Isolation of a Three‐Coordinate Boron Cation with a Boron–Sulfur Double Bond

The reaction of the bulky bis(imidazolin‐2‐iminato) ligand precursor (1,2‐(L^MesNH)₂‐C₂H₄)[OTs]₂ ( 1 ²⁺ 2[OTs]^?; L^Mes=1,3‐dimesityl imidazolin‐2‐ylidene, OTs=p‐toluenesulfonate) with lithium borohydride yields the boronium dihydride cation (1,2‐(L^MesN)₂‐C₂H₄)BH₂[OTs] ( 2 ⁺ [OTs]^?)_. The boronium cation 2 ⁺ [OTs]^? reacts with elemental sulfur to give the thioxoborane salt (1,2‐(L^MesN)₂‐C₂H₄)BS[OTs] ( 3 ⁺ [OTs]^?). The hitherto unknown compounds 1 ²⁺ 2[OTs]^?, 2 ⁺ [OTs]^?, and 3 ⁺ [OTs]^? were fully characterized by spectroscopic methods and single‐crystal X‐ray diffraction. Moreover, DFT calculations were carried out to elucidate the bonding situation in 2 ⁺ and 3 ⁺. The theoretical, as well as crystallographic studies reveal that 3 ⁺ is the first example for a stable cationic complex of three‐coordinate boron that bears a B?S double bond.     

Chemistry of Guanidinate‐Stabilised Diboranes: Transition‐Metal‐Catalysed Dehydrocoupling and Hydride Abstraction

Herein, we analyse the catalytic boron–boron dehydrocoupling reaction that leads from the base‐stabilised diborane(6) [H₂B(hpp)]₂ (hpp=1,3,4,6,7,8‐hexahydro‐2H‐pyrimido[1,2‐a]pyrimidinate) to the base‐stabilised diborane(4) [H₂B(hpp)]₂. A number of potential transition‐metal precatalysts was studied, including transition‐metal complexes of the product diborane(4). The synthesis and structural characterisation of two further examples of such complexes is presented. The best results for the dehydrocoupling reactions were obtained with precatalysts of Group 9 metals in the oxidation state of +I. The active catalyst is formed in situ through a multistep process that involves reduction of the precatalyst by the substrate [H₂B(hpp)]₂, and mechanistic investigations indicate that both heterogeneous and (slower) homogeneous reaction pathways play a role in the dehydrocoupling reaction. In addition, hydride abstraction from [H₂B(hpp)]₂ and related diboranes is analysed and the possibility for subsequent deprotonation is discussed by probing the protic character of the cationic boron–hydrogen compounds with NMR spectroscopic analysis.     

Synthesis of 1-indolyl substituted β-carboline natural products and discovery of antimalarial and cytotoxic activities

A series of 1-indolyl substituted β-carbolines including the natural products hyrtiosulawesine, pityriacitrin and pityriacitrin B were prepared via Pictet–Spengler condensation—oxidation strategy from the corresponding indolyl-acetaldehydes and substituted tryptamines. Efforts to prepare the C-1 methylene-linked β-carboline analogues for structure–activity relationship studies were unsuccessful. Biological evaluation revealed two analogues (5 and 41) to exhibit weak inhibition of phospholipase A₂ (IC₅₀ 171 and 131 μM, respectively), two to act as antioxidants (3 and 43), and 12 analogues with activity towards a chloroquine-resistant strain (FcB1) of Plasmodium falciparum (IC₅₀ 1.0–23 μM). Testing against a panel of 60 human tumour cell lines revealed a general lack of cytotoxic effect for most of the compounds with the exception of β-carboline 42 exhibiting modest antileukaemic activity towards the HL-60(TB) cell line (LC₅₀ 4.2 μM). In addition, two novel structures (30 and 32) resulting from aldol condensation followed by Pictet–Spengler cyclisation displayed cytotoxicity with pronounced subpanel specificities towards colon cancer (COLO 205 and HCC-2998) cell lines.     

