Palladium-Catalyzed Cleavage of α-Allenylic Aryl Ether toward Pyrazolemethylene-Substituted Phosphinyl Allenes and Their Transformations via Alkenyl C-P(O) Cleavage

Article abstract of DOI:10.1021/acs.orglett.7b00213

A palladium-catalyzed two-component coupling of allenylphosphine oxides with conjugated N-tosylhydrazones is revealed. For the first time, the cleavage of α-allenylic aryl ether toward pyrazolemethylene-substituted phosphinyl allenes enabled facile synthesis of combined motifs with pyrazole and allene. Moreover, the obtained adducts could be easily transformed to potential bioactive multifunctionalized phosphinates via a novel alkenyl C-P(O) cleavage.

Full text of DOI:10.1021/acs.orglett.7b00213

Letter
pubs.acs.org/OrgLett
Palladium-Catalyzed Cleavage of α‑Allenylic Aryl Ether toward
Pyrazolemethylene-Substituted Phosphinyl Allenes and Their
Transformations via Alkenyl C−P(O) Cleavage
Jie Zhu,†,§ Mao Mao,†,§ Huan-Jing Ji,† Jiang-Yan Xu,† and Lei Wu*,†,‡
†Jiangsu Key Laboratory of Pesticide Science and Department of Chemistry, College of Sciences, Nanjing Agricultural University,
Nanjing, China
‡Beijing National Laboratory for Molecular Sciences and Institute of Chemistry, Chinese Academy of Sciences, Beijing, P. R. China
S
* Supporting Information
ABSTRACT: A palladium-catalyzed two-component coupling of allenylphosphine oxides with conjugated N-tosylhydrazones is
revealed. For the first time, the cleavage of α-allenylic aryl ether toward pyrazolemethylene-substituted phosphinyl allenes
enabled facile synthesis of combined motifs with pyrazole and allene. Moreover, the obtained adducts could be easily transformed
to potential bioactive multifunctionalized phosphinates via a novel alkenyl C−P(O) cleavage.
substituents with electron-deficient functionalities as good
leaving groups, such as acetates, carbonates, halides, and
pseudohalides (Scheme 1a).13 The cleavage of fragments with
llenes, or 1,2-dienes, are core structures in many naturally
1
Aoccurring biomolecules and synthetic pharmaceuticals. In
addition to the cumulated diene structure, the potential for up to
four substituents also make them important building blocks in
organic synthesis, especially for bioactive compounds. The
importance of allenes necessitates extensive explorations on
their transformations and synthesis during the past two decades.2
On one side, various transformations from allenes to other
functionalities have been demonstrated including transition-
metal catalyzed intermolecular/intramolecular cycloaddition,3
nucleophilic addition,4 oxidative carbocyclization,5 and
others.6−10 The importance of allene moieties has, on the other
hand, stimulated substantial interest in constructing versatile
allene compounds. Traditional methods to prepare allenes
generally lie on monoalkenes, conjugated enynes, alkynes,
cyclopropanes, and propargylic fragments as starting materi-
als.2c,11 Due to the unique activity of cumulate diene, direct
functionalization of allenes to obtain allenes is appealing9a,12,13
but challenging, which would include maintenance of the 1,2-
diene moiety against forming alkenes in the presence of a
transition-metal catalyst and electrophilic/nucleophilic reagents,
generally via the generation of π-allylmetal species followed by a
β-hydride elimination process. For elegant studies, Ma’s group
developed palladium-catalyzed amination of allenyl phosphates
and allenyl N-tosylcarbamates respectively generating 2,3-allenyl
amines with central chirality, in which allenes remained
unchanged in the presence of palladium species.13d,e Of note,
the scope of allene precursors is currently limited to a few
Scheme 1. (a) Representative Allenes Synthesis with Electron-
Deficient Functionalities; (b and c) Allenes and N-
Tosylhydrazones involved Catalysis; (d) This Work
Received: January 20, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b00213
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Letter
electron-rich functionalities to generate π-allyl-Pd species is
challenging but attractive;14 however, this protocol has not been
established in allenes synthesis yet. Considering the importance
of allenes, as well as the limitation of reported methods, efficient
synthetic strategies for construction of multisubstituted allenes
from diverse functionalized allenes, along with their innovative
transformations, are still highly desirable.
