Double-bonded complexes

In principle, the direct cydopropanation of vinylallenes and related substrates also allows the preparation of cydopropylallenes. However, because of low or no discrimination between the differrent double bonds, complex product mixtures are to be expected. A case in point is provided by 1,2,4,5-hexatetraene (12), which yields the cyclopropylallene 149 on diazomethane/CuX treatment - but only in a (difficult to separate) mixture with all other cydopropanation products of 12 (Scheme 5.21) [61]. 

Ethynylcarbazole was apparently formed by sodamide treatment of 9-dichlorovinylcarbazole (97) and zinc reduction of 97 gave 98. Where no reaction occurred with the Z isomer, 99 gave the dimer 100 with boron trifluoride the process presumably involves Lewis acid-catalyzed alkylation of one double bond by another double bond, complexed to boron trifluoride, followed by intramolecular Friedel-Crafts type alkylation at the carbazole 1-position. ... 

C-l and C-6 carbon atoms in form (XLVb) would result in the formation of a terminal double bond complexed to the nickel atom and might be supposed to favor reaction through this isomer. 

The structural aspects of tin-transition metal complexes are well studied. Some recent examples of metal-tin single and double bonded complexes are provided in Tables 11 and 12. 

From the studies of the quadruple, triple, and double bonded complexes examined thus far, it will prove difficult to photo-chemically cleave multiple metal-metal bonds in a dissociative fashion. Photochemical cleavage of multiple bonds is not taboo, since the light induced cleavage of 02 and C02 is well known. 

In a novel use of photolytic addition of H2 across the metal-metal multiple bond, the two-H-atom-bridged double-bonded complex shown below has been prepared (equation 12). 

Many electrophilic agents have been employed for ring closure and the nature of the electrophile plays an important role. The mechanism of the electrophilic attack, however, has not been completely clarified13 15. Thus, in halogen attack, both the formation of an onium ion and a concerted mechanism with attack of the nucleophile on a halogen-double bond complex have been proposed for substrates containing an internal nucleophile. 

On the basis of these data, different mechanisms have been suggested. In fact, attack of the nucleophile on a bromonium ion is thought to be the rate determining step of bromocyclization, while in iodocyclization the rate determining step is attack of the nucleophile on the iodine-double bond complex. 

Less crowded double bonds complex more readily to the catalyst surface, resulting in faster reaction. 

In those cyclic olefins, having both cis and trans double bonds, complexing with silver ion occurs preferentially at the trans (372), undoubtedly as a result of the higher strain energy associated with the trans endooyclic double bond (416). The 2 1 and 3 1 stoichiometry of the... 

An elaborate strategy was needed for the synthesis of a palladium(O) complex with a quinonemethide ligand coordinated by the exocyclic double bond (complex 12 in Scheme 13), which involves aiming at the synthesis of the unstable palladium(n) complex of the zwitterionic form of the molecule 13. This non-observed intermediate spontaneously affords the desired complex. Treatment with dba produces the unstable free quinonemethide, which is a biologically relevant metabolite. The crystal structure of 12 was solved and showed a notably elongated exocyclic double bond (1.437 A) and a displacement of this bond out of the ring plane by 10.78° away from the palladium atom. 

Reaction of the Ru=Ru double bonded complexes [Cp2Ru2(M.-CO)(p,-C2R2)]. (R = Ph, CF3), with [Fe2(CO)9] afforded the new mixed metal trinuclear complexes [Cp2Ru2Fe(CO)3(p-CO)(p3-CO)( l3-C2R2)]. which was structurally characterised for R = Ph. Reactions of the products were reported. 

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