Rare-Earth-Catalyzed Selective 1,4-Hydrosilylation of Branched 1,3-Enynes Giving Tetrasubstituted Silylallenes

Article abstract of DOI:10.1021/jacs.1c04689

Allenes are versatile synthons in organic synthesis and medicinal chemistry because of their diverse reactivities. Catalytic 1,4-hydrosilylation of 1,3-enynes may present the straightforward strategy for synthesis of silylallenes. However, the transition-

Full text of DOI:10.1021/jacs.1c04689

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Rare-Earth-Catalyzed Selective 1,4-Hydrosilylation of Branched 1,3-
Enynes Giving Tetrasubstituted Silylallenes
Wufeng Chen, Chunhui Jiang, Jianying Zhang, Jiaqi Xu, Lin Xu, Xiufang Xu, Jianfeng Li,*
and Chunming Cui*
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ABSTRACT: Allenes are versatile synthons in organic synthesis and medicinal chemistry because of their diverse reactivities.
Catalytic 1,4-hydrosilylation of 1,3-enynes may present the straightforward strategy for synthesis of silylallenes. However, the
transition-metal-catalyzed reaction has not been successful due to poor selectivity and very limited substrate scopes. We report here
the efficient and selective 1,4-hydrosilylation of branched 1,3-enynes enabled by the ene-diamido rare-earth ate catalysts using both
alkyl and aryl hydrosilanes, leading to the exclusive formation of tetrasubstituted silylallenes. Deuteration reaction, kinetic study, and
DFT calculations were conducted to investigate the possible mechanism, revealing crucial roles of high Lewis acidity, large ionic
radius, and ate structure of the rare-earth catalysts.
Pd-catalyzed asymmetric 1,4-hydrosilylations of 1,3-enynes
that have been reported.3a,b,5 However, these precious-metal-
catalyzed reactions suffered from very limited substrate scopes.
Because allenes feature unique structures and diverse
reactivities and are valuable multifunctional synthons in
synthetic and medicinal chemistry,6 the development of
efficient and selective catalysts for 1,4-hydrosilylation of
enynes is highly desired.
Rare-earth-catalyzed hydrosilylation of alkenes and alkynes
exhibits unique reactivity and selectivity because of their high
Lewis acidity and large ionic radiis.7 In particular, we have
recently shown that ene-diamido rare-earth (RE) complexes
enabled highly regioselective hydrosilylation of internal alkenes
and alkynes.8 Stimulated by the high reactivity of these rare-
earth complexes, we are interested in the exploration of rare-
earth-catalyzed hydrosilylation of 1,3-enynes with the expect-
ation to realize some novel and selective transformations for
the synthesis of valuable synthons and organosilanes.
etal-catalyzed hydrosilylation of unsaturated organic
Mcompounds, especially alkenes and alkynes, represents
the most efficient and straightforward protocol for the
synthesis of various silyl-functionalized organic molecules,
which played important roles in organic and materials
chemistry.1 1,3-Enynes, readily available conjugated organic
compounds containing both alkene and alkyne moieties, are
basic building blocks in synthetic chemistry.2 However,
catalytic hydrosilylation of 1,3-enynes confronts the issues of
regio- and stereoselective control because there exist three
mainly competing pathways, namely, 1,2-, 4,3-, and 1,4-
hydrosilyation (Scheme 1).3 As a result, only a few cases of
transition-metal-catalyzed 4,3-hydrosilylation of 1,3-enynes for
the synthesis of silyldienes have been reported,4 while catalytic
1,4-hydrosilyation of 1,3-enynes to silylallenes has been met
with very little success. There were only a couple of Pt- and
Scheme 1. Catalytic Hydrosilylation of 1,3-Enynes
Herein, we report the employment of ene-diamido rare-earth
ate complexes for the successful catalytic 1,4-hydrosilylation of
branched 1,3-enynes, leading to the formation of tetrasub-
stituted silylallenes (Scheme 1). Mechanistic studies and DFT
calculations indicated that the high Lewis acidity, large ionic
radius, and unique ate structure are key factors for the
unprecedented transformation.
Catalytic hydrosilylation of 2-methyl-4-trimethylsilyl-1-bu-
tene-3-yne (7a) with nC6H13SiH3 (6a) was selected as model
reaction (Table 1). Initially, we investigated the ene-diamido
Received: May 5, 2021
Published: August 13, 2021
https://doi.org/10.1021/jacs.1c04689
J. Am. Chem. Soc. 2021, 143, 12913−12918
© 2021 American Chemical Society
12913

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Communication
a
hexane. In sharp contrast, almost no reaction was observed in
THF (entry 11) probably due to the interactions of the solvent
molecules with the lanthanum ion, which suppresses the
coordination−insertion process.
