C O M M U N I C A T I O N S
Acknowledgment. This work was partially supported by a
Grant-in-Aid for Scientific Research.
Supporting Information Available: Experimental procedures,
spectroscopic data of compounds (PDF). This material is available free
References
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Figure 2. Thermal stability of alkoxycarbenium ion 2 generated by the
direct method5a and the present indirect method.
Table 1. Indirect Generation of Cation Pools and Their Reactions
with Carbon Nucleophilesa
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45, 1844.
(3) To solve the problem, we have developed the cation flow method. (a)
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(10) Theoretical amount of electricity to convert ArSSAr to ArS(ArSSAr)+.
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a ArSSAr (Ar ) p-FC6H4, 0.40 mmol) was oxidized electrochemically
in 0.3 M Bu4NBF4/CH2Cl2 at -78 °C using 0.67 F/mol of electricity. The
solution thus obtained was allowed to react with 0.20 mmol of thioacetal
at -78 °C for 5 min. Then a nucleophile (0.50 mmol, 2.5 equiv) was added,
and the resulting solution was stirred at -78 °C for 15 min and the reaction
was quenched with Et3N. b Allyltrimethylsilane (10 equiv) was used.
c Reaction of thioacetal with ArS(ArSSAr)+: -78 °C, 10 min. Reaction
with allylsilane: -78 °C, 1 h. d Diastereomeric ratio.
(12) Yamaguchi, K. J. Mass Spectrom. 2003, 38, 473.
(13) Reactions of thioacetals with allylsilanes have been achieved by the action
of a Lewis acid, such as SnCl4 and BF3-OEt2. (a) Braga, L.; Dornelles,
L.; Silveira, C.; Wessjohann, A. Synthesis 1999, 562. (b) Sato, T.; Okura,
S.; Otera, J.; Nozaki, H. Tetrahedron Lett. 1987, 28, 6299. (c) Nishiyama,
H.; Narimatsu, S.; Sakuta, K.; Itoh, K. J. Chem. Soc., Chem. Commun.
1982, 459. However, when 1 (R ) C8H17) was treated with SnCl4 or BF3-
OEt2 in the absence of an allylsilane, 2 was not observed by 1H NMR,
indicating that the equilibrium lay so far to 1, and the concentration of 2
was extremely small with such acids.
(14) The reaction of 2 with 2 equiv of allyltrimethylsilane at -78 °C for 1
min followed by the treatment with 1-(trimethylsilyloxy)cyclohexene (2
equiv) at -78 °C for 10 min gave 3 in 81% yield. Only a trace amount
of the product derived from 1-(trimethylsiloxy)cyclohexene was obtained,
indicating that the reaction of 2 with allyltrimethylsilane was complete
within 1 min at -78 °C.
(15) The anodic oxidation of ArSSAr (Ar ) p-FC6H4) (0.40 mmol) in 0.3 M
Bu4NBF4/CH2Cl2 was carried out in an H-type divided cell equipped with
a carbon felt anode and a platinum plate cathode under the constant current
condition at -78 °C. To the anodic chamber containing electrogenerated
ArS(ArSSAr)+ was added 1 (0.20 mmol), and the mixture was stirred for
5 min at -78 °C. Allyltrimethylsilane (0.50 mmol) was added, and the
resulting solution was stirred at -78 °C for 15 min to obtain 3.
their reactions with several carbon nucleophiles (2.5 equiv) were
examined (Table 1).15 Allylsilanes, enol silyl ethers, and ketene
silyl acetals were effective as carbon nucleophiles. The p-MeC6H4S
and p-ClC6H4S groups were also effective as leaving groups. Alkyl-
and aryl-substituted thioacetals, including cyclic substrates, were
found to be effective.
In conclusion, we have developed a sequential one-pot indirect
method for the generation of alkoxycarbenium ion pools. The
reagent electrochemically generated from ArSSAr is well character-
ized spectroscopically, and its reaction with a thioacetal to generate
cation pools is very fast. The results obtained here add a new
dimension of the cation pool method, which serves as a powerful
tool for carbocation chemistry. Applications of the present method
to the generation and accumulation of highly unstable cations are
in progress.
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