indicated that bases played key roles in achieving high
regioselectivity when forming the C−N bond between allene
and pyrazole. The effect of inorganic bases adversely affected
regioselectivity, giving a mixture of 3aa and 3′ with ratios of
around 1:1. Organic bases, on the other hand, prominently
inhibited the generation of 3′, albeit only moderate yields of 3aa
were obtained. Considering both efficiency and regioselectivity,
DBN (1,5-diazabicyclonon-5-ene) was chosen for the following
optimizations. Further improvements were discovered from
screening substrate ratios, reaction temperature, and solvents (in
SI). The reaction was found to be significantly affected by the
solvents used, where a protic solvent led to no conversion of
allenes. To our delight, toluene was found to increase the yield of
3aa up to 81% without the detection of 3′ (entry 3). Other
transition-metal catalysts, including Pd(OAc)2, Pd(PPh3)4,
Ni(PPh3)Cl2, and Rh(PPh3)3Cl, were also tested, which were
observed to be ineffective with lower yields or less regioselectivity
(entries 4−7). Eventually, Pd(PPh3)2Cl2 was proven to be the
best catalyst to enable the reaction.
Over the past decade, N-tosylhydrazones, after pioneering
work by Barluenga and Valdes 15
have emerged as versatile
́
,
building blocks for transition-metal catalyzed coupling reactions,
most often with palladium complexes to access alkenes.16 In
general, the reaction intermediates undergo migratory insertion
of palladium carbenes, which is unambiguously compatible with
allene chemistry. Many achievements have been devoted to
converting N-tosylhydrazones into allenes,17 yet the transition-
metal catalyzed coupling reactions involving N-tosylhydrazones
and allenes are rather underdeveloped.18,19a Pioneering work was
reported by Wang’s group in 2013 (Scheme 1b), who discovered
the first synthesis of 1,3-dienes through three-component
coupling of allenes, aryl-iodides, and N-tosylhydrazones, possibly
through π-allyl-Pd-carbene intermediates.18a Very recently, we
advanced the synthesis of a new member of dendralenes family,
multisubstituted (Z)-selective phosphinyl [3]dendralenes, from
the palladium-catalyzed two-component coupling of allenyl-
phosphine oxides and N-tosylhydrazones (Scheme 1c).19a
Intriguingly, conjugatedN-tosylhydrazoneschangedthecoupling
pathwayagainsttheformationofconjugatedalkenes, asillustrated
in Scheme 1d. In continuation of our interest in organo-
phosphorus and heterocyclic chemistry,19 herein, we disclose a
palladium-catalyzed cleavage of allenyl electron-rich functionality
to finalize pyrazolemethylene-substituted phosphinyl allenes.
Theallenemoiety ismaintainedinthepalladium-catalysis system,
offering the first synthesis of combined motifs with pyrazole and
allene, to our knowledge.20 Besides, a novel palladium-catalyzed
alkenyl C−P(O) bond cleavage of the pyrazolemethylene-
substituted phosphinyl allenes to multifunctionalized phosphi-
nates is established (Scheme 1d).
Encouraged by the preliminary results, we next investigated the
substrate scope of various allenylphosphine oxides (1a−1n), as
listed in Scheme 2. In general, allenylphosphine oxides with
Scheme 2. Substrate Scopes
With allenylphosphine oxide (1a)21 and conjugated N-tosyl-
hydrazone (2a) as model substrates, the reaction was initially
carried out in the presence of bis(triphenylphosphine) palladium
dichloride, K2CO3, and refluxing 1,4-dioxane, furnishing 3aa in
41% yield, along with 38% of 3′ (entry 1). Systematic screenings
of the conditions were then performed, as shown in Table 1 (see
Supporting Information (SI) for more details). The results
terminal alkyl, cyclic, or aromatic substitutions afforded the
corresponding adducts (3aa−3la, 3oa) with moderate to good
yields. Though the cleavage of cyclopropane usually occurred in
palladium catalysis,22 cyclopropane substitution on allenes
remains intactin this protocol (3ca, 3da)excluding the possibility
of a metal−carbene mechanism. For allenes with aromatic
substitutions, both an electron-rich group (such as p-MeO) and
electron-deficientgroup(suchasp-Fandp-Cl)onthephenylring
were effective in achieving the reaction, without observation of a
distinct electronic effect. Moreover, the existence of the allene
moiety inthe molecule andthe regioselectivity of C−Nformation
wereexemplifiedbytheX-raycrystalstructureofracemic-3ka(see
details in the Supporting Information (SI)).23 It is worthy to
mention that dimethyl and cyclohexyl terminated substrates (1m,
1n) became slightly complicated, with only a trace amount of
products detected. Quite interestingly, in sharp contrast,
cycloheptanone derived allenes proceeded smoothly to furnish
a 33% yield of product (3oa). This differentiation might be
attributedtothecompetingratesofaside reactionandthedesired
coupling for specific substrates.
