A, >>-Diynes

Fig. 3a,b Typical diyne metathesis reactions ring-closing alkyne metathesis (RCAM, a) diyne cross metathesis (ACM, b)... 

The palladium-catalyzed cyclization reaction was used in the syntheses of several natural products such as siccanin [86], streptazolin [87], and ceratopi-canol (through a diyne, diene cascade) [80]. The production of the streptazolin precursor 149 through reductive cyclization of 150 is illustrative of the complexity that the reaction can provide (Eq. 29) [87]. 

The diyne reaction has been used extensively by Muller et al. in heterocyclic synthesis (see for example Scheme 86 in Section IV,C,2) a process using a diyne substrate and leading to the formation of an azuleno[l,2-d]-dibenzo[b,/]silepin is shown in Scheme 172.243fl... 

From the mechanistic point of view, the observed competitive reactions can be explained by considering two different pathways (Scheme 114). The intermediacy of ruthenacyclopentadiene 453 or biscarbenoid 452, formed from the reaction of a diyne and a ruthenium(ll) complex, is postulated in the proposed mechanism. Cyclopropanation of the alkene starts with the formation of ruthenacyclobutane 456, which leads to the generation of the vinylcarbene 457. Then, the second cyclopropanation occurs to afford the biscyclopropyl product 458. Insertion of the alkene 459 into the ruthenacyclopentadiene 453 affords the ruthenacycloheptadiene 454. The subsequent reductive elimination gives the cyclotrimerization product 455. The selectivity toward the bis-cyclopropyl product 458 is improved with an increasing order of haptotropic flexibility of the cyclopentadienyl-type ligand. 

A hydrosilylation/cyclization process forming a vinylsilane product need not begin with a diyne, and other unsaturation has been examined in a similar reaction. Alkynyl olefins and dienes have been employed,97 and since unlike diynes, enyne substrates generally produce a chiral center, these substrates have recently proved amenable to asymmetric synthesis (Scheme 27). The BINAP-based catalyst employed in the diyne work did not function in enyne systems, but the close relative 6,6 -dimethylbiphenyl-2,2 -diyl-bis(diphenylphosphine) (BIPHEMP) afforded modest yields of enantio-enriched methylene cyclopentane products.104 Other reported catalysts for silylative cyclization include cationic palladium complexes.105 10511 A report has also appeared employing cobalt-rhodium nanoparticles for a similar reaction to produce racemic product.46... 

The cobalt mediated [2 + 2 + 2] cycloadditions of a, >-diynes with indole were only accomplished when the nitrogen atom was substituted with an electron-withdrawing... 

Fiirstner succeeded in the synthesis of ambrettolide from a diyne using this catalyst system. Treatment of diyne 148a with Mo(CO)6 (5 mol%) and p-chlorophenol (1 equiv) in chlorobenzene at 140°C gave cycloalkyne 149a in 69% yield. Subsequent Lindlar reduction proceeded smoothly in a stereoselective manner to alford ambrettolide ... 

Fu and coworkers have discovered an intriguing franr-hydroacylation of alky-nals catalyzed by Rh complexes. During the course of these investigations, they observed high enantioselectivities in the desymmetrization of a diyne. With... 

Cook and co-workers took advantage of the molybdenum-based methodology and reported one of the most graceful applications of the allenic PKR. They intended to use PKR to synthesize a theoretically interesting tetracyclic compound 55, which has 14 vr-electrons and is supposed to be aromatic in principle. To this end, they have applied PKR to various alkynyl allenes, and finally employed a diyne-diallene substrate 54 successfully (Equation (27)). Double PKR by employing molybdenum hexacarbonyl in excess and DMSO afforded the requisite intermediate in a high yield. 

Titanium Mediated Cycl izat ion of a Diyne 1,2- E, E -bis(Ethylidene)cyclohexane... 

Cyclization of a, >-diynes in moderate to high yields with high regio- and stereoselectivity has been reported with palladium [29] and binudear ruthenium catalysts [30], to give exo- or mdo-cyclic (Z)-l-en-3-ynes, respectively (Scheme 11). 

Fig. 17.84. Chemoselective and trans-selective reduction of exactly one C=C triple bond of a diyne.

