Cyclopentadienyl-ring substituents

Cp derivatives (Scheme 185) to form -oxo-bridged dimeric complexes. The stabilities of the Af,0-bound carbamate intermediates depend mainly on the cyclopentadienyl ring substituents, but there is also a small dependence on the nature of the length of the (dimethylamino)alkyl chain for the C5H4Me systems. ... 

The aromatic complex can be a neutral t °-benzene derivative or an anionic ri -cyclopentadienyl ring. Substituents on these aromatic rings can greatly influence the effectiveness of these catalysts. For example, with benzene derivatives the unsubstituted benzene rings give lower ees and the use of hexamethylbenzene results in lower catalytic activities whilst the cumenyl or mesityl rings give optimum catalyst systems. The two types of chiral bifunctional hnkers that have been most practical are anionic ones based on monosulfonated diamines and amino alcohols. 

The effect of cyclopentadienyl-ring substituents on the reactivity of the group VII half-sandwich complexes (t7 -C5R5)M(CO)2L[M = Mn and Re R = H, Me, and Et (Mn only) L = Kr and Xe] toward CO in supercritical fluid solution at room temperature has been investigated (70). The reactivity of the corresponding alkane complexes ( 7 -C5R5)Mn(CO)2(n-heptane) (R = H, Me, and Et) toward small molecules such as CO, N2, and H2 in n-heptane solution steadily increased in the order H < Me < Et (71). These results indicated that steric rather... 

Introduction of substituents to the cyclopentadienyl ring of 7, or the use of complexes derived from other monodentate chiral alcohols, cause decreased enantioselectivity11,1 la,35a 36. 

Zirconocene dichloride 121 derived from (l-phenylethyl)cyclopentadienyl ligand is formed as a mixture of diastereomers from which the racemic form can be isolated by fractional crystallization. This complex was studied by X-ray diffraction methods and revealed a virtually chiral C2-symmetrical conformation in which the chiral ring-substituents are arranged in a synclinal position relative to the five-membered ring. It was proposed that this conformation is preserved in solution. Using 121 as catalyst the influence of double stereodifferentiation during isospecific polymerization of propylene (Eq. 32) was demonstrated for the first time [142],... 

In most cases oxidation of uncharged borabenzene complexes produces cations which can only be observed electrochemically. The iron compounds 62 and 63 may serve as an example. Oxidation is fully reversible in rigorously dried CH2C12 but irreversible in more basic solvents such as THF and acetonitrile (62). Preparative oxidation with Ce(IV) salts cleanly produces monosubstituted ferricenium cations 64 (Scheme 10) (66). In contrast to the above mentioned boranediyl extrusions, the substituent at boron is retained here in the newly formed cyclopentadienyl ring. 

Metallocenes with substituted cyclopentadienyl rings. Metallocenes with methylated rings were among the first heavy alkaline earth metallocenes to be structurally characterized, but many other substituents have been incorporated into bis(cyclopentadienyl) complexes. Under this classification are included compounds with indenyl ligands, which in... 

Better yields are obtained when polar solvents are utilized and an amine such as tetramethylethyl-enediamine is present, which also facilitates the formation of the dilithium compound. The lithium derivatives undergo a large number of reactions that can be used to produce the enormous number of ferrocene derivatives. Rather than trying to show a great number of reactions, a few of the common reactants and the substituents that they introduce on the cyclopentadienyl rings are shown in Table 21.3. 

The anra-cyclopentadienyl-amido initiators (CpA), often referred to as constrained-geometry initiators, retain one cyclopentadienyl ring of the metallocenes, but replace the other ring with a nitrogen substituent that coordinates with the metal center, usually a group 4 metal... 

Ferrocenyl-based polymers are established as useful materials for the modification of electrodes, as electrochemical biosensors, and as nonlinear optical systems. The redox behavior of ferrocene can be tuned by substituent effects and novel properties can result for example, permethylation of the cyclopentadienyl rings lowers the oxidation potential, and the chaige transfer salt of decamethylfer-rocene with tetracyanocthylene, [FeCpJ]" (TCNE], is a ferromagnet below = 4.8 K, and electrode surfaces modified with a pentamethylferrocene derivative have been used as sensors for cytochrome c These diverse properties have provided an added impetus to studies on ferrocene dendrimers. 

Various studies have examined the effect of substituents of the cyclopentadienyl ring on electrochemical behavior [14-16]. 

In 2001, Takahashi and his co-workers developed the first asymmetric ruthenium-catalyzed allylic alkylation of allylic carbonates with sodium malonates which gave the corresponding alkylated compounds with an excellent enantioselectivity (Equation (Sy)). Use of planar-chiral cyclopentadienylruthenium complexes 143 with an anchor phosphine moiety is essential to promote this asymmetric allylic alkylation efficiently. The substituents at the 4-position of the cyclopentadienyl ring play a crucial role in controlling the stereochemistry. A kinetic resolution of racemic allylic carbonates has been achieved in the same reaction system (up to 99% ee). ... 

We have seen how a substituent on one cyclopentadienyl ring in ferrocene markedly affects subsequent ring substitution reactions. While certain transformations of this type are at least partially determined by steric factors, it seems certain that resonance and inductive interactions also play a very important role in determining orientation. Electronic transmissions both within and across the cyclopentadienyl rings are not fully understood, since the precise nature of the bonding in ferrocene is still uncertain. 

Several additional studies have confirmed interannular electronic interactions in ferrocene. Early investigations by Nesmeyanov and coworkers and by others demonstrated the electron-donor properties of the ferrocenyl group (71). Aminoferrocene, for example, is approximately 20 times as strong a base as aniline, while benzoic acid is several times more acidic than ferrocenecarboxylic acid. Moreover, acidities and basicities of this sort are markedly affected by substituents on the opposite cyclopentadienyl ring. 

Metallocene derivatives may be named by either standard organic suffix (functional)7 or prefix nomenclature. Substituents are given the lowest numerical locants in the usual manner on the equivalent cyclopentadienyl rings of the ocene entity. The first ring is numbered 1 to 5 and the second ring is numbered V to 5. In metallocenes composed of multiple ocene groupings the cyclopentadienyl rings are further numbered 1" to 5", V" to 5", etc. The radical names -ocenyl, -ocenediyl, -ocenetriyl, etc. are used. For examples see Table 25. 

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