Shuji Tomoda

Molecular Motion and Conformational Interconversion of Azobenzenes in Crystals as Studied by X-ray Diffraction

Crystal structures of (E)-azobenzene (1), (E)-2,2′- dimethylazobenzene (2), (E)-3,3′-dimethylazobenzene (3) and (E)-4,4′-dimethylazobenzene (4) were determined by X-ray diffraction at various temperatures. An apparent shrinkage of the N=N bond and its temperature dependence were observed and are interpreted in terms of an artifact caused by the torsional vibration of the N—Ph bonds in crystals. In the crystal structures of (1), (3) and (4) the dynamic disorder was observed. The disorder is accounted for by the torsional vibration whose amplitude is large enough to give rise to the conformational interconversion. No disorder was observed for a crystal of (2). This is ascribed to the large difference in energy of the two conformers as free molecules. The true length of the N=N bond in azobenzenes was estimated to be 1.26–1.27 A.

Design of optically active selenium reagents having a chiral tertiary amino group and their application to asymmetric inter- and intramolecular oxyselenenylations

Abstract A new class of chiral selenium reagents 4–7 was synthesized on the basis of the concept that the strong intramolecular interaction between an electrophilic selenium and an optically modified tertiary amine (Se…N interaction) would induce asymmetric induction in the reaction between the selenium reagent and olefins. When 7, which showed the most powerful asymmetric induction, was applied to asymmetric methoxyselenenylation of (E)-phenylpropene under optimum reaction conditions, the highest diastereomeric excess (d.e.) of the trans-addition product was obtained (97 % d.e.). The same reaction conditions were employed in asymmetric methoxyselenenylation of various olefins (up to 97 % d.e.) and also in asymmetric intramolecular oxyselenenylation of terminal olefins (up to 59 % d.e.) and internal olefins (up to 98 % d.e.). The results show that this class of selenium reagents with a strong Se…N interaction is useful for asymmetric organic synthesis.

Synthesis of conjugated nitroalkenes via nitroselenenylation of alkenes

Abstract Addition of silver nitrite to 2-bromoalkyl phenyl selenide in the presence of mercury(II) chloride afforded 2-nitroalkyl phenyl selenide, which upon oxidative deselenenylation provided the conjugated nitroalkene in excellent yield.

Direct Observation of Intramolecular Interaction between a Divalent Selenium and a Tertiary Amine by Means of Single Crystal X-Ray Analysis and NMR Spectroscopy

Abstract Intramolecular interaction between a divalent selenium and a tertiary amino moiety has been observed in the solid state for 2,2′-diselenobis(N-cyclohexyl-N-methylbenzyl amine) (1). A similar interaction between an electrophilic selenium and a tertiary amine has also been observed in solution for the corresponding selenenyl bromide (6).

Theoretical study on the mechanism and diastereoselectivity of NaBH4 reduction.

As the first step to understand the reaction mechanism and diastereoselectivity of sodium borohydride reduction of ketones, ab initio Car-Parrinello molecular dynamics simulation has been performed on a solution of NaBH4 in liquid methanol. According to pointwise thermodynamic integration involving constrained molecular dynamics simulations, it was strongly suggested that Na+ and BH4(-) are associated in the solvent forming contact ion pairs. Thus we propose a new transition state structure model that contains complexation of the carbonyl oxygen with sodium cation. Predicted diastereoselectivity of the reduction of some substituted cyclohexanones applying this novel transition state model is in good agreement with experimental data, showing its validity and effectiveness to investigate the diastereoselectivity of NaBH4 reduction of other ketones.

Asymmetric methoxyselenenylation of olefins using an optically active diaryl diselenide derived from d-mannitol

Abstract The diastereomeric excess (d.e.) in asymmetric methoxyselenenylation of trans - β -methylstyrene was significantly enhanced by employing the selenohexafluoro-phosphate of an optically active diaryl diselenide derived from d -mannitol (97 %d.e.). The procedure was applied to the asymmetric methoxyselenenylation of various olefins.

