18-Crown-6, conformers

Crotonaldehyde complexes with group VIB metals, 12 221-223 18-Crown-6, conformers, 42 126, 128 Crown conformation, in polysulfidemetal complexes, 31 115... 

The conformation of uncomplexed [18]crown-6 found in the crystalline state has been shown by IR spectroscopy to be predominant also in solution and force-field calculations have essentially confirmed the same structure 241.2 9) interestingly, it was found that the ordered Dsa [18]crown-6 conformation observed in the potassium complex would be lower in energy by 7 kJ mor were it not for the aforementioned unfavorable electrostatic interactions The tendency to remove the latter by complexation of a cation and thus enable the ligand to adopt a low-energy conformation might contribute considerably to the high affinity of [18]crown-6 to potassium ions. 

Fig. 19 Representations of the C and solid 18-crown-6 conformations on the grid of the diamond lattice. Solid lines represent bonds of the macrocycle on the zero" grid and dashed lines represent bonds on the first grid above. Connections between the grids are made by vertical bonds (parallel to the z-axis). The atom numbering scheme begins with an oxygen on the x-axis [64].

The crown conformation is not necessarily the most stable structure for free (uncomplexed) crown ethers. Examine the lowest energy structure of 18-crown-6, and compare it to the crown structure. Explain why the crown structure is less stable. Use equation (1) to calculate the equilibrium ratio of lowest-energy and crown conformers of 18-crown-6 at room temperature. What causes a shift in conformation in the presence of metal cations ... 

Fig. 5. Conformational model of octamethyltetrasiloxane left tilted crown conformation right untilted crown conformation. Big circles represent the Si-atoms. (Ref. 10))...

The problem of the preferred conformation of cyclodecane has been extensively studied by Dunitz et al. (46). In the crystals of seven simple cyclodecane derivatives (mono- or 1,6-disubstituted cyclodecanes) the same conformation was found for the ten-membered ring (BCB-conformation, Fig. 9). It follows from this that the BCB-conformation is an energetically favourable conformation, possibly the most favourable one. Numerous force field calculations support this interpretation Of all calculated conformations BOB corresponded to the lowest potential energy minimum. Lately this picture has become more complicated, however. A recent force field calculation of Schleyer etal. (21) yielded for a conformation termed TCCC a potential energy lower by 0.6 kcal mole-1 than for BCB. (Fig. 9 T stands for twisted TCCC is a C2h-symmetric crown-conformation which can be derived from rrans-decalin by breaking the central CC-bond and keeping the symmetry.) A force field of... 

Fig. 12. trans-Cyclooctene a calculated (19) and observed (78) (in parentheses) torsion angles (deg) of the crown conformation b calculated torsion angles of the distorted chair conformation... 

In this sequence L, and L2 represent different crown conformations. 

The all-cis-l,4,7,10-cyclodecatetraene (1,4,7,10-CDT) molecule is of special interest as a tetrahomo-8jr-system, when all four TT-bonds are arranged in a way where maximum interaction is guaranteed. This arrangement is realized in the crown conformation, which is also the conformer observed in an X-ray study of the molecule. The mean C=C bond... 

The homologue c/s,ris,c -l,5,9-cyclododecatriene [111] adopts conformations in which homoconjugation is not possible. Any stabilization that may result from homoaromaticity obviously does not compensate for the increased strain in the crown conformation [112] (required for conjugation) (Untch and Martin, 1965 Anet and Rawdah, 1980). 

MM calculations are in agreement with the boat-chair conformation of symmetry being slightly more stable than the crown conformation [5]. 

More curious and unexpected reactions were observed when complex 219 was heated with triethylamine often used as an "inert" base in many reactions with sulfur monochloride. Thienopentathiepin 229 (30%) together with heptathiocane 230 (10%) were produced their polysulfur rings had the anticipated chair and crown conformations, respectively (20030L1939 Scheme 122). 

The value of V(13C-H) for the axial proton (174 Hz) in 1,3,5,7-tetroxocane is much larger than that for the equatorial proton (154 Hz) <77JST(36)329>. Crown conformations in general have large chemical shift differences between axial and equatorial OCH2O protons for 1,3,6-trioxocane and 1,3,5,7-tetroxocane these differences are 0.88 and 0.64 p.p.m., respectively. In contrast, the corresponding shift in the boat-chair of the former compound is only 0.07 p.p.m. (72JA1390). 

Cyclotriveratrilene 334 (also a cyclophane) [6] is an interesting building block for the synthesis of complex systems. Its obtaining was reported as early as in 1915 [7a] but its structure was established only 50 years later [7b]. The molecule can exist in one of the crown conformations separated by the barrier of ca. 27 kcal/mol [6]. 

Several methods are available for the synthesis of cycloarsoxanes (a) hydrolysis of organoarsenic(III) dihalides RASX2 (X = Cl, Br, I), (b) reduction of arsonic acids RAsO(OH)2 and (c) oxidation of primary arsines RASH2 or cyclopolyarsines (RAs) . The tetramer cyclo-(MeAsO)4 exhibits the boat-chair conformation, whereas the mesityl derivative cyclo-(MesAsO)4 adopts the crown conformation (Figure 11.24). The As-O bond distances are in the range 1.77-1.82 A, typical for single bonds. 

Figure 11.24 (a) Boat-chair and (b) crown conformations of the AS4O4 ring in... 

---