Static and Dynamic Stereochemistry of Push-Pull and Strained Ethylenes

Synthesis of Regular Ladder-Type Fluorenylidene-Xanthene Oligomers via Nucleophilic Aromatic Substitution Reaction

Fluorenylidene-xanthene and related polycyclic systems are typical examples of bistricyclic aromatic enes (BAEs); however, polycyclic systems of fluorenylidene-xanthene are relatively scarce. Here we synthesized a series of novel ladder-type polycyclic systems containing fluorenylidene-xanthene units, differing from the classic Barton's two-fold extrusion reaction. In the new synthetic scheme, the very popular Suzuki reaction was first used to couple the corresponding precursors, then a catalyst-free nucleophilic aromatic substitution reaction between Ar-F and Ar-OH groups in the backbone afforded ladder-type O-heteroarenes. Moreover, the new synthetic strategy is also suitable for the synthesis of other heterocyclic and polycyclic aromatic hydrocarbons. Some of these products showed reversible mechanochromism properties.

NHC-Supported 2-Sila and 2-Germavinylidenes: Synthesis, Dynamics, First Reactivity and Theoretical Studies

2-tetrelavinylidenes (C=EH2; E=Si, Ge) are according to quantum chemical studies the least stable isomers on the [E,C,2H] potential energy hypersurface isomerizing easily via the trans-bent tetrelaacetylenes HE≡CH to the thermodynamically most stable 1-tetrelavinylidenes (E=CH2). Consequently, experimental studies on 2-tetrelavinylidenes (C=ER2) and their derivatives are lacking. Herein we report experimental and theoretical studies of the first N-heterocyclic carbene (NHC) supported 2-silavinylidene (NHC)C=SiBr(Tbb) (1-Si: NHC=C[N(Dipp)CH]2, Dipp=2,6-diisopropylphenyl, Tbb=2,6-bis[bis(trimethylsilyl)methyl]-4-tert-butylphenyl) and the isovalent 2-germavinylidenes (NHC)C=GeBr(R) (1-Ge, 1-GeMind: R=Tbb, Mind (1,1,3,3,5,5,7,7-octamethyl-s-hydrindacene-4-yl)). The NHC-supported 2-tetrelavinylidenes were obtained selectively from the 1,2-dibromoditetrelenes (E)-(R)BrE=EBr(R) using the diazoolefin (NHC)CN2 as vinylidene transfer reagent. 1-E (E=Si, Ge) have a planar vinylidene core, a bent-dicoordinated vinylidene carbon atom (CVNL), a very short E=CVNL bond and an almost orthogonal orientation of the NHC five-membered ring to the vinylidene core. Quantum chemical analysis of the electronic structures of 1-E suggest a significantly bent 1-tetrelaallene and tetrelyne character. NMR studies shed light into the dynamics of 1-E involving NHC-rotation around the CVNL-CNHC bond with a low activation barrier. Furthermore, the synthetic potential of 1-E is demonstrated by the synthesis and full characterization of the unprecedented NHC-supported bromogermynes BrGe=C(EBr2Tbb)(NHC) (2-SiGe: E=Si; 2-GeGe: E=Ge).

Synthesis, structural analysis, and properties of highly twisted alkenes 13,13'-bis(dibenzo[a,i]fluorenylidene) and its derivatives

The rotation of a C = C bond in an alkene can be efficiently accelerated by creating the high-strain ground state and stabilizing the transition state of the process. Herein, the synthesis, structures, and properties of several highly twisted alkenes are comprehensively explored. A facile and practical synthetic approach to target molecules is developed. The twist angles and lengths of the central C = C bonds in these molecules are 36-58° and 1.40-1.43 Å, respectively, and confirmed by X-ray crystallography and DFT calculations. A quasi-planar molecular half with the π-extended substituents delivers a shallow rotational barrier (down to 2.35 kcal/mol), indicating that the rotation of the C = C bond is as facile as that of the aryl-aryl bond in 2-flourobiphenyl. Other versatile and unique properties of the studied compounds include a broad photoabsorption range (from 250 up to 1100 nm), a reduced HOMO-LUMO gap (1.26-1.68 eV), and a small singlet-triplet energy gap (3.65-5.68 kcal/mol).

Reversing the Bond Length Alternation Order in Conjugated Polyenes by Substituent Effects

We have synthesised several push-pull substituted conjugated polyenes and determined their accurate C-C bond lengths and charge-density distributions by utilising quantum crystallographic techniques. In a series of alkene, dienes, and triene bearing two (trifluoromethyl)sulfonyl (triflyl) groups on the terminal carbon atom, unique reversal of the bond-length alternation (BLA) order has been observed. This is a pronounced aberration from the molecular structure predicted by the Lewis-structure-based neutral resonance structure. Such reversal of BLA order has not been observed in push-pull compounds bearing conventional electron-withdrawing groups such as carbonyl and cyano groups instead of triflyl groups. Bonding behaviour of both normal and reversed bond length alternating systems has been revealed by complementary bonding analysis using several bond descriptors based on the experimentally fitted wavefunctions.

