Triyne reactions

Scheme 6.71 Dienyne (reaction a) and triyne (reaction b) domino ring-closing metathesis reactions.

The intermediate in this double 1,6-elimination may be the pentaene C1-CH=C=C=C=C=CH2, routes via other highly unsaturated compounds cannot be ruled out. Since the elimination is a very clean reaction, prior isolation of the explosive triyne is not always necessary 2-ethynylselenophen, for example, can... 

The terminal diyne 320 is prepared by coupling of the zinc acetylide 318 with /rfln.s-l-iodo-2-chloroethylenc (319), followed by elimination of HCI with sodium amide[231]. Similarly, terminal di- and triynes are prepared by using cw-l,2-dichloroethylene[232]. The 1-alkenyl or l-aryl-2-(perefluoroalkyl) acetylene 321 is prepared by the reaction of a zinc acetylide with halides[233]. 

The cyclization of the enediynes 110 in AcOH gives the cyclohexadiene derivative 114. The reaction starts by the insertion of the triple bond into Pd—H to give 111, followed by tandem insertion of the triple bond and two double bonds to yield the triene system 113, which is cyclized to give the cyclohexadiene system 114. Another possibility is the direct formation of 114 from 112 by endo-rype. insertion of an exo-methylene double bond[53]. The appropriately structured triyne 115 undergoes Pd-catalyzed cyclization to form an aromatic ring 116 in boiling MeCN, by repeating the intramolecular insertion three times. In this cyclization too, addition of AcOH (5 mol%) is essential to start the reaction[54]. 

As complex 40 proved to be active in cycloaddition reactions and is isoelectronic to Rh(+1), which is a potent catalyst for [2 + 2 + 2] cycloadditions [20, 21], it was expected that 40 might also be active in those reactions, which is indeed the case. Triyne 54 could be converted to the [2 + 2 + 2]-cycloaddition product 55 in good yield (eq. 3 in Scheme 11). Mechanistically, this reaction is also assumed to proceed via a metallacyclic intermediate. 

Cycloaddition of aUcynes catalysed by transition metals is one of the most efficient and valuable ways to prepare benzene and pyridine systems [12], Among the possible catalytic systems able to catalyse this reaction, cobalt and iron complexes containing NHCs as ligands have shown high catalytic activity in the intramolecular cyclotrimerisation of triynes 36 (Scheme 5.10) [13]. The reaction was catalysed with low loading of a combination of zinc powder and CoC or FeClj with two or three equivalents of IPr carbene, respectively. 

Occurring Acetylene Compounds. XXVII. The Synthesis of a Mixture of Trideca-1.3,1l-triene-5,7,9-triyne (II) and trans-l-Phenylhepta-l,3-diyn-5-ene (V). A Novel Cyclisation Reaction. Acta Chem. Scand. 13, 2101 (1959). 

As shown in the two examples described here, formation of the benzene nucleus by trimerization of alkynes is usually catalyzed by a Co-complex. However, Und-heim and coworkers [276] have recently shown that a Ru "-complex can also be used. Reaction of the triyne 6/4-9, which was prepared from SchollkopPs bislactim ether 6/4-8 [277] with Grubbs I catalyst 6/3-13, led to 6/4-10 in an excellent yield of 90%. Hydrolysis of 6/4-10 gave the desired as-indacene-bridged bis(a-amino acid) derivative 6/4-11 (Scheme 6/4.3). 

The reaction of triallylborane with silicon triyne 123 is interesting. A113B attacks both internal and external triple bonds giving rise to silole 124 and two heterocycles with bridgehead boron 125 and 126 in a 1 3 3 ratio as a result of competitive sequential reactions (Scheme 52). When 1,1-allylboration of the internal C C bond followed by intramolecular 1,1-vinyIboration takes place, the silole 124 is formed, while in another case 1,1-allylboration followed by a series of intramolecular 1,2-allylboration reactions leads to boron derivatives 125 and 126 <2002JOM(657)146>. 

In 2003, Efskind and Undheim reported dienyne and triyne domino RCMs of appropriately functionalized substrates with Grubbs type II or I catalysts (Scheme 6.71, reactions a and b, respectively) [151]. While the thermal processes (toluene, 85 °C) required multiple addition of fresh catalyst (3 x 10 mol%) over a period of 9 h to furnish a 92% yield of product, microwave irradiation for 10 min at 160 °C (5 mol% catalyst, toluene) led to full conversion. The authors ascribe the dramatic rate enhancement to rapid and uniform heating of the reaction mixture and increased catalyst lifetime through the elimination of wall effects. In some instances, use of the Grubbs I catalyst was more efficient than use of the more common Grubbs II equivalent. 

Trisila[l.l.l]orthocyclophane (46) was prepared by the reaction of 3,3,6,6,9,9-hexamethyl-3,6,9-trisilacyclonona-l,4,7-triyne and a-pyrone. The structure of 46 determined by X-ray crystallographic analysis (Fig. 28) indicates a twisted saddle conformation (10). 

The corresponding permethylzirconocene moiety is complexed by triynes RC = C—C = C—C=CR through two triple bonds in a refashion and complex 107 is obtained. Detailed NMR investigations have revealed a sliding of the permethylzirconocene along the triyne backbone 108 [55], This result is important because it shows there to be a lot of species in the mixture this is the reason for the low selectivity in some reactions. 