Origin of π-facial stereoselectivity of hydride reduction of cyclohexanones. A new interpretation based on quantitative analysis of exterior frontier orbital extension

Abstract A new theoretical interpretation for π-facial stereoselectivity of hydride reduction of cyclohexanones has been proposed based on the simple assumption that the lowest unoccupied molecular orbital (LUMO) of the substrates should play an essential role in the reaction: π-Facial stereoselectivity is described by relative magnitude of e xterior f rontier o rbital e xtension (EFOE) of the LUMO above or below the carbonyl plane, which is defined by integrated “hole density” of the LUMO at the carbonyl carbon over three-dimensional spatial points outside repulsive molecular surface, where the absolute total value of the gaussian functions belonging to the carbonyl carbon makes the maximum contribution to the total value of the LUMO wave function so that the driving force vector on hydride is maximally directed toward the carbonyl carbon. A good linear correlation is observed for ten alkylsubstituted cyclohexanones (NaBH4 in 2-propanol) between the difference in the square of EFOE densities for the axial (EFOE(ax)) and equatorial (EFOE(eq)) areas (λ = EFOE(ax)2 − EFOE(eq)2) and the difference in activation enthalpy for equatorial ( ΔH ‡ eq ) and axial ( ΔH ‡ ax ) attack ( ΔΔH ‡ = ΔH ‡ eq − ΔH ‡ ax ).

Structural characteristics of areneselenenyl bromide and areneselenenyl chloride stabilized by hypervalent coordination with a halide anion in the solid state

Abstract Solid-state molecular structures of 2-[(N-cyclohexyl-N-methylamino)methyl]benzeneselenenyl bromide (ArSeBr) and chloride (ArSeCl), stabilized by hypervalent coordination with a halide anion (Br− or Cl−, respectively), were determined by X-ray diffraction method. By comparing bond parameters of the observed hypervalent BrSe⋯Br fragment with those reported previously for other organoselenium compounds, which have a similar T-shaped selenium fragment, it was revealed that there is decent hyperbolic relationship between the two linear SeBr atomic distances in the solid state, reflecting the possibility of a pathway for the addition reaction of benzeneselenenyl bromide (PhSeBr) in solution. In addition, the existence of significant bond-shortening (∼0.1 A) due to intermolecular weak packing interactions was strongly suggested by comparison of the extrapolated SeBr covalent bond length (2.220 A) in the solid state with that reported for PhSeBr (2.325 A) in the gas phase. Ab initio molecular orbital calculations at the RHF/6-31G(d,p) level using Ahlrichs pVDZ basis sets for Se and Br estimated the perturbation energy in the solid state to be roughly 1.5 kcal mol−1 independent of the packing structure.

Origin of π-facial stereoselection in nucleophilic additions of adamantanones. A new interpretation based on the exterior frontier orbital extension model

The experimental data of π-facial stereoselection of 5-substituted adamantan-2-ones, 5-aza-adamantan-2-ones, and nor- and homo-adamantanones have been successfully rationalized by the exterior frontier orbital extension model (EFOE model). The values of π-plane-divided accessible space (PDAS), which represents simple summation of the π-plane-divided exterior three-dimensional space nearest to the reaction center outside the van der Waals surface, showed that the adamantanone system is sterically biased and sterically much more demanding than cyclohexanone. The PDAS values indicated that in most cases steric effects are responsible for π-facial stereoselection except for a series of 5-aryladamantan-2-ones, the facial stereoselection of which was found to be orbital-controlled. In most cases, the facial differences in the frontier orbital (LUMO) extension as quantified by the EFOE density were marginal, but were consistent with experimental stereoselectivity except for a few cases with electron-withdrawing substituents. The structures of the transition states of a few 5-substituted adamantan-2-ones with LiAlH4 (B3LYP/6−31+G(d)) have shown that (1) hyperconjugative anti-periplanar stabilization effects involving an incipient bond (AP effects) are much larger in adamantan-2-one than in cyclohexanone, in spite of enhanced reactivity with hydride of the latter, (2) the facial differences of AP effects are marginal and (3) they often operate against observed stereoselectivity.

Origin of π-facial stereoselectivity of nucleophilic addition to carbonyl compounds. Application of the exterior frontier orbital extension model to cyclohexanones with polar substituent

Abstract The application of the exterior frontier orbital extension model (EFOE model) to π-facial stereoselection of cyclohexanones with a polar substituent at C3 or C4 have been described. The theoretical prediction that a linear correlation should exist between the π-facial ( axial and equatorial ) difference in the square of the EFOE densities ( λ = EFOE( ax ) 2 − EFOE( eq ) 2 ) and experimentally obtained facial selectivity (ln( ax/eq )) has been demonstrated for 5-substituted 2-decalones ( 1 ), 4-substituted cyclohexanones ( 2 ) and 3-substituted cyclohexanones ( 3 ). In addition, facial stereoselection of 3 , where the substituent is located closer to the reaction center compared with those of 1 or 2 , showed remarkable dependence on steric effects evaluated by the π-plane-divided accessible space (PDAS), another important quantity of the EFOE model, which represents simple summation of the π-plane-divided exterior three-dimensional space nearest to the reaction center outside the van der Waals surface of the substrate.