a b
,
Table 1. Optimization of Reaction Conditions
entry
catalyst
base
solvent/temp (°C) yield (3aa/3′, %)
1
2
3
4
5
6
7
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(PPh3)2Cl2
Pd(OAc)2
K2CO3
DBN
DBN
DBN
DBN
DBN
DBN
1,4-dioxane/110
1,4-dioxane/110
toluene/80
toluene/80
41/38
42/0
81/0
34/<5
73/0
Pd(PPh3)4
toluene/80
Ni(PPh3)2Cl2
Rh(PPh3)3Cl
toluene/80
45/<5
42/<5
toluene/80
a
Reaction conditions: allenylphosphine oxide (1a, 0.2 mmol), N-
tosylhydrazone (2a, 0.4 mmol), 5 mol % catalyst, 0.6 mmol of base,
b
N2, 12 h. Isolated yield by chromatography.
B
DOI: 10.1021/acs.orglett.7b00213
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Letter
Next, the nature of N-tosylhydrazones was examined to verify
the generality of this protocol (Scheme 2). N-Tosylhydra-zones,
bearing OMe, CH3, NEt2, F, Cl, or Br groups, were all well
tolerated, affording the pyrazole functionalized allenes smoothly.
Para-substituted halogens (F, Cl, and Br) impaired the reactivity
to some extent, giving lower yields compared with electron-
donating groups (3af−3ah). Notably, naphthyl, biphenyl, and
aliphatic N-tosylhydrazones also proceeded efficiently in this
regime with yields of 64%, 59%, and 26% respectively (3aj, 3ak,
and 3al), which further extended the substrate scope greatly.
Besides, when R4 = H, cyclic or aromatic substitutions were also
found to be applicable in addition to methyl, with relatively lower
yields ranging from 15% to 45%.
Scheme 4. Transformations and the Tentative Mechanism
Based on the palladium-catalyzed allene chemistry, N-
tosylhydrazones, and previous reports,13,19a,24 a tentative
mechanism is proposed in Scheme 3. The reaction starts with
Scheme 3. Tentative Mechanism
C−P cleavage were conducted as well. As shown in eq a, the first
step between 3aa and NBS became slightly complicated in part
because of the addition of Br+ to the central carbon of allenes.26a
However, while the crude adducts containing the brominated
allene (3aa-Br) were treated with 1 equiv of NBS under the
standard conditions, a 40% yield of target compound (4a) was
isolated. Similarly, the coupling of brominated pyrazole and the
starting allene (1a) afforded 4a in 53% yield (eq b), which is
considerably lower than that from the nonbrominated procedure.
This decrease in yield might be attributed to the competing
couplingofheterobromide withallenes.26b Theabove-mentioned
experimental results collectively indicated two issues: (1) the
pyrazole moiety should be involved in the alkenyl C−P(O)
cleavage step; (2) NBS plays a dual role in bromination of the
pyrazolemoietyandgeneratingphosphinates. Apossiblepathway
is described in Scheme 4. The cleavage of the N−Br bond of NBS
by a palladium species followed by coordination will form a key
intermediate G,27,28 which undergoes an oxidative alkenyl C−
P(O) cleavage process to a Pd(IV) species (G′). Subsequently,
reductive elimination of G′ gives H, with the release of
diphenylphosphinic bromide. Spontaneous nucleophilic attack
of a diphenylphosphinate anion to the endmost position will lead
to the formation of the final product (4) and recycling of the
palladium catalyst.
