Chemistry and catalysis advances in organometallic chemistry and catalysis
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356 COORDINATIVE CHAIN TRANSFER POLYMERIZATION AND COPOLYMERIZATION Ln (BH
4 ) R R' Mg (BH 4 ) R Mg R R Ln (BH
4 ) R R' Mg (BH 4 ) R Mg R R Ln (BH
4 ) R R' Mg (BH 4 ) R Mg R R Ln (BH
4 ) R R' Ln (BH 4 ) R (BH 4 ) Mg R R' Ln (BH
4 ) R (BH 4 ) Mg R R' Ln (BH
4 ) (BH 4 ) R' MgR 2 + (BH 4 )MgR
Ln (BH
4 ) R (BH 4 ) Mg R R' MgR 2
B3 B2 B1 C3 C2 C1 Scheme 27.11 Equilibrium between putative active and dormant species in CCTP conditions. R = alkyl group; R = alkyl substituent (CpR features any cyclopentadienyl ligand); Ln = lanthanide. PI 1 [La] m PI 2 [Mg] n [La]
PI 1 + [Mg] PI 2 + PI 1 [Mg] m PI 2 [La] n + [La] p [Mg]
q PI 2 n PI 1 m + Transmetalation Scheme 27.12 Proposed mechanism to account for the increase in 3,4-stereoselectivity of isoprene polymerization under CCTP conditions. PI represents polyisoprene.
REFERENCES 357 be related to hard–soft considerations: the softer early lanthanide would be better “suited” to magnesium, while the late lanthanide, having a more pronounced Lewis acid character, gives more “efficient” combinations with aluminum alkyls. The metal-alkyl strength should be considered, as earlier proposed by Gibson, but in terms of dynamic heterometallic structures. 27.6 CONCLUSION— PERSPECTIVES The high versatility of lanthanide-based catalysts has allowed to extend the concept of CCTP from ethylene and α-olefins, to monomers such as styrene, and conjugated dienes, including the biosourced myrcene. Borohydrido derivatives are particularly powerfully combined with magnesium dialkyls to afford catalytic systems having up to 100% polymer chain transfer efficiency with these monomers. The CTA, which efficiently plays the role of a protecting agent versus β-H abstraction, also allows catalyst atom economy and high control of molecular weights. In the conditions of a statistical coordinative chain transfer copolymerization, dual borohydrido lanthanide/magnesium alkyl catalysts give access to a range of copolymers, which can be tuned by selectivity and the comonomer content, depending on the precatalyst/cocatalyst (CTA) ratio. The living macromonomers can subsequently be utilized as building blocks for the elaboration of unprecedented macromolecular block structures such as the PE- b-TPI copolymer. Future challenges in the frame of CCTP will consist of disclosing the most efficient catalytic combinations and by the way particular attention should be paid on the molecular nature of the CTA/cocatalyst. Indeed, the general strategy in polymerization catalysis is based on the search for new organometallic precatalyst architectures, whereas little is done regarding the CTA/cocatalyst itself. One can think about varying its molecular structure as well. In this regard, the in situ modification by the simple addition of a molecule that will change the properties of a CTA, in terms of alkylating ability, polymer chain transfer ability, or stereochemical induction, is of primary interest. In addition, although many polymerization processes have been claimed as living, the subsequent in situ functionalization of a growing polymer chain is generally hard to achieve with high yields, and it remains a goal to reach with stereoregular polydienes, for instance. Chain transfer could be more generally seen as a way to render a growing polymer chain active toward functionalization, as the chemistry of a [metal catalyst]-polymer is thus replaced by the chemistry of a [metal-CTA]- polymer. The degree of intervention of the CTA in selectivity is not clear up to now, and, in this regard, theoretical support would be helpful in getting more mechanistic insights into CCTP. Similarly, in the particular situation where more than two metals are involved in a polymerization process, the specific role of each one—alkylation, chain transfer, stereoregulation assistance—should also be clarified by means of joint theoretical/experimental studies. The versatility of chain transfer polymerization together with its easiness of use relative to the synthetic effort that would require a “one catalyst, one material” approach or to the difficulties encountered by other routes explored for fine-tuning polymerization selectivities makes this approach undoubtedly an attractive and promising field for the future of coordination polymerization. CCTP renders the emergence of the “one catalyst, several materials” paradigm a reality.
