Rhodium-Catalyzed 1,1-Hydroacylation of Thioacyl Carbenes with Alkynyl Aldehydes and Subsequent Cyclization

Article abstract of DOI:10.1021/acs.orglett.9b01003

A rhodium-catalyzed 1,1-hydroacylation of thioacyl carbenes with alkynyl and alkenyl aldehydes and subsequent 6-endo-trig/dig cyclization are realized, giving structurally diverse 4H-thiopyran-4-ones and 2,3-dihydro-4H-thiopyran-4-ones in moderate to good yields. The oxidative addition of Rh(I) to aldehydes is proposed to be the turnover-limiting step. Manipulations of estrones demonstrate the applications of our formal (3 + 3) transannulations in the structural modifications of natural products.

Full text of DOI:10.1021/acs.orglett.9b01003

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Rhodium-Catalyzed 1,1-Hydroacylation of Thioacyl Carbenes with
Alkynyl Aldehydes and Subsequent Cyclization
Bingnan Zhou, Qiuyue Wu, Ziyang Dong, Jiaxi Xu,* and Zhanhui Yang*
State Key Laboratory of Chemical Resource Engineering, Department of Organic Chemistry, Faculty of Science, Beijing University
of Chemical Technology, Beijing 100029, P.R. China
S
* Supporting Information
ABSTRACT: A rhodium-catalyzed 1,1-hydroacylation of thio-
acyl carbenes with alkynyl and alkenyl aldehydes and subsequent
6-endo-trig/dig cyclization are realized, giving structurally diverse
4H-thiopyran-4-ones and 2,3-dihydro-4H-thiopyran-4-ones in
moderate to good yields. The oxidative addition of Rh(I) to
aldehydes is proposed to be the turnover-limiting step.
Manipulations of estrones demonstrate the applications of our
formal (3 + 3) transannulations in the structural modifications of natural products.
n 1965, Tsuji and co-workers reported the reductive
decarbonylation of aldehydes with stoichiometric Wilkin-
reaction between carbene precursors and rhodium catalysts will
lead to undesired side products. Thus, aldehydes and carbene
precursors must be well-defined. Gratifyingly, after numerous
trials (see Supporting Information), the 1,1-hydroacylation of
thioacyl carbenes derived from 1,2,3-thiodiazoles (2)11,12 with
alkynyl aldehydes (1) occurred to deliver thioketones (3),
which undergo 6-endo-dig cyclization to give formal (3 + 3)
transannulation products 4H-thiopyran-4-ones (4) as impor-
tant structural subunits in a number of useful compounds.13
Similar reactions of alkenyl aldehydes also occur to deliver
useful products.14 In a proposed mechanism for the 1,1-
hydroacylation, oxidative addition of Rh(I) into aldehydes (1)
leads to rhodium hydrides (A),3−5,15 which further decompose
thiodiazoles (2) to give carbene complexes (B). Migratory
insertion of the hydrido into carbenic center yields (C),8b and
subsequent reductive elimination regenerates catalysts and
delivers 1,1-hydroacylation products (4).
The optimization of reaction conditions was performed by
using the reaction of 1,2,3-thiadiazole (2a) and 3-phenyl-
propiolaldehyde (1a) as the model (Table 1 and Table S1 in
Supporting Information). Under standard conditions, the
desired product 4aa was isolated in 36% yield (entry 1). The
structure of 4aa was unambiguously confirmed by the X-ray
crystallography of the sulfur-dioxidized product O-4aa (see
Table 2 and Table S2 and Figure S2 in Supporting
Information). Without ligands, no reaction occurred (entry
2). Further screening of other bidentate phosphine ligands
showed that no one was better than DPPF, although DPPP
rendered a 36% yield (entries 3−6). Monodentate phosphine
ligands gave poor results (see Table S1 in Supporting
Information). The addition of Ag(I) salts did not improve
the yields (entries 7−14). However, the reaction mixture with
AgSbF6 seemed much cleaner, as indicated by TLC analysis.
