Theoretical Study of the Mechanism of Cycloaddition Reaction between Silylene Silylene (H2SiSi:) and Acetone

The mechanism of the cycloaddition reaction between singlet silylene silylene (H₂Si?Si:) and acetone has been investigated with the CCSD (T)//MP2/6‐31G?? method. According to the potential energy profile, it can be predicted that the reaction has two competitive dominant reaction channels. The present rule of this reaction is that the [2+2] cycloaddition reaction of the two π‐bonds in silylene silylene (H₂Si?Si:) and acetone leads to the formation of a four‐membered ring silylene (E3). Because of the unsaturated property of Si: atom in E3, it further reacts with acetone to form a silicic bis‐heterocyclic compound (P7). Simultaneously, the ring strain of the four‐membered ring silylene (E3) makes it isomerize to a twisted four‐membered ring product (P4).     

A new synthetic route to cyclophosphadithiatriazenes: synthesis and X‐ray structural characterization of the first spirocycle containing thiadiazaphosphetidine and phosphadithiatriazene heterocycles

The phosphetidine 2,4‐Di‐tert‐butyl‐3‐chloro‐1λ⁶‐thia‐2,4‐diaza‐3‐phosphetidine‐1,1‐dioxide, O₂S (^tBuN)₂PCl, reacts with tetrasulfur tetranitride, S₄N₄, in benzene under reflux to afford the novel 4,6‐spirocycle in moderate yield. The deep‐blue crystals of the spirocycle are airstable and high melting in nature. The spiro phosphorus atom subtends a four‐membered P^VS^VIN₂ ring which is saturated, and a six‐membered P^VS N₃ ring which is unsaturated. The single‐crystal X‐ray structure of this first example of the spirocycle reveals a planar PSN₂ ring and a puckered PS₂N₃ ring and the molecule is symmetric in nature. Copyright © 2006 John Wiley & Sons, Ltd.     

Ring strain energies: substituted rings, norbornanes, norbornenes and norbornadienes

Ring strain energies (RSEs) are predicted using homodesmotic reactions at the B3LYP/6-31G^* level of theory. Substituents are conserved in the acyclic reference and any difference in energy between the ring and the acyclic reference corresponds exclusively to RSE. Small rings are stabilized by alkyl substituents and this stabilization decreases as the size of the ring increases. There is a destabilization of medium sized rings. Greater stabilization is found upon alkyl substitution at a double bond in an unsaturated ring and this stabilization decreases as ring size increases. The effects of cis-1,2-disubstitution on RSEs have been evaluated and indicate stabilization for both small and medium sized rings. RSEs of saturated and unsaturated polycyclic systems agree well with the RSEs derived from experimental thermochemical data. RSEs are reported for substituted norbornanes, norbornenes, and norbornadienes to complement experimental studies.     

Ab initio Study of Mechanism of Cycloaddition Reaction between Germylene Silylene (H2GeSi:) and Acetone

The mechanism of the cycloaddition reaction between singlet germylene silylene (H₂GeSi:) and acetone has been investigated with CCSD(T)/6‐31G*//MP2/6‐31G* method. From the potential energy profile, we could predict that the reaction has two competitive dominant reaction channels. The present rule of this reaction is that the [2+2] cycloaddition reaction of the two (‐bonds in germylene silylene and acetone generates a four‐membered ring silylene with Ge. Because of the unsaturated property of Si atom in the four‐membered ring silylene with Ge, it could further react with acetone, resulting in the generation of a bis‐heterocyclic compound with Si and Ge. Simultaneously, the ring strain of the four‐membered ring silylene with Ge makes it isomerize to a twisted four‐membered ring product.     

(Z)‐2‐[(1‐Phenylsulfonyl‐1H‐indol‐3‐yl)methylene]‐1‐azabicyclo[2.2.2]octan‐3‐one semicarbazone

In crystals of the title compound, C₂₃H₂₃N₅O₃S, the indole system is planar and the phenyl ring of the phenylsulfonyl group makes a dihedral angle with the best plane of the indole system of 77.18 (4)°. The olefinic bond connecting the azabicyclic and indole systems has Z geometry. The geometry adopted by the C=O bond with respect to the N—N bond is trans. The O atom of the carbonyl group of each molecule is hydrogen bonded to the hydrazidic H atom of an adjacent molecule to form an eight‐membered‐ring dimeric structure.     

