COMMUNICATION

Secondary a-deuterium kinetic isotope effects for the E2 reaction of the 2-phenylethyl halides with tert-butoxide ion in tert-butyl alcohol

PETER JAMES SMITH' A N D KANCHUGARAKOPPAL S. RANGAPPA Departmetzt of Chemistry. Utiiver.sity of Sa.skatchewarz, Sn.skrztoon, Sask., Canada S7N OW0

AND

KENNETH CHARLES WESTAWAY Department of Chemistry, Laurer~tiat~ University, Sudbury. Otrt., Cnrzada P3E 2B6

Received August 2, 1984

PETER JAMES SMITH, KANCHUGARAKOPPAL S. RANGAPPA, and KENNETH CHARLES WESTAWAY. Can. J. Chem. 63, 100 (1985).

Secondary a-deuterium kinetic isotope effects have been determined for the elimination reactions of 2-phenylethyl halides with tert-butoxide in tert-butyl alcohol at 40°C in the presence and absence of the crown ether 18C6. The second-order rate constant k, and the normal (kt'/kD), effect remained constant when the tert-butoxide concentration was varied for reaction of the iodo and bromo compounds. However, both the magnitude of k, and the secondary a-deuterium isotope effect were significantly dependent on [t-BuO-] when chlorine and fluorine are the leaving groups. It is noteworthy that (kH/k"),, is inverse for the reaction of both the chloro and fluoro compounds at "low" base concentrations and normal at "high" base concentrations. These results are discussed in terms of both syn- and anti-elimination pathways promoted by various associated and dissociated base species. I t is suggested that the (kH/k"),, effect may be useful as a criterion for determining the stereochemistry of E2 elimination reactions.

PETER JAMES SMITH, KANCHUGARAKOPPAL S. RANGAPPA et KENNETH CHARLES WESTAWAY. Can. J . Chem. 63, 100 (1985). Operant 3 40°C et en prtsence ou en I'absence de I'tther couronne 18C6, on a dttermink les effets isotopiques cinttiques

secondaires des deuterium en a sur les reactions d'klimination des halogtnures de phenyl-2 Cthyles par la tert-butylate dans I'alcool tert-butylique. Dans les cas des compos6s iodCs ou bromCs, la constante de vitesse du deuxikme ordre (kZ) ainsi que I'effet normal (kH/k"),, demeurent constants lorsqu'on fait varier la concentration du tert-butylate. Toutefois, lorsque les groupes nucltofuges sont soit Ie fluor ou le chlore, ['amplitude de k2 ainsi que I'effet isotopique secondaire des deuterium en a varient d'une f a ~ o n importante avec la concentration de I'ion tert-butylate. I1 est notable que, pour les rkactions des composts fluorCs ou chlorCs 3 de "basses" concentrations de base, la valeur (kl'/k"),, est inverse; toutefois, h des concentrations "ClevCes" de base, cette valeur est normale. On discute de ces resultats en fonction de voies d'eliminations syn et anti qui sont PavorisCes par diverses cspkces basiques associCcs et dissocites. On suggkre que I'effet (kk'/k"),, pourrait etre un critkrc utile pour dCterminer la sttrtochimie des rCactions dlCliminations E2.

[Traduit par le journal]

The elimination reaction of the 2-arylethyl derivatives with different leaving groups Y, ArCH,CH,Y, has been extensively studied (1 -6) under a variety of solvent/base conditions. It has been established that the majority of the reactions proceed by way of the E2 mechanism following the anti pathway (5,7). Experimental studies (8- 14) have demonstrated that the struc- tures of the respective E2 transition states vary significantly in accord with theoretical re dictions (15-20).

The reaction of the 2-arylethyl quaternary ammonium ions with base has been-shown to have considerable carbanionic character at the E2 transition state ( 1 , I 1,21), while the corre- sponding reaction of the 2-phenylethyl halides has been consid- ered (1 1,16) to involve a more central E2 transition state where the extent of the carbon-hydrogen and carbon - leaving group bond rupture processes has progressed in a more synchronous fashion.

