Ru-Catalyzed Geminal Hydroboration of Silyl Alkynes via a New gem-Addition Mechanism

Article abstract of DOI:10.1021/jacs.0c05334

While 1,2-addition represents the most common mode of alkyne hydroboration, herein we describe a new 1,1-hydroboration mode. It is the first demonstration of gem-(H,B) addition to an alkyne triple bond. With the superior [CpRu(MeCN)3]PF6 catalyst, a range of silyl alkynes reacted efficiently with HBpin under mild conditions to form various synthetically useful silyl vinyl boronates with complete stereoselectivity and broad functional group compatibility. An extension to germanyl alkynes and the hydrosilylation of alkynyl boronates toward the same type of products were also achieved. Mechanistically, this process features a new pathway featuring gem-(H,B) addition to form the key α-boryl-α-silyl Ru-carbene intermediate followed by silyl migration. It is believed that the orbital interaction between boron and Cβ in the coplanar relationship between the boron atom and the ruthenacyclopropene ring preceding boron migration is responsible for the new reactivity. Control experiments and DFT (including molecular dynamics) calculations provided important insights into the mechanism, which excluded the involvement of a metal vinylidene intermediate. This study represents a new step forward not only for alkyne hydroboration but also for other geminal additions of alkynes.

Full text of DOI:10.1021/jacs.0c05334

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Article
Ru-Catalyzed Geminal Hydroboration of Silyl
Alkynes via a New gem-Addition Mechanism
Qiang Feng, Hao-Nan Wu, Xin Li, Lijuan Song, Lung Wa Chung, Yun-Dong Wu, and Jianwei Sun
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c05334 • Publication Date (Web): 15 Jul 2020
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Ru-Catalyzed Geminal Hydroboration of Silyl Alkynes via a New gem-Addition
Mechanism
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Qiang Feng,# Haonan Wu,†,‖ Xin Li,‖ Lijuan Song,¶ Lung Wa Chung,‖,* Yun-Dong
Wu,†,¶,⊥,* and Jianwei Sun#,*
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#
Department of Chemistry, The Hong Kong University of Science and Technology,
Clear Water Bay, Kowloon, Hong Kong SAR, China
‖
Shenzhen Grubbs Institute, Department of Chemistry and Guangdong Provincial
Key Laboratory of Catalysis, Southern University of Science and Technology,
Shenzhen 518055, China
†
Lab of Computational Chemistry and Drug Design, State Key Laboratory of
Chemical Oncogenomics, Peking University Shenzhen Graduate School, Shenzhen
518055, China
¶
Shenzhen Bay Laboratory, Shenzhen 518055, China
⊥
College of Chemistry, Peking University, Beijing 100871, China
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Abstract
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While 1,2-addition represents the most common mode of alkyne hydroboration,
herein we describe a new 1,1-hydroboration mode. It is the first demonstration of
gem-(H,B) addition to an alkyne triple bond. With the superior [CpRu(MeCN)3]PF6
catalyst, a range of silyl alkynes reacted efficiently with HBpin under mild conditions
to form various synthetically useful silyl vinyl boronates with complete
stereoselectivity and broad functional group compatibility. An extension to germanyl
alkynes as well as the hydrosilylation of alkynyl boronates toward the same type
products was also achieved. Mechanistically, this process features a new pathway
featuring gem-(H,B) addition to form the key α-boryl-α-silyl Ru-carbene intermediate
followed by silyl migration. It is believed that the orbital interaction between boron
and Cβ in the coplanar relationship between the boron atom and the
ruthenacyclopropene ring preceding boron migration is responsible for the new
reactivity. Control experiments and DFT (including molecular dynamics) calculations
provided important insights into the mechanism, which excluded the involvement of
metal vinylidene intermediate. This study represents a new step forward not only for
alkyne hydroboration, but also for other geminal alkyne additions.
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Introduction
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Vinylboranes are versatile building blocks in organic synthesis, which can serve
as not only low-toxic robust nucleophilic partners in a range of C–C bond formation
processes with broad functional group compatibility, but also useful precursors
toward other versatile building blocks by simple transformations, such as oxidation
and reduction.1 Among the various methods for their synthesis, alkyne hydroboration
represents the most straightforward approach.1c,d Consequently, this transformation
has been a subject of intensive investigations over the past half century.1-7 Among
them, the 1,2-addition mode has been well-established, with the prototype concerted
syn-addition particularly known as a text-book reaction (Scheme 1).2 Recently,
various elegant catalytic systems have also been developed to achieve anti-
hydroboration.3,4 However, in contrast to these 1,2-additions, the 1,1-addition mode
(also called geminal or gem-addition), which adds H–B to the same terminal of the
triple bond and requires migration of one alkyne substituent, has been essentially
unknown. Notable exceptions are some formal trans-hydroborations of terminal
alkynes that proceed via metal vinylidene or metal acetylide as key intermediates,
which require prior substrate rearrangement and thus obviously cannot be applied to
internal alkynes.5,6 Moreover, direct geminal H–B addition to a triple bond remains
unknown. Herein, we introduce the first example of this type for silyl alkynes via a
new gem-hydroborated metal-carbene intermediate.
