Dearomative Cascade Photocatalysis: Divergent Synthesis through Catalyst Selective Energy Transfer.

The discovery and application of dearomative cascade photocatalysis as a strategy in complex molecule synthesis is described. Visible-light-absorbing photosensitizers were used to (sequentially) activate a 1-naphthol derived arene precursor to divergently form two different polycyclic molecular scaffolds through catalyst selective energy transfer.

Large-scale total synthesis of C-labeled citrinin and its metabolite dihydrocitrinone.

The analysis of the nephrotoxic mycotoxin citrinin in food, feed, and physiological samples is still challenging. Nowadays, liquid chromatography coupled with mass spectrometry is the method of choice for achieving low limits of detection. But matrix effects can present impairments for this method.

A hydroboration route to geminal P/B frustrated Lewis pairs with a bulky secondary phosphane component and their reaction with carbon dioxide.

The secondary aryl-P(H) phosphanyl substituted tert-butylacetylenes 7a,b (aryl: Mes or Mes*) undergo hydroboration with [HB(CF)] to give the geminal vinylidene-bridged P/B Lewis pairs 8a,b. The treatment of 8a,b with benzonitrile, N-sulfinylaniline, and phenyl isothiocyanate, respectively, gives the addition products 12a,b, 13a,b, and 14 with proton transfer from the phosphorus to the more basic nitrogen site. The reaction of the FLPs 8a,b with carbon dioxide yields a doubly boron bonded addition product.

Design and reactions of a carbon Lewis base/boron Lewis acid frustrated Lewis pair.

The conjugated dienamine selectively adds Piers' borane [HB(CF)] to give the enamine/borane system , which features a boratirane structure by internal enamine carbon Lewis base to boron Lewis acid interaction. Compound behaves as a C/B frustrated Lewis pair and undergoes typical addition reactions to benzaldehyde, several nitriles and to sulfur dioxide.This article is part of the themed issue 'Frustrated Lewis pair chemistry'.

CO-Reduction Chemistry: Reaction of a CO-Derived Formylhydridoborate with Carbon Monoxide, with Carbon Dioxide, and with Dihydrogen.

Treatment of the bulky metallocene hydride Cp*Zr(H)OMes (Cp* = pentamethylcyclopentadienyl, Mes = mesityl) with Piers' borane [HB(CF)] and carbon monoxide (CO) gave the formylhydridoborate complex [Zr]-O═CH-BH(CF) ([Zr] = Cp*Zr-OMes). From the dynamic NMR behavior, its endergonic equilibration with the [Zr]-O-CH-B(CF) isomer was deduced, which showed typical reactions of an oxygen/boron frustrated Lewis pair. It was trapped with CO to give an O-[Zr] bonded borata-β-lactone.

Why Does the Intramolecular Trimethylene-Bridged Frustrated Lewis Pair Mes2 PCH2 CH2 CH2 B(C6 F5 )2 Not Activate Dihydrogen?

The methyl labelled C3 -bridged frustrated phosphane borane Lewis pair (P/B FLP) 2 b was prepared by treatment of Mes2 PCl with a methallyl Grignard reagent followed by anti-Markovnikov hydroboration with Piers' borane [HB(C6 F5 )2 )]. The FLP 2 b is inactive toward dihydrogen under typical ambient conditions, in contrast to the C2 - and C4 -bridged FLP analogues. Dynamic NMR spectroscopy showed that this was not due to kinetically hindered P⋅⋅⋅B dissociation of 2 b.

1,3,5-Triazapentadienes by Nucleophilic Addition to 1,3- and 1,4-Dinitriles-Sterically Constrained Examples by Incorporation into Cyclic Peripheries: Synthesis, Aggregation, and Photophysical Properties.

1,3,5-Triazapentadienes usually show U- or twisted S-shaped conformations along the N-C-N-C-N skeleton due to dominating n/π* interactions. If, however, the 1,3,5-triazapentadiene unit is part of a ring, its W conformation might be restricted to the plane. Here, we describe the synthesis of 13 new 1,3,5-triazapentadienes 10-12, which are sterically restrained by incorporation into six- or seven-membered ring systems, by addition of a lithiated primary amine or hydrazine 5 to a dinitrile 7, 8, or 9 with the two cyano groups in 1,3 or 1,4 distance.

Insertion Reactions of Neutral Phosphidozirconocene Complexes as a Convenient Entry into Frustrated Lewis Pair Territory.