In summary, we reported here the first example of two-
component coupling between allenes and conjugated N-tosyl-
hydrazones, in which the allene structure was maintained in the
palladium-catalyzed cleavage of the allenyl electron-rich
functionality. The reaction tolerated various functional groups
and furnished a series of novel pyrazolemethylene-substituted
phosphinyl allenes in acceptable to good yields. The obtained
adducts can be easily transformed into multifunctionalized
phosphinates via a novel alkenyl C−P(O) cleavage. We believe
this novel protocol and transformations would enrich the allene
chemistry and provide novel scaffolds to constitute bioactive
compounds as well.
the cleavage of the C(sp3)−O(Ar) bond, which leads to the
formation of π-allyl-palladium species A. Simultaneously,
pyrazole compound C/C′ is formed from the in situ generated
diazo substrate B, with C′ likley being the major product owing to
the larger conjugation. Subsequent transmetalation between A
and C′ produces intermediates D and E. Afterward, reductive
elimination of palladium species E toward C−N bond formation
occurs successfully to finally generate the product 3, along with
recycling of the palladium(0) catalyst. Likewise, the reductive
elimination of D will furnish a 1,3-diene product (F), which is not
observed at all. On the other hand, the byproduct 3′ obtained
from C″ is inhibited under the optimized conditions.
To further demonstrate the synthetic applications of our
developedprotocol, furthertransformationswereperformedwith
pyrazolemethylene-substituted phosphinyl allenes. While the
substrates (3aa, 3ba, 3ea, 3ac, and 3al) were treated with 5 mol %
palladium acetate and 2 equiv of NBS under air, a series of novel
multifunctionalized phosphinates (4) formed in “one pot” with
acceptable to good yields, ranging from 34% to 87% (Scheme 4).
The X-ray crystal structure of 4a unambiguously displayed an
alkenyl C−P(O) cleavage and rearrangement of diphenylphos-
phine oxide to the endmost carbon of allenes.23 Although the
cleavage of aryl C−P(O) has been sporadically documented,25
this type of alkenyl C−P(O) bond cleavage is unprecedented and
interesting. Control experiments were then conducted to
determine the mechanism. In the case of allenes without a
pyrazole moiety (1a), transformations in the absence of NBS or
under anhydrous conditions led to negative results (see more
details in the SI). More importantly, sequential bromination and
C
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S.-I.; Naota, T. Org. Lett. 2005, 7, 5837. (c) Boutier, A.; Kammerer-
Pentier, C.; Krause, N.; Prestat, G.; Poli, G. Chem. - Eur. J. 2012, 18, 3840.
(d)Wan, B.;Ma, S. Angew. Chem., Int. Ed. 2013,52, 441. (e)Li, Q.;Fu, C.;
Ma, S. Angew. Chem., Int. Ed. 2014, 53, 6511.
(14) For representative cleavage of allyl fragments with electron-rich
functionalities to finalize π-allylpalladium species, see: (a) Nishikata, T.;
Lipshutz, B. H. J. Am. Chem. Soc. 2009, 131, 12103. (b) Huo, X.; Quan,
M.; Yang, G.; Zhao, X.; Liu, D.; Liu, Y.; Zhang, W. Org. Lett. 2014, 16,
1570.
ASSOCIATED CONTENT
* Supporting Information
TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/acs.orglett.7b00213.
■
S
Experimental procedures and spectral data for all new
compounds (PDF)
(15)(a)Barluenga, J.;Moriel, P.;Valdes
Ed. 2007, 46, 5587. (b) Barluenga, J.; Tomas
Valdes, C. Nat. Chem. 2009,1, 494.(c)Barluenga, J.;Tomas
Aznar, F.;Valdes, C. Angew. Chem., Int. Ed. 2010, 49, 4993. (d)Barluenga,
J.; Valdes, C. Angew. Chem., Int. Ed. 2011, 50, 7486.
(16) (a) Zhao, X.; Jing, J.; Lu, K.; Zhang, Y.; Wang, J. Chem. Commun.
2010, 46, 1724. (b) Xia, Y.; Hu, F.; Liu, Z.; Qu, P.; Ge, R.; Ma, C.; Zhang,
Y.; Wang, J. Org. Lett. 2013, 15, 1784. (c) Zhou, Y.; Ye, F.; Wang, X.; Xu,
S.; Zhang, Y.; Wang, J. J. Org. Chem. 2015, 80, 6109. (d) Arunprasath, D.;
Muthupandi, P.; Sekar, G. Org. Lett. 2015, 17, 5448.
(17) For representative synthesis of allenes from N-tosylhydazones/
diazo compounds, see: (a) Xiao, Q.; Xia, Y.; Li, H.; Zhang, Y.; Wang, J.
Angew. Chem., Int. Ed. 2011, 50, 1114. (b) Mundal, D. A.; Lutz, K. E.;
Thomson, R. J. J. Am. Chem. Soc. 2012, 134, 5782. (c) Neff, R.; Frantz, K.