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Scheme 1. Introduction to Alkyne Hydroboration
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R1
R2
R1
R2
B
B
H
1,2-addition
1,1-addition
+
B
H
R1
R2
migratory
syn or anti
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H
essentially unknown
well-known
(this work)
gem-addition
known pathway
new pathway
[M]
[M]
[M]
R1
R2
or
B
H
R
H
R
Recently, Fürstner and co-workers have reported a series of pioneering studies on
Ru-catalyzed trans-1,2-hydrogenation of alkynes, which proceeds via direct gem-H2
addition followed by 1,2-hydrogen migration.8 However, similar gem-(H,B) addition
pathway for alkyne hydroboration seemed to be not operative.3a-c Instead, the trans-
1,2-hydroboration was proposed to proceed via a ruthenacyclopropene intermediate, a
typical mechanism followed by other related trans-hydrometalation processes (e.g.,
hydrosilylation, hydrostannylation) without involving gem-addition.9 This mechanism
is also consistent with the subsequent DFT studies.3d
In continuation of our effort in alkyne hydrofunctionalizations,10 we have recently
discovered a gem-hydrogenation of silyl alkynes, in which the Fürstner intermediate
(gem-H2 carbene) was intercepted by silyl migration.10b Inspired by this study and
prompted by the important value of alkyne hydroboration, we were curious about the
possibility of forming gem-(H,B) Ru-carbene with silyl alkynes, which would be
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intercepted by silyl migration. This intriguing process would not only lead to
synthetically useful silyl vinyl boranes,11 but more importantly presents a different
hydroboration mechanism. However, additional challenges should be expected
compared with migratory gem-hydrogenation. For example, the gem-(H,B)-addition
step would experience higher steric repulsion with the silyl group (relative to gem-H2
addition). Secondly, silyl migration would require additional stereocontrol over the
resulting olefin configuration to achieve good stereoselectivity.
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Results and Discussion
We began our study with silyl alkyne 1a as the model substrate, in which the
phenyldimethylsilyl group (PhMe2Si) was used in view of its general versatility in
organic synthesis.11 The most robust and popular boron reagent, pinacolborane
(HBpin), was used as the reaction partner.2c,d We first evaluated the [Cp*Ru]-based
catalysts in view of their extraordinary performance in alkyne hydrometallation
reactions (Table 1, entries 1-4).3,8-10 Unfortunately, they uniformly exhibited low
catalytic activity, and the reaction in DCM proceeded with low conversion at room
temperature and gave a mixture of unidentifiable products. It is worth noting that
[Cp*Ru(MeCN)3]PF6 and [Cp*RuCl]4 have previously been identified as superior
catalysts for 1,2-trans-hydroboration of internal alkynes,3 but unfortunately they
resulted in no success in this case. However, to our delight, further screening
indicated that [CpRu(MeCN)3]PF6 showed dramatically high activity (entry 5). More
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surprisingly, a silyl-migrated gem-hydroboration product 2a was observed as the sole
product in 88% yield. Other Ru-based catalysts, such as those with a strong-
coordinating ligand PPh3 or without the Cp ligand, were completely ineffective
(entries 6-7). Remarkably, it is expected that both PhMe2Si and Bpin in 2a can be
easily and selectively transformed into other useful functional groups. Further
optimization of this reaction was performed using other reaction conditions. A higher
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temperature (50 C) led to slightly lower conversion, presumably due to catalyst
decomposition. Increasing the loading of HBpin boosted the reaction efficiency (entry
9). The use of other non-polar halogenated solvents, such as DCE and CHCl3, were
almost equally effective as DCM (entries 10-11). However, coordinating solvents like
THF, Et2O and toluene (as π-ligand) essentially shut down the reactivity (entry 12).
Notably, replacing the phenyldimethylsilyl group to trimethylsilyl (TMS) in the
substrate also failed to give the corresponding product (entry 13). Instead, slow
conversion to the regular 1,2-hydroboration products (Z/E mixture) was observed.
Finally, increasing the reaction concentration slightly improved the reaction efficiency
(entry 14).
Table 1. Condition Optimization for gem-Hydroborationa
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nBu
Bpin
H
O
catalyst (10 mol%)
SiMe2Ph
HB
+
nBu
O
DCM (0.1 M), r.t., 6 h
PhMe2Si
7
8
1a
(HBpin)
2a
(E only)
9
entry
catalyst or conditions
[Cp*Ru(MeCN)3]PF6
conversion (%)b
13
yield(%)b
<5
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1
2
[Cp*RuCl]4
[Cp*RuCl2]n
27
29
<5
<5
<5
88
<5
<5
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92
3
4
Cp*Ru(cod)Cl
<5
94
[CpRu(MeCN)3]PF6
CpRu(PPh3)2Cl
[Ru(cod)Cl]2
5
6
<5
<5
82
7
8c
9d
[CpRu(MeCN)3]PF6
[CpRu(MeCN)3]PF6
100
Deviation from entry 9
10
DCE as solvent
95
88
93
83
<5
11
12
CHCl3 as solvent
Other coordinating solvents
(e.g., Et2O, THF, MeCN, toluene)
<10
1a
13
14
TMS in place of PhMe2Si in
19
<5
94
c = 0.2 M (DCM)
100
a Reaction scale: 1a (0.1 mmol), HBpin (0.15 mmol), solvent (1.0 mL). b Determined
1
by analysis of the H NMR spectrum of the crude reaction mixture using CH2Br2 as
an internal standard. c Run at 50 oC. d Run with 3 equiv of HBpin.
Under the optimized conditions (entry 14, Table 1), a range of substituted silyl
alkynes participated in this gem-hydroboration process with high efficiency (Table 2).
