Photocatalytic Cross-Pinacol Coupling

Abstract

Cross-pinacol coupling of two different carbonyl compounds was achieved through successive one-electron transfer processes under photocatalytic conditions. In this process, an umpoled anionic carbinol synthon was generated in situ to react nucleophilically with a second electrophilic carbonyl compound. A wide variety of aromatic and aliphatic aldehydes and ketones were tolerated to afford the corresponding unsymmetric vicinal 1,2-diols.

Full text

Photocatalytic Cross-Pinacol Coupling
Shintaro Okumura,1,2,* Teruki Takahashi,1,2 Kaoru Torii,1 Yasuhiro Uozumi1,2,*
1 Institute for Molecular Science (IMS), Okazaki, Aichi 444-8787, Japan
2 Department of Functional Molecular Science, SOKENDAI (The Graduate University for Advanced
Studies), Okazaki, Aichi 444-8787, Japan
* Correspondence to: sokumura@ims.ac.jp, uo@ims.ac.jp
Abstract: Cross-pinacol coupling of two different carbonyl compounds was achieved through
successive one-electron transfer processes under photocatalytic conditions. In this process, an umpoled
anionic carbinol synthon was generated in situ to react nucleophilically with a second electrophilic
carbonyl compound. A wide variety of aromatic and aliphatic aldehydes and ketones were tolerated to
afford the corresponding unsymmetric vicinal 1,2-diols.
Main Text:
1,2-Diols have aroused considerable interest due to their presence in many therapeutically and
biologically active compounds, as well as their significance in synthetic chemistry. Reductive coupling
of carbonyls, the so-called pinacol coupling, has been recognized as a fundamental and straightforward
method for forming 1,2-diols.1-12 Several investigations have shown that the pinacol coupling of
carbonyls proceeds in the presence of strong metal reducing agents (e.g., Mg, Zn, Al, or Ti) via the
corresponding ketyl radical species, which undergo the radical–radical couplings to form 1,2-diol units
(Fig. 1a). However, owing to the mechanism involved in radical–radical coupling, conventional
intermolecular pinacol coupling is limited to the homo-coupling of carbonyl compounds to give
symmetric 1,2-diols. There is good reason to believe that the cross-coupling of two different carbonyl

compounds, i.e. cross-pinacol coupling, would offer an attractive alternative for forming unsymmetric
1,2-diols.1-5 However, ketyl radicals generated in situ are so reactive that one ketyl radical cannot
selectively couple with a second carbonyl radical; this results in the formation of homo-coupled and
cross-coupled 1,2-diols as products.11,13,14 Recently, the groups of Ohmiya,15 and Zhang16
independently developed an intermolecular cross-pinacol coupling of two different carbonyl
compounds through copper-catalyzed or electrochemical generation of nucleophilic carbinol species,
respectively. However, the cross-pinacol coupling reactions were controlled only when one of the
carbonyl components was electronically activated or had a markedly different structure from that of
the other carbonyl compound.14-22 Therefore, cross-pinacol coupling between two carbonyl
compounds bearing similar structures and comparable reactivities remains a major challenge.13
We recently developed a novel photocatalytic carboxylation of aromatic aldehydes and ketones to
give mandelic acid derivatives (Fig. 1b).23 In this reaction, the carbonyl substrates underwent
successive one-electron reduction under blue-light irradiation in the presence of an iridium photoredox
catalyst and a 1,3-dimethyl-2,3-dihydro-1H-benzimidazole-based reductant24,25 to generate
nucleophilic carbinol anion species that subsequently reacted with carbon dioxide to afford the
corresponding carboxylation products. If the inherent electrophilic nature of carbonyl groups is taken
into account, the photocatalytic process brought about an umpolung of the carbonyls serving as
nucleophilic carbinol synthons. These findings suggested to us that a cross-pinacol coupling might
take place through reaction of the umpoled carbinol species with a second carbonyl compound (Fig.
1c). Here, we report the first photocatalytic cross-pinacol coupling between two different carbonyl
compounds to afford unsymmetric 1,2-diols. The reaction proceeds by a novel umpolung pathway in
which one carbonyl compound serves as a carbinol anion nucleophile through rapid two-electron
reduction to react with a second relatively electron-rich carbonyl compound possessing inherent
electrophilic reactivity. Various combinations of reactants, two aldehydes, two ketones, or an aldehyde
and a ketone, have been coupled, including couplings of two carbonyl substrates with similar structures.

