In the Classroom
The Variable Transition State in Polar Additions to Pi Bonds Hilton M. Weiss Department of Chemistry, Bard College, Annandale-on-Hudson, New York, 12504
[email protected] The elimination of hydrogen halides (and similar acids) from alkyl halides is understood to occur by two basic mechanisms (1). The E1 mechanism involves an intermediate carbenium ion and occurs in polar solvents when no strong bases are present and the substrate possesses a good leaving group. In less polar solvents, the use of stronger bases and weaker leaving groups facilitate an E2 mechanism where the proton and leaving group are eliminated more-or-less simultaneously. The “moreor-less” is significant as this modifies the structure of the transition state and can lead to different products. For compounds having a poor leaving group, use of a strong base can lead to a transition state in which the proton is removed to a much greater extent than the leaving group is removed. Thus, the transition state has significant negative charge on the carbon being deprotonated and carbanionic stability determines which proton is removed (Scheme 1). This usually produces the less substituted pi bond and is referred to as a Hofmann elimination. Conversely, compounds with fairly good leaving groups undergo Saytzeff elimination when moderately strong bases are employed. In such cases, the E2 transition state shows a more simultaneous cleavage of both bonds and there is little charge developed at either of the carbon atoms (Scheme 2). The relative stability of the possible transition states is determined by the relative stability of the pi bonds being formed and this usually leads to the more substituted pi bond and the Saytzeff product. A similar situation can be found in the addition of protic acids to alkynes. In rare cases (2), strong acids can protonate an alkyne to generate a vinyl cation followed by the addition of the anionic component of the addition. In most cases, however, protonation and nucleophile addition to alkynes occur “more-or-less” simultaneously. This variable transition state can produce different products in some cases. We have studied (3, 4) the addition of HBr to 1-phenylpropyne by using a wide range of concentrations of tetrabutylammonium bromide in a 20% solution of trifluoroacetic acid in dichloromethane. Increasing the salt concentrations in this organic environment causes a large increase in activity coefficients and a resulting decrease in the effective acidity of the solution. This differential acidity leads to the predominant formation of three different addition products. In the most acidic solutions, 1-phenylpropyne is protonated to form a resonance-stabilized cation that adds the bromide ion in a second step. Steric factors put the bromide ion on the same side as the added proton and the principal product is the Markovnikov1 syn adduct, (E)-1-bromo-1-phenylpropene (Scheme 3). Increasing the bromide ion concentration lowers the activity of the acid making protonation of the alkyne less feasible. The acid forms a pi complex with the alkyne spreading some positive charge throughout the conjugated system. As a result, the rate of
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Scheme 1. E2 Mechanism with a Poor Halide-Leaving Group
Scheme 2. E2 Mechanism with a Good Halide-Leaving Group
Scheme 3. Addition Reaction under Highly Acidic Conditions Caused by Low Tetrabutylammonium Bromide Concentration
Scheme 4. Addition Reaction under Moderate Acidic Conditions Caused by Moderate Tetrabutylammonium Bromide Concentration
this reaction is strongly affected by substituents in the aromatic ring. The pi complex is subsequently attacked by a bromide ion on the opposite side of the R-carbon leading to a Markovnikov anti addition to the alkyne forming (Z)-1-bromo-1-phenylpropene (Scheme 4). Further increasing the bromide ion concentration lowers the acidity more and this alters the transition state much as the weaker base altered the transition state for the E2 mechanism described above. The weaker acidity results in a very weakly bonded pi complex with little charge developing in the pi system. As with the variable E2 transition state, the slower addition reaction leads to a transition state resembling the alkene product and there will be competition between this modified transition state occurring on the two orthogonal pi bonds of the triple bond. The more stable transition state will retain the conjugated
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r 2010 American Chemical Society and Division of Chemical Education, Inc. pubs.acs.org/jchemeduc Vol. 87 No. 12 December 2010 10.1021/ed900012v Published on Web 09/23/2010
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Journal of Chemical Education
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In the Classroom
Scheme 5. Addition Reaction under Low Acidic Conditions Caused by High Tetrabutylammonium Bromide Concentration
system and the HBr prefers adding to the isolated pi bond orthogonal to the conjugated system (Scheme 5). Since the aromatic ring is not conjugated to this pi bond, the partial positive charge developing in this transition state will primarily reside on the β-carbon stabilized by the adjacent methyl group and hence will be the site of bromide attachment. The steric demands of the coplanar phenyl ring also favor bromide attack at the β-carbon. The principal product is the anti-Markovnikov anti adduct, (Z)-2bromo-1-phenylpropene. Because the reaction is not taking place in the conjugated pi system, the rate of formation of this product is quite independent of para substituent groups. Other workers (5, 6) have explained their results by postulating reaction at the unconjugated pi bond of an alkyne, but this idea has met with considerable skepticism (7). The results noted here have extensive experimental support and have been met with no alternate explanation. These results show the requirements and rationale for this unusual and interesting behavior.
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Note 1. The term “Markovnikov” is used here to denote the involvement of an intermediate (or transition state) having the more stable positive charge.
Literature Cited 1. Carey, F. A.; Sundberg, R. J. Advanced Organic Chemistry, Part A; Springer-Verlag: New York, 2007; p 549. 2. (a) Peterson, P. E.; Duddey, J. E. J. Am. Chem. Soc. 1966, 88, 4990– 4996. (b) Weiss, H. M.; Touchette, K. M.; Jones, G. J. Chem. Soc., Perkin Trans. 2 2002, 756–758. (c) Griesbaum, K.; Rehman, Z. J. Am. Chem. Soc. 1970, 92, 1416–1418. 3. Weiss, H. M.; Touchette, K. M.; Andersen, F.; Iskhakov, D. Org. Biomol. Chem. 2003, 1, 2148–2151. 4. Weiss, H. M.; Touchette, K. M.; Angell, S.; Khan, J. Org. Biomol. Chem. 2003, 1, 2152–2156. 5. Hassner, A.; Ishister, R. J.; Friederang, A. Tetrahedron Lett. 1969, 10, 2939. 6. Okuyama, T.; Izawa, K.; Fueno, T. J. Org. Chem. 1974, 39, 351– 354. 7. Capon, B.; Rees, C. W. Organic Reaction Mechanisms 1969; Interscience: New York, 1970; p 179.
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