Dichotomy of Manganese Catalysis via Organometallic or Radical Mechanism: Stereodivergent Hydrosilylation of Alkynes

Article abstract of DOI:10.1002/anie.201710206

Herein, we disclose the first manganese-catalyzed hydrosilylation of alkynes featuring diverse selectivities. The highly selective formation of E-products was achieved by using mononuclear MnBr(CO)₅ with the arsenic ligand, AsPh₃. Whereas using the dinuclear catalyst Mn₂(CO)₁₀ and LPO (dilauroyl peroxide) enabled the reversed generation of Z-products in good to excellent stereo- and regioselectivity. Such a way of controlling the reaction stereoselectivity is unprecedented. Mechanistic experiments revealed the dichotomy of manganese catalysis via organometallic and radical pathways operating in the E- and Z-selective routes, respectively.

Full text of DOI:10.1002/anie.201710206

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Accepted Article
Title: Dichotomy of Manganese Catalysis via Organometallic or Radical
Mechanism: Stereodivergent Hydrosilylation of Alkynes
Authors: Congyang Wang and Xiaoxu Yang
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10.1002/anie.201710206
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Dichotomy of Manganese Catalysis via Organometallic or Radical
Mechanism: Stereodivergent Hydrosilylation of Alkynes
Xiaoxu Yang and Congyang Wang*
Abstract: Herein, we disclose the first manganese-catalyzed
hydrosilylation of alkynes featuring diverse selectivities. The highly
selective formation of E-products was achieved by using
mononuclear MnBr(CO)5 with the arsenic ligand, AsPh3. On the
contrary, the dinuclear catalyst Mn2(CO)10 together with LPO
(dilauroyl peroxide) enabled the reversed generation of Z-products in
good to excellent stereo- and regioselectivity. Such a way of
controlling the reaction stereoselectivity is unprecedented and unique.
Mechanistic experiments revealed the dichotomy of manganese
catalysis via organometallic and radical pathways operating in the E-
and Z-selective protocols respectively.
Hydrosilylation provides atom-economic methods for
accessing organosilicon compounds and has wide applications in
synthesis of organic molecules and functional materials.[1]
Transition-metal-catalyzed hydrosilylation reactions have made
great success over the past few decades.[2] In comparison with
Ru, Rh, Pt, and others, manganese-catalyzed hydrosilylation has
largely lagged behind (Scheme 1a). As early as in 1982, Yates,
for the first time, reported Mn2(CO)10-catalyzed hydrosilylation of
acetone, albeit only in 5% GC yield.[3] Later in 1990s, Cutler et al.
discovered
a range of acyl-manganese-complex catalyzed
hydrosilylation of carbonyls including organoiron acyl complexes,
ketones, and esters.[4] Since 2012, Pannell, Sortais, Darcel,
Lugan and others have demonstrated hydrosilylation of DMF,
carboxylic acids, aldehydes and ketones catalyzed mainly by
CpMnL(CO)2 (L = ligand) complex under photoirradiation.[5]
Recently, Du, Trovitch and Huang have achieved pincer-type
manganese catalyzed hydrosilylation of aldehydes, ketones, and
esters.[6 Of note, most manganese-catalyzed hydrosilylation has
been focused on the reduction of carbonyls so far, while only
sporadic substrates of terminal olefin was preliminarily tried for
hydrosilylation of C-C unsaturated bonds.[7]
Silyl-substituted alkenes are highly attractive building blocks
owing to their non-toxicity, stability, ease of handling and diverse
transformations. Among all the synthetic routes to
silyl-substituted alkenes, hydrosilylation of alkynes is
undoubtedly the most straightforward approach with 100% atom
economy. However, manganese-catalyzed hydrosilylation of
alkynes has remained elusive till now. We envision that the
underlying challenges have to be met for harnessing such a
process: (i) the lower polarity of the C-C triple bond compared to
the C=O bond of carbonyls; (ii) the control of stereo- and regio-
Scheme 1. Mn-catalyzed hydrosilylation of unsaturated molecules.
selectivity for unsymmetrical alkynes; (iii) the undesired formation
of hydrogenation byproducts. With these regard and as our
continous interest on earth-abundant manganese catalysis,[8,9]
herein
we
disclosed
the
first
manganese-catalyzed
hydrosilylation of alkynes with high stereo- and regioselectivity
(Scheme 1b). Remarkably, the catalytic use of mononuclear
MnBr(CO)5 with the ligand of AsPh3 could deliver the E-products
in excellent stereoselectivity. On the contrary, dinuclear
Mn2(CO)10 with LPO enabled the formation of Z-products in a
reversed stereoselectivity. Such a control on stereodivergency
highlights an unique feature of manganese catalysis, namely
dichotomy of organometallic and radical mechanism.
