More Like this

1. Total Synthesis of (±)‐Phyllantidine: Development and Mechanistic Evaluation of a Ring Expansion for Installation of Embedded Nitrogen‐Oxygen Bonds

   https://doi.org/10.1002/ange.202003829  

   Lambert, Kyle M. ; Cox, Joshua B. ; Liu, Lin ; Jackson, Amy C. ; Yruegas, Sam ; Wiberg, Kenneth B. ; Wood, John L. ( May 2020 , Angewandte Chemie)

   The development of a concise total synthesis of (±)‐phyllantidine (1), a member of thesecurinegafamily of alkaloids containing an unusual oxazabicyclo[3.3.1]nonane core, is described. The synthesis employs a unique synthetic strategy featuring the ring expansion of a substituted cyclopentanone to a cyclic hydroxamic acid as a key step that allows facile installation of the embedded nitrogen‐oxygen (N−O) bond. The optimization of this sequence to effect the desired regiochemical outcome and its mechanistic underpinnings were assessed both computationally and experimentally. This synthetic approach also features an early‐stage diastereoselective aldol reaction to assemble the substituted cyclopentanone, a mild reduction of an amide intermediate without N−O bond cleavage, and the rapid assembly of the butenolide found in (1) via use of the Bestmann ylide.

    

   more » « less
2. Cage Match: Comparing the Anion Binding Ability of Isostructural Versus Isofunctional Pairs of Metal‐Organic Nanocages

   https://doi.org/10.1002/chem.202303013  

   Dutton, Kaitlyn G. ; Jones, Taro J. ; Emge, Thomas J. ; Lipke, Mark C. ( November 2023 , Chemistry – A European Journal)

   Affinities of six anions (mesylate, acetate, trifluoroacetate,p‐toluenecarboxylate,p‐toluenesulfonate, and perfluorooctanoate) for three related Pt²⁺‐linked porphyrin nanocages were measured to probe the influence of different noncovalent recognition motifs (e. g., hydrogen bonding, electrostatics, π bonding) on anion binding. Two new hosts of M₆L₃¹²⁺(1b) and M₄L₂⁸⁺(2) composition (M=(en)Pt²⁺, L=(3‐py)₄porphyrin) were prepared in a one‐pot synthesis and allowed comparison of hosts that differ in structure while maintaining similar N−H hydrogen‐bond donor ability. Comparisons of isostructural hosts that differ in hydrogen‐bonding ability were made between1band a related M₆L₃¹²⁺nanoprism (1a, M=(tmeda)Pt²⁺) that lacks N−H groups. Considerable variation in association constants (K₁=1.6×10³ M⁻¹to 1.3×10⁸ M⁻¹) and binding mode (exovs.endo) were found for different host–guest combinations. Strongest binding was seen betweenp‐toluenecarboxylate and1b, but surprisingly, association of this guest with1awas only slightly weaker despite the absence of NH⋅⋅⋅O interactions. The high affinity betweenp‐toluenecarboxylate and1acould be turned off by protonation, and this behavior was used to toggle between the binding of this guest and the environmental pollutant perfluorooctanoate, which otherwise has a lower affinity for the host.

    

   more » « less
3. Density Functional Theory and ab Initio Computational Evidence for Nitrosamine Photoperoxides: Hammett Substituent Effects in the Photogeneration of the Nitrooxide Intermediate

   https://doi.org/10.1111/php.12941  

   Greer, Edyta M. ; Kwon, Kitae ( July 2018 , Photochemistry and Photobiology)

   Little attention has been focused on diradical and zwitterionic photoperoxides formed from nitrosamine compounds. Here, an attempt is made to probe the electronic character of the nitrooxide intermediate formed in photochemical reactions with triplet oxygen (³O₂). Theoretical studies have been conducted to screenpara‐substituted phenyl nitrosamines. In particular, we find that electron‐withdrawing substituents produce low‐lying triplet nitrooxide diradicals. A clear electronic dependence in theS₀–T₁andS₀–S₁energy gaps of nitrooxides was found using Hammett plots. Computed geometries show a twisted diradical triplet nitrooxide moiety, which contrasts to the nearly flat singlet zwitterionic ground state nitrooxide moiety; analyses of charges (natural bond order), molecular orbitals (HOMO/LUMO) and spin densities enable these assignments. Calculations predict the former triplet species is photogenerated initially from nitrosamine with O₂. The conversion of the triplet nitrooxide diradical to the singlet ground state is an example where longer‐lived zwitterionic nitrooxide structures become possible. The reaction mechanism is consistent with a zwitterionic ground state nitrooxide playing an important role in the bimolecular oxygen‐transfer reaction with phosphine and phosphite trapping agents as has been observed experimentally.