In order to gain a direct measure of the relative amount of hybridization change at the a carbon at the E2 transition state (double-bond character), we have determined the secondary a-deuterium kinetic isotope effects for the reaction of the 2-phenylethyl halides with tert-butoxide in tert-butyl alcohol at 40°C, eq. [I].

t-BuOH [I] PHCHZCHzX + t-BuO -PHCH=CH:

X = F, CI, Br, I

'Author to whom correspondence should bc addressed

The reactions, which were shown to give the styrene product in 100 2 1 % yield, were studied under pseudo-first-order conditions where the rert-butoxide concentration was at least in twenty-fold excess. 'The progress of the reactions was determined by monitoring the product spectrophotometrically (X,,,.,, = 248 nm). The analysis was done after the reaction solutions had been quenched at appropriate times with a dilute solution of sulfuric acid in 95% ethanol. 'This was done to eliminate the appreciable absorption of rerr-butoxide ion at this wavelength. For reactions involving the crown ether 18C6, the reference cell in the spectrophotometer contained a solution of potassium tert-butoxide and the crown ether 18C6 in tert- butyl alcohol, both at the same concentrations as used in the kinetic experiments in order to compensate for the absorption of the complexed t-BuOK at 248 nm.

The second-order rate constants for the reaction of the 2-phenylethyl halides with different tert-butoxide concentra- tions in rerr-butyl alcohol, both in the absence and presence of the crown ether l8C6, are given in Table I along with the observed values of the secondary a-deuterium kinetic isotope effects, (k"/kD),. The theoretical maximum values (22-25) of the isotope effect corresponding to complete hybridization change from sp3 to .up' at the transition state for the different leaving groups are also included.

It is-ienerilly considered that the reaction of the 2-phenyl- ethyl halides with ethoxide in ethanol proceeds by way of the E2 mechanism (I3,26-29) . Since a plot of log k2 (rate con-

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COMMUNICATION

TABLE I. Secondary a-deuterium isotope effects for reaction of the 2-phcnylcthyl halides with t - B u O in t-BuOH in thc abscncc and prcscncc of 18C6 at 40°C

Leaving (X1'/k"),, group [t-BuO-1. M [18C6], M x:, M ' s ' (~"lk"), , (theoretical maximum)

:':For two deuteriums. tThc crror in the isotope effect is calculated using the formul;~:

where A!iH and Ak" arc the standard deviations for the rate constants for the undeuteratcd and deuterated reactants. respectively.

stants found at "high" base concentration) for reaction with tert-butoxide in the present study against log kz (EtO-) (30) is linear,' it is reasonable to conclude that reaction with tert- butoxide ion also proceeds via the concerted mechanism.

It is seen (Table 1 ) that the second-order rate constants for both the iodo and bromo' substrates do not change significantly when the [t-BuO-] is doubled. However, there is a significant increase in kz in the reactions of the chloro and fluoro com- pounds when the tert-butoxide concentration is increased within certain limits; i.e., for the bromo compound, k2 = 1.92 x lo-' and 1.99 x lo-' M - ' s-' when [t-BuO-] =

0.01688 and 0.03376 M, respectively, while for the fluoro compound k, = 8.1 1 x lo-' and 9.80 x lo-%-' s f ' when [t-BuO-] = 0.1767 and 0.4066 M, respectively.

Addition of the crown ether 18C6 to the reaction of the chloro compound leads to the expected large increase in the magnitude of the second-order rate constant; i.e. k, = 5.43 x lo-"-' s - l when [t-BuO-] = 0.01320 M while k2 = 0.413 M-' s-' when [t-BuO-] = 0.01296 M in the presence of 0.01377 M 18C6. It is known (3 1 ) that 18C6 complexes the potassium of t-BuOK converting the ion pair or larger aggregates to non-associated ions which are more effective (stronger) bases.

It is seen that the secondary a-deuterium isotope effects, like the k, values, do not vary with base concentration for the reactions of both the iodo and bromo compounds. On the other hand, both the a-deuterium isotope effects and the k, values, for the reactions of both the chloro and fluoro com- pounds with t-BuO- in the absence of 18C6, vary over a par- ticular range of tert-butoxide concentrations. Of particular

significance, however, is the observation that the secondary a-deuterium i so to~e effects are inverse at low base concen- trations. To our knowledge, this is the first example of an inverse secondary a-deuterium isotope effect in an elimination reaction proceeding via the one-step concerted E2 pathway.

The reactions of both the chloro and fluoro substrates with tert-butoxide ion at the "higher" base concentrations, Table 1, have normal secondary a-deuterium isotope effects. As well, when the crown ether is added to the reaction solutions con- taining "low" concentrations of tert-butoxide, where inverse a-d, effects are found, the observed isotope effects are normal and are similar in magnitude to those found at "high" base concentrations.

An approximate measure of the degree of double bond for- mation (extent of C-X bond rupture) at the transition state for reaction of the four substrates can be determined by comparing the observed a-deuterium isotope effects, found for reaction carried out at "high" base concentrations, with the calculated theoretical maximum values for each leaving group, Table I. The percent of the theoretical maxima for the leaving groups are 7 , 5 1, 62, and 25 for F, CI, Br, and 1, respectively. 'This variation in the extent of double-bond formation with a change in leaving group can be rationalized in terms of the More O'Ferrall ( 1 7) - Jencks (19) energy surface diagrams if it is considered that the reaction of the fluoro compound proceeds via an Elcb-like E2 transition state while the iodo substrate reacts via a more central E2 activated c ~ m p l e x . ~

A reasonable explanation for the observed experimental results is that the reaction involving the best two leaving groups (1 and Br) proceeds via the usual anti pathway at all base concentrations even though the extent of base associa- tion varies. 'The reactions involving the more electronegative

'The irreversible Elcb mechanism is also consistent with a linear (poorer) ~1 and leaving groups proceed via two distinct plot. However, since it has been established (14, 39) that both the F and Br substrates react with t-BuO- in t-BuOH via the concerted E2 stereochemical pathways, the anti and syn, with the relative

pathway, it is reasonable to conclude that all four substrates react via proportions of each pathway depending On the tert-butoxide the same mechanism under these experimental conditions. concentration (32-34).

'Other workers (9, 39) also concluded that there was no significant drift in the second-order rate constants for reaction of the bromo 'A more detailed discussion can be found in a full paper to follow compound in the t-BuO- in t-BuOH. in this journal.

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102 CAN. 1. CHEM. VOL. 62. 1984

When the tert-butoxide concentration is "low" and t-BuOK exists to a significant degree as an associated base (ion pair), reaction of both the C1 and F substrates proceeds via a syn- elimination pathway where the loss of the leaving group and the P-hydrogen are assisted by the t-BuOK ion pair. For such a transition state 1 , which is expected to have considerable car- banionic character (3.3, sufficient negative charge could devel- o p on the P-carbon so that inversion occurs at both Cp and C,. Such a pathway is consistent with the observed inverse isotope effects which arise from an increase in the C,-H out-of-plane bending frequencies at the transition state (22-25, 36, 37, and footnote 4).

Inversion \OQ , / C C ; a a;\

The effect of tert-butoxide concentration on the magnitude of both k, and (k"/kD), is shown by the data in Table I for the reaction of the chloro substrate. It is seen that both the rate constant k, and the magnitude of (k"/kD), initially increase from their values at "low" base concentration, when tert- butoxide concentration is increased, and level off at "high" base concentration. A possible rationalization of these results is that an increase in tert-butoxide concentration leads to a de- crease in the amount of associated base species (which favours syn-elimination) and an increase in the concentration of multi- ple ions such as negatively charged ion triplets, (RO-),Mt, which may be considered as a free base (38). The bulky triple ion would presumably favour anti-elimination, for steric rea- sons, and a normal isotope effect and an increased rate of reaction would be observed because the triple ion is a stronger base than the associated species.

The effect of the addition of 18C6 to the reaction medium is consistent with the above explanation. The crown ether will complex the potassium ion and liberate a free tert-butoxide ion, an effective base, which removes the P-hydrogen via the anti- pathway. It is interesting to note that the magnitude of the secondary a-deuterium isotope effect is essentially the same for reaction of the chloro compound at "high" base concentration (triple ion as abstracting base) and under conditions of "low" base concentration in the presence of the crown ether where free t-BuO- acts as the base. This suggests that the transition state structure is similar for reaction under the above condi- tions, in agreement with studies on the reaction of 2-phenyl- ethyl bromide with dissociated and associated tert-butoxide ions (39). Finally, it appears that an inverse secondary a-deuterium kinetic isotope effect may be useful as a criterion for determining the stereochemistry of E2 elimination reac- tions; i.e., an inverse isotope effect is consistent with a syn- elimination and a normal kinetic isotope effect is diagnostic of a normal anti-coplanar E2 mechanism.

Acknowledgements The authors gratefully acknowledge the financial support

provided by the Natural Sciences and Engineering Research Council of Canada.

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