Table 1. Reaction Condition Optimization
Under the optimized conditions (Table 1, entry 9),
hydrosilylation of a range of branched 1,3-enynes 7a−r with
alkyl-substituted hydrosilanes nC6H13SiH3 6a and CySiH3 6b
was examined (Table 2). All of the branched enynes
a
Table 2. Hydrosilylation with Alkyl Hydrosilanes
b
entry
cat
solvent
time (h)
T (°C)
yield (%)
1
2
3
4
5
6
7
8
YC
1
3
2
4
5
2
KN
2
2
C6D6
C6D6
C6D6
C6D6
C6D6
C6D6
C6D6
C6D6
toluene
n-hexane
THF
12
12
12
4
4
4
8
24
8
8
24
80
80
80
rt
rt
rt
rt
80
rt
rt
80
<5
<5
<5
74
68
21
>99
29
>99
52
c
9
10
11
2
<5
a
nC6H13SiH3 6a (0.44 mmol), 2-methyl-4-trimethylsilyl-1-butene-3-
yne 7a (0.4 mmol), catalyst (0.02 mmol, 5 mol %), 0.6 mL of solvent.
b
1
Yields of 8a were determined by H NMR and GC-MS with the
c
crude mixture. KN = KN(SiMe3)2.
rare-earth alkyl and amide complexes LY(CH2SiMe3)(THF)2
(YC, L = ArNC(Me)C(Me)NAr, Ar = 2,6-iPr2C6H3, entry 1),9
LSm(CH2SiMe3)(THF)2 (1, entry 2),8b and LLa(N(SiMe3)2)-
(THF) (3, entry 3).8c However, only very low conversions
(<5% yield of geminal disilylallene 8a) have been observed in
12 h at 80 °C with these neutral catalysts. Prompted by our
recent successes in the hydrosilylation of internal alkynes with
rare-earth ate complexes, the ene-diamido lanthanum bis-
(amido) ate complex K[LLa(N(SiMe3)2)2] 2 (entry 4) was
employed. Indeed, 2 enabled highly selective hydrosilylation of
7a with 6a to give 1,4-hydrosilylation product 8a in high yield
(74%) in 4 h at room temperature without the contamination
of the other hydrosilylation products. The samarium and
yttrium ate complexes K[LSm(N(SiMe3)2)2] (4)8c and
K[LY(N(SiMe3)2)2] (5, see the Supporting Information
(SI)) were also examined (entries 5 and 6). Under same
reaction conditions, the samarium ate complex 4 gave 8a in
68% yield whereas the yttrium complex 5 only produced 8a in
21% yield. This activity trend appeared to be closely correlated
with the ionic radii of the elements: the larger the ionic radius
is, the higher the activity (La3+ > Sm3+ > Y3+). When the
reaction time was extended to 8 h (entry 7), the lanthanum ate
complex 2 gave 8a almost quantitatively. KN(SiMe3)2 (KN,
entry 8) is also active but only gave 8a in 29% yield at 80 °C in
24 h, and the reaction did not produce any more product after
24 h. These results indicated that the combination of a large
rare-earth ion with KN(SiMe3)2 forming ate structure is crucial
for the effective catalytic 1,4-hydrosilylation of enynes.
a
RSiH3 6a and 6b (R = nC6H13, R = cyclohexyl, 0.44 mmol), enynes
7a−r (0.4 mmol), catalyst 2 (0.02 mmol, 5 mol %), rt and 1 mL of
toluene. Isolated yield. 60 °C.
b
underwent selective 1,4-hydrosilylation to afford tetrasubsti-
tuted silylallenes 8a−r in excellent yields (83−94%), while the
other hydrosilylation products were not observable. When the
internal substituent R2 on the alkene moiety is alkyl, the
catalytic reaction could be applied to enynes containing various
silyl-substituted alkyne moieties (R1) to yield geminal
disilylallenes (8a−g); when the internal substituent R2 is
silyl, the reaction is viable for enynes containing alkyl-
substituted alkyne moieties (R1) to produce 1,3-disilylallenes
(8h−q). It is noteworthy that the 1,4-hydrosilylation under-
went smoothly in good yields for the enynes with sterically
demanding groups (86−87% yields of 8c, 8d, and 8m). The
Toluene (entry 9) and C6D6 are the superior solvents for the
hydrosilylation of the enyne, whereas the reaction in n-hexane
(entry 10) only resulted in the 1,4-hydrosilylation product in
52% yield probably because of the poor solubility of 2 in n-
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Communication
hydrosilylation of enynes with siloxyl and N-heterocyclic
substituents (8p and 8q) could also achieve good yields in 16
and 8 h. The enynes with chloro and hydroxyl groups were not
compatible with this reaction.
