The acid-catalyzed demetalation of 1 -(tri-n- butylstanny1)-2-phenylethyne. A surprisingly small P-stannyl effect

I. Egle, V. Gabelica, A.J. Kresge, and 1.1. Tidwell

Abstract: Rates of conversion of 1-(tri-11-butylstanny1)-2-phenylethyne to phenylethyne in HzO and D,O solutions of perchloric acid were found to be proportional to acid concentration, giving the hydronium ion rate constant kH+ = 1.85 x lo-' M-I s-I and the isotope effect kHJkD+ = 3.10. The magnitude of this isotope effect suggests that the reaction occurs by rate-determining hydron transfer to the substrate, producing a vinyl carbocation; this carbocation then loses its tributylstanny l group, giving phenylacetylene as the only detectable aromatic product. The hydronium ion rate constant, when compared to the rates of protonation of phenylethyne and I-(trimethylsily1)-2-phenylethyne, gives a P-stannyl stabilizing effect of FAG*= 6.6 kcal mol-I and a differential P-stannyVP-silyl effect of FAG*= 3.2 kcal mol-I. These stabilizations are very much smaller than previously reported P-stannyl effects. Possible reasons for the difference are suggested.

Key words: P-stannyl effect, P-silyl effect, carbocation stabilization, protodemetalation.

RCsumC : On a trouvt que les vitesses de conversion du I-(tri-n-butylstanny1)-2-phtnyltthyne en phCnylCthyne, en solutions d'acide perchlorique dans du H,O et dans du D20 , est proportionnelle B la concentration d'acide; la constante de vitesse de l'ion hydronium kH+ = 1,85 x 10-2 M-' s-I et I'effet isotopique kH+lkD+ = 3,10. L'amplitude de cet effet isotopique suggkre que la reaction se produit par une ttape dtterminant la vitesse au cours de laquelle il y a transfert d'un hydron au substrat conduisant B la formation d'un carbocation vinylique; ce carbocation perd alors son groupe tributylstannyle ne donnant que du phCnylacCtyl6ne comme seul produit aromatique pouvant &tre dttecte. Lorsqu'on compare la constante de vitesse de I'ion hydronium aux vitesses de protonation du phtnyltthyne et du I-(trimCthylsily1)-2-phCnyltthyne, on en dtduit l'effet stabilisant du groupe P-stannyle, FAG'= 6,6 kcal mol-I, ainsi que la difftrence entre les effets P-stannyle et P-silyle, FAG" 3,2 kcal mol-I. Ces stabilisations sont beaucoup plus faibles que celles rapportCes antkrieurement pour les effets P-stannyles. On suggkre diverses raisons possibles pour la difference.

Mots clgs : effet P-stannyle, effet P-silyle, stabilisation du carbocation, protodCmttallation.

[Traduit par la rtdaction]

It is well known that silyl substituents in the P-position stabi- lize carbocations strongly (for reviews, see ref. l), and other Group IV A metals lying below silicon in the periodic table have even more powerful effects. For example, a stannyl sub- stituent in the P-position was found to accelerate the rate of solvolysis of a cyclohexanol ester over that of the unsubsti- tuted substrate by a factor much in excess of 1 0 ' ~ (2), and the remarkably large stannyllsilyl rate ratio of lo8 was observed in the protonation of metalated acetylenes (3). We were surprised to discover, therefore, that the stannyl group in 1-(tri-rz-butyl- stanny1)-2-phenylethyne raises the rate of vinyl cation forma- tion by protonation of this substance, eq. [ l ] l (X = SnBu,), over that for phenylacetylene, eq. [ l ] X = H, by a factor of only

Received January 1 I, 1996.

I. Egle, V. Gabelica, A.J. Kresge,' and T.T. Tidwell. Department of Chemistry, University of Toronto, Toronto, ON M5S 3H6, Canada.