I
son’s catalyst.1 The mechanism involved the formation of acyl
rhodium hydrides as key intermediates via oxidative addition of
the formyl C−H bond (Scheme 1a, i). Subsequently, in 1972,
the interception of acyl rhodium hydrides was first realized by
Sakai’s group in an intramolecular mode, converting 4-alkenals
into cyclopentanones.2 Since then, transition-metal-catalyzed
hydroacylation with aldehydes has become an important and
intriguing concept, and many significant advances,3−5
especially in the field of asymmetric catalysis,4,5 have been
reported. Nowadays, transition-metal (Rh,3,4,5b,c Co,3c,e,f,h
Ir,3c,e,f Ni,3e,f,5a and Ru3e,f,h) catalyzed inter- and intramolecular
hydroacylations of alkenes,3,4 allenes,3e alkynes,3,4 aldehydes,5a
and ketones4a,5b,c are regarded as efficient, powerful, and atom-
economic methods to synthesize ketones, enones, and esters
(Scheme 1a, ii−iv). Despite these elegant advances, it is still of
great importance to explore new kinds of partners for
hydroacylations with aldehydes.
Previous hydroacylation reactions occurred across an
unsaturated CX (X = C or O) double or CC triple
bond (1,2-hydroacylation). Out of our cabene chemistry
background,6,7 we found that reports on hydroacylation at an
unsaturated carbene center (1,1-hydroacylation) are quite rare,
although it provides a novel access to carbon homologation
(Scheme 1a, v). It must be noted that in 2016 Yao and Lin
reported the annulation between salicylaldehydes and acyl
carbenes, and a 1,1-hydroacylation was proposed in the
mechanism (Scheme 1b).8a The 1,1-hydroacylation comple-
ments the Lewis-acid catalyzed Roskamp reaction of aldehydes
with active diazo compounds,9 but the chemistries of the two
kinds of reactions are distinctly different. To achieve the 1,1-
hydroacylation, the following problems should be taken care
of. The acyl rhodium hydrides are very susceptible to reductive
decarbonylation,8a,10 and it seems challenging to intercept
them with carbenes, especially without the assistance of
chelating/directing groups. On the other hand, the direct
Received: March 21, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01003
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 1. 1,2- and 1,1-Hydroacylations and Our Work
Table 1. Selected Optimization of the Reaction Conditions
a
entry
variation of standard conditions
yield (%)
1
2
none
No DPPF
36
0
Variations of Ligands (instead of DPPF)
3
4
5
6
DPPM
DPPE
DPPP
0
10
36
30
DPPPenta
Additions of Ag+ Salts (12 mol)
7
8
9
AgNO3
AgOAc
Ag2O
5
42
20
10
11
12
13
14
AgSbF6
CF3SO3Ag
AgBF4
NaBARF
AgNTf2
30
21
trace
14
17
Variations of Time (10 mol % of [Rh(COD)Cl]2, 24 mol % of DPPF, 24 mol
% of AgSbF6)
b
c
15
16
24 h
48 h
61 (40)
b
60
a
The yields were determined by 1H NMR with 1,3,5-trimethox-
b
ybenzene as an internal standard. Isolated yields after column
chromatography. Yield of 1 mmol scale reaction (for details, see
c
Supporting Information).
Scheme 2. Reactions of Different Alkynyl Aldehydes
Although addition of AgOAc delivered the highest yield, the
purification of 4aa by column chromatography seemed
tougher, due to the generation of unidentified impurities.
With 10 mol % of [Rh(COD)Cl]2, 24 mol % of DPPF, and 24
mol % of AgSbF6, the reactions for 24 and 48 h gave 61% and
60% yields, respectively (entries 15 and 16). Thus, the
conditions listed in entry 12 were selected as optimum.
With the optimal conditions, different alk-2-ynals were
tested (Scheme 2). The reactions of a variety of arylpropio-
laldehydes with electron donors (4ab, 4ac, 4ad, and 4ae),
halogens (4af, 4ag, 4ah, and 4ai), electron acceptors (4ak, 4al,
and 4am), fused, or hetero rings (4an, 4ao) gave the desired
products in satisfactory 45−70% yields, expect for 4-
iodophenylpropiolaldehyde (4aj). Possibly, the aromatic
C(sp2)−I bond was not well tolerated. Hept-2-ynal and hex-
2-ynal underwent the formal (3 + 3) transannulation to deliver
4H-thiopyran-4-ones 4ap and 4aq in 70% and 68% yields,
respectively. 4,4-Dimethylpent-2-ynal (4ar) and 3-cyclopro-
pylpropiolaldehyde (4as) gave moderate 39% and 40% yields,
respectively, probably because of the large steric hindrance of
tert-butyl in the former and the severe ring strain of
cyclopropyl in the latter. Good yields were obtained when
cyclopentyl and cyclohexylpropiolaldehydes were used (4at
and 4au).
a
b
Isolated yields after column chromatography. Oxidation of 4aa was
carried out on a 0.5 mmol scale for 4 h, and the X-ray crystallography
was simplified for clarity (for details, see Supporting Information).
