Padlocking dihydrofurannulation for the control of small degree of helicity built on a fused-tetracyclic core.

Enantioselective construction of small molecules displaying a configurationally stable helical shape built on a fused-tetracyclic core is a daunting synthetic challenge even more pronounced when five-membered rings are incorporated in the structure. The resulting higher configurational lability strongly hampers their access, and therefore the development of new efficient methodologies is timely and highly desirable. In this context, we describe a padlocking approach via the enantioselective organocatalytic domino furannulation of appropriately designed achiral fused-tricyclic precursors resulting in the synthesis of configurationally locked helically chiral tetracyclic scaffolds featuring one or two five-membered rings with the simultaneous control of central and helical chiralities.

A Cyclotriveratrylene Solvent-Dependent Chiral Switch.

Chiral molecular switches are attracting attention as they could pave the way to chiral molecular machines. Herein, we report on the design and synthesis of a single molecule chiral switch based on a cyclotriveratrylene scaffold, in which the chirality inversion is controlled by the solvent. Hemicryptophanes are built around a C3 cyclotriveratrylene chiral unit, with either M or P handedness, connected to another tripod and usually displaying an "out" configuration. Here, we demonstrate that solvents are able to control the "in" and "out" configurations of the CTV unit, creating a chiral molecular switch from (M/P)"in" to (P/M)"out" handedness. The full characterization of the "in" and "out" configurations and of the chirality switch were made possible by combining NMR, HPLC, ECD, DFT and molecular dynamics. Interestingly, bulky aromatic solvents such as 2-t-butylphenol favor the "in" configuration while polar aprotic solvents such as acetone favor the "out" configuration. This chiral switch was found to be fully reversible allowing the system to oscillate between two different M and P configurations several times upon the action of solvents stimuli.

Quantitative Structure-Activity Relationships for the Nucleophilicity of Trivalent Boron Compounds.

We describe herein the development of quantitative structure-activity relationships (QSAR) for the nucleophilicity of trivalent boron compounds covering boryl fragments bonded to alkali and alkaline-earth metals, to transition metals, and to sp3 boron units in diboron reagents. We used the charge of the boryl fragment (q[B]) and the boron p/s population ratio (p/s) to describe the electronic structures of boryl moieties, whereas the distance-weighted volume (Vw ) descriptor was used to evaluate the steric effects. The three-term easy-to-interpret QSAR model showed statistical significance and predictive ability (r2 =0.88, q2 =0.83). The use of chemically meaningful descriptors has allowed identification of the factors governing the boron nucleophilicity and indicates that the most efficient nucleophiles are those with enhanced the polarization of the B-X bond towards the boron atom and reduced steric bulk. A detailed analysis of the potential energy surfaces of different types of boron substituents has provided insight into the mechanism and established an order of nucleophilicity for boron in B-X: X=Li>Cu>B(sp3 )>Pd. Finally, we used the QSAR model to make a priori predictions of experimentally untested compounds.

Arene C(sp(2))-H Metalation at Ni(II) Modeled with a Reactive PONCPh Ligand.

Coordination of the reactive phosphinitopyridylphenyl PONCPh ligand L(H) to NiBr2 initially yields paramagnetic brown NiBr2(L(H)) (1), but addition of triethylamine results in fast and facile cyclometalation at Ni(II), giving NiBr(κ(3)-P,N,C-L) (2) as well-defined species. This is a rare example of direct cyclometalation at Ni(II) from a C-H bond in a ligand structure other than encumbering ligands (e.g., ECE pincers). Diamagnetic yellow complex 2 reacts instantaneously with HBF4 to give purple [NiBr(κ(3)-P,N-L(H))]BF4 (3). A very unusual (an)agostic Ni(CPh-H) interaction in the solid-state structure of 3 was unequivocally demonstrated using single-crystal X-ray crystallography and was interpreted by density functional theory calculations (quantum theory of atoms in molecules and electron localization function analysis). These compounds may be viewed as models for key intermediates in the Ni-catalyzed C-H functionalization of arenes.

How Can Linoleic Acid Be the Preferential Substrate of the Enzyme 15-Lipoxygenase-1? A QM/MM Approach.

