Palladium‐Catalyzed Fluoroarylation of gem‐Difluoroalkenes

A Pd‐catalyzed fluoroarylation of gem ‐difluoroalkenes with aryl halides is reported. By taking advantage of the in situ generated α‐CF₃‐benzylsilver intermediates derived from the nucleophilic addition of silver fluoride to gem ‐difluoroalkenes, this strategy bypasses the use of a strong base, thus enabling a mild and general synthetic method for ready access to non‐symmetric α,α‐disubstituted trifluoroethane derivatives.     

A new tribridged imidazolium cyclo­phane

The new imidazolium cyclo­phane 2,11,13,21,29,31‐hexamethyl‐5,16,24‐tri­aza‐8,19,27‐triazoniahepta­cyclo[10.10.6.1^1,3.­1^5,8.1^10,14.1^16,19.1^24,27]­tritriaconta‐1,3(29),6,8(30),10,­12,14(31),17,19(32),21,25,27(33)‐dodecene tri­bromide dihydrate, C₃₃H₃₉N₆³⁺.3Br^?.2H₂O, belongs to a type of cyl­indrical macrobicyclic salt containing imidazolium groups. It has mirror symmetry, the symmetry surface is the plane of the three 2‐C atoms of imidazoliums. Three bromine anions are associated with two water mol­ecules by hydrogen bonds. The O—H?Br bond lengths are 2.53 (3) and 2.46 (3) Å.     

1‐Chloro‐1,3,3,5,5‐penta­phenoxy­cyclo­triphosphazene: a precursor of functionalized cyclo­phosphazene derivatives

The crystal structure of the title compound, C₃₀H₂₅ClN₃O₅P₃, shows that the direct bonding between P and Cl does not modify the structural parameters in the vicinity of this P atom. It also confirms the structural difference between the two P atoms which are each bonded to two phenoxy groups, observed in the ³¹P NMR spectrum. The crystal packing consists principally of complex stacking interactions.     

gem‐Difluorocarbadisaccharides: Restoring the exo‐Anomeric Effect

Molecular mimicry is an essential part of the development of drugs and molecular probes. In the chemical glycobiology field, although many glycomimetics have been developed in the past years, it has been considered that many failures in their use are related to the lack of the anomeric effects in these analogues. Additionally, the origin of the anomeric effects is still the subject of virulent scientific debates. Herein, by combining chemical synthesis, NMR methods, and theoretical calculations, we show that it is possible to restore the anomeric effect for an acetal when replacing one of the oxygen atoms by a CF₂ group. This result provides key findings in chemical sciences. On the one hand, it strongly suggests the key relevance of the stereoelectronic component of the anomeric effect. On the other hand, the CF₂ analogue adopts the natural glycoside conformation, which might provide new avenues for sugar‐based drug design.     

Synthesis of gem‐Diaurated Species from Alkynols

A number of enol ether‐derived diaurated species were synthesized directly from different alkynols and cationic gold complexes in the presence of a non‐nucleophilic base (proton sponge). The reaction can be easily applied for in situ generation of diaurated species from all common types of hydroalkoxylation substrates: 5‐endo, 5‐exo/6‐endo, 6‐exo/7‐endo and intermolecular types. Six examples were also synthesized in individual state as stable hexafluoroantimonate salts. Whereas diaurated species are obtained reliably from all conventional mononuclear gold catalysts, application of binuclear ones often gave diaurated species with unusual properties. The preliminary results point to complexities of behavior of binuclear gold catalysts and would require more research in future for this subclass. The formation of diaurated species from various gold‐oxo compounds (LAu)₂OH⁺, (LAu)₃O⁺, and LAuOH (L=phosphine ligand) was also studied. Of these three types, only (LAu)₂OH⁺ is reactive, whereas (LAu)₃O⁺ and LAuOH are not reactive alone but require acidic promoters to enable the reaction. These differences in reactivity were explained by ability of these compounds to generate the necessary acetylene π‐complex intermediate.     

