Nucleophilic Attacks on Low Lowest Unoccupied Molecular Orbital

12 Nucleophilic Attacks on Low Lowest Unoccupied Molecular Orbital Compounds Downloaded by UNIV LAVAL on September 24, 2015 | http://pubs.acs.org Publication Date: July 1, 1987 | doi: 10.1021/ba-1987-0215.ch012

Shmaryahu Hoz Department of Chemistry, Bar-Ilan University, Ramat-Gan 52100, Israel

The nature of the transition state of nucleophilic reactions with LL [low lowest unoccupied molecular orbital (LUMO)] substrates is analyzed and reviewed. In cation-anion combination reactions, a partial radical character is developed on both the nucleophile and the substrate. Examination of a simple state diagram shows that this diradicaloid character is increased as the LUMO of the substrate is lowered. The model is further extended to other LL substrates such as carbonyl functions and activated olefins. Three empirical manifesta tions of the diradicaloid character of the transition state are dis cussed: (1) the correlation between the ionization potentials of the nucleophiles and their nucleophilicity toward LL substrates; (2) the α-effect phenomenon; and (3) the variations in the positional selec tivity of 9-nitromethylenefluorene in nucleophilic reactions as a func tion of the solvent.

T H E S W A I N - S C O T T E Q U A T I O N (I) is probably one of the most important empirical equations in the field of nucleophilic reactions: log (Uko) = sn

(1)

This equation relates the nucleophilicity of a nucleophile with its rate con stant in a given S 2 reaction. In this class of reactions, bond formation and bond cleavage are fused into a single transition state. A seemingly less complicated reaction is a reaction in which the status of only one bond is changed in the rate-determining step. Anion-cation combinations exemplify this type of reaction. The relationship between rate constants and nu cleophilicity in these reactions is given by the Ritchie equation (2, 3): N

0065-2393/87/0215-0181$06.00/0 © 1987 American Chemical Society

In Nucleophilicity; Harris, J., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1987.

182

NUCLEOPHILICITY

log (k/k ) = N 0

(2)

+

The two major differences between these two equations are as follows: (1) Unlike the Swain-Scott equation, the Ritchie equation lacks a selectivity parameter. Thus, a plot of log k versus N always gives a unity slope. (2) Nucleophilicity ranking by the two scales is different. This difference is especially noticeable for the three nucleophiles, C N ~ , C H 0 ~ , and N ~ . According to the N scale, the nucleophilicity order is N ~ > C H 0 ~ > C N ~ , whereas the reverse is true for the Swain-Scott η scale. To test the generality of the Ritchie equation, we reacted the olefins 9-(dinitromethylene)fluorene ( F D N ) , 9-(dicyanomethylene)fluorene (FDCN), and 9-(nitromethylene)fluorene (FN) with a series of nucleophiles (4, 5). The reaction of these three substrates resemble that of anion-cation combination reactions in the absence of leaving group departure at the transition state. Yet, in spite of the fact that these substrates are not positively charged, an excellent correlation was observed with the N scale. Similar results were observed for other uncharged substrates such as carbonyl func tions (6) and activated aryl halides (7). However, the three systems, F D N , F D C N , and F N , are unique in that the slopes of log k versus N are significantly larger than 1. This necessitates the incorporation of a selectivity parameter (S+) into the Ritchie equation. +

3

Downloaded by UNIV LAVAL on September 24, 2015 | http://pubs.acs.org Publication Date: July 1, 1987 | doi: 10.1021/ba-1987-0215.ch012

+

3

3

3

+

+

FDN

FDCN

FN

Recently, we found (unpublished results) that the reactions of the diphenyl analogues of these three substrates do not follow the Ritchie equation and the data are highly scattered. One major difference between the two sets of substrates is the extent of steric crowding around the activated carbon. However, the mechanism by which the steric effect induces this change is not entirely clear. Classification Ritchie (2) suggested that the differences between the reactions that follow the Swain-Scott and the Ritchie equations stem from the coupling between bond formation and bond cleavage in the S 2 process as opposed to only N


12.

