Boat conformation, conformational isomers

Bohlmann and Arndt (S3) have separated the possible stereoisomers of hexahydrojulolidine (78-80) and subjected them to mercuric acetate oxidation. The rates, which were followed by the precipitation of mercurous acetate, showed that isomer 78 reacted about five times faster than isomer 79, while isomer 80 reacted very slowly. The difference in rates between 78 and 79, both of which have tertiary a-hydrogens trans to the nitrogen electron pair, was explained by pointing out that greater relief of non-classical strain occurs in the oxidation of 78 as compared to 79. Isomer 80 has no tertiary a-hydrogens trans to the nitrogen electron pair except when it is in an unfavorable boat conformation. 

The thermodynamic stabilities of three possible annular tautomers of the parent 1,2,4,5-thiatriazine 134 were compared using ab initio HF/6-31G calculations (OOJOC931). The 4H isomer 134a appears to be the most stable (it is more stable than 134b by 11.6 kcal/mol and more stable than 134c by 15.5 kcal/mol), presumably because it allows low-energy distortion from planarity and formation of the boat conformation. 

An example of a compound having a preferred boat conformation is that of [2.2.2] bicyclooctane. This compound must necessarily exist as a boat because its cis-4 hydroxy cyclohexane-carboxylic acid easily forms lactose and the water elimination must proceed through the boat conformation. The trans isomer will not lactonize. 

Experimental results indicate that for the cis isomer of 1,2-di-ter/-butylcyclohexane (8a), either the chair (84) or the boat (85) conformation is the most stable, whereas for the trans isomer (8b) the diaxial chair and the twist (86) or boat (87) forms are supposed to be in equilibrium. According to MM calculations of the ds isomer, a boat form is preferred when Bartell s force field (36) is used, whereas with Lifson s force field (30) the chair form is predicted (87). Schafer performed ab initio calculations with the STO-3G, basis set on the final structures resulting from these MM calculations and found the boat conformer to be consistently more stable than the chair (87). For the trans isomer (8b), MM calculations again are inconsistent, but ab initio calculations on the final MM geometries result in almost... 

The geometry of the cis-alkylcyclohexanol is favorable for trans elimination since the hydroxyl and the neighboring trans hydrogen are coplanar, but this is not true for the l,i-trans isomer hence the molecular conformation has to flip over, to set the hydroxyl group in the axial position for the trans elimination to occur. This would require a few kilocalories of energy and for frans-lert-butylcyclohexanol it would be more difficult to achieve than for IroMs-methylcyclohexanol. It is, therefore, possible that the trans elimination from a boat conformation, or possibly even an epimerization from the trans to the cis isomer which then undergoes a trans elimination reaction. Such an epimerization was found to occur under conditions of dehydration of certain alcohols over alumina, as will be seen under 1,4-cyclohexanediol. The more facile elimination of the cis-i-tert-butylcyclohexanol system as compared with the trans system in solution was also reported in the literature 63). 

Also, 2,2,3,6-tetra-Me-5-Cl-l,3-dioxan, due to 1,3-diaxial interactions, prefers the 2,5-twist-boat form (76BSF563) the same conformation was reported for the stereoisomeric 2-Ph-4-(2 -furyl)-5-N02-6-Me-l,3-dioxanes and 2,2,6-tri-Me-4-(2 -furyl)-5-N02-l,3-dioxanes (75MI2), for 2-Alk-2,4,4-tri-Me-l,3-dioxane derivatives (78KGS1172) and for the cis isomers of 2-OR-4-Me-l,3-dioxane (R = Et, nPr, /Pr, nBu, n-CsHn) (81DOK116). The corresponding trans isomers adopt the chair conformation with di-eq substitution. The isomeric 2-OR-4,4-di-Me-l,3-dioxanes also prefer the 1,4-twisted-boat conformer (81DOK116). 

In the solid state of cis-l,4-dibenzoyl-2,5-dimethylpiperazine, the Z,Z-form is present, with the phenyl groups twisted (77AX(B)3568) with respect to the amide plane. In solution (77TL2895), the cis-isomer of the 2,5-dimethyl and 2,5-diethyl derivatives shows the presence of only one conformer (Z,Z, with the piperazine ring in the twist-boat conformation), while the tra s-form consists of an equilibrium mixture of nearly equal amounts of the four axial alkyl rotamers. 

Although these are well-defined conformational isomers, their energies are such that they are virtually unpopulated at room temperature. (The twist boat is an intermediate in the conversion of one chair form to the other.) At the same time die conversion of one chair form to the other occurs rapidly at room temperature, and both chair forms are in rapid equilibrium. 

The 3-benzyl-6-phenyl-l,4-diazepine-2,5-diones 21 and 22 exhibited complex NMR spectra indicative of limited conformational mobility in which the ring geometry is dictated by the two air-amide elements, which define individual planes <2003MI187>. Based on an analysis of the nuclear Overhauser effect (NOE) between protons on the ring, the preferred boat conformation in solution projects the 3-benzyl moiety pseudoequatorially with the 6-phenyl substituent disposed axially or equatorially, dependent upon the relative stereochemistry. This conformation is also observed in the solid state for the air-substituted isomer 21 in which the phenyl group is axial. In contrast, the bis-phenyl derivative 23 is conformationally mobile based on the H NMR spectmm where resonances were not resolved. 

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