Facile pinacol coupling of aliphatic ketones by Brook rearrangement in the presence of samarium species

Article abstract of DOI:10.1016/j.tetlet.2021.153069

Herein we report a practical pinacol coupling reaction, in which ketones (aldehydes) react smoothly with Sm and TMSBr to afford the diol products with Sm(II) or (III) siliyl species generated in situ. This reported method affords poor yields for aromatic ketone substrates and good yields for aliphatic ketones. Therefore, it distinguishes from most reductive coupling approaches that are more effective for aromatic carbonyl compounds and provides a facile and robust approach for the pinacol coupling of aliphatic ketones. Mechanistic studies also indicated the pinacolization probably proceeded via an anionic instead of radical coupling pathway involving the Brook rearrangement in the presence of samarium (II or III) silyl species.

Full text of DOI:10.1016/j.tetlet.2021.153069

Tetrahedron Letters 72 (2021) 153069
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Tetrahedron Letters
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Facile pinacol coupling of aliphatic ketones by Brook rearrangement in
the presence of samarium species
b,
Xincan Wang a,b, Guanqun Xie b, Yanfei Zhao b, Ke Zheng b, Yanxiong Fang a, , Xiaoxia Wang
⇑
⇑
a School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou, Guangdong 510006, China
b School of Materials Science and Engineering, Dongguan University of Technology, Dongguan 523808, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Herein we report a practical pinacol coupling reaction, in which ketones (aldehydes) react smoothly with
Sm and TMSBr to afford the diol products with Sm(II) or (III) siliyl species generated in situ. This reported
method affords poor yields for aromatic ketone substrates and good yields for aliphatic ketones.
Therefore, it distinguishes from most reductive coupling approaches that are more effective for aromatic
carbonyl compounds and provides a facile and robust approach for the pinacol coupling of aliphatic
ketones. Mechanistic studies also indicated the pinacolization probably proceeded via an anionic instead
of radical coupling pathway involving the Brook rearrangement in the presence of samarium (II or III)
silyl species.
Received 25 January 2021
Revised 19 March 2021
Accepted 6 April 2021
Available online 9 April 2021
Keywords:
Pinacol coupling
Sm/TMSBr
Brook arrangement
Anionic coupling
Ó 2021 Elsevier Ltd. All rights reserved.
Introduction
Sm(II) reagents has been well applied in promoting a variety of
coupling reactions [6], among which SmBr2 has showed more pow-
Pinacol coupling of carbonyl compounds is one of the most
important CAC forming reactions that has been widely used in
the construction of versatile important compounds, especially as
a key step for the synthesis of natural products [1]. It could also
be used as a probe reaction for the assessment of reductive CAC
bond formation potential of emerging reagents. Most routine pina-
colization methods involve the generation of keyl radicals and fol-
lowed by the radical homo-dimerization [2,3]. Among the various
methods, the homo-coupling of aromatic carbonyl compounds,
including photoredox- and electrochemcial protocols reported
recently, have been well developed [4]. However, despite the avail-
able reports of intramolecular couplings of aliphatic aldehydes and
ketones [5], efficient reagent systems that could achieve the read-
ily intermolecular pinacolization of aliphatic carbonyls are rela-
tively fewer. The high reductive potential of aliphatic carbonyls
makes the generation of aliphatic ketyl difficult. On the other hand,
the ketyl intermediates of the aliphatic carbonyls are highly unsta-
ble, thereby possessing a strong tendency to undergo H abstraction
to form simple alcohols.
erful reactivity than SmI2 [6f,g]. However, the generation of SmBr2
is not as convenient as SmI2, which limits its popularity to certain
extent. The available preparative methods include: (1) reduction of
SmBr3 with Li metal; [7a] (2) the reaction between Sm and 1,2-
dibromoethane [7b] or 1,1,2,2-tetrabromoehane; [7c] (3) addition
of four-fold excess of LiBr in the SmI2 solution [7d,e] The Ishii
group has attempted the preparation of Sm(II) regent from Sm/
TMSBr in CH3CN [7f], where 5 h under reflux conditions were
required and the exact Sm(II) species has not been addressed. It
is worth mentioning that large excess Sm and related bromo-
containing reagents were usually employed to ensure good
coupling efficiency.
