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(22) In selected cases, heating of crude reaction mixtures after
workup in toluene (90 °C, overnight) led to isolation of
thermodynamically privileged endo isomers, as shown in Scheme 2.
1
However, H NMR analyses of crude reaction mixtures prior to the
isomerization gave the following ratios of the isomers (endo:exo): 3t,
1.8:1.0; 3v, 2.0:1.0; 3w, 3.0:1.0; 3x, 2.4:1.0; and 3y, 4.9:1.0.
(23) Reaction of octyl phenyl selenone (2d) with 1,5-dinitronaph-
thalene was only moderately successful, giving a mixture of
benzonorcaradiene (26%), product of alkylation (13%), and
recovered nitroarene (31%).
(24) Reaction of 1a with 2e carried out in the presence 150 mol% of
TEMPO gave 3e in 84% of yield (72% endo + 12% exo). TEMPO was
recovered in 63%. No other defined products were isolated.
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(26) The effect is expectedly augmented for reactions of 2-
nitronaphthalene, where steric hindrance around the attacked position
1 (between NO2 and distant aromatic ring) hampers β-elimination of
both diastereoisomers of the σ-adduct, in favor of benzonorcar-
adienes. Indeed, yields of 3 produced from 2-nitronaphthalene were in
general higher than with 1-nitronaphthalene, in accordance with the
presented model.
(27) Sterically demanding substituents at the carbanionic center in
alkyl phenyl selenones (Scheme 3, top) are expected to inhibit the
cyclization step, running by the SN2-type mechanism. Compare:
Vayner, G.; Houk, K. N.; Jorgensen, W. L.; Brauman, J. I. Steric
Retardation of SN2 Reactions in the Gas Phase and Solution. J. Am.
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(28) The argument is no longer valid for typical VNS reagents, as,
for example, α-chlorosulfones, for which electron-withdrawing group
additionally stabilizes (delocalizes) anion of the product.
(29) In alkyl diphenyl sulfonium salts 7f (Et) and 7g (iPr), methyl
groups attached at the α-position (carbanionic center) are expected to
influence the cyclization step by steric hindrance but also electron
donation. Interestingly, it was demonstrated that in intramolecular
processes, where bond breaking is more advanced than bond making
at the transition state, electron donors accelerate SN2-type
substitution: Robiette, R.; Trieu-Van, T.; Aggarwal, V. K.; Harvey,
J. N. Activation of the SN2 Reaction by Adjacent π Systems: The
(17) An interesting exception is benzylation, where aromatic ring
acts as an electron-withdrawing group, for example, in deprotonation
of benzyl chloride at the α position: (a) Chhaly, L.; Pritzkow, W. 4-
Nitrobenzylchloride as a nucleophile in VNS reactions. J. Prakt.
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Chem./Chem.-Ztg. 1994, 336, 558−560. (b) Brzeskiewicz, J.; Loska,
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Nucleophilic Substitution with Benzylidene Dichloride: Umpolung of
the Friedel−Crafts Reaction. J. Org. Chem. 2018, 83, 8499−8508.
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(c) Kisiel, K.; Brzeskiewicz, J.; Loska, R.; Mąkosza, M. Transition
E
Org. Lett. XXXX, XXX, XXX−XXX