Use of Isopropyl Alcohol as a Reductant for Catalytic Dehydoxylative Dimerization of Benzylic Alcohols Utilizing Ti?O Bond Photohomolysis

Article abstract of DOI:10.1002/ejoc.202100286

Photohomolysis of Ti?O bonds is utilized in photocatalytic generation of titanium(III) species for dehydroxylative dimerization of benzylic alcohols under UV-light irradiation by using isopropyl alcohol (IPA) as a stoichiometric reductant. In this reaction, IPA works not as a single-electron donor as in the photo-redox catalyzed reactions but as an H-atom-donor. The reaction also proceeds under visible-light irradiation in the presence of thioglycolic acid as a ligand.

Full text of DOI:10.1002/ejoc.202100286

Communications
doi.org/10.1002/ejoc.202100286
Use of Isopropyl Alcohol as a Reductant for Catalytic
Dehydoxylative Dimerization of Benzylic Alcohols Utilizing
TiÀ O Bond Photohomolysis
Keiichi Sumiyama,[a] Naoyuki Toriumi,[a] and Nobuharu Iwasawa*[a]
expected photohomolysis of Ti(IV)À OR bonds would be a
promising strategy to develop a new type of Ti(III)-catalyzed
reductive functionalizations using alcohols as a stoichiometric
reductant (H-atom donor). To this end, we have focused on
dehydroxylative reactions of benzylic alcohols. While there have
been several reports on low-valent Ti-mediated or -catalyzed
dehydroxylative transformations, they required a stoichiometric
amount of metallic reductants (Scheme 1a).[9]
Photohomolysis of TiÀ O bonds is utilized in photocatalytic
generation of titanium(III) species for dehydroxylative dimeriza-
tion of benzylic alcohols under UV-light irradiation by using
isopropyl alcohol (IPA) as a stoichiometric reductant. In this
reaction, IPA works not as a single-electron donor as in the
photo-redox catalyzed reactions but as an H-atom-donor. The
reaction also proceeds under visible-light irradiation in the
presence of thioglycolic acid as a ligand.
Herein, we report a catalytic dehydroxylation dimerization
of benzylic alcohols by utilizing the photohomolysis of the
TiÀ OR bond for the generation of Ti(III) species from Ti(OiPr)4
using isopropyl alcohol (IPA) as an H-atom donor. The expected
catalytic cycle is shown in Scheme 1b. Photohomolysis of the
TiÀ OiPr bond of Ti(OiPr)4 under UV light irradiation would
generate an active Ti(III) species along with isopropoxy radical
Generation of carbon-centered radicals using trivalent titanium
species (Ti(III)) is an attractive approach to transform various
oxygen-containing substrates such as epoxides, aldehydes,
ketones, and alcohols owing to the high oxophilicity of Ti
species.[1] The use of a catalytic amount of Ti species has also
been studied by the combined use of a strong metallic
reductant such as Zn and Mn[2] or a unique dihydropyrazine
type organic reductant.[3] The recent rapid development of
photoredox reactions has enabled Ti(III) catalysis in combination
with organo- or Ir-photoredox catalysts with amines or
Hantzsch ester as an electron donor.[4] More recently, Gansäuer
et al. found that Cp2TiCl2 itself could work as a photoredox
*
OiPr. The generated Ti(III) species would reduce a benzylic
alcohol to give a benzylic radical, which would undergo
dimerization and Ti(IV) hydroxide. The generated Ti(IV)
hydroxide would undergo a ligand exchange with HOiPr to
*
regenerate Ti(OiPr)4. The isopropoxy radical OiPr would be
reduced by HOiPr via H-atom abstraction to afford HOiPr and
acetone. Here, HOiPr would act not as an electron donor but as
an H-atom donor, which is the key to accomplish Ti(III) catalysis
via photohomolysis using alcohol as a reductant and is different
i
catalyst, and in the presence of Pr2NEt, Cp2Ti(III)Cl is formed to
promote the reductive generation of radicals from epoxides.[4f]
However, utilization of easily available and less toxic, non-
metallic reductants like alcohols still remains to be realized due
to their low reducing ability.
