Preparation, Structure, and Properties of Symmetrically 1,3-Difunctionalized Penta- and Hexafluorobicyclo[1.1.1]pentanes

A Practical and Scalable Approach to Fluoro-Substituted Bicyclo[1.1.1]pentanes

After more than 20 years of trials, a practical scalable approach to fluoro-substituted bicyclo[1.1.1]pentanes (F-BCPs) has been developed. The physicochemical properties of the F-BCPs have been studied, and the core was incorporated into the structure of the anti-inflammatory drug Flurbiprofen in place of the fluorophenyl ring.

Bridge Functionalisation of Bicyclo[1.1.1]pentane Derivatives

"Escaping from flatland", by increasing the saturation level and three-dimensionality of drug-like compounds, can enhance their potency, selectivity and pharmacokinetic profile. One approach that has attracted considerable recent attention is the bioisosteric replacement of aromatic rings, internal alkynes and tert-butyl groups with bicyclo[1.1.1]pentane (BCP) units. While functionalisation of the tertiary bridgehead positions of BCP derivatives is well-documented, functionalisation of the three concyclic secondary bridge positions remains an emerging field. The unique properties of the BCP core present considerable synthetic challenges to the development of such transformations. However, the bridge positions provide novel vectors for drug discovery and applications in materials science, providing entry to novel chemical and intellectual property space. This Minireview aims to consolidate the major advances in the field, serving as a useful reference to guide further work that is expected in the coming years.

Polyhalogenated Bicyclo[1.1.1]pentane-1,3-dicarboxylic Acids

Herein we report the highly selective radical chlorination of 2,2-difluorobicyclo[1.1.1]pentane-1,3-dicarboxylic acid. Together with radical hydrodechlorination by TMS₃SiH, four new bicyclo[1.1.1]pentane cages carrying two fluorine and one to three chlorine atoms in bridge positions have been obtained. The exact positions of all halogen atoms have been confirmed by X-ray diffraction. The acidity constants (pK_a) for all new derivatives have been determined by capillary electrophoresis, and these experimental values show excellent agreement with pK_as predicted by DFT methods. Extensive DFT calculations have been used to rationalize the selective formation of four out of nine possible F₂Cl_1-4 isomers of bridge-halogenated bicyclo[1.1.1]pentanes and to obtain relative strain energies for all possible isomers.

Difluoro-Substituted Bicyclo[1.1.1]pentanes for Medicinal Chemistry: Design, Synthesis, and Characterization

A practical synthetic approach to the difluoro-substituted bicyclo[1.1.1]pentanes was developed. The key step was an addition of difluorocarbene (:CF₂) to electron-rich bicyclo[1.1.0]butanes by the CF₃TMS/NaI system. The obtained difluoro-bicyclo[1.1.1]pentanes are suggested to be used as saturated bioisosteres of benzene rings for the purpose of drug discovery projects.

A Selective Synthesis of 2,2-Difluorobicyclo[1.1.1]pentane Analogues: "BCP-F2"

The bicyclo[1.1.1]pentane (BCP) motif has been utilized as bioisosteres in drug candidates to replace phenyl, tert-butyl, and alkynyl fragments in order to improve physicochemical properties. However, bceause of the difficulty of synthesis, most BCP analogues prepared only bear 1,3-"para"-substituents. We report the first selective synthesis of 2,2-difluorobicyclo[1.1.1]pentanes via difluorocarbene insertion into bicyclo[1.1.0]butanes. Moreover, this methodology should inspire future studies on synthesis of other "ortho/meta-substituted" BCPs via similar mechanisms.

Gearing motion in cogwheel pairs of molecular rotors: weak-coupling limit

Investigation of the rotor dynamics by X-ray diffraction, spin–lattice relaxation, and DFT modelling of the two rotational barriers in arrays of rod-like molecules with 1,3-bis(ethynyl)bicyclo[1.1.1]pentane rotators conclude to gearing motion between two rotors in a pair.