The mild conditions were compatible with a diverse set of functional groups, such as
ester, ether, acetal (THP-protected alcohol), silyl ether, mesylate, halide, and
phthalimide. Incorporation of heterocycles, such as furan and thiophene, did not affect
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the high efficiency. Although the benzene ring was previously known as a good π-
ligand for such Ru-systems and can interfere with the catalytic activity,9g it was found
that the benzyl-substituted alkyne reacted successfully (2c). However, direct phenyl-
substitution on the alkyne resulted in low conversion. Substitution with a bulky group,
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t
such as Bu, also led to low reactivity. Notably, in all these successful examples,
exclusive gem-addition with silyl migration was observed, i.e., no regular 1,2-addition
product was detected. Moreover, this addition also features excellent stereoselectivity.
The corresponding vinylboronates 2 were all obtained as a single E-isomer. The
structure of product 2i was also confirmed by X-ray crystallography.
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Table 2. Reaction Scopea
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[CpRu(MeCN)3]PF6
(10 mol%)
R
Bpin
H
R
SiMe2Ph
+
HBpin
PhMe2Si
DCM (0.2 M),12 h, r.t.
1
2
single isomer
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nBu
PhMe2Si
2b
Bpin
H
nBu
Bpin
H
Ph
Bpin
AcO
Bpin
PhMe2Si
H
PhMe2Si
PhMe2Si
H
2a
2c
2d
, 80%
, 89%
, 87%
, 56%
AcO
THPO
TBSO
MsO
Bpin
H
Bpin
Bpin
Bpin
PhMe2Si
PhMe2Si
2f
H
PhMe2Si
2g
H
PhMe2Si
2h
H
, 87%
, 89%
, 70%
2e
, 86%
Cl
Br
BnO
Bpin
Bpin
H
Bpin
H
PhMe2Si
H
PhMe2Si
PhMe2Si
2i
2j
2k
, 88%
, 75%
, 85%
O
O
2
O
N
S
Bpin
H
Bpin
H
O
Bpin
H
O
PhMe2Si
PhMe2Si
PhMe2Si
2l
, 74%
2m
, 71%
2n
, 99%
a Reaction scale: 1 (0.3 mmol), HBpin (3 equiv), [CpRu(MeCN)3]PF6 (10 mol%),
DCM (1.5 mL); Isolated yield.
In addition to silyl alkynes, germanyl alkynes also worked well in this gem-
hydroboration process (eq 1). Under essentially the same conditions,
phenyldimethylgermanyl-substituted alkynes 3a and 3b reacted efficiently to form the
germanyl-migrated vinylboronates 4 with excellent stereoselectivity. The structure
was confirmed by X-ray crystallography. It is worth noting that organogermanes are
also important building blocks in organic synthesis.12
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[CpRu(MeCN)3]PF6
HBpin
+
R
Bpin
H
(10 mol%)
(1)
DCM (0.1 M), 3 h, r.t., N2
PhMe2Ge
GeMe2Ph
R
4a
4b
, 99%
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n
3a
3b
(R = Bu)
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, 67%
(R = OBn)
4b
To further investigate the robustness of this gem-hydroboration process, we further
employed additives with more diverse functional groups to examine their
compatibility using the standard reaction of 1a (Table 3).13 With aliphatic olefin 5a,
no obvious influence was observed. However, the standard reaction was retarded by
aryl olefin 5b, presumably due to its competing binding with the Ru-catalyst. Regular
alkynes 5c-d had competitive hydroboration reactivity, leading to low conversion of
1a. The standard reaction was not obviously affected by aldehydes or ketones 5e-g,
although these carbonyl groups could also react with HBpin. It was believed that these
carbonyls reacted at a slower rate than 1a. Moreover, free alcohol 5h, phenol 5i,
carboxylic acids 5j-k, azide-tethered phthalimide 5l and nitrile 5m also showed good
to excellent compatibility with this catalytic system. Unfortunately, the additives with
relatively strong-coordinating functionality, such as amide 5n, thiol 5o, oxazole 5p,
and thiazole 5q, significantly retarded this reaction.
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Table 3. Study of Functional Group Compatibilitya
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standard
conditions
2a
x% = yield of
2a
5
1a
5a
+
+
HBpin
5
1a
y% = remained
z% = remained
(additive)
x (y, z)
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5b
5c
5d
5e
O
nOct
Me
Br
9
tBu
Me
87 (100, 8)
59 (86, 36)
64 (24, 27)
0 (55, 100)
93 (38, 0)
5f
5g
5h
5i
Me
Me
Me
nOct
CHO
O
nDec
OH
OH
Me
Me
Me
81 (10, 0)
83 (20, 13)
83 (68, 18)
88 (60, 5)
5j
5k
5l
5m
O
CO2H
N3
CN
Ph
CO2H
N
Ph
4
O
94 (100, 0)
93 (100, 0)
96 (95, 0)
94 (95, 0)
5n
5o
5p
5q
Me
S
O
Ph
O
nDec
Ph
Ph
SH
Me
N
N
N
Me
H
52 (94, 44)
12 (82, 61)
9 (100, 81)
0 (39, 95)
a
Reaction scale: 1 (0.1 mmol), HBpin (3 equiv), 5 (0.1 mmol), DCM (0.5 mL), r.t.,
10 h. Yield was determined by analysis of the 1H NMR spectrum of the crude mixture
using CH2Br2 as an internal standard.
