Generation of Dicoordinate Boron(I) Units by Fragmentation of a Tetra‐Boron(I) Molecular Square

Reduction of carbene‐borane adduct [(cAAC)BBr₂(CN)] (cAAC=1‐(2,6‐diisopropylphenyl)‐3,3,5,5‐tetramethylpyrrolidin‐2‐ylidene) cleanly yielded the tetra(cyanoborylene) species [(cAAC)B(CN)]₄ presenting a 12‐membered (BCN)₄ ring. The analysis of the Kohn–Sham molecular orbitals showed significant borylene character of the B^I atoms. [(cAAC)B(CN)]₄ was found to reduce two equivalents of AgCN per boron center to yield [(cAAC)B(CN)₃] and fragmented into two‐coordinate boron(I) units upon reaction with IMeMe (1,3,4,5‐tetramethylimidazol‐2‐ylidene) to yield the corresponding tricoordinate mixed cAAC‐NHC cyanoborylene. The analogous cAAC‐phosphine cyanoborylene was obtained by reduction of [(cAAC)BBr₂(CN)] in the presence of excess phosphine.     

Homogeneous photochemical oxidation via singlet O2 in supercritical CO2

This communication describes the reaction chemistry of singlet oxygen in supercritical carbon dioxide, demonstrating rapid and quantitative conversion of alpha-terpinene to ascaridole.     

Synthesis and structures of crystalline Li, Al and Sn(II) 1-azaallyls and beta-diketiminates derived from [Li{mu,eta3-N(SiMe3)C(Ad)C(H)SiMe3}]2 (Ad = 1-adamantyl)

The crystalline dimeric 1-azaallyllithium complex [Li{mu,eta(3-N(SiMe3)C(Ad)C(H)SiMe3}]2 (1) was prepared from equivalent portions of Li[CH(SiMe3)2] and 1-cyanoadamantane (AdCN). Complex was used as precursor to each of the crystalline complexes 2-8 which were obtained in good yield. By 1-azaallyl ligand transfer, 1 afforded (i) [Al{eta3-N(SiMe3)C(Ad)C(H)SiMe3}{kappa1-N(SiMe3)C(Ad)=C(H)SiMe3-E}Me] (5) with [AlCl2Me](2), (ii) [Sn{eta3-N(SiMe3)C(Ad)C(H)SiMe3}2] (7) with Sn[N(SiMe3)2]2, and (iii) [Li(N{C(Ad)=C(H)SiMe3-E}{Si(NN)SiMe3})(thf)2] (8) with the silylene Si[(NCH(2)Bu(t))2C6H(4)-1,2] [= Si(NN)]. By insertion into the C[triple bond, length as m-dash]N bond of the appropriate cyanoarene RCN, gave the beta-diketiminate [Li{mu-N(SiMe3)C(Ad)C(H)C(R)NSiMe3}]2 [R = Ph (2), C(6)H(4)Me-4 (3)], and yielded [Al{kappa2-N(SiMe3)C(Ad)C(H)C(Ph)NSiMe3}{kappa1-N(SiMe3)C(Ad)=C(H)SiMe3-E}Me] (6). The beta-diketiminate [Al{kappa2-N(SiMe3)C(Ad)C(H)C(Ph)NSiMe3}Me2] (4) was prepared from 2 and [AlClMe2]2. The X-ray structures of 1 and 3-8 are presented. Multinuclear NMR spectra in C6D6 or C6D5CD3 have been recorded for each of 1-8; such data on 8 revealed that in solution two minor isomers were also present.     

A Triply [5]Helicene-Bridged (1,3,5)Cyclophane

A rigid propeller-shaped conjugated triple macrocycle consisting of two nearly perfectly stacked benzene rings and three linking [5]helicene moieties has been synthesized using a glyoxylic Perkin approach. Analysis of the electron delocalization in this atypical aromatic molecule revealed global aromaticity and a 78 π-electron circuit along the edge of its triple loop, to the detriment of the two 6 π-electron circuits in the two stacked benzene rings.     

Sequential ene, Diels-Alder reactions of AF4-yne with 1,3,5-cycloheptatriene

(4,5-Dehydro)-1,1,2,2,9,9,10,10-octafluoro [2.2]paracyclophane (AF4-yne) undergoes an ene reaction with 1,3,5-cycloheptatriene, the adduct of which subsequently undergoes a further Diels-Alder reaction with a second equivalent of AF4-yne to give two stereoisomeric 2:1 adducts. A very small amount of the classic 1:1 Diels-Alder adduct also can be isolated from the reaction. Structure assignments of all products were determined by NMR through a series of H1-H1, H1-C13 one bond, and H1-C13 two and three bond correlation experiments as well as H1-H1 NOE experiments.     