In continuation of our investigation on developing bromo con-
taining Sm(II) species such as allylSmBr [8], we incidentally found
the simple mixing of Sm, TMSBr and the carbonyl substrate (molar
ratio: 1:1:1) in THF under N2 atmosphere afforded a facile method
for the effective coupling of aliphatic ketones, and probe of the
mechanism indicates the possible involvement of samarium (II or
III) silyl species.
Results and discussion
2-Heptanone (1a, 1.0 mmol) was used as a model substrate,
which was added to a mixture of Sm (1.0 equiv.) and TMSBr (1.0
equiv.) in THF (3 mL) at r.t. under N2 atmosphere. The reaction pro-
⇑
Corresponding authors.
E-mail addresses: fangyx@gdut.edu.cn (Y. Fang), wangxx@dgut.edu.cn
(X. Wang).
https://doi.org/10.1016/j.tetlet.2021.153069
0040-4039/Ó 2021 Elsevier Ltd. All rights reserved.

X. Wang, G. Xie, Y. Zhao et al.
Tetrahedron Letters 72 (2021) 153069
Table 1
Examination of Sm/additive on the pinacol coupling of 2-heptanone 2a.a
Entry
Sm (equiv.)
Additive (equiv.)
Reaction time
Yield %b
1
2
3
4
5
6
7
8
9
10
1.0
1.0
1.0
1.0
1.0
1.0
0.33
1.0
1.0
1.0
TMSBr (1.0)
TMSCl (1.0)
TMSI (1.0)
BBr3 (1.0)
PBr3 (1.0)
Et3N∙HBr (1.0)
TMSBr (1.0)
TMSBr (0.1)
TMSBr (0.5)
TMSBr (2.0)
0.5 h
10 h
2.5 h
Over night
Over night
Over night
6 h
0.5 h
0.5 h
78%
0
71%
20%
0%
0
63%
16%c
60%
37%
0.5 h
a
Reaction conditions: Sm 1.0 mmol (0.15 g), 2-heptanone (1.0 mmol, 0.114 g) and the additive in dry THF 3 mL under N2 at the room temperature. The reaction mixture
was quenched with Bu4NF (1.0 M in THF, 1.2 mL) and allowed to be stirred for 1 h followed by usual workup (see SI).
b
Isolated yields.
With 71% of the starting material recovered.
c
cess was monitored by GC–MS. It is satisfying that 78% of isolated
yield of the desired aliphatic diol 2a was obtained in 30 min. under
the mild conditions (Table 1, entry 1). A blue slurry was gradually
formed in the reaction indicating the formation of Sm(II) species.
For comparison, the reaction promoted by Sm/TMSCl and Sm/TMSI
were examined. No color change could be observed using Sm/
TMSCl system and GC–MS analysis showed no formation of the
desired diol even in 10 h except a small amount of the 2-heptanol
(~5% yield). The result is consistent with the report by Yu and
Zhang [9], where aliphatic ketones are sluggish substrates in Sm/
TMSCl. Slightly lower yield of the diol was obtained using Sm/TMSI
(Table 1, entries 2 and 3), and in this case, the reaction mixture also
took on blue color. Besides, BBr3, PBr3 and Et3N∙HBr were also
examined as the bromo source. The reaction with Sm/BBr3 was
sluggish as most starting ketone remained unreacted and only
20% yield of the desired pinacol was detected even by prolonging
the time overnight (Talbe 1, entries 4). No desired reaction could
be observed at all using Sm/PBr3 or Sm/ Et3N∙HBr (Talbe 1, entries
5 and 6).
From the viewpoint of atom economy, 1/3 mmol of Sm, with its
complete transformation into Sm(III) oxidation state, would be
enough to achieve the complete coupling of 1 mmol of the ketone.
Thus the substoichiometric amount of Sm was attempted and it
was found relatively good yield was afforded (Table 1, entry 7).