HOZ

Nucleophilic Attacks on Low LUMO Compounds

183

bond formation in the second class of reactions. We (5), on the other hand, suggested that the origin of the differences probably lies in the nature of the lowest unoccupied molecular orbital ( L U M O ) of the substrate. The L U M O of a typical S 2 substrate being a σ* orbital is of high energy, whereas cations, carbonyls, activated olefins, and other substrates that obey the Ritchie equation have a relatively low L U M O , usually ττ*. The latter dichotomy is more in line with the generally accepted notion that highest occupied molecular orbital ( H O M O ) - L U M O interactions govern nucleophilic reac tion (8). So that these two approaches can be distinguished, a system should be found that undergoes an S 2 reaction and has a relatively low energy reactive L U M O . Because the vast majority of the substrates that possess both low L U M O and leaving group (e.g., aryl halides and esters) react with nucleophiles in a two-step process, an example that will meet the two demands is likely to be a borderline case in terms of classification. Two available examples are nucleophilic reactions with organometallic cations and with activated bicyclobutanes. Sweigart and co-workers (9, 10) showed that a linear correlation is obtained between the log k values for the reactions of nucleophiles (Nu) with various organic cations and the transition metal complexes of the same cations. The latter reactions resemble S 2 reactions if the carbon-metal bond cleavage is considered as a departure of the leaving group: N


N

N

+

Nu (3)

The second example involves nucleophilic attacks on activated b i cyclobutane. These attacks occur anti to the central bond, which is cleaved in the course of the reaction. The L U M O of this bond is lower than that of ethylene. Thus, if considered to be an S 2 reaction, the reaction complies with the two aforementioned demands. Unfortunately, data are available (11) for the reaction of this system with C N ~ and C H 0 ~ only: N

3

The order of reactivity of these two nucleophiles toward bicyclobutanecarbonitrile is C H 0 ~ > C N ~ . This order is for the nucleophilicity of the N and not of the η scale. Thus, in addition to the fact that frontier orbitals are usually considered to control nucleophilic reactions, the last two examples 3


+

184

NUCLEOPHILICITY

lend more credence to the classification of the substrates according to the energies of their L U M O s as a possible origin of the differences between the two types of behavior. Therefore, in the following discussion we will refer to the substrates acording to their L U M O s energies, namely, L L (low L U MO) or H L (high L U M O ) substrates.


Proposed Model According to the model developed (5, 12), the transition state of the reaction of L L substrates with nucleophiles should be characterized not only by partial charges ( δ ± ) but also by a partial diradicaloid character. In other words, at the transition state, both the nucleophile and the substrate acquire a partial radical character (δ·)· This finding emerges directly from the basic assumption of continuity between the two extreme situations that may take place upon a nucleophile-electrophile encounter. The first situation is the "normal" S 2 reaction, whereas the second extreme is an electron-transfer reaction that results in the formation of a radical pair. The transition state of a reaction located in the continuity zone between these two extremes can therefore be described as a combination of the two extreme electronic configurations in variable proportions. For a given nucleophile, the amount of the diradicaloid character will increase with the proclivity of the substrate to undergo electron-transfer reactions. Thus, an L L substrate will in general show more of the diradicaloid nature at the transition state than H L sub strates. In the following discussion nucleophilic reactions with L L substrate will be analyzed by making use of basic principles of electronic states. We will show that (a) under certain circumstances the transition state of nucleophilic reactions must be diradicaloid and (b) the diradicaloid nature of the transi tion state will in general increase with the increase in the electrophilicity of the substrate. To perform the analysis, we need to examine the classic state diagram (13, 14) shown in Figure 1. Curve a represents the ground state (S ) of a molecule A - B , which correlates with the diradical state ( D ) , and curve b is the excited state (S^, which correlates with the zwitterionic state (Z) (15, 16). In general, for cations and anions such as those studied by Ritchie, that is triarylmethyl cations, tropilium cations, diazonium cations, and common anionic nucleophiles such as C H 0 " , C N ~ , and N ~ , the zwitterionic state in the gas phase will always be of higher energy than the diradicaloid configura tion ( D ) . (In the case of A - B being N a - C l , the separation between curves a and b at the plateau region (7P — EA ) is small (ca. 35 kcal/mol). However, the Coulombic bonding is larger than the covalent bonding, which leads to curve crossing, which places the ionic bond well below the covalent bond. On the other hand, in the case of A - B = C H - C 1 , / F H ~~ ^ c i approximately 145 kcal/mol and the covalent bond is much stronger than the N