Design, Synthesis, SAR, Pharmacokinetic Prediction of New 4-Quinolones as Anti-Microbial Agents

s of new 4-quinolone derivatives was synthesized by conventional heating method. For the synthesized compounds, we performed pharmacokinetic prediction, SAR and antimicrobial assay. The presence of halogen elements plays a key role in the biological activity that is clear by in vitro analysis. Target compounds exhibit moderate to significant activity near to standard marketed drugs like amoxycillin, chloramphenicol, ciprofloxacin, norfloxacin, griseofulvin, and nystatin.

Ruthenium(II)-Catalyzed C-H (Hetero)Arylation of Alkenylic 1,n-Diazines (n = 2, 3, and 4): Scope, Mechanism, and Application in Tandem Hydrogenations

A general ruthenium(II)-catalyzed methodology enabling the (hetero)arylation of alkenylic C-H bonds utilizing a series of synthetically appealing diazines as directing groups is presented. Despite the presence of additional nitrogen lone pairs remote from the C-H bond activation site, which could eventually poison the catalyst, the reaction times are short (3 h), thus being suitable for selective double C-H bond arylation. Mixtures of E:Z isomeric products were observed in some cases, which were further hydrogenated in a tandem manner in the presence of the remaining ruthenium catalyst from the first step, representing an alternative approach to more difficult C(sp³)-H bond functionalization. According to mechanistic studies, the unexpected E:Z product formation seems to occur by thermal C═C bond isomerization after the reductive elimination step.

Quantification of the push–pull effect in 2-alkylidene-4-oxothiazolidines by using NMR spectral data and barriers to rotation around the CC bond

The rotational barriers around the CC bond are efficient parameters to quantify the strength of the push–pull effect in 2-alkylidene-4-oxothiazolidines.

Modulations in restricted amide rotation by steric induced conformational trapping

The rotation around the amide bond in N,N-diethyl-m-toluamide (m-DEET) has been studied extensively and often used in laboratory instructions to demonstrate the phenomenon of chemical exchange. Herein, we show that a simple modification to N,N-diethyl-o-toluamide (o-DEET) significantly alters the dynamics of the restricted rotation around the amide bond due to steric interactions between the ring methyl group and the two N-ethyl groups. This alters the classic two-site exchange due to restricted rotation around the amide bond, to a three-site exchange, with the third conformation trapped at a higher-energy state compared to the other two. This often overlooked phenomenon is elucidated using variable-temperature NMR, two-dimensional exchange spectroscopy and molecular modeling studies.

Use of 2-aminoprop-1-ene-1,1,3-tricarbonitrile for the synthesis of tetrahydronaphthalene, hexahydroisoquinoline and hexahydrocinnoline derivatives with potential antitumor activities

The aim of the work was to synthesize heterocyclic compounds from 2-aminoprop-1-ene-1,1,3-tricarbonitrile and to study their antitumor activities. The title reagent reacted with cyclohexanone to give the ethylidene derivative 2. The reactivity of the latter product towards different chemical reagents was studied to give tetrahydronaphthalene, hexahydroisoquinoline and hexahydrocinnoline derivatives. The newly synthesized products were screened as antitumor agents on the in vitro growth of three human tumor cell lines representing different tumor types, namely, breast adenocarcinoma (MCF-7), non-small cell lung cancer (NCI-H460) and CNS cancer (SF-268). It was found that some of these compounds showed inhibitory effects on the three cell lines, indicating their potential use in the development of oncology products.

Measurement and calculation of 13C and 15N NMR chemical-shift tensors of a push–pull ethylene

Methyl 3-dimethylamino-2-cyanocrotonate (MDACC) has a remarkably weak carbon–carbon double bond. It has strong electron-withdrawing groups on one end and electron-donating groups on the other: a so-called push–pull ethylene. To investigate this unusual electronic structure, we have determined the crystal structure and measured both the 13C and 15N NMR chemical-shift tensors. These measurements are supplemented by shielding-tensor calculations done with density functional methods. The large difference (approximately 100 ppm) between isotropic chemical shifts of the two alkenyl carbons reflects a large charge release from the electron-donating side of C=C double bond to the electron-withdrawing groups. Comparison of the calculated orientations of the principal components of the alkenyl carbons obtained from ab initio calculations shows that the primary changes in charge density occur in the molecular plane. On the other hand, smaller charge density changes above and below the plane of the C=C double bond establish the conjugation of donor and acceptor groups with π* and π molecular orbitals of the central double bond, respectively, which lowers the barrier to rotation about this bond.