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0040-4039/82/060615-04$03.00/O 01982 Pergamon Press Ltd. Tetrahedron Letters,Vol.23,No.6,pp 615-618,1982 Printed in Great Britain THEORETICAL Department of Department Laboratoire STUDIES of OF SN2 TRANSITION STATES, THE Saul Wolfe* and D.J. Mitchell Queen’s University, Kingston, Chemistry, Ontario, H. Bernhard Schlegel+ Wayne State University, Detroit, Chemistry, ALPHA Canada Michigan, C. Minot and 0. Eisenstein ThBorique, Bi?timent 490, Centre de Paris-Sud, de Chimie EFFECT K7L 3N6 U.S.A., 91405 48202 Orsay, France Swmnary: The quazitative HOMO-LVMO orbital interaction interpretation of the aZpha effect is found to be invaZid for anionic nucleophiles. In any event, gas phase SN2 reactions of HOO- and FO'Oshow no evidence of the effect. The role of a hydroxylic solvent requires greater consideration as the source of positive deviations from rate-equiZibrim plots. The frontier focuses upon the argument, the Therefore, molecular charge efficiency the of process I- > Br- > Cl- since iodide is seems thus observed experimentally solvent, the mides are Cl- > Br- that the relevance experimental result applicable to molecule of the clusters the rates the manner under then energy change. lose ent to anions to a gas of the water are phase usual is such ions to In the the the the to incorporate usual qualitative HOMO-LUMOenergy HOMOor the It changes theory,g and the is also effects the trends,’ leaving been is group ability, group, as is known5 that, nature inacetone and alkyl bro- the based the the reaction upon is strictly stable ion- heats of Moreover, reaction’ and leaving intrinsic treatment of barriers” of so any particular coordinate. with nucleophilicity Marcus to theory formation can be correlated of straightforward,6 trends that the of not ability uncertain concepts from group because ia in a treatment follows long best p-toluenesulfonates solvent the SN2 reactions has the gap. lower-lying order. SN2 reactions, phase distinction it several and leaving uncertain. conditions’ and also However, towards of gas phase gas by Marcus also of data solvent indicate are large. characteristics fore, quantitative that large,13 As discussed SN2 reaction bell-shaped solvation the X- + CH3Y + XCH3 + Y- CH3Y. in group and overall SN2 reactions conducted solvents.” A variety gas phase reaction higher-lying nucleophile, solvents.4 The same conclusion in hydroxylic the LUMOof proportional the nucleophilicity of of their of the are best halide reverse solution (efficiencies) ability be the FM0 method treatment suggested the predicted in inversely HOMOlevels to the the is X- to expected to follow electronegativity * * * * and the LUMO levels are u CI < ‘CBr < ‘CC1 ’ ‘CF.’ to conform to the concepts of nucleophilicity and leaving of of HOMOof more rapidly in hydroxylic > I-, Extension the predicted reactivities (FMO) treatment’ the reaction proceed > F-, The theory from the will LUMO. For X or Y = halogen, i.e., orbital transfer causes reaction of the enthalpies of of Clearly, SN2 reactions 615 transfer have in the of in heats of ion-molecule solvation profound solution.‘a effects of inorganic of addition by Bohme,“* disappearance anions, of differences recently coordinate. different treatment the and that anions solvation three between water complexes anions, upon the the differ- molecules and restoration and the rates, from different and, there- 616 is common to correlate the in solution It with Briinsted basicities of of the constant rate linear, RO- , or that all or RS-, correlations for of effect of the of of (iv) action plot. of of a normal cause of the of heteroatom found to lie nucleophiles of intrinsic barriers are the enhanced centre, this rate reactions the logarithm nucleophile, depends or i.e., all Br6nsted tertiary carbon.” be a special upon both With is centre, now known to relatively reactivity and is effect (i) the of normally has been factors: of ground electron case thermodynamic closely related invariant,” nucleophilic manifested discussed mem- and the reagents of state; (iii) containing as a positive extensively,” destabilization transition state pairs combination A lower the deviation and usually ground state of of