Journal of the American Chemical Society
Communication
yet to be explored. We reasoned that by applying a sufficiently
reducing potential, chlorosilanes could undergo single-electron
reduction and fragmentation, giving rise to polarity-reversed
nucleophilic silyl radicals. Traditionally, this chemistry has
been inaccessible due to the challenging reduction required of
the strong Si−Cl bonds (ca. − 0.5 V vs Mg0/2+, BDE ∼110
kcal/mol). Electrochemistry is capable of driving reactions far
from equilibrium under highly biased potentials, often
exceeding the limits of traditional chemical oxidants or
reductants.16 Indeed, early studies showed that electro-
reduction of chlorosilanes is possible toward the formation
of dimeric and polymeric silanes (Scheme 1B).17 However,
these reactions are proposed to undergo a silyl-anion pathway
and are predominantly limited to Si−Si coupling.18 In this
work, we employed a combination of synthetic and
mechanistic tools to establish the electroreductive activation
of chlorosilanes as a new and general strategy for the discovery
of new radical silylation chemistry.
disilanes as Si sources.20 We discovered that the combination
of TBAClO4 as the electrolyte, a magnesium sacrificial anode,
and a graphite cathode provided the optimal 94% yield under a
constant current of 10 mA (cathodic potential ∼ − 0.85 V vs
Mg0/2+). Notably, even TMSOAc with a very strong Si−O
bond (120−140 kcal/mol) can be activated, resulting in 31%
yield. Using TBA(TFSI) as the electrolyte instead of TBAClO4
attained comparable reactivity (77%). This electrochemical
protocol is easily scaled to 5 mmol without increasing solvent
volume.
We subsequently evaluated the scope and functional group
compatibility of our electroreductive strategy. Various
functionalities that are potentially sensitive to chemical redox
agents, such as boronate (4), tertiary amine (5), thioether (7),
alcohol (8), and ketone (9) were preserved. Several electron-
deficient and electron-rich heterocycles (10−11) and
ferrocene (12) were also compatible with the reaction
conditions. We also investigated other types of π-systems
such as allenes (15), internal alkynes (16), conjugated dienes
(17), and enynes (18) to generate a range of allyl and vinyl
silanes, which could be further derivatized using cross-coupling
and allylation reactions. Moreover, vinyl boronates proved to
be suitable substrates, providing products (19−23) with gem-
(B,Si) substitution, which are versatile functional groups in
organic synthesis.21 Simple aliphatic olefins suffer from lower
reactivity due to the lack of anion-stabilizing substituents
obtained using information gleaned from mechanistic analysis
(Scheme 5C).
We focused our initial exploration on the development of an
electroreductive alkene silylation reaction and discovered that
the electrolysis of a mixture of TMSCl and styrene (1) in THF
led to the formation of vicinal disilane 2 (Scheme 2 and Table
S1).19 Recently, Oestreich reported an elegant example of
alkene disilylation via silylium catalysis using homoleptic
Scheme 2. Electroreductive Disilylation of Alkenes
A diverse array of chlorosilanes proved to be effective for the
construction of value-added organosilanes (24−29). In
particular, dimethylsilane (24), vinyldimethylsilane (26), and
allyldimethylsilane (27) led to products that could be used as
monomers for silicon-containing polymers.22 Furthermore, the
incorporation of disilane groups vicinally to an alkene (25, 28)
demonstrated the potential utility of this reaction for the
preparation of parallel single-molecule silicon wires for
materials and electronic applications.23 Chlorotrimethylger-
mane could also react to furnish product 29. The success and
limitation of our reaction scope piqued our interest in
investigating the reaction mechanism with the objective of
expanding the reactivity to other synthetically useful trans-
formations.
The electrochemical disilylation is comprised of three
components that can be reduced at the cathodethe alkene,
chlorosilane, and anodically generated Mg2+. The reduction of
each of these components could contribute to the observed
disilylation (Scheme S6). For example, electrogenerated Mg0
could activate either styrene or TMSCl to form magnesiated
nucleophiles prior to C−Si formation.19e Alternatively,
cathodic reduction of styrene could lead to a radical anion
that initiates the disilylation. Finally, the direct reduction of
TMSCl followed by mesolytic Si−Cl cleavage could produce
TMS• prior to its addition to the alkene (Scheme 1A).
A series of electroanalytical experiments, multivariate linear
regression (MLR) analyses, and density functional theory
(DFT) calculations lent strong support to the silyl radical
pathway and provided more insights into the reaction
mechanism. First, control experiments using either Mg powder
(Table 1, entry 1) or electrogenerated Mg0 (entries 2−4) gave
no conversion. We also carried out divided-cell electrolysis that
separates the cathodic disilylation reaction from oxidation of
the sacrificial anode. Using either Mg or Zn as the anode
Reaction conditions: 1.0 mmol alkene, 3.0 equiv chlorosilane, 0.2 M
TBAClO4 in 9 mL THF, electrolysis at 22 °C under a constant
current of 10 mA (current density = 1.2 mA/cm2) until 3.5 F total
charge is passed. Abbreviations: TBA, tetrabutylammonium; TMS,
a
b
trimethylsilyl. Two equivalents of TMSCl. Five millimole scale.
c
Yield determined by 1H NMR using 1,3,5-trimethoxybenzene as
d
e
internal standard. 5.0 F total charge. TMSCl = 4.0 equiv., 4.0 F total
charge. TMSCl = 4.0 equiv., 2.2 F total charge. 2.2 F total charge.
f
g
B
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX