[(NHC)NiIIH]-Catalyzed Cross-Hydroalkenylation of Cyclopropenes with Alkynes: Cyclopentadiene Synthesis by [(NHC)NiII]-Assisted C?C Rearrangement

Article abstract of DOI:10.1002/anie.201901255

A cross-hydroalkenylation/rearrangement cascade (HARC), using a cyclopropene and alkyne as substrate pairs, was achieved for the first time by using new [(NHC)Ni(allyl)]BAr^F catalysts (NHC=N-heterocyclic carbenes). By controlling the (NHC)Ni^IIH relative insertion reactivity with cyclopropene and alkyne, a broad scope of cyclopentadienes was obtained with highly selectively. The structural features of the new (NHC)Ni^II catalyst were important for the success of the reaction. The mild reaction conditions employed may serve as an entry for exploring (NHC)Ni^II-assisted vinylcyclopropane rearrangement reactivity.

Full text of DOI:10.1002/anie.201901255

A Journal of the Gesellschaft Deutscher Chemiker
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International Edition Chemie
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Accepted Article
Title: (NHC)NiIIH-Catalyzed Cross-Hydroalkenylation of Cyclopropene
with Alkyne: A Straight Forward Cyclopentadiene Synthesis by
NHC-NiII Assisted C-C Rearrangement.
Authors: Jian-Qiang Huang and Chun Yu Ho
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201901255
Angew. Chem. 10.1002/ange.201901255
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COMMUNICATION
II
(
NHC)Ni H-Catalyzed Cross-Hydroalkenylation of Cyclopropene
with Alkyne: A Straight Forward Cyclopentadiene Synthesis by
II
NHC-Ni Assisted C-C Rearrangement.
[
a,b]
[a]
Jian–Qiang Huang , Chun–Yu Ho*
Abstract: Cross-hydroalkenylation-rearrangement cascade (HARC)
that using cyclopropene 1 and alkyne 2 as substrate pairs was first
cyclopentadienes 3 rather than gem-olefins. Instead of using
oxidative addition condition for 3-member ring opening by Ni(0)
in Scheme 1c, the CC bond cleavage was assisted possibly by
F
achieved by using a set of new [NHC-Ni(allyl)]BAr catalysts. By
II
II
controlling the NHC-Ni H relative insertion reactivity with 1 and 2, a
NHC-Ni in a subsequent rearrangement at mild condition. In
broad scope of cyclopentadiene 3 was obtained highly selectively for
sharp contrast to the typical 1 and 2 reactivity with metal hydride
as described in Scheme 1b, chemoselective insertion of 1 and 2
was achieved, where a Pd-cyclopropyl type of rearrangement
was avoided and two new CC bonds were formed at
II
the first time. The new NHC-Ni catalyst structural features were
found as the keys for the success. The mild condition employed may
II
serve as an entry for exploring NHC-Ni assisted vinylcyclopropane
rearrangement reactivity.
1 3
cyclopropene C and C -positions. When compared to the other
transition metal catalyst systems that rely on vinyl metal
[
10]
carbenoids
in Scheme 1a, unparalleled reactivity and new
Cyclopropene 1 is an important building block in organic
synthesis, significant advances for its synthesis have been made
recently by new transition metal catalysts and innovations. Other
scope were observed. The above, together with labelling
II
experiments, suggested the NHC-Ni -cyclopropanyl and the later
II
on NHC-Ni -vinylcyclopropenyl species might be the key
[
1]
[2]
[3]
than the use of traditional methods, catalysts like Au , Rh,
possible intermediates that account for the observations.
[
4]
[5]
[6]
[7]
Cu, Zn Pt and Pd are particularly useful in ring opening
[
8]
reactions of 1. The intermediates, such as the reactive vinyl
metal carbenoids and the cyclopropane ions, were often trapped
by other nearby functional groups on the skeleton or external
nucleophiles (Scheme 1a). That provides a general route to
substituted furans, pyrans and acyclic products. Transition metal
R2
R2
R1 R2
Cat.
Transition
Metals
R1
R1
various
products
(a)
-
or -
M
M
1
vinyl metal carbenoid like mode of activation
[
9]
hydride catalysts or their equivalents could potentially be one
of the most ideal approaches for cyclopropene functionalization
too, since it could insert to the olefin and allow subsequent
insertions of various reaction partners easily. However, their use
R1 R2
Cat. M-H
i) oligomerization/polymerization
2
(b)
1
2
R
R
R
ii)
R1
1
[
7]
NuPd
H
Nu
R2
in selective transformation of 1 is reported rarely. A number of
other possible reactivity of 1 with transition metal hydride,
leading to unfavorable 1 consumption, have severely limited the
progress of the cross-reaction development. With either too high
insertion activity of 1 or competing pathways from other reaction
partners such as alkyne 2, the non-selective oligomerization or
homo-reaction reactivity is often dominated (Scheme 1bi).
