Acetylenes with cycloalkenes

Table 10.7 Metathesis copolymerization of acetylenes with cycloalkenes...

Commercial development of a range of cycloalkene-cobalt homogeneous catalysts has prompted their application in the synthesis of pyridine and 2-substituted pyridines. Thus, bis(cyclopentadienyl)cobalt catalyzes the reaction of acetylene with hydrogen cyanide, acetonitrile or acrylonitrile to yield pyridine, 2-methylpyridine and 2-vinylpyridine respectively (Scheme 4 R = H, Me or CH=CH2) (76S26, 78AG(E)505, 75BEP846350). The high cost of the catalyst has so far limited full commercial realization of this route. Acrylonitrile... 

Acetylenes copolymerize with each other and with cycloalkenes under the influence of olefin metathesis catalysts. With non-living systems it is possible to make statistical... 

At 184.9 nm, the fragmentation as well as the isomerization of substituted cyclobutenes, namely bicyclo[3.2.0]hept-6-ene and bicyclo[4.2.0]oct-7-ene, were studied in pentane solutions. Acetylene and cycloalkenes are the major photoproducts, whereas the Woodward-Hoffmann allowed ring opening gives rise to 1,3-cyclodienes with lower quantum yields (90). 

The sequential addition method also allows the synthesis of many different block copolymers in which the two monomers have different functional groups, such as epoxide with lactone, lactide or cyclic anhydride, cyclic ether with 2-methyl-2-oxazoline, imine or episul-Hde, lactone with lactide or cyclic carbonate, cycloalkene with acetylene, and ferrocenophane with cyclosiloxane [Aida et al., 1985 Barakat et al., 2001 Dreyfuss and Dreyfuss, 1989 Farren et al., 1989 Inoue and Aida, 1989 Keul et al., 1988 Kobayashi et al., 1990a,b,c Massey et al., 1998 Yasuda et al., 1984]. 

Ziegler-Natta catalysts are not active at all in polymerization of disubstituted acetylenes.415 Mo- and W-based systems (for alkynes with small substituents) and Nb- and Ta-based catalysts (for alkynes with bulky groups), in turn, are very effective catalysts used to convert disubstituted acetylenes into polymers with very high molecular weight.414 415 A polymerization mechanism similar to that of metathesis polymerization of cycloalkenes are supported by most experimental observations.414 423 424... 

We have once earlier used the term cycloethene for acetylene in the context of defining the strain energy of acetylene in terms of ethylene, i.e. AHf(g, C2H2) vs 2AH g, C,H4) AH g, C2H6)], in order to compare that quantity with the strain energies of cyclopropene and other cycloalkenes cf Reference 4, p. 94. 

An interesting case is the coordination polymerisation of acetylene and higher alkynes. It may proceed by a mechanism quite similar to the metathesis polymerisation of cycloalkenes involving metal carbene and metallacycle (metallacyclobutene) species [45], The initiation and propagation steps in alkyne polymerisation (leading to a polymer of cis structure) in the presence of a catalyst with a diphenylcarbene initiating ligand are as follows ... 

Essentially the same methods of synthesis of open-chain acetylenes had been adopted for the preparation of cyclic acetylenes, e.g. cyclopentadecyne (1) and cyclohcptadecyne (2) were obtained by the reaction of potassium hydroxide with the corresponding 1 -bromo-cycloalkenes. ... 

Other cycloalkenes give mixed results. Cyclopropenes, unless heavily substituted, tend to react with each other rather than with acetylenes [lOS]. Cyclohexenes (and larger rings) lack useful reactivity in general, although an interesting exception is the homoallylic amine derivative shown in Eq. (45)]. Evidence from this and related systems suggests that prior com-plexation of the heteroatom to a cobalt atom in complex 12 (Scheme 5.2) facilitates complexa-tion and, ultimately, cycloaddition of the double bond [106]. 

Carbonylative cycloaddition of allyl bromides and acetylenes in methanol in the presence of nickel tetracarbonyl results in the formation of cyclopentenones. For instance, 3-bromocyclopentene and methyl but-2-ynoate afford the diester 493, the halogen atom being replaced by the methoxycarbonyl group. Nickel tetracarbonyl also catalyses the reaction of l-(bromomethyl) cycloalkenes 494 of ring size 5-8 with methyl but-2-ynoate in methanol to yield the spiro-compounds 495 (equation 54). ... 

Further evidence comes from the stmcture of statistical copolymers of cycloalkenes (Mj) with acetylenes (M2) in which the M1M2 junctions are present in sufficient proportion to be detectable by NMR. For example, a copolymer of cyclooctene and 2,4-dichlorophenylacetylene, containing 15% of Mi units, has been prepared using WCl4[OCH(CH2Cl)2]2/Et2AlCl as catalyst. The reactivity ratios are such that these units are nearly all flanked by M2 units and the olefinic... 

Unsaturated Ring Ketones.—Acetylene hexacarbonyldicobalt complexes react with ethene, alkenes, and cycloalkenes to provide a general synthetic route to cyclopentenones. Unfortunately most monosubstituted ethenes react non-specifically. 2-Bromoethylallene gives 2-methylcyclopent-2-enone on treatment with Collman s reagent. 

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