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28 CHARGE-NEUTRAL AND CATIONIC COMPLEXES OF LARGE ALKALINE EARTHS FOR RING-OPENING POLYMERIZATION AND FINE CHEMICALS CATALYSIS Jean-Franc¸ois Carpentier*, Bo Liu, and Yann Sarazin* Organometallics, Materials and Catalysis Department, Institut des Sciences Chimiques de Rennes, Universit´e de Rennes, Rennes, France 28.1 SYNTHESIS OF CHARGE-NEUTRAL HETEROLEPTIC RING-OPENING POLYMERIZATION CATALYSTS BASED ON LARGE ALKALINE EARTHS Polyesters derived from bioresourced cyclic monomers such as the ubiquitous l-lactide are enjoying growing attention as green materials, presenting an attractive combination of physical and mechanical properties [1]. Seminal efforts by Coates and Chisholm at the turn of the century triggered the development of many well-defined organometallic initiators for the controlled ring-opening polymerization (ROP) of cyclic esters, and systems based on aluminum, rare-earth metals, and zinc have since played a prominent role [2]; more recently, several breakthroughs have also been achieved with organic catalysts [3]. One of our contributions to the field of ROP catalysis has focused on the development of catalytic systems for the immortal ROP of cyclic esters based on the large alkaline earth (Ae) metals: calcium, strontium, and barium. Indeed, these innocuous metals have been largely neglected in the area of ROP catalysis. Such paucity owes much to the synthetic difficulties related to the high reactivity and kinetic lability of heteroleptic complexes of Ca, Sr, and Ba, both of which result from the large ionic radius of these elements (Ca(6): 1.00 ˚ A; Sr(6): 1.18 ˚ A; and Ba(6): 1.35 ˚ A) [4]. With divalent metals (M II ), typical single- site ROP initiators have the general formula {L
X }M
Nu, where {L
X }
is a (multidentate) monoanionic bulky ancillary ligand and Nu − is a reactive nucleophile (typically, OR − or NR
2 − ) that attacks the incoming monomer. Such heteroleptic complexes usually allow for the controlled ROP of cyclic esters according to a coordination-insertion mechanism [2, 5]. With large Ae metals, the stability of {L
X }AeNu complexes is usually very limited, as they readily decompose during deleterious Schlenk-type equilibria that eventually generate the poorly reactive and ill-defined homoleptic {L
X }
Ae and {AeNu
2 }
. The ease of ligand redistribution reactions increases with the ionic radius of the element, and whereas stable {L
X }MgNu complexes (Nu − = OR
− , NR
2 − , R − ) are available in the plenty (Mg(6): 0.72 ˚ A) [4], {L
X }AeNu (Ae = Ca, Sr, Ba) analogues are extremely scarce and known essentially for amido derivatives (Nu − = NR 2 − ). Early examples of discrete Ca-based ROP heteroleptic initiators include those developed by Chisholm involving sterically encumbered tris(pyrazolyl)borates [6] or Darensbourg’s complexes stabilized by Schiff-base ligands [7]. Other molecular Ae-based ROP initiators have been reported, but the associated polymerization mechanisms have not been elucidated, and the overall efficiency was rather limited [8]. We chose to develop heteroleptic Ca, Sr, and Ba complexes of the type {LO
}AeNu supported by multidentate, monoanionic phenolate {LO
x } − ligands (x = 1–5, Fig. 28.1), because we anticipated that these ligands would adequately satisfy the electrophilic nature of the Ae elements. Phenolates constitute ubiquitous ligand platforms for the stabilization of
First Edition. Edited by Armando J. L. Pombeiro. © 2014 John Wiley & Sons, Inc. Published 2014 by John Wiley & Sons, Inc.
