Journal of the American Chemical Society
ARTICLE
crowding by the anilino phenyl group in BAI is much larger than
that of the methyl group in BMI. Thus, it seems clear that for an
effective reaction the samarium has to coordinate to the lone pair
of the nitrogen. Most probably the coordination is to Sm ,
although in the surface catalysis mode it could be to a neighbor-
ing trivalent samarium ion. This model fits nicely the observed
HMPA effect. Parallel to the surface catalytic reaction, a direct
the SmI
2
(619 nm). Whenever proton donor was used, the proton donor
was mixed with the substrate solution. Each set of experiments was
repeated two to three times. Within a set, each measurement was
routinely repeated three times. At the end of each series, the first
measurement was repeated to ensure reproducibility within a set. The
deviation usually observed less than 5%. First-order kinetics were
analyzed using Kinet Asyst (v. 2.2 Hi-Tech Ltd.).
General Procedure for Product Preparation under Condi-
tions Similar to the Kinetic Measurements. A freshly prepared
2
solution of SmI (0.1 M) in THF was added in the glovebox to a
2+
reaction between the SmI and the substrates may also exist. This
2
11
inner sphere electron transfer is very fast and definitely plays an
important role in the early stages of the reaction before enough
3
+
homogeneous solution of the imine and TFE in dry THF. The total
volume of the reaction was 200 mL. The final concentrations were
Sm is generated to commence the surface catalysis. At certain
concentrations of HMPA, the surface catalysis is quenched, and
two reaction paths remain open, one through the direct coordi-
[
2
SmI ] = 2.5 mM, [imine] = 1 mM, and [TFE] = 0.1 M. After 5 min, the
solvent was evaporated under reduced pressure at 30 °C. The crude
reaction mixture was redissolved in DCM (50 mL), washed consecu-
nation of the SmI to the nitrogen lone pair, and the other is an
2
outer sphere mechanism that benefits from the higher reduction
tively with solutions of saturated NaHCO
10 mL), potassium dihydrogen phosphate buffer (20 mL), followed by
brine (20 mL), and dried over anhydrous Na SO . The solvent was
3 2 2 3
(10 mL), saturated Na S O
potential of the HMPAꢀSmI complex. In the case of BAI,
2
(
where the direct coordination is considerably hindered, the
2
4
coordination of SmI by the HMPA will enhance the reaction.
2
evaporated under reduced pressure. Under these conditions, BAI gave
5% reduced product and 75% of the coupled product. The two other
On the other hand, for the two other substrates, BPI and BMI,
2
the precoordination of the SmI to the substrate is more effective
2
imines gave only coupled products.
Preparation of SmI Solution in THF. To the homogeneous
solution of SmI
than the HMPAꢀSmI complex mechanism. Therefore, rate
2
3
retardation is caused by the further reduction of the small
amounts of free or partial HMPA-coordinated SmI2.
2 2
in THF was added a freshly prepared I solution in
THF. The concentration of I was one-half that of the SmI . The freshly
2
2
prepared yellowish-green solution of SmI in THF was used for the
3
’
SUMMARY AND CONCLUSIONS
kinetic experiments as well as dynamic light scattering, light microscopy,
and HRTEM analyses.
The three imines studied with SmI in THF all featured the
2
same autocatalytic behavior, suggesting autocatalysis to be a
general feature for imino substrates. After SmI transfers an
’ ASSOCIATED CONTENT
2
electron to the substrate, it becomes trivalent samarium, whose
salt is sparingly soluble in THF. Although no solids are visible to
the naked eye, the existence of microcrystals was proven by light
microscopy as well as by dynamic light scattering analysis. It turns
out that catalytic sites exist on the surface of the solid. Therefore,
as the reaction progresses, more of these catalytic sites are
formed, causing an autocatalytic reaction profile.
S
Supporting Information. Complete ref 10, light micro-
b
scopy of SmI with various filters, dynamic light scattering
measurement figures, kinetic traces of the reaction of BAI and
SmI in the presence of MeOH, HRTEM of SmI , and Gaussian
archives for the computed structures. This material is available
3
2
3
free of charge via the Internet at http://pubs.acs.org.
In addition, it was found that the reactivity order of the three
imines does not correlate with their electron affinity, as is
’ AUTHOR INFORMATION
common in the chemistry of SmI , but rather with the steric
accessibility to the nitrogen lone pair. Thus, in the noncatalytic
2
Corresponding Author
reaction, the SmI coordinates first to the nitrogen lone pair,
2
leading to a very efficient inner sphere electron transfer process.
As a natural continuation, we have initiated a study with
substrates where the binding site does not coincide with the
reaction site, such as in pyridine derivatives.
’
ACKNOWLEDGMENT
We thank Dr. J. Grinblat for the HRTEM analysis.
’
EXPERIMENTAL SECTION
’
REFERENCES
(
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General. THF was dried and freshly distilled over Na wire and
(
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12
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2
3
1
freshly prepared THF solution. The concentration of the SmI
2
solution was spectroscopically determined (λ = 619 nm; ε = 635).
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1
13
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2
1
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and the HRMS data with literature values.
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flow spectrophotometer in a glovebox under nitrogen atmosphere at
room temperature. The flow system was cleaned with 1 N HCl before
conducting a series of kinetic experiments to ensure that no catalytic
solids remain in the system. The reactions were monitored at the λmax of
Concell ꢀo n, J. M.; Rodríguez-Solla, H.; Concell ꢀo n, C.; Amo, V. Chem. Soc.
Rev. 2010, 39, 4103–4113. (n) Edmonds, D. J.; Johnston, D.; Procter,
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dx.doi.org/10.1021/ja205885q |J. Am. Chem. Soc. 2011, 133, 14795–14803