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Anomalous Behavior of the Ethyl Group in the Amin이ysis of S-Phenyl Acetate with Benzylamine in Acetonitrile

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Anomalous Behavior of the Ethyl Group in the Aminolysis Bull. Korean Chem. Soc. 2002, Vol. 23, No. 2 201

Anomalous Behavior of the Ethyl Group in the Amin이ysis of S-Phenyl Acetate with Benzylamine in Acetonitrile

IkchoonLee,* Hai WhangLee,* ByungChoonLeeJ andJinHeuiChoF

Department ofChemistry, Inha University, Inchon 402-751, Korea

‘Department of Chemistry, ChoongbukNation시 University, Chongju360-763, Korea Received November 16,2001

The rates of theaminolysis of S-phenylsubstituted-acetateseries(RC(=O)SCsH4Z, with R=Me,Et, i-Pr, t-Bu and Bn)with benzylamines(XC6H4CH2NH2) are not correlatedsimply with theTaft’s polar (b*)and/orsteric effect constants (Es) of the substituents due to abnormally enhanced rate of the substrate with R=Et.

Furthermore,thecross-interactionconstant,pxz, is thelargest with R=Et.Theseanomalousbehaviorscan only be explainedby invokingthe vicinal bond (b)-antibond (b*) charge transfer interactionbetweenC-C« and C-S bonds. In the tetrahedral zwitterionic intermediate, T,formed with R=Et the vicinal bC-C-b*C-s delocalization is thestrongest with anoptimum antiperiplanar arrangement and a narrowenergy gap, Ae =ea* -&. Due to thischargetransfer interaction, the stability of theintermediateincreases (withtheconcomitant increase in the equilibrium constant K(= kk-a)) and also theleavingability of the thiophenolate leavinggroup increases(and hence kb increases) sothat theoverall rate,=Kkb, is strongly enhanced. Theoretical support is providedbythenaturalbond orbital (NBO) analysesat theB3LYP/6-31+G* level.The anomaly exhibited by R=Etattests tothe stepwise reaction mechanism in which theleaving group departure is rate limiting.

Keywords: Aminolysis ofS-phenyl substituted-acetate series, Anomalous behaviors ofthe ethyl group, Naturalbond orbital (NBO) analysis, Cross-interaction constant.

Introduction

The nucleophilic substitution reactions of S-phenyl sub­ stituted-acetate (SPA) series, RC(=O)SAr where R=Me (CH3),1 EHCH3CH2),2 i-Pr((CH3)2CH), 3 t-Bu((CH3)3C) 3 and Bn(C6H5CH2),4 with a large excess amountofbenzylamines in acetonitrileare found to proceedby a stepwise mechanism through a tetrahedral zwitterionic intermediate, T ,with rate-limiting expulsion ofthe leaving group, thiophenolate anion (ArS-), kb in eq. 1. Since the reactions occur intwo steps,the overall rateconstants, kN, arecomplex,eq.2.

o_

RC(-O)SC6H4Z + 2XC6H4CH2NH2、稣 1 r-c-sc6h4z nh2ch2c6h4x

SPA + T± ⑴

如『RC(=O)NHCH2C6H4X + xc6h4ch2nh3 + sc6h4z

kN =(ka/k-a)•kb =Kkb (2) The reactivity and selectivity parameters including the cross-interaction constants,5 pxz, in eqs. 3 where X andZ represent substituentsinthenucleophile and leavinggroup,

log(奴z/kM= pxbx+pzbz + pxzbxbz (3a) pxz =dpz/dbx =dpx /dbz (3b) respectively, are summarized in Table 1. One notes im-

^Corresponding Authors. Tel: +82-32-860-7671; Fax: +82-32­

865-4855; e-mail: ilee@inha.ac.kr; hwlee@inha.ac.kr

mediately from this Table anomalously high rate (kN) and large magnitude ofpxz for the a-methyl substituted acetate, i.e., R=Et. The purpose ofthis workis to examine factors thatareresponsible for suchanomalousbehaviors exhibited by the S-phenyl acetate homologue (SPA) with R=Et. Since the Taft’s polar (b)6 as well as steric effect constant (Es)6 decreases, i.e., becomesmore negative, by a successive a- methylationfrom R=Me to R=t-Bu (Table 1), reactivity is expectedtochange(increase or decrease)successivelyalong with the increase in the number of methyl group onthe a- carbon when the same mechanism applies to all the membersinthe series, as normallyhave beenobserved and reported.6 The unexpected anomaly observed with R=Et therefore suggests some important stereoelectronic factors operative in the transition state that are not reflected in the Taft’s b*andEs substituent constants.

