Mechanism of Ring Inversion

Ring inversion, leading to interconversion of different ring conformers, is typically as facile a process as single-bond rotation. Particularly important are six-membered rings, where interconversion leads to interchange of axial and equatorial positions. 

One after the other, step through (or animate) the sequence of structures depicting ring inversion in cyclohexane, methylcyclohexane and trans-l,2-dimethylcyclohexane. Is the overall motion involved in the ring inversion similar in all three Describe any differences. 

For each molecule, calculate the overall energy barrier for ring inversion in each direction. Use this barrier to calculate the half-life (t./,) of an individual molecule at 298 K (use equation 2). Which molecule inverts most rapidly Most slowly Why (Hint What geometrical changes are required for inversion ) 

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This chapter assesses the ability of molecular mechanics and quantum chemical models to properly assign preferred conformation, and to account quantitatively for differences in conformer energy as well as for barriers to rotation and inversion. The chapter ends with a discussion of ring inversion in cyclohexane. 

The investigations of Lucas and his coworkers with 2,3-disubstituted butanes have contributed much to the clarification of the mechanism of ring closure and ring opening of ethylene oxides and ethylene imines. D-Mreo-2,3-Butanediol (XV) is converted into a chlorohydrin (XVII) via the diacetate (XVI).24 In the formation of the chlorohydrin (XVII) a Walden inversion occurs.26 The dehydrochlorination of XVII with... 

Theoretical estimations of energetic parameters agree quite reasonably with experimental data (Table 19.1). Value of ring inversion barrier is within 4.2-7.9 kcal/mol for molecular mechanics and 5.2-6.6 kcal/mol for quantum chemical methods. 

A relatively few oxirane ring openings have been reported to give retention or a mixture of inversion and retention (see the general reviews on the mechanisms of oxirane ring opening, and the specialized one devoted to retention (68RCR448)). 

Treatment of thiiranes with lithium aluminum hydride gives a thiolate ion formed by attack of hydride ion on the least hindered carbon atoms (76RCR25), The mechanism is 5n2, inversion occurring at the site of attack. Polymerization initiated by the thiolate ion is a side reaction and may even be the predominant reaction, e.g. with 2-phenoxymethylthiirane. Use of THF instead of ether as solvent is said to favor polymerization. Tetrahydroborates do not reduce the thiirane ring under mild conditions and can be used to reduce other functional groups in the presence of the episulfide. Sodium in ammonia reduces norbornene episulfide to the exo thiol. 

The zincblende (ZB), or sphalerite, structure is named after the mineral (Zn,Fe) S, and is related to the diamond structure in consisting entirely of tetrahedrally-bonded atoms. The sole difference is that, unlike diamond, the atoms each bond to four unlike atoms, with the result that the structure lacks an inversion center. This lack of an inversion center, also characteristic of the wurtzite structure (see below), means that the material may be piezoelectric, which can lead to spurious ringing in the free-induction decay (FID) when the electric fields from the rf coil excite mechanical resonances in the sample. (Such false signals can be identified by their strong temperature dependence due to thermal expansion effects, and by their lack of dependence on magnetic field strength). 

Besides, cyclic nitroso acetals have attracted considerable interest because [3+ 2]-cycloaddition of nitronates to olefins provides a simple and efficient procedure for the synthesis of five-membered cyclic nitrosals (see Scheme 3.153). Hence, knowledge of the configurations of these products allows conclusions about the mechanism of this very important process. However, it is impossible to distinguish kinetic and thermodynamic products of [3+ 2]-cycloaddition of nitronates to olefins without studying such a fundamental problem as the stereodynamics of five-membered cyclic nitroso acetals, that is, the ring and nitrogen inversions. 

Penicillins hydrolyze to penicilloic acids, which retain the same L5/f,6/f(-configuration (Fig. 5.10). In the subsequent step, however, penicilloic acids slowly epimerize in aqueous solution to their (5S,6f )-isomers [110]. The mechanism of C(5)-epimerization involves opening of the thiazol-idine ring by C-S bond fission, followed by reclosure with inversion of configuration at C(5) (Fig. 5.10) [111]. 

The conformational dynamics of furanose rings may be described by the mechanisms of pseudorotation and inversion. The former mechanism describes a continuous pathway of interconversion between twenty idealized non-planar (envelope, twist) conformers (Figures 1 and 2) that does not involve the planar form (e.g., - E4 — ... 

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