indicating the 1,4-hydrosilylation by D and RSiD2 groups,
1
2
which was strongly supported by the H, H, and 13C NMR
spectra. The synthetic potentials of silylallenes was demon-
strated by the desilylation of 8o with TBAF (Scheme 2), which
afforded 1,3-disubstituted allene 11 in 90% yield.13
The substituent effects of 1,3-enynes on the hydrosilylation
selectivity have been further examined with alkyl substituted
enynes and terminal enynes. 1,3-Enynes with t-butyl and
cyclohexyl groups could also been successfully hydrosilyated to
give 1,4-hydrosilylation products (8r and 8s, 91 and 92%
yields) in 12 and 6 h. However, n-hexyl (R1) substituted 1,3-
enynes (R2 = H, Me) exhibited poor regioselectivity with the
formation of 1,4- and 4,3-hydrosilylation products (Table S1 in
the SI). It is very likely that the steric effects of R1 group could
efficiently inhibit the 4,3-insertion but favor 1,4-selectivity.10 It
is noted that 8n was generated selectively from enynes with R1
= n-hexyl and R2 = silyl groups because of the inductive effects
of the silyl group on the alkene moiety, which could result in
high electron density on the silyl carbon atom and facilitate the
1,2-insertion.8c,11 However, with 1,3-enynes of R1 = aryl and R2
= silyl groups, the reactions only yielded polymeric materials
and hydrosilylation products could not be observed.8a,12
Aryl-substituted hydrosilanes could also be applied to the
catalytic reaction (Table 3) with satisfactory yields (81−90%)
Scheme 2. Derivatization of Silylallene
The reaction of lanthanum ate complex 2 with silane 6 was
reported to yield lanthanum hydride K[L((Me3Si)2N)La(μ-
H)2La(H)(THF)L]K.8c In contrast, the neutral complex 3 is
inactive to the hydrosilane. These results indicated that the
negative charge in anionic [LLa(N(SiMe3)2)2]− of ate complex
2 could facilitate the σ-bond metathesis of 2 with hydrosilane
to form active rare-earth hydride intermediate (Figure
1a).8,14,15 The catalytic cycle may initiate with the selective
1,2-insertion of enyne 7 into the lanthanum hydride A, giving
the η3-propargyl/allenyl lanthanum intermediate B, similar to
the Cu and Mg hydride-catalyzed 1,4-hydroelementation of
1,3-enynes.10 After that, σ-bond metathesis of B with
hydrosilane 6 led to the exclusive formation of silylallene 8
and regeneration of the active hydride A.
a
Table 3. Hydrosilylation with Aryl Hydrosilanes
To gain further support for the proposed mechanism, DFT
calculations were carried out. First, the LUMO of the 1,3-
enyne 7g was calculated using frontier orbital population
analysis (Figure S1 in the SI). The coefficients at the C1 and
C2 (0.40, 0.31) atoms of the alkene moiety are larger than
those at the C4 and C3 (0.27, 0.13) atoms of the alkyne
moiety, which explains, to a certain degree, the preference for
initial 1,2-insertion rather than 4,3-insertion.16 To further
understand the mechanism, the calculations were also
performed on the 1,4-hydrosilylation of 1,3-enyne 7a with
MeSiH3 using [LLa(H)(N(SiMe3)2]− as the catalyst (Figure
1b). It is predicted that the hydrosilylation was initiated by the
facile 1,2-insertion of an enyne via the four-centered TS1
(Figure 1c) with the energy barrier of 21.1 kcal/mol, affording
the η3-propargyl/allenyl lanthanum intermediate B. The 4,3-
insertion showed higher activation energy (ΔΔG⧧ = 0.4 kcal/
mol) than that of 1,2-insertion because of the strong steric
repulsion between substituents on the internal alkyne moiety
and bulky ene-diamido ligand in TS1′ (Figure S2 in the SI).