the stabilization in the products; of of destabilization is the an alpha-nucleophile.21 these electron HOMO-LUMOenergy of This pairs, difference result which results, orbital leads lies to higher inter- a HOMO than the and is considered to combination observed for HOMO be the reactivity. is incorrect is if also lower gas phase. anions. the When the taken FM0 theory second first into account, than the is valid, The out-of-phase order order the HOMOof perturbational Calculated HOMOEnergy effect (i.e., electronegativity’z) out-of-phase HOMOs of HO-, a decrease as seen of HOMOEnergies (au) from in reactivity The FM0 rationalization TABLE. Anion for represents centre. substantially in the of alpha-nucleophiles. adjacent reacting Therefore, carbon, is factors.g”8 following an alpha-nucleophile the addition RNH2, different secondary correlation the to FM0 theory, argument adjacent Table. to increased to carbon, a BrSnsted in which the or a plot e.g., barrier) stabilization nucleophile. attached family, activation reaction an out-of-phase This HOMOof the (ii) the the The origin solvation between that relationship refers to According comprised a given R = primary is substitution Typically, K refers to the conjugate acid of the a in question share the same nucleophilic describe (intrinsic one or more of reduced nucleophilic term diminishes. adjacent alpha-nucleophile; of nucleophiles.16 Even within nucleophiles, kinetic on a BrEnsted terms etc. rate-equilibrium The alpha-effect” a heteroatom where restrictions and kinetic a family the nucleophiles all these a more general (equilibrium) bers pK,, the RNH 2’ may be needed to One reason of versus provided rates (i) thus the is of the data for of the and FO- are presented predicted becomes heteroatom effect HOO-, ClO- the in the alpha- ambiguous. Oxyanions a,b of HOMOEnergy Anion HO- -0.0340 HOO- -0.0868 CH30- -0.0635 FO- -0.1372 (au) aAt the 4-31G level, with full geometry optimization (1 au = 627.5 kcal/mol); bin the computations of M.M. Heaton, J. Am. Chem. Sot., 100, 2004 (1978), the HOMOof ClO- was found to lie 19.9 kcal/mol below the HOMOof HO-, but the HOMOof HOO- was 6.3 kcal/mol above that of HO-. The latter is a result of Heaton’s use of the geometry of the hydroperoxyl radical rather than the anion. Recomputation of HOO-, using Heaton’s basis set and the fully optimized geometry, lowers the energy of the HOMOby 35.0 kcal/mol. In terms independent of the interpretations, Marcus treatment because these of reactivity, factors are (i), (ii) interrelated and (iii) when the do not constitute more general rate- 6 17 equilibrium correlation a Marcus-type involving are fifteen any of is employed to (eq exists relationship independent hydrogen, carbon, [l]) probe combinations nitrogen, for the alpha-effect. in the gas phase of X and Y, fluorine oxygen, We have for methyl in which or the sulfur, found transfer entering and in recently* that reactions and leaving groups which HOO- and FO- have been included. [II AEXY AEXyis In eq [ll, rated reactants, intrinsic of the the AE’ barriers of in completely not dynamic that this conform the type the alpha-effect. are clustered to eq the [l], for not accepted valid. around for the the transition state and the sepat and AE:X and AEyy are the reaction, As shown by Brauman,” gas phase since of This value of the shows and now does (r the AEXy is a good of to exists, it which and kinetic a plot alpha-nucleophiles relationship solvents, alpha-effect, conform = 0.99) of Marcus-type in hydroxylic Figure (-20 SN2 behavior no experimental thermodynamic relationship a single AEXY the definition RO- + CH3F + ROCH3 + F-, A linear between change reactions. reactions a combination is + AE;y)} measure SN2 reaction. However, exists difference energy degenerate of normal. than caveat the energy potential a gas phase yet rather the terms alpha-nucleophiles does ab initioz3 is for efficiency Thus, FO- is = Q(AExX + AE ) + $AE” + iAEo2/8(AE;, YY effects. AEXy versus the accepted because the could is incorporating be argued based Figure AE” for test for solely -4 AE” Figure 1. A plot of AEXy versus AE” for ab initio reactions of HO-, CH30-, HOO- and FO- with CH F. Data are in kcal/mol from 4-31G level computa 