Peter Henricus Maria Budzelaar

Understanding electronic descriptor strengths and weaknesses can indicate applicability domains in machine learning approaches and foreshadow model limitations.

The aluminum-alkyl borate (AAB) salt {[iBu₂(DMA)­Al]₂(μ-H)}⁺[B­(C₆F₅)₄]⁻ (AlHAl_DMA; DMA = N,N-dimethylaniline) is able of fully activating dichloride precatalysts for olefin polymerization and serving as an impurity scavenger, thus deserving to be called a molecular cousin of the well-established methylaluminoxane (MAO). With respect to MAO, it offers the advantage of having a well-defined molecular structure, which was exploited herein to investigate its mechanism of action as a cocatalyst. Particularly, the reaction of the precatalyst (Me₂SiCp₂)­ZrCl₂ with AlHAl_DMA and with stable [AliBu₂(L)]⁺, modeling the putative abstracting species [AliBu₂(DMA)]⁺, was studied. The latter reaction led to the isolation of a rare, singly bridged Zr–(μ-Cl)–Al heterodinuclear adduct (2), which is a plausible intermediate of chloride abstraction from the precatalyst. Addition of di-iso-butylaluminum hydride (DIBAL-H) to 2 yielded a mixture of several multinuclear Zr/Al adducts with bridging μ-Cl and μ-H fragments (3–6), which were fully characterized by in-depth 2D NMR spectroscopy. Analogous products were observed in the reaction between (Me₂SiCp₂)­ZrCl₂ and AlHAl_DMA, reinforcing the hypothesis that they are intermediates of chloride/hydride exchange, which generates a polymerization-active Zr–H species. The solid-state structure of [(Me₂SiCp₂)­Zr]₂(μ-H)­(μ-Cl)­(μ² -iBu₂AlH₂) (5) was determined by single-crystal X-ray diffraction. The presence of the μ-H fragment in AlHAl_DMA appears to be relevant also for determining the excellent impurity scavenging properties of this cocatalyst, as it was found to react more rapidly than Al–iBu moieties upon exposure of solutions of this cocatalyst to atmospheric oxygen and moisture.

The initial reaction steps in the formation of Heck‐type arylated alkenes catalyzed by P^N chelated Au(III) complexes have been studied computationally. Two mechanistic alternatives have been explored: (1) alkene coordination and insertion into a gold‐carbon bond, and (2) nucleophilic attack on a gold(III) alkene adduct. The common starting point, the [(P^N)AuPh(alkene)]²⁺ dication (alkene = C₂H₄, H₂C═CHEt, or H₂C═CEt₂), shows unequal bonding to the olefinic carbons of the 1‐alkenes (P^N = 1,2‐C₆H₄NMe₂(PR₂); R = H, Me, 1‐adamantyl). This polarization increases with steric hindrance and with the inclusion of an OTf^‐ anion in the model. While the reaction pathways are strongly governed by the trans‐influence of the ligand, the effects of steric hindrance in the ligand and alkene are remarkably small. In all cases the nucleophilic attack pathway is energetically favored. However, changing the ligand from a P^N to a P^P chelate, with a strongly electron‐donating ‐PMe₂ donor trans to Ph, sufficiently destabilizes the Au─Ph bond to make alkene insertion competitive. Alkene 1,2‐insertion regiochemistry is always preferred, unlike Pd‐catalyzed Heck reaction that requires a 2,1‐insertion. Based on these results, an alkene insertion pathway en route to Heck‐type olefins can therefore be ruled out.

Well-defined Al-alkyl borate (AAB) salts {[iBu₂(L)Al]₂(μ-H)}⁺[B(C₆F₅)₄]^- (AlHAl_L) with L = N-donor ligands have been recently reported as promising "complete" cocatalysts for olefin polymerization. Herein, we explore structural variations of AlHAl_L going beyond the class of N-donors like the prototypical N,N-dimethyl aniline (DMA). Thirteen P-, O-, and C-donor ligands were screened, allowing isolation of AAB salts with mono- and bidentate phosphines, alkyl-, aryl-, and silyl-ethers, and a N-heterocyclic carbene. Except for the diphosphine with the longest spacer between the P atoms [bis(diphenylphosphino)hexane, DPPH], all donors gave well-defined tetracoordinate or tricoordinate molecular species, which were characterized in solution (NMR) and solid state (XRD), and tested as cocatalyst in ethylene/1-hexene copolymerization with an ansa-zirconocene catalyst [rac-Me₂Si(2-Me-4-Ph-Ind)₂ZrCl₂]. The vast majority of novel AAB salts provided active catalytic systems, further demonstrating the broad tunability of these species. Consistent with previous studies, variability in productivity upon L variation is primarily related to the efficiency of precatalyst activation, determining the fraction of Zr active sites. Variations in polymer molecular weight and comonomer incorporation observed with some P-, O- and C-donor ligands indicate that also interactions between the L donors and the Zr active species might be relevant in determining catalytic performance in some cases.