0

3

Na

3

C]

i s

3

C

3



12. H O Z


185

A-B

Figure 1. State diagram for the reaction A + Β —* AB. Curve a is the covalent diradicaloid state; curve b is the excited (Sj) zwitterionic state; curve c is the zwitterionic state in condensed phase. The dashed line is similar to curve c and is for the more electrophilic A'.

ionic one (using the Pauling equation, the covalent bond energy of N a - C l can be estimated as 38 kcal/mol, whereas that of C H - C 1 is approximately 80 kcal/mol). Therefore, for C H C 1 and similar compounds, no curve crossing occurs in the gas phase. (For the relevant data, see references 17 and 18). In solution, to the first approximation the covalent diradicaloid curve a will not be significantly affected by the solvent, whereas curve b will be drastically lowered in energy (curve c) so as to appear in part below curve a. Thus, in solution, the A - B bond will be cleaved heterolytically by following curve a to the avoided crossing point with curve c and continuing along curve c to give A 4- Β~ as is indeed observed in solvolytic reactions. The reaction in the reverse direction is nothing but the anion-cation combination reactions studied by Ritchie. For a covalent bond to be formed from the ionic species A and B ~ , the ions must go over a barrier that peaks in the region of the intersection (avoided crossing zone) of the potential ionic and d i radical-covalent surfaces. More importantly, upon going from A · + Β · to the covalent bond A-B, a gradual decrease in the diradical character of the system occurs. At an intermediate point between the two extremes along 3

3

+

+


186

NUCLEOPHILICITY


curve a, the system, in a simple notation, will be described as À — Β. At the transition state, this configuration will be mixed with the ionic configuration in equal proportions to give the overall configuration, which using the same notation will be described as A — B. Thus, in this case, the transition state for anion-cation combination reaction is of a diradicaloid nature, and in order to reach this state, partial electron transfer (PET) from the nucleophile to substrate must take place. This model may also be appropriate to other L L substrates, such as carbonyl groups (6), activated double bonds (5, 19), and aryl halides (7), that also obey the Ritchie equation. This seems appropriate because these L L substrates are highly polar and because particularly in solution the zwitterion I is a major contribution to their overall resonance structure:

c=x^

+

c —χ ι

where X = O, C ( N 0 ) , C ( C N ) , C ( H ) N 0 , C(H)COPh, and so on. Thus, the carbon in I can be viewed as an equivalent to A of the previous discussion. This model is not applicable in cases where the transition state of the rate-limiting step cannot be identified with the point in which curves a and c intersect. Most of the data regarding the movement of a molecular system along the a-c combination path are obtained from solvolytic reactions. These studies indicate that curve c in Figure 1 is not necessarily a smooth line (as we have drawn for the sake of simplicity) but may have some energy minima along it that may correspond to intimate and solvent-separated ion pairs observed in the course of many solvolytic reactions. In cases where these two species are defined chemical entities, the species must be separated by a potential barrier. In many cases, the transfer from intimate to solventseparated ion pairs is assumed to comprise the rate-determining step in the solvolytic reaction (20). Therefore, by microscopic reversibility, the ratedetermining step for the same reaction in the reverse direction will not be the transition from intimate ion pairs to covalent compound but rather from the solvent separated to the intimate ion pair. Therefore, the transition state of the rate-determining step of this process will not contain the diradicaloid component, which is a crucial characteristic of the transition states of reac tions obeying the Ritchie equation, and hence will not be accommodated by this equation. 2