Next, with the hypothetical mechanism involving gem-(H,B) Ru-carbene IM1 in
mind, we further envisioned that this type of α-boryl-α-silyl Ru-carbene might also be
generated from gem-hydrosilylation of alkynyl boronate substrate 6 (eq 2). In a
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similar manner, the subsequent silyl migration should proceed to provide the exactly
same type of vinylborane product 2. If successful, this might lead to not only
expanded silyl scope beyond PhMe2Si, but also likely a new hydrosilylation
mechanism.
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+ HBpin
Si
R
R
[Ru]
R
Bpin
H
Si
1
6
R
(2)
H Bpin
Si
?
Bpin
Si
H
+
2
IM1
7
Intrigued by this possibility, we prepared alkynyl boronate 6a and subjected it to
the reaction with PhMe2SiH under the same conditions (Scheme 2). To our delight,
the formal syn-hydrosilylation product 2b was obtained in 85% yield. More
importantly, the spectral data indicated that this product was exactly same as that
obtained from gem-hydroboration. This process exhibited a broad scope with respect
to various types of silanes, including Ph2MeSi, Ph3SiH, (EtO)3SiH, and Et3SiH,
thereby providing an attractive complement to the above gem-hydroboration process.
Notably, these products were all obtained as a single isomer as well, highlighting the
excellent regio- and stereoselectivity.
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6
7
Scheme 2. Ru-Catalyzed Hydrosilylation of Alkynyl Boronatesa
8
9
[CpRu(MeCN)3]PF6
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nBu
Bpin
H
(10 mol%)
nBu
Bpin
+
Si
H
Si
DCM (0.1 M)
6 h, r.t., N2
2
6a
7
single isomer
nBu
Bpin
H
nBu
Bpin
H
nBu
Bpin
Ph3Si
Ph2MeSi
PhMe2Si
H
2p
, 70%
2o
, 75%
2b
, 85%
nBu
Bpin
nBu
Bpin
H
Et3Si
H
(EtO)3Si
2r
, 91%
2q
, 68%
a Reaction scale: 6a (0.3 mmol), 7 (1.5 equiv). Isolated yield.
The products obtained from this reaction can serve as useful precursors to other
stereodefined olefins (Scheme 3). The presence of two chemically distinct masking
groups (i.e., the silyl and boryl groups) is ideal for sequential transformations with
high chemoselectivity. Product 2a was used to demonstrate these applications. It
could serve as a nucleophile for Rh-catalyzed conjugate addition to chalcone, leading
to ketone 8 in 84% yield. Moreover, by Cu-catalysis, the boronate motif could
selectively react to form vinyl azide 9 and enol ether 10. With iodine, it could also be
converted to vinyl iodide 11. Furthermore, Pd-catalyzed Suzuki-Miyaura coupling
with aryl iodide proceeded selectively to form 12 with excellent efficiency. Finally,
the silyl group in 12 could be easily converted to iodide (13) upon treatment with NIS,
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which would allow further cross-coupling or other transformations when needed. It is
worth noting that these transformations not only feature high efficiency and
chemoselectivity, but also absolute integrity of the olefin configuration.
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Scheme 3. Product Transformations
O
nBu
I
Ph
Ph
a
d
nBu
PhMe2Si
H
78%
84%
PhMe2Si
H
8
11
nBu
PhMe2Si
Bpin
H
nBu
N3
H
nBu
Ar
H
b
e
12
99%
86%
PhMe2Si
PhMe2Si
2a
9
f
72%
Ar = p-MeC6H4
nBu
O
c
nBu
Ar
H
e,f
67%
PhMe2Si
H
I
13
10
o
Conditions: (a) (E)-chalcone, [Rh(cod)Cl]2, MeOH, H2O, 90 C; (b) NaN3, CuSO4,
MeOH, r.t.; (c) allyl alcohol, Cu(OAc)2, Et3N, r.t.; (d) I2, NaOH, THF, H2O, r.t.; (e) 1-
p-iodotoluene, PdCl2(dppf), dioxane, KOH, 90 oC; (f) NIS, 2,6-lutidine, (CF3)2CHOH,
0 oC, 30 min.
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Mechanistic Studies
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DFT calculations. The above experimental observations raise several mechanistic
questions: (1) Why and how does hydroboration proceed in the 1,1-addition fashion?
(2) How is the stereochemistry of the final product controled? To address these
questions, density functional theory (DFT) calculations have been carried out (with
SMD M06/6-31G*+SDD(Ru) method, see the SI for details).14,15 Also, DFT quasi-
classical molecular dynamic (MD) simulations16-18 on the rate-determining step by the
same method were performed.
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Oxidative hydrogen migration. Similar to the mechanism for the related Ru(II)-
catalyzed trans-hydrofunctionalization and gem-hydrogenation reactions proposed by
us and Fürstner,3,8-10 our DFT results suggested that the present hydroboration
reaction starts by ligand exchange with HBpin and alkyne followed by the rate-
determining oxidative hydrogen migration to the alkyne carbon substituted with the
silyl group via B1a-TS (Figure 1). Such oxidative hydrogen migration, featuring
considerable H…B interaction (1.82 Å, Figure 2), requires a barrier of about 23.3
kcal/mol above CAT and directly forms the key metallocyclopropene intermediate
B2a.19 In comparison, the other two pathways, oxidative boryl migration via B1c-TS
or oxidative hydrogen migration with opposite regioselectivity via B1b-TS, have
higher barriers by 1.8–3.5 kcal/mol. In addition, B1a-TS is lower in free energy than
the two intermediates involved in the classical metal vinylidene pathway10b by more
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than 4.1 kcal/mol (Figure S12). Notably, the computed B…C distance in B2a is quite
short (2.51 Å, shorter than its van der Waals distance, 3.62 Å).16a Also, the
metallocyclopropene plane and the Ru–B bond in B2a are oriented to be roughly
coplanar (Figure 2). These two structural features in B2a are crucial for the
subsequent boryl migration (vide infra).