It has long been known that photohomolysis of MnÀ X bonds
(X=halide, alkyl, hydroxide) affords reduced metallic species
*
M(nÀ 1) together with radical X , which typically undergo
*
dimerization of X , H-atom abstraction, photoisomerization of
metal complexes, etc.[5] More recently, photohomolysis of MÀ X
bonds has attracted much attention as a new method of
utilizing photo energy for synthetic catalytic reactions, and
several metallic compounds including nickel, copper, cerium,
iron, cobalt, etc. have been utilized for this purpose.[6]
Concerning photohomolysis of Ti complexes,[7,8] cleavage of a
TiÀ OH bond in a well-designed ligand[7b–d] or a CpÀ Ti bond for
initiation of radical polymerization[8] has been reported to date.
However, photohomolytic generation of Ti(III) species has not
yet been applied to catalytic CÀ C bond formation reactions. We
[a] K. Sumiyama, Dr. N. Toriumi, Prof. Dr. N. Iwasawa
Department of Chemistry, Tokyo Institute of Technology
O-okayama, Meguro-ku, Tokyo 152-8551, Japan
E-mail: niwasawa@chem.titech.ac.jp
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejoc.202100286
Scheme 1. a) Deoxygenative reaction of benzylic alcohols. b) Our design.
Eur. J. Org. Chem. 2021, 2474–2478
2474
© 2021 Wiley-VCH GmbH

Communications
doi.org/10.1002/ejoc.202100286
from the photoredox-mediated reduction using an amine as an
electron donor.
Based on the above consideration, we first examined the
dimerization reaction of benzhydrol 1 (Table 1). When benzhy-
donor. To suppress the formation of ketone 3, the reaction was
carried out in the presence of 5 equivalents of IPA as an H-atom
donor. Although 2 was not obtained under similar conditions
(entry 2), further examination of various additives revealed that
the addition of 20 mol% amount of acetic acid improved the
yield of 2 to 55% along with 3% of 3 (entry 3). When the
reaction time was extended to 8 h, 2 was obtained in 74% yield
(entry 4). Several control experiments confirmed that this
reaction necessitated both Ti(OiPr)4 and UV light (entries 5–7).[10]
The generality of this reaction was investigated using
10 mol% amount of Ti(OiPr)4 and 20 mol% amount of acetic
drol 1 was irradiated with 365 nm LED light in the presence of
i
°
10 mol% amount of Ti(OPr)4 in chlorobenzene at 90 C for 4 h,
the desired dimerized product 2 was obtained in 12% yield
along with ketone 3 in 6% yield (entry 1). The generation of
ketone 3 suggested that benzhydrol 1 acted as an H-atom
°
acid in chlorobenzene/IPA (25:1) at 90 C under irradiation of
Table 1. Screening of reaction conditions with UV-light irradiation.
365 nm LED light (Table 2). This reaction showed reasonable
generality, and the desired dimerized products were isolated in
up to 71% yield when benzhydrol derivatives containing alkyl,
halo, ether, and ester groups were employed as substrates.
Substrate 4e containing bromo groups partially decomposed
under the present reaction conditions, but afforded the
dimerized product 5e in moderate yield. Primary alcohols 4h
and 4i showed low reactivity and the desired dimerized
products 5h and 5i were isolated in about 20% yield when the
reactions were carried out using 10 mol% amount of Ti(OiPr)4
Entry
Additive
&
Yield[a]
2[b]
3
conditions
1
2
3
none
12%
0%
55%
6%
0%
3%
5 equiv. IPA
5 equiv. IPA
&
°
and 5 equivalents of acetic acid in chlorobenzene at 120 C
under the irradiation of 365 nm LED light.
