Triynes, aromatic

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]. 

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)). 

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... 

Many of the hyperbranched polymers contain aromatic chromophores in their 7r-conjugated structures and should show interesting optical properties [31]. This is indeed the case. For example, hb-P20 carries the TPA chro-mophore, which is often used as hole-transport materials in the fabrication of light-emitting diodes [120]. The Aab of triyne 20 appeared at 342 nm (Fig. 5),... 

Triynes 261 in which the alkyne linkages are properly spaced can undergo intramolecular cyclotrimerization to aromatics 262 and 263 in the presence of Ziegler-type catalysts-"h... 

Aromatic compounds can be prepared by cyclotrimerization of alkynes or triynes. Cyclotrimerization is possible by heating to 450-600°C with no catalyst. The spontaneous (no catalyst) trimeiization of t-BuC=CF gave 1,2,3-tri-tert-butyl, 5,6-trifluorobenzene (220), the first time three adjacent tert-butyl groups had been put onto a benzene ring. The fact that this is a head-to-head joining allows formation of 220 from two alkynes. The fact that 219 (a Dewar benzene) was also isolated lends support to this scheme. Three equivalents of 3-hexyne trimerized to hexaethylbenzene at 200°C in the presence of Si2Cl6. ... 

Numerous applications of this stereoselective transition metal catalyzed version of the tandem reaction principle have been reported3 32 6. Thus, the one-step cyclization of triynes of type 11 leads to the steroid nucleus 12, containing an aromatized B-ring, with complete control of stereochemistry38-3<). 

Inspired by the elegant approach developed by Stara, Stary, and coworkers (08JOC2074, 11EJ03849, 12AGE5857) for the synthesis of [ ]-heterohelicenes, Marinetti, Voituriez, and coworkers reported in 2014 the preparation of a new series of enantioenriched [6]-oxahelicenes 84 in which the terminal helical sequence ends with a phosphole oxide (14CC2199). The synthesis starts from the phosphindoles 82 and involves a metal-mediated diastereoselective [2+24-2] cycloisomerization of optically pure aromatic triynes 83 as a key step (Scheme 22). Under optimized... 

Roulland and coworkers utilized a [2-t2-t2] cycloaddition and an RCM-aromatization strategy in an innovative approach to the central core of the landomycinone natnral prodncts [46]. As shown in Scheme 17.24, the triyne 124 was successfully cycUzed with the catalyst RhCl(PPhj)j to afford compound 125, after which vinyUithium addition resulted in the hemiketal diene 126. Immediate... 

A mechanism has been proposed by Blechert for this metathesis cascade, which involved the formation of a number of carbon-carbon bonds (in principle, a ruthenium-mediated [2-I-2+2] cycloaddition is also plausible for this transformation [49]). This postulated mechanism, as shown for the conversion of triyne 141 into the substituted aromatic system 142, is depicted in Scheme 17.27 [50]. Initially, complex 1-Ru adds to the less hindered acetylene of 141 to afford the vinyl carbene complex 143, which then undergoes an intramolecular metathesis reaction to afford 144 via 145. The conjugated complex 144 can then undergo a further RCM reaction to yield the product 142. 

The sequential intramolecular hydroarylation of alkynes is applied to the synthesis of structurally complex extended x-systems. The gold-catalyzed sequential intramolecular hydroarylation of triynes followed by aromatization with DDQ proceeded to give triaryl-substituted diacenaphtho[l,2-y l, 2 -l] fluoranthenes, which can be used for organic light-emitting devices (Scheme 21.49) [55]. 

The cyclotrimerization can also be used to form unusual and challenging aromatic systems, including biaryls and helicenes (Scheme 11.22)." ° The triyne substrate 11.63 was constructed by a Sonogashira reaction of... 

Completely intramolecular alkyne cyclotrimerization was also achieved using triyne substrates to produce tricyclic aromatic compounds [18, 24] [Eq. (14)]. 

In 2010, Stara, Stary, and co-workers developed the synthesis of fully aromatic [5]helicenes via the cobalt-mediated intramolecular [2- -2-1-2] cycloaddition of triynes followed by the double silica gel-assisted acetic acid elimination (Scheme 10.4) [7]. A diazapentahelicene (X = N, R = H) was also synthesized in good yield by this method (Scheme 10.4) [7]. 

Typical substrates for Fritsch-Buttenberg-Wiechell rearrangements are chloro-ethenes and ge/w-dibromoethenes. They are readily prepared by Wittig reaction from the corresponding ketones. The method offers a versatile entry to aromatic or heterocyclic acetylenes. Moreover, ethynyl groups can embark also on 1,2-migrations, thus, making simple or functionalized diynes and triynes (e.g., 321) accessible (Scheme 1-250). ... 

---