Nickel(0) Complexes of Five‐Membered Titana‐ and Zirconacyclocumulenes as Intermediates in the Cleavage of CC Bonds of Disubstituted Butadiynes

Abstract

COMMUNICATIONSIn conclusion. we have demonstrated that the oxide anion accelerated retro-Diels-Alder reaction can be employed for the facile generation of cyclic enediynes from 1,5-diyne precursors. The use of 9-BOM-anthracene (2) to mask the double bond of the enediyne offers the advantages of excellent stability towards a variety of synthetic transformations and convenient removal when desired. Although the enediyne precursor is highly stable under acidic or neutral conditions, treatment with base leads to rapid cycloreversion and development of the enediyne. This methodology offers significant scope for the generation of new cyclic enediynes. Precursors such as 11 have potential as novel prodrugs when conjugated to appropriate delivery systems.

Stable Cyclopentynes—Made by Metals!?

“…A similar reaction was found earlier in the complexation of 1‐metallacyclo‐penta‐2,3,4‐trienes V with {Ni(PPh 3 ) 2 } 10d. The cyclotriene complexes that formed also had a planar C4 unit between the metals and were considered to be 1‐metallacyclo‐penta‐2,4‐dienes III 11b. These complexes are intermediates in the cleavage of the CC single bond of butadiynes.…”

What Do Titano- and Zirconocenes Do with Diynes and Polyynes?

“…On the other hand, they are a real complex-chemical proof for the metallacyclocumulene structure of the η 4 -complex. 24 …”

Oxidative Cleavage of the CarbonCarbon σ‐Bond Using Reusable Copper on Iron

“…Transition metal‐catalyzed cleavage of the carbon‐carbon bond,1 particularly the inert carbon‐carbon σ‐bond, is one of the most challenging tasks in organic chemistry. Until now, numerous unique and useful carbon‐carbon bond cleavage procedures have been developed using stoichiometric or catalytic amounts of transition metals, such as rhodium,2 ruthenium,3 gold,4 palladium,5 silver,6 nickel,7 copper8 or iron9 catalysts. Among these procedures, two general routes to the carbon‐carbon σ‐bond cleavage include (i) oxidative addition to the CC bonds with low‐valent metals through an insertion process,7,10 which often employs the strained ring as the reaction partner, and (ii) β‐carbon elimination of the carbon‐metal species (MCCC)3b,11 or heteroatom‐metal species (MYCC, Y=O, N) (Scheme ) 2i,5a–d,12…”