Ar Cl Bond Research Articles

Palladium-catalyzed selective Buchwald–Hartwig C–N coupling of chloroaryl triflates with amines

The chemoselective Buchwald–Hartwig amination developed here showed that the reactivity of the Ar–Cl bond is higher than that of the Ar–OTf bond. Catalyzed by PdCl2/P(t-Bu)3, the reaction selectively aminated the Ar–Cl bond in an aryl system containing both Ar–Cl and Ar–OTf with moderate yield and excellent chemoselectivity, and the significant effect of additives was also found. With the aid of this methodology, we also efficiently synthesized a bicarbazole scaffold that can serve as a valuable precursor for alkaloids such as murrastifoline A&B.

Palladium-catalyzed chemoselective direct α-arylation of carbonyl compounds with chloroaryl triflates at the C-Cl site.

This study described palladium-catalyzed chemoselective direct α-arylation of carbonyl compounds with chloroaryl triflates in the Ar–Cl bond. The Pd/SelectPhos system showed excellent chemoselectivity toward the Ar–Cl bond in the presence of the Ar–OTf bond with a broad substrate scope and excellent product yields. The electronic and steric hindrance offered by the –PR2 group of the ligand with the C2-alkyl group was found to be the key factor affecting the reactivity and chemoselectivity of the α-arylation reaction. The chemodivergent approach was also successfully employed in the synthesis of flurbiprofen and its derivatives (e.g., –OMe and –F).

Search for a photoinduced (site-selective) cleavage of the Ar-Cl bond in dichloroanisoles.

The site-selective cleavage of an Ar-X bond in polyhalogenated aromatics is an important tool in synthetic planning especially when more than one identical halogen atom is present. An alternative to the usual metal-catalyzed cleavage is represented by photochemistry although only a few examples have been reported. We then investigated the feasibility of the site-selective photodechlorination of some dichloroanisoles through a combined experimental and computational study. In the case of 2,4-dichloroanisole, selective detachment of the chlorine atom at the ortho position with respect to the OMe group was observed upon photohomolysis (in cyclohexane) or photoheterolysis (in MeOH) of the Ar-Cl bond. In the latter case, 5-chloro-2-methoxy-1,1'-biphenyl was exclusively formed upon reaction of the resulting phenyl cation with benzene. The substitution of an OH group for a OMe group was detrimental since a lower photoreactivity resulted with no improvement in the selectivity.

Sugar-Assisted Photogeneration of Didehydrotoluenes from Chlorobenzylphosphonic Acids.

Irradiation of the three isomeric chlorobenzylphophonic acids in aqueous buffer led to a pH-dependent photochemistry. Under acidic conditions (pH = 2.5), photocleavage of the Ar-Cl bond occurred and a phenyl cation chemistry resulted. Under basic conditions (pH = 11), a photoinduced release of the chloride anion followed by the detachment of the metaphosphate anion gave α,n-didehydrotoluene diradicals (α,n-DHTs), potential DNA cleaving intermediates. At a physiological pH (pH = 7.2), both a cationic and a diradical reactivity took place depending on the phosphonic acid used. It is noteworthy that the complexation exerted by a monosaccharide (glucose or methylglucopyranoside) present in solution induced an exclusive formation of α,n-DHTs. The mechanistic scenario of the different photoreactivities occurring when changing the pH of the solution and the role of the various intermediates (phenyl cations, diradicals, etc.) in the process was studied by computational analysis.

Observation of 36ArH37Cl, 38ArH35Cl and 38ArH37Cl in natural abundance using CP-FTMW spectroscopy

Multiple minor isotopologues of ArHCl have been observed in natural abundance using two CP-FTMW spectrometers. These include the first known microwave observation of 38Ar isotopologues in natural abundance. The parameters derived from fits of the spectrum have been utilized in calculating multiple molecular parameters for the system which are shown to be the best determined to date. From these new spectrum, it is shown that there is a mass-dependency of the derived effective ArCl bond distance and the large amplitude averaging angle of HCl is relatively invariant to mass substitution at 41.5°.

Aryl carbon–chlorine (Ar–Cl) and aryl carbon–fluorine (Ar–F) bond cleavages by rhodium porphyrins

Aryl carbon–chlorine (Ar–Cl) bond cleavage has been achieved with rhodium(III) tetrakis-4-tolylporphyrin chloride (Rh(ttp)Cl) to give Rh(ttp)Ar. For 4-chlorofluorobenzene, the aryl carbon–fluorine (Ar–F) bond cleavage competes with the Ar–Cl bond cleavage. Mechanistic investigations show that the Ar–Cl bond cleavage goes through metalloradical ipso-substitution mechanism, while the Ar–F bond cleavage goes through nucleophilic aromatic substitution. The selectivity of the Ar–F or Ar–Cl bond cleavage can be controlled by tuning the temperature and substrate concentration.