Neutral phosphidozirconocene complexes [Cp2Zr(PR2)Me] (Cp=cyclopentadienyl; 1a: R=cyclohexyl (Cy); 1b: R=mesityl (Mes); 1c: R=tBu) undergo insertion into the Zr-P bond by non-enolisable carbonyl building blocks (O=CR'R''), such as benzophenone, aldehydes, paraformaldehyde or CO2, to give [Cp2Zr(OCR'R''PR2)Me] (3-7). Depending on the steric bulk around P, complexes 3-7 react with B(C6F5)3 to give O-bridged cationic zirconocene dimers that display typical frustrated Lewis pair (FLP)/ambiphilic ligand behaviour. Thus, the reaction of {[Cp2Zr(μ-OCHPhPCy2)][MeB(C6F5)3]}2 (10a) with chalcone results in 1,4 addition of the Zr(+)/P FLP, whereas the reaction of {[Cp2Zr(μ-OCHFcPCy2)][MeB(C6F5)3]}2 (11a; Fc=(C5H4)CpFe) with [Pd(η(3)C3H5)Cl]2 yields the unique Zr-Fe-Pd trimetallic complex 13a, which has been characterised by XRD analysis.

Trichloromethylthiolation of N-Heterocycles: Practical and Completely Regioselective.

The first trichloromethylthiolation of a broad range of indoles and pyrroles is reported employing bench-stable N-trichloromethylthiosaccharin as reagent. This methodology is highly regioselective, exhibits high functional group tolerance, and provides access to two previously unknown classes of potentially bioactive compounds.

Frustrated Lewis Pair Chemistry Derived from Bulky Allenyl and Propargyl Phosphanes.

The dimesitylpropargylphosphanes mes2 P-CH2 -C≡C-R 6 a (R=H), 6 b (R=CH3 ), 6 c (R=SiMe3 ) and the allene mes2 P-C(CH3 )=C=CH2 (8) were reacted with Piers' borane, HB(C6 F5 )2 . Compound 6 a gave mes2 PCH2 CH=CH(B(C6 F5 )2 ] (9 a). In contrast, addition of HB(C6 F5 )2 to 6 b and 6 c gave mixtures of 9 b (R=CH3 ) and 9 c (R=SiMe3 ) with the regioisomers mes2 P-CH2 -C[B(C6 F5 )2 ]=CRH 2 b (R=CH3 ) and 2 c (R=SiMe3 ), respectively.

Profluorescent verdazyl radicals - synthesis and characterization.

The synthesis and characterization of various 6-oxo-verdazyl radicals and their diamagnetic styryl radical trapping products are presented. It is shown that styryl radical trapping products derived from -phenyl verdazyls show fluorescence whereas the -methyl congeners are non-fluorescent. In the parent -phenyl verdazyls fluorescence is fully quenched which renders these compounds highly valuable profluorescent radical probes.

Phosphido- and Amidozirconocene Cation-Based Frustrated Lewis Pair Chemistry.

Methyl abstraction from neutral [Cp2ZrMe(ERR')] complexes 1 (E = N, P; R, R' = alkyl, aryl) with either B(C6F5)3 or [Ph3C][B(C6F5)4] results in the formation of [Cp2Zr(ERR')][X] complexes 2 (X(-) = MeB(C6F5)3(-), B(C6F5)4(-)). The X-ray structure of amido complexes [Cp2Zr(NPh2)][MeB(C6F5)3] (2d) and [Cp2Zr(N(t)BuAr)][B(C6F5)4] (2e', Ar = 3,5-C6H3(CH3)2) is reported, showing a sterically dependent Zr/N-π interaction. Complexes 2 catalyze the hydrogenation of electron-rich olefins and alkynes under mild conditions (room temperature, 1.

Bifunctional Behavior of Unsaturated Intramolecular Phosphane-Borane Frustrated Lewis Pairs Derived from Uncatalyzed 1,4-Hydrophosphination of a Dienylborane.

Three unsaturated C4 -bridged phospane/borane frustrated Lewis pairs (P/B FLPs) are prepared by uncatalyzed hydrophosphination of a dienylborane. The systems are bifunctional. Consequently, two examples undergo clean hydroboration reactions with HB(C6F5)2 to yield B/B/P systems.

Hydroalumination of Ketenimines and Subsequent Reactions with Heterocumulenes: Synthesis of Unsaturated Amide Derivatives and 1,3-Diimines.

The series of differently substituted ketenimines 1 was hydroluminated using di-iso-butyl aluminum hydride. For the sterically congested ketenimine 1a, preferred hydroalumination of the C═N-bond was proven by X-ray crystallography (compound 5a). In situ treatment of the hydroaluminated ketenimines 5 with various heterocumulenes like carbodiimides, isocycanates, isothiocyanates and ketenimines as electrophiles and subsequent hydrolytic workup resulted in novel enamine derived amide species in case of N-attack (sterically less hindered ketenimines) under formation of a new C-N-bond or in 1,3-diimines by C-C-bond-formation in case of bulky substituents at the ketenimine-nitrogen atom.

Reaction of five-membered zirconacycloallenoids with the strong Lewis acid B(C6F5)3.

