Organometallics 1982, 1, 97-103
97
Kinetics of the Protodestannylation of Allenyitins John C. Cochran*la and Henry G. Kuivilalb University of New Hampshire, Durham, New Hampshire 03824 Received May 27, 198 1
Rate studies have been carried out on the protodestannylation reaction in methanol-4% water for five allenyltins. Substituents were varied in both positions on the allene skeleton as well as on tin. The reaction involves a competition between sE2 and SE2' mechanisms leading to allenic and acetylenic hydrocarbon products, respectively. Electrophilic displacements by chlorine, bromine, and 2,4-dinitrobenzenesulfenyl chloride have also been studied. The NMR spectra of a number of denic and isomeric propargylic derivatives are reported. In the protodestannylation reaction the allenyltins have been shown to be stable with respect to equilibration to the isomeric propargyltin. The 832' reactivity of the allenyltins is very similar to the S32' reactivity of dyltins. However, in the sE2 reaction the allenyltins are more reactive than comparable vinyltins. Hyperconjugation in the transition state of the h 2 reaction is proposed to account for the increased reactivity of the allenyltins. Also, a modified "steric sequence" is observed when substituents on tin are varied. This is consistent with an open transition state.
Introduction Some time ago we reported that allenyltins undergo electrophilic substitution by attack of hydrogen ion at both the cy-carbon (sE2) and the y-carbon (SE2')q2 In order to better understand the competitive nature of these two mechanisms, it was necessary to investigate the reactivity of vinyltins under these conditions. I t is presumed that both mechanisms involve an initial attack of the electrophile on the ?r electrons of the double bonds which are orthogonal in the allene system. This is the first instance in which an allenyltin has been shown specifically to react by an 832 mechanism. In previous cases, with electrophiles such as sulfur dioxide? dithiocyanogen,' iodine: mercuric chloride?' and chloral! only rearranged cleavage product was obtained. Likewise, the isomeric propargyltin derivatives gave only allenic product. Westmijze et al? have reported that allenylsilver compounds undergo electrophilic substitution to give >95% sE2 product with iodine, methyl methanethiosulfinate, allylbromide, carbon dioxide, and the trimethylmetal chlorides of silicon, germanium, and tin. Greater competition by the sE2' mechanism was found for cyanogen bromide, N-bromosuccinimide, N-chlorosuccinimide, and carbon disulfide. Also, Leung and Zweifel'O have reported that allenylboranes are cleaved by acetic acid to give allenes. However, this is a special case involving nucleophilic coordination with the carbonyl oxygen of the acid. As shown below, extent of 832 reaction compared to SE2' is dependent upon the nature of the substituents on tin and the substituents on the allene skeleton. In t h e preceding paper'l we reported protodestannylation of a number of vinyltins by hydrochloric (1)(a) Department of Chemistry, Colgate University, Hamilton, NY 13346. (b) Department of Chemistry, State University of New York at Albany, Albany, NY 12222. (2)H. G. Kuivila and J. C. Cochran, J. Am. Chem. SOC.,89,7152 (1967). (3)C. W. Fong and W. Etching, J. Organomet. Chem., 22,107(1970). (4)M. L. Bullpitt and W. Kitchmg, J. Organomet. Chem., 34, 321 (1972). (5)M. S.Simo,A. Jean, and M. Lequan,J.Organomet. Chem.,36,C23 (1973). (6)A. Jean, G. Guillerm, and M. Lequan, J. Organomet. Chem., 21, P1 (1970). (7)G. Guillerm, F. Maganem, M. Lequan, and K. R. Brower, J. Organomet. Chem., 67,43 (1974). (8) M. muan and G. Guillerm,J. Organomet Chem., 54,153 (1973). (9)H.Westmijze, H. Kleijn, H. J. T. Boa, and P. Venneer, J. Organomet. Chem., 199,293 (1980). (10)T.h u n g and G. Zweifel, J. AM. Chem. Soc., 96,5620 (1974). (11)J. C.Cochran, S. C. Bayer, J. T. Bilbo, M. S. Brown, L. B. Colen, F. J. Gaepirini, D. W. Goldsmith, M. D. Jamin, K. A. Nealy, C. T. Resnick, G. J. Schwartz, W. M. Short, K. R. Skarda, J. P. Spring, and W. L. Straws, in press.
