The Anomeric Effect and Associated Stereoelectronic Effects

Chapter 14

Stereoelectronic Effects in Pentaoxysulfuranes Putative Intermediates in Sulfuryl-Group Transfer Dale R. Cameron and Gregory R. J. Thatcher

Downloaded by UNIV OF GUELPH LIBRARY on June 5, 2013 | http://pubs.acs.org Publication Date: November 23, 1993 | doi: 10.1021/bk-1993-0539.ch014

Department of Chemistry, Queen's University, Kingston, Ontario K 7 L 3 N 6 , Canada

Ab initio calcluations have been performed on rotamers of the trigonal bipyramidal species (HO)3SO2- and CH3O(HO)2SO2- at the HF/3-21+G(*) and HF/6-31+G*//3-21+G(*) levels. Significant stereoelectronic effects on energy and bond length are observed, correlated with rotation about the equatorial S-O(H) bond in the non -methylated rotamers, in simile with those observed in phosphoranes. Three approaches are taken to decompose individual contributions to these effects: (1) rigid rotor conformational energy surface scans with Mulliken population analysis; (2) comparison of torsional profile periodicity with that calculated for HOSO3+; (3) full geometry optimization and natural bond order (NBO) analysis of charge transfer (CT) interactions for each rotamer. N B O analysis indicates that the combination of (a) internal hydrogen bonding and (b) the consequent increase in geminal σ->σ* CT interactions, contributes significantly to changes in geometry and stabilization of the rotamer with the equatorial O H bond in the apical plane. This analysis is confirmed by observation of the loss of stabilization in the methylated congener. In neither system does any evidence exist for the n->σ* interaction proposed to dominate in phosphoranes.

Sulfuryl group transfer is a reaction of some biological significance (7). However, mechanisms of nucleophilic substitution at S(VI) are poorly studied relative to the related reactions at phosphorus (2). As part of a study of the mechanism of sulfuryl group transfer, we have initiated computational studies on putative reaction intermediates. An associative process, depicted in Figure 1, results in a pentacoordinate trigonal bipyramidal (TBP) intermediate, for which H S 0 - (R=H) is a model. An investigation of stereoelectronic effects in the H3SO5" series of pentaoxasulfuranes was undertaken to compare with similar pentaoxaphosphorane systems (H3PO5 ") in which specific stereoelectronic effects have been postulated (3,4). 3

5

2

0097-6156/93/0539-0256$06.25/0 © 1993 American Chemical Society

In The Anomeric Effect and Associated Stereoelectronic Effects; Thatcher, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

14. CAMERON & THATCHER

Stereoelectronic Effects in Pentaoxysulfuranes

257

In the phosphoranes, the general trend is for a much longer apical bond as the equatorial OH is rotated from 90° to 0° (Figure 2). A corresponding bond shortening is also observed for the equatorial P - 0 bond. This has been rationalized as an n - x j * interaction between one of the non-bonding lone pairs (n) of the equatorial oxygen and the anti-bonding (σ*) orbital of the apical P-0 bond. Figure 2 depicts this proposed stabilization. This argument has been proposed as the possible driving force for endocyclic cleavage in the hydrolysis of methyl ethylene phosphate and a manifestation of the anti-periplanar lone pair hypothesis (ALPH) (3). H 0

OR


2

+

^

ROS0 3

ROH

HO—S^'ge i

•

+ HOSO3-

H

Figure 1. Reaction Scheme of Sulfate Esters with Putative Intermediate The interaction suggests an in-phase relationship with the lobe of the anti-bonding orbital, increasing the strength of the P-0 equatorial bond. The donation of electron density from the lone pair into the anti-bonding orbital would lead to a weaker P-0 apical bond, with longer bond length. Bond Shortens^ Oxygen Non-bonding Lone Pair (n)

^Bond Lenthens

P-0 Anti-Bonding Orbital (σ*)

Figure 2. Pentaoxaphosphorane System Showing η->σ* Derealization Three approaches are employed to define the cause of the observed stereoelectronic effects. Firstly, the conformational energy surface is examined using rigid rotor calculations, accompanied by Mulliken population analysis. Secondly, the torsional profile obtained by full optimization at each constrained torsion angle is compared with H O S 0 . Finally, N B O analysis and comparison with C H S 0 H - allows definition of the dominant orbital mixing effects contributing to the observed stereoelectronic effect. +