At the outset, diphenylacetylene 1a and phenylsilane 2a
were chosen as model substrates to screen reaction parameters
(Table 1).[10] The hydrosilylation products 3aa and 4aa were
formed in low yields with poor E/Z selectivity when 1a was
treated with 2a in the presence of Mn(CO)5Br (7) (entry 1). Of
note, the hydrogenation proucts 5 and 6 were also detected in
noticeable yields. To our delight, the addition of PPh3
dramatically increased the E/Z ratio to 15:1 (entry 2). The
preformed Mn(CO)4(PPh3)Br (8) complex showed a slightly
decreased selectivity (entry 3). Other ligands had inhibitory
effects on the reaction (entries 4-6).[10] Further variations on the
reaction concentration and ratio of substrates gave higher yields
of products, albeit with worse chemo- and stereoselectivity (entry
7). Gratifyingly, the use of triphenylarsine and triphenylantimony
as ligands led to clear increasements in both the chemo- and the
[*]
X. Yang, Prof. C. Wang
Beijing National Laboratory for Molecular Sciences, CAS Key
Laboratory of Molecular Recognition and Function, CAS
Research/Education Center for Excellence in Molecular Sciences,
Institute of Chemistry, Chinese Academy of Sciences, Beijing,
100190, China
E-mail: wangcy@iccas.ac.cn
Homepage: http://wangcy.iccas.ac.cn/
X. Yang, Prof. C. Wang
University of Chinese Academy of Sciences, Beijing 100049, China
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201710206
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COMMUNICATION
E/Z selectivity (entries 8-10). The optimized conditions were
Importantly, the industrially relavent HSi(OEt)3 and HSiMe(OEt)2
were successfully applicable to this protocol (3ah, 3ai).
Suprisingly, the Z-configured product was formed predominantly
when Et3SiH was used in the reaction despite the underlying
reason remaining unclear (4aj). Variations on the electronic
property of alkynes had no obvious effect on the reaction
outcome (3bb-hb). It is noteworthy that susceptible fluoro, chloro
and bromo groups could be preserved after reaction, which
provides handles for further synthetic elaborations (3cb-eb). The
existence of meta- and ortho-substituents on the phenyl moiety
of alkynes was well tolerated with the later showing enhanced
E-selectivity (3ib-kb). Phenyl alkyl alkynes were also amenable
to the reaction conditions leading to 3lb-nb in excellent E/Z
selectivity and moderate to good regioselectivity, the later of
which seemed depending on the steric properties of substituents
of the alkyne. The synthetic potentials of the hydrosilylation
products were illustrated by derivatization reactions of 3ab.[10]
Surprisingly, the E/Z selectivity was reversed when the
dinuclear manganese catalyst, Mn2(CO)10 (13), was used in the
reaction of 1a with 2a (Table 2, entries 1,2). The addition of PPh3
favored the E-product again though the selectivity was lower
than that using the mono-nuclear MnBr(CO)5 catalyst (entry 3),
which highlights the importance of manganese/ligand
cooperation in controling the E-selectivity. We envisioned that
the homolysis of the Mn-Mn bond in Mn2(CO)10 might occur in
the reaction. Therefore, radical promoters were tested and,
luckily, the use of a catalytic amount of dilauroyl peroxide (LPO)
delivered the desired Z-product in enhanced yields (entries 4-6).
Further variations on silanes showed that better Z-selectivity was
obtained with more sterically bulky silanes used (entries 7-10). In
view of both the yield and the selectivity, parameters of entry 9
were chosen as the optimized conditions for Z–selective
hydrosilylation.
defined as shown in entry 8 considering the ligand econony.
Table 1. Optimization of E-selective hydrosilylation[a,b,c]
Cat.
(5 mol%)
Ligand
(20 mol%)
Entry
5
6
3aa
4aa
E/Z
1
2
7
7
--
PPh3
6
3
5
3
6
3
1
1
8
2
4
2
14
31
29
1
11
2
1.3:1
15:1
10:1
--
3
8
7
--
6
3
4
PCy3
3
<1
--
7
7
7
7
7
7
5
dppe
1
1
--
6
2,2‘-bipyridine
PPh3
1
1
<1
7
--
7[d]
8[d]
9[d]
10[d,e]
12
2
61
70
76
85
8:1
AsPh3
SbPh3
AsPh3
3
25:1
19:1
43:1
5
4
3
2
[a] Reaction conditions unless otherwise noted: 1a (0.2 mmol), 2a (0.2 mmol),
catalyst (0.01 mmol), ligand (0.04 mmol), 150 oC, toluene (0.2 M), 12 h. [b]
Yields and E/Z were determined by GC-MS. [c] 7 = Mn(CO)5Br, 8 =
Mn(CO)4(PPh3)Br. [d] Toluene (2 M), 1a:2a = 1:1.5. [e] AsPh3 (50 mol%).
