Strained cycloalkynes

The reduction of medium-size cycloalkynes, however, always yields considerable amounts of the less strained cis-cycloalkenes (A.C. Cope, 1960 A M. Svoboda, 1965). Cyclo-decyne, for example, is reduced almost exclusively to cis-cyclodecene. 

Because of the linear geometry required of cumulated dienes cyclic allenes like cycloalkynes are strained unless the rings are fairly large 1 2 Cyclononadiene is the smallest cyclic allene that is sufficiently stable to be isolated and stored conveniently... 

In situ generation of strained alkenes or alkynes requires photoelimination of a leaving group toward which the targeted product is unreactive. Thus, cycloalkynes may be generated by elimination of CO from cyclopropenones, but these must in turn be generated in situ (e.g., from diazoketones). The generation of ace-naphthyne (Scheme 17.10) may serve as an illustration of this procedure that did, however, not work for the parent cyclopentyne, a compound that still awaits spectroscopic characterization. 

For reviews of triple bonds in rings, see Meier Adv. Strain Org. Chem. 1991, 1. 215-272 Krebs Wilke Top. Curr. Chem. 1983,109, 189-233 Nakagawa in Patai The Chemistry of the C—C Triple Bond. pt. 2 Wiley New York, 1978, pp. 635-712 Krebs, in Viehe Acetylenes-, Marcel Dekker New York. 1969. pp. 987-1062. For a list of strained cycloalkynes that also have double bonds, see Meier Hanold Molz Bissinger Kolshorn Zountsas Tetrahedron 1986, 42. 1711. 

Cycloalkynes display a deshielding with increasingly strained rings, as reported for a series of cyclooctynes [250] an outstanding alkyne carbon shift is found in cyclooctadie-nyne. 

This reaction is useful in conjunction with E2 elimination to prepare alkenyl halides, allenes, and alkynes. Cycloalkenyl halides are easily prepared, but because of angle strain the cycloalkynes and cycloallenes with fewer than eight atoms in the ring cannot be isolated (see Section 12-7) ... 

Strained cycloalkynes can be stabilized by coordination to one or more transition metal centers (198). The unusual vicinal defluorination reaction of perfluoro-l,3-cyclohexadiene with [Co2(CO)8] to give the /i-alkyne complex 45 (see Section III,E) prompted a study of the reactions of OFCOT with cobalt carbonyl precursors. 

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