Moreover, other possible rearrangements were reported as one
of the possible pathways for allyl-Nu formation, and that made
the hydride approach even more complicated (Scheme 1bii). As
a consequence, highly selective cross-transformations of 1 by
Oxidative
addition
Cat. Ni(0)
R1
R1 R2
1R2
R
Ni
(c)
Intramol.
This work:
R1 R2
R1
Cat.
R2
R1 R [NHC-Ni(allyl)]BArF
2
R3
R4
R3
R4
(d)
H
NHC Ni
Intermol.
R3
1
R4
2
3
[2b, 3d, 4, 10]
Scheme 1. Functionalized cyclopentadiene synthesis.
transition metal hydride catalyst and external -systems
remains elusive. Oxidative addition followed by rearrangement
or trapping is still dominating the field in Ni catalyzed 3-member
[
11] [12]
Our study commenced with cyclopropene 1a and terminal
ring opening (Scheme 1c).
,
F
alkyne 2a by using a sterically bulky [IPr-Ni(allyl)]BAr ] catalyst
Herein, a cross-hydroalkenylation-rearrangement cascade
[
15]
[13a]
reported in diene cycloisomerization and hydroalkenylation
(
HARC) of 1 and 2 was first achieved by using a new set of
F
with octene (Table 1). It is because we suspected that 1) the use
[NHC-Ni(allyl)]BAr catalyst (Scheme 1d). Unlike the cross-
[
9b, 13],[14]
of sterically bulky NHC might mitigate the high reactive of 1 and
hydroalkenylation of two different alkenes,
it provides a
II
2
towards Ni H and provide chemoselective insertion; and 2) a
highly efficient access to synthetically valuable, functionalized
possibly more selective insertion might let the HARC under
[
16][11b, 17]
F
control. While the use of either Ni(0)
or Ni(allyl)BAr
[
[
a]
b]
Dr. J.-Q. Huang, Prof. Dr. C.-Y. Ho*
favored either nonselective oligomerization or dimerization
(entries 10-12), 1a and 2a were converted into the desired
cyclopentadiene 3aa regioselectively and highly atom-
Shenzhen Grubbs Institute, Department of Chemistry, Southern
University of Science and Technology (SUSTech), China
E-mail: jasonhcy@sustc.edu.cn
Department of Chemistry and Molecular Sciences, Wuhan
University, China
F
economically in the presence of [IPr-Ni(allyl)]BAr ] catalyst for
[
11a, 11b]
the first time (entry 2).
This HARC was done simply at low
catalyst loading, at ambient condition and without adding
stoichiometric amount of additive. A slightly higher alkyne to
Supporting information for this article is given via a link at the end of
the document.
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Angewandte Chemie International Edition
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COMMUNICATION
cyclopropene ratio was proved effective to suppress undesired
a low terminal alkene isomerization (1-:2-alkene ratio) was
II
1
a consumption (entry 2), and the choice of ligand structure was
observed, which was deemed facile under Ni H conditions (Set 6,
[
22]
found very important. Among the NHCs examined (Figure 1), 6-
SIPr (L6) and IPent (L4) provided the best result while the IMes
and 7-SIPr provided the worst (entries 1-7), thus indicating that
the desired reactivity was brought by optimal NHC steric
bulkiness, either by the size of NHC core or N-aryl substituents.
The effect of steric increment method choice was dramatic only
when a linear unsubstituted terminal alkyne 1b was used. At
here, moderate yield of 3ab was obtained even when a large
excess amount of 2b (6 equiv) was used by having L6 as ligand,
Z = O, NTs).
That was attributed to the choice of NHC
employed, where the use of former hydroalkenylation catalyst
with L2 suffered from isomerization more than that of L6 in Z =
NTs. Interestingly, conjugated enyne was found also compatible
to the system, which allowed the synthesis of triene (Set 6).