Fig. 1. Cross-pinacol coupling to form unsymmetric 1,2-diol backbones. (a) Conventional radical
pinacol coupling. (b) Photocatalytic umpoled carbonyl carboxylation. (c) This work: anionic cross-
pinacol coupling via nucleophilic carbinol anions.
Results and discussion
First, the coupling reaction of two aromatic aldehydes was examined under photocatalytic
conditions. Methyl 4-formylbenzoate (1a, 1 equiv) was added to a solution containing p-anisaldehyde
(2A, 2 equiv), Ir(ppy)2(dtbbpy)PF6 (ppy =2-phenylpyridinato; dtbbpy = 4,4'-di-tert-butyl-2,2'-
bipyridine; 2 mol%) as a photocatalyst, and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzimidazole
(DMBI, 1.5 equiv) as a reductant in DMA. The mixture was subjected to blue-light irradiation (λmax =
462 nm) for 40 minutes at room temperature (r.t.) to afford the desired unsymmetric 1,2-diol 3aA in
26% yield, together with the dimeric diol 4aa in 52% yield as the major product (Table 1, entry1). The
product of dimerization of 2A (5AA) was not obtained, indicating that the ketyl radical from 2A was
hardly generated under the photocatalytic conditions. This suggested that the cross-pinacol coupling
proceeded via the carbinol anion of 1a, which reacted with the electrophilic coupling partner 2A to
give the cross-pinacol product 3aA. Surprisingly, when the reaction was carried out under a CO2
atmosphere, the cross-pinacol selectivity markedly improved, and 3aA was obtained in 77% NMR

yield, along with a 7% yield of the homo-coupled dimer 4aa and a 9% yield of the a-
hydroxycarboxylic acid 6a (entry 2).23 The presence of a small amount of CO2 (1 mL) was enough to
promote cross-pinacol coupling, without the formation of the carboxylic acid 6a as a byproduct, giving
3aA in 67% yield together with a 13% yield of the homo-coupled product 4aa (entry 3). By a thorough
screening of the reaction conditions, the NMR yield of 3aA was eventually increased to 82% along
with 5% of diol 4aa when CO2 (5 mL) was bubbled through the solution for 30 seconds before
irradiation under a N2 atmosphere; none of the a-hydroxycarboxylic acid 6a (entry 4) was formed [also
see Table S5 in the Supplementary Information (SI) for details]. The DMBI reductant was essential for
achieving an effective cross-pinacol coupling. When NEt3 or i-Pr2NEt was used instead of DMBI, the
yield of diol 3aA was less than 32% and the dimeric diol 4aa was obtained as the major product (entry
5). The best result was obtained when DMBI-CF3 was used as a reductant instead of DMBI; this gave
the desired cross-pinacol product 3aA (syn/anti = 80:20) in 81% isolated yield (84% NMR yield) (entry
6). Control experiments showed that all the reagents, as well as blue-light irradiation, were essential
for the formation of cross-coupled 1,2-diol 3aA (entries 7–9).

Table 1. Cross-pinacol coupling between methyl 4-formylbenzoate (1a) and p-anisaldehyde (2A)a
Entry
CO2 /mL
Variation
yield /%b
3aAc
4aa
5AA
6a
1
none
-
26
52
0
0
2
balloon
-
77
7
0
9
3
1d
-
67
13
0
0
4
5d
-
82
5
0
0
5
5d
NEt3 or i-Pr2NEte
<32
>51
0
0
6
5d
DMBI-CF3e
84 (81)f
5
0
2
7
5d
no Ir cat.
0
5
0
0
8
5d
no light
0
2
0
0
9
5d
no DMBI
0
0
0
0
a Reaction conditions: aldehyde 2A (0.4 mmol, 2 equiv), Ir(ppy)2(dtbbpy)PF6 (0.004 mmol, 2 mol%),
DMBI (0.3 mmol, 1.5 equiv), DMA (1.5 mL); dropwise addition of aldehyde 1a (0.2 mmol) over 20
min, then stirring for 40 min, blue LED irradiation (40 W, λmax = 462 nm), r.t..
b NMR yield unless otherwise noted.
c The major syn-3aA diastereomer is shown in the scheme.
d CO2 was bubbled for 30 s before irradiation.
e Instead of DMBI.
f Isolated yield; the diastereomeric syn/anti ratio for 3aA was 80:20.
Having identified an efficient cross-pinacol coupling system, we next examined the scope of the
reaction toward a wide variety of carbonyl compounds (Fig. 2). Aromatic aldehydes 2A–E bearing
electron-donating groups such as methoxy, methyl, or methylsulfanyl, when used as electrophilic
substrates, coupled with the nucleophilic aldehyde 1a under the photocatalytic conditions to afford the

