Bhaskar R. Aluri

An Alternative Mechanistic Concept for Homogeneous Selective Ethylene Oligomerization of Chromium‐Based Catalysts: Binuclear Metallacycles as a Reason for 1‐Octene Selectivity?

An alternative concept for the selective catalytic formation of 1-octene from ethylene via dimeric catalytic centers is proposed. The selectivity of the tetramerization systems depends on the capability of ligands to form binuclear complexes that subsequently build up and couple two separate metallacyclopentanes to form 1-octene selectively. Comparison of existing catalytic processes, the ability of the bis(diarylphosphino)amine (PNP) ligand to bridge two metal centers, and the experimental background support the proposed binuclear mechanism for ethylene tetramerization.

Sterically and Polarity-Controlled Reactions of tBuLi with P=CH-NR Heterocycles : Novel Heterocyclic P-and P,O-Ligands and Preliminary Tests in Transition-Metal Catalysis

(1R)-1,3-Benzazaphospholes 1 a-c, P=CH-NR heterocycles of the indole type, react with tBuLi in two ways, depending on the steric demand of the N-substituent and the polarity of the medium. The presence of small N-alkyl groups induces CH-deprotonation in the 2-position to give hetaryllithium reagents 2 a and 2 b, whereas bulky N-substituents and nonpolar solvents change the reactivity towards addition at the P=C bond. The preferred regioselectivity is tert-butylation at phosphorus, occurring with excellent diastereoselectivity for trans-adducts 3 b and 3 c, but the inverse tert-butylation at C2 to 5 b was also observed. N-Neopentyl groups, with intermediate steric demand, give rise to formation of mixtures in ethers but allow switching either to selective CH lithiation in THF/KOtBu or to addition in pentane. Bulkier N-adamantyl groups always cause preferred addition. Protonation, silylation, and carboxylation were used to convert the P=CLi-NR, (E)-tBuP-CHLi-NR, and LiP-CH(tBu)-NR species into the corresponding sigma(2)-P or sigma(3)-P compounds 4 b and 6 a,b, 7 b,c, or 8 b-10 b with additional N and/or O donor sites. Slow diffusion-controlled air oxidation of 10 b led to the meso-diphosphine 11 b. Preferred eta(1)-P coordination was shown for an [Rh(cod)Cl] complex 12 b, and the potential of the new ligands 4 b and 7 b in catalysis was demonstrated by examples of Pd-catalyzed C-N coupling and Ni-catalyzed ethylene oligomerization (TON>6300). Crystal structures of 6 b, 11 b, and 12 b are presented.

Activation and Deactivation by Temperature: Behavior of Ph2PN(iPr)P(Ph)N(iPr)H in the Presence of Alkylaluminum Compounds Relevant to Catalytic Selective Ethene Trimerization

Coordination, deprotonation, rearrangement, and cleavage of Ph(2)PN(iPr)P(Ph)N(iPr)H (1) by trialkylaluminum compounds R(3)Al (R=Me, Et) are reported that are relevant to the selective ethene trimerization system consisting of the ligand 1, CrCl(3)(THF)(3) and Et(3)Al that produces 1-hexene in more than 90% yield and highest purity. With increasing temperature and residence time first the formation of an adduct [Ph(2)PN(iPr)P(Ph)N(iPr)H][AlR(3)] (2), second the aluminum amide [Ph(2)PN(iPr)P(Ph)(AlR(3))N(iPr)][AlR(2)] (3) and third its rearrangement to the cyclic compound [N(iPr)P(Ph)P(Ph(2))N(iPr)][AlR(2)] (4) were observed. The cleavage of 3 by an excess of R(3)Al into an amidophosphane and an iminophosphane could be the reason for its rearrangement to complex 4, as well as to the cyclic dimer [R(2)AlN(iPr)P(Ph)(2)](2) (5). The chemistry of ligand 1 in the presence of alkylaluminum compounds gives hints on possible activation and deactivation mechanisms of 1 in trimerization catalysis.

