Lead cyclopentadienyl complexes

Several tri(cyclopentadienyl)tin(ll) and lead(ll) complexes have been prepared with alkali metal cations. The arrangement of Cp rings around the metal is in a paddle wheel configuration the alkali cation is bound to Cp and not Sn or Pb, further supporting the view of a weak alkali metal group 14 bond. Representative examples of these compounds include (77S-Cp)2E(/r-Cp)-Na(PMDTA) (E = Sn 230, Pb 231).239 240... 

The formal view. The fluorenyl-cyclopentadienyl complex contains a mirror plane. The two sites are therefore mirror images. One site co-ordinates to the re-face of propene, the other site to the si-face of propene. One site will therefore be enantiospecific for making R configured carbon atoms and the other site for S configured carbon atoms. This alternation of configuration leads to a syndiotactic polymer. 

Two communications on propene polymerization by non-metallocene catalysts that include DFT/MM calculations have been recently published [60, 61]. They deal with group 4 bidentate non-cyclopentadienyl complexes. In the first communication [60], the topic addressed is the fact that a C2-symmetric precatalyst of titanium leads to a syndiotactic polymer, contrary to observations of metallocene catalysts. The chirality at the metal center is found to play a key role in the stereocontrol of the process. The second communication [61] addresses the fact that a C2-symmetric precatalyst of zirconium very similar to the previous one produces an isotactic polymer, finds out that it is due to a complicated concourse of synergic steric and electronic effects, and emphasizes the key role that serendipity still plays in the design of new catalysts. 

In a third class of 7t-cyclopentadienyl complexes, compounds of type LVIII are bound to a transition metal via the lone pair at the germanium atom. The first compounds in this series, the pentacarbonyl-[chloro(pentamethylcyclopentadienyl)germylene]chromium(0) (LIXa) and -tungsten(O) (LIXb) were synthesized by alkylation of the ylide complexes (CO)5M—GeCl2 THF according to Eq. (22) (201). Nucleophilic substitution reactions of LIXb lead to the alkyl- and amino-substituted complexes LIXc-e [Eq. (23)] (202). 

There is a pronounced tendency for ruthenium cyclopentadienyl complexes with attached phosphorus ligands to undergo a wide variety of intramolecular metallation reactions. Heating (T75-C5H5)(PPh3)2RuMe in decalin for 24 hours leads to the orthometallated complex 135 in reasonable yield [Eq. (Ill)] (6,103). The mechanism presumably involves initial loss of... 

Arene Complexes Although the cyclopentadienyl group is the best-known aromatic ligand, there are several others of considerable importance. None leads to complexes as stable as the most stable metallocenes, however, and the chemistry of the complexes that do form is more severely limited. 

O-Methylation of mandelic acid leads to the enantiomers of a-methoxy-M-phcnylacetic acid (10), which are also commercially available. This methylation without noticeable racemiza-tion was achieved with diazomethane, using aluminum tris(tert-butanoate) as catalyst8. Alternatively, dimethyl sulfate/ sodium hydroxide has been used15, as described in detail for the racemic compound10. The acids have been used for the construction of quite sophisticated chiral auxiliaries, e.g., a rhodium cyclopentadienyl complex (Section 7.2.2.), and for chiral dienes applied in both normal and inverse Diels-Alder reactions (Section D.1.6.1.1.1.). Chiral dienes, e.g., 1, for normal Diels -Alder reactions were prepared by pyrolysis (460 C) of a tricyclic precursor cstcrified with (S)-O-methylmandeloyl chloride or with the free acid and dicyclohexylcarbodiimide/4-dimethylaminopyridine11 -13. 

Electrophilic attack. Reaction of cyclobutadienetricarbonyliron with electrophilic reagents leads to the substitution of hydrogen in the ring. This is an analogous reaction to hydrogen substitution in cyclopentadienyl complexes. Reactivities of Fe(C4H4)(CO)3 and Fecp2 are similar. 

The possibility that Ru(II) can form Jt-complexes with the aromatic side-chains of proteins is an intriguing one. Such complexes are known for simple amino acids derivatives of e.g. tryptophan [50], but have yet to be characterized for protein adducts ofRu(II) arenes. They are known for cyclopentadiene complexes. Reaction of strapped cyclopentadienyl complexes [(ri -C5,Ti -N-C5H4(CH2)nNH2)-Ru(CH3CN)2] (n = 2,3) with proteins such as the hormone secretin in water can lead to sandwich complexes by ruthenation of the arene side chain of the phenylalanine residue (Fig. 2.19) [103]. 

Bis(cyclopentadienyl) complexes CP2M (M = Ge, Sn, Pb) have been prepared by reaction of cyclopentadienylsodium and an appropriate dihalide in tetrahydro-furan. Electron diffraction shows that in the vapour these molecules have angular structures on account of the electron pair on the metal. In the solid state the germanium compound polymerizes within 3 h at ambient temperature, the tin compound within five days. The cyclopentadienyl groups are coordinated in a symmetrical pentahapto fashion (p. 191). Association through the rings occurs in the crystalline lead compound to give a polymeric chain structure (Fig. 4.8). 

Diels-Alder (DA) reaction is one of the most powerful synthetic approaches for the construction of six-membered carbocycles [144]. The most attractive feature of this reaction is the simrdtaneous, regioselective formation of two bonds leading to the potential creation of up to four chiral centers at the binding sites with largely predictable relative stereochemistry. Since the discovery of the accelerating effect of Lewis acids on the DA reaction of a,p-unsaturated carbonyl compounds, its broad and fine application imder mild reaction conditions has been amply demonstrated [144]. In addition to the acceleration effect on the reaction, other important role of Lewis acid in the DA reaction is its alteration of chemo-, regio-, and diastereoselectivity. The titanium compounds commonly used in DA reaction are titanium halides, alkoxides, or their mixed salts [145]. A cyclopentadienyl complex such as Cp2Ti(OTf)2 is also documented as a very effective promoter for the DA reaction [146] (Scheme 14.57). 

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