360 CHARGE-NEUTRAL AND CATIONIC ALKALINE EARTH COMPLEXES M Nu
n X General structure Metal Ionic radius (C. N. = 6) Synthetic precursors Ca Sr
1.18 Å 1.00 Å
1.35 Å Ca[N(SiMe 3 )
] 2 (THF) 2 , Ca[N(SiMe 2 H)
] 2 (THF) Sr[N(SiMe 3 ) 2 ] 2 (THF) 2 , Sr[N(SiMe 2 H) 2 ] 2 (THF) 2/3 Ba[N(SiMe 3 )
] 2 (THF) 2 , {Ba[N(SiMe 2 H)
] 2 } n OH N N O O OH N O OH t Bu t Bu t Bu t Bu t Bu t Bu t Bu t Bu t Bu O O O N O OH N OH N OMe
OMe MeO
{LO 1 }H {LO 2 }H {LO 3 }H {LO 4 }H {LO 5 }H Figure 28.1 Structures of the targeted heteroleptic complexes and selected phenol pro-ligands. a large variety of oxophilic metal centers and are readily amenable to the tuning of their steric and electronic properties. Yet, they have barely been used for the preparation of heteroleptic Ae complexes. We employed the pro-ligands {LO
}H depicted in Fig. 28.1, as they offer a broad range of electron-donating and structural features. All are readily prepared on large scales (5–20 g) by Mannich condensation. The reaction of {LO 1
3 ) 2 ] 2 (THF) 2 yielded the THF-free heteroleptic {LO 1
3 ) 2 (1) [9]. Remarkably, 1 was perfectly stable in solution at room temperature for days. Moreover, the NMR-scale reaction of 1 and 10 equiv of iPrOH in C 6 D 6 at 60
◦ C (i.e., under immortal ROP conditions, vide infra) indicated the clean and quantitative formation of {{LO
1 }CaOiPr} n with concomitant release of free HN(SiMe 3 )
. However, the analogous reactions between {LO
1 }H and Sr[N(SiMe 3 )
] 2 (THF) 2 or Ba[N(SiMe 3 )
] 2 (THF) 2 failed to return the desired heteroleptic complexes, instead yielding intractable mixtures of {LO
1 }AeN(SiMe 3 )
with {LO
1 } 2 Ae and Ae[N(SiMe 3 ) 2 ] 2 (Ae = Sr, Ba) containing various amounts of THF. Similarly, {LO
5 }CaN(SiMe 3 )
(2) and {LO
5 }SrN(SiMe 3 )
(THF) (3) were obtained in good yield (70–75%) using the tetradentate {LO 5
− , but the Ba congener could not be synthesized. Ligand redistribution reactions were also most troublesome with {LO
3 } − , and no heteroleptic complex was cleanly isolated. Hence, it rapidly emerged that owing to the kinetic lability of Ae complexes, procedures involving the archetypical Ae[N(SiMe 3 )
] 2 (THF) 2 were often ill-fated, and that other routes to stable heteroleptic complexes had to be devised. Inspired by Anwander’s [10] work with rare-earth elements, we sought to stabilize heteroleptic complexes by the means of intramolecular Ae ···H–Si agostic interactions between the large Ae metal and the N(SiMe 2 H)
amido group. Thus, the new precursors Ae[N(SiMe 2 H)
] 2 (THF) x (Ae
= Ca, x = 1; Sr, x = 2 3 ; Ba, x = 0) were synthesized [11]. The heteroleptic complexes {LO
3 }AeN(SiMe 2 H)
(Ae = Ca, 4; Sr, 5; Ba, 6) were subsequently formed in good yields (60–70%) by the addition of {LO
3 }H to the appropriate precursor (Scheme 28.1). The stability of 4–6 against redistribution reactions was attributed to the presence of β-agostic Si−H···Ae interactions with N(SiMe 2 H)
moieties. Complexes 4–6 could also be CHARGE-NEUTRAL HETEROLEPTIC RING-OPENING POLYMERIZATION Ae CATALYSTS Download 11.05 Mb. Do'stlaringiz bilan baham: 
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