Tripodal Phosphane Ligands with Novel Linker Units and Their Rhodium Complexes as Building Blocks for Dendrimer Catalysts

pKaH values and θH angles of phosphanes to predict their electronic and steric parameters

Phosphanes play an important role in various applications, serving as a class of organic bases with basicities spanning more than 30 orders of magnitude. Accessing comprehensive basicity data for phosphanes has been challenging due to scattered information across multiple sources and notable gaps in the existing data. In this report, we present basicities (pKaH values) of a diverse set of phosphanes, both newly measured or calculated and collected from the literature. We demonstrate that pKaH values can serve as an alternative to Tolman electronic parameters (TEP values) in evaluating the electronic properties of phosphanes. Additionally, we suggest parameters for assessing the steric properties of phosphanes without the need for preparation or calculation of metal-ligand complexes.

Synthesis and Reactivity of Poly(propyleneimine) Dendrimers Functionalized with Cyclopentadienone N-Heterocyclic-Carbene Ruthenium(0) Complexes

Ligand design in metal chemistry is a fundamental step when pursuing compounds with specific reactivity. In this paper, the functionalization of the OH group in the lateral chain of the N-heterocyclic-carbene (NHC) ligand bound to a bis-carbonyl cyclopentadienone NHC ruthenium(0) complex allowed the decoration of five generations of poly(propyleneimine) (PPIs) dendrimers with up to 64 organometallic moieties. The coupling was achieved by employing carbonyldiimidazole and the formation of carbamate linkages between dendritic peripheral NH2 and lateral OH groups on ruthenium complexes. The synthetic procedure, chemical purification, and spectroscopic characterization of the five generations of dendrimers (3g1–5) are here described. The ruthenium-modified dendrimers were activated as catalysts in the transfer hydrogenation of the model compound 4-fluoroacetophenone in the presence of cerium ammonium nitrate as their mononuclear congeners. The catalytic activity, being similar for the five generations, shows a decrease if compared to mononuclear complexes. This detrimental effect might be ascribed to the –CH2NH– functionalization, largely present in dendrimer skeleton and that can compete with the hydrogen transfer mechanism, but also partially to a dendritic effect caused by steric encumbrance.

Pentaerythritol: A Versatile Substrate in Organic Transformations, Centralization on the Reaction Medium

BACKGROUND

Pentaerythritol (2,2-bis (hydroxymethyl) propane-1,3-diol) as white crystalline odorless solid has been synthesized in 1891. Pentaerythritol is multifaceted species in many compounds, which are wildly utilized in medicine and industry. Also, multicomponent reactions (MCRs) play a crucial role in organic and medicinal chemistry. Hence, in these reactions, pentaerythritol is a versatile substrate for the synthesis of many polyfunctionalized products, because of the presence of the neopentane core and one hydroxyl group in each of the four terminal carbons.

OBJECTIVE

The review describes pentaerythritol multicomponent reactions in the presence of different solvents in the reaction medium to produce various compounds including pentaerythritols. This review covers the literature relevant up to 2018.

CONCLUSION

It is obvious from the provided review that a great deal of research has been done in this field, utilizing various mediums (solvent-free conditions, aqueous media, and organic solvents) for the synthesis of the products of containing pentaerythritols. This classification is based on the importance of economic and environmental friendly reactions. Due to the whole aforesaid reports, some reactions required heat for their progress, and some others were accompanied by microwave or ultrasonic waves.

Facile synthesis of supported Ru-Triphos catalysts for continuous flow application in selective nitrile reduction

Selectivity of immobilized Triphos-type catalysts can be tuned for application in nitrile hydrogenation in batch and continuous flow processes.

The selective catalytic hydrogenation of nitriles represents an important but challenging transformation for many homogeneous and heterogeneous catalysts. Herein, we report the efficient and modular solid-phase synthesis of immobilized Triphos-type ligands in very high yields, involving only minimal work-up procedures. The corresponding supported ruthenium–Triphos catalysts are tested in the hydrogenation of various nitriles. Under mild conditions and without the requirement of additives, the tunable supported catalyst library provides selective access to both primary amines and secondary imines. Moreover, the first application of a Triphos-type catalyst in a continuous flow process is presented demonstrating high catalyst life-time over at least 195 hours without significant activity loss.

Modular functionalized polyphosphines for supported materials: previously unobserved31P-NMR «through-space» ABCD spin systems and heterogeneous palladium-catalysed C–C and C–H arylation

Functionalized modular polyphosphines introduce a conceptual novelty in controlling both implantation and conformation of donors in immobilized catalysts.