Uncatalyzed Hydrogenation of First‐Row Main Group Multiple Bonds

Room temperature hydrogenation of an SIDep‐stabilized diboryne (SIDep=1,3‐bis(diethylphenyl)‐4,5‐dihydroimidazol‐2‐ylidene) and a cAAC‐supported diboracumulene (cAAC=1‐(2,6‐diisopropylphenyl)‐3,3,5,5‐tetramethylpyrrolidin‐2‐ylidene) provided the first selective route to the corresponding 1,2‐dihydrodiborenes. DFT calculations showed an overall exothermic (ΔG=19.4 kcal mol^?1) two‐step asynchronous H₂ addition mechanism proceeding via a bridging hydride.     

Reactivity of superoxide radical anion with cyclic nitrones: role of intramolecular H-bond and electrostatic effects

Limitations exist among the commonly used cyclic nitrone spin traps for biological free radical detection using electron paramagnetic resonance (EPR) spectroscopy. The design of new spin traps for biological free radical detection and identification using EPR spectroscopy has been a major challenge due to the lack of systematic and rational approaches to their design. In this work, density functional theory (DFT) calculations and stopped-flow kinetics were employed to predict the reactivity of functionalized spin traps with superoxide radical anion (O2*-). Functional groups provide versatility and can potentially improve spin-trap reactivity, adduct stability, and target specificity. The effect of functional group substitution at the C-5 position of pyrroline N-oxides on spin-trap reactivity toward O2*- was computationally rationalized at the PCM/B3LYP/6-31+G(d,p)//B3LYP/6-31G(d) and PCM/mPW1K/6-31+G(d,p) levels of theory. Calculated free energies and rate constants for the reactivity of O2*- with model nitrones were found to correlate with the experimentally obtained rate constants using stopped-flow and EPR spectroscopic methods. New insights into the nucleophilic nature of O2*- addition to nitrones as well as the role of intramolecular hydrogen bonding of O2*- in facilitating this reaction are discussed. This study shows that using an N-monoalkylsubstituted amide or an ester as attached groups on the nitrone can be ideal in molecular tethering for improved spin-trapping properties and could pave the way for improved in vivo radical detection at the site of superoxide formation.     

Oxidation,Coordination, and Nickel-Mediated Deconstruction of a Highly Electron-Rich Diboron Analogue of 1,3,5-Hexatriene

The reductive coupling of an N-heterocyclic carbene (NHC) stabilized (dibromo)vinylborane yields a 1,2-divinyldiborene, which, although isoelectronic to a 1,3,5-triene, displays no extended π conjugation because of twisting of the C₂B₂C₂ chain. While this divinyldiborene coordinates to copper(I) and platinum(0) in an η²-B₂ and η⁴-C₂B₂ fashion, respectively, it undergoes a complex rearrangement to an η⁴-1,3-diborete upon complexation with nickel(0).     

Synthesis and characterisation of trigonal C2-chiral di- and tetra-substituted bis(oxazoline) alkyl zinc complexes and their reactivity towards protic reagents

A series of zinc(ii) alkyl complexes stabilised by the C(2)-chiral bis(oxazoline) ligand ((R(1),R(2))BOX, with R(1) = (4S)-tBu, R(2) = H (a); R(1) = (4S)-Ph, R(2) = H (b); R(1) = (4R)-Ph, R(2) = (5S)-Ph (c)), has been synthesised and structurally characterised. ((R(1),R(2))BOX)H ligands react with ZnEt(2) in toluene to give the heteroleptic three-coordinate compounds of ((R(1),R(2))BOX)ZnEt, 1a, 1b and 1c in high yield. However, when the addition of (BOX)H ligands (a-b) over ZnEt(2) is "uncontrolled", the formation of homoleptic four-coordinate compounds are favoured (2a-b), but not for the more sterically crowded ligand (c). The zinc-ethyl derivatives (1a-c) react readily with protic reagents such as acetic acid (HOAc) and methanol (MeOH). For compounds 1a-c a redistribution of ligands is observed leading preferentially to homoleptic compounds, except for the bulkier ligand c providing a three-coordinate complex identified as ((Ph,Ph)BOX)Zn(OMe), 4c. The reaction of acetylacetone (acacH) with compounds 1a-c leads straightforwardly to the more stable four-coordinate compounds corresponding to ((R(1),R(2))BOX)Zn(η(2)-acac), 5a-c. The potential of these compounds as initiators for the copolymerisation of epoxides with CO(2) was investigated.