Richard Vaughan Williams

Semibullvalenes and Related Molecules: Ever Closer Approaches to Neutral Homoaromaticity

Since the first synthesis of semibullvalene by Zimmerman et al. more than three decades ago, this molecular system has been the focus of extensive study. A major factor in the continuing fascination with semibullvalenes is the potential for realizing a neutral homoaromatic ground state species. Dewar-Hoffmann electronic stabilization of the transition state of barbaralanes and semibullvalenes has been very successfully applied, principally by Quast, to yield negligible barriers to their Cope rearrangement. In our work we are attempting to “eliminate” the barrier to the Cope rearrangement of semibullvalenes through strain mediated destabilization of the localized forms. We predict, from the results of high level quantum mechanical calculations, that small ring annelation of the semibullvalene nucleus will result in homoaromatic ground state species. Experimentally, our small series of bisannelated semibullvalenes have proved to be localized Cope systems with exceptionally low activation barriers to the Cope rearrangement in the condensed phases.

Measuring aromaticity with the dimethyldihydropyrene ring current probe. experimental and computational studies of the fulvenes and the strongly antiaromatic cyclopentadienone reveal large mills-nixon-type Bond localization effects. synthesis of fulvene-fused dihydropyrenes

The synthesis of the methylfulvene- and phenylfulvene-annelated dihydropyrenes 10 and 22 from the cyclopentadiene-fused dihydropyrene 7 in 68% and 80% yields, respectively, are reported. However, the attempted formation of the parent fulvene-fused dihydropyrene 18 failed, both from the cyclopentadiene 7 with formaldehyde and from the cyclopentadienone 5 in Wittig-type reactions. Chemical shift data for the methylfulvene (35) and phenylfulvene (36)-fused dihydropyrenes 10 and 22 were used to estimate the reduction in the dihydropyrene nucleus aromaticity (DHPN) (relative to benzene fusion) in 10 and 22 (12-16% and 22-25% respectively). Calculations revealed that this reduction in diatropicity, contrary to the situation with benzene fusion, is not due to any aromaticity of the annelating fulvenes but instead is caused by Mills-Nixon-type effects. We conclude that methyl- and phenylfulvene are nonaromatic. An improved synthetic route to the cyclopentadienone 5 was found in an unprecedented cyclization of the trans-cinnamic acid analogue 29 in 80% yield. This enabled an X-ray structure of 5 to be obtained, for comparison to that of the saturated ketone 4. Even though crystals of 5 and 4 show diastereomeric disorder, when the average bond length data of cyclopentadienone 5 is compared with those of cyclopentenone 4 and the parent and benzo dihydropyrenes 6 and 33, it is clearly evident that 5 has the opposite bond-alternation pattern, consistent with a [4n] fused annulene. From the bond length data, cyclopentadienone has approximately 87% of the effect of a benzene ring on bond alternation, which is in reasonable agreement with the previously found NMR value (78%). Structure and nucleus-independent chemical shift calculations support these results.

The Experimental Realization of a Neutral Homoaromatic Carbocycle

Infrared spectra recorded for 1,5-dimethyl-2,8:4,6-semibullvalenetetracarboxylic acid dianhydride (12) in the condensed and vapor phase clearly prove that in the vapor phase the dianhydride 12 is a homoaromatic ground state semibullvalene.

Atmospheric reactions of vibrationally excited greenhouse gases : SH+N2O(n, 0, 0)

Abstract Rate coefficients for the reactions of SH + N 2 O(ν 1 , 0, 0) → HSO (A 2 A′) for ν 1 = 0 and 1, and HSO (A 2 A′) + N 2 O → products, are measured at room temperature. SH is formed by pulsed laser photolysis of H 2 S and vibrationally excited N 2 O is created using the stimulated Raman technique. The reaction kinetics are followed by chemiluminescence of HSO. The results show that the reaction between SH and unexcited N 2 O has a rate coefficient equal to 1.3 (±0.14) × 10 −11 cm 3 molecule −1 s −1 , while N 2 O (1, 0, 0) reacts with a rate coefficient at least 2.2. times larger.

Experimental verification of the homoaromaticity of 1,3,5-cycloheptatriene and evaluation of the aromaticity of tropone and the tropylium cation by use of the dimethyldihydropyrene probe.

