Phenylacetylene/norbornene

The side products of the reaction between benzoylnitromethane 279 and dipolarophiles (norbornene, styrene, and phenylacetylene) in the presence of l,4-diazabicyclo[2.2.2]octane (DABCO) were identified as furazan derivatives (Scheme 72). The evidence reported indicates that benzoylnitromethane gives the dibenzoylfuroxan as a key intermediate, which is the dimerization product of the nitrile oxide. The furoxan then undergoes addition to the dipolarophile, hydrolysis, and ring rearrangement to the final products (furazans and benzoic acid) <2006EJ03016>. 

An important procedure for the synthesis of cyclopentenones is the so-called Pauson-Khand reaction, which constitutes a formal [2 + 2 + 1] cycloaddition of an alkene, an alkyne, and carbon monoxide. Due to the increase in structural diversity of the available starting materials, the reaction has become an attractive target for scientific investigations [1-8]. The first successful example was reported by Pauson, Khand et al [9] in 1973 for the conversion of norbornene with the phenylacetylene-hexacarbonyldicobalt complex to give the corresponding cyclopentenone in 45% yield (Eq. 1). 

Interesting evidence supporting the mechanism of polymerisation of acetylenes via carbene species is provided by the block and random copolymerisation of acetylenic monomers with cycloolefins. For instance, block copolymers of acetylene and cyclopentene with the WC —AlEtCT catalyst [41] and block copolymers of acetylene and norbornene with the (MeA. Oj2W(=NAr)= CHMe3 catalyst [42] have been obtained moreover, random copolymers of phenylacetylene and norbornene with the WC16 catalyst have also been obtained [149, 150],... 

We could thus expect that this reaction terminated the palladium- and nor-bornene-catalyzed reaction sequence in place of the acrylic ester or terminal olefins in general. Considerable difficulties were met, however, because the alkyne interacted with all the palladium complexes of the sequence, giving rise to a number of by-products. Starting from 1 equivalent of aryl iodide, 2 equivalents of alkyl bromide, 1 equivalent of norbornene, 0.3 equivalents of aryl-acetylene, 8 equivalents of KOAc, and 0.1 equivalent of Pd(OAc)2 and adding gradually 2 equivalents of alkyl bromide and 0.7 equivalents of arylacetylene (to keep the concentration of the latter low) satisfactory results were obtained. Equation 30 reports the reaction withp-fluoroiodobenzene, n-propyl bromide, and phenylacetylene, which gave a 79% yield (71% with iodobenzene) [37]. 

The overall catalytic process, including both phenylacetylene coupling and diphenylacetylene insertion, is depicted in Scheme 2. The reaction proceeds according to the previously shown pattern until the o,o -dialkylated arylpalladium complex is formed. At this point coupling with phenylacetylene occurs to the extent allowed by the concomitant formation of diphenylacetylene as soon as phenylacetylene disappears diphenylacetylene is readily inserted. The resulting vinylpalladium species now reacts with norbornene and cyclization on one ring of diphenylacetylene affords the final product. It is worth noting... 

In 2010, Buchmeiser [56] developed a similar system that capitalized on the thermally reversible carboxylation [11] of NHCs (Scheme 31.13, inset). By employing the NHC-CO2 adduct (which essentially is a protected NHC), the reaction conditions did not have to be stringently air- and moisture-free to prevent NHC decomposition. Synthesis of the norbornene-functionalized monomer 37 allowed the molybdenum-catalyzed ROMP with l,4,4a,5,8,8a-hexahydro-l,4,5,8-exo-ewdo-dimethanonaphthalene (a ditopic norbornene) to produce crossHnked polymer 38 with pendant CO2-masked NHCs (Scheme 31.13). Upon heating in the presence of Rh, Ir, or Pd species, the NHC-metal-functionalized polymers 39 were formed and found to contain >20mol% metal, as determined with inductively coupled plasma optical emission spectrometry (ICP-OES). The C02-masked NHC material was found to catalyze the carboxylation of carbonyl compounds and the trimerization of isocyanates upon thermal deprotection (i.e., decarboxylation). Moreover, the NHC-metal-crosslinked materials were found to catalyze Heck reactions, transfer hydrogenations, and also the polymerization of phenylacetylene (M = 8.4 kDa, PDI = 2.45, as determined with GPC in DMF against PS standards). This modular system provides an array of options for catalysis from simple modifications of polymer-supported, C02-masked NHCs. 

The binuclear compounds as well as the mononuclear seven-coordinate bis(nitrile) compounds are catalysts for the polymerisation of terminal alkynes such as phenylacetylene (PA) or ferf-butylacetylene (f-BA) and for the ROMP of norbornene (NBE) and norbornadiene (NBD) [20, 38-42]. The first step in all catalytic reactions is the coordination of the organic substrate to metal and the formation of a complex. Sometimes it was possible to isolate the products in crystalline form but frequently such adducts were observed only in situ by NMR investigations. 

The reaction of l-aza-2-azoniaallene salts derived from coumarins with alkynes does not afford cycloadducts but rather leads to intramolecular cyclization. In contrast, a [3+2] cycloadduct is obtained in 92 % yield (mixture of diastereoisomes, ratio 2 1) from the same salt in the reaction with norbornene. From the camphor salt 41 (R = 2,4,6-trichlorophenyl) and phenylacetylene a 53 % yield of the cycloadduct 42 is obtained . 

Furthermore, ring-substituted phenylacetylenes were employed in reactions with norbornene in the presence of WClg-based catalysts to prepare the corresponding copolymers (111-113) (R = CH3, CF3, SiMe3) [81, 82] [Eqs. (43)-(45)]. 

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