Cumulene bridges

Hydrogenolysis of the acetylene-cumulenes 19 can be achieved by means of a partially poisoned palladium catalyst to yield the regular [22]porphyrin(2.2,2.2) 20 with cisjrans.cis,-trans configuration of the two-carbon bridges. 

Another [5]radialene ketone, the tetraallene 102a, has been prepared in 74% yield by treating the [5]cumulene 98a with stoichiometric amounts of Ni(CO)2(PPh3)267 the sterically very shielded ketone 102b is produced analogously in 32% yield from the corresponding terminally bridged [5]cumulene 98b (equation 11). In both cases the dimers of the [5]cumulenes, the peralkylated [4]radialenes are formed as well (see Section II.B, Scheme 11). 

While the even-numbered cumulene gives (Z)-111 of C2y symmetry and ( )-112 of C2h symmetry, the odd-numbered cumulene gives two enantiomers 113 and 114 both of C2 symmetry. Bridging these two substituents A and A in 113 or 114 by a chain gives rise to a single-bridged allene of C2 symmetry whose chirality can be specified following the axial chirality rule 11 ... 

As the ruthenium complex 109 is oxidized the carbon bridge becomes cumulenic, more in keeping with the Re complexes 96 than with the iron complex 107. As indicated above, this has been attributed to a greater interaction between the higher lying metal d orbitals of the heavier metals with the filled carbon 71-orbitals. The cumulenic dication is observed to be... 

The structure of the dilithiobutatriene (48) (67), prepared by lithium reduction of 1,4-di-t-butyldiacetylene, is a dimer with nearly parallel, C4 cumulene units. The four lithium atoms serve as bridges between the units. 

Finally, the cation of a fascinating /i-C=C—C=C-bridged diruthenium complex shows IVCT bands characteristic of a class-III Robin-Day system.45 The FTIR spectroelectrochemistry also showed a 100 cm-1 shift in u(C=C) to lower frequencies, characteristic of cumulene-type chains. Data were also gathered for the complex after further oxidation. 

A I Edited by Randolph P. Thummel, Department of Chemistry, University of Houston Volume 1,1989,467 pp. 97.50 ISBN 0-89232-869-X CONTENTS Introduction to the Series An Editor s Foreword, Albert Padwa. Preface, Randolph P. Thummel. Isobenzo-furans, Bruce Rickbom. Dihydropyrenes Bridged [14] Annu-lenes Par Excellence. A Comparison with Other Bridged Annulenes, Richard H. Mitchell. [I.m.n.] Hericenes and Related Exocyclic Polyenes, Pierre Vogel. The Chemistry of Penta-cyclo [SAO.O. O3,1 °.05,9] Undecane (PCUD) and Related Systems, Alan P. Marchand. Cyclic Cumulenes, Richard P. Johnson. Author Index. Subject Index. 

Just as vinylidene-type resonance forms constitute one extreme description for the oxidised forms of alkynyl complexes (Scheme 6.4), cumulenic structures may result from the oxidation of oligoynediyl-bridged dimetal complexes (Schemes 6.5 to 6.8). Interesting examples come from the work of Sato, who... 

As we have noted in a previous section of this chapter, cumulenic resonance forms present one extreme description for dimetal complexes linked by all carbon bridges. They are frequently encountered in some oxidation state (mostly the dioxidised one) and also contribute to intermediate ones between the purely cumulenic and oligoynediyl or the cumulenic- and the alkynyl-bridged dicarbyne forms (see Schemes 6.6-6.8). Purely cumulenic wires are encountered in ot,a)-di- or -tetraferrocenyl substituted cumulenes, but no evidence other than a splitting of the ferrocenyl-based redox waves has been presented to support the presence of electronic interactions between the fer-rocenyl end groups across the cumulenic ligands. Based on the results of ex-... 

In the D2h cumulenes, chirality is generated only by trans double bridging. The doubly bridged ethylene 43 ( bis(fn is-polymethylene)ethylene ) cited first in Cahn, Ingold, Prelog s paper (7) is the simplest example of a trans doubly bridged D2h cumulene (42) where p = 0 and m = n = 6. 

An interesting example of a p-q" complex in which both C=0 bonds of a single cumulene molecule are bonded to a metal centre was reported by Sadighi et al. in 2007 [31]. The p-q q -C02 coordination has been demonstrated to exist in a dinickel complex having the core [(IPr)Ni]2(p-C0)(p-q, q -C02) 6, where IPr is l,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene. The CO originates from CO2 and the two CO2 ligands bridge two nickel atoms via two Ni-CO bonds in a side-on way (Structure 2.4). 

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