Samarocene

An allyl samarocene catalyst, [(CMe2C5H4)2SmCl(C3H5)MgCl2(THF)4, was employed for the synthesis of trans-Vl-b-VCL copolymers and poly(fra s-isoprene-co-hex-l-ene)-fr-PCL terpolymers [111]. The copolymerizations... 

Among early and group III transition metals, the yttrocene catalysts have been studied in greatest detail. However, related metallocenes show great promise as catalysts for reductive cyclization. Neodymocene-catalyzed cyclization of 1,5- and 1,6-dienes 14a and 15a proceeds readily in the presence of silane to afford cyclopentanes 14b and 15b.37 Lutetocenes and samarocenes also catalyze silane-mediated cyclization of 1,5-diene 14a to cyclopentane 14b.38 39 In the case of the samarium-based metallocenes, the feasibility of borane-mediated cyclization has been established, as demonstrated by the highly diastereoselective conversion of phenyl-substituted diene 16a to cyclopentane 16b (Scheme ll).40... 

In the course of probing the range of reactivity accessible to decamethyl-samarocene, substrates containing C=N double bonds as part of 6-membered heterocycles have been tested. In the reaction with pyridazine reductive carbon-carbon bond formation occurred (compare Sect. 3.2) [99], The coupled ligand bridges two Sm(III) centers via the four nitrogen positions. In the phenazine reaction one phenazine ligand is placed between the Sm(III) centers (Fig. 20 Table 16) [203]. 

The applicability of organolanthanide metallocenes as polymerisation catalysts can also be seen from the results of the block copolymerisation of ethylene and methyl methacrylate. The persistence of the lanthanide-alkyl bond has been utilised to prepare ethylene copolymers with polar poly(methyl methacrylate) blocks. For this purpose, ethylene is introduced as the first monomer into the polymerisation system with the samarocene catalyst, and then methyl methacrylate is polymerised, which leads to block copolymer formation [532-534] ... 

The solid-state structure of the ethylaluminum oxide metallocene complex (Fig. 30) shows two trivalent samarocene units connected by an [(A EtsO ]2-ethylalumoxane ( EAO ) unit. The latter was described as an adduct of two molecules of triethylaluminum with two [AUfoO]- anions [116]. An unusual asymmetric fx-r q1 (side-on) ethyl coordination mode was observed, which previously has been found only in a small number of lanthanide complexes, i.e., (CsMeshYM/zy 1-Et)AlEt2(THF) [ 144], (C5Me5) 2Sm(THF) (fx-rj1 ijx-Et) AlEt3 (Fig. 9) [115] and (C5Me5)2Sm(THF) (/i-r]1 X-Et) (/z-Cl)AlEt2 [116]. 

The complex SmCpj was synthesized by metal vapour reaction of samarium with Cs(CH3)5H as a green product or by desolvation of the red SmCpj 2THF by sublimation. The crystal structure of the complex SmCpj shows it to be decamethyl samarocene with bent metallocene disposition [138], The Cg-Sm-Cg angle of 140.1° is greater than 136.7 in the desolvated analogue. The Sm-C bond distance is reduced, 2.79(1) A compared to 2.86 (3) A and the structure of SmCpj is shown in Fig. 6.12. 

A recent development in the low-valent lanthanide area is the synthesis and X-ray structural determination of the unsolvated complex (C5Me5)2Sm 121). This species is the first structurally characterized bis(cyclopentadienyl)lanthanide species which has no other ligands in the metal coordination sphere. As such, it is the closest lanthanide analogue of the bis(ring)metallocene sandwich compounds like ferrocene. Samarocene was obtained by desolvation of (C5Me5)2Sm(thf)2 under high vacuum and sublimation of the product [Eq. (58)]. The sublimed crystals have a bent... 

FIGURE 5. Energy profile for GXP catalyzed by (a) a neutral zirconocene, (b) a cationic zirconocene and neutral samarocene. Reprinted with permission from S. Tomasi, H. Weiss and T. Ziegler, Organometallics, 25, 3619. Copyright 2006 American Chemical Society... 

