Alkynyl N-Nosylhydrazones: Easy Decomposition to Alknynl Diazomethanes and Application in Allene Synthesis

Article abstract of DOI:10.1002/chem.201701462

The decomposition of N-tosylhydrazones is a safe and convenient method for the generation of donor carbenes. However, alkynyl carbenes cannot be isolated by this route because they readily undergo intramolecular cyclization to pyrazoles as soon as formed from alkynyl N-tosylhydrazones. Here, the use of alkynyl N-nosylhydrazones for the in situ generation of alkynyl carbenes and their coupling reaction with boronic acids under metal-free conditions is reported, giving rise to a wide array of di- and trisubstituted allenes. Preliminary mechanistic investigations demonstrated that γ-protodeboration of propargyl boric acid was responsible for the initial allene formation. This methodology based on the nosyl group allows for novel transformations that involve an alkynylcarbene transient species.

Full text of DOI:10.1002/chem.201701462

A Journal of
Accepted Article
Title: Alkynyl N-Nosylhydrazones: Easy Decomposition to Alknynl
Diazomethanes and Application for Allene Synthesis
Authors: Xihe Bi, Yang Yang, Zhaohong Liu, Alessio Porta, and
Giuseppe Zanoni
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DOI: 10.1002/chem.200((will be filled in by the editorial staff))
Alkynyl N-Nosylhydrazones: Easy Decomposition to Alknynl Diazomethanes and
Application for Allene Synthesis
Yang Yang,§ Zhaohong Liu,§ Alessio Porta, Giuseppe Zanoni* and Xihe Bi*
thermolysis of aziridinylimines,[10] (3) the dehydrogenation of
hydrazones,[11]
and
(4)
the
decomposition
of
Abstract: The dissociation of N-tosylhydrazones is a safe and
convenient method for the generation of donor carbenes.
However, alkynyl carbenes cannot be isolated by this route
because they readily undergo intramolecular cyclization to
pyrazoles as soon as formed from alkynyl N-tosylhydrazones.
Herein we report the use of alkynyl N-nosylhydrazones for
the in situ generation of alkynyl carbenes and their coupling
reaction with boronic acids under metal-free conditions,
giving rise to a wide array of di- and trisubstituted allenes.
Preliminary mechanistic investigations demonstrated that γ-
protodeboration of propargyl boric acid was responsible for
the initial allene formation. This methodology based on the
nosyl group allows for novel transformations that involve an
alkynylcarbene transient species.
alkynyldiazoacetates.[12] However, investigations into the
synthetically useful alkynyl carbenes generated in situ from diazo
precursors have lagged behind the established knowledge of
carbene chemistry. For example, N-tosylhydrazones are widely
used as diazo surrogates of alkyl-, alkenyl-, or aryldiazomethanes
and have significantly contributed to the development of carbene
chemistry.[13] In contrast, despite their long history,[14] alkynyl N-
tosylhydrazones have been scantily used as precursors of
alkynyldiazomethanes, because of the preferred intramolecular
cyclization to pyrazoles (Figure 1a).[8e-8g] Hence, alkynyl N-
tosylhydrazones, first pioneered by McMahon, were restricted to
work as triplet carbene precursors.[8] Notably, Thomson and
Schaus reported the synthesis of allenes from alkynyl N-
nosylhydrazones by catalytic Traceless Petasis reactions, which
not involves carbene intermediates.[14c] Myers,[15a] Thomson[15b-c]
and We[15d] have previously reported the readily decomposition of
ο-nosylhydrazides to allenes at room temperature. More recently
we have for the first time discovered the decomposition capability
of N-nosylhydrazones at room-temperature, as opposed to the
higher temperatures required for the dissociation of N-
tosylhydrazones.[16] As continued interest in the synthetic
potential of such kind of compounds, we propose the use of
alkynyl N-nosylhydrazones, instead of alkynyl N-tosylhydrazones,
for the expeditious in situ generation of alkynyl carbenes. With
this nosyl based strategy, the pyrazole formation was suppressed,
thus opening new opportunities for alkynylcarbene-based
transformations (Figure 1b). Herein, we report the new coupling
with boric acids to from allenes.
