Acetylene, functionalized

A frequent problem is selective reduction of an acetylene to the olefin in the presence of other easily reducible functions. Usually the reaction can be done without difficulty because of the relatively strong and preferential adsorption of the acetylenic function on the catalyst. Functions adjacent to the triple bond may cause special problems if the resulting allylic compound is itself susceptible to facile hydrogenolysis (18,23). The product composition is, as expected, sensitive to steric effects (82). 

Hydrogenation of 2,5-diacetoxy-2,5-dimethyl-3-hexyne 10 over 0% palladium-on-carbon is exceptionally complex. Seven different products are formed together with acetic acid. All are hydrogenolysis products arising from the initially formed 2,5-diacetoxy-2,5-dimethyl-3-hexene 11. One of these, 2,5-dimethyl-2-acetoxy-4-hexene 12 forms in as much as 4S yield. 

The authors suggested a concerted addition of hydrogen via a six-centered transition state that would produce 12 directly from the olefin (11) (61). Addition of pyridine or quinoline alters the reaction, but it remains complex. 

Hydrogenation of acetylenic carbinols is sometimes accompanied by isomerization to the ketone. In the case of 13, the isomerization to 15 was most pronounced when hydrogenations proceeded very slowly as when impure 13 was used (39). 

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In contrast to the previous method (equation 70), reaction 72 made possible the preparation of iodonium triflates from functionalized acetylenes bearing an electron-withdrawing group such as tosyl, cyano, or carbonyl [138]. Of special interest is the application of this method to the synthesis of the bisiodonium acetylenic salt [139, 140] (equation 73). 

F. Diederich, Functional Acetylenic Molecular Architecture , Pure Appl Chem. 1999, 71, 265. 

By cobalt-lithium exchange, the group of Sekiguchi and coworkers generated several dilithium salts of variously substituted cyclobutadiene dianions . By the reaction of the functionalized acetylenes (e.g. compound 137) with CpCo(CO)2 (136), the corresponding cobalt sandwich complexes, related to compound 138, were obtained (Scheme 50). These can be interconverted into the dilithium salts of the accordant cyclobutadiene dianions (e.g. dilithium compound 139) by reaction with metallic lithium in THF. Bicyclic as well as tricyclic (e.g. dilithium compound 141, starting from cobalt complex 140) silyl substituted systems were generated (Scheme 51) . ... 

This synthetic strategy proved to be unsuitable for [M(CO)5] (M = Cr, W) metal fragments due to the thermal instability of the corresponding non-donor-substituted allenylidenes [M(=C=C=CR R )(C0)5] (R, R = usually alkyl or aryl groups). An alternative general synthetic procedure using deprotonated functionalized acetylenes has been successfully applied (for example tris-amino or alkoxo prop-1-ynes see Equation 2.2) [4a]. 

The formation of liquid crystals (LC) from suitably functionalized acetylenic SPMs has often been the motivation for their synthesis. A number of groups have sought to capitalize on the planar or nearly planar disk-like geometry of SPMs to promote the formation one-dimensional columnar stacks based on a combination of n-n stacking, van der Waals, dipole, and hydrophobic interactions. For example, Heiney, Moore, and coworkers have reported the high-resolution X-ray diffraction analysis (XRD) of a tubular, discotic LC based on SPM 133, which has been synthesized via a Sonogashira reaction [106]. These studies reveal an unanticipated distortion and... 

Variation of the Procatalyst (Metal Component) and the Acetylenic Substrate. The in situ catalysts Co(acac)3-Et2AlCl-phosphine have proven to be well-suited for the synthesis of 4-aryl- and 4-alkyl-substituted deltacyclenes. The catalysts tolerate remote oxygen functionalities in the acetylenic substrate. However, they could not be used with functionalized acetylenes such as propargylic acid derivatives. 

A variety of functionalized acetylenes have been obtained by dehydrohalogenation with NaNHz in liquid ammonia with the functions remaining intact, as illustrated in several examples (equations 54-56). Stearolic acid and 2-butyn-l-ol were obtained... 

Tsu]i, J. Ring-closing metathesis of functionalized acetylene derivatives a new entry into cycloalkynes. Chemtracts 1999,12, 522-525. 

Functionalized Acetylenes in Organic Synthesis - The Case of the 1-Cyano- and the 1-Halogenoacetylenes... 

In toto, functionalized acetylenes have not been summarized recently [1]. However, there are some excellent reviews which may be consulted in order to become familiar with this class of unsaturated compounds. Monographs which have become classics in the field of acetylene chemistry have been compiled by Raphael [2], Rutledge [3], and Viehe [4]. There are furthermore excellent volumes on acetylenes in the Houben-Weyl-Mtlller [5] and in the Patai series... 

A recent article by E. Winterfeldt, Acetylenes in synthesis, in the Modern Synthetic Methods Series, Vol. 6, (Ed. R. Scheffold), Verlag Helvetica Chimica Acta, Basel, and VCH, Weinheim, 1992, pp. 103-226, assembles a wealth of interesting information on the use of functionalized acetylenes in modem organic chemistry, with, again, no attempt to cover this group of compounds totally. 

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