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email.gif - 0.3 KECHET96 Article 076: Masatomi Ohno
email.gif - 0.3 Kreply to question on article 76 by M. Ohno R.Alan Aitken

Construction of extended and polymeric 1,3-dithiolane and tetrathiafulvalene derivatives using cycloaddition of Bu3P*CS2

R. Alan Aitken and Lawrence Hill

School of Chemistry, University of St. Andrews, St. Andrews, Fife, UK KY16 9ST

Addition to strained double bonds and the Wittig reaction

The reaction of the zwitterionic tributylphosphoniodithioformate 1 with a number of alkenes has been investigated. No cycloaddition was observed with cyclohexene, styrene or stilbene. With norbornene a bright pink solid 2 was rapidly formed. The structure of this and its composition as a 1:1 adduct + CS2 were clear from analytical and spectroscopic data.

In particular the 13C NMR spectrum clearly showed the unusual structure 2. The intermediate ylide can be successfully intercepted by carrying out the reaction in the presence of benzaldehyde producing the new tricyclic dithiolane. The pink adduct 2 also reacts with benzaldehyde to give the same product. Most conveniently the one pot reaction of the four simple starting materials also gave this complex product directly in moderate yields. Norbornadiene gave similar results:

We have previously reported that this reaction can be carried out on a range of bicyclo[2.2.1]alkenes readily available from the Diels-Alder reaction of cyclopentadiene.1 In the remainder of this presentation, we will concentrate on more recent extensions of this chemistry.

Extension to dialdehydes

The reaction shown above for benzaldehyde also works with acetaldehyde and isobutyraldehyde but apparently not with simple ketones such as acetone. The scope of the reaction was broadened by the use of dialdehydes leading to the products shown.

For the ortho-substituted phthalic dialdehyde, interaction of the two alkylidenedithiolane functions occurs as shown to afford the unusual polycyclic product 3 as a 1:1 mixture of isomers (enantiomers). Its structure has been established by an X-ray structure determination.

Extension to polymeric systems

By combining a dialdehyde with norbornadiene a novel class of polymers can be obtained.

Reaction of 2 with acetylenic esters - an important new area

The pink adduct 2 also reacts with acetylenic esters such as dimethyl acetylenedicarboxylate by cycloaddition with elimination of tributylphosphine to give the tetracyclic dihydrotetrathiafulvalene structures 4.

The pink adduct derived from norbornadiene reacts similarly to give the corresponding hexacyclic products 5.

The X-ray structure of 5 (R = Me) shows that although the two dithiolane rings are exo there is a bend at the four central sulfurs to make the molecule approximately planar overall. Because of this they pack randomly with respect to whether the norbornane bridge is up or down.

The reaction of 2 with bis(acetylenic) esters leads to the formation of a variety of adducts such as those shown:

Potential applications

Possible applications of the above structures include the formation of charge-transfer complexes with suitable organic acceptors such as tetracyano-p-quinodimethane (TCNQ) or with suitable inorganic anions (e.g. I3-, PF6-, ClO4-) for use as potential organic conductors or superconductors. Some examples of the types of complexes potentially accessible are shown below. In addition, by starting from the bis-cyclopentadiene adduct of benzoquinone, we have reached the stage of 6 on the way to the compound 7 which has the donor and acceptor groups in the same molecule.

It will be noticed that all the systems mentioned up to now have been of the dihydro-TTF type which lack full conjugation. An important recent development has allowed us to use this method to get unsymmetrically substituted tetrathiafulvalenes.

Formation of substituted tetrathiafulvalenes

As mentioned in the above reaction, of the norbornadiene / Bu3P*CS2 adduct with DMAD or DEAD gives the extended bis-dihydro-TTF structures 5. However when the same compound reacts with methyl or ethyl propiolate or phenylpropiolate, i.e. less electron-deficient alkynes, the dihydro-TTF is formed on one side of the norbornene but the other remains as the double bond. These products 8 are important since they readily undergo very ready retro-Diels-Alder reaction upon FVP to give cyclopentadiene and the TTFs 9 shown.

In fact the "mono" products are also formed in isolated yields of ~10% for DMAD and DEAD together with the main "bis" products in 25-30 % yield. This implies that various species are present and available for reaction from interaction of 1 with norbornadiene. Most recently the "mono" Wittig product has also been isolated as a minor byproduct in the reaction with isobutyraldehyde.

The pyrolysis in all cases proceeds in almost quantitative yield. The ability to prepare the specifically mono- and di-substituted TTFs 9 in just two simple steps from the alkyne, norbornadiene, Bu3P and CS2 may be one of the most useful discoveries from this research programme.

Acknowledgements

We thank the EPSRC for a Research Grant (GR/J 38895) under the Innovative Polymer Synthesis Initiative and Dr Philip Lightfoot for the X-ray Structure of 5.

Reference

  1. R. A. Aitken, T. Massil and S. V. Raut, J. Chem. Soc., Chem. Commun., 1994, 2603.