unsymmetric 1,2-diols 3aA–aE in yields of 76–88%; aromatic C–F (3aF) and C–Cl (3aG) bonds were
also well tolerated. Aliphatic aldehydes were also suitable electrophiles. The primary and secondary
alkyl aldehydes 2H and 2I, respectively, gave the corresponding 1,2-diols 3aH and 3aI in yields of 78
and 71%. Even the sterically hindered tertiary alkyl aldehyde 2J reacted with 1a to afford the coupled
product 3aJ in 53% yield. A variety of aryl aldehydes 1a–m were then tested as nucleophiles to react
with p-anisaldehyde (2A) as an electrophile. Aryl aldehydes 1c and 1b bearing a tert-butoxycarbonyl
group or a carboxylic acid group, respectively, gave the desired 1,2-diols 3cA and 3bA in yields of
51% and 91% yield, respectively. When 4-acetylbenzaldehyde (1d) was used, the aldehyde part
coupled with 2A to afford 3dA in 61% NMR (46% isolated) yield. A 2-trifluoromethyl substituent,
which has a high inductive effect, promoted the generation of the carbinol anion to afford the cross-
coupled product 3eA in 62% yield. The coupling reaction of biphenyl-4-carboxaldehyde (3f), which
has a p-expanded aromatic moiety, gave the 1,2-diol 3fA in 54% yield.
The cross-pinacol coupling reactions of ketones afforded sterically hindered quaternary carbinol
carbons. Carbinol anions derived from electron-deficient aromatic aldehydes attacked aromatic or
aliphatic ketones. Methyl 4-formylbenzoate (1a) and 2-(trifluorometyl)benzaldehyde (1e) coupled
with 4-chloroacetophenone (2K) or acetone (2L), respectively, to give 1,2-diols 3aK and 3eL in yields
of 67 and 71%. Nucleophilic carbinol anions were successfully generated, not only from aromatic
aldehydes, but also from aromatic ketones. Electron-deficient acetophenone derivatives 1g–j bearing
methyl ester, trifluoromethyl, cyano, or methyl sulfone groups reacted with p-anisaldehyde to give the
corresponding products 3gA–jA in yields of 59–85%. Alkyl aryl ketone 1k bearing an isopropyl group
as an alkyl moiety was an eligible reactant and gave the corresponding diol 3kA in 69% yield. Diaryl
ketones such as benzophenone (1l) and 4-cyanobenzophenone (1m) selectively coupled with aromatic
aldehydes 2A and 2G, respectively, to afford the corresponding diols 3lA and 3mG in yields of 56 and
85%. Note that the presence of two large aryl groups on the diaryl ketones prevented dimerization of
the ketyl radical intermediates; consequently, these reactions could be carried out by mixing all the
reagents without using a high-dilution protocol.

We also achieved a cross-pinacol coupling between two different ketones; in these reactions, the
tertiary carbinol anions that were generated underwent subsequent addition to other ketones to
construct sterically hindered vicinal quaternary carbons. Methyl 4-acetylbenzoate (1g) reacted with 4-
fluoroacetophenone (2M) or acetophenone (2N) to give 1,2-diols 3gM and 3gN, respectively, in yields
of 67 and 64%. Cyano and chloro groups were well tolerated, and the unsymmetric 1,2-diol 3iO was
obtained in 51% yield. In the presence of an excess of acetone (2L), the tertiary carbinol anion also
reacted with the aliphatic ketone to give 3gL in 54% yield.