We expected that this reaction could be promoted by
visible-light irradiation with the addition of a suitable ligand. To
this end, several ligands were investigated for reductive
20 mol% AcOH
5 equiv. IPA
&
4
74%
0%
20 mol% AcOH for 8 h
w/o light
LED(425)
5[c]
6
0%
0%
0%
0%
5 equiv. IPA &
20 mol% AcOH
w/o [Ti] &
°
dimerization of 1 at 120 C under irradiation of 425 nm blue
LED light (Table 3).[11] When these ligands were mixed with
Ti(OiPr)4, the color of the solution changed from colorless to
yellow or orange, indicating visible-light active titanium com-
plexes were generated in situ.[12] However, the reaction did not
proceed well using phenol-type ligands such as 4-meth-
oxyphenol and BINOL (entries 1,2). Then, several thiocarboxylic
acids, which would form weak TiÀ S bonds but could strongly
coordinate to the titanium center with chelating carboxylate
moieties, were examined (entries 3–5). Gratifyingly, it was found
7[c]
0%
0%
w 20 mol% AcOH
1
°
[a] H NMR yields based on 1. [b] Based on monomer. [c] 120 C, 6 h.
Table 2. Generality of the reaction.
Product[a]
Table 3. Screening of reaction conditions with visible-light irradiation.
Entry
1
Ligand
Conversion[a]
13%
Yield[a,b]
0%
5a 71%
5b 61%
5c 58%[b]
2
3
81%
26%
4%
7%
5d 63%
5e 34%
5f 71%[b]
5i 20%[d]
5g 38%[c]
5h 24%[d]
4
2%
3%
[a] Isolated yields based on monomer. [b] Chlorobenzene/IPA (12.5:1) [c]
10 equivalents of MeOH and 20 mol% amount of PhCOOH were used
instead of IPA and AcOH, respectively. [d] 5 equivalents of AcOH were used
5
48%
100%
43%
81%
6[c]
1
°
without IPA at 120 C for 6 h.
[a] Determined by H NMR. [b] Based on monomer. [c] 12 h.
Eur. J. Org. Chem. 2021, 2474–2478
www.eurjoc.org
2475
© 2021 Wiley-VCH GmbH

Communications
doi.org/10.1002/ejoc.202100286
that the desired dimerized product 2 was obtained in moderate
reactivity of acetate 6 was examined to confirm whether the
reaction proceeded from acetate 6 generated in situ in the
presence of acetic acid. When the reaction was carried out
using acetate 6 as a substrate, the desired dimer product was
not obtained at all, and 6 was recovered completely (Figure 1a).
This result suggests that this reaction proceeded from alcohol 1
directly. Next, UV-spectra of Ti(OiPr)4 with acetic acid were
measured to confirm the effect of acetic acid as an additive
(Figure 1b). Although Ti(OiPr)4 or Ti(OiPr)4 with 2 equivalents of
benzhydrol showed a weak absorption at 365 nm, Ti(OiPr)4 with
2 equivalents of acetic acid showed much stronger absorption
at 365 nm. Although the exact active species was not obvious,
the generation of titanium acetates is thought to be important
for promoting the reaction efficiently due to the stronger
absorption of 365 nm light. We have also measured UV-vis
spectra of the mixture of Ti(OiPr)4 and AcOH in chlorobenzene/
IPA after irradiation with 365 nm LEDs at room temperature
(Figure 1c). A broad absorption band appeared at 659 nm after
photoirradiation, which corresponds to a color change from
colorless to blue. This low-energy absorption band is similar to
that of Ti(acac)3,[13] indicating that the Ti(III) species was
generated under the reaction conditions. In addition, to confirm
the role of IPA as an H-atom donor NMR study was conducted
in C6D6, and nearly the same amount of the dimerized product
yield when thioglycolic acid was employed as an additive
(entry 5). Further optimization of reaction conditions using
thioglycolic acid as a ligand resulted in the formation of the
desired dimerized product 2 in up to 81% yield when the
reaction mixture was irradiated with 425 nm LED light in the
presence of 20 mol% amount of Ti(OiPr)4 with 20 mol% amount
°
of thioglycolic acid in chlorobenzene/IPA (12.5:1) at 120 C for
12 h (entry 6). Under these conditions, the yields of the
products were improved in most cases (5b: 71%; 5d: 78%; 5e:
51%; 5g: 80%) (Table S2). Thus, titanium(III) species was
successfully generated catalytically by visible-light irradiation.
