Exploiting Noncovalent Interactions for Room-Temperature Heteroselective rac-Lactide Polymerization Using Aluminum Catalysts

Abstract

Whereas harnessing non-covalent interactions (NCIs) have largely been applied to late-transition metal complexes and to the corresponding catalytic reactions, there are very few examples showing the importance of NCIs in early-transition metal and main group metal catalysis. Here, we report on the effects of hydrogen bond donors in the catalytic pocket to explain the high activity and stereoselectivity of a series of aluminium catam complexes in rac-lactide ring-opening polymerisation (ROP). Four original aluminium catam catalysts have been synthetized and fully characterized. Structure-activity relationships and isotope effect show the importance of the NH moieties of the ligand in rac-lactide ROP. Computational studies highlight beneficial hydrogen bonds between the ligand and the monomer. Overall, structural characterization of the catalysts, mechanistic, kinetic and computational studies support the benefits of non-covalent interactions in the catalytic pocket.

Coordination Ring-Opening Polymerization of Cyclic Esters: A Critical Overview of DFT Modeling and Visualization of the Reaction Mechanisms

“…The importance of non-covalent interactions for the formation of catalytic complexes in lactide polymerization was demonstrated by Romain et al in a recent publication [131] involved the DFT modeling at the ωB97xD [88]/6-31G(d,p) level of theory of ( dl )-LA ROP catalyzed by aluminium complex 51 (Figure 19). The calculations established that the NH hydrogen bond donors are well positioned in the catalytic pocket to interact with the reactive species (lactide, growing polymer) and to form hydrogen bonds (Figure 26).…”

“…Mononuclear metal complexes studied in DFT modeling of lactide ROP [50,75,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140]. …”

The synthesis, characterisation and application of iron(iii)–acetate complexes for cyclic carbonate formation and the polymerisation of lactide

“…The importance of H-bonding interactions has recently been showed by Romain for lactide polymerisation. 81 For the Fe(III)-salen-acetate complexes, activity was increased by installing a planar phenyl ring into the Fe(III)-salen ethylene backbone {Fe(11)OAc, 52% vs. Fe(9)OAc, 28% and Fe(12)OAc, 59% vs. Fe(10)OAc, 43%}. Indeed Fe(12)OAc gave the second highest activity observed in this study, however, this was accompanied by a decrease in product selectivity to 84%.…”

Recent Advances in Metal‐Mediated Stereoselective Ring‐Opening Polymerization of Functional Cyclic Esters towards Well‐Defined Poly(hydroxy acid)s: From Stereoselectivity to Sequence‐Control

“…Whereas the latter trend is monotonous in the case of lactide (P r = 0.55 (R' = Me), 0.81 (tBu), 0.91 (CMe 2 Ph), 0.95 (CMe 2 tBu), 0.95 (CPh 3 )), the "most crowded" complexes hence being the best, it is not the case for the ROP of rac-BL Me (P r = 0.56 (R' = Me), 0.80 (tBu), 0.83 (CMe 2 C 6 H 4 (p-CF 3 )), 0.91 (CMe 2 Ph), 0.70 (CMe 2 tBu), 0.96 (CPh 3 )). In this latter ROP,a ne lectron-rich aryl group on the R' substituent is also beneficiala nd necessary for achieving high syndiotacticity.A sd iscussed in details in a recentf eature article, [10] this was accounted for by the occurrence of attractive CÀH···p(aryl) interactions between the aryl groups from the R' ortho-substituent andt he acidic CÀHo ft he methylene of the last inserted monomer unit within the growing poly(alkoxybutyryl) chain;s uch so-called "second-sphere" or "non-covalent" interactions (NCIs) [11] are illustrated in Fig In 2009, Coates and Thomas reportedt hat the ROP of rac-BL Me and related b-lactonesb earing alkyl side-groups higher than Me, using the yttrium-salan complex Y{ON Me N Me O tBu2 }a lso generates highly syndiotactic PBHAs (P r = 0.90-0.94). [7] In 2018, Cui and collaboratorsr evisited this chemistry by tuning the substituents on the Na toms of the salan ligand framework, and further using ytterbium(III) in addition to yttrium(III)c omplexes for the ROP of rac-BL Me .…”