Alkyne derivatives systems

The use of a PdCl2(PPh3)2-InBr3 reagent system catalyzes cross-coupling reactions of a variety of aryl iodides with several terminal alkynes. The corresponding functionalized alkyne derivatives are produced in good yields (Scheme 102) 351... 

If the M(CO)2 angle is acute, the interior vertical dir orbital (dyz in the coordinate system of Fig. 18) will be stabilized and occupied. A cis alkyne would then be in the plane bisecting the M(CO)2 unit so that 17] would see the filled dyz orbital while ir would encounter the vacant dxz orbital. Alternatively, an obtuse M(CO)2 angle would reverse the roles of dyz and dxz and lead to a predicted alkyne orientation orthogonal to the M(CO)2 bisector plane (Fig. 18). Note that these arguments are valid only for d4 monomers and completely irrelevant for d6 dicarbonyl alkyne derivatives such as W(CO)2(ZC=CZ)2(dppe) (30). 

The presence of halogens in norbornene-derived systems is a good way to control the regiochemistry of the PKR with alkynes. This has been observed with 7-oxa- and 7-azanorbornenes 246 and 249. Thus, the carbonyl is bonded with the olefinic carbon that bore the halogen, which is eliminated in the pro-... 

A characteristically different mechanism appears to operate in alkyne trimerization systems based on PdCb." Cationic metal complexes are involved in which initial halide transfer to an alkyne carbon is followed by sequential linear insertion of two more alkyne moieties. Metallacyclopentadiene intermediates are not involved in this sequence. Unique to this mechanism is the subsequent ring-closure to a cy-clopentadienylmethyl metal derivative, which, via halide transfer back to the metal, eventually leads to benzene via a bicyclo[3.1.0] system (Scheme 27). Support for this mechanism comes in part from the isolation of methylcyclopentadienyl-derived structures in several cases, including pentalene derivatives from further alkyne insertion followed by a second ring-closure." ... 

Among the metallacycloalkenes with different ring size, metallacyclopro-penes and -pentadienes are to be mentioned because they are involved as intermediates in the cyclotrimerization of alkynes to benzene derivatives, and in this section attention is mainly directed to such species. Cotrimeriza-tions also are possible with alkyne-like systems. 

The novel phospha-alkyne system (303) is formed in the reaction of lithium bis(trimethylsilyl)phosphide with 0,0 -diethyldithiocarbamate. C-chlorophosphaethyne, ClCsp, has been generated by the pyrolysis of trichloromethyldichlorophosphine over granulated zinc at 550°C, and characterised by infrared spectroscopy. A bimolecular proton transfer mechanism has been suggested for the base-promoted isomerism of alkynyl- and alkenyl-phosphines to the phospha-alkynes (304). The potential of phospha-alkynes as novel building blocks in heterocyclic chemistry has been reviewed. New examples of phospha-alkyne-derived... 

Miura. ° Reaction of allg nes with benzamides in dioxane at 100 °C proceeded under total regio- and stereoselective control to provide the ort/zo-alkenylated product (Scheme 9). It was essential to add stoichiometric amounts of AcOH to obtain optimal yield. Reaction of benzamide with various alkynes proceeded smoothly to produce the desired products in moderate to good yields. The reaction of 1-phenyl-2-(trimethylsilyl)acetylene proceeded efficiently but it underwent desilylation to afford the 1,1-diarylethene in 63% yield. In addition, the catalytic system was found to be suitable to the coupling of phenylazoles and phenylimidazoles with several alkyne derivatives giving dialkenylated products. In contrast, the sterically more hindered l-methyl-2-phenylimidazole provided the mono-alkenylated product selectively as the sole product. ... 

Riguera et al. [197] reported the conjugation of unprotected alkyne-derived carbohydrates to dendritic systems incorporating terminal azides using Click... 

When compared with C2 symmetric ligands, Ci symmetric and bidentate NHCs induced the highest ee values (<99% ee) with unsaturated C-C bonds and [B(pin)]2 in MeOH. For instance, in the p-boration of a- and p-unsatu-rated aldehydes and esters, which are more reactive than other C-C unsaturated substrates, a 213-derived system is more enantioselective than the 214-derived complex (74% ee vs. 59% ee, respectively) (Scheme 13.8). Improved results were obtained by Hoveyda and co-workers, using bidentate 217-220 with acyclic and cyclic internal alkenes, alkynes, 1,6-dienes and various allenes ee s of <98% were achieved (Scheme 13.8). For the regio-and enantioselective hydroboration of a,p-unsaturated carbonyl compounds... 

The benzene derivative 409 is synthesized by the Pd-catalyzed reaction of the haloenyne 407 with alkynes. The intramolecular insertion of the internal alkyne, followed by the intermolecular coupling of the terminal alkyne using Pd(OAc)2, Ph3P, and Cul, affords the dienyne system 408, which cyclizes to the aromatic ring 409[281]. A similar cyclization of 410 with the terminal alkyne 411 to form benzene derivatives 412 and 413 without using Cul is explained by the successive intermolecular and intramolecuar insertions of the two triple bonds and the double bond[282]. The angularly bisannulated benzene derivative 415 is formed in one step by a totally intramolecular version of polycycli-zation of bromoenediyne 414[283,284],... 

Benzene derivatives. Tbe nomenclature is a combination of the lUPAC system and traditional names. Many of the derivatives are named by the substituent group appearing as the prefbt. These may be considered a subclass of the aliphatic-aromatic hydrocarbon family, which contains both aliphatic and aromatic units in its structures. Thus, alkylbenzenes are made up of a benzene ring and alkane units alkenylbenzenes are Composed of a benzene ring and alkene units and alkynylbenzenes comprise a benzene ring and alkyne units. Examples of alkylbenzenes include... 