I Author to whom correspondence may be addressed. Telephone and Fax: (416) 978-7529. E-mail: akresge@alchemy .chem.utoronto.ca

6.5 x lo4, and that comparison with the corresponding silyl system, eq. [ I ] X = SiMe,, gives a stannyllsilyl rate ratio of only 2.1 x 10'.

Experimental section

Materials I-(Tri-n-butylstanny1)-2-phenylethyne was prepared by treat- ing lithium phenylacetylide with tributyltin chloride (4). All other materials were best available commercial grades.

Kinetics Rates of demetalation of 1-(tri-n-butylstanny1)-2-phenyl- ethyne were determined by monitoring the decrease in its absorbance at A = 266 nm. Measurements were made using a Cary 2200 spectrometer whose cell compartment was thermo- statted at 25.0 + 0.05"C. Reactions were initiated by adding 10-p,L aliquots of acetonitrile solutions of substrateto 3-mL quantities of perchloric acid solutions contained in quartz cuvettes, which had first been allowed to come to temperature equilibrium with the spectrometer cell compartment. The sub- strate was poorly soluble in both acetonitrile and aqueous acids, and dissolution was promoted by a few seconds' immer- sion in an ultrasonic bath. The kinetic data conformed well to the first-order rate law and observed rate constants were eval-

Can. J. Chem. 74: 13661368 (1996). Printed in Canada / ImorimC au Canada

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Egle et al.

uated by least-squares fitting of an exponential function. Acid concentrations of the reaction mixtures were determined by titrating weighed aliquots.

Results and discussion

Reaction mechanisms Rates of reaction of 1-(tri-n-butylstanny1)-2-phenylethyne were measured in H 2 0 and D,O solutions of perchloric acid over the concentration range [Acid] = 0.03 - 0.2 M. The data are summarized in Table ~ 1 ' and are displayed in Fig. 1. It may be seen that observed first-order rate constants are accu- rately proportional to acid concentration in both solvents. Lin- ear least-squares analysis of the data gave the hydronium ion rate constants k,+ = (1.85 f 0.04) x lo-' M-I s-I and k,+ = (5.96 0.20) x M-' s-', whose ratio provides the isotope effect kHlkD = 3.10 f 0.12.

Isotope effects on hydron transfer from the hydronium ion consist of an inverse (kHlkD < 1) secondary component in addi- tion to the normal (kHlkD > 1) primary component (51, and overall values consequently tend to be small. The effect deter- mined here is, in fact, close to the maximum expected value, and this provides strong evidence that the process under exam- ination is indeed a rate-determining hydron transfer from the hydronium ion to the substrate, as shown in eq. [I].

The cation formed in this process will then either be cap- tured by water, giving an en01 that will subsequently tautomer- ize to a stannyl-substituted ketone, as shown in eq. [2], or the cation will lose its stannyl group giving phenylacetylene and

+ [2] PhC =CHSnBu3

I H20 b PhC=CHSnBu3 -H+

tributylstannanol, as shown in eq. [3]. HPLC analysis of spent reaction mixtures showed phenylacetylene to be the only aro-

+ [3] PhC =CHSnBu3 H20 b PhC=CpH + HOSnBu3

-H+

matic product formed, indicating that the reaction is indeed a demetalation as shown in eq. 131.

Reactivity The presently determined rate constant for the protonation of l-(tri-n-butylstanny1)-2-phenylethyne, when compared to the rate of protonation of its unmetalated analog, phenylacetylene (6), shows that the stannyl group accelerates the rate of reac- tion some 65 000-fold and gives a P-stannyl effect of ~ A G ' = 6.6 kcal mol-I. Comparison with the corresponding trimethyl- silyl analog, l-(trimethylsilylj-2-phenylethyne (7), shows the stannyl substituent to be better than the silyl group by a factor of 210 and gives a differential P-stannyVP-silyl effect of 6AG' = 3.2 kcal mol-l.

Table SI of rate data may be purchased from: The Depository of Unpublished Data, Document Delivery, CISTI, National Research Council Canada, Ottawa, Canada K1A OS2.