Alkenyl aldehydes (5) were also viable substrates for the
rhodium-catalyzed formal (3 + 3) transannulations, with lower
B
DOI: 10.1021/acs.orglett.9b01003
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Letter
levels of catalysts and ligands in the absence of siver salts
(Scheme 3). The reason for the high activity of alkenyl
Scheme 4. Reactions of Different Thiodiazoles with 2-ynals
Scheme 3. Reactions of Different Alkenyl Aldehydes
a
b
Isolated yields after column chromatography. 0.6 mmol of 5j was
used.
a
Isolated yield after column chromatography.
aldehydes was obscure. Cinnamaldehyde (5a) and 2-
citronylacrylaldehyde (5b) were transformed to 2,3-dihydro-
4H-thiopyran-4-ones (6aa) and (6ab) in 61% and 62% yields,
respectively. 3-Methylbut-2-enal (5c), with two β-methyls, was
also susceptible to the transannulation, affording the desired
product 6ac in 70% yield. Next, we sought to synthesize spiro
2,3-dihydro-4H-thiopyran-4-ones. Spiro products (6ad, 6ae,
6af, and 6ag) with 4-, 5-, and 6-membered rings were prepared
in 65−82% yields, while those (6ah, 6ai, and 6aj) bearing 7-,
12-, and 15-membered rings were synthesized in 42−48%
yields. Bridged bicyclic product 6ak was accessed through the
(3 + 3) transannulation in 38% yield.
with 3-cyclohexylpropiolaldehyde (1u) gave 4qu and 4ru in
50% and 44% yields, respectively. This means that, to
guarantee the feasibility of the transannulations, the electronic
properties of thiodiazoles and 2-ynals must be well matched.
Several reports have disclosed that treating dialkynyl ketones
(7) with Na2S in protic solvents led to 4H-thiopyran-4-ones
(9), and the intermediates were enthiols (8) (Scheme 5a).13 In
the isotope tracing experiment, the reaction of 3-phenyl-
propiolaldehyde-d (d-1a) with thiadiazole (2a) under standard
conditions gave d-4aa in 54% yield with 33:67 H/D ratio
(Scheme 5b), revealing that the proton at the 5-position of
On the other hand, different 1,2,3-thiadiazoles were tested in
the reactions with 3-phenylpropiolaldehyde (1a) (Scheme 4).
Thiadiazoles with electron-rich 5-aryls readily underwent the
formal (3 + 3) transannulations, giving desired products 4ba−
4da in 50−58% yields. Halogens on the phenyl ring were well
tolerated under the rhodium catalysis. The transannulations of
thiadiazoles with halogen-substituted phenyls gave the
corresponding 4H-thiopyran-4-ones (4ea−4ia) in 57−72%
yields. Electron-deficient 4-trifluoromethylphenyl and 4-
cyanophenylthiadiazoles transannulated with ynal 1a to afford
thiopyran-4-ones (4ja) and (4ka) in 52% and 78% yields,
respectively. 2-Naphthalenyl, 2-furyl, and 2-thiophenyl thio-
pyran-4-ones (4la, 4ma, and 4na) were prepared in 43%, 68%,
and 45% yields, respectively. In addition, 5-methylthiadiazole
was converted to the desired product 4oa in 58% yield. In the
above synthesis, an electron-withdrawing ester group was
located at the 4-position of thiadiazoles to ensure its
decomposition into the corresponding carbenes when exposed
to rhodium catalysts. But the ester group was not mandatory.
For example, the reaction of 4,5-diphenyl-1,2,3-thiadiazole
(2p) readily produced 2,3,6-triphenyl-4H-thiopyran-4-one
(4pa) in 53% yield. The reaction of 5-cyclobutyl (2q) and
5-cyclopropyl thiadiazole (2r) with 3-phenylpropiolaldehyde
(1a) afforded no desired products; however, their reactions
Scheme 5. Mechanistic Studies
C
DOI: 10.1021/acs.orglett.9b01003
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Letter
product primarily came from the formyl proton of 2-ynal. The
incomplete D-incorporation indicated that an acid proton
might be involved. Control experiments were also conducted.
