Introduction of Bulky Substituents at the Bridgehead Position of a 9-Silatriptycene:  Pentacoordinate Hydridoorganylsilicates as Intermediates

2000 ◽  
Vol 19 (7) ◽  
pp. 1319-1324 ◽  
Author(s):  
Nicolette Rot ◽  
Tom Nijbacker ◽  
Rutger Kroon ◽  
Frans J. J. de Kanter ◽  
Friedrich Bickelhaupt ◽  
...  
Keyword(s):  
Bridgehead Position

Substitution of Bridgehead Halogens by a Free-Radical Electron-Transfer Mechanism

1996 ◽  
Vol 49 (5) ◽  
pp. 581 ◽  
Author(s):  
MC Harsanyi ◽  
PA Lay ◽  
RK Norris ◽  
PK Witting
Keyword(s):  
Electron Transfer ◽  
Nitro Group ◽  
Intramolecular Electron Transfer ◽  
Substitution Reactions ◽  
Radical Chain ◽  
Electron Transfer Mechanism ◽  
Regioselective Substitution ◽  
Leaving Group ◽  
Aromatic Nitro ◽  
Bridgehead Position

The reactions of 1-bromo-7-nitro- and 1-bromo-6-nitro-1,4-methanonaphthalene (2) and (3), and 9-bromo-2-nitro, 10-bromo-2-nitro-, 9,10-dibromo-2-nitro- and 9,10-diiodo-2-nitro-9,10-ethano-9,10-dihydroanthracene (4)-(7). respectively, with the sodium salt (1) of p-toluenethiol gave substitution products that were shown to be formed by an SRN1 or a related radical chain mechanism. In the relatively slow substitution reactions of the salt (1) with compounds (2)-(5). That contain bromine at bridgehead positions that are either meta- or para-benzylic to an aromatic nitro group, the rates of substitution in the isomers where the leaving group was meta- benzylic to the aromatic nitro group were slightly greater than those for the corresponding para-benzylic isomer. In compounds (6)and (7) the halogens are at bridgehead positions that are either meta- or para-benzylic relative to an aromatic nitro group within the same molecule. In the case of the reaction of the dibromide (6) with the thiolate (1), substitution was slow and occurred more rapidly at the benzylic -bridgehead position meta to the nitro group than at the corresponding para-benzylic position. In contast , the reaction of the diiodide (7) with the thiolate (1) gave substitution products which formed more rapidly than in the corresponding reaction of the dibromide (6) and the regioselectivity was reversed, with substitution occurring more readily at the bridgehead position para-benzylic to the nitro group than at the corresponding meta- benzylic position. The ratio of meta to para substitution products, determined for the reactions of compounds (2)-(6) with the salt (1), were in the range 1.15-2.5:1, while the reaction of (7) with the same nucleophile afforded a meta-to-para ratio of 1:2:3. These ratios contrast not only with each other, but also with the differences in reactivities determined for other nitrobenzylic systems, which are known to undergo SRN1 substitution reactions with the same nucleophile. The differences in first, the regioselectivity of substitution between the bridgehead systems, and secondly, the differences in the observed rates of regioselective substitution are compared with other simple nitrobenzylic halides. These differences are rationalized in terms of the effect of fixing the C-X bond at a bridgehead position to be orthogonal with the plane of the nitroaromatic group; this results in a reduction of the rate constants of intramolecular electron transfer, with significant consequences on the detailed overall mechanism for these reactions.

Electrochemical Studies on the Competition Between Intramolecular Electron Transfer and Intermolecular Protonation of Nitroaryl Radical Anions in the Reductions of Apical Carboxylic Acid Substituents of 2- and 3-Nitro- 9,10-dihydro-9,10-ethanoanthracene-9-carboxylic Acids

1997 ◽  
Vol 50 (10) ◽  
pp. 999 ◽  
Author(s):  
Peter A. Lay ◽  
Robert K. Norris ◽  
Paul K. Witting
Keyword(s):  
Electron Transfer ◽  
Carboxylic Acid ◽  
Carboxylic Acids ◽  
Acid Group ◽  
Intramolecular Electron Transfer ◽  
Radical Anions ◽  
Intermolecular Electron Transfer ◽  
Electron Transfer Mechanism ◽  
Reduction Potentials ◽  
Bridgehead Position

The results obtained from variable scan rate cyclic voltammetry (c.v.) on 2-nitro- and 3-nitro-9,10- dihydro-9,10-ethanoanthracene-9-carboxylic acids [(4) and (5), respectively], combined with simulations of various c.v. responses, are consistent with reduction of a benzylic acid group being facilitated by an intramolecular electron transfer process. This intramolecular process involves a one-electron reduction of the nitroaromatic group, followed by a rapid and irreversible π*(ArNO2)•- → π*(RCO2H)•- intramolecular electron transfer to the carboxylic acid group at a benzylic bridgehead position of the acids (4) and (5). The reduction potentials of the acid groups are shifted more than 0·3 V to positive potentials at slow scan rates (20-100 mV s-1) compared with the unnitrated acid derivative (6). The reduction potentials and the relative peak currents for the reductions of the nitro and acid groups for each of compounds (4) and (5) are dependent on the concentrations of the reactants. At concentrations of substrate >1 mM, reduction of the acid moiety is increasingly dependent on complex intermolecular processes. These intermolecular processes compete with intramolecular electron transfer from the nitroaryl anion to the apical acid group at the benzylic bridgehead position. Digital simulations of the voltammetric data were confined to substrate concentrations <1 mM, and show that the intramolecular reductions of the apical carboxylic acid protons of (4) and (5) are complicated by competing intermolecular electron transfer and intermolecular self-protonations of the nitro radical anions. The value of the intramolecular electron transfer rate constant for the meta compound is an order of magnitude larger than that for the para compound, which is the opposite reactivity pattern to that generally found in the SRN1 reactions of m- and p-nitrobenzyl halides. This indicates that there is likely to be an important contribution from an intramolecular through-space electron transfer mechanism for the former reaction

1,5-Dihydropyrrol-2-ones from (1,4-diaza-1,3-diene)tricarbonyliron and alkyne. 2. Structure of a [2.2.2] bicyclic intermediate with iron at the bridgehead position

1985 ◽  
Vol 4 (5) ◽  
pp. 948-949 ◽  
Author(s):  
Hans Werner. Fruehauf ◽  
Frank. Seils ◽  
Richard J. Goddard ◽  
Maria J. Romao
Keyword(s):  
Bridgehead Position

Synthesis of bicyclo[n.2.1] bridgehead alkenes acetoxy substituted at the opposite bridgehead position

1984 ◽  
Vol 49 (11) ◽  
pp. 2012-2015 ◽  
Author(s):  
Yoshito Tobe ◽  
Yasushi Fukuda ◽  
Kiyomi Kakiuchi ◽  
Yoshinobu Odaira
Keyword(s):  
Bridgehead Position
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