Table 2. Optimization of Z-selective hydrosilylation[a,b]
Additive
(20 mol%)
Entry
Silane
5
6
3
4
E/Z
1[c,d]
PhSiH3
PhSiH3
--
13
8
5
2
5
1
1
1
1
1
9
6
7
3
3
2
1
2
1
3
13
16
29
19
18
19
15
13
8
41
60
11
58
68
77
81
67
82
25
1:3
1:4
2
--
3[c,d]
4[d]
5[d]
6
PhSiH3
PPh3
AIBN
LPO
LPO
LPO
LPO
LPO
LPO
3:1
PhSiH3
1:3
PhSiH3
1:4
PhSiH3
1:4
7
Ph2SiH2
PhMe2SiH
Ph2MeSiH
(TMS)3SiH
1:5
8
1:5
9
1:10
1:30
Scheme 2. Mn-catalyzed E-selective hydrosilylation of alkynes. Reaction
conditions: 1 (0.5 mmol), 2 (0.75 mmol), Mn(CO)5Br (5 mol%), AsPh3 (20
mol%), toluene (2 M), 150 oC, 10 h. Combined isolated yield was shown. E/Z
ratio was determined by GC-MS. r.r. = regioisomeric ratio. [a] AsPh3 (50 mol%).
[b] No AsPh3.
10
1
[a] Reaction conditions unless otherwise noted: 1a (0.2 mmol), 2 (0.4 mmol),
Mn2(CO)10 (0.02 mmol), additive (0.04 mmol), decalin (0.2 M), 120 oC, 10 h. [b]
Yields and E/Z were determined by GC-MS. [c] Toluene (0.2 M). [d] 1a:2a =
1:1. AIBN = azobisisobutyronitrile, LPO = dilauroyl peroxide, (n-C11H23COO)2.
Then, the substrate scope for E-selective hydrosilylation of
alkynes was investigated (Scheme 2). A wide range of mono-, di-,
and tri-substituted aromatic or aliphatic silanes was successfully
applied to this protocol giving the E-configured products in high
yields with good to excellent stereoselectivity (3aa-g).
Next, the generality of Z-selective hydrosilylation was tested
(Scheme 3). Internal alkynes bearing varied substituents are
suitable for this reaction, thus leading to the Z-products in good
yields and high Z/E selectivity (4af-of). For aryl alkyl alkynes, not
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only good to excellent stereoselectivity but also very high
regioselecitvity were achieved favoring products with the silyl
group placed adjacent to the alkyl moiety (4lf, 4of). Remarkably,
a series of functional groups on the phenyl moiety of terminal
alkynes including delicate halogens were amenable to this
protocol affording the Z-products in good yields and excellent
selectivity (4pf-vf). 3-Ethylylthiophene was also compatible to the
reaction conditions giving 4wf smoothly with high level of control
on stereoselectivity.
To test whether the Mn-Si species was also a key
intermediate for the dinuclear Mn2(CO)10-catalyzed Z-selective
hydrosilylation, the Mn-Si complex 23 was prepared from
Mn2(CO)10 and silane 2b in 62% isolated yield (Scheme 5a).
Then a reaction of 23 and alkyne 1a was examined, however,
only a negligible amount of 3ab were detected (Scheme 5b),
which excludes 23 as a possible intermediate in Z-selective
Scheme 3. Mn-catalyzed Z-selective hydrosilylation of alkynes. Reaction
conditions: 1 (0.5 mmol), 2f (1.0 mmol), Mn2(CO)10 (10 mol%), LPO (20 mol%),
decalin (0.2 M), 120 oC, 10 h. Combined isolated yield was shown. E/Z ratio
and regioisomeric ratio (r.r.) were detected by GC-MS.