D. E. ACS Catal. 2014, 4, 519. (d) Poh, J. S.; Tran, D. N.; Battilocchio, C.;
Hawkins, J. M.; Ley, S. V. Angew. Chem., Int. Ed. 2015, 54, 7920.
(18) (a) Xiao, Q.; Wang, B.; Tian, L.; Yang, Y.; Ma, J.; Zhang, Y.; Chen,
S.; Wang, J. Angew. Chem., Int. Ed. 2013, 52, 9305. (b) Gao, Y.; Xiong, W.;
Chen, H.; Wu, W.; Peng, J.; Gao, Y.; Jiang, H. J. Org. Chem. 2015, 80,
7456.
́
, C.;Aznar, F. Angew. Chem., Int.
-Gamasa, M.; Aznar, F.;
-Gamasa, M.;
AUTHOR INFORMATION
■
́
Corresponding Author
*E-mail: rickywu@njau.edu.cn.
ORCID
́
́
́
́
Lei Wu: 0000-0001-9130-6619
Author Contributions
§J.Z. and M.M. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This project is supported by the Foundation Research Project of
Jiangsu Province (The Natural Science Foundation, No.
BK20141359) and the Fundamental Research Funds for the
Central Universities (Grant No. KYTZ201604).
(19) (a) Mao, M.; Zhang, L.; Chen, Y.-Z.; Zhu, J.; Wu, L. ACS Catal.
2017, 7, 181. (b) Luo, K.; Chen, Y.-Z.; Yang, W.-C.; Zhu, J.; Wu, L. Org.
Lett. 2016, 18, 452. (c) Luo, K.; Chen, Y.-Z.; Chen, L.-X.; Wu, L. J. Org.
Chem. 2016, 81, 4682. (d) Chen, Y.-Z.; Zhang, L.; Lu, A.-M.; Yang, F.;
Wu, L. J. Org. Chem. 2015, 80, 673.
(20) For selected synthesis of pyrazoles from allenes: (a) Nair, V.; Biju,
A. T.; Mohanan, K.; Suresh, E. Org. Lett. 2006, 8, 2213. (b) Chakravarty,
M.;Kumar, N.N. B.;Sajna, K.V.;Swamy, K. C.K. Eur. J.Org.Chem. 2008,
2008, 4500. (c)Haydl, A. M.;Xu, K.;Breit, B. Angew. Chem., Int. Ed. 2015,
54, 7149.
(21) 2,6-Dimethyl substitution was chosen for its solid state and
operational simplicity; please see details in our previous study (ref 19d).
(22) (a) Braese, S.; de Meijere, A. Angew. Chem., Int. Ed. Engl. 1995, 34,
2545. (b) Shi, M.; Chen, Y.; Xu, B. Org. Lett. 2003, 5, 1225. (c) Zhou, L.;
Ye, F.; Zhang, Y.; Wang, J. Org. Lett. 2012, 14, 922.
(23) CCDC 1525328 (rac-3ka) and CCDC 1525329 (4a) contain the
supplementarycrystallographicdataforthispaper(ORTEPinSI). These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
(24) (a) Ye, J.; Ma, S. Acc. Chem. Res. 2014, 47, 989. (b) Li, X.; He, L.;
Chen, H.; Wu, W.; Jiang, H. J. Org. Chem. 2013, 78, 3636.
(25) Selected aryl C−P(O) bond cleavage: (a) Inoue, A.; Shinokubo,
H.; Oshima, K. J. Am. Chem. Soc. 2003, 125, 1484. (b) Derrah, E. J.;
Ladeira, S.; Bouhadir, G.; Miqueu, K.; Bourissou, D. Chem. Commun.
2011, 47, 8611. (c) Lu, E.; Chen, Y.; Zhou, J.; Leng, X. Organometallics
2012, 31, 4574.
REFERENCES
■
(1) Hoffmann-Roder, A.; Krause, N. Angew. Chem., Int. Ed. 2004, 43,
̈
1196.
(2) For reviews of allenes: (a) Ma, S. Chem. Rev. 2005, 105, 2829.
(b) Ma, S. Acc. Chem. Res. 2003, 36, 701. (c) Yu, S.; Ma, S. Chem.
Commun. 2011, 47, 5384.
(3) (a) Alcaide, B.; Almendros, P.; Aragoncillo, C. Chem. Soc. Rev. 2010,
39, 783. (b) Saito, S.; Hirayama, K.; Kabuto, C.; Yamamoto, Y. J. Am.
