ol with [Fe( /2-CH2=CMe2)(CO)2( -C5H5)]+, which afford a mixture of dihydrofuran complex (64) and the oxacyclopentylidene complex (65) (84). The formation of these two derivatives involves a common tp-alkyne intermediate, which either forms 64 directly by internal nucleophilic attack of the oxygen on the complexed C=C triple bond, or rearranges to the vinylidene. This forms 65 by a similar attack of the hydroxy group on the a-carbon, followed... 

In the alkyne dimerization catalyzed by palladium systems, all proposed mechanisms account for an alkynyl/alkyne intermediate with cis addition of the alkynyl C-Pd bond to the alkyne in a Markovnikov fashion, in which the palladium is placed at the less-substituted carbon, both to minimize steric hindrance and to provide the most stable C-Pd bond (Scheme 2a). The reverse regioselectivity in the palladium-catalyzed dimerization of aryl acetylenes has been attributed to an agostic interaction between the transition metal and ortho protons of the aromatic ring in the substrate (Scheme 2b) [7, 8],... 

Terminal acetylenes readily add to low-valent metal centers. The addition of PhC=CH to [Ir(RCN)(CO)(PPh3)2]+ requires RCN dissociation to give an r -alkyne intermediate.131... 

Mori, Y, and Hayashi, H. 2002. Synthetic studies of yessotoxin, a polycyclic ether implicated in diarrhetic shellfish poisoning convergent synthesis of the BCDE ring system via an alkyne intermediate. Tetrahedron 58, 1789-1797. 

The dehydrogenation of bis(a-bromobenzyl) sulfide (41) with Fe2(CO)9 in the presence of furan affords (43). The iron-stabilized sulfur ylide (42) has been proposed as intermediate. Reaction of C -di-phenylnitrone (44) with l,2-bis(methylene)-3,3.4,4,5,5-hexamethylcyclopentane (45) in benzene at 80 C gives a [4 -I- 3] cycloadduct (46 18%), along with some other products. Landor et al. have shown that the dehydrobromination of allenyl bromide (47) in the presence of furan affords bicyclic ketone (49) in 9% yield. Apparently, a strain alkyne intermediate reacts with r-butyl alcohol to afford an enol ether (48), which is then converted to (49) (Scheme 11). 

The two-alkyne annuation reactions shown in Scheme 43 are actually reactions of this class since mechanistically they can be viewed as the interception of vinyl carbene complex intermediate (248) in Scheme 36 by an aU ne. In addition to alkynes, intermediates from the reactions of Fischer complexes... 

Alkynic intermediates serve as important functional groups in organic synthesis. Many important reactions exploiting the unique and versatile chemistry of the carbon-carbon triple bond have been devised over the last few years. A general strategy for the synthesis of substituted alkynes involves substitution and addition reactions of propargylic anion equivalents this approach is particularly well suited for the preparation of homopropargylic alcohols (Scheme 28). 

An interesting carbene-alkyne intermediate was postulated for the conversion of carbene complexes to chrysene derivatives, which again illustrates the potential of cr,Tr-activation of bridging ligands by metal centers [Eq. (32)] (225). The thermal decomposition of the carbene complex Cr-... 

W.J. Kerr and co-workers carried out the total synthesis of (+)-taylorione starting from readily available (+)-2-carene and using a modified Pauson-Khand annulation with ethylene gas as the key step. The key terminal alkyne intermediate was prepared by the Corey-Fuchs reaction. Interestingly, the ketal protecting group was sensitive to the excess of CBr4, so the addition of this reagent had to be monitored carefully to cleanly transform the aldehyde to the desired dibromoolefin. 

Alkynic intermediates derived from sp -sp coupling reactions can be applied to the syntheses of a variety of aromatic and heterocyclic molecules. " For example, a one-step synthesis of the aminoindoli-zines (44a-e) and the 5-aza analog (44f) from cx-haloaza aromatics has been reported as outlined in Scheme 31. ° The mechanism of this reaction appears to involve the initial formation of pyridyl-acetylene (45), followed by ring closure via the intermediate (46 Scheme 32). ° ... 

Scheme 40. Infermolecular [2 + 2 + 2] cyclotrimerization of reactive aryne and alkyne intermediates [137c, 138,139 c]...

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]. 

A tentative mechanism for the A -coupling is depicted in Scheme 5.74. Firstly, the acetylene reacts with the Ag catalyst, forming it-metal-alkyne intermediate 168, which leads to the formation of species 169. This process is still poorly understood and presumably is the key step of the reaction. 

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