Molecular Tweezers as Synthetic Receptors in Host—Guest Chemistry: Inclusion of Cyclohexane and Self‐Assembly of Aliphatic Side Chains

Abstract

L W q ) MezC-O-UC14(THF) 1 2-= I >uci4 + U C I~ + Licl -1 Me2C-O-UC14(THF) 1 2 0.5 1 Scheme 2. The role of 1 as an intermediate i n the formation of 2two unsuspected points of the reaction mechanism. Firstly, acetone is not involved in the transformation 1 + 2, which is induced by Li/Hg reduction and generates UCI, and LiCI. Secondly, 2 is the true precursor of the alkene, tetramethylethylene. Indeed, 2 was smoothly reduced at 20 "C by Li/Hg to give a not yet identified uranium(rr1) alkoxide, and complete formation of the alkene could be achieved after the mixture was heated at reflux for 24 h. In agreement with these observations, the reductive coupling of acetone to give pinacol or tetramethylethylene was performed in a one-pot reaction by using the proper quantities of UCl, and Li/Hg; the diol was formed by hydrolysis after 8 h at room temperature, whereas the alkene was obtained by heating for 24 h under reflux (quantitative yields by NMR). When the UCl, and Na/Hg system was used, the yield of tetramethylethylene did not exceed 10% after 48 h at 65°C. This difficulty in forming the alkene could be clearly related to the sluggish reduction of the pinacolato intermediate 4, which was quite inert towards Na/Hg at 20 "C; even after 30 h at 65 "C only about 20 % was converted into tetramethylethylene. In contrast, 4 was readily reduced by Li/Hg and transformed completely into the expected alkene after 10 h in refl uxing tetrahydrofuran.We have found that in the reductive coupling of acetone, quite distinct conditions are necessary for both the coupling process leading to the pinacolate intermediates and for the subsequent deoxygenation step giving tetramethylethylene. These differences can be used to achieve remarkable selectivity like that observed with some titanium systems." 21 The results also underline the major role of the reducing agent, which determines the structure of the intermediates and is of particular importance in the deoxygenation step. If the first intermediate, a bimetallic species with a bridging OCMe,CMe,O ligand, readily reacts with the reducing agent to give a cyclic mononuclear metallopinacol, only this intermediate is transformed under more forcing conditions into the corresponding alkene. Experimental Procedure'H NMR (TMS int.): Bruker WP60 (60 MHz). All experiments were carried out under argon ( < 5 ppm oxygen and water) using standard Schlenk-vessel and vacuum-line techniques or in a glove box. Solvents were thoroughly dried and deoxygenated by standard methods and distilled immediately before use. 1 A mixture of UCI, (354 mg. 0.93 mmol). 1 % Li/Hg (640 mg, 0.93 mmol of Li), and acetone (68 gL, 0.93 mmol) in THF (15 mL) was stirred for 3 h at 20°C. The green solution was filtered and its volume reduced to 5 mL. The green microcrystals of 1 that precipitated upon addition of pentane (10 mL) were filtered off, washed with pentane, and dried under vacuum (464 mg, 90%). 2: By using the same procedure as for 1 , green microcrystals of 2 were isolaled in 76% yield from the reaction of UCI, (3...

Molecular tweezers as synthetic receptors: molecular recognition of cationic substrates; an insight into the mechanism of complexation

“…The difference ∆E = 13.3 kcal/mol can be considered to be a rough estimate of the expected activation barrier. These calculations are in good agreement with the experimental findings and the results from crystal structure analyses and earlier molecular mechanics and semiempirical calculations [7,15].…”

“…The experiments presented here prove that the hydrocarbon tweezer 1a due to its electronic and topological properties [9] has the ability to selectively bind not only neutral electron-deficient aromatic and aliphatic substrates [7] but also cationic substrates in organic media by the use of multiple cation-π interactions. We are currently investigating the binding poperties of a water-soluble derivative of 1a to transfer this novel binding motive to aqueous solutions [16].…”

“…The magnetic anisotropy of these benzene units makes the 1 H NMR spectroscopy to be a very sensitive probe to detect the complexation of substrate molecules bound inside the cavity of 1a [7]. From the results obtained in binding studies with small electrondeficient aliphatic substrates (CH 3 CN, CH 2 (CN) 2 , CH 3 NO 2 ) [7a] and intramolecular complexation of flexible aliphatic sidechains [7b] we already know that the cavity of 1a has an almost ideal size for the binding of methyl or methylene groups.…”

“…Besides the frequently used cyclic receptors of the cyclophane-type [5] non-cyclic receptors with cavities of flexible size proved to be very effective [6]. We have recently reported on the synthesis and some supramolecular properties of the hydrocarbon compounds 1a and 1b, which due to their ability to selectively bind electron-deficient aromatic and aliphatic substrates as well as organic cations can be regarded as molecular tweezers [7]. Here we report on the binding properties of the benzene-spaced molecular tweezers 1a towards various aliphatic and aromatic cations in organic solution and attempts to use this binding motive for the template synthesis of mechanically interlocked supermole- …”

Molecular Tweezers as Synthetic Receptors: Molecular Recognition of Electron-Deficient Aromatic and Aliphatic Substrates

“…Repetitive Diels ± Alder reactions of bisdienophile 5 with diene 6 [7] proceed stereospecifically to yield the bisadduct 7, which can be converted to molecular tweezer 1 by oxidative dehydrogen-ation with 2, in an overall yield of 30 % (Scheme 1). [8] The tweezer 2, containing only benzene spacer units, can be synthesized following an analogous route. Repetitive Diels ± Alder reactions of the known bisdienophile 8 [6] with diene 6 and subsequent DDQ dehydrogenation of bisadduct 9 produce the substituted tweezer 10 a with an overall yield of 60 %.…”