3

3

5

2

Method The structural and electronic nature of penatoxasulfiiranes was investigated by looking at various structures using RHF/3-21+G(*) level (5-8) geometry optimization.. The starting point for the rigid rotation of H3SO5" was obtained by full optimization using HONDO-8 (9) on an I B M RISC-6000/320E workstation. The subsequent rotations were performed as single point calculations and bond orders were calculated (10). The surfaces obtained were generated with the Surfer (77) series of programs. The optimized rotational points were calculated using Spartan 2.1 (12) on a Silicon Graphics Indigo workstation at the RHF/3-21+G(*) level. Natural Bond Orbital



258

THE ANOMERIC EFFECT AND ASSOCIATED STEREOELECTRONIC EFFECTS

(NBO) populations were also calculated using Spartan (13-16). Each of these points was recalculated with Gaussian 92 at the RHF/6-31+G*//3-21+G(*) level using the standard basis set supplied by Gaussian (77). The Gaussian 92 calculations were performed on an I B M RISC 6000/320E workstation. Pre-orthogonal N B O overlap matrices, second order perturbational analyses and full NBO analyses were performed with Gaussian 92. Both the eqec and apec structures were also optimized at the 631G* and 6-31+G* levels and N B O analysis was performed to investigate basis set trends. The sulfurane series H3SO5" is depicted in Figure 3. The structures located were distorted trigonal bipyramids (TBPs). TBP structures are characterized by the positioning of three ligands, around the central sulfur atom, in a plane with angles close to 120°. This is called the equatorial plane. There are two more ligands, one above and one below the equatorial plane, each bonded to sulfur. These are called the apical ligands. We have further named the two rotamers in Figure 3 depending on the position of the equatorial O H (01-H7). When 01-H7 is parallel to one of the apical bonds (S-05) and thus, has a torsion angle (05-S-01-H7) of 0°, it is eclipsing the apical bond and is called apec. Similarly, when the 01-H7 is eclipsing the S-04 equatorial bond, it has a torsion (05-S-01-H7) of 90° and is called eqec. Optimizations were performed along the rotational pathway from eqec to apec at 0°, 4.8° ,20° ,50° ,70° ,75° ,80° ,85° and 90° by freezing the 05-S-01-H7 dihedral angle and optimizing the rest of the internal coordinates. Bond lengths and angles, as well as charges, derived from the electrostatic potential (CHELP) (18) were recorded for each of these points. Full N B O analysis was not calculated for the 75° ,80° or 85° points. Two other points were calculated. The equatorial proton of both apec and eqec was replaced my a methyl substituent and the geometry of these was optimized to compare with the proton species. Both structures (apecMe and eqecMe) were located by full optimization with the Spartan package and the N B O analysis was performed using Gaussian 92. The fully optimized eqecMe species is not located at a torsion angle of 90° but at 75.740.

Apical

apec

eqec

Equatorial Plane

Figure 3. Pentaoxasulfiirane Trigonal Bipyramidal (TBP) Structures Results and Discussion 1. Rigid Rotation. The optimized structure for the eqec species was located and the apical (01-S-05-H9) and equatorial (05-S-01-H7) dihedral angles were rotated in 30° increments generating 144 rigid structures from which, energies and bond orders were obtained. The important surfaces generated are shown in Figures 4 and 5.


14. CAMERON & THATCHER

Stereoelectronic Effects in Pentaoxysulfuranes

259

Energy. There are three main features of the energy surface to note (Figure 4a). First, there is a large energy maximum when both the apical and equatorial dihedral angles are 0°. This is due to steric repulsion of the two hydrogens. This barrier is overestimated as the geometries were not optimized. The next feature to note concerns the location of minima on the surface. It is clear that there is a minimum located when the equatorial dihedral angle is at 0° and the apical dihedral angle is ~ 180°. There is clearly a much stronger dependence of energy on equatorial rotation than on apical rotation. This relative insensitivity to apical rotation has also been noted in the phosphorane case (2).


(a)

(b) ~

-1B0 -9 0 Equatorial

0 90 180 (05-S-01-H7)

1B0

-1B0 -9 0 Equatorial

0 90 180 (05-S-01-H7)

(d)

(c) 1B0

ca m ι in ο I CO

ι -90

The Anomeric Effect and Associated Stereoelectronic Effects

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