42447-90-5

The allylic carbocation formed from 3-(2-X-propan-2-yl)indene (1-X) (X = Cl, OAc, or OH2+), or from 2-acetoxy-1-isopropylideneindan (2-OAc) in aqueous solvents containing ca. 75 vol percent water reacts rapidly with nucleophiles.The selectivity is very low: βnuc ca. 0.07 with alcohols as nucleophiles and βnuc ca. 0.03 with substituted acetate anions.The nucleophile attacks at both ends of the allylic system but preferentially at the exocyclic carbon atom, giving the thermodynamically more stable product.Azide anion reacts with the carbocation about 50 times as fast as does water.The solvolysis of the allylic isomers (1-OAc) and (2-OAc) is accompanied by intramolecular rearrangement of the acetates as well as hydron abstraction by the leaving acetate anion, yielding the 1,2- and 1,4-elimination products.The elimination product compositions are quite different, which indicates two discrete ion-pair intermediates.It is concluded also that the isomerization proceeds by an ion-pair route, since in less polar solvents it has only been observed along with solvolysis and elimination.The experimental results for the acetates accomodate well a mechanism in which solvolysis, rearrangement, and elimination are connected via two contact ion-pair intermediates.

The allylic carbocation intermediate formed from 3-(2-chloropropan-2-yl)indene (1-Cl) and from (1,1-2H2)-(1-Cl) in 75 vol percent water-acetonitrile at 35 deg C reacts rapidly with solvent water to give the allylic alcohols but is also subject to hydron abstraction by general bases to produce 1-isopropylideneindene (3) and 3-isopropenylindene (4).The Bronsted parameters measured with substituted acetate anions were found to be small, β 0.16 for formation of (3) and 0.14 for production of (4).The kinetic deuterium isotope effect on hydron abstraction from the intermediate with acetate anion to form the olefin (3) (kH/kD) is 3.0 +/- 0.4.The corresponding intramolecular elimination of acetic acid from the contact ion pair formed from 3-(2-acetoxypropan-2-yl)indene (1-OAc) to yield (3) shows a substantial isotope effect, kH/kD = 5.2 +/- 1.0.The allylic isomer 2-acetoxy-1-isopropylideneindan (2-OAC) yields a contact ion pair that reacts to give (3) with a considerably smaller isotope effect, kH/kD = 2.9 +/- 0.3.The free carbocation generated from protonated 3-(2-hydroxypropan-2-yl)indene (1-OH) and its (1,1-2H2)-analogue undergo hydron abstraction to give the olefin (3) with an isotope effect kH/kD = 4.1 +/- 0.3.These isotope effects do not include the isotope effect on the rate-limiting ionization step, which were found to be kH/kD = 1.07 and 1.22 in the reactions of (1-OAc) and (2-OAc), respectively.The elimination isotope effects for the ion pairs are evaluated by assuming that the formation of alcohols from the intermediates has an isotope effect of unity.

The reaction of 1-(2-acetoxy-2-propyl)indene (1-h) or 1-(2-acetoxy-2-propyl)indene (1-d) with p-NO2PhO- in methanol buffered with p-NO2PhOH results in base-catalyzed 1,3 proton transfer, yielding 3-(2-acetoxy-2-propyl)indene (2-h) and 3-(2-acetoxy-2-propyl)indene (2-d), respectively, in competition with base-promoted 1,2-elimination producing 1-isopropylideneindene (3-h) and 1-isopropylideneindene (3-d), respectively.The overall deuterium isotope effect on the reaction of 1 was measured as (k12H + k13H)/(k12D + k13D) = 5.2, which is composed of the rearrangement isotope effect k12H/k12D = 12.2 +/- 1.0 and the elimination isotope effect k13H/k13D = 3.6.The enlarged rearrangement isotope effect shows that the intramolecularity of the 1,3 proton-transfer reaction is substantial.The intramolecularity was determined as ca. 87percent for = 0.24 M and ca. 80percent for = 0.71 M by analyzing the 2H content of the product 2-d.The amplified isotope effect on the 1,3-prototropic shift together with the attenuated elimination isotope effect shows that the two reactions are coupled via at least one common intermediate, which is concluded to be a tightly hydrogen-bonded complex between the protonated base and the carbanion.An increase in basicity of the oxyanion favors elimination at the expense of rearrangement.Reaction of 2-h and 2-d predominantly give 1,4 elimination accompanied by a trace of competing 1,3 proton transfer.The isotope effect k23H/k23D was measured to 2.5; it is small owing to a large amount of internal return.