C O M M U N I C A T I O N S
Scheme 1. Regioselective Dehydrohalogenationa
Table 3. Reactions of Other Alkyl Halides and Pseudohalides
entry
1
time /h
1 /%
2a /%
3a /%
1
2
3
4
5
6
1a-I
0.25
3.7
2
4
0.5
1.5
0
83
82
97
0
87
13
0
0
96
79
3
1
0
0
2
8
1a-Cl
1a-OMs
1a-OTs
1k-I
1k-Br
0
a Co-catalyzed conditions: 5 mol% CoCl2, 5 mol% IMes·HCl, 2 equiv
of Me2PhSiCH2MgCl, dioxane, 25 °C
The interesting reaction mechanism of the cobalt-catalyzed
dehydrohalogenation offered unique transformations that are oth-
erwise difficult to attain (Scheme 1). High regioselectivity was
observed in the dehydrobromination reaction of 1l, whereas the
reaction of 1l with t-BuOK showed no regioselectivity. The cobalt-
catalyzed dehydrohalogenation of homoallyl iodide 1m selectively
yielded unconjugated diene 11. In contrast, treatment of 1m with
t-BuOK provided conjugated diene 12 exclusively. The selective
formation of unconjugated alkene 13 in the reaction of 1n also
highlights the synthetic utility of the cobalt catalysis. Notably, most
of trans-1n remained untouched in the t-BuOK-mediated dehy-
droiodination, due to the difficulty in forming the anti periplanar
transition state for the E2 elimination.
Figure 1. Plausible reaction mechanism and conformational analysis of 7
that undergoes ꢀ-elimination.
mixture of 2-methyl-1-tridecene (35% yield) and 2-methyl-2-
tridecene (51% yield).
Acknowledgment. This work was supported by Grants-in-Aid
for Scientific Research from MEXT and JSPS. H.O. acknowledges
JSPS for financial support.
Based on our studies on cobalt-catalyzed reactions6 as well as
the reactivity trend observed in Table 3, we are tempted to assume
the reaction mechanism as follows (Figure 1). Single electron
transfer would take place from electron-rich cobalt complex 4 to
an alkyl halide to generate the corresponding alkyl radical.7 The
radical would be then captured by cobalt complex 6 to afford
alkylcobalt intermediate 7. Intermediate 7 would then undergo
ꢀ-hydride elimination to afford 1-alkene. Degrees of steric repulsion
in the transition states of the ꢀ-hydride elimination account for the
regioselective formation of 1-alkenes. The ꢀ-hydride elimination
should proceed via a syn periplanar conformation 7a, 7b, or 7c.8
Conformation 7a is most preferable, minimizing the total steric
repulsion.
Supporting Information Available: Experimental details, additional
experimental data, and characterization data of products. This material
References
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When the elimination reactions were performed at 50 °C, the
corresponding (E)-2-alkenes were obtained selectively (eqs 3 and
4). Notably, further isomerization into 3-alkenes is negligible,
probably because of the sufficient bulkiness of the cobalt catalyst.
The reactions initially afforded 1-alkenes 2 within 15 min, and
prolonged heating induced gradual isomerization to (E)-alkenes 3.
(7) The single electron transfer mechanism is justified by an additional
experiment. For details, see the Supporting Information.
(8) Kova´cs, I.; Ungva´ry, F.; Marko´, L. Organometallics 1994, 13, 1927–1933.
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