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Inductive CC-Hyperconjugative and Frangomeric Effects in Solvolytic Fragmentation Reactions.

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Inductive, CC-Hyperconjugative, and Frangomeric
Effects in Solvolytic Fragmentation Reactions
By Cyril A . Grob, Martin Bolleter, and Walter Kunz"]
l-Azabicyclo[3.2.l]octylendo-4-p-toluenesulfonate(la), 1azabicyclo[3.3.1Inonyl endo-4-p-toluenesulfonate(2a), and
the equatorial''] 4-bromo-I-azaadamantane (3a) undergo
quantitative fragmentation in 80% ethanol to give the methy~
('1 Prof. Dr. C. A. Grob, Dipl.-Chem. M. Bolleter, Dr. W. Kunz
Institut fur Organische Chemie der Universitat
St. Johanns-Ring 19. CH-4056 Basel (Switzerland)
leneimmonium salts (7), (S), and (9), respectively[*! Moreover, they react some lo3, lo4, and lo5 times faster than the
nitrogen-free parent compounds (4a), {5a), and (6a). This
comparison demonstrates the pronounced frangomeric acceleration of the ionization of (la), (2a), and (3a)"l. The azabicyclics thus react by a concerted mechanism, as was to be expected from the antiperiplanar arrangement of the C(4)-R'
bond and the electron pair at N(l) relative to the C(2)-C(3)
On the other hand, the axial''] epimer ( l b ) in which this
stereoelectronic condition is not fulfilled (cf. ref.[31)reacts
slower than the parent compound (4b), giving the substitution and elimination products (lc), (Id),and (12), respectively, along with the fragmentation product (7). The fivefold
slower reaction of (16) relative to (4b) and the formation of
several products suggest the intermediacy of the cation (lo),
whose formation is rendered more difficult by the electronwithdrawing inductive ( - 0 effect of the nitrogen atom
(SOH = solvent).
Surprisingly, the likewise axial"] epimer (2b) reacts twice
as fast, and (3b) even 1230-times asfast as (Sb) and (6a)I5],respectively, although the stereoelectronic conditions for a concerted fragmentation are again not fulfilled141. In the case of
(2b) and (3b), fragmentation forming (8) and (9), respectively, is accompanied by substitution giving (2c) and (3c), and
with (2b) by elimination giving the olefin (13); this indicates
occurrence of the cationic intermediates (11) and (14), respectively. This assumption is supported by the observation
that addition of ten mol. equiv. of sodium azide to an aqueous solution of (3b) hardly affects the reaction rate, while the
yield of (9) falls from 75 to 58%. The question thus arises as
to how the nitrogen atom accelerates the ionization of the axial bromide (3b) to form (14).
Fundamental studies by Schleyer et al.161have shown that
nucleophilic participation of the solvent in the ionization of
secondary p-toluenesulfonates (SN2 with intermediate) is
highly dependent upon steric hindrance at the rear of the
reaction site. This hindrance is only slight in simple acyclic
tosylates, significantly stronger in the cyclic compounds, and
so strong in 2-adamantylp-toluenesulfonate (6b) that nucleophilic participation of the solvent is no longer detectable1’].
This participation should therefore be practically ruled out
in the ionization of the axial 4-bromo-I -azaadamantane (36)
but not in the case of (Ib) and (26).
( a ) R = H ; R’=Br
R=OTS ; R’=H
( b ) R = B r ; R’=H
R = H ; R ’ = O H , OC2H5
( c ) R=H ; R’=OH
R = O H , OC2Hg ; R’ = H
and (3a), in which nucleophilic solvation is likewise hindered
but which can undergo concerted fragmentation. Thus exclusion of solvent triggers a reaction-promoting hyperconjugative or frangomeric effect. However, the stereoelectronic requirements for CC hyperconjugation are evidently less rigorous than for concerted fragmentation, and also less rigorous
than conventional orbital models would suggest.
Received: April 30, 1980 [Z 547 IE]
German version: Angew Chem. 92. 734 (1980)
[i] “Equatorial” and “axial” relative to the piperidine ring.
[2] Or their products formaldehyde and secondary arnine; cf. C. A . G o b . W.
