Search

Relatively scarce literature data on BN/CC isosterism in 4nπ-electronic systems have prompted us to investigate theoretically the influence of BN pair position within the central butadiene fragment of dibenzo[a,e]pentalene on two fundamental molecular properties: stability and (anti)aromaticity. It was found that stability and aromaticity follow the same trend only for BN-orientational isomers. The source of different stability of other isomers was examined first by an analysis of bond types and their dissociation energies and then by isomerization energy decomposition analysis and was explained in terms of classical electrostatic interactions, quantum-mechanical orbital interactions, structural changes and electronic changes (transition from charge-separated π-system to the neutral one). (Anti)aromaticity was investigated by using three kinds of indices, HOMA, FLUπ, and NICS(1)zz, which indicated that delocalization at the central pentalene motif is almost unaffected by various BN arrangements, molecular perimeter is slightly affected, while the most affected subunits are five-membered rings and benzene-fused five-membered rings containing only one heteroatom. Supplementary information: [https://cer.ihtm.bg.ac.rs/handle/123456789/4477]

This is a theoretical (DFT) study of the impact of electronic structural changes, induced by B-N/C-C isosterism, on two basic properties of 4-electron antiaromatic system, that is, stability and antiaromaticity. The main driving force for the nonplanarity of B2N2 rings is electrostatic energy, and that for a ring with one B-N unit is the relief of Pauli repulsion. The charge-separation instability, inherent for a 1,3-B,N relationship, turns the ground state of the BCNC system to an aromatic triplet, which is less stable than the isomeric BNCC system, mostly because of larger Pauli interactions. The alternating BNBN connectivity is favoured primarily by orbital interaction energy and, secondarily, by better electrostatic attraction. The C-C B-N substitution weakens the antiaromatic character, except that for a 1,3-B,N relationship, which results in increased antiaromaticity in the closed-shell state relative to that of cyclobutadiene. This is the peer-reviewed version of the following article: Baranac-Stojanović, M. 4-Electron B-N Monocycles: Stability and (Anti)Aromaticity. European Journal of Organic Chemistry 2017, No. 34, 5163–5169. [https://doi.org/10.1002/ejoc.201700959]

Information about the strength of donor–acceptor interactions in push–pull alkenes is valuable, as this so-called “push–pull effect” influences their chemical reactivity and dynamic behaviour. In this paper, we discuss the applicability of NMR spectral data and barriers to rotation around the C[double bond, length as m-dash]C double bond to quantify the push–pull effect in biologically important 2-alkylidene-4-oxothiazolidines. While olefinic proton chemical shifts and differences in 13C NMR chemical shifts of the two carbons constituting the C[double bond, length as m-dash]C double bond fail to give the correct trend in the electron withdrawing ability of the substituents attached to the exocyclic carbon of the double bond, barriers to rotation prove to be a reliable quantity in providing information about the extent of donor–acceptor interactions in the push–pull systems studied. In particular all relevant kinetic data, that is the Arrhenius parameters (apparent activation energy Ea and frequency factor A) and activation parameters (ΔS‡, ΔH‡ and ΔG‡), were determined from the data of the experimentally studied configurational isomerization of (E)-9a. These results were compared to previously published related data for other two compounds, (Z)-1b and (2E,5Z)-7, showing that experimentally determined ΔG‡ values are a good indicator of the strength of push–pull character. Theoretical calculations of the rotational barriers of eight selected derivatives excellently correlate with the calculated C[double bond, length as m-dash]C bond lengths and corroborate the applicability of ΔG‡ for estimation of the strength of the push–pull effect in these and related systems.

Configurational isomerization of stereo-defined 5-substituted and unsubstituted 2-alkylidene-4-oxothiazolidines (1) in the solid state, giving the Z/E mixtures in various ratios, was investigated by 1H-NMR spectroscopy, X-ray powder crystallography and differential scanning calorimetry (DSC). The Z/E composition can be rationalized in terms of non-covalent interactions, involving intermolecular and intramolecular hydrogen bonding and directional non-bonded 1,5-type S∙∙∙O interactions. X-Ray powder crystallography, using selected crystalline (Z)-4-oxothiazolidine substrate, revealed transformation to the amorphous state during the irreversible Z→E process. A correlation between previous results on the Z/E isomerization in solution and now in the solid state was established. Konfiguraciona izomerizacija stereodefinisanih 5-supstituisanih i nesupstituisanih 2-alkiliden-4-oksotiazolidina 1 u čvrstom stanju, pri čemu se stvara Z/E smesa u različitim odnosima, proučavana je pomoću 1H-NMR spektroskopije, rendgenske kristalografije praha i diferencijalne skenirajuće kalorimetrije (DSK). Odnos Z/E izomera može se objasniti u kontekstu nekovalentnih interakcija, koje obuhvataju intermolekulsko i intramolekulsko vodonično vezivanje i usmerene nevezivne S∙∙∙O interakcije 1,5-tipa. Rendgenska kristalografija praha odabranog kristalnog (Z)-4-oksotiazolidinskog supstrata, potvrdila je transformaciju u amorfno stanje u toku ireverzibilnog Z→E procesa. Postavljena je korelacija između prethodnih rezultata koji se odnose na Z/E izomerizaciju u rastvoru, i sada, u čvrstom stanju.