2.Ions with full geometry optimization. that eq [l] on thermo- 1 demonstrates four the RO- intrinsic kcal/mol). HOO- and reactions existence barriers” of of 618 Unfortunately, all experimental data relating to the alpha-effect refer to hydroxylic solvents, in which the role of (iv) is expected to be maximal. According to recent work by Benson,24 the difference in the heats of aquation of HO- and HOO- is 21.5 kcal/mol. Although the existence of such a large solvation effect is not surprising,13 the possibility that this factor alone may be responsible for the alpha-effect seems generally to have been discounted, despite the recognition otherwise given to the important role of solvation effects. It is known that the magnitude of the alpha-effect depends upon the reaction type, and is smallest in an SN2 reaction.25 Clearly, it is necessary to extend the theoretical treatment of rate-equilibriumrelationships from SN2 reactions to the more complex problems of additions to sp and sp2 centres. It is also necessary to extend experimental investigations of the rate-equilibrium behavior of alpha-nucleophilesto a range of dipolar non-hydroxylic solvents. Finally, it appears that the validity of the FM0 treatment of certain bimolecular reactions requires reevaluation. ACKNOWLEDGMENTS: This research has been supported by grants from the Natural Sciences and Engineering Research Council of Canada, and the Petroleum Research Fund, administered by the American Chemical Society. REFERENCES AND NOTES t Fellow of the Alfred P. Sloan Foundation, 1981-1983. 1) I. Fleming, Frontier Orbitals and Organic Chemical Reactions, John Wiley, New York, 1976. 2) F.A. Carey and R.J. Sundberg, Advanced Organic Chemistry, Part A, Plenum Press, page 13. 3) R.C. Bingham, J. Am. Chem. Sot., 97, 6743 (1975). 4) T.H. Lowry and K.S. Richardson, Mechanism and Theory in Organic Chemistry, Second Edition, Harper and Row Publishers, page 333. 5) S. Winstein, L.G. Savedoff, S. Smith, I.D.R. Stevens, and J.S. Gall, Tetrahedron Lett., No. 9, 24 (1960); A.J. Parker, J. Chem. Sot. A, 220 (1966); A.J. Parker, Chem. Rev., 69, 1 (1969); F.G. Bordwell and D.L. Hughes, J. Org. Chem., 46, 3570 (1981). 6) C. Minot and Nguyen Trong Anh, Tetrahedron Lett., 3905 (1975); C. Minot, PhD Thesis, UniversitG de Paris Sud, Centre d'0rsay. 7) J.M. Riveros, A.C. Breda, and L.K. Blair, J. Am. Chem. Sot., 95, 4066 (1973). 8) S. Wolfe, D.J. Mitchell, and H.B. Schlegel, J. Am. Chem. Soc.,in press. 9) R.A. Marcus, J. Phys. Chem., 72, 891 (1968); A.O. Cohen and R.A. Marcus, ibid., 72, 4249 (1968). 10) W.J. Albery and M.M. Kreevoy, Adv. Phys. Org. Chem., 16, 87 (1978); M.J. Pelleriz and J.I. Brauman, J. Am. Chem. Sot., 102, 5993 (1980). 11) For a gas phase SN2 reaction, the intrinsic barrier associated with X is defined as the energy difference between the ion-molecule cluster [X.*.CH3X]- and the transition state [X*.CH3**X]-. In the present work, these energies have been obtained at the 4-31G level, with full optimization of all structures.8 The calculated intrinsic barriers of CH30-, HO-, HOO- and FO- are (kcal/mol): 23.5, 21.2, 18.5 and 18.2, respectively. 12) W.J. Albery, Ann. Rev. Phys. Chem., 31, 227 (1980). Conway, Ion Hydration in Chemistry and Biophysics, Elsevier, New York, 1981. 13) B.E. 14) D.K. Bohme and G.I. Mackay, J. Am. Chem. Soe., 103, 978 (1981). 15) J.F. Coetzee and C.D. Ritchie, Editors, Solute-Solvent Interactions,Volume 2, Marcel Dekker, Inc., 1976,Chapter 12. 16) J.E. Leffler and E. Grunwald, Rates and Equilibria of Organic Reactions, John Wiley and Sons, Inc., New York, 1963. 17) G. Klopman and R.C. Evans, Tetrahedron, 34, 269 (1978). 18) N. Agmon, Int. J. Chem. Kinetics, 13, 333 (1981). 19) N.J. Fina and J.O. Edwards, Int. J. Chem. Kinetics, 2, 1 (1973). 20) A.P. Grekov and V.Y. Veselov, Usp. Khim., 47, 1200 (1978). 21) G. Klopman, Editor, Chemical Reactivity and Reaction Paths, John Wiley, New York, 1974. 22) This point will be elaborated upon in a forthcoming full paper. 23) Computed at the 4-31G level with full optimization of all geometries. 24) S.W. Benson and P.S. Nangia, J. Am. Chem. Soe., 102, 2843 (1980). 25) E. Buncel, C. Chuaqui, and H. Wilson, J. Org. &em., 45, 3621 (1980). (Received in USA 8 October 1981)