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Cp
Ru
H
Cp
Ru
Gsoln
(Esoln
kcal/mol
Cp
Si
Me
L
L
)
Cp
Ru
Cp
Ru
H
Bpin
L
pinB
Bpin
Si
B1c-TS
Cp
Ru
H
Me
L
Ru
Bpin
L
9
Bpin
Si
B1b-TS
25.1(15.4)
Bpin
L = MeCN
Si = SiMe2Ph
Me
L
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Me
H
B2b
Me
H
26.8(15.1)
H
25.6(13.1)
PhMe2Si
Me
PhMe2Si
PhMe2Si
B1a-TS
23.3(14.5)
Cp
C1b-TS
19.4(7.3)
C3b-TS
18.4
(6.2)
A2a
13.5
(3.8)
C2b
16.1(5.4)
B2a
14.8(3.1)
Cp
Cp
Ru
C1a-TS
16.0(3.4)
Cp
Me
Ph
2
HBpin
Ru
L
Me
+
2L
H
A2b
12.0(4.1)
H
L
SiMe
pinB
Si
Si
Ru
C2a
6.3
(-7.1)
Bpin
Bpin
Me
C3a
L
Ru
Bpin
L
Cp
C3a
2.6(-10.0)
H

Me

Me
Cp
Ru
Si
H
Me
CAT
0.0(0.0)
Cp
Ru
L
PhMe2Si
C3b
-3.1(-16.3)
L
H
Si
Bpin

pinB
Cp
Ru
Me
SiMe2Ph
A2a
H
O
B
Ru
L
L
L
O
C4b
-10.9
(-26.4)
Cp
L
Me
PhMe2Si
H
H
Ru
Bpin
L

PhMe2Si
Me
Reductive Boryl Migration to C
Reductive Boryl Migration to C
Oxidative Hydrogen Migration
Figure 1. Free energy surface of the most favorable pathway for the initial stage of the gem-hydroboration in solution by the SMD M06 method.
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Figure 2. Computed geometries and relative free energies of the key transition states and
intermediates for the Ru-catalyzed gem-addition of MeC≡CSiPhMe2 by the SMD M06 method.
Next, B2a could potentially undergo reductive 1,2-boryl migration to the C position and
form the regular 1,2-cis-hydroboration product C4b (Figure 1).3d However, this path would
require a prior rearrangement to form metallocyclopropene C2b (via C1b-TS), in which the Ru–
B bond becomes roughly perpendicular to the metallocyclopropene plane. Alternatively, a
reductive boryl migration to the C position in B2a via C1a-TS to form a new gem Ru(II)-
carbene intermediate C2a preferentially occurs with a lower barrier (~16.0 kcal/mol above CAT;
only ~1.2 kcal/mol above B2a). C1a-TS was computed to be lower in free energy than C1b-TS
by ~3.4 kcal/mol. It is believed that the interaction between the empty p(B) orbital and the filled
(C=C) orbital in C1a-TS as well as their coplanar structural relationship should promote this
reductive boryl migration to C leading to C2a. Although the related reductive hydrogen
migration leading to a gem-H2 Ru(II)-carbene intermediate was first discovered by Fürstner and
coworkers, such a reductive boryl migration to form a gem-(H,B) Ru(II)-carbene is uncommon,
which is crucial for the ultimate gem-hydroboration product formation (vide infra).
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Reaction dynamics of oxidative hydrogen migration. Encouraged by the abovementioned very
low barrier for the reductive boryl migration via C1a-TS (~1.2 kcal/mol above B2a) and the
recent DFT quasi-classical MD studies on ultrafast dynamics in organic reactions,16a-c we
envisioned that the initial process from B1a-TS to the Ru(II)-carbene intermediate C2a might be
very fast. Pleasingly, our DFT MD simulations show that roughly 17 trajectories (~24 %) out of
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the 71 productive trajectories were found to pass by C1a-TS and essentially form the C–B bond
(<1.6 Å) within 1 ps in solution (Figures 3-4 and S3-S9). Also, considerable C–B bond
formation (<2.18 Å, shorter than that in C1a-TS) was observed in ~31% of the trajectories,
…
whereas the C B distance is mainly within 2.8-3.13 Å (~56%, Figure 4). Moreover, the time
gap between the C–H and C–B bond forming processes in these successful trajectories is ~430-
965 fs, in which the initial C–H bond formation is extremely fast (9-51 fs, see Figures S3-S4).
Overall, our DFT MD simulations suggest that an ultrafast formation (~ps) of the gem-(H,B) Ru-
carbene directly from the initial oxidative hydrogen migration could be possible.
Figure 3. The 17 successful trajectories leading to B2a from the rate-determining transition state
region (B1a-TS) in solution by the SMD M06 method.
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…
Figure 4. Histogram of the minimum C B distance in all the productive trajectories in solution
by the SMD M06 method.
