Pd-catalyzed one-pot multicomponent coupling reaction for the highly regioselective synthesis of polysubstituted benzenes

[Reaction: see text] Tetrasubstituted benzenes can be efficiently synthesized in a regioselective manner from alkynes and 2-bromoacrylates by palladium-catalyzed cascade Sonogashira coupling-benzannulation reaction.     

DMAP catalyzed reaction of β-ketoesters and dimethyl acetylenedicarboxylate: efficient synthesis of polysubstituted benzenes and biaryls

A DMAP catalyzed tandem addition-cyclization-dehydration sequence involving dimethyl acetylenedicarboxylate and β-ketoesters leading to polysubstituted benzene/biaryl derivatives is presented.     

On the proton affinity of pteridine

All-valence-electron and perturbation calculations suggest that pteridine may preferably be protonated at the pyrazine moiety. Correlation between these results and some experimental data is presented.     

Additivity of long-range spin-spin coupling constants J(13C-H) in substituted benzenes

Theoretical and Experimental Chemistry -     

Efficient method for the synthesis of polysubstituted benzenes by one-pot tandem reaction of vinyl malononitriles with nitroolefins

The one-pot synthesis of polysubstituted benzene derivatives was achieved via vinylogous Michael addition of vinyl malononitriles and nitroolefins as the key step and a sequential tandem reaction. A series of complex aryl compounds such as biphenyls and terphenyls can be obtained with satisfactory yields.     

Regioselective synthesis of polysubstituted benzenes from Baylis-Hillman adducts via [4+2] annulation protocol

A new and regioselective [4+2] benzannulation protocol toward polysubstituted benzenes was developed. A nitroalkane derivative, which was prepared from Baylis-Hillman adduct, served as the four-carbon unit and a Michael acceptor as a two-carbon unit. Vinyltriphenylphosphonium salt could also be used as a Michael acceptor efficiently.     

Protonation of group 14 oxides: the proton affinity of SnO

Molecular orbital calculations are reported for the monoxides, XO, of group 14 elements (X = C, Si, Ge and Sn) and for both isomers, XOH⁺ and HXO⁺, of the protonated monoxides. Structure optimisation has been carried out using the Density Functional Theory employing the B3LYP procedure and at both Hartree-Fock and MP2 (full) levels, all with a variety of medium-sized Gaussian basis sets. In all XO molecules the oxygen atom is the preferred site for protonation, except when X = C where HCO⁺ is the lower energy isomer. Barriers to interconversion between the two isomers XOH⁺ and HXO⁺ are over-estimated by the Hartree-Fock calculations, but with wave functions that include electron correlations they generally fall into the range 27-44 kcal mol⁻¹. Proton affinities increase as the atomic number of X increases, and values calculated by averaging over all wave functions that include electron correlation, give the following proton affinities: for CO, 141.5; for SiO, 189.3; for GeO, 196.1; and for SnO, 215.6 (all in kcal mol⁻¹).     

Reactions of polysubstituted thiopyrans with proton acids and structural studies of the resulting thiacyclohexanes

Depending on the number and nature of substituting groups, 4H-thiopyrans under the influence of perchloric and trifluoroacetic acids either undergo disproportionation or form thiapyrylium salts in quantitative yields with the participation of air oxygen; in the case of ionic hydrogenation with CF₃COOH-SiHEt₃ they are converted to thiacyclohexanes, regardless of their structure. To ascertain the reason for the difference in the mechanisms of the reaction of polysubstituted thiopyrans with proton acids the structures of the polysubstituted thiacyclohexanes were investigated jointly by NMR spectroscopy and x-ray diffraction analysis; this made it possible to form a judgment regarding the effect of the degree and character of substitution of 4H-thiopyrans on the mechanism of their reaction with proton acids. The tetrahydrothiopyran ring has a chair conformation. The phenyl substituents are equatorially oriented, while the methyl substituents are axially oriented.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 324–327, March, 1980.     

Absolute proton affinity of some polyguanides

The problem of the absolute proton affinity (APA) of some polyguanides is addressed by the MP2(fc)/6-311+G//HF/6-31G theoretical model. It is shown that the linear chain polyguanides exhibit increased basicity as a function of the number of guanide subunits. However, the saturation effect yields an asymptotic APA value of 254 kcal/mol. Branched polyguanides on the other hand have higher APAs than their linear counterparts. The largest proton affinity is found in a doubly bifurcated heptaguanide, being as high as 285 kcal/mol, thus potentially representing one of the strongest organic bases. Finally, it is found that all polyguanides protonate at imino nitrogen atoms, since they are apparently susceptible the most to the proton attack. The origin of their very high intrinsic basicity is traced down to a dramatic increase in the resonance interaction of the corresponding conjugate bases. For instance, the increase in the resonance energy in the protonated guanidine is estimated to be in a range of 24-27 kcal/mol, which is higher than the aromatic stabilization in benzene. The proton affinity of some polycyclic guanides including Schwesinger proton sponge and porphine is briefly discussed.     

