Molecular Rearrangements of Triphenylmethane Derivatives I. General Discussion

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Whereas the hydrocarbon parent systems, the [ n ]dendralenes, have long been a neglected class of oligoenes [4] , the recent preparative accomplishments of the Sherburn group have changed the situation fundamentally [5,6]. These cross-conjugated hydrocarbons are now known up to [13]dendralene, and many of these potentially very valuable compounds are available in gram quantities, allowing, often for the first time, comprehensive chemical studies [6].

Notwithstanding modern progress, the phenomenon of cross-conjugation is as old as scientific organic chemistry. Many of the organic compounds that played an important role in the dawn of industrial organic chemistry are cross-conjugated systems or were converted into cross-conjugated organic salts during the color-generating process. Scheme 1: From indigo to heteroindigo derivatives and all-carbon-indigo. As far as we are aware, however, no attempt to prepare an all-carbon equivalent of indigo has ever been described. A system that could qualify as such an all-carbon analog of 1 is the bis-anion 5 , which itself should be obtainable by anionization of the linearly conjugated triene 4.

Although we have to accept for the time being that our different approaches to preparing 4 and 5 have so far been unsuccessful see below , we think that our initial efforts to attain this goal are worth publication. Furthermore, we are convinced that 5 will eventually become available. The methylation of 7 to 8 was achieved by the treatment with trimethylsulfonium iodide in the presence of n -butyllithium in THF [16].

Also the second route, starting with the reaction of 2-indanone 9 with the Eschenmoser salt 10 according to [17] to give the iodide 11 , was unsuccessful, since the attempted Hofmann elimination to 12 failed. Of course 12 , if formed, might not have survived the isolation process. Since the target molecule 12 was expected to be a reactive compound, we decided to increase its stability by the introduction of two methyl substituents at its exocyclic double bond. Indeed, when 9 was first metalated with LDA and the resulting enolate quenched with acetone, the resulting ketol could be dehydrated in situ by treatment with diethyl chlorophosphate to yield 13 [18].

Derivative 13 is a crystalline solid that can be kept in the refrigerator for longer periods of time without decomposition. Slow evaporation of the solvent of a chloroform solution provided single crystals of 13 that were suitable for X-ray structure analysis. Other H atoms are omitted for clarity.

Palladium-catalysed synthesis of triaryl(heteroaryl)methanes

The layers of the second molecule not shown are not topologically equivalent; they do not contain an equivalent interaction. Compound 13 crystallizes with two independent but closely similar molecules rmsd 0.

Scheme 3: Dimerization of 13 under McMurry conditions. To rationalize its formation we propose that the deoxygenation of 13 does indeed take place, but provides a carbenoid intermediate 15 rather than the vicinal diol complex that is usually postulated to be formed during the McMurry dimerization. Possibly the dimerization of the substrate molecule cannot take place because of steric hindrance by the gem -dimethyl group.

Intermediate 15 is a vinylcarbene that, in principle, has several options to react further. It could dimerize to the intended product 14 or its diastereomer , cyclize to a cyclopropene derivative or react via its resonance structure 16 to the isolated dimer Clearly, among these alternatives, the last route is preferred. In 17 the two benzene rings are anellated in anti -orientation, i.

There exists an alternative structure, however, in which the two aromatic rings point in the same direction. To distinguish between these two possibilities based on spectroscopic evidence alone would not be easy. Only the asymmetric unit is numbered.

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H atoms are omitted. Only th The molecule of 17 exhibits crystallographic inversion symmetry, but the true symmetry is close to C 2 h rmsd 0. The ring system is planar, with a mean deviation of only 0. The molecular packing involves a herringbone pattern in layers perpendicular to the a -axis. There are no noticeably short intermolecular interactions. Scheme 4: Dimerization of indanone 18 by a stepwise approach.

Enolization of 18 was carried out with sodium hydride in THF and on subsequent oxidative dimerization the expected bis-ketone 19 was obtained.