The stability and reactivity of {[iBu₂(L)Al]₂(μ-H)}⁺[B(C₆F₅)₄]⁻ (AlHAl), recently identified as a promising cocatalyst for olefin polymerization, can be modulated by varying the structure of the neutral N-donor (L).

A set of metallocene olefin polymerization catalysts bearing triptycene moieties in either position 4–5 (complexes Ty1–Ty5) or in position 5–6 (complexes Ty6–Ty8) of the basic dimethylsilyl-bridged bis(indenyl) system has been tested in propene polymerization and in ethene/1-hexene copolymerization. Comparison of the results with QSPR (quantitative structure–property relationship) predictions not parametrized for these exotic ligand variations demonstrates that trends can still be identified by extrapolation. Interestingly, Ty7, upon suitable activation, provides a highly isotactic polypropylene with an exceptional amount of 2,1 regio-errors (8%). The previously developed QSPR type models successfully predicted the low regioselectivity of this catalyst, despite the fact that the catalyst structure differs significantly from the benchmark set.

In zwei vielbeachteten Veröffentlichungen wurde vor kurzem über die Bildung von arylierten Heck‐Olefinen durch Katalyse mit MeDalPhosAuCl/AgOTf (J. Am. Chem. Soc. 2023, 145, 8810) und ihre Cyclisierung zu Tetralinen berichtet (Angew. Chem. Int. Ed. 2023, e202312786). Es wurde behauptet, es handle sich dabei um den erstmaligen Nachweis von Gold‐katalysierten Alken‐Insertionen in Au‐Aryl‐Bindungen, β‐H‐Eliminierung und Kettenwanderung durch Au−H. Wir zeigen hier, dass es sich dabei tatsächlich aber um einen Zweistufenprozess handelt, wobei nur der erste Schritt, die Bildung eines Alkyltriflat‐Esters als primäres organisches Produkt durch die wohlbekannte Alken‐Heteroarylierungssequenz, einen Gold‐Katalysator involviert. Die nachfolgende Umsetzung zu Heck‐Olefinen sowie deren Cyclisierung zu Tetralinen ist klassische, H⁺‐initiierte Carbokationenchemie. Diese Schritte finden auch unter Gold‐freien Bedingungen statt, mit identischen Resultaten. Literaturberichte über neuartige Reaktivität von Gold‐Katalysatoren, wie Kettenwanderung durch das hypothetische [LAuH]²⁺ Dikation, entbehren jeder faktischen Grundlage.

Two recent high‐profile publications reported the formation of Heck‐type arylated alkenes catalysed by MeDalPhosAuCl/AgOTf (J. Am. Chem. Soc. 2023, 145, 8810) and their cyclisation to tetralines (Angew. Chem. Int. Ed. 2023, e202312786). It was claimed that these were the first demonstrations in gold catalysis of alkene insertion into Au‐aryl bonds, β‐H elimination and chain‐walking by Au−H dications. We show here that in fact this chemistry is a two‐stage process. Only the first step, the production of an alkyl triflate ester as the primary organic product by the well‐known alkene heteroarylation sequence, involves gold. The subsequent formation of Heck‐type olefins and their cyclisation to tetralines represent classical H⁺‐triggered carbocationic chemistry. These steps proceed in the absence of gold with identical results. Literature claims of new gold reactivity such as chain walking by the putative [LAuH]²⁺ dication have no basis in fact.

The stereoelectronic factors responsible for stereoselectivity in propene polymerization with several metallocene and post–metallocene transition metal catalysts have been revisited using a combined approach of DFT calculations, the Activation Strain Model, Natural Energy Decomposition Analysis and a molecular descriptor (%V_Bur). There are in most cases two different paths leading to the formation of stereoerrors (SE), and the classical model does not suffice to fully understand stereoregulation. Improving stereoselectivity requires raising the energies of both SE insertion transition states. Our analyses show that the degrees of deformation of the active site (catalyst+chain) and the prochiral monomer differ for these two paths, and between different catalyst classes. Based on such analyses we discuss: a) the subtle differences in SE formation between stereoselective catalysts with different ligand frameworks; b) the reason for exceptional stereoselectivity reported for a special ansa–metallocene catalyst; c) the (double) stereocontrol origin for isoselective catalysts; d) the electronic contribution for isoselective catalysts generating SE by a modification of the ligand wrapping mode during the polymerization. Although this study will not immediately suggest new catalyst structures, we believe that understanding stereoregulation in great detail will increase our chances of success.