Although hydrogenation of A-benzylideneaniline in the presence of 11 afforded the corresponding product (eq. 1 in Scheme 11), the a,(3-unsaturated ketone was converted into a mixture of unsaturated and saturated alcohols in the 42 56 ratio (eq. 2 in Scheme 11). Several substrates (nitrile derivatives, epoxides, esters, internal alkynes, and terminal alkenes), which are shown in Fig. 4, are not hydrogenated in this catalytic system. 

Enynes 71 react with aldehydes 61 in the presence of the [Ni(COD)J/SIPr catalytic system to afford two distinct products 72 and 73 (Scheme 5.20) [20b], The enone 72 is derived from aldehyde addition with the alkyne moiety while the adduct 73 arises from the aldehyde addition with the alkene moiety. The product distribution is dependent on the substituent on either the alkyne or alkene moieties. The reaction between 71 and ketones 74 led to the unprecedented formation of pyrans 75 (Scheme 5.20). The reaction showed to be highly regioselective in aU the cases, the carbonyl carbon was bound to the olefin. 

The use of imidazolium salts for in situ catalyst formation was shown to be optimal for the coupling of TMS-protected alkynes even with sterically demanding aryl bromides and avoids the formation of homocoupling-derived products. For this reaction, Nolan reported that the activation of chlorobenzene by this catalytic system was possible in moderate yield [125] (Scheme 6.41). 

The key feature of the first total synthesis of (+)-homopumiliotoxin 223G 418 was a Lewis acid-induced, chelation-controlled propargylation of the trifluoroacetate salt of (. )-2-acetyl pi peri dine 415, derived from iV-Cbz-L-pipecolinic acid. Alkyne 416 thus formed was transformed after several steps into 417, which was cyclized by activation of the primary hydroxyl with the carbon tetrabromide-triphenylphosphine system to give the natural product (Scheme 98) <1998TL2149>. 

The synthesis of multivalent neoglycoconjugates by 1,3-dipolar cycloaddition of nitrile oxides and alkynes has been reported (493). The nitrile oxides have been generated in situ in the presence of alkynyl derivatives, allowing the access to homo and hetero multivalent systems containig O- and C-linked glycosides and isoxazole bridges. 

Numerous studies aimed at the understanding of the mechanism of these processes rapidly appeared. In this context, Murai examined the behavior of acyclic linear dienyne systems in order to trap any carbenoid intermediate by a pendant olefin (Scheme 82).302 A remarkable tetracyclic assembly took place and gave the unprecedented tetracyclo[6.4.0.0]-undecane derivatives as single diastereomer, such as 321 in Scheme 82. This transformation proved to be relatively general as shown by the variation of the starting materials. The reaction can be catalyzed by different organometallic complexes of the group 8-10 elements (ruthenium, rhodium, iridium, and platinum). Formally, this reaction involves two cyclopropanations as if both carbon atoms of the alkyne moiety have acted as carbenes, which results in the formation of four carbon-carbon bonds. 

This system also worked well in ionic liquids.518 Li etal. found silver-phosphine complexes to promote aldehyde-alkyne coupling in water. When triphenylphosphinesilver chloride was used as a catalyst in water, the only detected product was the aldehyde addition product instead of the adduct derived from imine (Scheme 111).519... 

Insertion of the alkyne into the Pd-H bond is the first step in the proposed catalytic cycle (Scheme 8), followed by insertion of the alkene and /3-hydride elimination to yield either the 1,4-diene (Alder-ene) or 1,3-diene product. The results of a deuterium-labeling experiment performed by Trost et al.46 support this mechanism. 1H NMR studies revealed 13% deuterium incorporation in the place of Ha, presumably due to exchange of the acetylenic proton, and 32% deuterium incorporation in the place of Hb (Scheme 9). An alternative Pd(n)-Pd(iv) mechanism involving palladocycle 47 (Scheme 10) has been suggested for Alder-ene processes not involving a hydridopalladium species.47 While the palladium acetate and hydridopalladium acetate systems both lead to comparable products, support for the existence of a unique mechanism for each catalyst is derived from the observation that in some cases the efficacies of the catalysts differ dramatically.46... 

The metal-mediated and metal-catalyzed [6 + 2]- and [6 + 4]-cycloaddition reactions, pioneered by Pettit and co-workers105 106 and Kreiter and co-workers,107 respectively, involve the cycloaddition of metal-complexed cyclic trienes with 7r-systems such as alkenes, alkynes, and dienes. The [6 + 2]-reactions produce bicyclo[4.2.1]nonadiene derivatives and the [6 + 4]-reactions produce bicyclo[4.4.1]undecatrienes (Scheme 32). Trienes complexed to chromium, which can be prepared on large scale (40 g) as reported by Rigby and co-workers,108 react with 7r-systems upon thermolysis or irradiation.109-111 Chromium and iron-catalyzed [6 + 2]-reactions of cycloheptatrienes and disubstituted alkynes... 

The high-valent metal species required for activation of an alkyne has also been generated by the oxidative addition to an allylic or propargylic system. For example, with an allyl aryl ether as the substrate, this type of reaction achieves a cycloisomerization that occurs through an 0- to C-allyl migration (Equation (92)) 323,324 similarly, (9-propargyl derivatives lead to a mixture of allenyl and propargyl products (Equation (93)).325,326... 

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