Pentadiene complexes

Pentadentate 2,2 6, 2"-terpyridine, 30 107 Pentadiene complexes with iron, 12 255-257 with rhodium, 12 295 with silver, 12 340 Pentadienyl complexes with group VIB metals, 12 224, 225 with group VIIB metals, 12 240 JV,Af,/V,lV",N"-Pentaethyldiethylenetriamine, 42 76... 

The analogous cycloheptadienyl complex (5 equation 1) was similarly prepared by Dauben and Ber-telli,4 but the acyclic pentadienyl systems were a little more difficult to obtain. The triphenylmethyl cation does not remove hydride from tricarbonyl(frans-pentadiene)iron (6 equation 2). The corresponding c/s-pentadiene complex (7 equation 3) cannot be prepared directly from the diene and an iron carbonyl,... 

P(CH3)3, P(OCH3)3], For each of the [Cr(CO)3(j/4 CH-diene)] complexes 39g, 37n, and 37o two enantiomers are expected. As a consequence of the C—H—Cr bridge from C-5 or C-6 of the 1,3-cyclohexadiene, and from C-5 or C-7 of the 1,3-cycloheptadiene, both ligands lose their inherent mirror planes. The situation is somewhat different with the 2,4-dimethyl-1,3-pentadiene complex (37n). Here only the 4Z methyl group is able to form the C—H—Cr bridge. Due to the two coordination modes of the diene, R or S, a pair of enantiomers is present for 37n. Finally, 37r forms two diastereomeric pairs of enantiomers, because the C—H—Cr bridge can be formed from C-5 or C-6 and the diene can be coordinated in the R or S mode. 

Pentadienyls may be considered as vinylogues of allyl complexes and accordingly parallels exist, both in their synthesis and reactivity. These are most widely prepared by transmetallation from lithium or Grignard reagents, nucleophilic attack at pentadienyl halides or via hydride abstraction from rj4-pentadiene complexes (Figure 7.13). 

Scheme 4. Cyclization of 2-amino-l-metalla-l,3,5-trienes to cyclopentadienes via r 1 cyclo-pentadiene complexes.

These diene dimerizations might start with the formation of cationic Ru(IV) bis(jt-allyl) complexes rather than ruthenacyclopentanes such as 137 (Scheme 4.49). This was confirmed by the stoichiometric reaction of the 1,3-pentadiene complex... 

In helium quantitative yield of HCI. remainder residue and hydrocarbons, benzene is major volatile hydrocarbons product aliphatic hydrocarbons, benzene (major product), toluene, ethylbenzene, o-xyiene, monochlorobenzene styrene, vinyl tcriuerre. p-dichlorobenzene, o-dichlorobenzene, indene, 1,3.5-trichlorobenzene 1.2.4. richlorobenzene. naphthalene, u-methylnaphthalene. p-methylnaphthatene effect of ZnO. SnOj, and Ab03 on the yields of products Is also recorded HCI. CO2. ethene. ethane, propane, 1-butene. 2-butene. 1-pentene. cydopentene, n-pentane, 2-methylbulane, 1,3-pentadiene. 2-methyl-1,3-pentadiene, complex series (60 Identlfled) of aromatic and polyaromatic species including benzene, styrene, methylstyrenes, toluene, o-xytene, m-xylene, p-xylene, biphenyl, naphthalene, anthracene, phenanthrene. pyrene, etc. 

Metallacycles have been suggested as intermediates in many transition-metal catalyzed reactions of olefins, acetylenes, and cyclopropanes. Metallacyclobutane complexes are invoked in olefin and cyclopropane isomerization schemes (Scheme as well as in olefin metathesis schemes. Metallacyclopentane, -pentene, and -pentadiene complexes can all be invoked in olefin and acetylene dimerization and polymerization (Scheme Many of these involve early transition metals and do not include phosphine ligands. 

Cyclopentadienones are reactive and versatile diene systems that can be used to construct polycyclic compounds and polymeric materials. The early preparation of cyclopentadienones via Rh-catalyzed [2+2+1] cycloaddition of two alkynes and CO often required a stoichiometric amount of a metal complex and a stepwise process, as shown in Scheme 2-18. The rhodium-pentadiene complex... 

Metallocyclopentadiene complexes are often involved in cyclotrimerization reactions of alkynes. Studies on the complex (36) reveal that these alkenyl-metallocyclo-pentadiene complexes are more efficient catalysts for cyclotrimerization reaction than (37). The significant enhancement of catalysis by the alkenyl ligand may be a... 

The probable role of tricarbonyl complexes, e.g., Fe(CO)3(alkene)2, in these isomerization cycles has been demonstrated by preparing these species photolytically from Fe(CO)5 [66,79]. Both Fe(CO)3(alkene)2 (and Ru(CO)3(alkene)2) proved active thermal catalysts for the isomerization of 1-pentene at 293 K. However, a competing dehydrogenation process which leads to the formation of stable and catalytically inactive (at 298 K) M(CO)3(Tl -l,3-pentadiene) complexes serves to deactivate these... 

X-ray studies have revealed a long iron-iron bond (3.14 A) in [ Fe(CO)a-(i -allyl) 2], in agreement with the ability of the dimer to dissociate into Fe(CX))3-(allyl) radicals. The complex slowly decomposes in solution to give ferracyclo-pentadiene complexes such as (25) via hydrogen abstraction and carbon-carbon coupling reactions. "... 

Thus raM -l,3-pentadiene-Fe(CO)3 fails to react with the triphenyl-methyl fluoroborate under conditions favorable for hydride abstraction in the cyclic series. On the other hand ctf-l,3-pentadiene-iron tricarbonyl (LXXXV) reacts readily under the identical conditions to yield pentadienyl-iron tricarbonyl fluoroborate (LXXXVI) 32). The same cation is obtained from reaction with strong acids (37). Hydrolysis of the cation (LXXXVI) regenerates the alcohol complex while reduction with NaBH4 produces (LXXXV) this latter reaction constitutes the only means to date of making the ar-pentadiene complex (32). These reactions are given in Eq. 14. 

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