J. Am. Chem. Soc. 1997, 119, 4834-4840
Stepwise and Concerted Solvolytic Elimination and Substitution Reactions: E1 Reaction via a Primary Carbocation
Qingshui Meng and Alf Thibblin*
Contribution from the Institute of Chemistry, UniVersity of Uppsala, P.O. Box 531, S-751 21 Uppsala, Sweden ReceiVed February 3, 1997X
Abstract: Solvolysis of 9-(X-methyl)fluorene (1-X, X ) I, Br,OBs) in 25 vol % acetonitrile in water gives the elimination product 9-methylidenefluorene and the substitution products 9-(hydroxymethyl)fluorene (1-OH) and 9-(acetamidomethyl)fluorene (1-NHCOMe). Kinetic studies of the corresponding ring-substituted compounds 2-X and 3-X show that the rate of elimination increases with increasing acidity of the substrate, Brønsted R > 0. The small kinetic deuteriumisotope effects measured for the elimination reactions of the brosylates 1-OBs and 3-OBs, kH/kD ) 2.0 ( 0.1 and 2.8 ( 0.1, respectively, suggest significant amounts of E1 reaction. The bimolecular reactions of the brosylates with added bases may be of irreversible E1cB type in contrast to the reactions of the halides which exhibit E2 reaction with added bases as well as with solvent water.Introduction In some recent papers we have reported results on concerted solvent-promoted elimination reactions.1-5 These E2 reactions with water as the hydron-abstracting base require relatively acidic substrates. For example, it was found that 9-(1-X-ethyl)fluorene (X ) I, Br) in a highly aqueous solvent reacts by a solvent-promoted E2 reaction in competition with a minor reaction through the ionpair.2,3 In contrast, the corresponding brosylate gives stable alkene mainly via the ion pair as shown in Scheme 1. We now report on a study in which we have decreased the stability of the putative ion pair by removing the methyl group of the exocyclic carbon (Scheme 2). Does this primary ion pair exist, i.e., does it have a significant lifetime, under solvolytic conditions? Our results indicatethat it has a significant lifetime and that the solvolytic elimination of the brosylates occurs through the ion pair. There seems to be no precedence for E1 reaction of a primary substrate. The reasons for this surprisingly high stability of the primary carbocation are discussed. The paper also addresses the question about the mechanism of the competing solvent-promoted elimination of the brosylate:is it of E2 type or of irreversible E1cB type? Results The solvolysis of 9-(X-methyl)fluorene (1-X, X ) I, Br, OBs) in 25 vol % acetonitrile in water provides the alkene 9-methylidenefluorene and the substitution products 9-(hydroxymethyl)fluorene (1-OH) and 9-(acetamidomethyl)fluorene (1NHCOMe, Scheme 2). Solvolysis in mixtures of water with methanol affords 9-(methoxymethyl)fluorene (1-OMe)instead of the amide. The solvolyses of 2-bromo-9-(X-methyl)fluorene (2-X, X ) Br, OBs) and 2,7-dibromo-9-(X-methyl)fluorene (3X, X ) Br, OBs) yield analogous products. The kinetics of the reactions were studied by a sampling-quench highX Abstract published in AdVance ACS Abstracts, May 1, 1997. (1) Meng, Q.; Thibblin, A. J. Am. Chem. Soc. 1995, 117, 1839-1840. (2) Meng, Q.; Thibblin, A. J. Am. Chem.Soc. 1995, 117, 9399-9407. (3) Meng, Q.; Gogoll, A.; Thibblin, A. J. Am. Chem. Soc. 1997, 119, 1217-1223. (4) Meng, Q.; Thibblin, A. J. Am. Chem. Soc. 1997, 119, 1224-1229. (5) Meng, Q.; Thibblin, A. J. Chem. Soc., Chem. Commun. 1996, 345346.
Table 1. Rate Constants for the Solvolyses of 1-X, 2-X, and 3-X in 25 vol % Acetonitrile in Watera at 70 °C substrateb 1-I 1-Br 1-OBs 2-Br2-OBs 3-Br 3-OBs
106kobs, s-1 44.5 8.11 32.7 21.7 12.5 68.0 17.3
106kS, s-1 0.38 29.9 0.25 9.3 5.0
106kE, s-1 44.5 7.73 2.77 21.4 3.24 68.0 12.3
[HClO4] ) 1 mM. b Substrate concentration 0.01-0.1 mM.
performance liquid chromatography procedure. The product compositions are strongly dependent on the nature of the leaving group. For example, 1-Br and 3-Br yield 95% and almost 100%...