Scope and limitations of the Pd(II)-catalyzed cycloisomerization of functionalized 1,5-hexadienes have been studied. In situ NMR experiments indicate a challenging competition between various reaction pathways. A careful balance between substrate structure, nature of the precatalyst, and reaction conditions was required to gain access to a useful building block for sesquiterpene total synthesis.

The Pd(II)-catalyzed cycloisomerization of 3-alkoxycarbonyl-3-hydroxy-substituted 1,5-hexadienes has been studied experimentally and computationally. Experimentally, the reaction is characterized by a rapid room temperature formation of monomeric as well as dimeric cycloisomerization products using the commercially available precatalyst [(CH(3)CN)(4)Pd](BF(4))(2). In situ NMR measurements indicate the initial kinetic advantage of the desired cycloisomerization pathway to methylene cyclopentanes; however, double bond isomerization, elimination, and dimer formation are competitive undesired pathways. Evaluation of the obtained product structures by NMR spectroscopy and X-ray crystallography indicates that the sole determinant for the monomer/dimer ratio is the regioselectivity of the initial hydropalladation in favor of the allylic (monomer formation) or the homoallylic double bond (dimer formation). In order to account for the experimental results, we propose the coexistence of two product-forming catalytic cycles, an open, monomer generating, as well as an interrupted and redirected, dimer generating, hydropalladation/carbopalladation/β-hydride elimination (HCHe) process. Results from computational studies of the proposed competing catalytic cycles are supportive to our mechanistic hypothesis and pinpoint the pivotal importance of Pd(II)-hydroxo-chelate complexes for the reactivity-stability interplay of on- and off-pathway intermediates.

Various physical measurements and quantum-mechanical computations to characterize molecular 2-propanone(2,4,6-trinitrophenyl)hydrazone, alias acetone-picrylhydrazine (AH), are reported, including an X-ray diffraction structural determination, an 1H and 13C NMR study of its internal hindered reorientation, and a theoretical (SCF-MO) interpretation of these observations. The structure of AH was determined by X-ray crystallography. The space group is Pbar over 1, with a = 10.1768(9) Å, b = 7.7968(18) Å, c = 8.0018(5) Å, α = 92.102(6)°, β = 99.919(7)°, γ = 105.926(6)°, Z = 2, wR2(F2) = 0.1995 based on all 2748 unique reflections. The (picryl) proton NMR thermal work yielded a Gibbs activation energy ΔG‡ = 46.9 ± 0.4 kJ mol-1 in acetone-d6 and 48.1 ± 0.2 kJ mol-1 in chloroform-d, whereas 13C NMR (two pairs in the picryl ring) yielded 46.6 ± 1.0 and 46.4 ± 1.0 kJ mol-1 in acetone-d6. The SCF-MO computations yielded a detailed model of the conformerization path. Various model conformations and tautomers of AH have been considered, as has removal of H+ or of H0 from its hydrazinic linkage.Key words: dynamic NMR, picrylhydrazone, hindered rotation, activation parameters, SCF-MO model.

In the title compound, C21H18Si, the coordination geometry around the Si atom is a slightly distorted tetra­hedron with C—Si—C angles in the range 106.05 (11) to 110.58 (10) ° and Si–C bond lengths in the range 1.855 (2) to 1.883 (3) Å. The alkyne C—C bond length is 1.167 (4) Å. The dihedral angles between the three phenyl rings are 63.89 (7), 86.38 (7) and 70.51 (8)°. In the crystal, mol­ecules inter­act only by van der Waals forces.

The title compound, C12H18N4O2SSi, was synthesized to be employed in a Julia–Kocieński olefination. In the mol­ecule, the dihedral angle between the phenyl ring and the tetra­zole ring is 41.50 (5)°. The significantly longer Si—C(methyl­ene) bond [1.8786 (13) Å] and the shortened adjacent C—C bond [1.5172 (18) Å], as well as the significant deviation of the corresponding Si—C—C angle [114.16 (9)°] from the ideal tetra­hedral angle, can be attributed to the β-effect of silicon. In the crystal, mol­ecules are held together by van der Waals inter­actions.