Fig. 1. Relationship between rates of protonation of 1-(tri-n- butylstanny1)-2-phenylethyne and acid concentration in aqueous perchloric acid solution at 25OC; 0: H,O, A: D,O.

These are substantial effects, but they pale in comparison to the much larger P-stannyl stablizations observed in other sys- tems (2, 3). The difference between the present result and the previously reported stannyl/silyl rate ratio of 5.1 x lo7, corre- sponding to ~ A G ' = 10.5 kcal mol-', is especially striking, for in both cases the reaction examined involved proton addition to a carbon-carbon triple bond.

There are, however, important differences between the present and this previous system. The previous study involved proton addition to a trimethylsilylacetylene with either an- other trimethylsilyl group or a tributylstannyl group at the other end, eq. [4], while in the present case addition was to a phenylacetylene with the metallic group at the other end, eq. [5]. In the previous system therefore, the vinyl cation formed had its positive charge next to a silyl substituent, whereas in

the present case the charge was next to a phenyl group. Since silicon is not very effective at stabilizing an adjacent positive charge (11, while phenyl stabilizes such charge very well, there would be less demand for additional stabilization from the metallic substituent in the phenyl system and the p-metal effect would consequently be reduced. A similar phenyl- induced reduction of the P-silyl effect has been found in the protonation of additionally substituted trimethylsilylacety- lenes, eq. [6], where a change from R = Me or n-Bu to R = Ph

cut the P-silyl effect in half, from ~ A G ' = 6.5 kcal mol-' to SAG'= 3.4 kcal mol-' (8).

Another difference between the previous study involving proton addition to acetylenes and the present work is that the previous reactions were carried out in chloroform solution while in the present case the solvent was water. Water is much more polar than chloroform and consequently much better at solvating ions such as the vinyl cations formed in these reac-

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Can. J. Chem. Vol. 74, 1996

tions. The stabilization provided by the better solvation in the present case would once again reduce the demand for additional stabilization from the metal and lead to a smaller p-metal effect.

A reaction mechanism in which proton addition to the acet- ylene is concerted with metallic group loss would also lead to a reduced p-metal effect, for such a process would lower the amount of positive charge generated on the substrate in the position p to the metal. It is unlikely, however, that the metal- lic group would leave unassisted as a free cation, and such a process would consequently probably involve nucleophilic displacement at the metal by a water molecule, as shown in eq. [7]. In the transition state of this process, the attacking water

molecule would be taking on positive charge, and that would contribute an additional normal (k,lk, > 1) component to the solvent isotope effect. The large solvent isotope effect actually observed is consistent with this explanation. On the other hand, it is not clear why such nucleophilic assistance should not have occurred in the previously studied systems, reducing the magnitude of the P-silyl effect there as well. Nucleophilic assistance by the carboxylic acid proton donor was in fact sug- gested in one of the previous studies (3) as a possible explana- tion for the observation that the rate of reaction decreased with increasing bulk of the aliphatic ligand attached to the metal,

Table 1. Summary of silyl and stannyl effects."

Substrate k, N-' s-' Relative rate 6AGi/kcal mol-I

P~C-CH' 2.86 x lo-' I .oo o PhC-CSiMe,' 8.93 x loM5 312 3.4 PhCeCSnBu, 1.85 x lo-' 64,700 6.6

"Aqueous solution at 25OC. hReference 6 "Reference 7.

i.e., that the tributylstannyl substrate, eq. [4] R = Bu, was less reactive than the trimethylstannyl substrate, eq. [4] R = Me.

Acknowledgement We are grateful to the Natural Sciences and Engineering Research Council of Canada for financial support of this work.

References

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2. J.B. Lambert, G.-t. Wang, and D.H. Teramura. J. Org. Chem. 53,5422 (1988).

3. C. Dallaire and M.A. Brook. Organometallics, 12, 2332 (1993). 4. M.W. Logue and K. Teng. J. Org. Chem. 47,2549 (1982). 5. A.J. Kresge, R.A. More O'Ferrall, and M.F. Powell. In Isotopes

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