Medium-sized 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. 

Medium-size cycloalkynes.1 In the presence of BF, etherate, a cobalt com-plexed propargylic ether can undergo an intramolecular alkylation with an allylic silane to provide six-, seven-, and eight-membered complexed cycloalkynes. This reaction is an extension of propargylation of allylsilanes to provide 1,5-enynes (10, 129-130). 

The carbonyl Co2(CO)6 forms stable 7i-complexes of alkynes 0/2 complexes). Four effects on alkyne reactivity are expected from this coordination (i) protection of the triple bond (ii) stabilization of the carbonium ion on the a-carbon (or propargylic cations (iii) syntheses of common and medium-size cycloalkynes and (iv) steric effects. 

Inspection of molecular models reveals that cyclononyne should be slightly strained, while all smaller cycloalkynes should be strongly strained. In fact up to the present time, cycloheptyne is the smallest cycloalkyne to have been isolated -Since the reactivity of medium-sized cycloalkynes increases with decreasing ring size (see Section III), one may expect the formation of smaller cycloalkynes as highly reactive reaction intermediates. 

A range of different synthetic procedures is known to obtain medium-sized cycloalkynes, as covered by several reviews [38] Tochtermann [25]. In this paragraph, only the most relevant procedures will be covered. 

Fig. 1 Oxidative decomposition of cycloalka-l,2-dione dihydrazone, leading to medium-sized cycloalkynes (n = 1-5)...

Meier H (1972) Synthesis of medium-sized cycloalkynes. Synthesis 235-253... 

Only a year later, Gagosz and coworkers [32] published a synthesis of 10-membered medium-sized cycloalkynes 28 from 1,10-diynes 27. Like in the earlier investigation of Houk and Toste, the ligand was a bulky phosphane ligand. Two important mechanistic proposals were included in this publication the one shown in Scheme 4.10a which is based on the proposal of Houk and Toste [31], and also the alternative pathway shown in Scheme 4.10b. Both pathways are in accordance with the deuterium isotope labeling experiments conducted. The Houk/Toste catalyst transfer step was not considered by Gagosz. On the other hand, Gagosz already addressed a possible role of the counterion as a proton acceptor, a motive later discussed by Corma [33] and Widenhoefer [34] for the formation of species related to I and Q. 

The valence-angle constraints imposed by cyclization lead to very high strain energies (Es) in the small- and medium-ring cycloalkynes. Members of the series below ring size 6... 

Selenadiazoles 176, which are readily formed by the action of selenium dioxide on semicarbazones of a-methylene ketones , decompose thermally to acetylenes (equation 25)106 the Lalezari reaction, is widely applicable. Recent uses are illustrated by the synthesis of four medium-sized cycloalkadiynes cyclonona-l,5-diyne (177), cyclodeca-1,5-diyne (178), cycloundeca-l,5-diyne (179) and cycloundeca-l,6-diyne (180) (Scheme 5) . Strained cycloalkynes with additional double bonds, e.g. the cyclododeca-3,7,11-triene-l-yne 181, have also been obtained by the Lalezari method. Compound 181 isomerizes spontaneously to the bicyclic hydrocarbon 182 . ... 

Ring enlargement of cycloalkenes by conversion into the corresponding geminal dihalogenocyclopropanes, followed by solvolysis with silver salts, has provided a useful approach to medium-sized l-bromo-rra 5-cycloalkene derivatives, themselves useful precursors of medium-ring cycloalkynes (Scheme 32). ... 

Using the decarboxylative photocyclization, the synthesis of medium- and macrocyclic amines, lactones, polyethers, lactams, as well as cycloalkynes were accessible in ring sizes up to 26 (Scheme 14) and even in multigram quantities. The regioselectivity of the radical cyclization step was studied for unsymmetrically substituted pyridinecarboximides and trimeUitic acid imides, respectively, where the site of the C-C-bond formation is controlled by the maximum spin density of the radical anion intermediates. ... 

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