Flyover complex

In the presence of higher concentrations of phosphines, di- and trisub-stituted products may be obtained (727), and interesting examples of the stereospecific formation of isomers by electron-transfer catalysis have been reported (123,124). The flyover complex [Co2(CO)4 /u,-C6(CF3)6 ] (53) forms the bis(phosphite) derivative [Co2(CO)2 P(OMe)3 2(/u-C6(CF3)6 ] when reduced in the presence of excess P(OMe)3, but two different isomers can be exclusively produced depending on the applied electrolysis potential. At the potential of the one-electron reduction of 53 (E = -0.1 V vs Ag/AgCl), isomer 54 is formed, but, at a potential appropriate for the reduction of 54 ( = —0.7 V), quantitative conversion to isomer 55 is achieved. The reaction is electrocatalytic in both cases. 

A number of other mechanistic sequences are thought to occur in alkyne trimerizations mediated by other metal systems. Catalysts derived from Co2(CO)s follow a characteristic sequence of steps that involves exclusively dinuclear complexes (Scheme 26). The nature of the final (alkyne)3C02(CO)4 intermediate, a so-called flyover complex, is supported by both X-ray crystal structure data as well as its involvement in forming other products besides benzenes. These dinuclear cobalt systems are note-... 

MeC2Me)2(Tl -indenyl)]+ gives a flyover complex, [Mo2( J.-C6Me6)(T -indenyl)2], in which an alkyne trimer bridges the two metal atoms in this case the Q chain begins at one metal and ends at the second. 

It has long been known that reaction of Fe2(GO)9 with ethyne produced tropone as one of the products and using the dppm-stabilized complex, Fe2(CO)6(/ -GO)(/x-dppm) 11, this process has been shown to occur in a stepwise manner. Thus, the tropone complex 39 results upon prolonged photolysis of 11 and ethyne, while over shorter timescales dimetallacyclopentenone 37 and the flyover complex 38 are the major products (Scheme 4). " ... 

Reactions of alkynes with iron carbonyl species afford a variety of products (usually with a low selectivity) depending on the structure of the alkyne and the reaction conditions as already described in COMC(1982). Major products are the trinuclear /X3-alkyne clusters with the - 1 and -coordinated alkyne ligands 2, the trinuclear di(/X3-alkyne) complexes 3, the di- 4 and trinuclear ferrole complexes 5, and the diiron flyover complexes 6 (Scheme 1). 

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