Preparation, Properties, and Reactions of Metal‐Containing Hetercycles, XC. Reactivity of Differently Activated Alkynes toward Ruthenium and Osmium Complexes of the Type (η 2 ‐C 2 H 4 )M(CO) 4

Depending on the substituents, the reaction of the activated alkynes ZC≡CZ ( 2a ‐ g ) [Z = CO 2 R: R = Me ( a ), Et ( b ); Z = R 1 : R 1 = CF 3 ( c ), CH 2 Cl ( d ), CH 2 Br ( e ); Z = CH 2 OC(O)R 2 : R 2 = Me ( f ), CCl 3 ( g )] with the labile ruthenium complex (η 2 ‐C 2 H 4 )Ru(CO) 4 ( 1 ) results in the formation of three different types of heterocycles. While the reactions of the dialkyl acet‐ylenedicarboxylates 2a , b lead to the dimeric tricarbonylru‐thenacyclopentadienes 3a , b , being catalytically active in the cyclotrimerization of alkynes like 2a , b , the application of the 1,4‐halogeno‐2‐butynes 2c‐e yields the bicyclic heterocycles 4c ‐ e. The esters of 2‐butyne‐1,4‐diol 2f , g are converted into the tetracarbonylruthenacyclopentadienes 5f , g. Investigation of the primary attack of the alkynes 2a ‐ g at complex 1 leads to the conclusion, that an ionic mechanism is preferred in the ruthenium‐catalyzed cyclotrimerization of electron‐poor alkynes. If, instead of 1 , (η 2 ‐C 2 H 4 )Os(CO) 4 ( 6 ) is allowed to react with the acetylenes 2a , b the osmium complexes (η 4 ‐C 6 Z 6 )Os(CO) 3 ( 7a , b ) are isolated. In the presence of CO at 2 bar 7a, b release the benzene derivatives C 6 Z 6 ( 8a, b ) with the formation of Os 3 (CO) 12 .