2

2

2

+

The second point that is worth noting is the effect of the electrophilicity of A on the magnitude of the diradicaloid character of the transition state. Replacing A by A ' , which is of a higher electrophilicity, will diminish the separation (V) between curves a and c in Figure 1. Because in general the effect of such a change on the energy of the covalent bond as well as on the coulombic interaction will be secondary to the effect on V, the intersection +

+



12. Hoz


187

point of curves a and c (and therefore the transition state) will move to the right (Figure 1). As we have previously pointed out, the diradicaloid nature of the A , Β system decreases gradually upon moving from right to left (Figure 1, curve a). Thus, if the transition state is shifted to the right (an early transition state), it will acquire a larger diradicaloid character. Shifting the transition state further to the extreme right will result in a complete electron transfer that will precede the coupling of A · and Β ·. This finding, in addition to confirming our previous intuitive conclusion, also implies that the magni tude of the diradicaloid character at the transition states of a series of reactions in which either the substrate or the nucleophile is varied is not constant. At the intersection point, the electronic configurations represented by curves a and c are mixed in equal proportions to yield the final electronic configuration of the transition state. Empirical Manifestation of the Model In the following sections, three possible empirical manifestations of the proposed model are presented: (1) correlation between the nucleophilicity toward L L substrate and the ionization potentials of the nucleophiles, (2) the α effect, and (3) the positional selectivity of F N as a function of the solvent. Nucleophilicity and Ionization Potentials. Previously, the suggestion that the differences in the nucleophilic ranking between the η and the N scale may stem from a relatively large amount of electron transfer present in the transition state described by the N scale was made. If this statement is true, then some correlation between the N scale and the solution ionization potentials of the nucleophiles should exist. Because these data are not available, we have calculated (5) the energy associated with K using the thermodynamic cycle shown in Scheme I. +

+

+

3

H- + -Nu Scheme I The energy associated with K is the energy required to transfer an electron from the nucleophile to H . According to the data obtained this way, the 3

+


188

NUCLEOPHILICITY

ease with which an electron can be removed from the nucleophile is in the order N ~ > O H ~ > C N ~ . A s expected, this order is indeed the order of the N scale; thereby, the suggested model is confirmed. Subsequent to this calculation, using the same thermodynamic cycle, Ritchie (21) [however, at the conference, Ritchie reported that this correlation holds only for aqueous solutions and not for reactions in (CH ) SO] showed that activation energies for nucleophilic reactions of the pyronin cation correlate linearly with the solution ionization potentials of the nucleophiles. These two examples indi cate that the transition state of the reactions of nucleophiles with L L sub strates reflects in part features typical of electron-transfer processes. 3

+


3

2

α Effect, the α effect is defined as a positive deviation of an α nu cleophile from a Br0nsted-type plot (22). Several different origins exist for the α effect. The proposed model offers a consistent explanation of this effect based on transition-state stabilization (12). This effect manifests itself mainly with unsaturated substrates (23), which according to our terminology are L L substrates. In light of the previous discussion, the nucleophile in the transi tion state of these reactions acquires a radicaloid character. That radicals are relatively highly stabilized when located α to a lone pair of electrons is wellknown (24). This statement is easily explained in terms of the molecular orbital (MO) diagram shown in Scheme II.

