Retro Diels-Alder reactions of 5,6-disubstituted-7-oxabicyclo

Several 5,6-disubstituted-7-oxabicyclo[2.2.1]hept-2-enes (1-4) were synthesized on > or = 0.1 mol scale. The heat-induced retro Diels-Alder (rDA) decomposition of these derivatives was studied by thermal analysis, and the kinetics of the rDA were measured for 4. First-order rate constants (k = 1.91-14.2 x 10(-5) s(-1)), measured at four temperatures between 124 and 150 degrees C, were used to calculate Arrhenius activation parameters Ea (34.5 +/- 0.5 kcal/mol) and ln A (1.77 +/- 0.03 x 10(4)). The observed activation energy was significantly larger (by 9.5 kcal/mol) than that previously measured for the maleic anhydride adduct 1, and this was attributed to the difference in LUMO energies for the two dienophiles. Modeling of the activation parameters found for 4 with density functional theory (DFT) calculations for similar compounds 5 and 6 gave close quantitative correlations for deltaH double dagger, deltaG double dagger, deltaS double dagger. The rDA reactions studied were found to be entropy-driven.

Formation of 1,10-disubstituted benzo

The first 1,10-heterodisubstituted benzo[c]cinnoline derivative 1 was prepared from the trinitrobiphenyl 2. Investigation of the mechanism of ring closure in 2, 5, and 8 revealed a complex reduction-oxidation-cyclization sequence. The mechanism is discussed in light of the stereoelectronic demands of the substituent functionalities. Benzo[c]cinnoline derivative 1 [C15H15N3S, monoclinic, P2(1)/c: a = 7.4063(3) A, b = 10.3739(5) A, c = 16.7642(8) A, beta = 91.816(1) degrees, Z = 4] and its 5-N-oxide 7(N5) [C18H18N3OS, triclinic, Pi: a = 8.1510(7) A, b = 8.6106(7) A, c = 12.102(1) A, alpha = 86.262(1) degrees, beta = 83.364(1) degrees, gamma = 74.711(1) degrees, Z = 4] were structurally characterized and showed a significant helical distortion of the heterocyclic ring. Oxidation of 1 with NCS or triamine 12 with PhI(OAc)2 led to a new heterocyclic ring system, ylide 13. Both benzo[c]cinnoline 1 and ylide 13 were characterized spectroscopically and the absorption spectra were correlated with the results of ZINDO calculations.

A new thiatriazine isomer: synthesis, tautomerism, and molecular structure of 3,6-diphenyl-4H-1,2,4,5-thiatriazine as a precursor to the 1,2,4,5-thiatriazinyl radical

The diphenyl derivative of 4H-1,2,4,5-thiatriazine (5) was prepared by oxidative cyclization of 9. The molecular structure of 5, obtained by X-ray diffraction [orthorhombic, Pna2(1), a = 9.7746(13) A, b = 21.692(2) A, c = 5.6580(8) A], compares favorably with that predicted with ab initio calculations. The thiatriazine 5 was used as a precursor to the 3,6-diphenyl-1,2,4,5-thiatriazinyl radical (4) through either oxidation with PbO(2), or conversion to and reduction of sulfiminyl chloride 6 with Ph(3)Sb. The weak ESR quintet (a(N) = 1.03 mT, g = 2.0103) observed in the latter case correlates well with the molecular structure of 4, but the results of DFT calculations are ambiguous. Ab initio calculations show that 4H-1,2, 4,5-thiatriazine (I-4H) is the most stable tautomer and is the second most stable isomer among the six possible thiatriazines. All isomeric thiatriazinyl radicals exhibit similar spin distribution patterns. 1,2,4,5-Thiatriazinyl radical (I-R) is calculated to be 23. 1 kcal/mol less stable than the most stable 1,2,4,6 isomer II-R.

Triphenylsilyl as a protecting group in the synthesis of 1, 12-heterodisubstituted p-carboranes

A triphenylsilyl group is used as an auxiliary in the synthesis of heterodisubstituted p-carboranes via triphenylsilyl-p-carborane (1). The preparation of 1 is statistical, but with recovery of the starting p-carborane, the effective conversion to 1 is about 90%. Carborane 1 has been easily converted to its lithium and copper derivatives, which were reacted with a range of electrophiles including alkyl halides: an aryl iodide, an acetylene bromide, and a sulfenyl chloride. The derivatives of 1 are crystalline and UV active, which facilitates their isolation and purification. The Ph(3)Si group is efficiently removed with fluoride to give monosubstituted p-carboranes I, which upon further nucleophilic substitution yield p-carboranes II. The yields of heterodisubstituted products II are higher than for direct, "statistical" syntheses. For example, 12-pentyl-1, 12-dicarba-closo-dodecaborane-1-carboxylic acid was synthesized from p-carborane via 1 in 62% overall yield, a considerable improvement over the 44% yield obtained by direct methods.

Carborane-containing liquid crystals: synthesis and structural, conformational, thermal, and spectroscopic characterization of diheptyl and diheptynyl derivatives of p-carboranes.

Molecular and electronic structures for four p-carborane derivatives were studied in the context of their liquid crystalline properties. Thus molecular and crystal structures of diheptyl and diheptynyl derivatives of 10- and 12-vertex bi-p-carboranes were determined by X-ray crystallography and compared to the results of ab initio calculations at the HF/6-31G level of theory. Experimentally observed significant positional disorder of one of the substituents in the 10-vertex derivatives, 2[2]a and 2[2]b, was related to conformational properties of the alkyl-carborane bond. Experimental and theoretical studies of the electronic structures were conducted for the four compounds using UV and NMR spectroscopies. The nature of the unique long wavelength absorption band at 232 nm in the diheptynyl derivative 2[2]b was explained using INDO/2//HF/6-31G analysis. The complete assignment of the (13)C signals was accomplished using a long-range coupling technique and was supported by the calculated (HF/6-31G) isotropic shielding tensors. Analysis of absorption spectra, NMR substituent effects, and trends in bond lengths shows generally strong cage-acetylene electronic interactions for the 10-vertex p-carborane, while the 12-vertex p-carborane remains largely electronically isolated. Ab initio calculations revealed that 12-vertex p-carborane has significantly larger electronic polarizability and quadrupole moments than the 10-vertex analogues, which are larger than those for bicyclo[2.2.2]octane compounds. All these results on packing, conformational, and electronic properties form the basis for the discussion of thermal behavior of the four carborane compounds, bicyclo[2.2.2]octane analogues, and some related compounds.