Fig. 2. Substrate Scope. Reaction conditions: carbonyl compound 2 (2 equiv), Ir(ppy)2(dtbbpy)PF6 (2
mol %), DMBI (1.5 equiv), CO2 (5 mL), DMA (0.13 M), r. t., dropwise addition of 1 (0.2 mmol) over
20 min followed by stirring for 40 min. aDMBI-CF3 was used instead of DMBI. b1.0 mmol scale.
cdropwise addition of 1 over 50 min followed by stirring for 10 min. dCO2 (10 mL). eCO2 (20 mL), f2
(5 equiv). g2 (10 equiv), hdropwise addition of 1 over 50 min followed by stirring for 1 h. iDMA
solution of all reagents and 1 irradiated with blue light for 1 h. j2 (50 equiv).

The resulting 1,2-diols are useful as building blocks for synthesizing various unsymmetric
compounds (Fig. 3). The unsymmetric 1,2-diketone 7aA was obtained in 92% yield by Swern
oxidation of diol 3aA.26 A reaction involving a cross-pinacol coupling of 1a and 2a followed by
treatment with TsOH afforded ketone 8aA in 45% yield (two steps). The reaction of 3aB with 2,2-
dimethoxypropane in the presence of TsOH gave the cyclic acetal 9aB in 96% yield.27 A unsymmetric
epoxide was also synthesized through the internal nucleophilic substitution of a 1,2-diol: treatment of
3aB with benzyl(triethyl)ammonium chloride (TEBA), MsCl, and NaOH afforded the corresponding
epoxide 10aB in 71% yield, with inversion of the stereochemistry. 28
Fig 3. Synthetic applications of unsymmetric 1,2-diols
A radical-trapping experiment and a deuteration experiment were performed to gain mechanistic
insights into the photocatalytic cross-pinacol coupling. When the coupling reaction of aldehyde 1a
with 2A was carried out in the presence of TEMPO as a radical scavenger, the formation of the cross-

coupled 1,2-diol 3aA was completely inhibited, indicating the involvement of a radical intermediate
(see SI; Section 2-10). The deuteration experiment was performed by using D2O, which barely reacts
with radical species but reacts readily with anionic species. Biphenyl-4-carboxaldehyde (1f) reacted
with D2O in the presence of a CO2 additive to give the a-C-deuterated alcohol 11-D in 76% NMR
yield with 96% benzylic incorporation of deuterium (Fig. 4a).23 To rule out a radical–radical coupling
pathway, we confirmed the inertness of the electron-rich aldehydes and ketones undecanal (2H), 4-
fluoroacetophenone (2M), and acetophenone (2N) under the standard conditions (Fig. 4b and SI, Table
S9). These results support the supposition that the cross-pinacol coupling proceeds through a radical–
polar crossover pathway.

Fig. 4. Mechanistic Studies. a, Deuteration experiments using D2O in the presence or absence of CO2.
b, Inertness of carbonyl compounds serving as electrophiles in the reaction. c, Cyclic voltammogram
of aldehyde 1a with various amounts of CO2 on a glassy carbon (GC) electrode in DMF–Et4NBF4. See
SI for details. d, Theoretical studies of the reduction step [B3LYP/6-311++G(d,p) with the CPCM
solvation model (DMF)].
As we reported previously, it is assumed that CO2 plays a key role in promoting the generation of
the carbinol anions.23 When the deuteration experiment was performed under a N2 atmosphere without
a CO2 additive, the NMR yield of alcohol 11-D decreased to only 12%, whereas that of the dimer 4ff
increased (Fig. 4a). To elucidate the role of CO2, we performed an electrochemical analysis of aldehyde
CO2
0 mL
1 mL
saturated