Screening on the loading of TMSBr showed 0.5 equiv. of TMSBr
could afford reasonable yields, while 0.1 equiv. or 2.0 equiv. of
TMSBr gave unsatisfactory yields (Table 1, entries 8–10). The
decrease in the yield of diol with excess TMSBr could be rational-
ized by the competitive formation of hexamethyl disilante, as has
been monitored by GC–MS.
trast, aromatic ones are less efficient. For acetophenone 1k 65% of
the starting material remained in 30 min. together with the diol in
22% yield (Table 2, entries 1–10 vs entry 11). Sterically hindered
ketone 1l is not a suitable substrate and only trace amount of the
diol was detected (Table 2, entry 12). As far as we know, the avail-
able reductive coupling methods always realize the pinacolization
of aromatic ketones and aldehydes better than that of their alipha-
tic counterparts [10]. Therefore, the above results can not be satis-
factorily explained by initiation via the reduction of the carbonyl to
form ketyl since the reduction of aliphatic carbonyls is more diffi-
cult than aromatic ones. The same phenomenon was observed
between aliphatic and aromatic aldehydes (Table 2, entries 13–
19 vs entries 20–21). Unfortunatley, aldehydes usually afforded
lower yields under the standard conditions, where simple reduced
products was the main side reaction together with several minor
by-products mixture. For aromatic aldehydes, 1u with the elec-
tro-withdrawing F group afforded better yield of the diol than 1t
with an electro-donating methyl group.
Measurement of cyclic voltammetry of TMSCl, TMSBr and TMSI
found the reductive potentials were ca. À2.58, À1.67 and À0.488 V
respectively (vs Ag/AgCl with Fc/Fc+ as internal standard, see sup-
porting information), which could account for the facts that Sm
reacts with TMSBr and TMSI to give a green suspension of Sm(II)
species while the reaction between Sm and TMSCl is sluggish.
In view of the more negative reductive potentials of carbonyl
compounds (-2.05 to À2.23 V vs Ag/AgCl) and aliphatic carbonyl
compounds (~3.00 V) [11], it is reasonable that Sm may reduce
TMSBr preferentially in the coexistence of ketones and aldehy-
des. Furthermore, the more negative reductive potentials of ali-
phatic carbonyls could not rationale why the reaction of
aliphatic carbonyls is more readily than aromatic ones in the
system. A mechanism rather than ketyl related coupling should
exist.
Considering the simplicity of the condition and the good yield
(entry 1, Table 1), no further optimization of the reaction was con-
ducted and the ratio of Sm/TMSBr/substrate (1:1:1) was estab-
lished as the standard condition. Subsequently, the scope of the
reaction was examined by extending the substrates to a variety
of ketones and aldehydes. The results are shown in Table 2.
It is noteworthy the Sm/TMSBr system afforded good to high
yields of the diols rapidly for aliphatic ketones (in 30 min.). In con-
According to the research on the pinacolization with Yb/TMSBr
by Takaki and Fujiwan et al. [12], where YbBr2 was proposed to be
the active species, it seems reasonable to propose the key Sm(II)
species produced here maybe SmBr2 (Scheme 1, path a). However,
treatment of 2-heptanone with SmBr2 (1.0 equv.) or SmBr2 (1.0
2

X. Wang, G. Xie, Y. Zhao et al.
Tetrahedron Letters 72 (2021) 153069
Table 2
Sm/TMSBr promoted pinacol coupling of carbonyl compounds.a
Entry
1
Substrate 1
1a
Product 2
Yield (%) of 2b
Dl : mesoc
52:48
78%
2a
2b
2
3
58%
54%
58:42
49:51
1b
1c
2c
4
5
61%
78%
52:48
56:44
1d
2d
1e
2e
6
7
74%
76%
–
1f
1g
2f
50:50
2g
8
63%
–
2h
2i
1h
9
70%
75%
22%
–
1i
1j
10
11
–
2j
57:43
1k
1l
2k
12
13
14
1%d
49%
50%
–
2l
48:52
68:32
1m
2m
1n
1o
2n
15
16
17
51%
60%
64%
60:40
2o
60:40
1p
1q
2p
56:13:12:19e
2q
18
48%
0:100
1r
2r
19
20
21
46%
16%
25%
51:49
49:51
43:57
1s
2s
1t
2t
2u
1u
a
Reaction conditions: Sm 1.0 mmol (0.15 g), the carbonyl compound (1.0 mmol), TMSBr (1.0 mmol) in dry THF 3 mL under N2 at the room temperature. The reaction
mixture was quenched with Bu4NF (1.0 M in THF, 1.2 mL) and allowed to be stirred for 1 h followed by usual workup (see SI).
b
Isolated yields.
