Communication
and organosilanes, under metal-free conditions. We show
herein that a wide range of heteroaromatic silanes can be
used in the direct synthesis of esters from CO2, organohalides,
and a fluoride source; the methodology has been transposed
to the formation of a polyester material from CO2. Mechanistic
studies show that this reactivity can be enabled by a unique
catalytic influence of CO2 in the activation of the CÀSi bond.
In the absence of any transition metal, the controlled activa-
tion of commercially available phenyltrimethylsilane (1a) and
2-(trimethylsilyl)pyridine (2a) with fluoride ions is challenging,
À
because the corresponding C6H5 and C5H4NÀ anions are un-
stabilized carbanions with pKa of 45 and 44, respectively.
Indeed, phenyltrimethylsilane (1a) exhibited no reactivity
when subjected to an atmosphere of CO2, in the presence of
CH3I and a variety of fluoride sources, such as CsF, TASF (tris-
(dimethylamino)sulfonium difluorotrimethylsilicate), and TBAT
(tetrabutylammonium difluorotriphenylsilicate). In contrast,
traces of the methyl picolinate (4aa) were detected when 2a
was reacted with CO2, CH3I, and CsF, TBAF·3H2O, or TMAF as
a fluoride source (Table 1, entries 1–3). Whereas Brønsted
bases, such as Cs2CO3 or tBuOK, were unable to promote the
carboxylation of the substrate (entries 4 and 5); other anhy-
drous fluoride salts, such as TASF and TBAT, allowed for the for-
mation of the methyl picolinate (4aa) in moderate to excellent
yields (46% and 93%, respectively, entries 6 and 7). It is worth
noting that a procedure to recycle the Ph3SiF byproduct is
possible by the addition of TBAF to reform TBAT.[14] Carrying
the reaction in THF or CH2Cl2 allowed for a nearly quantitative
conversion into the ester.
Scheme 2. Synthesis of picolinic esters by fluoride-mediated carboxylation
of 2-(trimethylsilyl)pyridine (2a) with various electrophiles. NMR yield given;
[a] average isolated yield over two runs on 1 mmol scale; [b] 3 h (0.5 equiv).
organosilane is compatible with both CO2 and the halogenoal-
kane, and the reaction proceeds in one pot. In fact, changing
the nature of the electrophile enables the formation of a large
diversity of picolinic esters (Scheme 2). Primary and secondary
alkyl and allyl iodides provided very good yields to the desired
esters (4ab, 90%; 4ac, 85%; 4ae, 93%; 4af, 99%) after 40 h at
room temperature (RT) or 18 h at 408C (Scheme 2). Iodoben-
zene proved to be unreactive in this reaction, indicating that
the formation of the OÀC bond occurs through a nucleophilic
substitution. Importantly, the alkylation of the pyridine ring
with the organic halide was not observed, and the carbon nu-
cleophile, released by CÀSi bond activation, selectively reacted
with CO2. The utilization of less electrophilic bromoalkanes and
chloroalkanes required longer reaction times at RT (72 h) or
mild heating (708C), as exemplified by the formation of 4ae,
4af, and 4ag in >93% yield from various organic halides
(Scheme 2). Interestingly, using a difunctional electrophilic spe-
cies, such as 1,4-diiodobutane, diester 4ah formed in 85%
yield and crystals suitable for X-ray diffraction analysis could
be isolated (see the Supporting Information and Figure S6).
The substituents borne by the silicon center have a notable
influence on the reactivity of the pyridylsilane reagent and dif-
ferent kinetic rates were measured when one methyl group in
2a was replaced with a vinyl (2a2), allyl (2a3) or phenyl (2a4)
group. Although the presence of an allyl group slows down
the activation of the CÀSi bond, introducing a phenyl or vinyl
group at the silicon center enhances the kinetics of the
carboxylation reaction (see the Supporting Information).
The formation of 4aa represents the first example of the
direct carboxylation of a pyridylsilane reagent. Importantly, the
Table 1. Screening of the reaction conditions for the direct esterification
of 2a using CO2 and CH3I.
[b]
Entry[a]
Additive
Solvent
Yield [%]
1
CsF
THF
traces
2
3
4
5
6
7
8
9
TBAF·3H2O
TMAF
Cs2CO3
tBuOK
TASF
TBAT
TBAT
TBAT
TBAT
–
THF
THF
THF
THF
THF
THF
CH3CN
CH2Cl2
toluene
THF
7
traces
0
0
46
93 (82)[c]
12
95
2
Taken together, these data suggest that the formation of pi-
colinic esters proceeds by a fluoride-mediated activation of the
CÀSi bond, which releases a carbanion able to undergo car-
boxylation in the presence of CO2 prior to the esterification
step with the electrophile. Noticeably, no reaction occurred for
arylsilanes that do not feature a nitrogen atom on the ortho
position of the silyl group, and both phenyldimethylvinylsilane
10
11
0
[a] Reaction conditions: NMR tube, 2a (0.1 mmol), additive (0.1 mmol), 3a
(0.1 mmol), solvent (0.3 mL), CO2 (1 bar). NMR yield determined with me-
sitylene as internal standard; [b] TBAF=[nBu4N][F], TMAF=[Me4N][F];
[c] Isolated yield.
Chem. Eur. J. 2016, 22, 2930 – 2934
2931
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