Willem H. de Wolf

Synthesis and conformational analysis of 68,11-dihalo[5]metacyclophanes

Abstract The synthesis of the 8,ll-dihalo[5]metacyclophanes 1b-1d (halogen = chlorine or bromine) from 1,2-dimethylenecycloheptane ( 2 ) is described. Spin saturation transfer and line shape analysis of the temperature dependent 1H NMR spectra showed 1b-1d to occur in two conformations A (85-89%) and B (15-11%, respectively). The conformation of the pentamethylene bridge was fully analyzed from vicinal proton coupling constants which obey the Karplus relation inspite of severe distortions of the carbon valence angles. The thermodynamic parameters of the ground and transition state were determined for the equilibrium between the conformers A and B and are discussed in terms of steric interactions in these highly strained molecules.

1,4-Addition of a Terminal Phosphinidene Complex to [5]Metacyclophane

Three novel aspects emerge for the reaction of [5]metacyclophane (1) with the (intermediate) phenylphosphinidene complex 2 to give the 7-phosphanorbornadiene 3. It is the first 1,4-addition of a phosphinidene complex to an unsaturated system, the first addition of a phosphinidene complex to a benzene ring, and the first [4+1] cycloaddition to an aromatic compound.

1,4-Addition of a Phosphinidene Complex to Cisoid 1,3-Dienes

Abstract The first genuine 1,4-additions of the terminal phosphinidene complex PhPW(CO)5 (2) to 1,3-dienes 3 are reported. Two of the 1,4-adducts, 15 and 17, were characterised by X-ray crystal structure determinations. A cisoid conformation, but not coplanarity, of 3 appears to be a prerequisite for the occurrence of a 1,4-addition. The reaction of 2 with the sterically hindered 3B yields 66.5% of the 1,4-addition product 13, the highest percentage of 1,4-addition thus far observed for the reaction of an electrophilic species with a 1,3-diene. The product composition is almost independent of the temperature, implying that the usual preference for the 1,2-mode is mainly entropy controlled. Steric factors appear to be the main cause of the higher tendency towards 1,4-addition of phosphinidines as compared to carbenes.

Synthesis and anomalous structure–reactivity relationship of 8,11-dichloro[5]metacyclophan-3-one

Abstract The synthesis, structural characterisation, and reactivity of the title compound are reported. It was prepared in order to investigate the effects of incorporation of an sp2-centre in the bridge of a [5]metacyclophane on its structure and reactivity. An X-ray crystal structure of the cyclophanone demonstrates that the benzene ring is distorted to the same extent as in the hydrocarbon parent compound, while there is less strain in the bridge. Nevertheless, the cyclophanone displays a reduced reactivity in Diels–Alder reactions, which is tentatively ascribed to transition state effects. Calculations indicate that in spite of the close proximity between the ketone functionality and the benzene ring, there are no π–π interactions between them.

The synthesis of hetero-bridged [5](3,6)oxepinophanes

The [5](3,6)oxepinophanes (8) are the oxepines with the shortest bridge reported so far. They were obtained starting from diethyl furan-3,4-dicarboxylate (3) which, with lithium aluminium hydride, afforded the diol (4); this was converted into the seven-membered siloxanes (5a and b) by reaction with dichlorodimethylsilane or dichlorodimesitylsilane, respectively. By the Prinzbach–Tochtermann sequence, compounds (5) were converted into the oxepinophanes (8) in three steps: Diels–Alder reaction of (5) with dimethyl acetylenedicarboxylate to give the fused 7-oxanobornadienes (6), photochemical transformations of compounds (6) to the fused 7-oxaquadricyclanes (7), and thermal isomerization of compounds (7) to (8). Attempted conversion of (8a) into (11a), a derivative of [5]paracyclophane, was unsuccessful.

Some Investigations on [6]Metacyclophanes

[6]Metacyclophanes 2 with chlorine or fluorine substituents have been synthesized by a novel, convenient version of the base-catalyzed double elimination of hydrogen chloride from the corresponding halopropellanes 7. Conformational analysis by 1H and 13C NMR spectroscopy revealed remarkable analogies of certain partial structures of the hexamethylene bridge of 2 with the known exo and endo conformers of [5]metacyclophanes 1. The Diels−Alder reactivity of 2 towards tetracyanoethene was investigated in order to clarify seemingly contradictory reports in the literature. As anticipated, it was observed that the reactivity of 2 is strongly reduced compared to that of the more strained lower homologues 1, but there is a subtle fine tuning: While the unsubstituted parent compound 2a still forms adducts with strong dienophiles, chlorine substitution at the aromatic ring completely blocks the Diels−Alder reactions.

Structures and strain energies of small [n]metacyclophanes

The structures and strain energies of various derivatives of [5]- and [6]metacyclophanes have been computed by employing the density functional method (ADF). They showed a good agreement with the few available X-ray crystal structures. The structure of 9,12-dichloro[6]metacyclophane was experimentally determined at 200/233 K. Contrary to intuition, the incorporation of sp2-hybridised carbon atoms in the oligomethylene chain was found to have a minor influence only on the structure and strain energy of these compounds. It is concluded that small metacyclophanes with potentially interesting interactions between unsaturated fragments in the bridge and the aromatic ring or between two aromatic rings, e.g. the so far elusive [1.1]metacyclophane, are realistic synthetic targets.

New one-pot synthesis of a benzonorcaradiene derivative by reduction of naphthalic anhydride with LiAlH4

Reaction of naphthalic anhydride (naphthalene-1,8-dicarboxylic anhydride) (1) or of 1,8-bis(hydroxymethyl)naphthalene (3) with LiAlH4 in refluxing THF yields the benzonorcaradiene derivatives 2 in yields of up to 50%. It is proposed that in the formation of this (strained) product an anti-hydroalumination reaction is involved, which is facilitated by the assistance of a neighbouring alkoxide function.

Flash vacuum thermolysis of spirocyclohexadienones

Abstract In an attempt to prepare short-bridged hydroxymetacyclophanes 1b-d , the spirocyclohexadienones 2b-d were pyrolyzed by flash vacuum thermolysis (FVT). Instead of 1b-d , variable amounts of 4-(5-hexenyl)phenol ( 4b ), β-hydroxybenzocycloalkenes ( 5b-d ) and 4-(trans-1-alkenyl) phenols ( 6c-d ) were obtained. The formation of these products is explained by invoking cleavage of a spiro bond in 2 under formation of the intermediate diradical 3 which, depending on the length of the aliphatic chain and on the temperature, has several pathways open for isomerization to spin-paired products.

11-t-Butyl-8-chloro[5]metacyclophane

The reaction of 8,11-dichloro[5]metacyclophane with t-butyl-lithium gave the very crowded title compound in an unusual substitution reaction at the most hindered position of the molecule, C-11.