Conformation, skew-boat

A potential energy diagram for nng inversion m cyclohexane is shown m Figure 3 18 In the first step the chair conformation is converted to a skew boat which then proceeds to the inverted chair m the second step The skew boat conformation is an inter mediate in the process of ring inversion Unlike a transition state an intermediate is not a potential energy maximum but is a local minimum on the potential energy profile... 

The various conformations of cyclohexane are in rapid equilibrium with one another, but at any moment almost all of the molecules exist in the chair confor mation. Not more than one or two molecules per thousand are present in the skew boat conformation. Thus, the discussion of cyclohexane confor-mational analysis that follows focuses exclusively on the chair confor mation. 

The conformation of two 4-phenylsulfonyl-tetrahydropyran-2-one derivatives both in solution and in the solid state was studied (95JP0747) the tetrahydropyran-2-one ring proved to adopt a skew-boat conformation. 

Heavily substituted piperidones, for example l-tert-butyl-rrans-2,5-dime-thylpiperidone (141), may adopt skew-boat conformations.197... 

Figure 2 (a) Chair and (b) enantiomorphic skew-boat conformations of the [M(P207)] chelate ring... 

In many cases a type of deterministic search is conducted, where all probable structures are entered individually and minimized separately. The possible structures are generated by drawing all possible combinations of the chair, boat and skew boat conformers for six-membered rings, and X and S conformers for five-mem-beredrings116 37 64-107 1081. 

Three other cyclic compounds are depicted in Table XVII as entries 25- 27 the first two compounds show a skew-boat conformation, while the last approximates to a chair form. 

The proposition 23a is surprising. Beside the skew-boat conformation of the central chelate ring determined by the RRa)-conformation of the secondary nitrogen atoms, the substituent is placed in a axial position. An equatorial position could be obtained... 

This hexaamine cage ligand is derived from sar (see Section 17.16) but the three annular chelate rings are not five-membered but six-membered chelate rings and the central carbon has two methyl substituents. These chelate rings have skew-boat conformations, and their orientation is lel3 for one and ob3 for the other of the two conformers. Build these two structures in HyperChem and refine them as cobalt(III) and cobalt(II) compounds. Compute the reduction potentials and compare them with the data in Table 17.18.2. 

Six-membered chelate rings commonly adopt the chair or symmetrical-skew boat conformations in Co(III) complexes (Fig. 4). More distorted conformations are also possible for example, the unsymmetrical-skew boat (68). The symmetric boat conformation has never been observed and is predicted to be extremely strained. The... 

The optically active ligands, RR-ptn and SS-ptn, adopt skew boat conformations in which both methyls are equatorial (76). Thus RR-ptn adopts the X conformation, and SS-ptn forms the 8 conformation of the skew boat. The meso form of the ligand, RS-ptn, however, must adopt a chair conformation if both methyls are to be equatorially oriented. Tris complexes composed solely of RR- (or SS-) ptn are therefore restricted to the lela and 083 isomers. Molecular mechanics calculations have predicted that the lel conformation of complexes with the racemic ligands (e.g., A XXX) is significantly (4-7 kJ mol ) more stable than the 083 (A XXX) (77, 78). The tris chair form of RS-ptn was estimated to be very similar in strain energy to the RR-ptn leh complex (78). 

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