Correlations of Additions to Alkenes.

    We have developed a technique for correlating measurable characteristics in addition reactions of alkenes in order to gain information which is useful mechanistically and synthetically.  We have applied this technique to many reactions:   hydroboration,^1-4 oxymercuration,^2-4 bromination,³ epoxidation,⁴ sulfenyl halide addition,⁴ mercuric chloride complexation,⁴ silver ion complexation,⁵ diimide addition,⁵ oxidation with permanganate,⁶ carbene addition,⁷ nitrosyl chloride addition,⁷ oxidation with osmium tetroxide,⁷ chlorination,⁸ complexation with iodine,⁸ palladium chloride (Wacker) oxidation,⁹ hydrogenation using Wilkinson's catalyst RhCl(PPh₃)₃,¹⁰ chromyl chloride oxidation,¹¹ and chromic acid oxidation,¹¹ iodine thiocyanate,¹² and dipolar cycloadditions.¹³ This technique offers (1) a procedure to determine relative magnitudes of steric and electronic effects in the rate-determining step, (2) a relatively simple way to predict the effects of substituents on reaction rates for synthetic purposes, and (3) a method to choose between alternative proposed reaction mechanisms in some cases.
    In these investigations, we apply this simple method by correlating the logs of the relative reaction rates (log k_rel values) with the alkene ionization potentials (IPs), with their highest occupied molecular orbital energies (HOMOs), and in some cases with their lowest unoccupied molecular orbital energies (LUMOs).  Reactions with similar mechanisms give correlation plots which are similar in appearance, and those with different mechanisms give dissimilar correlation plots.  For example, the correlation plots of hydroboration and bromination show similar slopes caused by similar electronic effects but different groupings of alkenes due to steric effects; hydroboration exhibits a natural separation into sterically similar groups while bromination has a single line of correlation regardless of the steric requirements of the alkene.

    On the other hand, plots of hydroboration and palladium chloride (Wacker) oxidation show similar groupings due to steric effects, but opposite slopes caused by opposite electronic effects.  In order to develop further this new technique and to elucidate synthetically and mechanistically important information from experimental or from computational data, we apply the technique to additional important reactions with a variety of mechanisms.

References
1.  An Experimental and Theoretical Investigation of the Influence of Alkene HOMO Energy Level Upon the Hydroboration Reaction.  Additional Evidence Supporting an Early Transition State Which has Retention of Alkene Character.  Nelson, Donna J.; Cooper, Penny J. Tetrahedron Lett. 1986, 4693-4696.
2.  A  Comparison of the Relative Reaction Rates of Alkenes Toward Hydroboration and Toward Oxymercuration.  Nelson, Donna J.; Cooper, Penny, J.; Coerver, Judy A. Tetrahedron Lett. 1987, 943-944.
3.  A Simplified Method of Ascertaining Steric Effects in Electrophilic Addition Reactions.  A Comparison of Bromination, Oxymercuration, and Hydroboration. Nelson, Donna J.; Cooper, Penny J.; Soundararajan, R. J. Am. Chem. Soc. 1989,111, 1414-1418.
4.  Using the Comparison of Steric Versus Electronic Effects to Infer Mechanistic Information in Stepwise Electrophilic Addition Reactions Involving Three-Membered Cyclic Intermediates.  Nelson, Donna J.; Soundararajan, R. Tetrahedron Lett. 1988, 6207-6210.
5.  Diimide Reduction of Some Representative Alkenes and the Correlation of Their Relative Rates of Reduction with Their Corresponding Ionization Potentials. Nelson, Donna J.; Yao, Ziyun; Henley, Robert L.; Smith, Terrill D. Tetrahedron Lett. 1993, 5835-8.
6.  Relative Rates of Permanganate Oxidation of Functionalized Alkenes and the Correlation with the Ionization Potentials of Those Alkenes.  Nelson, Donna J.; Henley, Robert L. Tetrahedron Lett. 1995, 6375.
7.  Selected Electrophilic Addition Reactions of Representative Acyclic Alkenes.  Nelson, Donna J.  Tetrahedron Lett. 1999,  5823.
8.  Correlation of Ionization Potentials or HOMO Energies Versus Relative Reactivities of Cl₂, of Br₂, and of I₂ with Representative Acyclic Alkenes. Comparison with Other Additions to Alkenes. Nelson, Donna J.; Li, Ruibo; Brammer, Christopher. J. Org. Chem. 2001, 66, 2422.
9.  The Correlation of Relative Rates of PdCl₂ Oxidation of Functionalized Acyclic Alkenes Versus Alkene Ionization Potentials, HOMO's, and LUMO's.  Nelson, Donna J.; Li, Ruibo; Brammer, Christopher. J. Am. Chem. Soc. 2001, 123,1564-1568.
10.    (a) Correlation of Relative Rates of Homogeneous Hydrogenation of Acyclic Alkenes in the Presence of Wilkinson’s Catalyst vs Alkene IPs, HOMOs, and LUMOs. Nelson, Donna J.; Li, Ruibo; Brammer, Christopher N. J. Org. Chem. 2005, 69, 761 - 767; 2004, online http://pubs.acs.org/cgi-bin/asap.cgi/joceah/asap/pdf/jo048968r.pdf (b) Correlation of Relative Rates of Homogeneous Hydrogenation of Acyclic Alkenes in the Presence of Wilkinson’s Catalyst vs Alkene IPs, HOMOs, and LUMOs. Nelson, Donna J.; Li, Ruibo; Brammer, Christopher N. Cover art for Journal of Organic Chemistry. 2005, 69. (c) Correlation of Relative Rates of Homogeneous Hydrogenation of Acyclic Alkenes in the Presence of Wilkinson’s Catalyst vs Alkene IPs, HOMOs, and LUMOs. Nelson, Donna J.; Li, Ruibo; Brammer, Christopher N. Cover art for American Chemical Society Division of Organic Chemistry Calendar, Sep 2006.

11. Correlation of Relative Rates of Chromyl Chloride Oxidation and Chromic Acid Oxidation of Acyclic Alkenes Versus Alkene IPs and HOMOs. Nelson, Donna J.; Li, Ruibo; Brammer, C. N. J. Phys. Org. Chem. 2004, 17, 1033 - 1038.

12. Substituent effects in additions of iodine thiocyanate to alkenes. Brammer, C. N.; Nelson, Donna J.; Li, Ruibo. Tetrahedron Lett. 2007, In press.

13. Correlations of relative rates of selected 1,3-dipolar cycloadditions to alkenes versus alkene LUMO energies and IPs. Brammer, C. N.; Nelson, Donna J.; Li, Ruibo. Tetrahedron Lett. Submitted.