Determined by GC–MS.
c
d
Together with simple reduced alcohol (12%) and the starting ketone (87%) as determined by GC–MS.
Four diastereomers were detected.
e
3

X. Wang, G. Xie, Y. Zhao et al.
Tetrahedron Letters 72 (2021) 153069
pinacolization efficiently [6a,c], however, in the co-existenc of Sm,
SmBr2 may reduce TMSBr more efficiently [15] to generate Me3-
SmBr2 V and SmBr3, rather than reduce the carbonyl into a ketyl
to give the pinacols. Me3SmBr2 could undergo the same nue-
cleophilic addition to the carbonyl of the ketone to produce an
organosiliyl intermediate VI. Brook rearrangement of VI trans-
forms into organosamarium(III) reagent VII. In a similar way, the
momosamarium(III) pinacolate VIII would be produced. Cation
exchange with TMSBr gives disilyl pinacolate IV and regenerates
SmBr3, which is then reduced by Sm readily to regenerate SmBr2.
With such a cycle, the piancol coupling of aliphatic carbonyls with
Sm metal will be more efficient, since the formation of
organosamarium intermediate and the key coupling process both
involved the nucleophilic addition to the carbonyls. It is well
known that alihphatic carbonyls are more electrophilic and steri-
cally less hindered than their aromatic counterparts, therefore
the former are more active towards nucleophiles.
In conclusion, a novel and efficient protocol for the pinacol cou-
pling of aliphatic ketones has been developed with readily avail-
able reagents. The in-advance preparation of the active Sm(II)
species is avoided, and the amount of the reagents and solvent
could be significantly decreased [16]. A unique Me3SiSmBr species
was proposed, and an anionic pinacol coupling pathway via samar-
ium Brook rearrangement was suggested. Further studies concern-
ing the catalytic version of the novel reductive system with proper
ligand to further improve the diastereoselectivities of the coupling
reaction would be underway.
Scheme 1. Proposed generation of Sm(II) species from Sm and TMSBr.
equiv.)/TMSBr (1.0 equiv.) afforded the desired diol only in 16% and
20%, respectively. Therefore, the reaction between Sm and TMSBr
should proceed in an alternative way. The samarium(II) species A
(Me3SiSmBr) may be involved (Scheme 1, path b).
Accordingly, the mechanism shown in Scheme 2 is proposed for
the Sm/TMSBr system mediated pinacol coupling.
Thus Sm may reduce the Si-Br bond to form Me3SmBr A, which
then undergoes nuecleophilic addition [13] to the carbonyl of the
ketone to produce an organosiliyl intermediate I, which would
undergo samarium Brook rearrangement [14] to form
organosamarium(II) reagent II. Subsequently, II attacked another
ketone molecule to afford the momosamarium(II) pinacolate III.
By exchange of the trimetylsily with samarium cation, the disilyl
pinacolate IV was produced and at the same time releasing SmBr2.
Although SmBr2 is a powerful reducing agent and could achieve the
Scheme 2. Proposed mechanism for the generation of diols.
4

X. Wang, G. Xie, Y. Zhao et al.
Tetrahedron Letters 72 (2021) 153069
H. Org. Lett. 2005, 7, 2365–2368. (c) Zhang, X. M.; Tu, Y. Q.; Zhang, F. M.; Shao,
H.; Meng, X. Angew. Chem., Int. Ed. 2011, 50, 3916–3919. (d) Li, H.; Wang, X.;
Lei, X. Angew. Chem., Int. Ed. 2012, 51, 491–495. (e) Marcos, I. S.; Cubillo, M. A.;
Moro, R. F.; Carballares, S.; Díez, D.; Basabe, P.; Llamazares, C. F.; Beneitez, A.;
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Sanz, F.; Broughton, H. B.; Urones, J. G. Tetrahedron 2005, 61, 977–1003. (f)
´
Nicolaou, K. C.; Ding, H.; Richard, J. A.; Chen, D. Y. K. J. Am. Chem. Soc. 2010,
132, 3815–3818. (g) Kingsbury, J. S.; Corey, E. J. J. Am. Chem. Soc. 2005, 127,
13813–13815. (h) Foster, S. L.; Handa, S.; Krafft, M.; Rowling, D. Chem.