Decompostions of thiodiazole (2a)11 and 2-ynal (1a)
occurred, when they were individually treated with catalysts
(see Schemes S11 and S12 in the Supporting Information).
According to Gevorgyan’s and Lee’s reports, an alternative
mechanism in which the decomposition of 1,2,3-thiodiazoles
occurs prior to oxidative addition to 2-ynals is also possible.
However, according to published results on Rh-catalyzed
hydroacylations,3−5,8a we prefer a somewhat different one in
which the oxidative addition to 2-ynal takes place first (Scheme
1c).
To gain more mechanistic details, we performed the kinetic
isotope effect (KIE) study by the initial rate method using two
parallel reactions of ynals 1a and d-1a with thiodiazole 2a
(Scheme 5c and Scheme S15, Figures S3 and S4, Table S2 in
the Supporting Information). A primary KIE of 3.07 was
observed. In several cases of Rh(I)-catalyzed hydroacylations
of ketones16a and alkenes16b,c in which KIEs of 2.4−2.9 were
measured, the direct oxidative cleavage of aldehyde C−H bond
with Rh(I) catalysts was also proposed to be turnover-
limiting.16 Assuming that oxidative addition is turnover-
limiting, Madsen et al. calculated out a KIE of 2.85 for Rh-
catalyzed aldehyde decarbonylation.15 Our KIE result is
consistent with those reported; thus, the mechanism proposed
in Scheme 1c is more reasonsable, and the irreversible
oxidative addition of Rh(I) into formyl C−H bond to form
acyl rhodium hydride A is suggested to constitute the turnover-
limiting step. Previous reports also revealed that KIEs of 1.4−
1.7 for other hydroacylation reactions were indicative of
migratory insertion or reductive elimination as a turnover-
limiting step.17
(14) and reduction with DIBAL-H, delivered enal (15). Spiro
2,3-dihydro-4H-thiopyran-4-one (16) was easily constructed in
75% yield by our rhodium-catalyzed transannulation.
In summary, we have realized a rhodium-catalyzed 1,1-
hydroacylation of thioacyl carbenes with alkynyl and alkenyl
aldehydes to generate thioketone intermediates, which under-
go an intramolecular cyclization to complete formal (3 + 3)
transannulations. 1,2,3-Thiodiazoles are used as thioacyl
carbene precursors. Structurally diverse 4H-thiopyran-4-ones
and 2,3-dihydro-4H-thiopyran-4-ones are prepared in moder-
ate to good yields from 2-enals and 2-ynals, respectively. In the
mechanistic proposal, acylrhodium hydrides are key inter-
mediates, and their formation via oxidative addition is the
turnover-limiting step. The 1,1-hydroacylation/cyclization
protocol is demonstrated to be useful in structural
modifications of naturally occurring products.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01003.
Experimental details, compound characterization, and
spectra (PDF)
Accession Codes
CCDC 1895642 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
Our 1,1-hydroacylation/cyclization sequence is useful in the
structural modifications of natural products (Scheme 6).18 For
example, submission of estrone (10) to a sequence of triflation,
Sonogashira coupling, and deketalization afforded alkynal (12),
which was transformed to 4H-thiopyran-4-one (13) in 62%
yield by the rhodium-catalyzed formal (3 + 3) transannulation
with thiodiazole 2a. O-Methylation of estrone, followed by
Horner−Wadsworth−Emmons olefination with phosphonate
■
*E-mail: zhyang@mail.buct.edu.cn.
*E-mail: jxxu@mail.buct.edu.cn.
ORCID
Jiaxi Xu: 0000-0002-9039-4933
Zhanhui Yang: 0000-0001-7050-8780
Notes
Scheme 6. Structural Modifications of Estrone
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the National Natural
Science Foundation of China (no. 21602010 to Z.Y.), the
BUCT Fund for Discipline Construction and Development
(project no. XK1533 to Z.Y.), and the Fundamental Research
Funds for the Central Universities (XK1802-6 to Z.Y. and J.X.;
no. 12060093063 for Z.Y.).
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