Boryl migration versus silyl migration. After the reductive boryl migration to C2a, C–C
rotation takes place to give two more stable Ru(II)-carbene isomers C3a (with 2-coordination of
the benzene to the metal) and C3b (with 1-coordination of the boryl oxygen atom to the metal,
see Figure 5a). Although C3a was computed to be less stable than C3b by 5.7 kcal/mol, the
former has a lower barrier for the 1,2-silyl migration from C to C via D1a-TS than the latter
one via D1b-TS by 11.3 kcal/mol.10b In addition, D1a-TS is lower in free energy than D1a-TS2
(with opposite boryl and hydrogen positions) by 2.2 kcal/mol. D1a-TS with the bulky boryl
group away from the Cp ring and the silyl group is energetically more favorable and thus leads to
the desired and stable (E)-type complex D2a (Figure 5b). It is worth noting that the silyl
migration results in slight bending of the Cα–Ru part (along Cα–Cβ bond) away from the
incoming silyl group (Figure 5b), which causes Ru to approach Cβ from the opposite side
(relative to silyl migration). Therefore, this step (C3a to D2a) can also be viewed as cooperative
hopping of the silyl and Ru on the opposite side of the Cβ=Cα plane. In contrast, silyl migration
via either D1a-TS2 or D1b-TS leading to the less stable (Z)-type complex D2b is disfavored due
to steric repulsion between the boryl and CpRu moieties (Figure 5a). Moreover, the 1,2-silyl
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migration overcomes a lower barrier than the related boryl or hydrogen migration by at least 6.5
kcal/mol. Overall, our computational results are in qualitative agreement with the observed
exclusive formation of the gem-hydroborated (E)-olefin products.
9
Cp
Ru
Cp
Ru
Cp
Ru
Cp
Ru
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Ph
2
(a)
Gsoln(Esoln
L
L
Bpin
H
Ph
2
L
B
L
L
O
B
)
SiMe
O
Me
kcal/mol
Me
Me
SiMe2Ph
Me
SiMe2Ph
SiMe
H
H
L = MeCN
H
Bpin
D1a-TS-H
34.0 (24.7)
D1a-TS2
D1c-TS-B
17.0 (-8.8)
B
D1b-TS
22.8(9.7)
= Bpin
Cp
Ru
Cp
H
H
Ru
Me
L
D1a-TS2
13.7(0.3)
L
Bpin
Me
Bpin
C3a
Cp
Ru
L
D1a-TS
11.5(-1.4)
Cp
O
C3a
2.6
(-10.0)
SiMe2Ph
B
O
Me
2L
Ph
H
2
Ru
SiMe2Ph
Me P2
Bpin
L
H
D2b
-2.7
(-17.4)
SiMe

C3b
-3.1
(-16.3)
H
Me

Bpin
P2
P2 + CAT
-15.4
(-34.4)
Cp
Ru
Cp
Ru
D2a
-13.0(-24.7)
O
L
L
B
O
H
Me
P1 + CAT
-16.3
(-34.7)
2L
PhMe2Si
Me
P1
H
Bpin
SiMe2Ph
H
P1
Me
Bpin
D1a-TS
D1a-TS2
(b)
migrate
backward
H
[Ru]
Bpin
[Ru]
migrate
backward
bend toward
left & front side
bend toward
right & front side
Si

Si

 BCCRu = 164.1o
 HCCRu = 13.7o
 SiCCRu = 81.9o
 BCCRu = o
 HCCRu = 179.0o
 SiCCRu = 68.0o
Me
Bpin
Me
H
Figure 5. (a) Free energy surface of the most favorable final 1,2-migration pathways in solution
by the SMD M06 method. The less favorable pathways for different migrations are given in
Figure S10. (b) Newman projection along Cβ–Cα in the two key silyl migration transition states
with the computed key dihedral angles and relative free energies by the SMD M06 method.
Migration of other groups. To get further insights on the two uncommon migration steps (i.e.
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the reductive boryl migration from Ru to C in B2a and the subsequent silyl migration from C
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to C in C3a), additional calculations were carried out to probe the migratory propensity of other
groups in a similar system (Tables 4 and S7).
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As to reductive migration of the X group to C (Table 4), our calculations show that the boryl
and hydrogen migrations have the lowest migration barriers relative to their preceding
metallocyclopropenes (1.2 and 3.2 kcal/mol, respectively). For comparison, the migration of
trimethylsilyl (TMS) requires a higher barrier (12.0 kcal/mol). In fact, this barrier is even higher
than its alternative migration to C leading to 1,2-trans-addition according to our previous
studies (~5-7 kcal/mol).3d,9c,10 Moreover, the silyl migration is thermodynamically unfavorable.
Furthermore, the migrations of the carbon-based groups, such as -CH3, -COMe, and -C≡CMe,
were also found to have high barriers (10.7-18.6 kcal/mol). Finally, the good -donating NMe2
group results in a classical Ru(II) -vinyl intermediate with a high migration barrier of 34.8
kcal/mol. Overall, these computational results suggest that the presence of a suitable orbital (e.g.
s(H), p(B) or (CC)) on the migrating group to interact with the filled (C=C) orbital should
play a pivotal role in facilitating migration (see Figure S11b).