Photoredox‐Coupled F‐Nucleophilic Addition: Allylation of gem‐Difluoroalkenes

A novel strategy for the expedient construction of CF₃‐embeded tertiary/quarternary carbon centers was developed by taking advantage of photoredox catalysis. Thanks to a key step of single‐electron oxidation, electron‐rich gem‐difluoroalkenes, which otherwise are essentially reluctant towards F‐nucleoplilic addition, now readily participate in this fluoroallylation reaction. Furthermore, this strategy provides an elegant example for the generation, as well as functionalization, of α‐CF₃‐substituted benzylic radical intermediates using cheap and readily available starting materials.     

gem‐Dimethylcyclopropanation of dibenzylideneacetone using triisopropyl sulfoxonium tetrafluoroborate

The reaction between dibenzylideneacetone (dba) and triisopropyl sulfoxonium tetrafluoroborate has been reinvestigated. The stereochemistry of the major diasteromeric bis(gem‐dimethylcyclopropane) adduct has now been assigned as [(1RS,3RS)‐2,2‐dimethyl‐3‐phenylcyclopropyl][(1SR,3SR)‐2,2‐dimethyl‐3‐phenylcyclopropyl]methanone, C₂₃H₂₆O, by X‐ray crystallographic studies on a twinned crystal. The asymmetric unit contains two molecules of the adduct, the conformations of which differ in the orientation of the phenyl ring relative to the adjacent cyclopropanated double bond. The carbonyl groups of each adduct are aligned approximately along the a axis and in opposite directions to each other. The molecules pack to give a sinusoidal pattern along the b axis. This is the first acyclic bis(dimethylcyclopropyl) ketone for which an X‐ray crystal structure determination has been reported, and is also the first bis‐cyclopropanated dba analogue. The knowledge that the major diastereomer has the meso structure (and therefore the confirmation that the minor isomer is the racemate) will prove invaluable in future studies to utilize bis(dimethylcyclopropyl) ketones as reagents, in rearrangement processes, and as potential ligands and ligand precursors in organometallic chemistry.     

Some Reactions of gem‐Dibromocyclopropanes and Metal Carbonyls

A series of gem‐dibromocyclopropanes were treated with various metal complexes. Among the metal complexes, Ru(CO)₂(PPh₃)₃, Ru(CO)₃(PPh₃)₂, and Mo(CO)₆ were able to remove a bromine atom from 1,1‐dibromo‐2‐phenylcyclopropanes ( 1 ) to yield a series of corresponding of 1‐bromo‐2‐phenylcyclopropanes ( 2 ). Upon the treatment of 1 with Cr(CO)₆ in DMSO, a series of allenes were obtained in good yields. The correlation between the rate of formation of allenes and the substituents on the benzene gives a negative coefficient which suggests the dibromocyclopropanes possesses as an electrophile toward to Cr(CO)₆. In the presence of Cr(CO)₆, gem‐dibromobicyclo[n,1,0]alkanes ( 4 ) in DMF or DMSO solution underwent the cleavage of carbon‐bromine bond followed by ring‐expansion and coupling reaction to form bicycloalkenes 7 .     

On‐Surface Formation of Cumulene by Dehalogenative Homocoupling of Alkenyl gem‐Dibromides

The on‐surface activation of carbon–halogen groups is an efficient route to produce radicals for constructing various hydrocarbons and carbon nanostructures. To date, the employed halide precursors have only one halogen attached to a carbon atom. It is thus of interest to study the effect of attaching more than one halogen atom to a carbon atom with the aim of producing multiple unpaired electrons. By introducing an alkenyl gem‐dibromide, cumulene products were fabricated on a Au(111) surface by dehalogenative homocoupling reactions. The reaction products and pathways were unambiguously characterized by a combination of high‐resolution scanning tunneling microscopy and non‐contact atomic force microscopy measurements together with density functional calculations. This study further supplements the database of on‐surface synthesis strategies and provides a facile manner for incorporation of more complicated carbon scaffolds into surface nanostructures.     

Why is benzyl‐gem‐diacetate remarkably stable in aqueous solution?