Results andDiscussion

The rate constants,kN in Table 1,arecorrelated(excluding R=Et) with the Taft’s polar substituentconstants, b*,eq.4,6 wherek0 is thekN value with R=Me forwhich b*=0.

log(kN/k0) = p*b* (4) Figure 1 shows that the p* value obtainedfor 4 R’s except R=Et is p*=2.97 士0.22 with correlation coefficient r= 0.994. The observed reactivity of R=Et is higher by ca. 16 times than thatderived fromeq. 4. In contrast tothe good correlationof log kN vs. b, inclusionofthesteric effect (eq.

5a)6 gave unsatisfactory correlation, eq. 5b, with r =0.779 for the5R’s.

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202 Bull. Korean Chem. Soc. 2002, Vol. 23, No. 2 Ikchoon Lee et al.

Table 1. Reactivity parameters for the reactions of RC(=O)SC6H4Z with XC6H4CH2NH2 in acetonitrile at 45 oC

aX = Q-MeO andZ = p-Me at 45 oC. bZ=H. cX=H.dX=Q-MeO andZ = p-Me at 45 oC. ^Extrapolatedvalue fom Arrhenius plot.

R d* k2X103a

(M-1s-1) pX (庭

PZC

(位) Pxz ah

(kcal mol-1)

-AS*

(e.u.) kn/kn ref.

Me, CH3 0.00 0.00 1.65 -1.40 5.32 0.90 5.2-6.2 46-56 1.25-1.39 1

(1.36) (-2.21) (1.33)d

Et, C2H5 -0.10 -0.07 13.6 -2.09 2.74 2.36 3.9-4.6 53-60 1.18-1.24 2

(2.11) (-1.18) (1.23)

z-Pr, (CH3)2CH -0.19 -0.47 0.320 -1.33 4.35 0.82 10.6-13.2 30-41 1.22-1.53 3

(1.30) (-1.82) (1.22)

t-Bu, (CH3)3C -0.30 -1.54 0.221e -1.35 3.65 1.05 9.9-11.5 39-46 1.23-1.51 3

(1.30) (-1.49) (1.23)

Bn, C6H5CH2 0.22 -0.33 6.68 -1.50 1.61 0.92 4.2-5.5 49-60 1.21-1.72 4

(1.55) (-1.66) (1.36)

Figure 1. The plot log [kN(R)/kN(Me)] vs. a for the reactions of RC(=O)SC6H4-Q-CH3 with Q-CH3OC6H4CH2NH2 at 45 oC where R= Me, Et, z-Pr, t-Bu and Bn.

log(kNko) =p d"+SEs (5a) log(kNko) = 1.74 士 2.19d* +0.55 士 0.70Es (5b) Forthethreepoints (R=Me, Z-Pr, t-Bu) correlation of log

(知/k°)with the stericeffectconstants, (Es), gave S =0.30 士 0.05 (r=0.987);thetwo, R=Etand Bn,deviated wildlyfrom the linearity.Comparison of thep andS valuessuggests that the steric effect (Es) contributes much less (ca. one tenth) thanthepolar effect (d) tothe overall rates.One reason for this could be that the steric effect is compensated by its opposite influence on the rate. In the additionstep, ka, the bulkier thesubstituentR,therate becomes moreretarded by steric hindrance, whereas the rate is enhanced by steric relieving effectin the rate-limiting expulsion oftheleaving group (kb)from the intermediate, T士. The negligible steric effecthas led, ofcourse, to the satisfactory correlationwith polar effect alone (eq. 4). The p d correlation (eq. 4) is expected to apply for the reaction where steric and conjugative effects donot play anyrole.6,7 Forexample the Sn1 and SN2 reactivities of alkyl derivatives are normally well correlated with eq. 4.7

Theanomalous reactivity found with R=Et cannotbedue

to the steric effect since there is no reason to believe the ethyl group has ananomalouslyrate enhancing streic effect.

We therefore think that it is caused by a rather strong conjugative effect within theTS. Sincethe rate constant 知 is composed of two constants,知=K-kb, the rate enhancing effectcan be oneither K orkb, or on both.

