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The Acetolysis of exo ‐ and endo ‐5,6‐Dimethylidene‐2‐Norbornyl p ‐Bromobenzenesulfonates and of their Optically Active and Deuterium‐Labelled Derivatives
Author(s) -
Sonney JeanMarie,
Vogel Pierre,
Burger Ulrich
Publication year - 1980
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19800630430
Subject(s) - 2 norbornyl cation , chemistry , racemization , solvolysis , medicinal chemistry , stereochemistry , reaction rate constant , organic chemistry , kinetics , hydrolysis , physics , quantum mechanics
The buffered (AcOK) acetolyses of exo , (11) and endo ‐5, 6‐dimethylidene‐2‐norbornyl brosylate (12) yielded exo 5, 6‐dimethylidene‐2‐norbornyl (16) and (3‐methylidene‐2‐nortricyclyl)methyl acetates (18) . Endo ‐5, 6‐dimethylidene‐2‐norbornyl (17) and 2‐methylidene‐3‐tricyclo [3.2.1.0 3,6 ]octyl acetates (20) could not be detected. The titrimetric rate constants of the acetolysis of 11 ( k t( exo ) =4.49 ± 0.02) · 10 −5 s −1 at 25°, Δ H ≠ =23.6 ±0.7 kcal mol −1 , Δ S ≠ =0.7 ±2 calmol −1 K −1 and 12 ( k t( endo ) =1.9 ±0.08) · 10 −9 s −1 at 25°, Δ H ≠ =27 ±1 kcal mol −1 , Δ S ≠ =‐8 ±2.5 calmol −1 K −1 ) were measured and compared with the polarimetric rate constants ( k α / k (exo) =6.8 at 25°,( k α / k (exo) =1.0 at 121°) of the buffered acetolyses of the optically active brosylates (+)‐ 11 and (+)‐ 12 . Neither a common‐ion (KOBs) nor a special ion effect (LiClO 4 ) on k t(endo) could be detected, although external return might well intervene as some exo ‐5,6‐dimethylidene‐2‐norbornyl tosylate (21) was formed upon solvolysis in the presence of KOTs. Acetolysis of (+)‐ 11 yielded completely racemized products, whereas (+)‐ 12 led to incomplete racemization. The buffered acetolysis of exo ‐(3 exo ‐D)‐5,6‐dimethylidene‐2‐norbornyl brosylate (24) furnished (3 exo ‐D)‐( 26 :37.5%), exo ‐(7 syn ‐D)‐5,6‐dimethylidene‐2‐norbornyl brosylate (27 : 37.5%) and [(5 anti ‐D)‐3‐methylidene‐2‐nortricyclyl]methyl acetates (28 : 25 %). The acetolysis of endo ‐(2 exo ‐D)‐5,6‐dimethylidene‐2‐norbornyl brosylate (25) yielded (2 endo ‐D)‐( 29 : 54%), exo ‐(1‐D)‐5,6‐dimethylidene‐2‐norbornyl ( 30 : 36%) and [(6‐D)‐3‐methylidene‐2‐nortricyclyl]methyl acetates ( 31 : 10%). Product analysis and deuterium label distribution was established by a combination of GC., 1 H‐NMR., 2 H‐{ 1 H}‐NMR. and MS. techniques. The results are rationalized by invoking anchimerically assisted ionization of the exo ‐brosylate 11 to symmetrical ion‐pairs (cyclopropylcarbinyl cation intermediates) which undergo internal (and probably also external) return. Acetolysis of the endo ‐brosylate 12 is not anchimerically assisted and leads initially to non‐symmetrical ion pairs. These evolve to symmetrical ion pair intermediates or, to a minor extent, are intercepted by solvent.