Angewandte
Communications
Chemie
Proton Donors
High-Affinity Proton Donors Promote Proton-Coupled Electron
Transfer by Samarium Diiodide
Abstract: The relationship between proton-donor affinity for
SmII ions and the reduction of two substrates (anthracene and
benzyl chloride) was examined. A combination of spectro-
scopic, thermochemical, and kinetic studies show that only
those proton donors that coordinate or chelate strongly to SmII
promote anthracene reduction through a PCET process. These
studies demonstrate that the combination of SmII ions and
water does not provide a unique reagent system for formal
hydrogen atom transfer to substrates.
dination to SmII ions is a prerequisite for reduction through
PCET.
To examine the importance of proton-donor coordination
to SmII ions, we chose several proton donors: diethylene
glycol (dg), diethylene glycol monomethyl ether (dgme),
ethylene glycol (eg), ethylene glycol monomethyl ether
(egme), water, and 2,2,2-trifluoroethanol (TFE). Proton
donors dg, eg, and water are known to coordinate strongly
to SmII ions, and the monomethyl ethers dgme and egme were
chosen since the replacement of a hydroxy proton with
a methyl group has a deleterious impact on the affinity of
proton donors for SmII ions.[5] The proton donor TFE does not
coordinate to SmII ions even at high concentrations.[2d,e,h]
Anthracene and benzyl chloride were chosen as substrates
since studies would not be complicated by competition with
proton donors for coordination sites on SmII ions. Addition-
ally, benzyl chloride is reduced through a rate-limiting
dissociative electron transfer,[6] whereas anthracene has
been shown to be reduced through a PCET by SmII–water.
If proton-donor coordination is important for PCET from
a SmII donor complex, we expect high-affinity donors to have
a larger relative impact on the rate of reduction of anthracene
than benzyl chloride.
To test the assertion described above, the rate of reduction
of anthracene and benzyl chloride by SmI2 containing
increasing amounts of proton donor were measured under
pseudo-first-order conditions with substrates in a 12.5-fold
excess with respect to [SmI2]. Unfortunately, the use of dg led
to significant precipitation, which prevented rate studies,
although substrate reduction was fast as observed by rapid
decoloration of the solution, consistent with oxidation of SmII
ions. Additionally, dgme and egme did not facilitate the
reduction of anthracene in the timescale of the rate studies
although dgme had a modest impact on the rate of benzyl
chloride reduction by SmII ions. A plot of proton-donor
concentration versus kobs for reactions where rates could be
measured is displayed in Figure 1.
T
he presence of additives in reactions of samarium diiodide
has a profound impact on their rate and selectivity.[1] There
are three classes of additives used in reactions of SmI2: Lewis
bases, proton donors, and transition-metal salts. Among these
groups of additives, proton donors have been shown to be
quite effective in accelerating substrate reduction and altering
the selectivities of reactions.[2] In particular, the combination
of SmI2 and water is unusual in that it is effective in reacting
with substrates that are difficult to reduce through single
electron transfer.[3] The distinguishing feature of this class of
reductions is that they are significantly endergonic. This raises
the question as to whether substrate reduction proceeds
through an electron transfer followed by a proton transfer, or
whether the process occurs through a formal hydrogen-atom
transfer (HAT). We recently examined this question through
a series of rate and mechanistic studies to examine the
reduction of anthracene by SmI2–water. These studies showed
that substrate reduction occurred through initial proton-
coupled electron transfer (PCET).[4] The key component of
this transformation was strong coordination of water to SmII
À
ions that significantly weakens the O H bond. Proton donors
such as methanol, which do not have a high affinity for SmII
ions, are ineffective for arene reduction. Since initial studies
showed that the affinity of water for SmII is critical, it raises
several important questions: 1) Does the combination of SmI2
and water provide a unique combination for HAT to
substrates? 2) Can high-affinity proton donors be used in
place of water to promote reductions? 3) Is there a relation-
ship between proton-donor affinity for SmII ions and initial
HAT to substrate? Herein we present studies designed to
answer these important questions. Overall, the experiments
presented here demonstrate that strong proton-donor coor-
There are several interesting features of the data dis-
played in Figure 1. Among the reductions that could be
measured by stopped-flow experiments, only water and eg
facilitate the reduction of anthracene by SmI2, whereas all
proton donors (except TFE) slightly accelerate the reduction
of benzyl chloride. The reduction of benzyl chloride through
an initial ET from SmI2 is exergonic by approximately
À3.7 kcalmolÀ1, whereas an initial ET to anthracene is
endergonic by approximately 25 kcalmolÀ1. While the
impact of proton donors on the reduction of benzyl chloride
is apparent, they are not as significant as those that facilitate
reduction of anthracene, a substrate considerably harder to
reduce through ET.
[*] T. V. Chciuk, W. R. Anderson Jr., Prof. R. A. Flowers II
Department of Chemistry, Lehigh University
6 E. Packer Ave., Bethlehem, PA 18015 (USA)
E-mail: rof2@lehigh.edu
Supporting information and the ORCID identification number(s) for
Angew. Chem. Int. Ed. 2016, 55, 6033 –6036
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
6033