Gadolinium Photocatalysis: Dearomative [2+2] Cycloaddition/Ring-Expansion Sequence with Indoles

Lanthanide photocatalysts are much less investigated in synthetic chemistry than rare and expensive late transition metals. We herein introduce Gd^III photocatalysis of a highly regioselective, intermolecular [2+2] photocycloaddition/ring-expansion sequence with indoles, which could provide divergent access to cyclopenta[b]indoles and indolines. A simple and commercially available Gd(OTf)₃ salt is sufficient for this visible-violet-light-induced transformation. The reaction proceeds either through a transient or start-to-end dearomatization cascade and shows excellent regioselectivity (usually >95:5 r.r.), broad scope (59 examples), good functional group tolerance and facile scale-up under mild, direct visible-light-excitation conditions. Mechanistic investigations reveal that direct excitation of the Gd(OTf)₃/indole mixture gives an excited state intermediate, which undergoes the subsequent [2+2] cycloaddition and cyclobutane-expansion cascade.     

Structural analysis of synthetic homo- and copolyesters by electrospray ionization on a fourier transform ion cyclotron resonance mass spectrometer

The molecular structure of a series of homo- and copolyesters was studied using sustained off-resonance irradiation collisionally activation dissociation on a Fourier transform ion cyclotron resonance mass spectrometer. Electrospray ionization was used as an ionization technique. The most important fragmentation pathways of the homopolyesters poly(dipropoxylated bisphenol-A/adipic acid) and poly(dipropoxylated bisphenol-A/isophthalic acid) were studied. Six different dissociation mechanisms were observed which are very similar to the mechanisms found to occur during pyrolysis of these compounds. Four of these mechanisms are a result of cleavages of the ester bond and the others are due to cleavages of the ether bond or bisphenol-A unit. Some of the fragments expected are not present in the spectrum, indicating that each fragment has a specific sodium affinity. Sequence-specific fragments of two of the three copolyester sequences that theoretically can exist were experimentally observed. Fragments that originate from the third sequence are not unique and can also be formed from other sequences. Therefore, it was not possible to determine the presence of the third sequence. Copyright 2000 John Wiley & Sons, Ltd.     

An Ethylene‐Bridged Phosphane/Borane Frustrated Lewis Pair Featuring the ‐B(Fxyl)2 Lewis Acid Component

Hydroboration of dimesitylvinylphosphane with bis[3,5‐bis(trifluoromethyl)phenyl]borane [HB(Fxyl)₂] gave the intramolecular ethylene‐bridged P/B frustrated Lewis pair (FLP) Mes₂PCH₂CH₂B(Fxyl)₂. The new compound underwent a variety of typical FLP reactions such as P/B‐addition to the carbonyl group of p‐chloro‐benzaldehyde. Cooperative N,N‐addition to nitric oxide gave the respective persistent P/B FLPNO^. radical, which readily reacted with 1,4‐cyclohexadiene by H‐atom abstraction to yield the corresponding P/B FLPNOH product. The B(Fxyl)₂‐containing FLP reacted as a template for the HB(C₆F₅)₂ reduction of carbon monoxide to the formyl stage to give the respective FLP(η²‐formylborane) product. Most products were characterized by single‐crystal X‐ray crystal structure analysis.     

隐函数曲面的实时采样与三角化技术

采用逐步蔓延采样点和三角形的方法，给出了3个算法用于隐函数曲面的采样和三角化，这些算法使得隐函数曲面的重复绘制和控制都能实时进行，其采样方法具有局部适应性，能随着曲率的变化自动控制采样点的疏密程度，从而使得采样点尽量少，但又不至于遗漏表面细节，提出的三角化方法能用于其它散乱数据点的表面重构，它的算法复杂度仅为O(n)。     

Coupling of Carbon Monoxide with Nitrogen Monoxide at a Frustrated Lewis Pair Template

Coupling of carbon monoxide with nitrogen monoxide was achieved at a frustrated Lewis pair template. This unique reaction uses hydride as an auxiliary, which reductively activates carbon monoxide at the frustrated Lewis pair. The CO/NO coupling reaction then takes place through a pathway involving a radical reaction in which the hydrogen atom auxiliary is eventually removed again.     

Asymmetric Hydrogenation of Vinylthioethers: Access to Optically Active 1,5‐Benzothiazepine Derivatives

A novel asymmetric hydrogenation of vinylthioethers was developed using a ruthenium(II) NHC complex. This method provides an efficient approach to optically active 1,5‐benzothiazepines featuring stereocenters with C?S bonds. Excellent enantioselectivities (up to 95 % ee) and high yields (up to 99 %) were obtained for a variety of substrates bearing a range of useful functional groups. Moreover, this methodology could be directly applied to the synthesis of the antidepressant drug R‐(?)‐thiazesim.     

Can acyclic conformational control be achieved via a sulfur–fluorine gauche effect?

The gauche conformation of the 1,2-difluoroethane motif is known to involve stabilising hyperconjugative interactions between donor (bonding, σ_C–H) and acceptor (antibonding, σ*C–F) orbitals. This model rationalises the generic conformational preference of F–C_β–C_α–X systems (φ_FCCX ≈ 60°), where X is an electron deficient substituent containing a Period 2 atom. Little is known about the corresponding Period 3 systems, such as sulfur and phosphorus, where multiple oxidation states are possible. Conformational analyses of β-fluorosulfides, -sulfoxides and -sulfones are disclosed here, thus extending the scope of the fluorine gauche effect to the 3rd Period (F–C–C–S(O)_n; φ_FCCS ≈ 60°). Synergy between experiment and computation has revealed that the gauche effect is only pronounced in structures bearing an electropositive vicinal sulfur atom (S⁺–O^–, SO₂).