In the molecular orbital theory, the proximate (geminal and/or vicinal) bond-antibond (a-d*) charge transfer de­ localization stabilizations are well established effects.8 The charge transfertakes place from a bonding orbital (which can be n, a or lone pair, n) to an unoccupied antibonding orbital(which can be n or d*), represented in generalasd- d, and the stabilization energy is given by a second-order perturbation energy, AE^2)^-^*, in eq. 6 where Ae is the energy gap betweenthetwo, bonding (n, n or d type) and AE-d= -2F2g*/( &f =- 2(kSd)2 /Ae (6) antibonding, orbital levels (n* or d type) and Fg* is the Fock matrix element whichis proportional(with a constant k) totheoverlap (Sg*) betweenthetwoorbitals.8 Nowif we look atthe Tintermediatestructures, 1 and 2 inScheme 1, for thecaseof (b) theC2-R2 d bond is locatedantiperiplanar to thevicinalC1-LG antibondingdorbital,ascan be readily seen in structure 2. The vicinal d-d* charge transfer interaction isstronger when theyare antiperiplanar thanthey are synperiplanar to eachother.8 Wetherefore expect that in

Scheme 1. R1=C6H5CH2 and LG二SC6H4Z, (a) Me : R2=R3=R4=H, (b) Et : R2=CH3, R3=R4=H, (c) z-Pr : R2=R3二CH3, R4=H, (d) t-Bu : R2=R3=R4=CH3.

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Anomalous Behavior of the Ethyl Group in the Aminolysis Bull. Korean Chem. Soc. 2002, Vol. 23, No. 2 203

Figure 2. The structure of tetrahedral intermediate (T)bound with aniline and S-phenyl methyl (A) and ethyl (B) acetate at the RHF/3- 21G*//RHF/3-21G* level.

the case of(b) with R=Et, there will be a strong charge transfer delocalization and the structure, i.e., the T±

intermediate, is strongly stabilized (and hence K islarge in eq. 2). The natural bond orbital (NBO) theory8b-c predicts thatthe Oab-o*cdNBO delocalizaionleads to a decrease in A-B and C-D bond orders (and hence causes stretching of the bonds) anda simultaneous increase in B-c bond order (causescontraction of B-C bond). In otherwords the OC-C- gc-sinteraction in (b) leads to overall stabilization ofthe intermediate, T±,(increase in K), but stretchesthe C-S bond somewhat, which facilitates, of course,theexpulsion of the leaving group, ZC6H4S-, in therate-limitingstep(increase in kb). As a result,the rate is enhancedsince both K and kb in eq. 2 are increased. An antiperiplanar arrangememt intermediate structure is presented in Figure2.

Whatabout in other compounds? Since the OC-H level is lower than the OC-C level,8a the g-g interaction in (a) with R=Mewillbesmallerdue to alargerenergy gap, △&in eq.

6. ForR=i-Pr (c) and t-Bu (d) cases, the electron-donating abilityincreases with anincrease in the number of Megroup ontheC2atom (as reflected in thealmost 2 and 3timesmore negative Taft polar effect (O*) constants6 inTable 1). Asa resultof this greater electrondonation toward the C-Sbond, the oc-sorbitallevel is raised9 and leads to a wider energy gap,匡 ineq. 6 and reducesthe charge transferstabilization energies, AE⑵o-g*. Furthermore as the a-carbon is successively methylated, steric inhibition increases in the intermediate, T ±, which will cause stretching ofthe C1-C2 bond8b,d and in effect theoverlap, S" decreases. Thesetwo effects, (i) wider energy gap, (Ae=large), and (ii) weaker overlap, (Soo*=small) cause weak and insignificant stabilizationof the T± structure and the rate enhancingeffect dueto oC-c-o* c-sinteraction becomes small (much smaller than that for R=Et case). For R=Bn, the phenyl group is a relativelystrong electron-acceptor(o*=+0.22) so that the oc-c

level is depressed9 and hence AE?)*, decreases due to a wider energy gap, Ae, in eq. 6.

We have performed natural bond orbital (NBO) analyses8b-e to calculate charge transfer energies using a model system (3)with R1=Hand LG=F in 1 and2.Forthe

ethyl case, R=Et, the g-gc-f vicinal charge transfer stabilization8is the largest with AE^2)g-o*= -4.9 kcal mol-1at

R--- --- nh3 R=Me,Et,z-Pr and t-Bu

theB3LYP/6-31+G* levelof theory.For theR=Me, i-Prand t-Bu, thecorrespondingstabilizationenergiesarelower with -4.5, -2.9 and -3.2 kcal mol-1 respectively. Albeit the differences in the charge transfer stabilization energies are small due to the adoption of a model calculation, the expected trend is borne out in the result of our density functional theory(DFT)calculation.10

In conclusion, the vicinal oC-c-o*c-s charge transfer delocalization is the strongest with R=Et due to the optimum, antiperiplanar, arrangement between g and g orbitalsaswellasthenarrowenergy gap, Ae.