2,2′‐Anhydro‐1‐(3′,5′‐di‐O‐acetyl‐β‐D‐arabinofuranosyl)uracil,a cyclouridine nucleoside with a C4′‐endo furanosyl conformation

2,2′‐Anhydro‐1‐(3′,5′‐di‐O‐acetyl‐β‐D‐arabinofuranosyl)uracil, C₁₃H₁₄N₂O₇, was obtained by refluxing 2′,3′‐O‐(methoxymethylene)uridine in acetic anhydride. The structure exhibits a nearly perfect C4′‐endo (⁴E) conformation. The best four‐atom plane of the five‐membered furanose ring is O—C—C—C, involving the C atoms of the fused five‐membered oxazolidine ring, and the torsion angle is only −0.4 (2)°. The oxazolidine ring is essentially coplanar with the six‐membered uracil ring [r.m.s. deviation = 0.012 (5) Å and dihedral angle = −3.2 (3)°]. The conformation at the exocyclic C—C bond is gauche–trans which is stabilized by various C—H...π and C—O...π interactions.     

Adduct Formation,B−H Activation and Ring Expansion at Room Temperature from Reactions of HBcat with NHCs

We report the reactions of catecholborane (HBcat; 1 ) with unsaturated and saturated NHCs as well as CAAC^Me. Mono‐NHC adducts of the type HBcat?NHC (NHC=nPr₂Im, iPr₂Im, iPr₂Im^Me, and Dipp₂Im) were obtained by stoichiometric reactions of HBcat with the unsaturated NHCs. The reaction of CAAC^Me with HBcat yielded the B?H activated product CAAC^Me(H)Bcat via insertion of the carbine‐carbon atom into the B?H bond. The saturated NHC Dipp₂SIm reacted in a 2:2 ratio yielding an NHC ring‐expanded product at room temperature forming a six‐membered ?B?C=N?C=C?N? ring via C?N bond cleavage and further migration of the hydrides from two HBcat molecules to the former carbene‐carbon atom.     

A study of NMR parameters of cyclobutenones and benzocyclobutenones

An NMR study of ketones 5–12 was undertaken to gain insight into the low electrophilicity of the carbonyl moiety of butenones 9–11. Initial IR studies on compounds 9–12 indicated that there is relatively strong double bond character (and hence low electrophilicity) in the carbonyl of saturated and unsaturated cyclobutyl ketones. The ¹³C chemical shifts confirm that the carbonyl moiety is highly conjugated with the fused benzene ring in 9, and with the olefinic linkage in 10 and 11. Partial positive charge is distributed away from the carbonyl carbon, which is also expected to lower the electrophilicity of the carbonyl carbon atoms of 9–11. One‐bond carbon–proton coupling constants (¹J_CH) depend directly on carbon hybridization. In the four‐membered ring ketones 9–12 the experimental values are larger than in cyclobutane, probably as a result of the additional strain of the extra trigonal centers in the ring. A similar trend is seen in the case of the olefinic CH in 10 and 11 (ca 175 Hz), for which the coupling constant is larger than for the corresponding carbon in cyclobutene. ¹J_CC values between the ring carbon atoms of the cyclobutenones are some 20% lower than in the models—a bigger difference than in cyclobutanes, again indicative of the increased ring strain. The very low 42.4 Hz coupling between C‐1 and C‐2 in 9 might well indicate a measure of bond localization. ²J_CC and ³J_CC values are also discussed. Copyright © 2003 John Wiley & Sons, Ltd.     

