Ruthenium(II)-Catalyzed C-H Functionalizations with Allenes: Versatile Allenylations and Allylations

Article abstract of DOI:10.1002/chem.201502785

Ruthenium(II)-catalyzed direct C-H functionalization of aromatic compounds with allenes was achieved under exceedingly mild reaction conditions to yield trisubstituted allenes. The reactions of N-methoxybenzamides proceeded smoothly in an isohypsic fashio

Full text of DOI:10.1002/chem.201502785

DOI: 10.1002/chem.201502785
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CÀH Activation |Hot Paper|
Ruthenium(II)-Catalyzed CÀH Functionalizations with Allenes:
Versatile Allenylations and Allylations
Sachiyo Nakanowatari and Lutz Ackermann*[a]
Abstract: Ruthenium(II)-catalyzed direct CÀH functionaliza-
tion of aromatic compounds with allenes was achieved
under exceedingly mild reaction conditions to yield trisubsti-
tuted allenes. The reactions of N-methoxybenzamides pro-
ceeded smoothly in an isohypsic fashion at ambient temper-
ature with high chemo- and regioselectivity, thereby provid-
ing a versatile means of accessing trisubstituted allenes. De-
tailed mechanistic studies were suggestive of a kinetically
relevant CÀH metalation step, which occurs by the assis-
tance of a carboxylate moiety; this also set the stage for un-
precedented CÀH allylations with removable directing
groups in a step-economical fashion.
Introduction
During the last decades, allenes have attracted the interest of
many chemists due to their unique structural and electronic
features. The allene moiety is present in a number of natural
products, pharmaceutical compounds, and materials;[1–3] fur-
thermore, allenes are particularly valuable precursors[4] in or-
ganic synthesis.[5] However, despite their importance, the use
of allenes in the field of the transition-metal-catalyzed CÀH ac-
tivation[6] is underdeveloped, whereas alkynes and alkenes
have been actively studied for catalytic CÀH functionaliza-
tions.[7] Indeed, achieving chemo-, regio-, and diastereoselectiv-
ity[8] in CÀH functionalizations with allenes is particularly diffi-
cult. An early example of CÀH functionalization with allenes
was reported by Krische in 2009, in which a cationic iridium
catalyst was used to yield allylated arenes.[9] Thereafter, the use
of allenes in tandem cyclizations,[10] annulations,[11] allyla-
tions,[12] dienylations,[13] and allenylations[14] has been devel-
oped through aromatic CÀH bond activation by exploiting rho-
dium, rhenium, or palladium catalysis. However, to the best of
our knowledge, an economically favorable[15] ruthenium-cata-
lyzed CÀH functionalization[16] with allenes has so far proven to
be elusive. Within our research on sustainable CÀH activa-
tion,[17] we now report a novel ruthenium(II)-catalyzed CÀH
functionalization with gem-disubstituted allenylsilanes to ach-
ieve aromatic CÀH allenylation as well as an unprecedented al-
lylation of an aromatic compound with a removable directing
group (Scheme 1).
Scheme 1. Ruthenium(II)-catalyzed CÀH functionalization by insertion into
unsaturated CÀC bonds.
Results and Discussion
Optimization studies
We commenced our studies on the envisioned ruthenium-cata-
lyzed CÀH bond functionalization with allenes by probing vari-
ous Lewis basic directing groups. We selected the gem-disub-
stituted silylallene 2a with the expectation that allenes would
preferentially react at the terminal position due to the steric re-
pulsion between the substituents on the allene and the ruthe-
nium complex, thereby ensuring a predictable regioselectivi-
ty.[9,11h,12,14] Also, we expected that the silyl substituents would
enhance the inherent reactivity of the allenes. To our delight,
we found that ruthenium complexes were indeed capable of
catalyzing the desired allenylation reaction[14] of N-methoxy-
benzamide (1a) by an isohypsic CÀH functionalization strategy
[a] S. Nakanowatari, Prof. Dr. L. Ackermann
Institut für Organische und Biomolekulare Chemie
Georg-August-Universität Gçttingen
Tammannstrasse 2, 37077 Gçttingen (Germany)
E-mail: Lutz.Ackermann@chemie.uni-goettingen.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201502785.
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Table 1. Optimization of CÀH bond allenylation of amide1a.[a]
Table 2. Influence of the amide 1 substitution pattern.[a]
Entry [Ru]
Additive Solvent
Yield [%]
Entry
1
Yield of 3 [%]
1
2
3
4
5
6
7
8
RuCl3·(H2O)n
[Cp*RuCl2]n
[RuCl2(benzene)]2