Overall, L6 was found suitable for structurally flexible substrates
(e.g. 1 has mixed small alkyl and aryl, 2 has terminal propargylic
X), while L4 was found more suitable for substrates with lower
structural flexibility (e.g. 1 & 2 have bulky substituents, like Y =
F
1
1
2
[23]
however, just 1:2 substrate ratio at 1 mol% [L4-Ni(allyl)]BAr
Ph; R = iPr; R = R = Ph or spiroskeleton; and internal 2).
catalyst could provide up to 82% yield (entries 8 and 9).
II
[a]
Table 1 Screening NHC for desired Ni H, 1 and 2 relative insertion reactivity.
[
b]
2
Condition NHC =
IMes (L1)
Yield (3)
Yield
50%
10%
15%
<1%
15%
<1%
30%
30%
<1%
1
2
23%
[
c]
[d]
IPr (L2) 56%; 35%; 61%
Cl
Figure 1. NHC structures employed in this work.
3
IPr (L3)
IPent (L4)
SIPr (L5)
64%
88%; 76%
63%
[
e]
4
5
2a
II
The fine-tuned Ni H reactivity with 1, which was brought by
NHC optimization, came with also a broad scope of 2 (Table 2).
It provided a new general access to functionalized 3 in one step
simply at mild reaction condition (30 C), low substrate ratio (as
6
7
6‐SIPr (L6)
7‐SIPr (L7)
6‐SIPr (L6)
IPent (L4)
82%
[
f]
o
25% (60%)
[
h]
low as equimolar) and low catalyst loading (1-4 mol%). It
complements to a few shortcomings by the intramolecular
8
9
10
46%; 52%
84%; 82%
[
g]
2
b
[i]
[
18]
[10,
approaches, tailor-made ynamides with Au vinyl carbenoids,
1
9]
[20]
[j]
cycloaddition,
and approaches that using stoichiometric
Only Ni(COD)
2




[
21]
amount of organometallic reagents.
High yield, functional
[j]
1
1
2a/b
L4 + Ni(COD)
2

group compatibility and regioselectivity (up to 46:1, > 20:1 in
general) were noted in cases with commercially available or
simple terminal 2 (Set 1-3). A vast number of general and
functionalized acetylenes, both sterically demanding (Set 2, Y =
[
j]
1
2
No NHC

[a] Screening condition: Substrate (1a: 2 = 1: 2) was added to 4 mol% [NHC-
F
Ni(allyl)BAr ] catalyst (0.02 mmol) with 16 mol% 1-octene in toluene and
stirred for 1 hr at 30 C. Conversion of 1a and 2 were 100% except otherwise
indicated. Yield was determined by H NMR. [b] Yield of low MW oligomers
estimated by GCMS. [c] 1: 2 = 1: 1. [d] An extra 1 equiv of 2a was added after
TMS, tBu, Cy, CH
2
Cy, aryl) and electronic activated (Set 3, Y =
o
1
acyl, ferrocenyl, 3-thiophen, ester) were also identified as good
substrates for the first time. Lower yield was noted in some
electron-poor acetylenes, which might be related to their
insertion reactivity (Y = acyl and ester). Yet, their product
regioselectivity remained high (> 20:1). This HARC system is
also compatible with the internal alkynes (Set 4, both neutral and
hetero). This allowed the preparation of higher substituted and
hetero-substituted 3 for joining other common transformations.
0
.5 h. [e] 0.5 mol% catalyst (0.025 mmol), 1:2 = 1:1.2. [f] conversion of 1a. [g]
2b = 1-Octyne. [h] 6 equiv of 2b was added. [i] 1 mol% cat., 1 mL toluene, 3
hrs. [j] Sticky gel was obtained, yield of 3 was not determined.
The robust condition that we obtained also allowed us to
carry out gram-scale HARC reaction without difficulty. By using
0.5 mol% catalyst with L4 (Table 1, entry 4), 1.05 g of 3aa was
prepared which prompted us to explore the product reactivity.
The product was found compatible with a number of (hetero)
II
The fine-tuned NHC-Ni H insertion reactivity is also compatible
with other commonly employed 1 in the literature. Symmetric
and unsymmetric, dialkyl and diaryl, as well as spiro-substitution
patterns on 1 were also accepted in this new system (Set 5),
hence indicating that the desired reactivity was not limited by
steric bias from Ph and Me. Notably, by making use of a) the
higher reactivity of terminal alkyne than alkene and internal
alkyne; as well as b) the slightly lower reactivity of catalyst with
L6 than L4, a highly chemoselective formation of 3 was
achieved even in the presence of nearby -systems
[
24]
Diels-Alder reactions (Scheme 2).