To obtain information on the mechanism of this reaction,
several experiments were carried out (Figure 1). First, the
1
and acetone was detected in H NMR (Figure 1d).[14] This result
suggests that IPA acted as a sacrificial reducing reagent as
shown in Scheme 2.
From these results, the reaction mechanism is proposed as
shown in Scheme 2. The ligand exchange of Ti(OiPr)4 with acetic
acid occurs to generate (AcO)nTi(OiPr)4-n in situ. This ligand
exchange is thought to be in equilibrium under the reaction
conditions. The titanium(III) species along with the alkoxo
radical is generated from (AcO)nTi(OiPr)4-n by irradiating 365 nm
light.[15] The generated titanium(III) species reduces benzhydrols
to give diarylmethyl radicals with (AcO)n(iPrO)3-nTiOH and the
radical dimerizes to give products. The alkoxo radical abstracts
hydrogen atoms from IPA to give acetone. (AcO)n(iPrO)3-nTiOH
Figure 1. Mechanistic studies. a) Reaction of acetate 6. b) UV-spectra of
Ti(OiPr)4 (35 μmol) (blue), Ti(OiPr)4 (35 μmol) with benzhydrol (70 μmol)
(green), and Ti(OiPr)4 (35 μmol) with acetic acid (70 μmol) (red) in
chlorobenzene (3.5 mL). c) UV-spectra of Ti(OiPr)4 (3.5 μmol) with acetic acid
(7.0 μmol) in IPA (28 μl) and chlorobenzene (0.35 mL) before (green) and
after (blue) irradiation of LEDs (365 nm) at room temperature for 1.5 h. d)
NMR experiment for confirmation of acetone formation.
Scheme 2. Proposed mechanism.
Eur. J. Org. Chem. 2021, 2474–2478
www.eurjoc.org
2476
© 2021 Wiley-VCH GmbH

Communications
doi.org/10.1002/ejoc.202100286
Keywords: CÀ O cleavage · Photocatalysis · Radical reactions ·
Synthetic methods · Titanium
[1] For recent reviews of reactions utilizing Ti(III) complexes, see: a) A.
Gansäuer, H. Bluhm, Chem. Rev. 2000, 100, 2771–2778; b) J. Streuff,
Chem. Rec. 2014, 14, 1100–1113; c) A. Rosales, I. Rodríguez-García, J.
Muñoz-Bascón, E. Roldan-Molina, N. M. Padial, L. P. Morales, M. García-
Ocaña, J. E. Oltra, Eur. J. Org. Chem. 2015, 4567–4591; d) M. Castro R-
odríguez, I. Rodríguez-García, R. N. Rodríguez Maecker, L. Pozo Morales,
J. E. Oltra, Org. Process Res. Dev. 2017, 21, 911–923; e) J. M. Botubol-Ares,
M. J. Durán-Peña, J. R. Hanson, R. Hernández-Galán, I. G. Collado,
Synthesis 2018, 50, 2163–2180; f) T. McCallum, X. Wu, S. Lin, J. Org.
Chem. 2019, 84, 14369–14380; g) A. Rosales Martínez, L. Pozo Morales, E.
Díaz Ojeda, M. Castro Rodríguez, I. Rodríguez-García, J. Org. Chem. 2021,
86, 1311–1329.
[2] For selected examples of Ti(III)-catalyzed reactions with Zn and Mn, see:
a) A. Gansäuer, H. Bluhm, M. Pierobon, J. Am. Chem. Soc. 1998, 120,
12849–12859; b) A. Gansäuer, B. Rinker, M. Pierobon, S. Grimme, M.