Chem. Soc. 2000, 122, 10776. (c) Sankar, M. G.; Garcia-Castro, M.; Golz,
C.; Strohmann, C.; Kumar, K. Angew. Chem., Int. Ed. 2016, 55, 9709.
(4) (a) Zhang, Z.; Widenhoefer, R. A. Org. Lett. 2008, 10, 2079.
(b) Kuppusamy, R.; Muralirajan, K.; Cheng, C.-H. ACS Catal. 2016, 6,
3909.
(5) (a) Piera, J.; Persson, A.; Caldentey, X.; Backvall, J.-E. J. Am. Chem.
Soc. 2007, 129, 14120. (b) Persson, A. K. Å.; Backvall, J.-E. Angew. Chem.,
̈
̈
Int. Ed. 2010, 49, 4624. (c) Yamamoto, H.; Ueda, M.; Yamasaki, N.; Fujii,
A.; Sasaki, I.; Igawa, K.; Kasai, Y.; Imagawa, H.; Nishizawa, M. Org. Lett.
2016, 18, 2864.
(6) (a) Pritzius, A. B.; Breit, B. Angew. Chem., Int. Ed. 2015, 54, 15818.
(b) Huang, Z.; Lu, Q.; Liu, Y.; Liu, D.; Zhang, J.; Lei, A. Org. Lett. 2016,
18, 3940.
(7) (a) Trost, B. M.; Jakel, C.; Plietker, B. J. Am. Chem. Soc. 2003, 125,
4438. (b) Trost, B. M.; Simas, A. B. C.; Plietker, B.; Jakel, C.; Xie, J. Chem.
- Eur. J. 2005, 11, 7075.
(8) (a) Ma, S. M.; Yu, Z. Q. Angew. Chem., Int. Ed. 2002, 41, 1775.
(b) Miao, M.; Wang, W.; Yang, W.; Xu, L.; Ma, J.; Ren, H. Chem. - Eur. J.
2015, 21, 14447. (c) Haydl, A. M.; Breit, B. Angew. Chem., Int. Ed. 2015,
54, 15530. (d)Sakashita, K.;Shibata, Y.;Tanaka, K. Angew. Chem., Int. Ed.
2016, 55, 6753.
(9) (a) Yu, F.; Lian, X. D.; Ma, S. M. Org. Lett. 2007, 9, 1703. (b) Braun,
M.-G.; Katcher, M. H.; Doyle, A. G. Chem. Sci. 2013, 4, 1216.
(10) (a) Ma, S.; Wei, Q.; Wang, H. Org. Lett. 2000, 2, 3893. (b) Ma, S.
Acc. Chem. Res. 2009, 42, 1679. (c) Wang, M.; Li, J.; Fu, C.; Ma, S. Org.
Lett. 2014, 16, 4976.
̈
̈
(26) (a) Kong, W.; Guo, B.; Fu, C.; Ma, S. Eur. J. Org. Chem. 2011, 2011,
2278. (b) Ma, S.; Zhang, J. Chem. Commun. 2000, 117.
(27) Oxidation of NBS/NCS to palladium precursors: (a) Crawforth,
C. M.; Burling, S.; Fairlamb, I. J. S.; Taylor, R. J. K.; Whitwood, A. C.
Chem. Commun. 2003, 2194. (b) Whitfield, S. R.; Sanford, M. S. J. Am.
Chem. Soc. 2007, 129, 15142.
(28) Selected coordination of (P)O with transition metals: (a) Yang,
Y.; Qiu, X.; Zhao, Y.; Mu, Y.; Shi, Z. J. Am. Chem. Soc. 2016, 138, 495.
(b)Yang, Y.;Li, R.;Zhao, Y.;Zhao,D.;Shi,Z. J. Am. Chem. Soc.2016,138,
8734.
(11) Brandsma, L. Synthesis of Acetylenes, Allenes and Cumulenes;
Elsevier, Oxford, 2004.
(12) Tsuji, J. Palladium Reagents and Catalysis−New Perspectives for the
21st Century; John Wiley & Sons: New York, 2004.
(13) (a) Trost, B. M.; Fandrick, D. R.; Dinh, D. C. J. Am. Chem. Soc.
2005, 127, 14186. (b) Imada, Y.; Nishida, M.; Kutsuwa, K.; Murahashi,
D
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