Commun. 2007, 4791–4793. (i) Chiara, J. L.; García, A.; Sesmilo, E.; Vacas, T.
Org. Lett. 2006, 8, 3935–3938. (j) Hagiwara, K.; Himuro, M.; Hirama, M.; Inoue,
M. Tetrahedron Lett. 2009, 50, 1035-1037. (k) Urabe, D.; Nagamoto, M.;
Hagiwara, K.; Masuda, K.; Inoue, M. Chem. Sci. 2013, 4, 1615–1619.
[6] Selected reviews (a) Just-Baringo, X.; Procter, D. J. Acc. Chem. Res. 2015, 48,
1263–1275. (b) Szostak, M.; Fazakerley, N. J.; Parmar, D.; Procter, D. J. Chem.
Rev. 2014, 114, 5959–6039. (c) Szostak, M.; Spain, M.; Procter, D. J. Chem. Soc.
Rev. 2013, 42, 9155–9183. (d) Beemelmanns, C.; Reißig, H.-U. Chem. Soc. Rev.
2011, 40, 2199–2210. (e) Chciuk, T. V.; Floweres, R. A. In Science of Synthesis
Knowledge Updates; Marek, I., Ed.; Thieme: Stuttgart, 2016; 2, 177–265.
Selected examples: (f) Shabangi, M.; Flowers, R.A. Tetrahedron Lett. 1997, 38,
1137–1140. (g) Knettle, B.W.; Flowers, R.A. Org. Lett. 2001, 3, 2321–2324.
[7] (a) A. Lebrun, J.L. Namy, H.B. Kagan, Tetrahedron Lett. 34 (1993) 2311–2314;
(b) A. Lebrun, E. Rantze, J.-L. Namy, H.B. Kagan, New J. Chem. 19 (1995) 699–
705;
Acknowledgment
This work was financially supported by Science Foundation for
Distinguished Scholars of Dongguan University of Technology (No.
196100041).
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tetlet.2021.153069.
References
(c) F. Hélion, M.I. Lannou, J.L. Namy, Tetrahedron Lett. 44 (2003) 5507–5510;
(d) J.R. Fuchs, M. Mitchell, M. Shabangi, R.A. Flowers II, Tetrahedron Lett. 38
(1997) 8157–8158;
(e) R.S. Miller, J.M. Sealy, M. Shabangi, M.L. Kuhlman, J.R. Fuchs, R.A. Flowers II,
J. Am. Chem. Soc. 122 (2000) 7718–7722;
[1] (a) L.D. Guo, Y. Zhang, J.P. Hu, C.Q. Ning, H.F. Fu, Y.Y. Chen, J. Xu, Nature
Commun. 11 (2020) 3538–3544;
(b) Y. Imamura, S. Yoshioka, M. Nagatomo, M. Inoue, Angew. Chem. Int. Ed. 58
(2019) 12159–12163;
(c) Y. Sasano, J. Koyama, K. Yoshikawa, N. Kanoh, E. Kwon, Y. Iwabuchi, Org.
Lett. 20 (2018) 3053–3056;
(d) D. Chen, P.A. Evans, J. Am. Chem. Soc. 139 (2017) 6046–6049;
(e) B. Werner, M. Kalesse, Org. Lett. 19 (2017) 1524–1526;
(f) M. Buccini, K.A. Punch, B. Kaskow, G.R. Flematti, B.W. Skelton, L.J. Abraham,
M.J. Piggott, Org. Biomol. Chem. 12 (2014) 1100–1113;
(g) H. Li, X. Wang, X. Lei, Angew. Chem. Int. Ed. 51 (2012) 491–495;
(h) F.X. Wang, P.L. Zhang, H.B. Wang, G.B. Zhang, C.A. Fan, Science China-
Chemistry. 59 (2016) 1188–1196.