In order to probe the possible mechanism of mononuclear
Mn(CO)5Br-catalyzed E-selective hydrosilylation reaction,
a
series of experiments was carried out. Firstly, HMn(CO)4(L) and
Mn(CO)4(L)(SiHPh2) species were assumed as the key reaction
intermediates. Therefore, two representive manganese
complexes 14 and 15 using PPh3 as a ligand were prepared
respectively (Scheme 4a). The structure of 15 was
unambiguously confirmed by single-crystal X-ray analysis.[11]
Next, we examined the stoichiometric reactions of alkyne 1a
and silane 2b with the Mn-H species 14 and Mn-Si species 15
respectively (Scheme 4b). The hydrogenation products, stilbenes
5 and 6, were detected as the major ones (60% GC-MS yield
together), which suggested the preferred formation of
hydrogenation products than hydrosilyation products from
alkenyl-Mn species 17, which was generated from 14 and 1a
through intermediate 16. Consequently, the hydrogenation
products 5 and 6 should be the major products if the Mn-H
species 14 is the active species in the catalytic reaction, which is
contrary to our experimental results (Table 1, entry 2). On the
other hand, the Mn-Si species 15 reacted with 1a and 2b
affording the desired hydrosilylation products 3ab and 4ab in
excellent E/Z selectivity with the hydrogenation products 5 and 6
nicely repressed. It clearly indicated that the Mn-Si rather than
Mn-H species was the active intermediate in Mn-catalyzed
Scheme 4. Mechanistic studies on E-selective hydrosilylation. [a] Isolated
yields. [b] GC-MS yields.
E-selective hydrosilylation reaction. Therefore,
a plausible
mechanism was depicted in Scheme 4c. The reaction starts with
a ligand replacement and silylation giving the Mn-Si species 18.
It undergoes a CO-alkyne exchange (19) and insertion of alkyne
into the Mn-Si bond giving a (β-silyl)alkenyl-Mn intermediate 20.
The coordination and σ-bond metathesis of 20 with silane via 21
leads to product 3 and regenerates the Mn-Si species 18, thus
closing the catalytic cycle (path A). Alternatively, species 21
could release product 3 directly affording intermediate 22, which
reacted with alkyne 1 to regenerate complex 19 (path B).
Scheme 5. Mechanistic studies on Z-selective hydrosilylation. [a] Isolated
yields. [b] GC-MS yields.
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might operate in the reaction based on the fact that the radical
initiator, LPO, could promote the reaction. Therefore, a radical
scavenger, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), was
added and it completely inhibited the reaction (Scheme 5c).
Besides, it was reported that (CO)5Mn· was generated via
homolysis of Mn2(CO)10 under UV irradiation.[12] As such, the Z
hydrosilylation reaction was tested without LPO under 380 nm
UV irradiation at room temperature and the hydrosilylation
products were formed in 75% yield with consistent Z-selectivity
(Scheme 5d). In view of these results, a radical mechanism was
shown in Scheme 5e. The homolysis of Mn2(CO)10 occurs to
generate (CO)5Mn·, which reacts with silane giving a silyl radical
and HMn(CO)5. The silyl radical adds to the alkyne to afford E-
and Z-configured alkenyl radicals, which undergo hydrogenolysis
with HMn(CO)5 to form the desired product 4 and regenerate the
(CO)5Mn· species. The hydrogenolysis of the Z-alkenyl radical
was preferred presumably due to the steric hindrance of the
diphenylmethylsilyl group.[13]
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In summary, the first manganese-catalyzed hydrosilylation
of alkynes is developed, which is highlighted by divergent E/Z
stereoselectivity. The cooperative use of the mononuclear
MnBr(CO)5 and the AsPh3 ligand enables highly selective
formation of E-products while the catalytic combination of
dinuclear Mn2(CO)10 with LPO delivers the Z-products in good to
excellent stereo- and regioselectivity. Mechanistically, the herein
observed dichotomy of manganese catalysis through either
organometallic or radical pathways is unique and holds great
potentials for developing new synthetic transformations, which
are underway in our laboratory.
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Acknowledgements
Financial support from NSFC (21472194, 21772202, 21521002)
are gratefully acknowledged. We also thank the Alexander von
Humboldt Foundation for the Equipment Subsidy (GC-MS).
Keywords: hydrosilylation • alkynes • manganese • selectivity •
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Dichotomy of Manganese Catalysis
via Organometallic or Radical
Mechanism: Stereodivergent
Hydrosilylation of Alkynes
Dichotomy of Man(ganese): The first manganese-catalyzed hydrosilylation of
alkynes featuring diverse selectivities is developed. Highly selective formation of
E-products was achieved by using mononuclear MnBr(CO)5 with AsPh3. On the
contrary, the dinuclear catalyst Mn2(CO)10 together with LPO (dilauroyl peroxide)
enabled the reversed generation of Z-products. Mechanistic experiments revealed
the dichotomy of manganese catalysis via organometallic and radical pathways.
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