Gerenkamp, C. Mück-Lichtenfeld, Angew. Chem. Int. Ed. 2003, 42, 3687–
3690; Angew. Chem. 2003, 115, 3815–3818; c) A. Gansäuer, S. Hilde-
brandt, A. Michelmann, T. Dahmen, D. von Laufenberg, C. Kube, G. D.
Fianu, R. A. Flowers II, Angew. Chem. Int. Ed. 2015, 54, 7003–7006;
Angew. Chem. 2015, 127, 7109–7112; d) W. Hao, X. Wu, J. Z. Sun, J. C.
Siu, S. N. MacMillan, S. Lin, J. Am. Chem. Soc. 2017, 139, 12141–12114;
e) K.-Y. Ye, T. McCallum, S. Lin, J. Am. Chem. Soc. 2019, 141, 9548–9554;
f) P. Funk, R. B. Richrath, F. Bohle, S. Grimme, A. Gansäuer, Angew. Chem.
Int. Ed. 2021, 60, 5482–5488; Angew. Chem. 2021, 133, 5542–5548.
[3] T. Saito, H. Nishiyama, H. Tanahashi, K. Kawakita, H. Tsurugi, K. Mashima,
J. Am. Chem. Soc. 2014, 136, 5161–5170.
[4] For recent reviews and selected examples of Ti-catalyzed photoredox
reactions, see: a) Y. Chen, S. Lin, F. Li, X. Zhang, L. Lin, L. Shi,
ChemPhotoChem 2020, 4, 659–663; b) A. Fermi, A. Gualandi, G.
Bergamini, P. G. Cozzi, Eur. J. Org. Chem. 2020, 6955–6965; c) Z. Zhang,
R. B. Richrath, A. Gansäuer, ACS Catal. 2019, 9, 3208–3212; d) M.
Parasram, B. J. Shields, O. Ahmad, T. Knauber, A. G. Doyle, ACS Catal.
2020, 10, 5821–5827; e) A. Gualandi, F. Calogero, M. Mazzarini, S. Guazzi,
A. Fermi, G. Bergamini, P. G. Cozzi, ACS Catal. 2020, 10, 3857–3863; f) Z.
Zhang, T. Hilche, D. Slak, N. R. Rietdijk, U. N. Oloyede, R. A. Flowers II, A.
Gansäuer, Angew. Chem. Int. Ed. 2020, 59, 9355–9359; Angew. Chem.
2020, 132, 9441–9445; g) S. Lin, Y. Chen, F. Li, C. Shi, L. Shi, Chem. Sci.
2020, 11, 839–844; h) F. Li, S. Lin, Y. Chen, C. Shi, H. Yan, C. Li, C. Wu, L.
Lin, C. Duan, L. Shi, Angew. Chem. Int. Ed. 2021, 60, 1561–1566; Angew.
Chem. 2021, 133, 1585–1590.
[5] a) J. K. Kochi, J. Am. Chem. Soc. 1962, 84, 2121–2127; b) R. A. Sheldon,
J. K. Kochi, J. Am. Chem. Soc. 1968, 90, 6688–6968; c) A. Barbieri, A.
Droghetti, S. Sostero, O. Traverso, J. Photochem. Photobiol. A 1999, 129,
137–141; d) C. Bartocci, F. Scandola, A. Ferri, V. Carassiti, J. Am. Chem.
Soc. 1980, 102, 7067–7072; e) D. N. Hendrickson, M. G. Kinnaird, K. S.
Suslick, J. Am. Chem. Soc. 1987, 109, 1243–1244; f) L. A. Wickramasinghe,
P. R. Sharp, J. Am. Chem. Soc. 2014, 136, 13979–13982; g) S. J. Hwang,
D. C. Powers, A. G. Maher, B. L. Anderson, R. G. Hadt, S.-L. Zheng, Y.-S.
Chen, D. G. Nocera, J. Am. Chem. Soc. 2015, 137, 6472–6475; h) C. L.
Pitman, A. J. M. Miller, Organometallics 2017, 36, 1906–1914.
