Rearrangements of Carbocation Sulfinate Ion Pairs

Abstract

The chirality of alkyl p-toluenesulfinates and of I80-labeled p-toluenesulfinate ions was utilized to study the stereoselectivity of ion-pair recombinations. The diastereomers of 2-norpiny1 (13), 2-norbornyl (16), 2-methyl-2-norbornyl (25), and exo-4-protoadamantyl (34) p-toluenesulfinates were rearranged in formamide or trifluoroacetic acid (TFA). Solvolysis competed to a minor extent. Predominant return of the carbocations to the oxygen atoms of ArS02-was observed if the isomeric p-toluenesulfinates persisted (kinetic control). On repeated ionization (thermodynamic control), sulfones were eventually formed. With the exception of 25, 1,2 shifts of the p-toluenesulfinate anion proceed faster than oxygen exchange. The migration origin of the carbocation returns preferentially to the oxygen atom of ArS02-from which the migration terminus departed. Conversely, the sulfinate anion discriminates between positions 1 and 2 of the symmetrical, bridged 2-norbornyl cation in favor of the carbon atom from which it departed. The selectivity of ion-pair recombination decreases in the order 2-norpinyl = 4-protoadamantyl > 2-norbornyl > 2-methyl-2-norborny1, i.e., with increasing stability of the carbocation. The rearrangements of 13 and 34 proved to be more selective in TFA at 0°C than in formamide at 120-130°C. The p-toluenesulfinates 13 and 34 were compared with the analogous tosylates and 3,5-dinitrobenzoates. More oxygen scrambling was observed with less nucleophilic anions (tosylate % p-toluenesulfinate > 3,5-dinitrobenzoate).Oxygen scrambling is also enhanced if the anion migrates over a longer distance (2-norpinyl + exo-2-norbornyl vs. 2-The concept of ion pairs in nucleophilic substitution, introduced by Winstein ['], is now generally acceptedl2] (Eq. 1). Contact (intimate) as well as solvent-separated ions pairs have been invoked to explain the products, stereochemistry, and kinetics of solvolysis reactions. Evidence for the recombination of ion pairs (internal return) was obtained from rearrangement or racemization of the cationic component (RX + R'X) and from exchange processes (isotopic scrambling) of the anionic component (RX + RX'). Most often, either technique was applied. More insight can be gained from the combination of both methods, as illustrated by the seminal work of Goering and Thiesr'] on lXO labeled, enantiomerically enriched bicyclo[3.2.l]oct-endo-2-yl tosylate (4). The intervening carbocation was found to return preferentially to the oxygen atom from which it departed (4a'/4b' ca. 2:l).The substrates used in previous studies were largely carboxylates (l)L41 and sulfonates (2) [3,5]. Our efforts were directed to sulfinates (3) since we anticipated opportunities with 3 that are not provided by 1 and 2l61. The ambident character of the sulfinate ion suggests that sulfones as well as sulfinates should arise from ion-pair recombination. Owing to the chirality of 3, migration of R from oxygen to oxygen results in racemization (if R is achiral) or diastereomerization (if R is chiral). The rearrangement of a...

Rearrangements of Spirocyclobutane-Substituted 2-Norbornyl Cations

“…The unprecedented exo Ǟ endo shift, 32a Ǟ 39a, proceeds with complete scrambling of the 18 O label between all oxygen atoms of the sulfonate ion. The present data confirm the notion that the scrambling of 18 O increases with the distance by which the counterion migrates [22] .…”

“…In the course of the exo Ǟ exo shift, 32a Ǟ 34a, the carbocation returns preferentially to the oxygen atom of the tosylate ion from which it departed. This type of selectivity is generally observed if dissociation and recombination occur on the same side of the molecular plane [22] . The unprecedented exo Ǟ endo shift, 32a Ǟ 39a, proceeds with complete scrambling of the 18 O label between all oxygen atoms of the sulfonate ion.…”

Stereoselectivity of Destabilized Carbocations: 1-Cyano-2-adamantyl and 3-Cyano-4-protoadamantyl Cations

“…The analogous rearrangement of the parent exo-4-protoadamantyl tosylate was associated with 52% equilibration of the tosyloxy group [19] even though the conditions (pyridine, 0°C) were milder than those applied to 10c. Comparison of the data points to tighter ion pairing in the case of the less stable, CNThe tosylation (TsCl, pyridine) of 10a proceeded reluc-substituted carbocation.…”

“…In ion-pair gested tosylate 11c (64%). It should be noted that the sub-recombination processes, the counterion returns preferenstituent X of 10 assumes the "flagpole" position of a tially to the side of the molecular framework from which it slightly distorted cyclohexane boat, as compared to the departed [18] [19] . Hence the (2R) configuration is assigned to equatorial (linear) orientation of X in 11.…”