Two alkyl and aryl substituted five-membered zirconacycloallenoids underwent a typical σ-alkyl metallocene reaction with B(C6F5)3, namely cleavage of the Zr-C(sp(3)) bond with formation of zwitterionic (η(2)-allenyl)zirconium/alkylborate products. Both products were characterized by X-ray crystal structural analyses.

Stereospecific formal [3+2] dipolar cycloaddition of cyclopropanes with nitrosoarenes: an approach to isoxazolidines.

The MgBr2-catalyzed formal [3+2] cycloaddition of donor-acceptor activated cyclopropanes with nitrosoarenes offers a novel approach to various structurally diverse isoxazolidines. The reactions, which are experimentally easy to conduct, occur with complete stereospecificity and perfect control of regioselectivity. Product isoxazolidines can be readily transformed into α-amino lactones by reductive or decarboxylative N-O cleavage and subsequent lactonisation, and the N-aryl bond cleavage is also possible under oxidative conditions.

Ethene/alkyne exchange reaction at an intramolecular frustrated Lewis pair.

A vicinal ethylene bridged S/B frustrated Lewis pair (FLP) in situ generated by a hydroboration reaction of phenyl vinyl sulfide, reacts with p-tert-butylphenylacetylene by ethene/alkyne exchange and subsequent 1,2-addition of a second alkyne equivalent to give a zwitterionic sulfonium/borate product.

Modification and unexpected reactivity of 2-borylbenzaldimines: acylated and silylated derivatives as well as dimeric compounds.

Various novel N-alkyl and N-benzyl 2-borylbenzaldimines 3 were prepared by condensation of 2-(dimesitylboryl)benzaldehyde (1) with amines. Further functionalization of compound 3e was possible by deprotonation and subsequent regioselective reaction with electrophiles to give compounds 4. Applying similar conditions to 3a led to the unexpected formation of hitherto unknown dimeric compounds (5 and 6).

Activation of isocyanates and carbon dioxide by a monomeric aluminium hydrazide as an active Lewis pair.

The monomeric aluminium hydrazide H10 C5 NN(AltBu2 )Ad (4; Ad=adamantyl, NC5 H10 =piperidinyl) was obtained in high yield by hydroalumination of the corresponding hydrazone derivative 1. Compound 4 has a strained AlN2 heterocycle formed by a donor-acceptor bond between the β-nitrogen atom of the hydrazide group and the aluminium atom. Opening of this bond resulted in the formation of an active Lewis pair that was able to cooperatively activate carbon dioxide or isocyanates.

Formation and structural characterization of a five-membered zirconacycloallenoid.

A conjugated enyne reacts with in situ generated zirconocene to yield a five membered zirconacycloallenoid that was characterized by X-ray diffraction.

Transannular O-heterocyclization: a useful tool for the total synthesis of Murisolin and 16,19-cis-Murisolin.

Transannular O-heterocyclization is applied as a key step in a total synthesis. This highly stereoselective and metal-free transformation introduces four stereocenters in one step. It was chosen to be the pivotal step in the synthesis of Murisolin and 16,19-cis-Murisolin, two annonaceous acetogenins.

Frustrated Lewis pair addition to conjugated diynes: formation of zwitterionic 1,2,3-butatriene derivatives.

The frustrated Lewis pair B(C(6)F(5))(3)/P(o-tolyl)(3) (4a) reacts with 4,6-decadiyne to give the trans-1,2-addition product 5. In contrast, the B(C(6)F(5))(3)/P(t)Bu(3) FLP (4b) reacts with this substrate to give the trans-1,4-adduct trans-6. The cumulene trans-6 undergoes trans-/cis-isomerization upon photolysis to give a ca.

Synthesis and photophysical properties of aryl-substituted 2-borylbenzaldimines and their extended π-conjugated congeners.

Novel N-aryl-substituted 2-borylbenzaldimines 6 and related systems with extended π-framework 7 based on two borylbenzaldimine units linked by a spacer moiety were synthesized by condensation reactions of 2-(dimesitylboryl)benzaldehyde 3 with various amines 4 and diamines 5. All compounds were completely characterized including X-ray diffraction, especially in view of Lewis acid-base B-O and B-N interactions. The electronic as well as the photophysical properties of bisimines 7 were determined using cyclic voltammetry, UV/vis, and fluorescence spectroscopy and quantum chemistry.

Ring closure reactions of 2,6-diazaheptatrienyl metal compounds: synthesis of 3-aminoindole derivatives and 14-membered macrocyclic dimers.

2,6-Diazaheptatrienyl metal compounds 6(-)K(+) are easily accessible from the corresponding diimines 6 by deprotonation using KO-t-Bu as base. According to quantum chemical calculations, they are, in comparison to other isomeric species with nitrogen atoms in other positions, highly reactive intermediates, which undergo in dilute solution at 50 °C ring closure reactions to form 3-aminoindole derivatives 8/10. In contrast, in more concentrated solution at room temperature, the formation of 14-membered macrocyclces 13 as a result of formal dimerization is observed.