Scheme 3. Use of toluene as an H-atom donor instead of IPA. The yields
were determined by 1H NMR based on monomer.
regenerates (AcO)nTi(OiPr)4-n by a ligand exchange reaction with
IPA.
Finally, to utilize the alkoxo radicals generated from photo-
homolysis of TiÀ (OiPr) bond, the reaction was conducted in the
presence of a suitable H-atom donor instead of IPA to promote
a hetero-coupling reaction. When the reaction of benzhydrol
derivative 4g with Ti(OiPr)4 was conducted in chlorobenzene/
toluene (4:1) without IPA, a dehydroxylative coupling product
7 with toluene was obtained in 20% yield along with 31% yield
of homo-coupling product 5g (Scheme 3). This result suggests
that toluene acted as an H-atom donor of alkoxo radical to
generate benzyl radical under the reaction conditions. Although
primitive, this reaction would afford a new possibility for the
cross-coupling between benzylic alcohols and hydrocarbons
containing reactive C(sp3)À H bonds.
In summary, we developed a catalytic dehydoxylative
dimerization of benzylic alcohols using a catalytic amount of
Ti(OiPr)4 with 365 nm LED light irradiation using IPA as an H-
atom donor based on the photohomolysis of the TiÀ O bond.
This is a rare example that titanium-catalyzed reduction is
achieved without using metallic reductants. By adding thiogly-
colic acid as a ligand, this reaction could be carried out by
visible light irradiation. The application of this reaction to other
radical reactions is now in progress.
[6] For a recent review and selected examples of photohomolysis of metal-
substrate complexes, see: a) Y. Abderrazak, A. Bhattacharyya, O. Reiser,
Angew. Chem. Int. Ed. 2021, doi: 10.1002/anie.202100270; b) C. F. Harris,
C. S. Kuehner, J. Bacsa, J. D. Soper, Angew. Chem. Int. Ed. 2018, 57, 1311–
1315; Angew. Chem. 2018, 130, 1325–1329; c) A. Hu, J.-J. Guo, H. Pan, Z.
Zuo, Science 2018, 361, 668–672; d) S. Xia, K. Hu, C. Lei, J. Jin, Org. Lett.
2020, 22, 1385–1389; e) S. K. Kariofillis, B. J. Shields, M. A. Tekle-Smith,
M. J. Zacuto, A. G. Doyle, J. Am. Chem. Soc. 2020, 142, 7683–7689; f) Y.
Hirao, Y. Katayama, H. Mitsunuma, M. Kanai, Org. Lett. 2020, 22, 8584–
8588; g) P. Lian, W. Long, J. Li, Y. Zheng, X. Wan, Angew. Chem. Int. Ed.
2020, 59, 23603–23608; Angew. Chem. 2020, 132, 23809–23814.
Acknowledgements
This work was supported by a JSPS KAKENHI Grant Number
19K22182. The numerical calculations were carried out on the
TSUBAME3.0 supercomputer at Tokyo Institute of Technology
supported by the MEXT Project of the Tokyo Tech Academy for
Convergence of Materials and Informatics (TAC-MI).
[7] a) M. Kessler, S. Schüler, D. Hollmann, M. Klahn, T. Beweries, A.
Spannenberg, A. Brückner, U. Rosenthal, Angew. Chem. Int. Ed. 2012, 51,
6272–6275; Angew. Chem. 2012, 124, 6377–6380; b) M. Kessler, S.
Hansen, C. Godemann, A. Spannenberg, T. Beweries, Chem. Eur. J. 2013,
19, 6350–6357; c) C. Godemann, L. Dura, D. Hollmann, K. Grabow, U.
Bentrup, H. Jiao, A. Schulz, A. Brückner, T. Beweries, Chem. Commun.
2015, 51, 3065–3068; d) C. Godemann, D. Hollmann, M. Kessler, H. Jiao,
A. Spannenberg, A. Brückner, T. Beweries, J. Am. Chem. Soc. 2015, 137,
Conflict of Interest
The authors declare no conflict of interest.