Divalent samarocene complexes also show unique reactivity toward a-olefins [55]. Bis-Cp complexes of type 13 display high polymerization activity toward ethylene, but yield rather low molecular weights (M < 25 000) [56]. The polymerization mechanism was investigated using field desorption mass spectroscopy (FD-MS) and proposed as outlined in Scheme 2. A 2 1 complex of 13 with ethyl-... 

Scheme 2. Proposed a-olefm polymerization mechanism for divalent samarocene complexes.

The dual function of the precatalysts 4 opened the way to well-controlled block polymerization of ethylene and MMA (eq. (5)) [89, 90]. Homopolymerization of ethylene (Mn = 10000) and subsequent copolymerization with MAA (Mn 20000) yielded the desired linear AB block copolymers. Mono and bis(alkyl/silyl)-substituted flyover metallocene hydride complexes of type 8 gave the first well-controlled block copoymerization of higher a-olefins with polar monomers such as MMA or CL [91]. In contast to the rapid formation of polyethylene [92], the polymerization of 1-pentene and 1-hexene proceeded rather slowly. For example, AB block copolymers featuring poly( 1-pentene) blocks (M 14000, PDI = 1.41) and polar PMMA blocks (M 34000, PDI = 1.77) were obtained. Due to the bis-initiating action of samarocene(II) complexes (Scheme 4), type 13-15 precatalysts are capable of producing ABA block copolymers of type poly(MMA-co-ethylene-co-MMA), poly(CL-co-ethylene-co-CL), and poly(DTC-co-ethylene-co-DTC DTC = 2,2-dimethyltrimethylene carbonate) [90]. 

The hexa(/-butyl)metallocenes of Sm, Eu, and Yb have been prepared following the routes depicted in Scheme 44. Adducts of the samarocene with THF and of the samarocene and ytterbocene with 2,6-Me2C6H3NC have also been prepared and characterized. The deca-i-propylmetallocenes have been synthesized from the metal and the free penta-i-propylcyclopentadienyl radical (Scheme 45).218... 

More substituted double bonds are less reactive, and therefore, regioselective hydrosilylations of dienes are possible (8) and (9) [45], Yttrocene catalysts are less reactive in case of 1,1-disubstituted alkenes however, the larger samarocene is better suitable to catalyze this transformation with excellent diastereoselectivity (10) [47],... 

Initial studies on the hydrosilylation/carbocyclization of terminal dienes carried out in the early 1990s have demonstrated that five-membered ring formation proceeds smoothly in the presence of lutetocene [40], neodymocene [43], and samarocene [29] catalysts (18). 

A tandem carbocyclization/hydroboration sequence has been disclosed by Molander [83], The carbocyclization of a,co-dienes proceeds smoothly in the presence of the divalent samarocene Cp 2Sm(THF) and the metal-alkyl intermediate is trapped by a l,3-diaza-2-boracyclopentaneto afford the cyclic hydroboration product (29). 

Scheme 6 Samarocene-catalyzed hydroamination/cycUzation of aminoalkynes [105,106]...

Barbier-Baudry [147,148,185-188] studied in detail the ani a-samarocene allyl and chloride complexes 91-93 (Fig. 14) as the single-component catalysts for the copolymerization of a series of a-oleflns with conjugated dienes. The resultant copolymers contained about 6% linear a-oleflns. Each olefin unit randomly inserted between two tran5-l,4-isoprenes. It was worth noting that the afforded copolymers with different precatalysts were characterized with almost the same properties, denoting the same catalytic active species formed during polymerization [147,186]. In addition, these catalysts were able to copolymerize isoprene with e-caprolactone to form diblock copolymers [186,187] and triblock copolymer poly[isoprene-ct -... 

Hou and Wakatsuki [197] reported a cationic ternary system composed of samarocene aluminate Cp 2Sm( j,-Me)2AlMe2 (95) and TIBA and [PhsC] [B(C6Fs)4], showing living mode for the copolymerization of butadiene and styrene... 

---