A DBU–diheteroaryl halide adduct as the fastest current N-diheteroarylating agent

An unexpected diazabicyclo[5.4.0]undec-7-ene (DBU) catalysed rate enhancement of N-arylation of amines with diheteroaryl halides is reported. DBU is found to activate the Ar–Cl bond of a diheteroaryl halide, forming a green coloured adduct under neat conditions. The activated green coloured adduct was used for the arylation of amines under neat conditions and was found to be the fastest diheteroarylating agent reported to date.

Acetalization Allows the Photoheterolysis of the Ar–Cl Bond in Chlorobenzaldehydes and Chloroacetophenones

The nonaccessibility of phenyl cations by irradiation of electron-poor aryl chlorides was circumvented by transforming the carbonyl group of aromatic ketones or aldehydes into the corresponding 1,3-dioxolanes and the carboxyl group of benzoate ester into an orthoester functionality. This transformation allowed the heterolytic photoactivation of the Ar-Cl bond in protic media and the generation of phenyl cations. In the presence of π-bond nucleophiles, arylated products were obtained in good to excellent yields.

The β Effect of Silicon in Phenyl Cations

Irradiation of chloroanisoles, phenols, and N,N-dimethylanilines bearing a trimethylsilyl (TMS) group in the ortho position with respect to the chlorine atom caused photoheterolysis of the Ar-Cl bond and formation of the corresponding ortho-trimethylsilylphenyl cations in the triplet state. The beta effect of silicon on these intermediates has been studied by comparing the resulting chemistry in alcoholic solvents with that of the silicon-free analogues and by computational analysis (at the UB3LYP/6-311+G(2d,p) level in MeOH). TMS groups little affect the photophysics and the photocleavage of the starting phenyl chlorides, while stabilizing the phenyl cations, both in the triplet (ca. 4 kcal/mol per group) and, dramatically, in the singlet state (9 kcal/mol). As a result, although triplet phenyl cations are the first formed species, intersystem crossing to the more stable singlets is favored with chloroanisoles and phenols. Indeed, with these compounds, solvent addition to give aryl ethers (from the singlet) competed efficiently with reduction or arylation (from the triplet). In the case of the silylated 4-chloro-N,N-dimethylaniline, the triplet cation remained in the ground state and trapping by pi nucleophiles remained efficient, though slowed by the steric bulk of the TMS group. In alcohols, the silyl group was eliminated via a photoinduced protiodesilylation during the irradiation. Thus, the silyl group could be considered as a directing, photoremovable group that allowed shifting to the singlet phenyl cation chemistry and was smoothly eliminated in the same one-pot procedure.

Versatile behaviour of iron–arylcarbene complexes towards alkoxides: CCl and CC bond activation reactions

The Cp-containing carbene complexes [ Fe(C 5 H 5 )(CO){κ 2(C,O)&0xF700; C(OMe)C 6 H 4-o -O Me}][OTf] ( 2) and [Fe(C 5H 5)(CO)(CH 3CN){C(OMe)C 6H 4- o-Cl}[OTf] ( 3) have been synthesised and characterised. Complex 2 has been characterised by a single-crystal X-ray diffraction analysis; the Fe–O distance of 2.045(5) Å suggests a rather strong bonding of the methoxy group. Substitution of one carbonyl ligand by PPh 3 allows the isolation of the chelate complex [ Fe(C 5 H 5 )(PPh 3){ κ 2(C,Cl)&0xF700; C(OMe)C 6 H 4-o -C l}][OTf] ( 4). The reaction of [ Fe(C 5 Me 5 )(CO){κ 2(C,Cl)&0xF700; C(OMe)C 6 H 4-o -C l}][OTf] ( 5) with alkoxides RONa (R=Me, Et) affords the chelate complexes [ Fe(C 5 Me 5 )(CO){κ 2(C,O)&0xF700; C 6 H 4-o -C(OMe)(O R)(OR)}] 7 (R=Me) and 8 (R=Et), respectively. Similarly, the reaction of 3 with EtONa affords [ Fe(C 5H 5)(CO){κ 2( C, O)−C 6H 4-o-C(O Et)(OEt)2}] ( 9), for which the ArCl bond has been cleaved. No activation of the ArCl bond is observed for the unchelated complex [Fe(C 5Me 5)(CO) 2{C(OMe)C 6H 4- o-Cl}][OTf]. In contrast, treatment of the anisyl derivative [ Fe(C 5 Me 5 )(CO){κ 2(C,O)&0xF700; C(OMe)C 6 H 4-o -O Me}][OTf] ( 6) with EtONa gives selectively the carbene complex [Fe(C 5Me 5)(CO)(C 6H 4- o-OMe){C(OEt) 2}] ( 12). The ArOMe bond remains intact, but cleavage of the CαCAr bond occurs. This rearrangement process, i.e. α-elimination of the anisyl substituent, is favoured by the lability of the ortho-OMe group. Treatment of 2 with NaOEt gives as the only identified compound the ethyl ester [MeOC 6H 4- o-(CO)OEt]. Clean formation of the methyl ester derivative [MeOC 6H 4- o-(CO)OMe] is observed upon oxidation of 2 with C 6H 5I +O −.