(f) A. Noripki, H. Tadashi, K. Hiroko, N. Yutaka, I. Yasutaka, J. Org. Chem. 59
(1994) 7902–7907.
[8] (a) Y.W. Tu, L.J. Zhou, R.F. Yin, X. Lv, R.A. Flowers II, K.A. Choquette, H.L. Liu, Q.
S. Niu, X.X. Wang, Chem. Commun. 48 (2012) 11026–11028;
(b) M.M. Shen, Y.W. Tu, G.Q. Xie, Q.S. Niu, H. Mao, T.T. Xie, R.A. Flowers II, X. Lv,
X.X. Wang, J. Org. Chem. 80 (2015) 52–61;
(c) B.X. You, M.M. Shen, G.Q. Xie, H. Mao, X. Lv, X.X. Wang, Org. Lett. 20 (2018)
530–533.
[9] M.X. Yu, Y.M. Zhang, Org. Prep. Proced. Int. 33 (2001) 187–190.
[10] (a) H.C. Aspinall, N. Greeves, C. Valla, Org. Lett. 7 (2005) 1919–1922;
(b) Xu; Hirao, T., The Journal of Organic Chemistry. 70 (2005) 8594–8596;
(c) H. Kronenwetter, J. Husek, B. Etz, A. Jones, R. Manchanayakage, Green
Chemistry. 16 (2014) 1489–1495;
[2] (a) Q. Xia, J.Y. Dong, H.J. Song, Q.M. Wang, Chem. Eur. J. 25 (2019) 2949–2961;
(b) J. Streuff, Synthesis 45 (2013) 281–307;
(c) B. Rossi, S. Prosperini, N. Pastori, A. Clerici, C. Punta, Molecules. 17 (2012)
14700–14732;
(d) B.S. Terra, F. Macedo, Arkivoc. (2012) 134–151;
(e) K.C. Nicolaou, S.P. Ellery, J.S. Chen, Angew Chem Int. Ed. 48 (2009) 7140–
7165;
(d) S.-Z. Yuan, J. Zhang, Green Chemistry Letters and Reviews. 8 (2015) 13–16;
(e) N. Sotto, C. Cazorla, C. Villette, M. Billamboz, C. Len, The Journal of Organic
Chemistry. 81 (2016) 11065–11071;
(f) S. Kramer, F. Hejjo, K.H. Rasmussen, S. Kegn
754–759;
æs, ACS Catalysis. 8 (2018)
(f) Y.-G. Li, X.-B. Chi, Chin. J. Org. Chem. 27 (2007) 431–437;
(g) D. Riber, R. Hazell, T. Skrydstrup, J. Org. Chem. 65 (2000) 5382–5390.
[3] (a) T. Wirth, Angew. Chem. Int. Ed. 35 (1996) 61–63;
(b) H.L. Pedersen, T.B. Christensen, R.J. Enemaerke, K. Daasbjerg, T. Skrydstrup,
Eur. J. Org. Chem. 1999 (1999) 565–572;
(g) R. Nomura, T. Matsuno, T. Endo, J. Am. Chem. Soc. 118 (1996) 11666–
11667;
(h) H.C. Aspinall, N. Greeves, S.L.F. Hin, Tetrahedron Lett. 51 (2010) 1558–
1561.
[11] H. Maekawa, Y. Yamamoto, H. Shimada, K. Yonemura, I. Nishiguchi,
Tetrahedron Lett. 45 (2004) 3869–3872.
[12] Y. Taniguchi, M. Nakahashi, T. Kuno, M. Tsuno, Y. Makioka, K. Takaki, Y.
Fujiwara, Tetrahedron Lett. 35 (1994) 4111–4114.
(c) A. Gansauer, Chem. Commun. (1997) 457–458;
(d) T. Hirao, B. Hatano, M. Asahara, Y. Muguruma, A. Ogawa, Tetrahedron Lett.
