Bicyclo nonanes conformation

Zefirov, N. S., Conformational Analysis of Bicyclo[3.3.1]nonanes and their Hetero Analogs, 20, 171. 

The validity of Eq. (6.1) has been carefully established for linear and branched paraffins, cyclohexane and methylated cyclohexanes, including molecules consisting of several cyclohexane rings in the chair conformation (e.g., cis- and frani-decalin, bicyclo[3.3.1]nonane, adamantane, and methylated adamantanes) as well as in boat conformation (e.g., iceane and bicyclo[2.2.2]octane) [44]. No special effect seems to contribute to the chemical shift because of the presence of cyclic stmctures. 

The basic skeleton of these compounds is 2,9-dioxa-bicyclo[3.3.1]nonane (57), which has limited steric flexibility. 1,7-Anhydroheptoses adopt predominantly the twin-chair conformation 57a, provided that no endo substituents are present at positions C-3 and C-7 to cause steric interactions. Otherwise, flattened boat-like and skew conformations may also play an important role. 

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... 

The boat-chair conformation may be due to the bond shortening at C1-O2 and O4-C5, i.e., the hydrogen repulsion at C3 and C7 increases in comparison with that in bicyclo [3.3.1] nonane. In this class, XV has no polymerizability13), suggesting that the conformation of XV is a double chair and that the replacement of -CHj- by O-atom at the -C3-position lessens the strain energy. 

Secondly, make a mode of the hindered alkene 43 this shows that it is sterically impossible for bromine to bond with the alkene carbons other than by a top or bottom approach. To construct a molecular model of 43, make two models of bicyclo[3.3.1]nonane, with both cyclohexane rings in a chair conformation. Identify carbons C(3) and C(7) in each of these structures then take a molecule of ethene and use C(l) of ethene to bond to C(3) and C(7) of one bicyclic structure, and C(2) of the alkene to bond to C(3) and C(7) of the other structure. The result is the hindered alkene 43, Examination of this model, together with an understanding of the orientation of the double bond, indicates that steric factors permit isolation of the bromonium ion. 

It has been claimed that the parent hydrocarbon corresponding to 58, bicyclo[3.3.1]nonane, exists in a twin-chair conformation. The basis for this proposal came from the H NMR spectrum of the hydrocarbon (see Peters et alP) and also from the infrared (IR) spectrum, which showed a distinct band at ca. 1490 cm1, indicative of a strong transannular interaction of the methylene groups at C(3) and C(7) (see Eglinton et alP). Although results from IR spectra are probably less incisive than those from NOE experiments, they are mutually supportive. 

Conformational Analysis of Bicyclo [3.3.1] nonanes and Their Hetero Analogs... 

III. Conformational Analysis of Carbocyclic Bicyclo[3.3.1]nonane Systems Experimental and Computational Data... 

C. Influence of 1,5- and 9-Substituents on the Conformational Behavior of Bicyclo[3.3.1]nonanes... 

IV. Conformational Studies of Hetero Analogs of Bicyclo[3.3.1]nonane... 

Significant progress in conformational studies of bicyclo [3.3.1] nonanes in the past two decades have revealed the main conformational features of carbocyclic compounds bearing different substituents, including the ways in which both chair-boat and boat-boat conformations could be stabilized. The conformational behavior of some heterobicyclo[3.3.1]nonanes was interpreted in terms of conformational effects (8). Naturally occurring compounds that include the bicyclo[3.3.1]nonane moiety in their structure have also been studied extensively (9-15). 

There are hundreds of publications on virtually all conceivable aspects of the conformational analysis of bicyclo [3.3.1] nonanes. The most fundamental of these studies have been reviewed by Zefirov in 1975 (1) and, for the 3-aza-bicyclo[3.3.1]nonanes, by Jeyaraman and Avila in 1981 (2). Reviews on the synthesis of bicyclo[3.3.1]nonanes (16-18) also have been published. 

In the present chapter, the basic principles of the conformational analysis of substituted bicyclo[3.3.1]nonanes and their hetero analogs are considered. These principles are illustrated with experimental and computational data with attention concentrated on those results believed most important to the understanding of the conformational behavior of bicyclo[3.3.1] nonanes. Some results previously reviewed in references 1 and 2 are not discussed here. Conformational studies of polycyclic compounds that incorporate the bicyclo [3.3.1] nonane skeleton as part of their structure are generally beyond the scope of this chapter. 

The following conformations, free from angular strain, can be postulated for bicyclo[3.3.1]nonane itself chair-chair, CC (2a), chair-boat, CB (2b) boat-boat, BB (2c). Although conformations 2a-2c are free from angular strain, none is free from strong destabilizing interactions between nonbonded atoms. In order to discuss the experimental and computational data more systematically, it is necessary to consider the factors that stabilize and destabilize conformation 2a-2c. 

Figure 1. The designations of substituents positions for the rings in a chair (a) and in a boat (b) conformation in the substituted bicyclo [3.3.1] nonanes.

Considering the factors favoring the CB conformation, it should be taken into account that the introduction of an endo-3 substituent (11) causes one of the CB forms (11a) to be destabilized by the resulting extra 1,3-diaxial interactions. Indeed, for each six-membered ring in the bicyclo[3.3.1]nonane skeleton, the second ring is equivalent to two axial substituents, the introduction of an endo-3 substituent being equivalent in this case to a third axial one. It leads not only to the absence in the equilibrium of the chair-chair conformation lib (which is mostly destabilized by the 3 - 7 repulsion) but also to the minor content of the chair-boat form 11a. The alternative boat-chair conformation 11c with the substituent in the bowsprit position dominates in this case (24,25). 

The introduction of three trigonal atoms in a wing of the bicyclo[3.3.1] nonane skeleton leads to the planarity of the wing. Such molecules tend usually to adopt envelope-chair conformations exemplified by 16. In the case of six such trigonal atoms, the envelope-envelope form 17 would be expected to become the only stable one. 

The factors thus far considered determine the conformational behavior of alkyl-substituted bicyclo[3.3.1]nonanes and their hetero analogs in molecules in which the interaction of heteroatoms is approximately the same in different conformations. In other cases, the interaction of heteroatoms or functional groups, especially at 3-, 7-, and 9-positions, may lead to significant deviations from that of the conformational behavior of alkyl-substituted carbocyclic analogs (vide infra). Thus, not only the dipole-dipole interactions but also lone pair interactions must be taken into account. In molecules bearing several substituents and heteroatoms in different positions, a very careful analysis is necessary in making predictions since different factors can cause opposite effects and the overall conformational behavior will be determined by their balance. 

III. CONFORMATIONAL ANALYSIS OF CARBOCYCLIC BICYCLO[3.3.1]NONANE SYSTEMS EXPERIMENTAL AND COMPUTATIONAL DATA... 

---