Benzyl‐gem‐diacetate is synthesized and performed its solvolysis in water at 25°C. It did not solvolize even for a year, whereas its counterparts benzyl‐gem‐diazide and dihalides underwent spontaneous cleavage through a S_N1 mechanism in aqueous solution to give benzaldehydes as the final product through α‐azido benzyl and α‐halo benzyl carbocation intermediates, respectively. The possible explanations are offered for the extraordinary stability of the benzyl‐gem‐diacetate in water. © 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 554–557, 2009     

Stereoselective Palladium‐Catalyzed C−F Bond Alkynylation of Tetrasubstituted gem‐Difluoroalkenes

A stereoselective Pd(PPh₃)₄‐catalyzed C?F bond alkynylation of tetrasubstituted gem‐difluoroalkenes with terminal alkynes has been developed. This method gives access to a great variety of conjugated monofluoroenynes bearing a tetrasubstituted alkene moiety with well‐defined stereochemistry. Chelation‐assisted oxidative addition of Pd to the C?F bond is proposed to account for the high level of stereocontrol. An X‐ray crystal structure of a key monofluorovinyl Pd^II intermediate has been obtained for the first time as evidence for the proposed mechanism.     

Visible‐Light‐Induced Selective Defluoroborylation of Polyfluoroarenes,gem‐Difluoroalkenes,and Trifluoromethylalkenes

Fluorinated organoboranes serve as versatile synthetic precursors for the preparation of value‐added fluorinated organic compounds. Recent progress has been mainly focused on the transition‐metal catalyzed defluoroborylation. Herein, we report a photocatalytic defluoroborylation platform through direct B?H activation of N‐heterocyclic carbene boranes, through the synergistic merger of a photoredox catalyst and a hydrogen atom transfer catalyst. This atom‐economic and operationally simple protocol has enabled defluoroborylation of an extremely broad scope of multifluorinated substrates including polyfluoroarenes, gem‐difluoroalkenes, and trifluoromethylalkenes in a highly selective fashion. Intriguingly, the defluoroborylation protocol can be transition‐metal free, and the regioselectivity obtained is complementary to the reported transition‐metal‐catalysis in many cases.     

Cobalt‐Catalyzed Enantioselective Synthesis of Chiral gem‐Bis(boryl)alkanes

We report an asymmetric synthesis of enantioenriched gem‐bis(boryl)alkanes in an enantioselective diborylation of 1,1‐disubstituted alkenes catalyzed by Co(acac)₂/(R)‐DM‐segphos. A range of activated and unactivated alkenes underwent this asymmetric diborylation in the presence of cyclooctene as a hydrogen acceptor, affording the corresponding gem‐bis(boryl)alkanes with high enantioselectivity. The synthetic utility of these chiral organoboronate compounds was demonstrated through several stereospecific derivatizations and the synthesis of sesquiterpene and sesquiterpenoid natural products.     

7‐Oxobi­cyclo­[2.2.1]­heptane‐1‐carboxylic acid and ()‐7‐oxobi­cyclo­[2.2.1]­hept‐5‐ene‐2‐endo‐carboxylic acid: hydrogen bonding in two norbornyl keto acids

Molecules of the title β‐keto acid, 7‐oxobi­cyclo­[2.2.1]­heptane‐1‐carboxylic acid, C₈H₁₀O₃, exhibit chirality due to the bridgehead carboxyl group, which is partially ordered and has a slightly asymmetric conformation. The mol­ecules form centrosymmetric hydrogen‐bonded carboxyl dimers [O?O 2.639 (2) Å]. The title alkenoic γ‐keto acid, ()‐7‐oxobi­cyclo­[2.2.1]­hept‐5‐ene‐2‐endo‐carboxylic acid, C₈H₈O₃, also forms typical centrosymmetric hydrogen‐bonded carboxyl dimers [O?O 2.660 (3) Å]. There is partial disorder of the carboxyl group in each compound.     

Stereoselective Synthesis of Z Fluoroalkenes through Copper‐Catalyzed Hydrodefluorination of gem‐Difluoroalkenes with Water

A copper catalytic system was established for the stereoselective hydrodefluorination of gem‐difluoroalkenes through C−F activation to synthesize various Z fluoroalkenes. H₂O is used as the hydrogen source for the fluorine acceptor moiety. This mild catalytic system shows good‐functional group compatibility, accepting a range of carbonyls as precursors to the gem‐difluoroalkenes, including aliphatic, aromatic, and α,β‐unsaturated aldehydes and even ketones. It serves as a powerful synthetic method for the late‐stage modification of complex compounds.