Table 4. Computed Reductive Migration of X to C in the Metallocyclopropene
Intermediatea
Cp
Cp
Cp
L
Ru
L
Ru
Ru
L
X
X
X
Me
Me
Me
H
H

H
SiMe2Ph
SiMe2Ph
SiMe2Ph
B2a(X)
C1a-TS(X)
C2a(X)
X
B2a(X)
C1a-TS(X)
C2a(X)
Bpin
0.0
1.2
-8.5
H
CH3
0.0
0.0
3.2
18.6
-10.0
-17.6
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0.0
0.0
0.0
0.0
11.9
10.7
12.0
34.8
-17.8
-21.2
0.3
-C≡CMe
COMe
SiMe3
b
NMe2
-11.9
a
b
Computed relative free energies (kcal/mol) in solution by the SMD M06 method. A Ru(II)-
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vinyl intermediate was formed instead of the metallocyclopropene.
Control Experiments. Although efforts to directly characterize the key gem-(H,B) Ru-carbene
intermediate or intramolecular trap it were not successful, we carried out some control
experiments to further substantiate this mechanism. First of all, it might be possible for silyl
alkynes to undergo prior silyl migration to form a metal vinylidene intermediate, which can
potentially lead to gem-hydroboration. While our DFT calculations suggested this pathway is
unfavorable (see Figure S12), we also designed experiments to distinguish between these two
paths (Schemes 4 and S1). Silyl alkyne 14, which bears a propargylic silyl group, was subjected
to the standard hydroboration conditions. In addition to the major 1,2-hydroboration product 15a,
two gem-hydroboration products 15b and 15c were also formed, albeit in low yield (Scheme 4A).
In fact, both 15b and 15c were likely generated from the common gem-(H,B) Ru-carbene IM2
by the two different silyl migration paths. In contrast, if the reaction proceeds via Ru-vinylidene
IM3, only product 15c (but not 15b) should be expected, which is obviously inconsistent with
the experimental results. Furthermore, to favor gem-addition (over 1,2-addition) and minimize
the perturbation by the migratory ability of different silyl groups, alkyne 16 bearing two same
PhMe2Si groups was prepared and subjected to the standard conditions. Expectedly, gem-
hydroboration products 17a and 17b were both observed (Scheme 4B). Similarly, the presence of
both products is more consistent with the intermediacy of carbene IM4 than vinylidene IM5,
which would only lead to 17b. Finally, alkyne 18, with carbon in place of the silicon atom in 16,
was also examined (Scheme 4C). It should be unlikely to involve a metal vinylidene intermediate
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for 18. While an unidentifiable mixture was observed with [CpRu(MeCN)3]PF6 as catalyst, the
use of [Cp*RuCl]4 led to product 19, which is consistent with the involvement of gem-(H,B) Ru-
carbene IM6 as well as the high migration barrier calculated for the PhMe2C group (Table S7).
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59
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Scheme 4. Differentiation between Ru-Vinylidene and gem-Hydroborated Ru-Carbene
Intermediates
A. Reaction of 14
1,2-addition
gem-addition
TMS
SiMe2Ph
standard
conditions
TMS
pinB
SiMe2Ph
PhMe2Si
Bpin
H
TMS
H Bpin
+
+
H
TMS
15c
SiMe2Ph
15a
15b
, 58% (NMR)
, 11% (NMR)
Ru-vinylidene
, 2% (NMR)
14
Ru-carbene
(unlikely)
a
[Ru]
[Ru]
15b
PhMe2Si
TMS
VS
15c
only
b
PhMe2Si
H Bpin
15c
TMS
IM2
IM3
B. Reaction of 16
SiMe2Ph
SiMe2Ph
SiMe2Ph
PhMe2Si
Bpin
H
standard
conditions
53% (isolated)
+
SiMe2Ph
H Bpin
17a
17a/17b
(
= 1:17)
PhMe2Si
17b
16
Ru-carbene
[Ru]
Ru-vinylidene
(unlikely)
a
b
[Ru]
17a
17b
PhMe2Si
SiMe2Ph
17b
only
VS
PhMe2Si
IM5
H Bpin
SiMe2Ph
IM4
C. Reaction of 18
HBpin
[Ru]
[Cp*RuCl]4
(25 mol%)
Me Me
Ph
Me Me
Ph
Me Me
SiMe2Ph
Ph
SiMe2Ph
H Bpin
H Bpin
DCE (0.2 M)
N2, r.t., 23 h
SiMe2Ph
IM6
19
, 33% (isolated)
18
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Another possible mechanism for silyl migration might involve dissociation of the substrate to
form a silyl cation (due to β-Si effect) followed by re-association (Scheme S1). To distinguish
from this possibility, we carried out a cross-over experiment (Scheme 5). The standard
hydroboration of a 1:1 mixture of alkynes 1n and 1s led to exclusive formation of vinyl
boronates 2n and 2s. The cross-over products 2n′ and 2s′ were not observed, thereby ruling out
the dissociative pathway.