0276-7333/82/2301-0097$01.25/0
acid in methanol-water. In this paper we report the acid cleavage of a series of allenyltins under similar conditions. The protodestannylation reaction is more complex in this system because the allenyltin compound undergoes competitive sE2 and 832' reactions, the former leading to an allenic hydrocarbon and the latter to a rearranged acetylenic hydrocarbon. R
R'
R
R-CH2-C=C-R'
R'
+
R'iSnCI
(1)
Results As do allyltind2 and vinyltins," allenyltins exhibit intensive UV absorption a t approximately 206 nm (E > 10000). Protonolysis of these compounds leads to a disappearance of this absorption, in accordance with Beer's law, and thus it is assigned to the presence of a sp2carbon to tin bond. Thus the decrease in concentration of the allenyltin could be followed as a function of time. The electrophilic displacement reactions were run in methanol-4% water solution under both second-order and pseudo-first-order conditions (excess acid). Combined second-order rate constants were obtained a t 25,35, and 45 "C. The hydrocarbon composition of the product mixtures were determined a t each temperature either by sweeping the volatile hydrocarbons from the reaction mixture into a cooled trap with nitrogen or by treating the reaction mixture with potassium fluoride followed by fractional distillation of the solvent. The hydrocarbon residues were then analyzed by gas chromatography. With use of the ratio of allene to acetylene in the product mixture, the measured rate constants could be factored into sE2 and sE2' components. In Table 1 are listed the second-order rate constants a t the three temperatures followed in each case by the 332 and SE2' partial rate constants. The activation parameters for each reaction, calculated from the appropriate partial rate constants, are also listed in Table I. For each double entry, the value of the 832 reaction is listed above that for the sE2' reaction. The combined second-order rate constants agreed to within *3% in multiple determinations, and the error in the activation parameters, calculated by the computer leastsquares technique, was generally no greater than 10% as indicated by the standard deviation of the slopes of the (12)(a) J. A. Mangravite, J. A. Verdone, and H. G. Kuivila, J. Organomet. Chem., 104,303(1976). (b) J. A. Verdone, J. A. Mangravite, N. M. Scarpa, and H. G. Kuivila, J. Am. Chem. Soc., 97,843 (1975).
0 1982 American Chemical Society
98 Organometallics, Vol. 1, No. 1, 1982
Cochran and Kuiuila
Table I. Measured Second-Order Rate Constants, sE2 and 832' Partial Rate Factors, and sE2 and 832' Activation Parameters for Protonolysis of AllenyltinsC k,, M-l s-* A H * , kcal AS*,cal compd 25 "Ca 35 "Cb 45 "Cb mol-' dee-' mol-' (0.266) 0.519
[ 0.253 1
(0.484) 0.939 [0.455]
(0.108)
(0.190) 0.558
0.299
(0.000525)
[ 0.3681
(0.00190) 0.0183 [ 0.01641
[0.00687] (0.0425)
(0.111)
0.125
(0.206) 0.242
[ 0.01361
[0.899]
[ 10.91
[-25.11
(0.335)
(9.6.)
(-30.8)
[0.675]
[ 10.81
[-25.71
(0.00400)
(17.8)
(-13.8)
[ 0.0 242 ]
[10.8]
[-32.21
(0.145)
(10.5)
(-29.7)
[ 0.2071
L9.41
[-32.41
(0.327)
(9.2)
(-32.2)
[13.7]
[-21 .o 1
0.352 [ 0.1271
[ 0.06851
(-32.2)
0.0282
(0.0850) 0.212
0.111
(8.7)
1.01
[0.191]
0.00739
(0.741) 1.64
[0.0358]
0.393
[ 0.06641
These values are the average of at least four determinations at 25.0 f 0.1 "C. The average deviations of k , were < 3%. These values are the average of at least three determinations at 35.0 ?: 0.1 "C and 45.0 2 0.1 "C. The average deviations of k, were