Kunz, P. R. Marbet. Tetrahedron Lett. 1975. 2613; E. Krauchi, Dissertation,
Universitat Basel 1977.
131 Cf. review: C. A. G o b , Angew. Chem. 81,543 (1969); Angew. Chem. Int. Ed.
Engl. 8, 535 (1969).
p-to[4] Unusual behavior is also observed with 6-exo-amino-2-exo-norbornyl
luenesulfonale (cf. W. Fischer, C. A. Grob, G. von Sprecher. A. Waldner, Tetrahedron Lett. 1979, 1901) and 2-chloro-2,3.3-trimethyl-I-buianol(cf. W.
Fischer, C. A. Grob, Helv. Chim. Acta 61,2336 (1978)).
IS] Owing to the plane of symmetry there is only one isomer of (6).
161 T W. Bentley, P. uon R. Schleyer. J. Am. Chem. SOC.98, 7658 (1976); F. L.
Schadt, T W. Bentley, P. von R. Schleyer, ibid. 98, 7667 (1976).
[71 This steric hindrance probably accounts for the fact that (46) and (Sb) react
ca. 500 times faster than (66) in 80% ethanol.
[8] The electron shift is indicated in only one of the equivalent bridges for the
sake of clarity.
191 C. A. G o b , Angew. Chem. 88, 621 (1976); Angew. Chem. Int. Ed. Engl. 15,
569 (1976); W.Fischer, C. A Grob, Helv. Chim. Acta 61, 1588 (1978); C. A.
Grob. A. Waldner, ibid. 62. 1736 (1979); W. Fischer, C. A. Grob, G. von
Sprecher. A. Waldner, Tetrahedron Lett. t979. 1905.
(R,NPO),: A Novel Heterocycle with h3-Phosphorus
by Trimerization of an Aminooxophosphane[”’
By Edgar Niecke, Hans Zorn, Bernt Krebs, and Gerald
The Staudinger iminophosphorane/carbonyl reaction and
its sulfinyl variant”] are of preparative importance for the
coupling of nitrogen-carbon and nitrogen-sulfur double
bonds, respectively.
However, since the positive charge at the reaction site is
largely transferred to the solvent by nucleophilic solvation,
the electron deficiency at C(4) is only slight in the ion pairs
(10) and (11) so that only the ( - Z) effect of the nitrogen
atom should be effective. But if access of solvent is hindered,
as in (14), then the positive charge spreads over the molecule,
and particularly to the nitrogen atom if, as in (1.5)lx1,it is hyperconjugated via a CC single bond with the cationic site at
C(4)I9l. This also applies to the equatorial epimers (la), (2a),
Angew. Chem. Int. Ed. Engl. 19 (1980) No. 9
In the case of iminophosphanes, however, this reaction
provides a possible approach for the generation of phosphorus compounds with pm(P)-pn(0) bonding. This class of
compounds is gaining more and more interest in connection
with investigations on highly reactive species containing element-element multiple bonds1*].
We report here on a pseudo-Wittig reaction of the irninophosphane (I), which leads to the previously unknown 1,3,5trioxa-2h3,4h3,6X3-triphosphorin
Diisopropylamino-tert-butyliminophosphane (I) already
reacts with sulfur dioxide at -25 OCP1to give tert-butyliminosulfur oxide (2) and a product o f the composition
[*] Prof. Dr. E. Niecke, DiplLChem. H. Zorn
Fakultat fur Chemie der Universitat
Postfach 8640, D-4800 Bielefeld 1 (Germany)
Prof. Dr. B. Krebs, Dr. G . Henkel
Anorganisch-Chemisches Institut der Universitat
Gievenbecker Weg 9, D4400 Miinster (Germany)
[**I Phosphazanes of Coordination Number 2 and 3. Part 18. This work was
supported by the Fonds der Chemischen Industrie. We thank Dr. T. Wirthlin,
Varian AG. Zug/Switzerland for carrying out the NMR measurements.-Part
17: E. Niecke, D:A. Wildbredt, Chem. Ber. 113, 1549 (1980).
0 Verlag Chemie, GmbH, 6940 Weinheim, 1980
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effect, reaction, inductive, hyperconjugative, fragmentation, frangomeric, solvolytic
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