Acetylenic carbenes, first obtained by Skell and Klebe in 1960,[1]
are found in extreme chemical environments such as in interstellar
space and in the atmosphere of Titan.[2] In addition, acetylenic
carbenes and their derivatives are frequently used as versatile
synthetic intermediates in organometallic chemistry and organic
synthesis.[3-6] These carbene species are most commonly
generated from Fischer carbene complexes,[4] from diynes by
ring-closing and cross metathesis,[5] and from oligoynes
possessing propargyl ester moieties by isomerization.[6] The
preferred prevalent methods for the generation of free- or metal
carbene species are photolytic, thermal or through metal-induced
extrusion of nitrogen from diazo compounds.[7] Over the past
decades, considerable efforts have also been put into the in situ
generation of alkynyl carbenes from diazo precursors. Established
synthetic methods include (1) the base treatment of
sulfonylhydrazones[8] and N-nitroso-alkynylamines,[9] (2) the
[]
Y. Yang, Z. Liu, Prof. X. Bi
Jilin Province Key Laboratory of Organic Functional
Molecular Design & Synthesis
Department of Chemistry, Northeast Normal University
Changchun 130024, China
E-mail: bixh507@nenu.edu.cn
Homepage: http://www.bigroup.top/
Prof. X. Bi
State Key Laboratory of Elemento-Organic Chemistry
Nankai University, Tianjin 300071, China
Prof. A. Porta, Prof. G. Zanoni
Department of Chemistry, University of Pavia
Viale Taramelli, 10-27100 Pavia, Italy
E-mail: gz@unipv.it
§ These two authors make equival contributations.
Figure 1. [2+1] Cylopropenation of alkynes with diazo surrogates.
[] Supporting information for this article is available on the
WWW under
1
http://dx.doi.org/10.1002/anie.201xxxxxx.((Please delete if
not appropriate))
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Allenes are crucial structural motifs in a variety of bioactive
molecules and natural products,[17] and useful building blocks or
intermediates in organic synthesis.[18] Despite great achievements
in the allene synthesis by metal-mediated direct coupling of two
fragments,[19] an analogous metal-free reactions remain extremely
limited to date.[14c, 20] Taking cue from the carbon-carbon bond-
forming reaction between boronic acids and diazo compounds
under metal-free conditions,[21] we envisaged that a base could
promote the coupling of alkynyl N-nosylhydrazones with boronic
acids to afford allenes. In an initial experiment, alkynyl N-
nosylhydrazone 1a was reacted with phenylboronic acid 2a, in the
presence of K2CO3 in 1,4-dioxane at 110 oC, to afford an
inseparable mixture of propyne 3a and allene 4a in 64% yield
allenes in good yields (4k-4n, 62% to 89%, Scheme 1). In
addition, we found that alkynyl N-nosylhydrazones with an alkyl
or alkenyl moiety smoothly converted to the corresponding
disubstituted allenes in synthetically useful yields (4o-4q, 65% to
85%, Scheme 1). Of note, the conjugated alkenyl allene 4q is an
important synthon in organic synthesis.[22]
1
(measured by H-NMR; for experimental details see supporting
information). Product 3a was conveniently converted to allene 4a
by stirring the crude reaction mixture at room temperature in the
presence of KOH and tert-butylammoiunm bromide (TBAB).
After screening of a variety of bases, solvents, and reaction
temperatures, the best yield of the expected allene was achieved
with sodium hydride (NaH) as base in toluene at 60 oC (Figure 2a).
Notably, the reaction between alkynyl N-tosylhydrazones 1a' and
phenylboronic acid 2a, performed under identical conditions,
afforded pyrazole 5a' in 90% yield with a trace amount of alkyne
3a (Figure 2b). Therefore, the type of sulphonyl group in the
hydrazone plays a decisive role on the reactivity of the alkynyl N-
sulphonylhydrazones.
Scheme 1. Substrate scope. Reaction conditions: Step 1: N-nosyhydrazone 1 (0.3
mmol), boronic acid 2 (0.6 mmol), NaH (0.6 mmol) and PhCF3 (3 mL) were stirred
at 60 °C for 2 h under inert atmosphere. Step 2: KOH (1.0 equiv) and TBAB (30
mol %) in toluene at RT under inert atmosphere for 12 h.
Figure 2. Different reactivity of N-tosyl and N-nosylhydrazones.
With these optimized conditions in hand, we investigated the
scope of this novel transformation (Scheme 1). Using alkynyl
N-nosylhydrazone 1a, a productive conversion to allenes was
achieved with a variety of aryl, heteroaryl, and alkyl boronic acids.