By use of a dimethyldihydropyrene experimental probe for aromaticity, 1,3,5-cycloheptatriene (16) is demonstrated to be a neutral homoaromatic hydrocarbon! On the basis of (1)H NMR results, 16 is judged to be ~30%, tropone 18 ~20%, and tropylium 22 ~50% as aromatic as benzene. The latter result may be an underestimation because of charge delocalization. The B3LYP/6-31G* calculated geometries and GIAO-HF/6-31G*//B3LYP/6-31G* calculated NMR chemical shifts and nucleus-independent chemical shifts (NICS) support these conclusions. These estimates were obtained by synthesis of the annelated dihydropyrenes 7 (tropone fused), 9 (1,3,5-cycloheptatriene fused), and 10 (tropylium fused). [4 + 3] Cycloaddition of the isofuran 5 with an oxyallyl cation (prepared from 2,4-dibromopentan-3-one) gave the C7 fused dihydropyrene 6 in 77% yield. Elimination of water gave tropone 7 in 61% yield, which, via LiAlH(4) reduction to alcohol 8 (48% yield) and treatment with HBF(4), gave quantitatively tropylium cation 10. When ketone 7 was reduced with AlH(3) (generated from AlCl(3)/LiAlH(4)) in ether/benzene at 25 °C, the isomeric cycloheptatrienes 11 (70% yield) and 9 (15% yield) were obtained.

An experimental estimate of the relative aromaticity of the cyclooctatetraene dianion by fusion to dimethyldihydropyrene

The synthesis of the cyclooctatetraene dianion (COT(2-)) fused at the [e]-position of trans-10b,10c-dimethyl-10b,10c-dihydropyrene (DHP) is described, and by comparison of (1)H NMR properties and NICSAv to the analogous benzene fused DHP, the relative aromaticity of the dianion is found to be at least as great as that of benzene, and substantially larger than that of the cyclopentadienide anion.

B3LYP calculations on bishomoaromaticity in substituted semibullvalenes

B3LYP/6‐31G* calculations on the degenerate rearrangements of substituted semibullvalenes spuriously predict the relative enthalpies of the bishomoaromatic TSs to be lower than the experimental values. However, the calculations do make the useful and experimentally testable prediction that the two cyano and two phenyl substituents in 2,6‐dicyano‐4,8‐diphenylsemibullvalene (9d) are more likely than four cyano substituents in 2,4,6,8‐tetracyanosemibullvalene (9f) or the four phenyl substituents in 2,4,6,8‐tetraphenylsemibullvalene (9g) to produce a semibullvalene that has a bishomoaromatic equilibrium geometry in the gas phase. The major reason for the surprising finding that 9d is more likely to be bishomoaromatic than 9g is shown to be steric interactions between the phenyl groups at C‐2 and C‐8 and at C‐4 and C‐6 in bishomoaromatic structure 10g. These interactions inhibit the conjugative stabilization of 10g; but they are absent in bishomoaromatic structure 10d, where cyano groups replace the phenyl groups at C‐2 and C‐6 in 10g.

Pyramidalized olefins: The stereospecific conjugate reduction of a bicyclo[2.2.2]octadiene

Abstract Structure determination on 2-benzenesulphonyl-3-trimethylsilybicyclo[2.2.2]octa-2,5-diene ( 3 ) demonstrates that the double bonds are pyramidalized in the exo direction. Nucleophilic attack (conjugate reduction) on 3 parallels this pyramidalization and occurs exclusively from the endo face.

New ketene equivalents for the Diels–Alder reaction. Vinyl sulphoxide cycloadditions

1-Phenylsulphinyl-1-trimethylsilylethene and phenyl vinyl sulphoxide have been shown to be effective ketene equivalents for the Diels–Alder reaction.

The reaction between bis(trimethylsilyl)cyclopentadiene and dichloroketen, and the Diels–Alder reactions between N-phenylmaleimide and two silylated methylcyclopentadienes

Bis(trimethylsilyl)cyclopentadiene reacts with dichloroketen to give only the adduct (7) derived from the 2,5-disilylated isomer (5). Methyl(trimethylsilyl)cyclopentadiene (18) and trimethylsilylmethylcyclopentadiene (21) give Diels–Alder adducts (19) and (20) and (22) and (23) in which the regioselectivity is unaffected by the presence of the silyl group. Epoxidation of the adduct (22) and acid-catalysed opening of the epoxide lead to formation of the exo-methylenenorbornane (26) in a rearrangement controlled by the presence of the silyl group.