Reactivity and catalytic activity of a robust ruthenium(II)-triphos complex

The ruthenium(II)-triphos acetato complex [RuCl(OAc)(kappa3-triphos)] (triphos = (PPh2CH2)3CMe) has been found to be an active catalyst precursor for the hydrogenation of 1-alkenes under relatively mild conditions (5-50 bar H2, 50 degrees C). In contrast to related triphenylphosphine complexes, [RuCl(OAc)(kappa3-triphos)] is much less air sensitive and high catalytic activities were achieved when catalyst samples were prepared without exclusion of air or moisture. Substitution of the acetato ligand can be effected by treatment of acid, affording [Ru2(mu-Cl)3(kappa3-triphos)2]Cl and [RuCl(kappa3-triphos)]2(BF4)2 with aqueous HCl and [Et2OH]BF4, respectively, or by heating with dmpm in the presence of [NH4]PF6, resulting in formation of [RuCl(kappa2-dmpm)(kappa3-triphos)]PF6 (dmpm = PMe2CH2PMe2). A hydride complex, [RuHCl(kappa3-triphos)], formed by acetato-mediated heterolytic cleavage of dihydrogen is proposed as the active catalytic species. An inner-sphere, monohydride mechanism is suggested for the catalytic cycle, with chloro and triphos ligands playing a spectator role. These mechanistic proposals are consistent with reactivity studies carried out on [RuCl(OAc)(kappa3-triphos)] and [RuH(OAc)(kappa3-triphos)] and supported by a computational analysis. The solid-state structures of [RuCl(OAc)(kappa3-triphos)], [RuCl(kappa3-triphos)]2(BF4)2, and [RuCl(kappa2-dmpm)(kappa3-triphos)]PF6 have been established by X-ray diffraction.

Shaping and Enforcing Coordination Spheres: The Implications ofC3 andC1 Chirality in the Coordination Chemistry of 1,1,1-Tris(oxazolinyl)ethane (“Trisox”)

A key feature of tris(oxazolinyl)ethane ("trisox") ligands, which have shown broad scope in asymmetric catalysis, is the orientation and steric demand of their oxazoline substituents. This, along with the modularity of their synthesis determines their coordination chemistry. The possibility to combine oxazolines, in which the stereogenic centers adjacent to the N-donor atoms have different absolute configuration, whilst retaining their ability to coordinate as tripodal ligands, has been demonstrated by the synthesis of the enantiomerically pure C3-symmetric iPr-trisox(S,S,S) and C1-symmetric iPr-trisox(S,S,R) and their reaction with [Mo(CO)3(NCMe)3] yielding [Mo{iPr-trisox(S,S,S)}(CO)3] (1 a) and [Mo{iPr-trisox(S,S,R)}(CO)3] (1 b), respectively. The non-autocomplementarity of two homochiral trisox ligands at one metal center has been demonstrated by reaction of rac-C3 iPr-trisox with one equivalent of [Co(ClO4)2].6 H2O, giving the centrosymmetric heterochiral complex [Co(iPr-trisox)2](ClO4)2 (3), whereas an analogous reaction with the enantiopure ligand yielded a mixture of Co(II) complexes, which is characterized by the total absence of a [(trisox)2Co](+/2+) ion. The scope of the trisox ligand in terms of facial coordination to both early and late transition metals was demonstrated by the synthesis and structural characterization of the mononuclear complexes [ScCl3(iPr-trisox)] (4), [Fe(tBu-trisox)(NCMe)3](BF4)2 (5), and [Ru(eta6-p-cymene)(iPr-trisox)](PF6)2 (6). The facial coordination of their three ligating atoms to a metal center may be impeded if the transition-metal center stereoelectronically strongly favors a non-deltahedral coordination sphere, which is generally the case for the heavier d8-transition-metal atoms/ions. Reaction of iPr-trisox with [Rh(cod)2]BF4 led to the formation of the 16-electron d8-configured complex [Rh(iPr-trisox)(cod)](BF4) (7), which is oxidized by CsBr3 to give the Rh(III) complex [RhBr3(iPr-trisox)] (8) possessing a C3-symmetric structure with a kappa3-N-trisox ligand. The crystalline salts [M2(mu-Cl3)(iPr-trisox)2](PF6) (M=Fe(II): 9, Co(II): 10, Ni(II): 11), were prepared by addition of one molar equivalent of iPr-trisox and an excess of KPF6 to solutions of the anhydrous (FeCl2) or hydrated metal halides (CoCl2.6 H2O, NiCl2.6 H2O). All dinuclear complexes display weak magnetic coupling. For the mononuclear species [CuCl2(iPr-trisox)] (12) the removal of a chloride anion and thus the generation of a dinuclear chloro-bridged structure failed due to Jahn-Teller destabilization of a potential octahedral coordination sphere.

Tuning "kappticity" of tripodal ligands

The synthesis and structural characterization of a series of tripodal tris(phosphine) ligands, containing SiMe2 elbow groups, is described. The significant steric congestion in these ligands, due to the silylmethyl substituents, is manifest both in the solid-state structures and in the solution NMR spectra of the free ligands. Variable temperature 1H{31P} NMR studies of one of the ligands, CH3C(SiMe2PEt2)3 (4b) gave an estimated barrier to rotation around the Si-Capical bonds of approximately 10.4 kcal mol(-1). Octahedral kappa2- and kappa3-molybdenum complexes of these ligands also demonstrate the impact of the additional bulk imparted by the SiMe2 substituents, and the high Lewis basicity of these phosphines, with subtle changes at the apical and phosphine substituents changing the overall coordination chemistry observed.