In previous works this group had observed a competition between the PKR and a [2 + 2 + 2] cyclization in the second reaction step of three triple bonds. Thus, when reacting linear triynes 174 under catalytic, high CO pressure, cobalt mediated PKR conditions, they obtained mixtures of products 175 coming from two [2 + 2 + 1] cycloadditions, and 176 from a [2 + 2 + 1]/ [2 + 2 + 2] tandem reaction. When the triple bonds were ether linked, the latter was the favored reaction, while with substrates lacking oxygen atoms, the iterative PKRs was the major pathway (Scheme 51) [166]. When the reaction was performed intramolecularly between a diyne and an alkyne, the only reaction products were the result of a [2 + 2 + 1 ]/[2 + 2 + 2] tandem cycloaddition [167,168]. 

Fig. 50. Molecular structure of [Fe2(CO)6(Se2) jU.-HC — C(CCMe) Ru3(CO)10] (150). [Reprinted with kind permission from Mathur, P. Dash, A. K. Hossain, M. M. Satyanarayana, C. V. V. Rheingold, A. L. Liable-Sands, L. M. Yap, G. P. A. Diyne-bridged metal clusters Synthesis and spectroscopic characterisation of [(CO)6Fe2Se2 /i-HC=C(CCR) M), (R = Me and Bu" M = Cp2Mo2(CO)4, Co2(CO)6, Ru3(CO)i0 and Os3(CO)10. Structural characterisation of [(CO)6Fe2Se2 /i-HC=C(CCBun) Cp2Mo2(CO)4] and [(CO)6Fe2Se2 /t-HC=C(CCMe) Ru3(CO)io]. J- Organomet. Chem. in press. Copyright 1997 Elsevier Science S. A., P.O. Box 564,1001 Lausanne, Switzerland.]...

The reactions of Fischer carbene complexes with alkynes can under certain conditions lead to products that result from the incorporation of two alkynes, the carbene ligand and a carbon monoxide. In inter-molecular reactions, this is most commonly observed for acetylene itself or for sterically unhindered al-kynes. °2 As can be anticipated by the mechanism in Scheme 36, two-alkyne incorporated products of the type (258) are also favored for high alkyne concentration. Synthetically, the two-alkyne reactions are most useful in intramolecular reactions, two of which have been reported and are exemplified by the reactions in Scheme 43. The typical product from the reaction of a Fischer carbene complex with a diyne, such as (308), is a bicyclic phenol of the type (309). ° These products are apparently the result of the assembly of pieces indicated by (311). Under some conditions, dienones of the type (310) and (314) can be isolated, and it is thought they are the immediate precursors of the phenol products via an in situ reduction by a chromium(O) species. This reaction is completely regioselective with diyne (308) and the phenol (309) results from incorporation of the terminal alkyne of (308) before the disubstituted alkyne. Phenols of the type (309) have also been observed from the reaction of diynes with carbyne complexes. ... 

As is the case with totally intermolecular cocycloadditions, chemoselectivity is a potential problem in partially intramolecular systems where the desired process, combination of a diyne with a monoyne, may face competition from cycloadditions involving either exclusively the diyne or exclusively the monoyne. In practice chemoselectivity is readily achieved. Phosphite complexes of Ni° chemoselectively catalyze rapid and efficient cycloaddition of alkynes to heteroatom-containing diynes in good yields (equation... 

Histrionicotoxin is a diyne isolated in small quantities from the skin of Dendrobates histrioni-cus, a colorful South American frog (Figure 11.3). This toxin, secreted by the frog as a natural defense mechanism, was used as a poison on arrow tips by the Choco tribe of South America. 

Polyynes have served as starting materials for the synthesis of a wide variety of heterocyclic ring systems. The reactions used involve addition to triple bonds, and any of the common mechanistic pathways may be followed, i.e. nucleophilic, electrophilic or free radical attack as well as concerted cycloadditions. Although the evidence does not permit unequivocal classification of many of the reactions into one of these categories, the ones considered here are those which most likely involve nucleophilic attack at some stage. In a formal sense the reactions amount to successive additions of a divalent nucleophile to two triple bonds the first involves intermolecular and the second intramolecular attack, as illustrated in equation (19) for the addition of HoS to a diyne. 

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