This compound is produced as a mixture of two diastereomers in roughly ratio NMR analysis , which can be separated by column chromatography. Since, however, in the next step the connecting single bond is transformed into a double bond, we used the diastereomeric mixture for the subsequent oxidation. This was carried out by metalating 19 with sodium hydride again, and then oxidizing the presumably resulting carbanion to 21 a known compound [19] with Cu OTf 2 , which gave cleaner results than the chloride employed previously.

Unfortunately, all efforts to convert 21 into 4 failed. Nor was the tetramethyl derivative of 4 , hydrocarbon 14 , obtained when 21 was subjected to a crossed McMurry coupling with excess acetone. In a stepwise approach, 21 was treated with either methyllithium in ether or methylmagnesium bromide in the hope of either preparing a mono- or the bis-tertiary alcohol derivative 20 , which subsequently could be subjected to a dehydration reaction. In both cases only minute amounts of products could be obtained. When trying to purify these by column chromatography on silica gel, the stationary phase turned blue, but no defined products could be isolated.

Since it might have been the central conjugated butenedione core that caused all these preparative difficulties, we next decided to investigate the behavior of diketone 19 , in which this conjugation is interrupted. Scheme 5: Methylenation of 19 and bisalkylation of the product 23 with 1,2-dibromoethane. Figure 3: The molecule of compound 23 in the crystal.

Only the a The molecule of 23 is planar mean deviation 0. The central C—C bond seems slightly short at 1. The molecular packing is devoid of striking features. As far as the mode of formation of 23 is concerned, the isomerization formally is a [3. Hydrocarbon 23 is, in fact, a known compound. It has been prepared previously from indene 6 by other routes [25,26] , but always in the form of mixtures also containing other isomers.

As far as we are aware, the above route is the only one that provides isomerically pure 23 , not surprising in view of its route of formation. These isomeric hydrocarbons are useful ligands for the preparation of bridged Ziegler—Natta catalysts employed for olefin polymerization [26,27]. Since compound 23 contains doubly activated methylene positions, it should be easy to alkylate or bis-alkylate it. Furthermore, use of a bis-electrophile such as 1,2-dibromoethane could lead to the [2.

The other conceivable spiro isomer 28 seems to be a less likely product since its E -configurated double bond within a seven-membered ring should result in considerable strain anti-Bredt hydrocarbon. When 23 was treated with tert -BuLi in THF and the presumably resulting bis-anion quenched with 1,2-dibromoethane, a complex product mixture consisting largely of polymeric material was formed.

Surface-Hopping Dynamics Simulations of ... - ACS Publications

Further column chromatographic attempts to obtain preparative amounts of the two isomers in analytically pure form failed, however. An unambiguous structure determination had to await X-ray crystallographic analysis. Unfortunately, because of lack of material, no high quality NMR spectra of these two hydrocarbons could be obtained.

Figure 4: a The molecule of compound 24 in the crystal. H atoms not involved in these contacts are omitted. Figure 5: One of the two independent molecules of compound 25 in the crystal. The molecule 24 displays crystallographic inversion symmetry, although the true symmetry is close to C 2 h rmsd 0.

The structure determination of 25 was of limited accuracy because of twinning and disorder problems indeed, there may be a small amount of contamination by 24 and we therefore do not discuss it in detail. Both independent molecules display non-crystallographic mirror symmetry rmsd 0.

Scheme 6: Cross-conjugated hydrocarbons by Thiele condensation. These data are compared with those of the 4,5,9,tetrahydropyrene radical anion. The ESR of the 4,5-dihydropyrene radical anion is also presented. The rate of the reaction is proportional to the nitric oxide pressure and independent of the nitrogen pressure. The ReO 3 and Re 2 O 7 were formed at lower temperatures and at higher temperatures respectively.

The Re 2 O 7 was presumed to be formed by the oxidation of the ReO 3 deposited on the vessel wall with atomic oxygen evolved in a high-temperature range. The shifts in the temperature of the maximum density of water produced by the addition of small amounts of oligomers of polyethylene glycol and polyethylenimine and other related compounds were determined by dilatometry.

These effects were interpreted in terms of the change in the structure of water around the various solute molecules in connection with their chain length.

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