High yield, reactivity and
selectivity were obtained by slightly modified conditions for
simpler cyclic dienes. Functionalized bicycle[2.2.1]heptanes and
derivatives were obtained in 2 steps from 2a, which are common
units in many drug candidates and bioactive compounds.
[
2b]
competitions (Set 6, internal alkynes and alkenes). In addition,
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COMMUNICATION
F
[a]
Table 2 Scope of the cyclopentadiene 3 synthesized by [NHC-Ni(allyl)]BAr catalyzed HARC.
Set 1a
= OBn 82% (30/1 ); 88% (31/1c)
c
b
X
X = CH2OBn
68% (>20/1)
= CH2OTIPS 81% (>20/1)
= CH OH.
40% (>20/1).
Heterosub.
OMe 73% (25/1c)
OCy 80% (>20/1)
OPh 71% (39/1c)
NHTs 75% (>20/1)
=
=
=
=
Ph
2
X = nHex 51% (32/1 ); 82% (32/1c)
c
b
X
Set 2b
Y
= CH2Cy 89%d (46/1c)
Set 3
b
81%d (>20/1)
Y = ferrocene
Branching
93%d (28/1c)
Electronic
Activated
c
=
=
=
=
Cy
= 3-thiophene 76% (27/1 )
= COCH3
= CO2Et.
Ph
82% (23/1c)
b
tBu
25% (>20/1)
b
TMS 73% (>20/1)
56% (>20/1)
74% (10/1 ); 53% (10/1e)
e
a
Ph
Y
Set 4b
R3 = Et;
R = Et
4
73%
Internal
=
nPr;
= nPr
76%
R3
Ph
R3 = Me;
4
80% (r.r: 3/1)f
R = iPr
= nHex
= c-propyl.
=
OEt;
78% (r.r: 2.8/1)f
R4
Set 5a
= Br;.
30% (r.r: 1.2/1)f.
R1 = Me;
R2 = p-OMeC H
69% (>20/1)
72% (>20/1)
75% (>20/1)
6
4
Sym. & Unsym.
=
Me;
= Me;
Me;
R1 = CH2OTBS; R2 = Ph
= p-CF3C6H4
R1
= 2-Naphthyl
Spiro
R2
= o-OMeC6H4 53% (>20/1) (4.8/1e) BnO
73% (35/1)c
77%b (35/1)c
=
74% (>20/1)
51% (>20/1);
67% (31/1c);
b,d
=
=
=
iPr;
Me;
Ph;
= Ph
75% (>20/1)
81% (35/1c)
b,d
= CH2Ph
= Ph
OBn
31% (20/1)(1/1 ); 52% (>20/1)(1:1e)
e
b
Set 6a
Ph
Ph
Ph
Ph
Chemoselective
Triene
O
Z
O
Z = O
63% (30/1)c
Ph
g
89%b,d (22/1)c
7
5% (35/1)c
= NTs. 75% (14/1)g
72% (>20/1)
5
0%h (10:1)g
o
Cross-HARC condition: 1:2 = 1:2, at 30 C. Isolated yield of the preferred regioisomer 3 was shown. Values in parenthesis are the regioselectivity of 3, which was
1
F
F
determined by H NMR except otherwise indicated. [a] Method A: [L6-Ni(allyl)]BAr (4 mol%, 0.02 mmol) in 3 mL toluene for 1 hr. [b] Method B: [L4-Ni(allyl)]BAr
1 mol%, 0.005 mmol) in 1 mL toluene for 3 hrs. [c] By GCMS (ratio of 3: others with same MW); [d] Equimolar ratio of 1 and 2 was used. [e] Conformational
(
isomers ratio. [f] Ratio of regioisomers. [g] > 20/1 regioisomer ratio. Values in parenthesis are the ratio of 3 with 1-:2-alkene (E/Z mixture). [h] By Method A, except
L2 (IPr) was used instead of L6.
Scheme 2. Convenient route for substituted bicycle[2.2.1]heptane derivatives. Syn:Anti‐product ratio was in parenthesis. The
[
25]
contrasteric structures obtained were assigned by a comparison that used 5‐methyl‐5‐phenylcyclopentadiene.
At this stage, the keys for the straightforward synthesis of
than a homo-insertion of 1 (by steric interaction of 1 with bulky
3
with broad scope were attributed mainly to optimal insertion
NHCs as depicted in Table 1) or premature ring-opening in Pd.
II
II
reactivity of 1 and 2 by their steric interactions with L4 / L6-Ni ,
and to the high rearrangement reactivity of vinylcyclopropane
assisted by NHC-Ni , both under mild condition (Scheme 3).