Aryl boronic acids with different electron-donating (Me) and
electron-withdrawing groups (CF3, CN, Cl) at the para, meta, or
ortho positions efficiently provided 1,3-diarylallenes (4b-4e) in
good yield (55% to 82%). Gratifyingly, the reactions with
naphthyl (1f) and two heteroaryl boronic acids (1g and 1h) also
proceeded smoothly, producing the corresponding allenes in good
yields. The reaction performed well with aliphatic boronic acids
(1i and 1j), too. We then investigated the reaction scope for N-
nosylhydrazones with a variety of substituents on the phenyl ring,
obtained from a variety of alkynals, with p-bromo-phenylboronic
acid. Aryl substituents such as methyl, chloro, and nitro groups
did not negatively impact on the reaction, affording the target
Encouraged by these results, we attempted to expand the
scope of this metal-free synthetic methodology to the synthesis of
trisubstituted allenes. A wide range of N-nosylhydrazones of
alkynones reacted smoothly with p-bromo-phenylboronic acid,
providing various trisubstituted allenes in moderate to good yield
(4r-4w, 31% to 76%, Scheme 1). A diaryl-substituted alkynyl
N-nosylhydrazone gave the triaryl allene 4t in somehow reduced
yield, presumably because of steric hindrance that would result in
an increased proclivity to undergo the intramolecular cyclization
to pyrazole. Phenyl, 3-thiophenyl, and isopropyl boronic acid
participated in the coupling reaction efficiently, to give the
corresponding trisubstituted allenes in synthetically meaningful
yields (4x-4z, 42% to 58%). The overall excellent scope of
reactivity and the broad tolerance of functional groups make this
2
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metal-free reaction amenable to routine chemical synthesis. We
have therefore devised a novel approach to di- and trisubstituted
allenes by the coupling under metal-free conditions of readily
available alkynyl N-nosylhydrazones with commercially available
boronic acids.
Mechanistic studies were undertaken to unravel the reaction
pathway. When alkyne 3a was independently synthesized and
subjected to the standard basic conditions for the known
rearrengement to allene,[23] only a trace amount of allene 4a was
observed (Scheme 2, eq. 1). Interestingly, the ratio of allene 4a
and alkyne 3a did not change with reaction time (Scheme 2, eq. 2).
These results seem to refute the notion that allene 4a would form
by rearrangement of alkyne 3a. Reaction of 1a with 2a in
anhydrous 1,4-dioxane at 110 oC in the presence of D2O (2.0
equiv) led to the formation of deuterated allene D-[4a] with 100%
deuterium content, and of deuterated alkyne D-[3a] with 70%
deuterium content (Scheme 2, eq. 3). These results clearly
indicated that the adventitious water in the reaction solvent was
the proton source of the products.
Scheme 3. Postulated reaction pathway.
In conclusion, with our work the expediency of the
dissociation of alkynyl N-sulfonylhydrazones into alkynyl
diazomethanes was realized for the first time by incorporating the
nosyl group in the substrate. The synthetic potential of
alkynyldiazomethanes was demonstrated by their unprecedented
coupling with boronic acids in the absence of metals, giving an
easy access to a wide range of allenes. Preliminary mechanistic
studies implied a γ-protodeboration of the intermediate alkynyl
propargyl boric acid may be responsible for the formation of the
allene product.
Acknowledgements
This work was supported by the NSFC (21522202, 21372038), the Ministry of
Education of the People’s Republic of China (NCET-13-0714), the Jilin Provincial
Research Foundation for Basic Research (20140519008JH), and Fundamental
Research Funds for the Central Universities (2412015BJ005).
Keywords: acetylenic carbene • N-nosylhydrazone •
boronic acid • allene • coupling reaction
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N-Nosylhydrazones
Yang Yang, Zhaohong Liu, Alessio
Porta, Giuseppe Zanoni* and Xihe
Bi*..........…...… Page – Page
Alkynyl N-Nosylhydrazones:
Easy Decomposition to Alknynl
Diazomethanes and Application
for Allene Synthesis
Formation of pyrazole by-product is means of introduction of nosyl
the frequently encountered side
reaction in the dissociation of
alkynyl-N-sulfonylhydrazones,
which represents a long-standing
challenge for chemists to generate
acetylenic carbenes for thereafter
applications. We resolved this by
group in the place of tosyl moiety of
alkynyl-N-tosylhydrazones, thus
generating the required acetylenic
carbenes for the
coupling/isomerization reactions
with aryl/alkyl boronic acids.
5
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