Sun exposure and Protection Behavior of Danish Farm Children: Parental Influence on Their Children

Healthy sun habits acquired in childhood could reduce skin cancer incidence. We examined the sun exposure and protection behavior of an expected high‐exposure group of children, and the association to their parents. Open, prospective cohort study. One hundred and thirty nine participants (40 families) kept daily sun behavior diaries (sun exposure, sunscreen use, sunburns) over a 4‐month summer period (15 985 diary days). The Pigment Protection Factor (PPF), an objective measure of sun exposure, was measured at two body sites, before and after summer. All participants presented data from the same 115 days. Risk behavior (sun exposure of upper body) took place on 9.5 days (boys) and 15.6 days (girls). Sunburn and sunscreen use were infrequent. Boys’ sun exposure resulted in an increased photo protection over the study period of 1.7 SED (upper arm) and 0.8 SED (shoulder) to elicit erythema. Corresponding values for girls were as follows: 0.9 SED (upper arm) and 0.5 SED (shoulder). Boys’ sunscreen use correlated to their mothers’ (r = 0.523, P = 0.02). Girls’ number of risk days (r = 0.552, P = 0.005) and sun exposure (upper arm: r = 0.621, P < 0.001) correlated to their mothers’. The children's sun exposure was substantial. Only mothers influenced children's sun behavior and exposure. This may be of relevance in future sun protection campaigns.     

What Makes a Strong Organic Electron Donor (or Acceptor)?

Organic electron donors are of importance for a number of applications. However, the factors that are essential for a directed design of compounds with desired reduction power are not clear. Here, we analyze these factors in detail. The intrinsic reduction power, which neglects the environment, has to be separated from extrinsic (e.g., solvent) effects. This power could be quantified by the gas‐phase ionization energy. The experimentally obtained redox potentials in solution and the calculated ionization energies in a solvent (modeled with the conductor‐like screening model (COSMO)) include both intrinsic and extrinsic factors. An increase in the conjugated π‐system of organic electron donors leads to an increase in the intrinsic reduction power, but also decreases the solvent stabilization. Hence, intrinsic and extrinsic effects compete against each other; generally the extrinsic effects dominate. We suggest a simple relationship between the redox potential in solution and the gas‐phase ionization energy and the volume of an organic electron donor. We finally arrive at formulas that allow for an estimate of the (gas‐phase) ionization energy of an electron donor or the (gas‐phase) electron affinity of an electron acceptor from the measured redox potentials in solution. The formulas could be used for neutral organic molecules with no or only small static dipole moment and relatively uniform charge distribution after oxidation/reduction.     

From an Isolable Acyclic Phosphinosilylene Adduct to Donor‐Stabilized SiE Compounds (E=O,S, Se)

Reaction of the arylchlorosilylene‐NHC adduct ArSi(NHC)Cl [Ar=2,6‐Trip₂C₆H₃; NHC=(MeC)₂(NMe)₂C:] 1 with one molar equiv of lithium diphenylphosphanide affords the first stable NHC‐stabilized acyclic phosphinosilylene adduct 2 (ArSi(NHC)PPh₂), which could be structurally characterized. Compound 2 , when reacted with one molar equiv selenium and sulfur, affords the silanechalcogenones 4 a and 4 b (ArSi(NHC)(?E)PPh₂, 4 a : E=Se, 4 b : E=S), respectively. Conversion of 2 with an excess of Se and S, through additional insertion of one chalcogen atom into the Si?P bond, leads to 3 a and 3 b (ArSi(NHC)(?E)‐E‐P(?E)Ph₂, 3 a : E=Se, 3 b : E=S), respectively. Additionally, the exposure of 2 to N₂O or CO₂ yielded the isolable NHC‐stabilized silanone 4 c , Ar(NHC)(Ph₂P)Si?O.