Thermochemistry of N‐heterocyclic carbenes with 5‐, 4‐, 3‐, and 2‐membered rings

N‐heterocyclic carbenes (NHCs) based on imidazole‐2‐ylidene ( 1 ) or the saturated imidazolidine‐2‐ylidene ( 2 ) scaffolds are long‐lived singlet carbenes. Both benefit from inductive stabilization of the sigma lone pair on carbon by neighboring N atoms and delocalization of the N pi lone pairs into the nominally vacant p‐pi atomic orbital at the carbene carbon. With thermochemical schemes G4 and CBS‐QB3, we estimate the relative thermodynamic stabilization of smaller ring carbenes and acyclic species which may share the keys to NHC stability. These include four‐membered ring systems incorporating the carbene center, two trivalent N centers, and either a boron or a phosphorus atom to complete the ring. Amino‐substituted cyclopropenylidenes have been reported but three‐membered rings containing the carbene center and two N atoms are not known. Our calculations suggest that amino‐substituted cyclopropenylidenes are comparable in stability to the four‐membered NHCs but that diazacyclopropanylidenes would be substantially less effectively stabilized. Concluding the series are acyclic carbenes with and without neighboring N atoms and a series of “two‐membered ring” azapropadienenylidene cations of form :C?N?W with W = an electron‐withdrawing agent. We have studied W = NO₂, CH₂(+), CF₂(+), and (CN)₂C(+). Although these systems display a degree of stabilization and carbene‐like electronic structure, the stability of the NHCs is unsurpassed. © 2014 Wiley Periodicals, Inc.     

N‐Cinnamo­ylsaccharin

The title compound [systematic name: 2‐cinnamoyl‐1,2‐benzisothiazol‐3(2H)‐one 1,1‐dioxide], C₁₆H₁₁NO₄S, contains both saccharin and cinnamo­yl groups. The mol­ecule is approximately planar in the solid state, and adjacent mol­ecules are connected by C—H·O and C—H·π(phen­yl) inter­actions. In the C—H·π inter­action, the C·CgA distance is 3.916 (4) Å (CgA is the non‐fused benzene ring centroid) and the C—H·π angle is 156 (2)°. A feature of the mol­ecular geometry is the narrow C—S—N angle of 92.51 (9)° in the five‐membered ring. This angle relieves strain from the ring and makes it possible for the whole saccharin group to become quite planar.     

The Driving Force for the Acylation of β-Lactam Antibiotics by L,D-Transpeptidase 2: Quantum Mechanics/Molecular Mechanics (QM/MM) Study

β-lactam antibiotics, which are used to treat infectious diseases, are currently the most widely used class of antibiotics. This study focused on the chemical reactivity of five- and six-membered ring systems attached to the β-lactam ring. The ring strain energy (RSE), force constant (FC) of amide (C−N), acylation transition states and second-order perturbation stabilization energies of 13 basic structural units of β-lactam derivatives were computed using the M06-2X and G3/B3LYP multistep method. In the ring strain calculations, an isodesmic reaction scheme was used to obtain the total energies. RSE is relatively greater in the five-(1a–2c) compared to the six-membered ring systems except for 4b, which gives a RSE that is comparable to five-membered ring lactams. These variations were also observed in the calculated inter-atomic amide bond distances (C−N), which is why the six-membered ring lactams C−N bond are more rigid than those with five-membered ring lactams. The calculated ΔG^# values from the acylation reaction of the lactams (involving the S−H group of the cysteine active residue from L,D transpeptidase 2) revealed a faster rate of C−N cleavage in the five-membered ring lactams especially in the 1–2 derivatives (17.58 kcal mol⁻¹). This observation is also reflected in the calculated amide bond force constant (1.26 mDyn/A) indicating a weaker bond strength, suggesting that electronic factors (electron delocalization) play more of a role on reactivity of the β-lactam ring, than ring strain.     

Borane–Lewis Base Complexes as Homolytic Hydrogen Atom Donors

Radical stabilization energies (RSE)s have been calculated for a variety of boryl radicals complexed to Lewis bases at the G3(MP2)‐RAD level of theory. These are referenced to the B? H bond dissociation energy (BDE) in BH₃ determined at W4.3 level. High RSE values (and thus low BDE(B? H) values) have been found for borane complexes of a variety of five‐ and six‐membered ring heterocycles. Variations of RSE values have been correlated with the strength of Lewis acid–Lewis base complex formation at the boryl radical stage. The analysis of charge‐ and spin‐density distributions shows that spin delocalization in the boryl radical complexes constitutes one of the mechanisms of radical stabilization.     

