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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...

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The dynamics in the host-guest complexes of the molecular tweezers 1 a,b and clips 2 a,b with 1,2,4,5-tetracyanobenzene (TCNB, 3) and tropylium tetrafluoroborate (4) as guest molecules were analyzed by temperature-dependent 1H NMR spectroscopy. The TCNB complexes of tweezers 1 a,b were found to be particularly stable (dissociation barrier: DeltaG(++)=16.8 and 15.7 kcal mol(-1), respectively), more stable than the TCNB complexes of clips 2 a,b and the tropylium complex of tweezer 1 b (dissociation barrier: DeltaG(++)=12.4, 11.2, and 12.3 kcal mol(-1), respectively). A detailed analysis of the kinetic and thermodynamic data (especially the negative entropies of activation found for complex dissociation) suggests that in the transition state of dissociation the guest molecule is still clipped between the aromatic tips of the host molecule. The 1H NMR analysis of the TCNB complexes 3@1 b and 3@2 a at low temperatures (T<-80 degrees C) showed that 3 undergoes fast rotation inside the cavity of tweezer 1 b or clip 2 a (rotational barrier: DeltaG( not equal)=11.7 and 8.3 kcal mol(-1), respectively). This rotation of a guest molecule inside the host cavity can be considered to be the dynamic equilibration of noncovalent conformers. In the case of clip complex 3@2 a the association and rotational barriers are smaller by DeltaDeltaG(++)=3-4 kcal mol(-1) than those in tweezer complexes 3@1 a,b. This can be explained by the more open topology of the trimethylene-bridged clips compared to the tetramethylene-bridged tweezers. Finally, the bromo substituents in the newly prepared clip 2 b have a substantial effect on the kinetics and thermodynamics of complex formation. Clip 2 b forms weaker complexes with (TCNB, 3) and tetracyanoquinodimethane (TCNQ, 12) and a more stable complex with 2,4,7-trinitrofluoren-9-ylidene (TNF, 13) than the parent clip 2 a. These results can be explained by a less negative electrostatic potential surface (EPS) inside the cavity and a larger van der Waals contact surface of 2 b compared to 2 a. In the case of the highly electron-deficient guest molecules TCNB and TCNQ the attractive electrostatic interaction is predominant and hence responsible for the thermodynamic complex stability, whereas in the case of TNF with its extended pi system, dispersion forces are more important for host-guest binding.

Molecular tweezers and clips of type 1−3 substituted with OAc, OH, OCONHPh, OMe, OCH 2 COOR and OCH 2-CONHR groups in the central spacer units have been synthesized by modification, by standard methods, either of the known diacetoxy-substituted derivatives 1b, 2b and 3b, or of the correspondingly substituted bis-dienophiles 4b and 5b. The synthesis of the dimethoxy-diacetoxy-substituted tweezer 1d could be accomplished through pressure-induced repetitive Diels−Alder reactions between the bis-dienophile 4b and the newly prepared diene 6b and subsequent DDQ oxidation. The thermodynamic parameters (K a and ∆G) of complex formation between the new receptors and aromatic substrates such as DCNB 21, TCNB 22, TCNQ 24 and Kosower salt 25 and the maximum complexation-induced 1 H NMR shifts (∆δ max. ) were determined by 1 H NMR titration experiments. It was found that the presence of substituents OH, OAc and OCONHPh in the central spacer units of the tweezers and clips 1−3 favours complex formation, whereas that of the substituents OMe, OCH 2 COOR and OCH 2 -CONHR disfavours it. This finding can be explained in terms