Theoretical proton affinity and fluoride affinity of nerve agent VX

Proton affinity and fluoride affinity of nerve agent VX at all of its possible sites were calculated at the RI-MP2/cc-pVTZ//B3LYP/6-31G* and RI-MP2/aug-cc-pVTZ//B3LYP/6-31+G* levels, respectively. The protonation leads to various unique structures, with H(+) attached to oxygen, nitrogen, and sulfur atoms; among which the nitrogen site possesses the highest proton affinity of -ΔE ～ 251 kcal/mol, suggesting that this is likely to be the major product. In addition some H(2), CH(4) dissociation as well as destruction channels have been found, among which the CH(4) + [Et-O-P(═O)(Me)-S-(CH(2))(2)-N(+)(iPr)═CHMe] product and the destruction product forming Et-O-P(═O)(Me)-SMe + CH(2)═N(+)(iPr)(2) are only 9 kcal/mol less stable than the most stable N-protonated product. For fluoridization, the S-P destruction channel to give Et-O-P(═O)(Me)(F) + [S-(CH(2))(2)-N-(iPr)(2)](-) is energetically the most favorable, with a fluoride affinity of -ΔE ～ 44 kcal. Various F(-) ion-molecule complexes are also found, with the one having F(-) interacting with two hydrogen atoms in different alkyl groups to be only 9 kcal/mol higher than the above destruction product. These results suggest VX behaves quite differently from surrogate systems.     

Substituent effects on the proton spin-spin coupling of benzenes with side-chain interacting groups

Sixty MHz NMR proton spectra of 15 2,4-substituted benzaldehydes and 12 methylbenzenes dissolved in different solvents have been accurately analyzed. Substituent effects on the coupling constants have been determined and have been found to be additive in those trisubstituted compounds where there are neither internal hydrogen bonds or strong interacting groups present.     

Topological incremental scheme for calculating13C NMR chemical shifts in polysubstituted benzenes with substituents of one kind

A new topological approach to predicting the¹³C NMR chemical shifts of polysubstituted benzenes has been proposed (in the example case of compounds with substituents of one kind of the type C₆H_6–nX_n). The collective interactions of several substituents X [X=CH₃, C₂H₅, iso-C₃H₇, CF₃, COOH, f, Cl, Br, Si(CH₃)₃] have been expressed in the framework of a regression treatment in terms of two-particle increments. The chemical shift of each carbon atom has been represented in the form of an equation containing 17 parameters. The calculation scheme can be transformed and expanded for use even in the calculation of the spectra of compounds not previously studied. The calculated shifts for some previously investigated compounds and some compounds not previously investigated have been presented. The error in reproducing chemical shifts is equal to 0.2–0.3 ppm (and may be as high as 0.8 ppm for only a few measurements).V. I. Vernadskii Institute of Geochemistry and Analytical Chemistry, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 32, No. 2, pp. 62–71, March–April, 1991.     

SiH+5 and the proton affinity of monosilane

Tandem mass spectrometric studies show that SiH⁺₅ is formed in bimolecular reactions of SiH₄ and NH⁺₂, C₂H⁺₃, C₂H⁺₆ and C₃H⁺₈ ions. The dependence of the reaction cross sections on ion energy indicates the formation of SiH⁺₅ from NH⁺₂, C₂H⁺₃, and C₂H⁺₆ to be exothermic reactions, while formation from C₃H⁺₈ is endothermic. Using known thermochemical data, these facts permit the assignment of 150 and 156 kcal/mole to the lower and upper limits of the proton affinity of monosilane.     

Additivity of proton and carbon nmr chemical shifts in protonated benzene carboxylic acids

The additivity of the proton and carbon chemical shift increments due to structural changes in a series of conjugate acids derived from benzene polycarboxylic acids is reported. In the acids, two sets of increments had been applied, one to the ortho diacids and one to acids which do not bear ortho substituents. In the conjugate acids only one set of increments is required. The disappearance of the ‘ortho’ effect may be the outcome of properties inherent to these conjugate acids.     

One stone two birds: construction of polysubstituted benzenes from the same starting material and precatalyst by switching the active sites of catalyst with different additives

Two different tetrasubstituted benzenes were selectively constructed from the same starting materials by tuning the active sites of the Grubbs second generation catalyst (GC-II) with CuI or AgOTf as the additive.