/—ι—\

ι ι

\ \

A

V-

\

ι I

\

*—H—' Scheme II As can be seen from Scheme II, the fact that two electrons drop in energy whereas only one electron goes up leads to a net stabilization. This stabiliza tion effect will be partly reflected in the transition state of the reaction of an α effect nucleophile with a L L substrate. Thus, an α effect is likely to be observed because this stabilization mechanism is unique to α nucleophiles and is unavailable to any "normal" nucleophile. In principle, the magnitude of the α effect is expected to increase with the radicaloid character developed on the α nucleophile at the transition state. So that the validity of this hypothesis can be assessed, first, a probe for the degree of electron transfer at the transition state should be found. A possible probe in this case is the β value. As was pointed out by Bordwell and Clemens (25), it acquires values of 0.3-0.5 for S 2 reactions and much higher values (1.1-1.5) for electron-transfer reactions. Thus, if the β value can indeed be correlated with the radical character in the nucleophile, a η ι ι ε

N

η υ ε


12. H O Z

189


correlation between the α effect and β values should exist. Indeed, such a correlation was observed by Dixon and Bruice (26) in the reactions of hydrazines with a variety of substrates. Further support for the hypothesis that β values can be used as a measure of the radical character of the nucleophile can be gained from Fukuzumi and Kochis studies (27) on electrophilic aromatic substitution reactions: η ι ι ε

ηι10

ArH + E + — ^ A r E + H

+

(5)

+


The p value (equation 6) in these reactions was shown to reflect the mean separation of the reactants at the transition state (27): log k = p > +

+ C

e

(6)

The proton affinity (pK ) of arenes is also a linear function of σ: fl

ArH

+

Η

+

Ξ = ^ Α Γ Η

+

(7)

2

ΔρΚ = \σ+ α

Thus, substituting σ

+

(8)

9

for ΔρΚ in equation 6 results in α

+

log Κ = (ρ V p . ) A p K . + C

(9)

The last equation is in fact a Br0nsted-type equation in which β is replaced by p /p . Because p is constant, β , like p , should also be interpreted as a measure of the mean distance between the reactants at the transition state. It is clear from Figure 1 that as β increases, that is, as the separation of A and Β at the transition state grows larger, the degree of electron transfer will also increase. This analysis predicts that for large enough β values (probably larger than 1), a complete electron transfer is expected. As noted, Bordwell found this situation indeed to be the case. The proposed model does not imply that an α effect will be observed in the gas phase. Moving from right to left on curve b (Figure 1) gives an ionic bond that may later on decay to the ground state of the covalent compound. Because neither the exact mechanism nor the identity of the rate-limiting step is known for this process, no definite conclusion regarding the existence of the α effect in the gas phase can be derived on the basis of this model. ηι10

+

+

e

+

a

+

a

η υ ε

e

η ι ι ε

ηι10

Positional Selectivity in F N . The reactions of nucleophiles with F N (for structures, see the introduction) present an interesting problem. In water, the nucleophile interacts with position 9 of the fluorene ring, whereas in dipolar aprotic solvents such as (CH ) SO, the nucleophile attacks C - a ; 3

2


190

NUCLEOPHILICITY

this reaction leads ultimately to the formation of II (5), probably via a nucleophilic vinylic substitution mechanism (equation 10). In a solvent of intermediate polarity, for example, C H O H , both mechanisms are operative.


3

In general, the reactivity of olefirtic substrates is known to correlate with the electron-withdrawing power of the activating group (28). A good measure for the latter is the pK of the methano derivative of the activating group. When this criterion is employed for the two sites of F N , in water, the pK of C H N 0 is 10.2 (29), whereas that of fluorene is 21-22 (30). Thus, in water, nucleophilic attack will take place at C-9 and the negative charge will be located on the nitromethide moiety. In (CH ) SO, the pK of C H N 0 rises to 17.2 and that of fluorene remains essentially unchanged (30). Prima facie, this result could have been taken as a satisfactory explanation for the ob served shift in the site of the nucleophilic attack. However, a detailed structure-reactivity analysis shows this argument to be fallacious. This find ing becomes apparent from a comparison of F D C N with F N . In spite of the fact that nitromethane is more acidic than malononitrile by approximately 1 pK unit (29), F N is less reactive than F D C N by 2 log units (5). On the other hand, a linear correlation does exist between the log k for the nucleophilic attacks on the three substrates and log k (deprotonation reaction) for the corresponding methano derivatives. A similar correlation was reported by B e r n a s c o n i et a l . (31) for the reactions of β - n i t r o s t y r e n e , b e n zylidenemalononitrile, and Meldrum s acid derivative. Pearson and Dillon (29) have shown that a plot of log fc versus log K for many carbon acids in water is linear, with the marked exceptions of nitromethane and nitroethane, which deviate strongly from that line. Extrapolation using this plot shows that the kinetic acidity of nitromethane should be correlated with a pK of 17 rather than 10.2. Thus, the effective pK of nitromethane that should be used a