Coupling between substituents as a function of cage structure: synthesis and valence ionized states of bridgehead disubstituted parent and hexafluorinated bicyclo[1.1.1]pentane derivatives C5X6Y2

He(I) photoelectron spectroscopy was used to examine the valence-shell electronic structure of three new and seven previously known bicyclo[1.1.1]pentane derivatives, 1,3-Y2-C5X6 (for X = H, Y = H, Cl, Br, I, CN; for X = F, Y = H, Br, I, CN). A larger series (X = H or F, Y = H, F, Cl, Br, I, At, CN) has been studied computationally with the SAC-CI (symmetry-adapted cluster configuration interaction) method. The outer-valence ionization spectra calculated by the SAC-CI method, including spin-orbit interaction, reproduced the experimental photoelectron spectra well, and quantitative assignments are given. When the extent of effective through-cage interaction between the bridgehead halogen lone-pair orbitals was defined in the usual way by orbital-energy splitting, it was found to be larger than that mediated by other cages such as cubane, and was further enhanced by hexafluorination. The origin of the orbital-energy splitting is analyzed in terms of cage structure, and it is pointed out that its relation to the degree of interaction between the bridgehead substituents is not as simple as is often assumed.

cis-endo-Bicyclo[1.1.1]pentane-1,2,3,4- tetracarboxylic Acid and Its Derivatives

[reaction: see text] We report the synthesis of cis-endo-bicyclo[1.1.1]pentane-1,2,3,4-tetracarboxylic acid and several of its derivatives with differentiated bridgehead and bridge positions, starting with tetracyclo[3.2.0.0(1,6).0(2,6)]heptane.

DFT-GIAO calculations of19F NMR chemical shifts for perfluoro compounds

The (19)F NMR shieldings for 53 kinds of perfluoro compounds were calculated by the B3LYP-GIAO method using the 6-31G(d), 6-31+G(d), 6-31G(d,p), 6-31++G(d,p), 6-311G(d,p), 6-311++G(d,p), 6-311G(2d,2p), 6-311++G(2d,2p), 6-311++G(2df,2p), 6-311++G(3d,2p), and 6-311++G(3df,2p) basis sets. The diffuse functions markedly reduce the difference between the calculated and experimental chemical shifts. The calculations using the 6-31++G(d,p) basis set give the chemical shifts within 10 ppm deviations from experimental values except for the fluorine nuclei attached to an oxygen atom, a four- and a six-coordinated sulfur atom, and FC(CF(3))(2) attached to a sulfur atom.

Partially bridge-fluorinated dimethyl bicyclo[1.1.1]pentane-1,3-dicarboxylates: preparation and NMR spectra

Direct fluorination of dimethyl bicyclo[1.1.1]pentane-1,3-dicarboxylate, obtained from [1.1.1]propellane prepared by an improved synthetic procedure, furnished esters of 14 of the 15 possible bridge-fluorinated bicyclo[1.1.1]pentane-1,3-dicarboxylic acids, isolated by preparative GC. Calculated geometries reflect the substitution pattern in a regular fashion compatible with Bent's rules. Considerable additional strain is introduced into the bicyclo[1.1.1]pentane cage by polyfluorination; it is calculated to be as high as 33-35 kcal/mol for hexasubstitution. Three arrangements of the fluorine substituents are especially strain-rich: geminal, proximate, and W-related. The (1)H, (13)C, and (19)F NMR spectra exhibit a striking variety of chemical shifts and long-range coupling constants. These are in good agreement with results calculated with neglect of the bridgehead substituents for all of the chemical shifts by the GIAO-RHF/6-31G//RHF/6-31G and GIAO-RHF/6-31G//MP2/6-31G methods and for many of the coupling constants by the EOM-CCSD/6-311G//MP2/6-311G method. The proximate (4)J(FF) constants are particularly large (50-100 Hz) and show an inverse linear dependence on the calculated F-F distance in the range 2.43-2.58 A.

Manifestations of Bridgehead-Bridgehead Interactions in the Bicyclo[1.1.1]pentane Ring System

A series of 3-halo-substituted bicyclo[1.1.1]pentane-1-carboxylic acids 1 (Y = COOH; X = F, Cl, Br, I, and CF(3)) as well as the parent compound 1 (Y = COOH, X = H) have been prepared, and a study of some of their properties have been made. It was found that their reactions with xenon difluoride cover a wide range of reactivities. On one hand, the fluoro acid 1 (Y = COOH, X = F) displayed no apparent reaction at all while, on the other, the bromo acid 1 (Y = COOH, X = Br) and parent compound 1 (Y = COOH, X = H) underwent ready reaction with complete disintegration of the ring system. A possible explanation is advanced based on polar kinetic and thermodynamic effects governing the lifetime of an intermediate acyloxy radical species. The relative ease of oxidation of the carboxylates 1 (Y = COO(-); X = H, F, Cl, Br, I, CF(3), and COOCH(3)), as mirrored by their peak oxidation potential values (E(p)) determined by cyclic voltammetry, also covers a wide range. These data coupled with the dissociation constants (pK(a)) of some of the acids 1 (Y = COOH; X = H, F, Cl, and CF(3)) reflect significantly on the modes of transmission of electronic effects acting through the bicyclo[1.1.1]pentane ring system.