Hemilabile Properties of Chelate Iron Complexes: Synthesis and Structure of Oxametallacycles

Oxametallacycles [Fe(C5Me5)(CO){C3(C6H4-o-Cl)(CO2Me)(R)Oa}(Fe–Oa)] (2, R = OMe; 4, R = Me) are accessible from the chelate (chloroaryl)carbene complex [Fe(C5Me5)(CO){C(OMe)C6H4-o-Cla}(Fe–Cla)][OTf] (1) upon treatment with the appropriate carbanions. Their formation arises from the lability of the chlorine atom. The related phosphonium salt [Fe(C5Me5)(CO){C3(C6H4-o-PMe3)(CO2Me)(OMe)Oa}(Fe–Oa)][OTf] (3) is formed only for the bis(ester) derivative, via Ar–Cl bond activation. No reaction occurs for 4, for which the coordination of the acetyl group has been supported by an X-ray analysis.

Resonances in the photodissociation of HCl in the Ar–HCl van der Waals complex: How prominent are they?

We investigate the cage effect in the ultraviolet (UV) photodissociation of the Ar...HCl van der Waals complex, especially the possibility of resonance structures caused by trapping of the hydrogen atom between its heavy partners as recently highlighted by Garcia-Vela and Gerber [J. Chem. Phys. 98, 427 (1993)]. The dynamics is described by solving the time-dependent Schrödinger equation employing the standard Jacobi coordinates which are routinely used for triatomic systems. Due to the large size of the required grid, exact three-dimensional (3D) wave packet calculations are extremely time consuming and could be followed up to 20 fs only. This time is sufficient for calculating the absorption spectrum, but too short for determining the final kinetic energy distributions of the fragment atoms. Therefore, the photodissociation dynamics is mainly treated in a vibrationally sudden approximation, in which the dynamical calculations are performed for a range of fixed ArCl bond distances, and the results averaged over this bond length. 3D classical trajectory calculations show that the energy transfer out of the dissociative HCl mode is very weak (∼5% of the total energy), supporting the application of the sudden approximation. In this approximation, both the absorption spectrum and the kinetic energy distribution associated with the dissociating HCl motion exhibit very weak diffuse structures (resonances) which, following the work of Garcia-Vela and Gerber, can be assigned to the transient vibrational motion of hydrogen between Ar and Cl. However, in our calculations these structures are much less pronounced than in the work of Garcia-Vela and Gerber. The very small amplitudes of the resonance features indicate that trapping in the dissociation of HCl in Ar...HCl is marginal, and much less important than suggested by the previous studies of Garcia-Vela et al. Furthermore, in contrast to the work reported by Garcia-Vela et al., we do not find any evidence for the narrow, irregular features superimposed on the resonance structures.

PdCl2-catalyzed hydrogenolysis of an Ar-Cl bond by sodium phosphinate in an aqueous alkaline medium

The hydrogenolysis of the Ar-Cl bond in chlorinated benzoic acids, anilines, and nitrogen-containing heterocycles by the action of NaH2PO2 in the presence of catalytic amounts of PdCl2 and NaOH in an aqueous medium was studied. Under these conditions aryl chlorides were quantitatively converted into the corresponding arenes. Chloropyridines, such as 2-chloropyridine and 3, 5-dichloropicolinic acid, did not undergo hydrogenolysis even in the presence of an equivalent amount of NaI. A distinctortho effect was observed forortho-substituted substrates.

The structure and bond energy of ArCl2

The geometry and bond energy of ArCl2 are obtained by the technique of parity selected excitation spectroscopy. This is a pump–probe technique in which the probe laser is set to detect specific rotational levels of the B state of Cl2 while the pump laser is tuned through an ArCl2 B–X band. Since the excitation spectrum obtained depends on the parity of the detected Cl2 rotational level, the ArCl2 complex must have a symmetrical, T-shaped geometry on average over the time scale of the experiment, ∼100 ps. Simulation of the observed spectra, which show partially resolved rotational structure, yields an argon to chlorine center of mass distance of 3.7 Å for both the X and B electronic states of the complex. By measuring the threshold for dissociation to the v=6, j=8 level of the Cl2 B state, the Ar–Cl2 bond energy was determined to be 178 cm−1 in the B state and 188 cm−1 in the X state. These results are quite different from those on the isovalent complex ArClF which is a linear molecule with an Ar–Cl bond length of 3.33 Å, and a bond energy of 230 cm−1.