a

3

2

3

2

a

3

2

a

ion

ion

a

a

fl


12. H O Z


191

both in water and ( C H ) S O is 17 (using this value in a plot of log k for nucleophilic reactions with F D N , F D C N , and F N in water versus pK values yields the expected linear correlation). Hence, the origin of the different positional selectivities in water and ( C H ) S O cannot be explained on the grounds of variations of the pK of nitromethane. A more suitable explanation can be suggested for this case, which makes use of the present model. Because at the transition state the substrate acquires a partial radical-anionic character, the radical anion of the substrate should be examined. The positional selectivity will most likely be deter mined by the location of the unpaired spin population in the model radical anion, toward which the radicaloid nucleophile will be attached in order to complete bond formation. A similar argument was invoked by Kochi (27) to explain the positional selectivities observed in electrophilic aromatic sub stitution. To determine the spin population distribution, we (32) performed semiempirical ( M N D O ) calculations on the radical anion of F N with and without two water molecules hydrogen bonded to the nitro group. The optimized bond lengths are shown in Figures 2-4 (the geometry of the water molecules was not optimized and the N O · · · H O H separation was arbitrarily set to 2.0 A). The M N D O coefficients of the S O M O on C-9 and C-α are given in Table I. Geometry optimization of F N (the neutral molecule) was performed with the aromatic portion of the molecule kept planar. The results show that the rest of the molecule is coplanar (within 1°) with the aromatic moiety. No 3

2

fl

3

2


a

Figure 2. Bond lengths in the MNDO-optimized structure of FN.



NUCLEOPHILICITY

Figure 4. Bond lengths in the MNDO-optimized structure of FN ~2H 0 (see the text). 2


12. HOZ Table I.

Nucleophilic Attacks oh Low LUMO Compounds

Total Energies, MNDO, and R H F Open Shell STO-3G Coefficients of the SOMO on C-9 and C-α in F N FN^

Method MNDO MNDO STO-3Gfr a

b


c

193

FN2H 0 2

Energy (eV)

C-9

C-a

Energy (eV)

C-9

C-a

-2764.408 -2764.564 -730.017'

0.414 0.436 0.458

-0.443 -0.404 -0.630

-3466.816 -3467.133 -879.958

-0.438 -0.461 -0.489

0.426 0.376 0.572

Calculation performed on M N D O - o p t i m i z e d geometry of F N . Calculation performed on M N D O - o p t i m i z e d geometry of F N . E n e r g y in au. 7

restrictions were imposed in the geometry optimization of the radical anion (FN* ). In its final structure, all carbon atoms are found essentially in a single plane. However, the dihedral angle around the C - 9 - C - a bond is 23°-26° with a slight pyramidalization about C-α. R H F open shell S T O - 3 G ( H O N D O ) (33, 34) calculations were performed at the M N D O optimized geometries. The R H F coefficients at the S O M O are also given in Table I. Obviously, these calculations provide an indication of general trends rather than reliable absolute data. However, the calculations show that adding water molecules to the system indeed causes a shift of spin population from C-α to C-9. This shift in spin population is probably the governing factor in deter mining the positional selectivity in the reactions of F N with nucleophiles. -

