Double-boat conformation

The most stable conformation of bicyclo [3.3.1] nonane is the chair-chair conformation117-11. The energy barriers between the double-chair and chair-boat and between the chair-boat and double-boat conformations are 27 and 43 kJ/mol, respectively11. The... 

From the analysis of the factors thus far considered, boat-boat conformation 2c should be stabilized by 1,3-diaxial repulsion. The introduction of bulky substituents both in endo-3 and in endo-1 positions (13) leads to the predominance of the double boat conformation 13c in the equilibrium (27,28). Analogously, the introduction of four bulky substituents in exo-2,4,6, and 8 positions (14) could lead to the destabilization of the chair form for each of the six-membered rings and to a preference for boat-boat conformation 14c because of the strong 2 4 and 6 8 repulsion of the substituents. However, compounds of this kind have not been studied yet. It is necessary to mention that the boat-boat conformation should correspond in fact to a maximum on the energy curve, while a twist-twist, TT, form approximated by 15 would be expected to correspond to a minimum. In the TT conformation the flagpole and eclipsed interactions are diminished (the BB conformation is significantly more flexible than the CB one and is easily twisted). 

Participation of transannular sulphur atoms has also been suggested to operate in the facile ring-opening of a dibenzodithiecin by brominolysis. The crystal-structure determination of (180) revealed a double boat conformation, with all sulphur atoms nearly coplanar. ... 

The stmcture of the compound 67b was determined by X-ray (95JA6388). The length of the Te=Se bond (2.445 A) is 0.1 A shorter that in bis(2-ethylcarboxy-phenyltellurenyl) selenide (81CSC1353), which points to a greater double bond character of this bond in the former compound. Selenoxide 67b has a boat conformation. The length of the transannular Te- N bond (2.620 A) is appreciably shorter than the corresponding van der Waals contact (3.70 A). The intramolecular coordination Te- N bond thus formed contributes to the stabilization of the boat conformation of the compounds 67. 

AMJnsubstituted compounds are, in general, too unstable to be isolated. The X-ray crystal structure of 1-tosyl-l //-1,2-diazepine shows that the molecule adopts a boat conformation with localized double bonds.69... 

Introduction of a-substituents into the 3- or 7-position of 12a leads to a shift of the prevailing conformation from chair-chair (cc) to boat-chair (be), and finally to twist double boat (16) as judged by H NMR spectra (121). The changes in the H H coupling constants in these derivatives can be interpreted with skewed cc conformations as well. MM calculations, however, ruled out the latter possibility (121). The results of acetolysis of exo- and endobicyclo[3.3.1]nonan-3-yl tosylates were also interpreted with the aid of MM calculations (122). 

For each of the five groups, many derivatives were found and a comparison of the experimental bond lengths for the 1,3-dioxane ring system with representatives of the different classes are presented in Tables 2 and 3. The chair proved to be the most stable conformer and was obtained in all kinds of structures, though often some were in fact twisted. In addition to twist and boat conformers, also sometimes twisted when exocyclic double bonds were present, the corresponding half-chair conformers were also obtained. 

The X-ray crystal structure of the 1 -tosyl-1H-1,2-diazepine (10) shows a boat conformation with N(l) at the prow (Figure 1) (72T581). Double bonds are clearly localized at N(2)—C(3) 1.255 A, C(4)—C(5) 1.326 A, and C(6)—C(7) 1.333 A, and the imine bond is isolated, whereas bond delocalization is present in the butadiene-like part C(4)—C(5)—C(6)—C(7). In contrast, when the diazepine is complexed with Fe(CO)3, for example (11), the iron atom is bound to the butadiene moiety C(4) to C(7), as also occurs in the lf/-azepine and l-methoxycarbonyl-l//-azepine complexes the N(l) atom adopts a planar sp2 configuration and the seven-membered ring consists of the two planar parts bent along the C(4)... C(7) line by 140° (70AG(E)958). 

X-Ray structure shows that Pd is coordinated to four S 55 atoms in a square planar fashion only long range weak axial interactions are observed with the two remaining S atoms, i.e. the conformation of the macrocycle is an S-shaped double boat, with the Pd11 ion at the molecular inversion centre... 

Whereas 4-ethyl-3,5-dimethyl-2,4,6-triphenyl-4//-thiopyran forms orthorhombic crystals, the corresponding 4 butyl derivative is monoclinic. Both C-4 and S lie above the plane of the double bonds and the alkyl groups are oriented towards the S atom <2001NCS47>. These and other 47/-thiopyrans have 67t electrons and can be considered homoaromatic p-jt overlap involving the heteroatom improves such character. Ab initio calculations with HF/STO-3G, HF/6-31G, and B3LYP/6-31G basis sets indicate a near planar or shallow boat conformation. Bulky 4-substituents increase the dihedral angles C-C-C=C and C=C-S-C <2004JST(678)171>. 

In the previous subsection, it was shown that the Ferrier reaction offers an opportunity to convert glycal derivatives into unsaturated sugar derivatives, which have an isolated double bond between C(2) and C(3). The Tipson-Cohcn reaction is another important reaction for the introduction of isolated double bonds.29 In this procedure, a cis or tram diols are converted into disulfonates (mesylates or tosylates) which are reductively eliminated with sodium iodide and zinc in refluxing DMF (Scheme 3.6a). In this reaction, the C(3) sulfonate is substituted by an iodide, which then is reductively removed by zinc with concomitant elimination of the second sulfonate moiety, introducing a double bond. Stereoelectronic effects make nucleophilic substitutions at C(3) more favourable than similar reactions at C(2) (see Section 3.2.3). Probably, the elimination proceeds through a boat conformation. In this case, the iodide and tosylate are in a syn relation. In most cases, E2 elimination proceeds via a transition state involving an anti orientation. Nevertheless, syn elimination becomes the dominant mode of reaction when structural features prohibit an anti orientation. 

Both planar and boat conformations for 1,4-cyclohexadiene and its derivatives are possible. Addition of Eu(fod)3 allowed the determination of the conformation of 1,4-dihydrobenzyl alcohol [41]. Addition of Eu(fod)3 and double and triple irradiation experiments made the determination of coupling constants possible. The vicinal coupling constants 73,4 and 73,4/ were equal indicating planarity of the ring. Other coupling constants also indicated a planar conformation for the ring. Possible rapid boat-to-boat inversion was ruled out by measurement of coupling constants of 3-fluoro- 1,4-dihydrobenzyl alcohol-d2 from unaltered spectra and spectra obtained after addition of Eu(fod)3. 

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