One of the possible reaction sequences might involve a NHC-Ni
After a NHC-Ni assisted vinylcyclopropane rearrangement at
o
II
[11b,
just 30 C as compared to Scheme 1c (alkenyl-Ni activation),
II
17, 26]
II
[27]
NHC-Ni -allyl 1,3-shift and catalyst regeneration by a β-H
elimination/transfer,
II
[27c-e]
the desired product 3 was obtained. In
directed selective cross-hydroalkenylation of 1 with 2. Here, a
chemoselective insertion of 1 occurred (favored by ring-strain
release), followed by a fast regioselective insertion of 2, rather
this way, this HARC could occur without slow addition of
substrates, and fairly free from non-selective oligomerization
competitions of either 1 or 2. D-labelling experiments support the
[
8]
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Angewandte Chemie International Edition
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COMMUNICATION
hypothesis in part: i) < 4% D exchange was observed when D
1
-
Acknowledgements
2
a was used and produced the corresponding D-labelled 3aa
II
successfully (Scheme 3a). This supported that NHC-Ni -
acetylene species was not involved. ii) Mono-deuteriated
CYH thanks R. H. Grubbs (Shenzhen Nobel Prize Scientists
Laboratory Project C17213101), SZ research fund (JCYJ
cyclopropane was obtained by an early CD
3
OD quenching
2
0170817105041557) and NSFC (21602099). CYH and JQH
(
Scheme 3b). This suggested the insertion of 1 to catalyst is fast
thank Yang HU for substrate preparation.
II
and the NHC-Ni -cyclopropyl rearrangement analogous to the
Pd or Au systems is relatively slow. Overall, this new mode of
reactivity and selectivity expanded the scope and efficiency of 3
preparation significantly, which is a persistent challenge in many
systems.
Keywords: N-heterocyclic carbenes • hydroalkenylation • nickel
•
cyclopentadiene • cyclopropene • alkyne.
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1]
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o
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Ph
D
Ni(L4)
Ph
Ni(L4)
Rearrang.
R
D
R = CH2OBn
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d.r. = 6.2:1
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In conclusion, a highly efficient cross-HARC of 1 and 2
was first established by manipulating the relative insertion
reactivity of 1, 2 and NHC-Ni . Unlike the hydroalkenylation
studies that using [NHC-Ni(allyl)]BAr with two alkenes, cyclic
dienes rather than gem-olefins were obtained by subsequent
rearrangements for the first time and just at fairly mild condition.
This route provides versatile access to functionalized 3 and
bicycle[2.2.1]heptane derivatives, both are difficult to obtain
expediently and atom-economically with this broad substrate
scope before. This report also represents the first applications of
1
those easily accessible substrates in organic synthesis. The new
reactivity preference offered new insights for achieving other
applications and rearrangements by NHC-T.M. hydrides.
II
F
[
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o
stirred at 30 C for 1-3 hrs. A spatula of Na
2 3
CO (s) was added after the
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Angewandte Chemie International Edition
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COMMUNICATION
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[
22] We were surprised to notice that the 1-alkene was not isomerized to 2-
alkene under the Method A condition. However, part of the 1-alkene
was isomerized when less bulky NHC was employed.
.
23] The reason for why the L6 is not good for 1-octyne case is unclear at
this stage. Presumably, the heteroatoms on 2 might somehow involve
in guiding the desired reactivity in Set 1.
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COMMUNICATION
COMMUNICATION
Jian-Qiang Huang, Chun–Yu Ho*
Page No. – Page No.
II
(
NHC)Ni H-Catalyzed Cross-
Hydroalkenylation of Cyclopropene
with Alkyne: A Straightforward
Cyclopentadiene Synthesis by NHC-
II
Ni Assisted Rearrangement.
Hydroalkenylation-rearrangement cascade. Functionalized cyclopentadiene 3
was prepared by catalytic cross-hydroalkenylation-rearrangement cascade in high
yield and step-economy. The keys for the success and broad substrate scope of
cyclopropene 1 and alkyne 2 are the optimal insertion reactivity and high
II
rearrangement reactivity enabled by the new NHC-Ni design under mild condition.
F
Two new [NHC-Ni(allyl)]BAr catalysts were developed, they showed the potentials
of NHC in tuning transition metal hydride reactivity towards the high reactivity of 1
and 2. This discovery opens up new opportunities in NHC directing cascade, and
expands the number of possible combinations significantly.
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