[2‐(Isopropylthio)benzoato‐O,S]phenyl(triphenylphosphine)palladium(II) tetrahydrofuran solvate

In the title compound, [Pd(C₆H₅)(C₁₀H₁₁O₂S)(C₁₈H₁₅P)]·C₄H₈O, an iso­propyl­thio­benzoate ligand is coordinated in a bidentate fashion to the central Pd atom through the O and S atoms. The square‐planar geometry of the Pd atom is completed by a phenyl ligand and a tri­phenyl­phosphine group but is distorted by the bidentate ligand, which forms a six‐membered chelate ring. This ring deviates strongly from planarity, which is illustrated by the plane through the phenyl portion of the chelate ring forming a dihedral angle of 62.3 (1)° with the coordination plane.     

Crystallographic report: Bis{µ‐[O‐cyclopentyl(4‐methoxyphenyl)dithiophosphonato]1κ:S,2κ:S‐[O‐cyclopentyl(4‐methoxyphenyl)dithiophosphonato]‐1κ2S,S′}dicadmium(II)

The centrosymmetric title compound, [Cd₂{CH₃OC₆H₄P(OC₅H₉)S₂}₄], features an eight‐membered [? Cd? S? P? S? ]₂ ring owing to the presence of bridging dithiolate ligands. Tetrahedral coordination geometries for cadmium are completed by chelating ligands. Copyright © 2004 John Wiley & Sons, Ltd.     

The geometry of small rings—V : Geometric variations and hybridization in three-membered heretocycles C2X (X=N,O, Si,P, S) and CXY (X,Y=N,O)

Structural data relating to 369 organic derivatives of C₂X(X=N, O, Si, P, S) and CXY(X, Y=N, O) heterocycles have been retrieved from the Cambridge Crystallogrphic Database and analysed in conjuction with pertinent gas-phase results. Heterocycle geometries are compared with each other, and with those for the ‘parent’ carbocycles cyclopropane and cyclopropene. For saturated C₂X rings the previously observed (gas phase) decrease in CC bond length (d_cc) and bent-back angle (γ) with increasing heteroatom electronegativity (χ_x) are confirmed as linear relationships using mean solid state geometry for X=C, N, O, S. The CX bonds shows an effective increase in length with increasing χ_x, in line with their facile cleavage in ring opening reactions. A model for hybridization changes at C in saturated C₂X rings is derived empirically and is in broad agreement with theoretical studies. There is no evidence for geometric variations in the heterocyclic rings induced by π-acceptor substituents, but π-donor substituent effects are directly comparable to those occurring in cyclopropane and cyclopropene. Geometric variations in unsaturated heterocycles are analogous to those in cyclopropene derivatives; CC double bond lengths in available C₂X systems appear to indicate a π_x dependence. Heteroatom-heteroatom bonds in CYX systems are weak, with N weaker than NN, in agreement with thermochemical reasoning.     

Synthesis and Spectroscopic Study of Some New Phosphoramidates,Crystal Structures of N‐Benzoyl‐N′,N″‐bis(azetidinyl)phosphoric Triamide and N‐Benzoyl‐N′,N″‐bis(hexamethylenyl)phosphoric Triamide

Some new N‐carbonyl, phosphoramidates with formula C₆H₅C(O)N(H)P(O)R₂ (R = NC₃H₆ ( 1 ), NC₆H₁₂ ( 2 ), NHCH₂CH=CH₂ ( 3 ), N(C₃H₇)₂ ( 4 )) and CCl₃C(O)N(H)P(O)R′₂ (R′ = NC₃H₆ ( 5 ), NHCH₂CH=CH₂ ( 6 )) were synthesized and characterized by ¹H, ¹³C, ³¹P NMR and IR spectroscopy and elemental analysis. The structures were determined for compounds 1 and 2 . Compound 1 exists as two crystallographically independent molecules in crystal lattice. Both compounds 1 and 2 produced dimeric aggregates via intermolecular ‐P=O…H‐N‐ hydrogen bonds, which in compound 2 is a centrosymmetric dimer. In compounds with four‐membered ring amine groups, ³J(P,C)>²J(P,C), in agreement with our previous studies about five‐membered ring amine groups. Also, ³J(P,C) values in compounds 1 and 5 are greater than in compounds with five‐, six‐ and seven‐membered ring amine groups.     

