Richard J. Staples

Coordination Chemistry of Lanthanides at “High” pH: Synthesis and Structure of the Pentadecanuclear Complex of Europium(III) with Tyrosine

A five-coordinate chloride ion is believed to template the assembly of a pentadecanuclear lanthanide complex of europium(III). This cluster (see picture) has been prepared by coordination of europium(III) perchlorate with tyrosine at about pH 6. Single crystal X-ray analysis established an unprecedented structure in which 15 constituent europium(III) ions are organized into three parallel pentagonal layers.

A Catalytic Asymmetric Chlorocyclization of Unsaturated Amides

Stereodefined carbon–halogen bonds are ubiquitous in nature with several natural products exhibiting this motif. While the biogenetic origins of this unique chiral functionality has been a subject of several investigations in the past, attempts by organic chemists to forge the carbon–halogen bond stereoselectively have largely been unsuccessful. This problem has come into focus only recently. Several elegant reports of asymmetric halogenations of alkenes and alkynes followed by an intramolecular attack of a pendant nucleophile have appeared in the literature in the last decade. Kang et al. reported a cobalt–salen catalyzed iodoetherification reaction. An asymmetric fluorocyclization of allyl silanes mediated by a cinchona alkaloid dimer was reported by Gouverneur and co-workers. Tang and co-workers disclosed an asymmetric bromolactonization of enynes catalyzed by a cinchona alkaloid derived urea; other bromolactonizations have also appeared following the disclosure of their report. More recently, Veitch and Jacobsen reported an asymmetric iodolactonization reaction mediated by chiral thiourea catalysts. Polyene cyclizations induced by chiral halonium ions have also been realized as reported by the research groups of Ishihara and Snyder. However, given the fledgling nature of this research area, one may find it easy to highlight the many drawbacks and limitations even in the present state of the art—for example, the relatively large catalyst loadings (superstoichiometric quantities in some cases) to achieve meaningful levels of enantioselectivity or the lack of a robust catalytic system that can catalyze a number of diverse reactions rather than one specific reaction. Moreover, efficient asymmetric chlorocyclizations have remained underdeveloped. This situation is attributable, at least in part, to the highly reactive nature of chloronium ions, which are known to exist in equilibrium with the corresponding carbocation rather than exclusively as cyclic chloronium ions, thus making the development of chlorocyclizations a formidable challenge. Our research group has recently reported the catalytic asymmetric chlorolactonization of alkenoic acids. Herein, we disclose the efficient halocyclization of unsaturated amides to furnish chiral heterocycles. Furthermore, these heterocycles have been transformed into useful chiral building blocks such as amino alcohols. Chiral heterocycles such as oxazolines and dihydrooxazines are commonly encountered motifs in natural products, molecules of pharmaceutical interest, and in several chiral ligands. Their syntheses, however, usually employ stoichiometric quantities of chiral amino alcohols. With only one precedented method to access these molecules in a catalytic asymmetric fashion, we were intrigued by the possibility of one-step access to these versatile chiral heterocycles by a catalytic asymmetric halocyclization of easily accessed unsaturated amides. We chose the conversion of benzamide 1 into oxazoline 2 as our initial test reaction. Among the several ligands screened for the test reaction, (DHQD)2PHAL emerged as the best candidate, thus affording the desired oxazoline 2 in 57% ee (Table 1, entry 1) with DCDMH as the terminal chlorine source. Reactions with other chlorenium sources

On the Chlorenium Source in the Asymmetric Chlorolactonization Reaction

N-Acylated N-chlorohydantoins are shown to be competent chlorenium sources in the (DHQD)(2)PHAL-mediated asymmetric chlorolactonization. The derivatives demonstrate the exact role of the N1 and N3 chlorine atoms in the parent dichlorohydantoins with the N1 chlorine serving as an inductive activator and the N3 chlorine being delivered to the substrate. The putative associated catalyst/chlorine source complex was experimentally demonstrated through a series of matched/mismatched experiments employing chiral N-chlorinated hydantoins.

Luminescent Chains Formed from Neutral, Triangular Gold Complexes Sandwiching TlI and AgI. Structures of {Ag([Au(μ-C2,N3-bzim)]3)2}BF4·CH2Cl2, {Tl([Au(μ-C2,N3-bzim)]3)2}PF6·0.5THF (bzim = 1-Benzylimidazolate), and {Tl([Au(μ-C(OEt)═NC6H4CH3)]3)2}PF6·THF, with MAu6 (M = Ag+, Tl+) Cluster Cores

