[1,5] Sigmatropic shifts in a 1,4-diphospholylbenzene

The central phenylene ring of 1,4-bis(3,4-dimethylphosphol-1-yl)benzene undergoes [1,5] shifts around both phospholyl rings above 150°C to give 1,4-bis(3,4-dimethyl-5H-phosphol-2-yl)benzene, which can be trapped by tolane, [CpFe(CO)₂]₂, or Mn₂ (CO)₁₀ to yield the corresponding bis-1-phosphanor-bornadiene, bis-phosphaferrocene, or bis-phosphacymantrene respectively.     

Sigmatropic hydrogen shifts in aryltetraphenylcyclopentadienes

Dynamic NMR has revealed intramolecular migrations of hydrogen atom over the periphery of the five-membered ring in 5-(p-tolyl)-1,2,3,4-tetraphenylcyclopentadiene in a deuteronitrobenzene solution with energy barrier ΔG ₁₈₀ ^≠ = 24.8 kcal/mol. Quantum-chemical DFT calculations B3LYP/6-311++G** have shown that such migrations in 1,2,3,4,5-pentaphenylcyclopentadiene in the gas phase occur in a chiral conformation of propeller type by the mechanism of 1,5-sigmatropic hydrogen shifts with retention of configuration through asymmetric transition state with energy barrier ΔE _ZPE ^≠ = 25.9 kcal/mol. Enantiomers P and M can readily interconvert into each other (ΔE _ZPE ^≠ = 3.9 kcal/mol) owing to synchronous flip rotations of the phenyl groups.     

Gas-phase kinetics and activation parameters for thermal [1,5] hydrogen shifts interconverting monodeuterium- labeled 1,3-cycloheptadienes

The kinetics of thermal equilibrations among monodeuterium-labeled 1,3-cycloheptadienes in the gas phase followed from 154 to 190 degrees C provide activation parameters for the [1,5] shift of a single hydrogen: E(a) = (27.5 +/- 0.9) kcal/mol and log A = 9.7 [corrected] +/- 0.4. These activation parameters imply a comparatively low E(a) barrier balanced by demandingly specific geometric constraints, for DeltaS (170 degrees C) = -17 [corrected] e.u.     

Thermal [1,5] hydrogen sigmatropic shifts in cis,cis-1,3-cyclononadienes probed by gas-phase kinetic studies and density functional theory calculations

The kinetics of gas-phase thermal [1,5] hydrogen shifts interconverting the five isomeric mono-deuterium-labeled cis,cis-1,3-cyclononadienes have been followed at four temperatures from 240 to 287 degrees C. The activation parameters found were Ea = 37.1 +/- 0.8 kcal/mol, log A = 11.6 +/- 0.3, DeltaH++ = 36.0 +/- 0.8 kcal/mol, and DeltaS++ = -9.0 +/- 0.3 eu. Density functional theory based calculations have provided geometries and energies for the ground-state cyclononadiene conformational isomers, for the transition states linking one to another, and for the transition states for [1,5] hydrogen shifts responsible for isomerizations among the five labeled dienes. A generalized formulation of the Winstein-Holness equation is presented and applied to the complex system, one that involves 11 ground-state conformers, 10 transition states separating them, and five transition states for [1,5] hydrogen shifts. The value for the empirical Ea derived from calculated mole fractions of ground-state conformers and calculated energies for specific ground-state conformers and [1,5] hydrogen shift transition structures was 37.5 kcal/mol, in excellent agreement with the experimentally obtained activation energy. The significance of conformational options in various ground states and transition structures for the [1,5] hydrogen shifts is considerable, an inference that may well have general applicability.     

Sulfoxide-induced stereoselection in [1,5]-sigmatropic hydrogen shifts of vinylallenes. A computational study

The sulfoxide-induced preference for a migrating trajectory in the vinylallene [1,5]-H sigmatropic shift (resulting in stereodefined trienes in the conceptual equivalent of torquoselectivity in electrocyclizations), originally reported by Okamura, has been computationally studied at the B3LYP/6-311++G(3df,2p)//B3LYP/6-31++G(d,p) level. The face selectivity this group induces in the [1,5]-H shift is enhanced by bulky geminal substituents and is not reproduced by any of the other (more than 20) substituents tested. Analysis of transition-state geometries or charges and evaluation of steric effects did not show any correlation with the preferences. The origin of this selectivity is thought to lie in a secondary orbital interaction (SOI) involving the termini of the pericyclic array and the sulfinyl group which is only observed for this substituent. This secondary orbital interaction, arising from the favorable energies of the orbitals involved, is enhanced in the transition structure due to a better orbital overlap (piC2-C3-->sigma*C1-S), which correlates with a piC2-C3-->sigma*C6-H SOI, which is more important in the transition structure, that weakens the C-H bond, thus lowering the energy of the corresponding transition structure.     