39 (1998) 5247–5248;
(e) M.C. Barden, J. Schwartz, J. Org. Chem. 62 (1997) 7520–7521;
(f) H.C. Aspinall, N. Greeves, S.L.F. Hin, Tetrahedron Lett. 51 (2010) 1558–
1561;
(f) M.C. Barden, J. Schwartz, J. Am. Chem. Soc. 118 (1996) 5484–5485;
(g) R. Nomura, T. Matsuno, T. Endo, J. Am. Chem. Soc. 118 (1996) 11666–
11667.
[13] Several nucleophilic Si reagents has been reported: (a) Bonini, B. F.; Comes-
Franchini, M.; Fochi, M. Polyhedron 2000, 19, 529–531. (b) Clark, C. T.;
Milgram, B. C.; Scheidt, K. A. Org. Lett. 2004, 6, 3977–3980. (c) Azuma, H.;
Okano, K.; Tokuyama, H. Chem. Lett. 2011, 40, 959–961.
[14] A chromium Brook rearrangement was proposed for the reductive olefination
of aldehydes, where diols was suggested to be the intermediates: Baati, R.;
Mioskowski, C.; Barma, D.; Kache, R.; Falck, J. R. Org. Lett. 2006, 8, 2949–2951.
[15] It was reported the Sm/SmI2 system was able to even reduce
organochlorosilanes under reflux conditions. See: Li, Z.; Iida, K.; Tomisaka,
Y.; Yoshimura, A.; Hirao, T.; Nomoto A.; Ogawa, A. Organometallics 2007, 26,
1212À1216.
[4] (a) M. Nakajima, E. Fava, S. Loescher, Z. Jiang, M. Rueping, Angew. Chem. Int
Ed. 54 (2015) 8828–8832;
(b) S.H. Huang, R. Hong, Science China-Chemistry. 61 (2018) 509–510;
(c) M.X. Liu, L.D. Tan, R.T. Rashid, Y.N. Cen, S.B. Cheng, G. Botton, Z.T. Mi, C.J. Li,
Chem. Science. 11 (2020) 7864–7870;
(d) X.L. Yang, J.D. Guo, H.Y. Xiao, K. Feng, B. Chen, C.H. Tung, L.Z. Wu, Angew.
Chem. Int. Ed. 59 (2020) 5365–5370;
[16] Typical procedure: An oven-dried two-necked flask (10 mL) containing finely
powdered samarium (0.15 g, 1 mmol) was evacuated and backfilled with N2
for three times. Under a positive pressure of nitrogen, dry THF (3 mL) was
added via a syringe followed by the addition of the carbonyl compound (1
mmol) in 1 mL dry THF and TMSBr (0.132 mL, 1.0 mmol). The reaction mixture
was stirred at r.t. for 30 min until the completion of the reaction (monitored by
TLC). The reaction was quenched by n-Bu4NF solution (1 mol/L in THF, 1.2 mL)
and allowed to be strirred for 1 h. The mixture was extracted by ethyl ether (3
 15 mL). The combined extracts were washed with brine, dried over Na2SO4,
and concentrated under reduced pressure. The residue was purified by
chromatography on silica gel (200–300 mesh) using petroleum/EtOAc (8/1, v
: v) as eluent to afford the corresponding product.
(e) Z.H. Qiu, H.D.M. Pham, J.B. Li, C.C. Li, D.J. Castillo-Pazos, R.Z. Khaliullin, C.J.
Li, Chemical Science. 10 (2019) 10937–10943;
(f) A. Caron, E. Morin, S.K. Collins, ACS Catal. 9 (2019) 9458–9464;
(g) R. Naumann, M. Goez, Green Chem. 21 (2019) 4470–4474;
(h) K. Nakahara, K. Naba, T. Saitoh, T. Sugai, R. Obata, S. Nishiyama, Y. Einaga, T.
Yamamoto, Chemelectrochem. 6 (2019) 4153–4157;
(i) A. Gualandi, G. Rodeghiero, E. Della Rocca, F. Bertoni, M. Marchini, R.
Perciaccante, T.P. Jansen, P. Ceroni, P.G. Cozzi, Chem. Commun. 54 (2018)
10044–10047.
[5] Selected intramolecular coupling of alihphatic carbonyls: (a) Paquette, L. A.;
Lai, K. W. Org. Lett. 2008, 10, 3781–3784. (b) Ueda, T.; Kanomata, N.; Machida,
5