SummaryThe cycloadditions of methyl propynoate and methyl vinyl ketone to 5,6-dimethylidene-2-norbornanone (6) are 'para 'regio~elective~). A smaller 'para Lregioselectivity is observed for the addition of methyl propynoate to 5,6-dimethylidene-2-bicyclo[2.2.2]octanone (10). No regioselectivity is observed with 5,6-dimethylidene2exo-norbornyl alcohol (3), acetate (5) and 5,6-dimethylidene-2 exo-bicyclo[2.2.2]0~-tanol(9). PMO arguments based on the shape of the HOMO'S and subHOMO's of the dienes allow to rationalize these observations. Unpredictable 'para-or 'metat regioselectivities are found for the Diels-Alder additions of 5,6-dimethylidene-2en-do-norbornyl alcohol (2), acetate (4) and 5,6-dirnethylidene-2endo-bicyclo[2.2.2]0~-tanol(8). The carbonyl group ofP,y-unsaturated ketones such as 6 and 10 can act as an electron donating homoconjugated substituent. The n(CO)++o[C( l),C(2)]t+x [C(5),C(6)] hyperconjugative interaction can override the usual electron-withdrawing effect of this function. ~~Introduction. -The Diels-Alder reactivity of an exocyclic s-cis-butadiene6) grafted onto norbornane [2] [3] and bicyclo[2.2.2]octane [ 11 [4] skeletons (s. 1 and 7, resp.) can be affected by remote substitution of the bicyclic systems. The carbonyl group in 5,6-dimethylidene-2-norbornanone (6) and in 5,6-dimethylidene-2-bicycIo[2.2.2]0~-tanone (10) causes a significant rate retardation effect on the cycloadditions of the diene to strong dienophiles. We report now on the regioselectivity of the Diels-Alder additions of the homoconjugated dienones 6 and 10, and of the related endo-and exo-alcohols 2,8, 3 and 9, and acetates 4 and 5.Since our exocyclic dienes are grafted onto rigid bicyclic skeletons, geometry and steric factors should not play a dominant role in determining the regioselectivity
The I3C n.m.r. spectra of 11 derivatives of 2,3-dimethylenenorbornane, 1-11, of 5 derivatives of 2,3-dimethylene-7-oxanorbornane, 12-16, and of 2,3,5,6-tetramethylene-7-oxanorbornane (
SummaryLow temperature (-130" to -110") addition of exo-norborn-5-en-2-01 (7) to excess HS03F in S02CIF yielded a mixture of exo-5-(fluorosulfonyloxy)-exo-2-and endo-2-norbornylhydroxonium ions (9 + 10) under kinetic control that was different from the mixture of 9 + 10 obtained by addition of endo-norborn-5-en-2-01(8) to HS03F under kinetic control. These mixtures differed from the mixture of 9+ 10 observed at higher temperature (-80" to -60") (thermodynamic control). Addition of 3-nortricyclanol(23) or exo-2,3-epoxynorbornane (24) to HS03F at -120"k 10" yielded a mixture containing the exo-2-(fluorosulfonyloxy)-anti-7-and syn-7-norbornylhydroxonium ions (26+ 27) as major adducts. Qualitative rates of the isomerization of 26 + 27 to the more stable ions 9 + 10 and of the isomerization 9 % 10 were evaluated. The solvolysis of 9 + 10 in HS03F yielded the exo-2, exo-5-and exo-2, endo-5-norbornanediyl bis (fluorosulfates) (21 + 22). Norbornadiene and quadricyclane added 2 equivalents of HS03F and furnished kinetically a mixture of exo-2, anti-7-and exo-2, syn-7-norbornanediyl bis (fluorosulfates) (36 + 37) as major adducts. The latter 36+37 were isomerized into a kinetic mixture of the more stable isomers 21 + 22. The rates of these isomerizations were compared. The use of DS03F and (exo-2-D)-norborn-5-en-2-01 (15) confirmed that heterolyses of the fluorosulfates were responsible for the observed isomerization; elimination-addition processes occurred but much more slowly. The results are interpreted in terms of substituted classical and o-bridged secondary 2-norbornyl cation intermediates. It appears that the electron withdrawing substituents FSO, and H20+ (HO) destabilize the o-bridged 2-norbornyl cation more at C(5) than C(7). If the o-bridged ions 5-Z substituted at C(5) by Z=FSO, or H20+ (HO) are transition states in the isomerization of the corresponding classical ions 3-Z,4-Z, the free enthalpy difference between the 'non-classical' o-bridged ion and the classical ions is not higher than the energy barrier to the quenching of the latter intermediates by FSOj in HSO,F/SO*ClF. I) 2,Present address: Lonta AG, CH-3930 Visp, Switzerland. Author, to whom correspondence should be addressed. 336HELVETICA CHIMICA ACTA -Vol. 62, Fasc. 1 (1979) -Nr. 36 Introduction. -The nature of the secondary 2-norbornyl cation has stirred a controversy that is not yet settled [l]. A long-standing problem is whether this carbocation exists as a pair of 'rapidly equilibrating' enantiomorphic structures 1 1' (double minimum energy hypersurface) or as a 'non-classical' o-bridged carbonium ion 2 with C,-symmetry in its ground state (single minimum energy hyper~urface)~).