The most stable intermediate, T ,with R=Et should provide the strongest C-N bond and hence the largest magnitude of Px and §xvalue is observed experimentally (Table 1). The strongest g-o interaction should leadto a facile C-S bond cleavage in the rate-determining step, kb, and the TSis reachedatanearlystage with lowerdegree of bond cleavage. The lower degree of bond cleavage is reflectedin the smaller magnitude of (orp£) value, and alsoin thelowactivation enthalpy, AHin Table 1. It is well known that the maincomponent ofthedeformation energy, AEdef, which is required totransform the reactant to the TS structure, is the stretching ofthe cleaving bond.11 Thus a tightest TSisrealized with R=Et as evidenced bythelargest positivecross-interactionconstant,5 pxz, in Table 1.

The kinetic isotope effects kH/kD, involving deuterated benzylamine (XC6H4CH2ND2) nucleophiles are all greater than unity12 and the magnitude is similar so that the kH/kD

values do not provide a very sensitive measure of the TS structure.

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204 Bull. Korean Chem. Soc. 2002, Vol. 23, No. 2

The anomalous reactivity and large magnitude of pxz found with R=Etinthe S-phenylseries are alsofound in the dithioseries,RC(=S)SC6H4Z. The rateis the fastest and the Pxzvalue is thelargest with R=Et in thedithioseries studied so far (R=Me,13 Et14 and Bn15). If our interpretation is correct, the same anomaly willbe found with R=Et in the phenolate series, RC(=O)OC6H4Z, as well as in the thiono series, RC(=S)OC6H4Z . Substitution of O by S, or of S by O, does not change the strongest vicinal 0Cc-S c-lgcharge transfer interaction expected with R=Et, as can be seen in Scheme 1. The same argumentpresented above for the S- phenyl series should applyto the other series, providedthe samestepwisemechanism appliestoallthemembers inthe respective series.We stress that the anomaly observedwith R=Etisa manifestation of the stepwise mechanismthrough a tetrahedral zwitterionic intermediate with rate-limiting expulsion of theleavinggroupon the groundsthat : (i) Ifthe bond formation step, in which polar effect alone is important, were rate-limiting, the rate order should have been in the orderof the Taft’s o* constant; R= t-Bu <z-Pr<

Et < Me < Bn. (ii) If the reaction rates were affected exclusively by steric effects of thetype presentinreactions usedto define E3, the rate (log kg) should have been well correlated with the steric effectconstant, Es, alone with the susceptibilityconstantS.(iii)Ifthereactionsproceeded bya simple concerted (S^2) process, the Taft equation (5a) should have been valid.7 (iv) If the bond cleavage were solely responsible in determining the rate, the rate order should have been in the reverse order of the Taft’s o*

constants since electrondonating R groupfacilitatesleaving group expulsion16;R=Bn<Me<Et < Z-Pr < t-Bu.

We conclude that theanomalyobserved withR=Etprovides evidencefor a stepwisemechanismin which therateconstant (kN) is composed of two factors as expressed by eqn. (2).

Thus the observed rate is determined by both the stability (K) ofthe intermediate,T, andnucleofugality of theleaving group (kb). Both ofthese factors are outstandinglyfavorable for the reactions with R=Et due tothe strong vicinal Oc-c-

o*c-s charge transfer interaction in the intermediate, T . Further experimental as well as theoretical works are in progresstosupportourargumentspresented in thiswork.

Cculations.Ab initio MO calculationswere performed with Gaussian 98 system of programs.17 Geometries were optimized at the B3LYP/6-31+G* level of theory. The natural bond orbital (NBO) analyses were carried out to obtainproximate bond- antibond(o-o*) orbitalinteraction energies (AE(2)oo) attheNBO-B3LYP/6-31+G* level.

Acknowledgment. This paper is dedicated to Prof.

Kyung-HoonJung on the occasion of his 65thanniversary.

This work was supported by grant No. R-01-1999-00047 fromtheBasic ResearchProgram of theKoreaScience and Engineering Foundation. Wealso thank InhaUniversityfor support of thiswork.

Ikchoon Lee et al.

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