Two isomeric 2‐[4‐chloro‐2‐fluoro‐5‐(prop‐2‐ynyloxy)phenyl]hexahydro­isoindole‐1,3‐dione compounds

The molecular structures of 2‐[4‐chloro‐2‐fluoro‐5‐(prop‐2‐ynyloxy)phenyl]‐1,3,4,5,6,7‐hexahydro­isoindole‐1,3‐dione, C₁₇H₁₃ClFNO₃, (I), and the isomeric compound 2‐[4‐chloro‐2‐fluoro‐5‐(prop‐2‐ynyloxy)phenyl]‐cis‐1,3,3a,4,7,7a‐hexahydro­isoindole‐1,3‐dione, (II), are, as anticipated, significantly different in their conformations and in the distances between the farthest two atoms. The six‐membered ring of the 1,3,4,5,6,7‐hexahydro­isoindole‐1,3‐dione moiety in (I) adopts a half‐chair conformation. The dihedral angle between the five‐membered dione ring of (I) and the benzene ring is 50.96 (7)°. The six‐membered ring of the cis‐1,3,3a,4,7,7a‐hexahydro­isoindole‐1,3‐dione moiety in (II) adopts a boat conformation. The dihedral angle in (II) between the five‐membered dione ring and the benzene ring is 61.03 (13)°. In the crystal structures, the molecules are linked by C—H⋯O hydrogen bonds and weak π–π interactions. Compound (I) is a much more potent herbicide than (II). The Cl⋯H distances between the farthest two atoms in (I) and (II) are 11.37 and 9.97 Å, respectively.     

Redetermination of 4‐(phenyl)thio­semicarbazide

The structure of the title compound, C₇H₉N₃S, comprises twisted mol­ecules that associate via N—H?N and N—H?S hydrogen‐bonding interactions. The dihedral angle between the phenyl ring and the five‐membered thio­semicarbazide plane is 67.56 (5)°. This structure was previously determined using data from visual estimation of photographic intensity (measurements at room temperature, with R = 0.112).     

New silicon-mediated ring expansion of n-sized conjugated cycloalkenones into homoallylic n+3 lactones

Silicon nucleophilic β-addition to various 2-cycloalkenones, followed ultimately by mild and rapid α-alkylation of the corresponding cycloalkanone enolates using diverse epoxides and BF₃·OEt₂, produces useful γ-lactols and γ-hydroxyketones. Hypervalent iodine-promoted oxidative fragmentation then yields regiospecifically unsaturated, 3-atom ring expanded, 8-10 membered homoallylic lactones with good control of alkene geometry.     

Large‐ring P/O chelate nickel complex catalyzed oligomerization of ethylene to linear α‐olefins

1,1‐(Bicyclononyl‐9‐phosphino)hendecanoic acid and potassium 1,1‐(biscyclohexylphosphino)­hendecylate were synthesized. A model nickel complex [η³−C₈H₁₃]Ni[(C₈H₁₄)P(CH₂)₁₀COO] containing a 14‐membered chelate ring was also synthesized. The catalytic activity of this large chelate ring nickel complex for the oligomerization of ethylene was studied and compared with that of six‐membered ring chelate nickel complexes. The influence of the chelate ring was rationalized in terms of intramolecular rotation. The 14‐membered ring P/O chelate nickel complex was shown to have efficient catalytic activity for the oligomerization of ethylene to α‐olefins. Copyright © 2000 John Wiley & Sons, Ltd.