Compounds containing 4-oxo-1,3-thiazolidine ring attract attention due to their chemical reactivity, biological activity and diverse application. Among them, 2-alkylidene-4-oxothiazolidines are of interest as precursors for the synthesis of different heterocyclic compounds. They are also a constituent of more complex structures with different application, whereas biological activity makes them of interest for pharmacology. This article describes the synthesis of 2-alkylidene-4-oxothiazolidines, their physico-chemical properties and selected reactions, performed in our laboratory. These synthetically useful reactions lead to the formation of new heterocyclic systems, or to the C(5) functionalization of 4-oxothiazolidine ring based on the new bromine migration reaction. Jedinjenja koja sadrže 4-okso-1,3-tiazolidinski prsten privlače pažnju hemičara zbog raznolike hemijske reaktivnosti, kao i zbog širokog biološkog dejstva i raznovrsne primene. U poslednje vreme, velika pažnja hemičara je usmerena i ka 4-oksotiazolidinima koji u položaju C(2) sadrže egzocikličnu dvostruku vezu. Ovi, 2-alkiliden-4-oksotiazolidini se koriste kao prekursori za sintezu različitih heterocikličnih jedinjenja i ulaze u sastav kompleksnijih molekula koji mogu imati različitu primenu. Oni takođe poseduju i različito biološko dejstvo. U ovom članku će, bez detaljne analize i diskusije eksperimentalnih rezultata, biti prikazana sinteza 2-alkiliden-4-oksotiazolidina, njihove fizičko-hemijske osobine, kao i odabrane reakcije urađene u našoj laboratoriji. Ove sintetički korisne reakcije vode stvaranju novih heterocikličnih sistema ili C(5) funkcionalizaciji ovih jedinjenja koja se zasniva na novoj reakciji premeštanja broma.

Introducing students to the regioselective synthesis of a stereodefined push-pull alkene from inexpensive chemicals can be facilicated through experiment. In the experiment, preparation of the precursor, such as diethyl mercaptosuccinate from mercaptosuccinic acid and ethanol in toluene as a cosolvent, demonstrates the educational significance of binary and tertiary azeotropes in practical organic synthesis. The utility of 1H NMR spectroscopy is illustrated as a tool to identify the configurational isomers and to study the stereodynamic behavior of the isolated heterocyclic product.

Pyridinium hydrobromide perbromide (PHBP) is a highly efficient reagent for the conversion of 5-substituted-2-alkylidene-4-oxothiazolidine derivatives to the corresponding thiazolidines with two fully delocalized exocyclic double bonds at the C(2) and C(5) positions. This conversion as a two-step bromination-rearrangement process occurs in acetonitrile under homogeneous reaction conditions. Piridinijum-hidrobromid-perbromid (PHBP) je veoma efikasan reagens za konverziju 5-supstituisanih-2-alkiliden-4-oksotiazolidinskih derivata u odgovarajuće tiazolidine sa dve potpuno delokalizovane egzociklične dvogube veze u položajima C(2) i C(5). Ova konverzija je dvofazni proces bromovanja i premeštanja koji se odvija u acetonitrilu pod homogenim reakcionim uslovima.

Application of dynamic H-1-NMR spectroscopy added to the understanding of the hydrogen bonds existing in the structurally related 5-substituted-2-alklidene-4-oxothiazolidines in polar and apolar solvents. The equilibrated mixtures of these topical push-pull alkenes in CDCl3 consist of the intramolecularly H-bonded (E)-isomer and intermolecularly H-bonded (Z)-isomer in varying proportions which depend on the solvent polarity. For the representative of the series. (Z)-2-(5-ethoxycarbonylmethyl-4-oxothiazolidin-2-ylidene)-1-phenylethanone, a concentration effect on the degree of intermolecular hydrogen bonding in apolar CDCl3 has been studied. Primenom dinamičke 1H-NMR spektroskopije došlo se do boljeg razumevanja o vrsti vodoničnih veza koje postoje u strukturno sličnim 5-supstituisanim 2-alkiliden-4-oksotiazolidinima u polarnim i apolarnim rastvaračima. Uravnotežene smese ovih tipičnih push-pull alkena u CDCl3 sadrže (E)-izomer vezan intramolekulskom vodoničnom vezom kao i intermolekulski vodoničnom vezom vezan (Z)-izomer u različitim odnosima, koji zavise od polarnosti rastvarača. U slučaju tipičnog predstavnika serije (Z)-2-(5-etoksikarbonilmetil-4-oksotiazolidin- 2-iliden)-1-feniletanona uticaj koncentracije na stepen stvaranja intermolekulske vodonične veze u apolarnom CDCl3 je takođe proučavan.

A study on the regiospecificity of the base-catalyzed reaction of activated beta-oxonitriles 1 with diethyl mercaptosuccinate affording the title compounds 3 is reported. Other competitive heterocyclic products. that is 4-oxo-1.3-thiazinanes 4. derivatives of tetrahydrothiophene 5 and/or thiacyclohexane 6 which on the ggrounds of mechanistic considerations could be formed were not observed. Spectroscopic and experimental evidence. together with theoretical considerations. provides a reasonable explanation for the observed regiospecificity. Proučavana je regiospecifičnost bazno-katalizovane reakcije aktiviranih β-oksonitrilado 1 sa dietil-estrom merkaptoćilibarne kiseline u kojoj se grade u naslovu navedena jedinjenja 3. Mogući heterociklični proizvodi 4-okso-1,3-tiazini 4, derivati tetrahidrotiofena 5 i/ili tijacikloheksana 6 koji mogu biti nagrađeni na osnovu mehanističkog razmatranja, nisu detektovani. Spektroskopski i eksperimentalni dokazi sa teorijskim obrazloženjem daju prihvatljivo objašnjenje za uočenu regiospecifičnost.