    

   more » « less
4. Small Molecule Activation with Intramolecular “Inverse” Frustrated Lewis Pairs

   https://doi.org/10.1002/chem.202005143  

   Manankandayalage, Chamila ; Unruh, Daniel K. ; Krempner, Clemens ( March 2021 , Chemistry – A European Journal)

   The intramolecular “inverse” frustrated Lewis pairs (FLPs) of general formula 1‐BR₂‐2‐[(Me₂N)₂C=N]‐C₆H₄(3–6) [BR₂=BMes₂(3), BC₁₂H₈, (4), BBN (5), BBNO (6)] were synthesized and structurally characterized by multinuclear NMR spectroscopy and X‐ray analysis. These novel types of pre‐organized FLPs, featuring strongly basic guanidino units rigidly linked to weakly Lewis acidic boryl moieties via anortho‐phenylene linker, are capable of activating H−H, C−H, N−H, O−H, Si−H, B−H and C=O bonds.4and5deprotonated terminal alkynes and acetylene to form the zwitterionic borates 1‐(RC≡C‐BR₂)‐2‐[(Me₂N)₂C=NH]‐C₆H₄(R=Ph, H) and reacted with ammonia, BnNH₂and pyrrolidine, to generate the FLP adducts 1‐(R₂HN→BR₂)‐2‐[(Me₂N)₂C=NH]‐C₆H₄, where the N‐H functionality is activated by intramolecular H‐bond interactions. In addition,5was found to rapidly add across the double bond of H₂CO, PhCHO and PhNCO to form cyclic zwitterionic guanidinium borates in excellent yields. Likewise,5is capable of cleaving H₂, HBPin and PhSiH₃to form various amino boranes. Collectively, the results demonstrate that these new types of intramolecular FLPs featuring weakly Lewis acidic boryl and strongly basic guanidino moieties are as potent as conventional intramolecular FLPs with strongly Lewis acidic units in activating small molecules.

    

   more » « less
5. Reaction Pathway for the Aerobic Oxidation of Phosphines Catalyzed by Oxomolybdenum Salen Complexes

   https://doi.org/10.1002/ejic.202300506  

   Rusmore, Theo A. ; Lander, Chance ; Nicholas, Kenneth M. ( December 2023 , European Journal of Inorganic Chemistry)

   Catalysis ofO‐atom transfer (OAT) reactions is a characteristic of both natural (enzymatic) and synthetic molybdenum‐oxo and ‐peroxo complexes. These reactions can employ a variety of terminal oxidants, e. g. DMSO,N‐oxides, and peroxides, etc., but rarely molecular oxygen. Here we demonstrate the ability of a set of Schiff‐base‐MoO₂complexes (cy‐salen)MoO₂(cy‐salen=N,N’‐cyclohexyl‐1,2‐bis‐salicylimine) to catalyze the aerobic oxidation of PPh₃. We also report the results of a DFT computational investigation of the catalytic pathway, including the identification of energetically accessible intermediates and transition states, for the aerobic oxidation of PMe₃. Starting from the dioxo species, (cy‐salen)Mo(VI)O₂(1), key reaction steps include: 1) associative addition of PMe₃to an oxo‐O to give LMo(IV)(O)(OPMe₃) (2); 2) OPMe₃dissociation from2to produce mono‐oxo complex (cy‐salen)Mo(IV)O (3); 3) stepwise O₂association with3via superoxo species (cy‐salen)Mo(V)(O)(η¹‐O₂) (4) to form the oxo‐peroxo intermediate (cy‐salen)Mo(VI)(O)(η²‐O₂) (5); 4) theO‐transfer reaction of PMe₃with oxo‐peroxo species5at the oxo‐group, rather than the peroxo unit leading, after OPMe₃dissociation, to a monoperoxo species, (cy‐salen)Mo(IV)(η²‐O₂) (7); and 5) regeneration of the dioxo complex (cy‐salen)Mo(VI)O₂(1) from the monoperoxo triplet³7or singlet¹7by a concerted, asynchronous electronic isomerization. An alternative pathway for recycling of the oxo‐peroxo species5to the dioxo‐Mo1via a bimetallic peroxo complex LMo(O)‐O−O‐Mo(O)L8is determined to be energetically viable, but is unlikely to be competitive with the primary pathway for aerobic phosphine oxidation catalyzed by1.

    

   more » « less