Meanwhile, the calculations suggest that propargyl/allenyl
intermediate B from 1,2-insertion is more stable (ΔΔG = −4.7
kcal/mol) than dienyl intermediate B′ from 4,3-insertion. It is
worth noting that the η1-propargyl lanthanum species has not
been predicted on the basis of 1,2-insertion, which is probably
caused by the large lanthanum ion that could favor the η3-
fashion.17 In fact, the short distance of La−C4 (2.67 Å)
compared to that of La−C2 (3.37 Å), along with the almost
linear C2−C3−C4 angle of 171.9°, suggests that the geometry
of intermediate B lies between η3-propargyl/allenyl and η1-
allenyl structures. Finally, intermediate B underwent σ-bond
metathesis with hydrosilane via the TS2 to give the silylallene
with the regeneration of the hydride intermediate. This σ-bond
metathesis step requires a higher energy barrier (24.1 kcal/
mol) and thus becomes the rate-limiting step of the overall
reaction.
a
ArSiH3 6c−j (0.44 mmol), 1,3-enyne 7a, 7g, and 7m (0.4 mmol),
catalyst 2 (0.02 mmol, 5 mol %), rt and 1 mL of toluene. Isolated
yield. 60 °C.
b
and excellent regioselectivity. The catalytic protocol is
tolerated to some functional groups (chloro 8v and alkoxy
8w). In addition, the bis(allene) 8y can be obtained. It is noted
that the substituents of hydrosilanes have significant effect on
the catalytic activity: aryl hydrosilanes (phenyl 8t) are more
active than alkyl hydrosilanes (n-hexyl 8a and cyclohexyl 8b),
and bulky hydrosilanes are less active (naphthyl 8x). To our
delight, the 1,4-hydrosilylation with the secondary silane
Ph2SiH2 proceeded successfully to give the expected product
9 in 84% yield. The hydrosilylation with PhSiD3 yielded the
deuteration 1,3-disilylallenes (10) exclusively (90% yield),
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Communication
Figure 1. (a) Proposed mechanism. (b) Gibbs free-energy profile. (c) Calculated structures of TS1, B, and TS2.
Kinetic studies on 1,4-hydrosilylation were carried out
(Table S2 and Figure S3 in the SI) which indicated that the
rate law was of first order dependence on the hydrosilane and
enyne.8c The results suggested that the reaction rate was
influenced by both substrates. It is probably attributed to the
steric hindrance of the branched 1,3-enynes, which leads to the
intermediate B with bulky η3-propargyl/allenyl features having
a relatively slow σ-bond metathesis reaction with hydrosilane.
In summary, we have disclosed that the lanthanum ate
complex 2 enabled highly efficient and selective 1,4-hydro-
silylation of branched 1,3-enynes with various primary and
secondary hydrosilanes to yield tetrasubstituted silylallenes.
The high activity for sterically demanding branched enynes
could be attributed to the high Lewis acidity and large ionic
radius of lanthanum ion as well as the highly reactive ate
hydride intermediate. Further applications of ene-diamido rare-
earth ate catalysts in hydroelementation of bulky unsaturated
molecules are in progress.
Chunhui Jiang − State Key Laboratory of Elemento-Organic
Chemistry and College of Chemistry, Nankai University,
Tianjin 300071, People’s Republic of China
Jianying Zhang − State Key Laboratory of Elemento-Organic
Chemistry and College of Chemistry, Nankai University,
Tianjin 300071, People’s Republic of China; orcid.org/
0000-0001-6119-2497
Jiaqi Xu − State Key Laboratory of Elemento-Organic
Chemistry and College of Chemistry, Nankai University,
Tianjin 300071, People’s Republic of China
Lin Xu − State Key Laboratory of Elemento-Organic
Chemistry and College of Chemistry, Nankai University,
Tianjin 300071, People’s Republic of China
Xiufang Xu − State Key Laboratory of Elemento-Organic
Chemistry and College of Chemistry, Nankai University,
Tianjin 300071, People’s Republic of China; orcid.org/
0000-0002-3510-3267
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.1c04689
ASSOCIATED CONTENT
■
sı
* Supporting Information
Notes
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.1c04689.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Experimental procedures, DFT calculations, and spec-
troscopic data (PDF)
■
We are grateful to the National Natural Science Foundation of
China (Grant No. 21890722, 22071123 and 21632006) for
financial support. Dedicated to the 100th Anniversary of
Chemistry at Nankai University.
AUTHOR INFORMATION
Corresponding Authors
■
Jianfeng Li − State Key Laboratory of Elemento-Organic
Chemistry and College of Chemistry, Nankai University,
Tianjin 300071, People’s Republic of China; orcid.org/
0000-0003-0996-1679; Email: lijf@nankai.edu.cn
Chunming Cui − State Key Laboratory of Elemento-Organic
Chemistry and College of Chemistry, Nankai University,
Tianjin 300071, People’s Republic of China; orcid.org/
0000-0002-1493-7596; Email: cmcui@nankai.edu.cn
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