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13
14
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16
17
18
19
20
21
22
23
24
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Scheme 5. Cross-over Experiment
nBu
Bpin
H
PhthN
Bpin
H
PhthN
3
SiMe2Ph
HBpin
PhMe2Si
3
PhMe2Si
2n
1n
2n'
, 0%
+
, 78%
standard
conditions
nBu
Bpin
nBu
SiMe2(p-Tol)
PhthN
Bpin
3
1s
1n 1s
(p-Tol)Me2Si
H
(p-tol)Me2Si
2s'
H
, 0%
2s
, 80%
/
= 1:1
crossover products not observed
Finally, control experiments were also designed to understand the mechanism of 1,2-
hydrosilylation of alkynyl boronate 6 shown in Scheme 2, which led to the same vinyl boronates
from gem-hydroboration of silyl alkynes. Under the standard conditions, hydrosilylation of
alkynyl boronate 20 bearing a propargylic silyl group led to exclusive formation of 1,2-addition
product 17b as a Z/E mixture (Scheme 6A). If the gem-addition intermediate IM4 is involved, a
mixture of 17a and 17b (see Scheme 4B) should be expected. However, the migratory product
17a was not observed, suggesting that this hydrosilylation may not proceed via gem-addition
mechanism. In retrospect, we also suspected that our previous discovery on hydrosilylation of
silyl alkynes may also possibly proceed via gem-addition mechanism.10a Therefore, two pairs of
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silyl alkynes (1a and 22) with different silanes (Et3SiH and PhMe2SiH) were employed for cross-
check (Scheme 6B). If gem-hydrosilylation is operative, the common intermediate IM7 should
be involved and the subsequent silyl migration is expected to give the same products. However,
these two reactions proceeded cleanly and selectively to form their own 1,2-syn-addition
products 21 and 23, respectively. These results ruled out the gem-addition mechanism and
further supported the previously established hydrosilylation mechanism, which is also consistent
with our calculations shown in Table 4 and highlights the importance of orbital interaction when
forming the gem-addition Ru-carbene intermediates.9c,10a
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12
13
14
15
16
17
18
19
20
21
22
23
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Scheme 6. Possibility of the Related gem-Hydrosilylation
A. Hydrosilylation of Alkynyl Boronate 20
Bpin
PhMe2Si
PhMe2Si
Bpin
H
PhMe2SiH
1,2-addition only
PhMe2Si
84% isolated
20
standard
conditions
17b
(Z/E = 4:1)
X
SiMe2Ph
Bpin
[Ru]
expected if
via gem-addition
but not observed
PhMe2Si
Bpin
H SiMe2Ph
(unlikely)
H
SiMe2Ph
17a
IM4
B. Hydrosilylation of Silyl Alkynes 1a and 22
nPent
Et3Si
SiMe2Ph
H
unlikely intermediate
[Ru]
SiMe2Ph
[CpRu(MeCN)3]PF6
(10 mol%)
+
HSiEt3
nPent
DCM, r.t., N2, 5 h
H
nPent
1a
21
(E only)
30% yield @ ~35% conv.
Et3Si SiMe2Ph
SiEt3
+
same
as above
nPent
SiEt3
H
IM7
common intermediate
if via gem-addition
PhMe2SiH
nPent
PhMe2Si
23
91% yield
22
=
(E/Z 14:1)
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Conclusion
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We have developed the a new migratory geminal hydroboration of alkynes. With the proper
choice of a [CpRu]-based catalyst, a range of silyl alkynes overcame the intrinsic 1,2-addition
propensity observed for general internal alkynes and reacted in a 1,1-addition mode with
concomitant silyl migration, providing efficient access to various vinyl boronates with excellent
stereoselectivity. These stereodefined boryl- and silyl-substituted olefin products have been
demonstrated as useful precursors toward other diversely substituted olefins with defined
configuration. The mild reaction conditions can tolerate a wide range of functional groups,
including free acid and alcohol, as proved by our scope study and additive compatibility study.
This process can also be successfully extended to germanyl alkynes. Mechanistically, unlike the
established Ru-catalyzed 1,2-trans-hydrometallation, this process features a new pathway
involving the key α-boryl-α-silyl Ru-carbene intermediate (Scheme S1). It is noteworthy that this
is the first demonstration of such gem-addition beyond hydrogenation. DFT (including molecular
dynamics) calculations and a series of control experiments provided important insights into
mechanistic understanding. The coplanar relationship between the boron atom and the
ruthenacyclopropene ring preceding boron migration enables an interaction between the empty
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p(B) orbital and the filled -type C–C orbital, which is the key bonding feature responsible for
the subsequent unusual reactivity. Control experiments with a range of substrates bearing
propargylic silyl group together with the cross-over experiments provided diagnostic information
toward the carbene pathway and ruled out the vinylidene and dissociative pathways. Finally,
although hydrosilylation of vinyl boronates provided access to the same type products, control
experiments indicated that these reactions proceed via normal 1,2-addition pathway, rather than
gem-addition followed by silyl migration. This study represents a new step forward not only for
alkyne hydroboration, but also for more general geminal additions of alkynes.
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ASSOCIATED CONTENT
7
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Supporting Information
9
The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.
Experimental and computational details and NMR data (PDF).
X-ray crystallography data (CIF)
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Movie of one successful trajectory (MP4).
AUTHOR INFORMATION
Corresponding Author
*Email to
J. S.: sunjw@ust.hk
Y.-D. W.: wuyd@pkusz.edu.cn
L. W. C.: oscarchung@sustech.edu.cn
Notes
The authors declare no competing financial interest.
Acknowledgment: We gratefully acknowledge the financial support from the Research Grants
Council of Hong Kong (GRF 16302719), National Natural Science Foundation of China
(21672096, 21873043, 21933003, 21933004, 91956114, 21572192) and Shenzhen Science and
Technology Innovation Committee (JCYJ20170412150507046, JCYJ20170817104736009), the
Shenzhen Nobel Prize Scientists Laboratory Project (C17783101), and Guangdong Provincial
Key Laboratory of Catalysis (2020B121201002). We thank Profs. Guochen Jia and Xinhao
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Zhang for helpful discussion and the Center for Computational Science and Engineering at the
Southern University of Science and Technology for partial support on this work.
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References
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