Lanthanocenes

Due to its marked atom economy, the intramolecular hydroamination of alkenes represents an attractive process for the catalytic synthesis of nitrogen-containing organic compounds. Moreover, the nitrogen heterocycles obtained by hydroamination/cyclisation processes are frequently found in numerous pharmacologically active products. The pioneering work in this area was reported by Marks et al. who have used lanthanocenes to perform hydroamination/cyclisation reactions in 1992. These reactions can be performed in an intermolecular fashion and transition metals are by far the more efficient catalysts for promotion of these transformations via activation of the... 

Scheme 13 Generalized mechanism for the alkene insertion and cr-bond metathesis reactions catalyzed by lanthanocenes...

Metal complexes of lanthanides beyond lanthanocenes were used to catalyze the reductive coupling reaction of dienes. La[N(TMS)2h was found to effect the cyclization of 1,5-hexadiene in the presence of PhSiH3 (Eq. 13) [50]. Cyclized products 88 and 89 were isolated in a combined yield of 95% (88 89 = 4 1). It was suggested that the silacycloheptane 89 resulted from competitive alkene hydrosilylation followed by intramolecular hydrosilylation. 

Scheme 21 Formation of five- and six-membered ring heterocycles by lanthanocene complexes...

The lanthanocene alkyls (190) and (191) are also highly active initiators for MMA polymerization. These too are syndioselective, producing 82-85% rr PMMA at 0°C with high initiator efficiencies and narrow molecular weight distributions. Ln11 complexes such as (192)-(194) also generate syndiotactic PMMA, but exhibit much lower efficiencies (30 10%). 

The lanthanocene initiators also polymerize EtMA, PrMA and BuMA in a well-controlled manner, although syndiotacticity decreases as the bulk of alkyl substituent increases. Reactivity also decreases in the order MMA EtMA > PrMA > BuMA. Chain transfer to provide shorter polymer chains is accomplished by addition of ketones and thiols.460 The alkyl complexes (190) and (191) also rapidly polymerize acrylate monomers at 0°C.461,462 Both initiators deliver monodisperse poly(acrylic esters) (Mw/Mn 1.07). An enolate is again believed to be the active propagating species since the model complex (195) was also shown to initiate the polymerization of MA. 

The polymerization of MMA has been shown to be subject to enantiomorphic site control when the Ci-symmetric a .va-lanthanocene complexes (196) and (197) are employed as initiators.463 When the (T)-neomenthyl catalyst (196) is used, highly isotactic PMMA is produced (94% mm at — 35 °C), whereas the (-)menthyl derived (197) affords syndiorich PMMA (73% rr at 25 °C). NMR statistical analysis suggests that conjugate addition of monomer competes with enolate isomerization processes, and the relative rate of the two pathways determines the tacticity. 

Many other lanthanide-based initiators have been shown to polymerize MMA, including lanthanocene amides,464 168 alkoxides,469 substituted indenyl and fluorenyl bivalent ytterbocenes,470,471 hexamethylphosphoric triamide thiolates,472 and allyl, azaallyl, and diazapentadienyl complexes.473... 

As described in Section 9.1.2.2.3, several lanthanocene alkyls are known to be ethylene polymerization catalysts.221,226-229 Both (188) and (190) have been reported to catalyze the block copolymerization of ethylene with MMA (as well as with other polar monomers including MA, EA and lactones).229 The reaction is only successful if the olefin is polymerized first reversing the order of monomer addition, i.e., polymerizing MMA first, then adding ethylene only affords PMMA homopolymer. In order to keep the PE block soluble the Mn of the prepolymer is restricted to <12,000. Several other lanthanide complexes have also been reported to catalyze the preparation of PE-b-PMMA,474 76 as well as the copolymer of MMA with higher olefins such as 1-hexene.477... 

As described previously for aluminum, in order to obviate the kinetic complexity arising from aggregation, several groups have examined potentially less complicated single-site monoalkoxides. For example, complexes (299)-(301) are active for the ROP of CL, 6-VL and even /3-PL at 0 °c.888 Polydispersities are low (< 1.10) even up to 90% conversion and Mn increases linearly with conversion for CL, although initiator efficiencies are typically 50-60%. Lanthanocene alkyls, such as (302) and (303), and hydrides, (304), exhibit almost identical reactivity for the polymerization of CL and <5-VL to the alkoxides (299)-(301) (although no activity for /3-PL was observed). 

The lanthanocene systems have been extended to cover a range of monomers including LA,890 cyclic carbonates891 and even MMA.454 Block copolymers of MMA and lactones, with Mw/Mn = 1.11-1.23, may be prepared but only if the vinyl monomer is polymerized first. The... 

Without the third component no activity at all was observed. They proposed a mechanism similar to the one given by Yasuda et al. [216, 217] for polymerization of methylmethacrylate by lanthanocenes which are isoeleetronie with alkylzirconocenium ions. The role of the third component in this mechanism is not very clear. Nevertheless polymerization of polar monomers by metallocene catalysts is an open field of research and investigations are just beginning. 

Insertion of di-t-butylcarbodiimide into organolanthanite complexes (La = Er, Y, Gd) to give organolanthanide amidinates is also observed. Likewise, diisopropylcarbodiimide undergoes insertion into lanthanocene amides. However, diisopropylcarbodiimide does not react with lanthanocene guanidate complexes In the reaction of Et2 Y(N-i-Pr2 )2 with... 

Shen, Q., Wang, Y.R., Zhang, K.D. etal. (2002) Lanthanocene amide complexes as single-component initiators for the polymerization of (dimethylamino)ethyl methacrylate. Journal of Polymer Science, Part A Polymer Chemistry, 40, 612. 

The previous procedure can be employed to prepare the following related compounds [lanthanocene(III) chlorides], as summarized in Table I. Instead... 

In spite of countless applications of rare earth activation in industrial heterogeneous catalysis, most soluble complexes have long been limited to more or less stoichiometric reactions. An early example is the Kagan C-C coupling mediated by samarium(II) iodide [126]. Meanwhile, true catalytic reactions have become available. Highlights are considered the organolanthanide-catalyzed hydroamina-tion of olefins [127], the living polymerization of polar and nonpolar monomers [128], and particularly the polymerization of methyl methacrylate [129]. In the first case, lanthanocene catalysts of type 27 are employed [127]. 

The synthesis of [ MeX(CH2CH2CsI I/ )2 Ln(/i-Cl)]2 (Ln = Y, Nd, Sm, Yb) in high yields has been achieved by treatment of anhydrous LnCljlTHF), (n 0, 3, or 4) with Na2[MeN(CH2CH2C5H4)2] (Scheme 152).629 Closely related dimeric tf/Mtf-lanthanocene chlorides have been reported with pyridine-bridged630,631 and furan-bridged505 bis(cyclopentadienyl) ligands. 

Chiral lanthanocene chlorides with an ether-functionalized indenyl ligand were obtained by the reaction of l-(2-methoxyethylindenyl) potassium (in situ) with the corresponding anhydrous lanthanide chlorides in THF (Ln = Y, La, Nd, Gd, Ho, Lu) (Scheme 183).685... 

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