Trans diaxial geometry

Evidence for structure 318 was provided by desulfurization of 318 with Raney nickel, with formation of the deoxy derivative 320. Additional confirmation of the trans- diaxial geometry of the thiocyanate was the formation of episulfide 321 from mesylate 319. Guthrie and Murphy131,132... 

Figure 8.8 conformation X (equatorial Cl) The trans diaxial geometry for the E2 elimination in... 

Trans-diaxial elimination (Section 9.3) Elimination of axial groups on adjacent atoms on a cyclohexane ring. This is the preferred geometry for the E2 reaction with cyclohexane derivatives. 

Nearly all cyclohexanes are most stable in chair conformations. In the chair, all the carbon-carbon bonds are staggered, and any two adjacent carbon atoms have axial bonds in an anti-coplanar conformation, ideally oriented for the E2 reaction. (As drawn in the following figure, the axial bonds are vertical.) On any two adjacent carbon atoms, one has its axial bond pointing up and the other has its axial bond pointing down. These two bonds are trans to each other, and we refer to their geometry as trans-diaxial. 

Trans diaxial (Section 8.8B) In an elimination reaction of a cyclohexane, a geometry in which the P hydrogen and the leaving group are trans with both in the axial position. 

An important feature of the Evans system is the insight provided into catalyst structure. The X-ray crystal structure of the [Sn(Ph-pybox)](OTf)2 complex displays the central Sn(II) atom in square pyramidal geometry. The three amino donors occupy a meridional position with the triflate counter ions trans -diaxially bound. These are slightly distorted away from the stereochemically relevant, Sn-centered lone pair that resides in the meridional plane. Electrospray ionization studies reveal that the cationic complex is readily generated by dissociation of the triflate counter ions, underscoring the kinetic lability of [Sn(Bn-box)](OTf)2 and [Sn(Ph-pybox)](OTf)2 towards ligand exchange, the sine qua non of a catalytic process. 

The X-ray crystal structure of 9a confirms the octahedral coordination geometry in which the two PEta ligands assume trans diaxial positions. The aromatic ring is nearly planar with the iridium atom lying just 0.13 A out of the ring carbon plane (Cl/... 

Halogen atoms in 1,4-dioxane prefer to occupy an axial position, favored by an anomeric effect For this reason, trans-2,3-dichloro-l,4-dioxane prefers the diaxial geometry 1 ... 

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... 

If the ring system has a substituent in the 3-position, then the molecule has two asymmetric centers, and two pairs of diastereoisomers are possible (see Fig. 2). The most commonly encountered system is that obtained from the naturally occurring l-a-amino acids. These acids have the S absolute configuration and thus provide the 3S, 8a S structure 79a, which has the hydrogens at positions 3 and 8a cis-diaxially and the 3-substituent pseudoequatorially disposed. This isomer is frequently referred to as the cis isomer and is the mirror image of the dd isomer 79d. Both the dl derivatives, 79b and 79c, are sometimes described as having trans geometry and have the 3-substituent in a pseudoaxial orientation. 

The synthesis of pyranoid sugar derivatives having an episulfide group fused to the ring involves requirements of molecular geometry more critical than for episulfides on an acyclic chain. Conversion of an epoxide in a locked-ring system to an episulfide by the action of thiourea or thiocyanate ion is diflicult, because reactions of this type involve a trans-fused, cyclic intermediate (91), formation of which requires a [Pg.179]

The required anti and coplanar transition state geometry can also be used to predict the regiochemistry of E2 elimination in halocyclohexanes such as chlorocyclohex-anes. In these molecules, anti and coplanar correspond to trans and diaxial. Consider the E2 reaction of the cis isomer of l-chloro-2-isopropylcyclohexane.The major product is 1-isopropylcyclohexene, the more substituted cycloalkene. 

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