Summwy. Mass spectrometrical and 1H-NMIL-analyses of the exo-5-norbomen-2-yl acetate, formed by acctolysis of endo-5-norbonen-2-yl-2-exo-d brusylatc, demonstrate that thc deuterium initially on C(2) migratcs partially (30%) onto C(l) (mechanism l a or Ib). No dcuterium could bc detccted on the other positions, which shows that C(1-7) migration is insignificant. W-NMR.-analysia of th? deutcriatcd nortricyclyl acetate obtained as main product shows that the deuterium is equally and uniquely distributed between positions C(l) ancl C(G). This indicates that the nortricyclyl derivatives do not arise frotri nucleopliilic attack on C(5) of asymmetrical norbornenyl interrnediatcs, but from the reaction of a symmctrical nortricyclyl cation interrnediatc with solvent (mechanism Tb).Since the pionecring work of Roberts [11 and Winskin 121 on thc solvolysis of a m -and endo-5-norborncn-2-yl derivatives 1-X and 2-X many papers have dealt with the cationic intermediate, thc nature of which ha5 still not been cstablished satisfactorily [3J. Wc discuss briefly the main fcatuws of this homoallylic system and preacnt cxperirnental results that allow, for the first time, it clear distinction bctween five possible mechanisms Is, Ib, 11, 111 and I V of the dcgencrate rcarrangemcnt of the cationic intermcdiatc formed in the acetolysis of the endo-5-norbornen-2-yI brosylate.Introduction. -The participation of the C(5-6) double bond in the solvolysis of em-S-norbornen-2-yl derivatives 1-X was indicated by the relative rate of acetolysis ef exo-5-norbornen-Z-yl 9-bromobenzenesulfunate (1-OBs), which was found to be of the norpinenyl cation 7 in the interconvertion of dehydronorbonyl cations cannot therefore be excluded a prioris).In the acetolysis and formolysis of 1-and 2-OBs or deamination of 2-NH,, Rob& [12] observed that 30 to 48% of the initial 14C label at C(2,3) had migrated into the other psiitions C(1,4,5,6,7) of the exo-5-norbornen-2-yl products. These results suggested the occurance of an equilibrium betwcen two asymmetrical cationic intermediates 4 arid 4' via a symmetrical intermediate or transition state of type 10, i. e. a Wagaer-Meerweirt rearrangement involving the migration of the C( 1-6) bond.Depending upon the reaction conditions and particularly upon the reactivity of the nucleophile, the label scrambling was partial or complete. This interpretation is supported by the solvolysis' of 2-dcuterio-1-X derivatives [13] and by DC1 additions to norbornadiene and quadricyclane [14].In the acetolysis of em-3-deuterio-1-OBs CristoE 1151 observed that the deuterium is distributed equally between the e x 0 3 and syw7 positions of the 1-OAc isolated.It was further shown that the deuterium migration in the starting brosylatc was faster than nucleophilic attack by solvent yielding I-OAc and 3-OAc. In contrast to Robert's conclusion it was suggested that a symmetrical cation intermediate of type 10 is primarily formed, thus removing the necessity of an asymmetrical norbornenyl cation intermcdiateY). The participation and migration...
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