A Kinetic Model for Selective Ethene Trimerization to 1‐Hexene by a Novel Chromium Catalyst System

A numerical model for the kinetics of the selective trimerization of ethene to 1‐hexene has been developed on the basis of mechanistic investigations and extensive experimental parameter studies. The reaction is catalyzed by a homogeneous catalyst system, comprising the chromium source [CrCl3(thf)3], a Ph2PN(iPr)P(Ph)N(iPr)H ligand, and triethylaluminum as activator. The kinetic model is designed as a tool for laboratory data evaluation, design and planning of meaningful experiments in the multidimensional parameter space, and parameter identification, and, moreover, it includes all features needed to eventually facilitate the transfer of the laboratory results into the technical environment. In particular, the model is designed to deliver the intrinsic chemical kinetics of the homogeneous catalytic system and to rule out any undetected influence of phase‐transfer limitations. Key kinetic parameters are determined by fitting the numerical simulations to the experimental results. In general, the model calculations and experimental data are in excellent agreement. In conjunction with mechanistic investigations, the model helps to elucidate the complex reaction network.

Immobilized Chromium Catalyst System for Selective Ethene Trimerization to 1‐Hexene with a PNPNH Ligand

Immobilization of transition metal complexes on solid supports is well known as an efficient method to handle and recover catalysts. Among these supports, polystyrene-based materials are widely used to create recyclable catalysts. The immobilization of transition metals on these supports offers a number of advantages over traditional solution-phase chemistry. Covalent binding is the most commonly used technique for immobilization, due to its broad applicability, the fact that significant leaching does not usually occur, and that stable, active catalysts are formed. Binding is usually achieved in one of two ways: a) by grafting the catalyst (via a ligand) onto the prederivatized support, or b) by copolymerization of the active species with styrene and divinylbenzene (DVB). Linear a-olefins have found extensive applications in the manufacture of fine chemicals such as detergents, plasticizers and lubricants, in addition to being used as comonomers for the production of linear low-density polyethene. Industrially prevalent in the production of linear a-olefins are mainly the SHOP, Chevron, and Amoco processes. In the field of selective production of linear a-olefins, only the Chevron–Philips selective trimerization process is currently industrially used, although there are several other known selective oligomerization systems. Even fewer examples of immobilized chromium-based selective trimerization catalysts for the production of 1-hexene have been reported to date. We recently described the development of a novel homogeneous catalyst for selective ethene trimerization to 1-hexene, which is based on a new class of aminophosphine ligands with a Ph2PN(iPr)P(Ph)N(iPr)H (PNPNH) backbone, in conjunction with [CrCl3(thf)3] and Et3Al as a cheap and well-defined aluminum–alkyl activator (Scheme 1). It is important to mention that especially the terminal secondary amine function is crucial for selectivity towards 1-hexene. Deprotonation of this group by the aluminum–alkyl activator, to yield an amide ligand backbone, is very likely to occur under catalytic conditions. Detailed kinetic investigations of this new homogeneous trimerization system were very recently reported. Herein we report the immobilization of this homogeneous system to yield an effective heterogeneous catalyst. The latter combines the advantages of the homogeneous catalyst system, such as selectivity and moderate conditions, with the advantages of a heterogeneous process setup, such as easy catalyst separation and recyclability. An excess of the ligand precursor Ph2P(iPr)NP(Ph)Cl [7] reacted with the amino groups of the amino-modified styrene polymer to yield the desired PNPNH ligand structure (Scheme 2). Among various supports investigated, such as aminomethyl polystyrene resin, TentaGel MB NH2, aminomethyl ChemMatrixR resin, and diethylenetriamine polystyrene resin, tris(2-aminoethyl)amine polystyrene resin gave the best results in terms of loading and catalysis. Gel-phase P NMR spectroscopy of the solid support suspended in C6D6 gave clear evidence for the covalently bonded PNPNH ligand (d= 40.6, 72.5 ppm). According to elemental analysis, approximately Scheme 1. Composition according to the homogeneous system.

Ambident PCN Heterocycles: N- and P-Phosphanylation of Lithium 1,3-Benzazaphospholides