1a by cyclic voltammetry in DMF in the presence or absence of the CO2 additive (Fig. 4c, and SI, Fig.
S3). Under a N2 atmosphere, aldehyde 1a showed two successive reduction peaks (Fig. 4c, red). The
first reversible reduction peak at –1.51 V (Ep) can be assigned to the first electron transfer to afford the
ketyl radical, whereas the second irreversible peak at –2.11 V (Ep) is assigned as a second electron
transfer to give the anionic species. As a rough estimate of the redox potential E01/2, we used the half-
peak potential Eh (the potential at half the current in Ep).29 The first reduction potential was
approximately –1.45 V (vs SCE), whereas the second reduction potential was approximately –2.03 V
(vs SCE), which is too high for reduction by Ir(ppy)2(dtbbpy)PF6 (–1.51 V vs SCE in MeCN);30
therefore, one-electron reduction of aldehyde 1a proceeded as the main pathway in the absence of CO2,
resulting in the formation of dimer 4aa through conventional radical–radical coupling. Bubbling a
certain amount of CO2 over 30 seconds resulted in a significant change in the cyclic voltammogram
of 1a. As the amount of CO2 additive increased, the first peak current increased and the second peak
current decreased (Fig. 4c green). Finally, the second peak disappeared and the first peak had twice the
intensity of that observed in the absence of CO2 and became irreversible, indicating that the rapid
reaction of the ketyl radical with CO2 generated a new intermediate (Fig. 4c blue). The large current
suggests that a second electron transfer also occurred in the first-peak region to produce an anionic
species. Similar effects of CO2 have been observed in the electrocarboxylation of ketones and imines
with CO2.31–35
We next performed DFT calculations on the reduction step to support the hypothesis that CO2
promotes the generation of carbinol anion species (Fig. 4d).33 The calculated first reduction potential
(Ecal) of aldehyde 1a to the ketyl radical I was –1.21 V (vs SCE), which is similar to the experimental
potential observed in CV (Eexp = –1.45 V vs SCE). The direct successive reduction of the generated
ketyl radical I to the dianion species II shows a markedly high potential (Ecal = –2.14 V vs SCE),
consistent with the experimental result (Eexp = –2.03 V vs SCE). The negative charge on the ketyl
radical I is localized on the oxygen derived from the aldehyde moiety, and the spin density is highest
at the neighboring carbon atom. Attack by the anionic oxygen on CO2 therefore gives the intermediate

III; the activation barrier for this process is 12.1 kcal/mol. A subsequent one-electron reduction of
intermediate III affords the dianion species IV, which serves as a carbinol anion nucleophile. The
calculated reduction potential of intermediate III is –1.35 V (vs SCE), which is more positive than the
direct reduction potential of the ketyl radical I; this is consistent with the value observed in CV (Eexp
= –1.39 V (vs SCE)). The resulting negative charge on IV is delocalized across the aromatic ring
containing the electron-withdrawing ester group. CV studies and DFT calculations support the view
that carbon dioxide promotes a successive one-electron reduction to generate carbinol anion
nucleophiles, suppressing the formation of the dimer by reacting rapidly with the ketyl radicals.
A plausible mechanism is shown in Fig. 5. The excited Ir(III) photocatalyst is reduced by the DMBI
to give an Ir(II) species and the DMBI radical cation C. Previous Stern−Volmer fluorescence
quenching experiments have shown that the excited iridium photocatalyst is quenched by DMBI.23
The Ir(II) species reduces the electron-deficient aromatic carbonyl compound 1 to give the ketyl radical
A with regeneration of the Ir(III) photocatalyst. According to the CV measurements and DFT
calculations, the ketyl radical A is smoothly transformed into the carbinol anion F through reaction of
the oxygen anion in A with CO2, followed by one-electron reduction by the DMBI radical D (–1.68 V
vs SCE for DMBI in MeCN)24,36 produced by deprotonation of the DMBI radical cation C. The
carbinol anion F then reacts nucleophilically with the other carbonyl compound 2; this is followed by
protonation and decarboxylation to afford the final product 3.

Fig. 5. Proposed mechanism
Conclusions
In conclusion, we have developed the first photocatalytic cross-pinacol coupling between two
different carbonyl compounds to give unsymmetric 1,2-diols in up to 91% yield. The reaction is
applicable to couplings between aldehydes and aldehydes, aldehydes and ketones, or ketones and
ketones to give the corresponding 1,2-diols, useful as building blocks for conversion into unsymmetric
ketones, epoxides, or other products. Electron-deficient carbonyl compounds are converted
nucleophilic carbinol anions through successive one-electron reduction, and the resulting anions attack
the more-electron-rich carbonyl compounds, which serve as electrophiles. CV and DFT calculations
revealed that the CO2 additive plays a key role in the second reduction and suppresses undesired
dimerization.

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Acknowledgements
This work was supported by JSPS KAKENHI (Grant JP21K14635 and JP21K18968).
Author contributions
Y. U . a n d S . O . c o n c e i v e d t h e c o n c e p t a n d p r e p a r e d t h e m a n u s c r i p t w i t h f e e d b a c k f r o m all authors. T.T. and K.T.
performed the laboratory experiments. S.O. performed the computational study
Competing interests
The authors declare no competing financial interest.