A variety of palladium-catalyzed dimerizations of conjugated enynes and their additions to diynes and triynes gave rise to styrene and phenylacetylene derivatives, respectively. Inter alia, 1,2,4-cyclohexatrienes have been invoked as intermediates in these reactions [134], 5,6-Diphenyl-l,2,4-cyclohexatriene has been proposed as an intermediate in the rearrangement of 4,4-diphenylcyclohexa-2,5-dienylidene to o-ter-phenyl and its possible existence was supported by quantum-chemical calculations [135],... 

Before leaving the diacetylenes we must note some of the stereochemical varieties available in their polymers. Triynes, 138, also polymerize in the crystal by 1,4-addition (213). Also, cyclic di- and polyenes give polymeric products on irradiation. The exact structure of the polymer, however, has been established only in the polymer 135. Note that alternate side chains in this polymer are on opposite sides of the plane of the main chain the polymer is thus meso. However, in principle such a reaction could give rise to optically active polymers in suitable structures. The cyclic tetradiyne 139 crystallizes in a polymerizable phase containing interstitial chloroform (209). The polymerization reaction, which involves... 

Two precedent examples had been reported of the enantioselective [2+2+2] cycloaddition of alkynes. In one case, an enantioposition-selective intermolecular reaction of a triyne with acetylene generated an asymmetric carbon at the benzylic position of a formed benzene ring [19]. In the other case, an intramolecular reaction of a triyne induced helical chirality [20]. Both reactions were developed by chiral Ni catalysts. 

The Ir-MeDUPHOS catalyst also functioned efficiently in an intramolecular reaction, where triynes, which possessed ortho-substituted aryl groups on their termini, were transformed into ortho-diarylbenzene derivatives, which have adjacent two axial chiralities (Scheme 11.14) [22]. 

It is possible to carry out the [2+2+2] cyclotrimerization reaction in a regioselective manner by using a partially or completely intramolecular approach. Rhodium-catalyzed intramolecular cyclotrimerization of 1,6,11-triynes, which construct fused 5-6-5 ring-systems, has been studied extensively [33-36]. Cyclization of 1,6,11-triyne 47 catalyzed by RhCl(PPh3)3, gives the tricyclic benzene 48 in good yield (Eq. 14) [33a]. 

A proposed mechanism for the SiCaT reaction using the 1,6,11-triyne system as an example is illustrated in Scheme 7.21. The reaction proceeds through insertion of one of the terminal alkynes into the Si-[Rh] bond of the hydrosilane-[Rh] oxidative adduct, generating an ethenyl-[Rh] intermediate, which undergoes addition to the second and third alkyne moieties to form intermediate III.2a. Subsequent carbocyclization followed by /9-hydride elimination gives the tricyclic silylbenzene derivative 70. Alternatively, ethenyl-[Rh] intermediate can be isomerized to the thermodynamically more... 

In the contrast to the SiCaT reaction of triynes vide supra), when an enediyne has an olefin at the terminal position a novel silicon-initiated carbonylative carbotricyclization (CO-SiCaT) reaction takes place, incorporating CO into the product [52], As Scheme 7.24 indicates, the reaction of dodec-ll-ene-l,6-diynes 82 catalyzed by Rh(acac)(CO)2 under 1 atm. CO affords the corresponding cyclopenta[e]azulenes 83 in good to excellent yield. 

An example of the next higher homolog in the series of odd-numbered cumuleny-lidene complexes, heptahexaenylidene complexes, was already proposed in 1996 as an intermediate in the formation of the alkenyl(dimethylamino)pentatetraenylidene tungsten complex 27 (Scheme 3.21) in the sequential reaction of triyne Me3Si-(C = C)3-C(NMe2)3 with LiMe LiBr, (CO)5W(thf)], and BFj OEtz [37]. The putative heptahexaenylidene complex 26 could neither be isolated nor spectroscopically... 

However, by very careful control of the reaction conditions and the purity of the triyne, using lower temperatures and substituting SiO, for BF3-OEt2 it was recently possible to synthesize, isolate and characterize the first heptahexaenylidene tungsten complex 26 and its chromium analog [38]. 

Rhodium carbonyl complexes catalyze the silane-initiated cascade cyclization of 1,6,11-triynes to form fused aromatic tricyclic compounds. For example, reaction of 83 [X = G(G02Et)2] with methyldiphenylsilane catalyzed by the tetrarhodium carbonyl cluster Rh4(GO)i2 in toluene at room temperature gave an 88 12 mixture of the silylated and unsilylated fused tricycles 84a and 84b [X = G(G02Et)2] in 85% combined yield (Equation (55)). The ratio of silylated to unsilylated tricyclic product formed in the reaction of 1,6,11-triynes was dependent on the nature of the substrate (Equation (55)). For example, Rh4(GO)i2-catalyzed reaction of diaminotriyne 83 (X = NBn) with methyldiphenylsilane gave unsilylated tricycle 84b (X = NBn) in 92% yield as the exclusive product (Equation (55)). 

Cascade silylcarbocyclization of (E)- or (Z)-dodec-6-ene-l,ll-diyne (355) with HSiMe2Ph catalyzed by Rh(acac)(CO)2 gives the bis(exo-methylenecyclopentyl) (356) with complete stereospecificity (equation 144)342. The reaction of dodec-l,6,ll-triyne (357) under the same conditions affords a 3 1 mixture of 358 and 359 in 75% yield... 

This [2+2 + 2]cycloaddition is useful for synthesis of highly substituted aromatic compounds since substitution reactions with arenes are seldom regiospecific. An example is the synthesis of calomelanolactone (2) from triyne l.5... 

Using aromatic triynes Ar(C = CH)3 as monomeric building blocks, hb-PYs can be readily constructed (Scheme 8) [34]. The fast polycoupling reaction could be controlled by terminating the propagating species before the... 

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