Synthetic and structural aspects of the phosphanylation of 1,3-benzazaphospholides 1(Li), ambident benzofused azaphosphacyclopentadienides, are presented. The unusual properties of phospholyl-1,3,2-diazaphospholes inspired us to study the coupling of 1(Li) with chlorodiazaphospholene 2, which led to the N-substituted product 3. Reaction of 1(Li) with chlorodiphenyl- and chlorodicyclohexylphosphane likewise gave N-phosphanylbenzazaphospholes 4 and 5, whereas with the more bulky di-tert-butyl- and di-1-adamantylchlorophosphanes, the diphosphanes 6 and 7 are obtained; in the case of 7 they are isolated as a dimeric LiCl(THF) adduct. Structural information was provided by single-crystal X-ray diffraction and solution NMR spectroscopy experiments. 2D exchange spectroscopy confirmed the existence of two rotamers of the aminophosphane 5 at room temperature; variable-temperature NMR spectroscopy studies of 6 revealed two dynamic processes, low-temperature inversion at ring phosphorus (DeltaH( not equal)=22 kJ mol(-1), DeltaS( not equal)=2 J K(-1) mol(-1)) and very low-temperature rotation of the tBu(2)P group. Quantum chemical studies give evidence that 2-unsubstituted benzazaphospholides prefer N-phosphanylation, even with bulky chlorophosphanes, and that substituents at the 2-position of the heterocycle are crucial for the occurrence of P-N rotamers and for switching to alternative P-substitution, beyond a threshold steric bulk, by both P- and 2-position substituents.

Contributions to the Chemistry of Twofold-Coordinated Group 15/14 Element Heterocycles (A Personal Account)

Synthesis, properties, and reactivity towards metal compounds of benzo- and pyrido-anellated 1,3-azaphospholes and 1,3,2λ 2 -diazaelement-2-ylidenes are described. They present heterocycles bearing a twofold-coordinated (σ 2 ) group15 element in 3- or a σ 2 -group14 element in 2-position. Migration of substituents between the 2- and 3-position that would make them isomers was so far not observed. Nevertheless, they are relatives with respect to stabilization of the twofold-coordinated element by inclusion into a cyclodelocalized 10π-electron system and by weak donor but notable π-acceptor properties.

Ambident Reactivity of P˭CH‒N‒Heterocycles: Lithiation and Substitution Sites

Abstract Benzofused 1H-1,3-azaphospholes are lithiated at the N-atom by tBuLi but phosphinylation takes place at either the N- or the P-atom. Smaller chlorophosphines react at nitrogen, bulkier react at phosphorus. Substituents at C2 promote the latter mode. N-Substituted 2H-1,3-benzazaphospholes undergo CH-metalation or addition at the P˭C bond, depending on the conditions, and allow access to 2-functionally substituted benzazaphospholes or their 2,3-dihydro derivatives, new σ2P,X or σ3P,X hybrid ligands (X=O,P).

2-Hydroxy- and 2-Amino-Functional Arylphosphines—Syntheses, Reactivity, and Use in Catalysis

We present the synthesis, resolution, structures and catalytic activity of selected representatives of 2-(1S)-camphanoyloxy-phenyl- and biphenylphosphines and the access to novel asymmetric P,N-heterocyclic ethylenebis(phosphine) ligands.

Synthesis of Differently Substituted N- and P-Aminodiphosphinoamine PNPN-H Ligands

Abstract On the basis of the known aminodiphosphinoamine ligand Ph2PN(i-Pr)P(Ph) N(i-Pr)-H (3a), differently substituted aminodiphosphinoamine PNPN-H ligands (3) were prepared. By using different synthetic methods, the N-substituted ligands Ph2PN (i-Pr)P(Ph)N(c-Hex)-H (3b), Ph2PN(c-Hex)P(Ph)N(i-Pr)-H (3g), and Ph2PN(i-Pr)P(Ph) N[(CH2)3Si(OEt)3]-H (3c), in addition to the formerly described Ph2PN(n-Hex)P (Ph)N (i-Pr)-H (3h), Ph2PN(i-Pr)P(Ph)N(Et)-H (3d), Ph2PN(i-Pr)P(Ph)N(Me)-H (3e), and Ph2PN(c-Hex)P(Ph)N(c-Hex)-H (3f), were obtained. In addition, Ph2PN(i-Pr)P(Me)N(i-Pr)-H (3i), (cyclopentyl)2PN(i-Pr)P(Ph)N(i-Pr)-H (3j), (-O-CH2-CH2-O-)PN(i-Pr)P(Ph)N(i-Pr)-H (3k), and (1-Ad)2PN(i-Pr)P(Ph)N(i-Pr)-H (3l) were prepared with different P-substitutions. All compounds were characterized and the molecular structures of the intermediates Ph2PN(i-Pr)P(Ph)Cl (1a) and (cyclopentyl)2PN(i-Pr)P(Ph)Cl (1e) and the ligand (1-Ad)2PN(i-Pr)P(Ph)N(i-Pr)-H (3l) were investigated by single-crystal X-ray diffraction. Supplemental materials are available for this article. Go to the publishers online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file. GRAPHICAL ABSTRACT