The most common substrate of mammalian lipoxygenases (LOXs) is arachidonic acid (AA). However, 15-LOXs can present dual substrate specificity. These LOXs catalyze the peroxidation of AA, initiated by a H-abstraction step (mainly H13-abstraction) by the Fe(III)-OH(-) cofactor, and the peroxidation of linoleic acid (LA) after H11-abstraction. In this paper, QM(B3LYP)/MM(CHARMM) calculations of the rate-limiting H11-abstraction process of LA catalyzed by rabbit 15-LOX-1 (15-rLOX-1) have been carried out using a complete model of the solvated 15-rLOX-1:LA complex. A total of 26 QM/MM potential energy profiles as a function of the H-transfer reaction coordinate have been computed along with one QM/MM free energy profile obtained using the Free Energy Perturbation method. The molecular origin of substrate specificity of 15-rLOX-1 for LA in comparison with AA has been analyzed. In many of the QM/MM reactive 15-rLOX-1:LA energy minima, LA adopts more elongated conformations than AA, although having a shorter carbon chain, because LA has one double bond between C1 and C11 whereas AA has three double bonds between C1 and C13. Consequently, C11 of LA can be located in the same region of the active site as C13 of AA, a zone where H11-abstraction from LA as well as H13-abstraction from AA is not hindered by bulky residue side chains. This explains at a molecular level how 15-LOXs might accommodate and recognize for catalysis two substrates that are different in length by two carbons. Our results also explain why (9Z,11E)-13-hydro(pero)xyoctadeca-9,11-dienoic acid is the major product of the peroxidation and why LA is the preferential substrate of 15-rLOX-1.

Correction: Unsymmetrical 1,1-diborated multisubstituted sp(3)-carbons formed via a metal-free concerted-asynchronous mechanism.

Correction for 'Unsymmetrical 1,1-diborated multisubstituted sp(3)-carbons formed via a metal-free concerted-asynchronous mechanism' by Ana B. Cuenca et al., Org. Biomol. Chem., 2015, 13, 9659-9664.

Carbon-nitrogen bond construction and carbon-oxygen double bond cleavage on a molecular titanium oxonitride: a combined experimental and computational study.

New carbon-nitrogen bonds were formed on addition of isocyanide and ketone reagents to the oxonitride species [{Ti(η(5)-C5Me5)(µ-O)}3(µ3-N)] (1). Reaction of 1 with XylNC (Xyl = 2,6-Me2C6H3) in a 1:3 molar ratio at room temperature leads to compound [{Ti(η(5)-C5Me5)(µ-O)}3(µ-XylNCCNXyl)(NCNXyl)] (2), after the addition of the nitrido group to one coordinated isocyanide and the carbon-carbon coupling of the other two isocyanide molecules have taken place. Thermolysis of 2 gives [{Ti(η(5)-C5Me5)(µ-O)}3(XylNCNXyl)(CN)] (3) where the heterocumulene [XylNCCNXyl] moiety and the carbodiimido [NCNXyl] fragment in 2 have undergone net transformations. Similarly, tert-butyl isocyanide (tBuNC) reacts with the starting material 1 under mild conditions to give the paramagnetic derivative [{Ti3(η(5)-C5Me5)3(µ-O)3(NCNtBu)}2(µ-CN)2] (4). However, compound 1 provides the oxo ketimide derivatives [{Ti3(η(5)-C5Me5)3(µ-O)4}(NCRPh)] [R = Ph (5), p-Me(C6H4) (6), o-Me(C6H4) (7)] upon reaction with benzophenone, p-methylbenzophenone, and o-methylbenzophenone, respectively. In these reactions, the carbon-oxygen double bond is completely ruptured, leading to the formation of a carbon-nitrogen and two metal-oxygen bonds. The molecular structures of complexes 2-4, 6, and 7 were determined by single-crystal X-ray diffraction analyses. Density functional theory calculations were performed on the incorporation of isocyanides and ketones to the model complex [{Ti(η(5)-C5H5)(µ-O)}3(µ3-N)] (1H). The mechanism involves the coordination of the substrates to one of the titanium metal centers, followed by an isomerization to place those substrates cis with respect to the apical nitrogen of 1H, where carbon-nitrogen bond formation occurs with a low-energy barrier. In the case of aryl isocyanides, the resulting complex incorporates additional isocyanide molecules leading to a carbon-carbon coupling. With ketones, the high oxophilicity of titanium promotes the unusual total cleavage of the carbon-oxygen double bond.

Unsymmetrical 1,1-diborated multisubstituted sp(3)-carbons formed via a metal-free concerted-asynchronous mechanism.

We have experimentally proved the unsymmetrical 1,1-diboration of diazo compounds, formed in situ from aldehydes and cyclic and non-cyclic ketones, in the absence of any transition metal complex. The heterolytic cleavage of the mixed diboron reagent, Bpin-Bdan, and the formation of two geminal C-Bpin and C-Bdan bonds has been rationalised based on DFT calculations to occur via a concerted-asynchronous mechanism. Diastereoselection is attained on substituted cyclohexanones and DFT studies provide understanding on the origin of the selectivity. The alkoxide-assisted selective deborylation of Bpin from multisubstituted sp(3)-carbon and generation of a Bdan stabilized carbanion, easily conducts a selective protodeboronation sequence.