Acknowledgmen t The assistance of D . Cohen in performing the M N D O calculation is grate fully acknowledged. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Swain, G. C.; Scott, C. B. J. Am. Chem. Soc. 1953, 75, 141. Ritchie, C. D. Acc. Chem. Res. 1972, 5, 348. Ritchie, C. D.; Virtanen, P. Ο. I. J. Am Chem. Soc. 1972, 94, 4966. Hoz, S.; Speizman, D. Tetrahedron Lett. 1978, 1775. Hoz, S.; Speizman, D. J. Org. Chem. 1983, 48, 2904. Ritchie, C. D. J. Am. Chem. Soc. 1975, 97, 1170. Ritchie, C. D.; Sawada, M . J. Am. Chem. Soc. 1977, 99, 3754. Fujimoto, H . ; Fukui, K. In Chemical Reactivity and Reaction Paths; Klopman, G., Ed.; Wiley: New York, 1974; Chapter 3. Alavosus, T. J.; Sweigart, D. A. J. Am. Chem. Soc. 1985, 107, 985. Kane-Maguire, L. A. P.; Honig, E. D.; Sweigart, D. A. Chem. Rev. 1984, 84, 525. Hoz, S.; Aurbach, D. J. Org. Chem. 1984, 49, 4144. Hoz, S. J. Org. Chem. 1982, 47, 3545. Evans, M . G.; Polanyi, M . Trans. Faraday Soc. 1938, 34, 11. Warshel, Α.; Weiss, R. M . J. Am. Chem. Soc. 1982, 102, 6218. Michl, J. Top. Curr. Chem. 1974, 46, 1.



194

NUCLEOPHILICITY

16. Dauben, W. G.; Salem, L.; Turro, N . J. Acc. Chem. Res. 1975, 8, 41. 17. Pauling, L. The Nature of the Chemical Bond; Cornell University: Ithaca, NY, 1960. 18. Turner, D. W. Adv. Phys. Org. Chem. 1966, 4, 31. 19. Ritchie, C. D.; Kawasaki, A. J. Org. Chem. 1981, 46, 4704. 20. Harris, J. M . Prog. Phys. Org. Chem. 1974, 11, 89. 21. Ritchie, C. D. J. Am. Chem. Soc. 1983, 105, 7313. 22. Hoz, S.; Buncel, E. Isr. J. Chem. 1985, 26, 313. 23. Fina, N . J.; Edwards, J. O. Int. J. Chem. Kinet. 1973, 5, 1. 24. Nelson, S. F. In Free Radicals; Kochi, J. K., Ed.; Wiley: New York, 1973; Vol. 2, Chapter 21. 25. Bordwell, F. G.; Clemens, A. H . J. Org. Chem. 1981, 46, 1035. 26. Dixon, J. E . ; Bruice, T. J. Am. Chem. Soc. 1972, 94, 2052. 27. Fukuzumi, S.; Kochi, J. K. J. Am. Chem. Soc. 1981, 103, 7240. 28. Patai, S.; Rappoport, Z. In The Chemistry of Alkenes; Patai, S.; Ed.; Wiley: New York, 1964; Chapter 8. 29. Pearson, R. G.; Dillon, R. L. J. Am. Chem. Soc. 1953, 75, 2439. 30. Matthews, W. S.; Bares, J. E.; Bartmess, J. E . ; Bordwell, F. G.; Comforth, F. J.; Drucker, G. E . ; Margolin, Z.; McCallum, R. I.; McCollum, G. J.; Vanier, N . R. J. Am. Chem. Soc. 1975, 97, 7006. 31. Bernasconi, C. F.; Fox, J. P.; Fornarini, S. J. Am. Chem. Soc. 1980, 102, 2810. 32. Dewar, M . J. S.; Thiel, W. J. Am. Chem. Soc. 1977, 99, 4899. 33. Dupuis, M . ; King, Η. F. Int. J. Quantum Chem. 1977, 11, 613. 34. Dupuis, M . ; King, Η. F. J. Chem. Phys. 1978, 68, 3998. RECEIVED

for review October

2 1 , 1985.

ACCEPTED

February

10, 1 9 8 6 .


Nucleophilic Attacks on Low Lowest Unoccupied Molecular Orbital

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