Tris(pyrazolyl)borate carbosilane dendrimers and metallodendrimers

A modified tris(pyrazolylborate) ligand has been prepared in two steps. First, reaction of triisopropylborate with allylmagnesium bromide and further treatment with benzoyl chloride gave CH(2) = CHCH(2)B(O(i)Pr), which was then reacted with potassium pyrazolate and pyrazole to give the compound K[CH(2) = CHCH(2)Bpz(3)]. The new allyl-containing scorpionate anion of acts as a bi- or tri-dentate ligand, as shown by the mononuclear complexes [CH(2) = CHCH(2)Bpz(3)M(LL)] (M = Rh, LL = nbd, ; LL = tfb, ; LL = (CO)(PPh(3)), ; M = Ir, LL = cod, ), obtained from reactions of the chlorido-bridged dinuclear complexes [{M(mu-Cl)(LL)}(2)] with 2. Furthermore, the borate represents a key material to achieve the attachment of tris(pyrazolyl)borate groups to the peripheries of carbosilane dendrimers. Thus, the platinum-catalyzed hydrosilylation reactions of compound with the dendritic cores Si[(CH(2))(3)SiMe(2)H](4) (G(0)-(SiH)(4)), (G(1)-(SiH)(8)), and (G(2)-(SiH)(16)) gave the corresponding borate-containing dendrimers Si[(CH(2))(3)SiMe(2)(CH(2))(3)B(O(i)Pr)(2)](4) (G(0)-B(4)), Si[(CH(2))(3)SiMe{(CH(2))(3)SiMe(2)(CH(2))(3)B(O(i)Pr)(2)}(2)](4) (G(1)-B(8)), and Si[(CH(2))(3)SiMe{(CH(2))(3)SiMe[(CH(2))(3)SiMe(2)(CH(2))(3)B(O(i)Pr)(2)](2)}(2)](4) (G(2)-B(16)) selectively in the anti-Markovnikov direction. Further reactions of G(0)-B(4), G(1)-B(8) and G(2)-B(16) with potassium pyrazolate and pyrazole rendered the corresponding polyanionic dendrimers K(4)[Si{(CH(2))(3)SiMe(2)(CH(2))(3)Bpz(3)}(4)] (G(0)-(Bpz(3))(4)), G(1)-(Bpz(3))(8), and G(2)-(Bpz(3))(16), respectively, which contain 4, 8, and 16 tris(pyrazolyl)borate groups symmetrically located around the dendritic peripheries. These unusual polyanionic dendrimers are excellent scaffolds to support metal centres, as shown by the reactions of G(0)-(Bpz(3))(4), G(1)-(Bpz(3))(8), and G(2)-(Bpz(3))(16) with [{Rh(mu-Cl)(nbd)}(2)] to give the neutral rhodadendrimers [Si{(CH(2))(3)SiMe(2)(CH(2))(3)Bpz(3)Rh(nbd)}(4)] G(0)-(Bpz(3)Rh)(4), G(1)-(Bpz(3)Rh)(8) and G(2)-(Bpz(3)Rh)(16) as stable solids in excellent yields. Following this protocol, mixed rhodium/iridium metallodendrimers can be prepared.

“A posteriori” modification of carbosilane dendrimers and dendrons: their activation in core and branch positions

The introduction of phenyl groups at different points on carbosilane dendrimers allows their acidolytic conversion to highly reactive triflato groups which in turn are readily substituted by anionic nucleophiles. Core phenylated first-fourth generation dendrimers were synthesized from tri(allyl)phenylsilane by an alternating sequence of hydrosilylation and allylation steps. Similarly, carbosilane dendrimers containing phenyl-Si groups at the branching points and in the periphery were prepared from tetraallylsilane which was hydrosilylated with PhHSiCl2. Reaction of the phenylated dendrimers with triflic acid in toluene cleanly gave the silyl triflate derivatives, provided that the correct stoichiometry of the reagents was used. In the presence of a large excess of triflic acid the SiMe3-end groups are slowly converted to SiMe2(OTf)-units. The proof of concept was provided by the fixation of a {Ph2PCH2} group using the lithiated diphenylphosphinomethanide Ph2PCH2Li, obtained by cleavage of Ph3SnCH2PPh2 with PhLi, as well as a lithiated ether-alcohol functionalized triphos derivative to the core of a third generation carbosilane dendrimer.

Syntheses of monohydroxy benzyl ethers of polyols: tri-O-benzylpentaerythritol and other highly benzylated derivatives of symmetrical polyols

Symmetrical polyols can be converted into benzyl ethers with one free hydroxyl group in good yield by reaction of the monodibutylstannylene acetal with excess benzyl bromide in the presence of tetrabutylammonium bromide and diisopropylethylamine in xylene. The reaction pathway involves initial benzylation of the dibutylstannylene acetal to give benzyl and bromodibutylstannyl ethers; if a hydroxyl group remains unsubstituted, the latter ether ring closes and reacts further.