It has been found that several trinuclear complexes of AuI interact with silver and thallium salts to intercalate Ag+ and Tl+ cations, thereby forming chains. The resulting sandwich clusters center the cations between the planar trinuclear moieties producing structures in which six AuI atoms interact with each cation in a distorted trigonal prismatic coordination. The resultant (B3AB3B3AB3)∞ pattern of metal atoms also shows short (∼3.0 Å) aurophilic interactions between BAB molecular centers. These compounds display a strong visible luminescence, under UV excitation, which is sensitive to temperature and the metal ion interacting with the gold. X-ray crystal structures are reported for Ag([Au(μ-C2,N3-bzim)]3)2BF4·CH2Cl2 (P [Formula: see text] , Z = 2, a = 14.4505(1)Å; b = 15.098(2)Å; c = 15.957(1)Å; α = 106.189(3)°; β = 103.551(5)°; γ = 101.310(5)°); Tl([Au(μ-C2,N3-bzim)]3)2PF6·0.5C4H8O (P [Formula: see text] , Z = 2, a = 15.2093(1)Å; b =15.3931(4)Å; c = 16.1599(4)Å; α = 106.018(1)°; β = 101.585(2)°; γ=102.068(2)°); and Tl([Au(μ-C(OEt)═NC6H4CH3)]3)2PF6·C4H8O (P2(1)/n, Z = 4, a = 16.4136(3)Å; b = 27.6277(4)Å; c = 16.7182(1)Å; β = 105.644(1)°). Each compound shows that the intercalated cation, Ag+ or Tl+, coordinates to a distorted trigonal prism of six AuI atoms. The counteranions reside well apart from the cations between the cluster chains.

Electronic effects in iridium C-H borylations: insights from unencumbered substrates and variation of boryl ligand substituents.

Experiment and theory favour a model of C-H borylation where significant proton transfer character exists in the transition state.

Silyl phosphorus and nitrogen donor chelates for homogeneous ortho borylation catalysis.

Ir catalysts supported by bidentate silyl ligands that contain P- or N-donors are shown to effect ortho borylations for a range of substituted aromatics. The substrate scope is broad, and the modular ligand synthesis allows for flexible catalyst design.

Development of a Formal Catalytic Asymmetric [4 + 2] Addition of Ethyl-2,3-butadienoate with Acyclic Enones

Allene esters are unique not only as excellent electrophiles but also because of their ability for subsequent reactivity after the initial nucleophilic attack. A mechanistically inspired cyclization using ethyl-2,3-butadienoate and acyclic enones to provide dihydropyrans in excellent yields and enantioselectivity under solvent-free conditions at room temperature is reported.

Nickel(ii) and copper(ii) complexes with pyridine-containing macrocycles bearing an aminopropyl pendant arm: synthesis, characterization, and modifications of the pendant amino groupElectronic supplementary information (ESI) available: colour versions of Figs. 4, 5 and 7. See http://www.rsc.org/suppdata/dt/b2/b211489e/

The synthesis of three five-coordinate nickel(II) complexes with pendant arm-containing macrocycles has been achieved by the reduction of CN bonds in the Schiff base precursors derived from diacetyl- or diformyl-pyridine and a tripodal tetramine. Demetallation of the nickel(II) macrocycles yielded stable pentadentate ligands that were used for the preparation of the copper(II) complexes. The structures of three nickel(II) complexes and two copper(II) complexes were determined by X-ray crystallography. Protonation of the pendant arm (pKa = 6.3–6.6 for the nickel complexes, and 6.5–7.3 for the copper complexes) produced four-coordinate macrocycles, one of which was structurally characterized. The primary amino group of the pendant arm coordinated to the nickel(II) reacted with acetic anhydride or benzoyl chloride. The resulting mono-functionalized nickel(II) complexes and their copper(II) counterparts obtained by transmetallation displayed square-planar geometry in the solid state, as determined by X-ray crystallography, and remained four-coordinate in solutions below pH 11.

Dinuclear gold(I) dithiophosphonate complexes: formation, structure and reactivity

Abstract 2,4-Diaryl- and 2,4-diferrocenyl-1,3-dithiaphosphetane disulfide dimers (1a–1c) react with a variety of alcohols, silanols and trialkylsilylalcohols to form dithiophosphonic acids in a facile manner. Their corresponding salts react with chlorogold(I) complexes to produce the first dinuclear gold(I) dithiophosphonate complexes of the type [AuS2PR(OR′)]2 (R=Ph, R′=Et (2); R=p-C6H4OMe, R′=SiPh3) (3)) in high yield (>70%). The dinuclear gold(I) complexes react with dppm (Ph2PCH2PPh2) or dppa (Ph2PNHPPh2) to form new heterobridged dinuclear gold(I) complexes of the type [Au2(dppm)S2P(O)R] (R=Ph (4)). Moreover, 1a–1c react with Au2(dppm)Cl2 to form new heterobridged trithiophosphonate gold(I) complexes of the type [Au2(dppm)S2P(S)R] (R=ferrocenyl (5)).

Getting the sterics just right: a five-coordinate iridium trisboryl complex that reacts with C-H bonds at room temperature

Five-coordinate boryl complexes relevant to Ir mediated C-H borylations have been synthesized, providing a glimpse of the most fundamental step in the catalytic cycle for the first time.