Eur. J. Org. Chem. 2021, 2474–2478
www.eurjoc.org
2477
© 2021 Wiley-VCH GmbH

Communications
doi.org/10.1002/ejoc.202100286
16187–16195; e) L. Jin, J. Wang, W. Ye, W. Fang, X. Chen, J. Phys. Chem.
C 2018, 122, 18412–18421.
[8] M.-A. Tehfe, J. Lalevée, F. Morlet-Savary, B. Graff, J.-P. Fouassier,
Macromolecules 2012, 45, 356–361.
11408–11409; b) G. R. Kasner, C. Boucher-Jacobs, J. M. McClaine II, K. M.
Nicholas, Chem. Commun. 2016, 52, 7257–7260; c) C. Boucher-Jacobs, P.
Liu, K. M. Nicholas, Organometallics 2018, 37, 2468–2480; d) E. Steffen-
smeier, K. M. Nicholas, Chem. Commun. 2018, 54, 790–793; e) T.
Sakuramoto, Y. Donaka, M. Tobis, T. Moriuchi, New J. Chem. 2019, 43,
17571–17576; f) C. Bandari, K. M. Nicholas, J. Org. Chem. 2020, 85, 3320–
3327; g) S. E. Griffin, L. L. Schafer, Inorg. Chem. 2020, 59, 5256–5260.
[9] For examples of reactions using Ti complexes with stoichiometric
metallic reductants, see: a) E. E. van Tamelen, M. A. Schwartz, J. Am.
Chem. Soc. 1965, 87, 3277–3278; b) E. E. van Tamelen, B. Akermark, K. B.
Sharpless, J. Am. Chem. Soc. 1969, 91, 1552–1554; c) J. E. McMurry, M.
Silvestri, J. Org. Chem. 1975, 40, 2687–2688; d) F. Sato, Y. Tomuro, H.
Ishikawa, T. Oikawa, M. Sato, Chem. Lett. 1980, 9, 103–106; e) H. R.
Diéguez, A. López, V. Domingo, J. F. Arteaga, J. A. Dobado, M. M.
Herrador, J. F. Quílez del Moral, A. F. Barrero, J. Am. Chem. Soc. 2010,
132, 254–259; f) C. Prieto, J. A. González Delgado, J. F. Arteaga, M. Jaraíz,
J. L. López-Pérez, A. F. Barrero, Org. Biomol. Chem. 2015, 13, 3462–3469;
g) T. Suga, S. Shimazu, Y. Ukaji, Org. Lett. 2018, 20, 5389–5392; h) T.
Suga, Y. Ukaji, Org. Lett. 2018, 20, 7846–7850; i) H. Xie, J. Guo, Y.-Q.
Wang, K. Wang, P. Guo, P.-F. Su, X. Wang, X.-Z. Shu, J. Am. Chem. Soc.
2020, 142, 16787–16794.
°
[11] The conversions were much lower at lower temperature (100 C). See
Table S1 for the details.
[12] J. Jones, J. A. Douek, Polyhedron 1989, 8, 2893–2895.
[13] D. W. Barnum, J. Inorg. Nucl. Chem. 1961, 21, 221–237.
[14] For details of formation of acetone from IPA via radical mechanism, see
ref. 5a and Scheme S2.
[15] Computational studies suggest the photohomolysis occurs at the
TiÀ OiPr bond, not at the TiÀ OAc bond. See Figure S2 for the details.
[10] Similar reactions were reported by using oxo-vanadium or oxo-rhenium
complexes, but these reactions often required harsh reaction conditions
Manuscript received: March 8, 2021
Revised manuscript received: April 15, 2021
Accepted manuscript online: April 19, 2021
°
such as high temperature (150 C) and longer reaction time (over 24 h.
See: a) E. Arceo, J. A. Ellman, R. G. Bergman, J. Am. Chem. Soc. 2010, 132,
Eur. J. Org. Chem. 2021, 2474–2478
www.eurjoc.org
2478
© 2021 Wiley-VCH GmbH