Synthesis of 1-nitro-1,3-dienes via nitrotrifluoroacetoxylation of 1,3-dienes

Nitrotrifluoroacetate adducts, obtained from 1,3-dienes by reaction with ammonium nitrate in trifluoroacetic anhydride, readily eliminate to afford 1-nitro-1,3-dienes in high yield.     

Synthesis of Chiral Pyrazoles: A 1,3‐Dipolar Cycloaddition/[1,5] Sigmatropic Rearrangement with Stereoretentive Migration of a Stereogenic Group

The reactions between terminal alkynes and α‐chiral tosylhydrazones lead to the obtention of chiral pyrazoles with a stereogenic group directly attached at a nitrogen atom. The cascade reaction includes decomposition of the hydrazone into a diazocompound, 1,3‐dipolar cycloaddition of the diazo compound with the alkyne, and [1,5] sigmatropic rearrangement with migration of the stereogenic group. This strategy has been successfully applied to the synthesis of structurally diverse chiral pyrazoles through α‐chiral tosylhydrazones, obtained from α‐phenylpropionic acid, α‐amino acids, and 2‐methoxycyclohexanone. Notably, the stereoretention of the [1,5] sigmatropic rearrangements represent very rare examples of this stereospecific transformation.     

Hexachloroacetone as a precursor for a tetrachloro-substituted oxyallyl intermediate: [4+3] cycloaddition to cyclic 1,3-dienes

The enol phosphate 2,2-dichloro-1-(trichloromethyl)ethenyl diethyl phosphate (1), easily available by a Perkow reaction between hexachloroacetone and triethyl phosphite, reacts with sodium trifluoroethoxide/trifluoroethanol in the presence of cyclic 5-membered 1,3-dienes to furnish alpha,alpha,alpha',alpha'-tetrachloro-substituted[3.2.1]bicyclic ketones 2. A [4+3] cycloaddition of a tetrachloro-oxyallyl intermediate is postulated.     

Enantioselective silicon-boron additions to cyclic 1,3-dienes catalyzed by the platinum group metal complexes

Silaborations of 1,3-cyclohexadiene and 1,3-cycloheptadiene were achieved using catalysts prepared from different combinations of phosphorus ligands and group 10 metal compounds. For the six-membered compound, 1,4-adducts with up to 82% ee were obtained employing Pt(0) and phosphoramidite ligands. For the seven-membered diene optimal conditions were found using catalysts based on Ni(0), but the highest selectivity observed was merely 22% ee. No improvement of the chiral induction was obtained using chiral silylboranes in combination with chiral phosphoramidite ligands in the additions to 1,3-cyclohexadiene. The adduct obtained from cyclohexadiene was used in allylborations of aldehydes under microwave irradiation to produce homoallylic alcohols with moderate to good diastereoselectivity.     

1,2 Asymmetric induction in the cyclic hydroboration of 1,5-dienes. A synthesis of the Prelog-Djerassi lactone

Cyclic hydroboration of an appropriately substituted 1,5-diene yields a 1,5 diol as the major product which can be converted to the racemic Prelog-Djerassi lactone.     

Hydricity-promoted [1,5]-H shifts in acetalic ketenimines and carbodiimides

2-monosubstituted 1,3-dioxolanes and dithiolanes act as hydride-releasing fragments, transferring intramolecularly their acetalic H atom to the central carbon of ketenimine functions. The presumed products of these migrations, o-quinomethanimines, undergo in situ 6pi-electrocyclization. A computational study supports this mechanism and the hydride-shift character of the first step. Carbodiimides were also suitable substrates, although less reactive. [reaction: see text].     

The coordination and polymerisation of cyclic 1,3-dienes by gold(I) cations

Cyclopentadiene and 1,3-cyclohexadiene are readily polymerised by [LAu][X]. With specific ancillary ligands polymerisation was suppressed and a molecular species involving an Au(I) cation η(2)-bonding CpH was isolated and whose structure was probed in both the solid state and in solution.