[RuBr2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[Ru(OAc)2(p-cymene)]
[Ru(OAc)2(p-cymene)]
[Ru(OAc)2(p-cymene)]
[RuCl(OAc)(p-cymene)]
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
–
NaOAc
KCl
–
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH/H2O (20:1) 75
DCE
PhMe
MeOH
MeOH
–
26
58
52
75
77[b]
24
41
46
–
1
2
3
4
5
6
R1 =Me (1a)
R1 =Et (1b)
R1 =iPr (1c)
R1 =tBu (1d)
R1 =Bn (1 f)
R1 =Ph (1e)
3aa: 75
3aa: 71
3aa: 8[b]
–
3aa: 18[b]
3aa: 5[b]
9
10
11
12
13
14
15
16
17
[RuCl(OAc)(p-cymene)] NaOAc
65
3[c]
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2
–
–
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
34
5[c]
7
8
–
56[d]
–
[a] Reaction conditions: 1a (0.50 mmol), 2a (0.53 mmol), [Ru] (10 mol%),
additive (30 mol%), solvent (3.0 mL), 228C, 18 h, under N2, yields of isolat-
ed products. [b] With 0.75 mmol of allene 2a. [c] Conversion determined
by 1H NMR spectroscopy with CH2Br2 as the internal standard. [d] At 08C.
–
[a] Reaction conditions:
1
(0.50 mmol), 2a (0.53 mmol), [RuCl2(p-
cymene)]2 (5.0 mol%), NaOAc (30 mol%), MeOH (3.0 mL), 228C, 18 h,
under N2, yields of isolated products. [b] Conversion determined by
1H NMR spectroscopy with CH2Br2 as the internal standard.
(Table 1 and Tables SI1 and SI2, Supporting Information).[18]
Among a set of representative ruthenium complexes, [RuCl2(p-
cymene)]2 catalyzed the reaction to deliver the desired product
3aa in the highest yield (Table 1, entries 1–5). The well-defined
ruthenium(II) biscarboxylate complex [Ru(OAc)2(p-cymene)][19]
delivered a moderate yield of product 3aa, even when NaOAc
or KCl[20] were used as additives (entries 7–9). [RuCl(OAc)(p-
cymene)][21] did not furnish the desired product 3aa, but the
catalytic activity could be restored through the addition of
NaOAc (entries 10 and 11). After testing a variety of cocatalytic
additives and solvents, we found that acetates and protic sol-
vents proved to be superior, and optimal results were obtained
with NaOAc and MeOH (entries 12–15). The reaction proceed-
ed smoothly under exceedingly mild reaction conditions of
228C;[22] in fact, the catalyst was also operative at 08C
(entry 16). In the absence of a ruthenium complex, the sub-
strates 1a and 2a did not react (entry 17).
Subsequently, we examined the influence of the substituents
on the N-alkoxyl moiety (Table 2). N-Methoxy- and N-ethoxy-
substituted amides 1a and 1b reacted with comparable con-
version (Table 2, entries 1 and 2). Amides with a more hindered
benzyl or iso-propyl group reacted sluggishly (entries 3 and 5),
and substrates with phenyl or tert-butyl groups did not partici-
pate in the CÀH activation reaction (entries 4 and 6), probably
due to unfavourable steric interactions. Similarly, no conversion
was observed with the tertiary Weinreb benzamide (1g,
entry 7), which suggests that the coordination by an anionic
amide is essential. The amides that bear other potential leaving
groups, such as acetyl (entry 8), pivaloyl, tert-butyloxycarbonyl,
or benzoyl substituents, were found to be ineffective.
Substrate scope
To evaluate the versatility of the ruthenium(II)-catalyzed pro-
cess, the optimized reaction conditions were applied to vari-
ously substituted N-methoxybenzamides 1i–t (Scheme 2). Ac-
cordingly, high yields of the desired allenylated products 3ia–
na were obtained with electron-rich substituents, whereas
electron-deficient aromatic compounds 1 proved to be more
challenging. Amides with meta-methyl, meta-trifluoromethyl,
or meta-chloro substituents (1o–q) furnished a single isomer
through the functionalization at the less sterically encumbered
CÀH bond. The disubstituted amide 1r and the naphthoic
amide 1s also underwent the CÀH functionalization process
with excellent site-selectivity. Notably, the hindered di-meta-
substituted aromatic compound 1t delivered the desired prod-
uct 3ta with high catalytic efficacy.
Subsequently, differently decorated allenes 2b–i were sub-
jected to the optimized reaction conditions (Scheme 3). Under
the mild reaction conditions, a variety of silylated gem-disubsti-
tuted allenes reacted to give products 3ab–ai, despite the
bulk of the silyl groups. We observed that the reactivity of the
allenes was strongly influenced by the substituents on the
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Scheme 2. Scope of ruthenium(II)-catalyzed allenylation: N-methoxyaryla-
mides 1.
Scheme 4. Intermolecular competition experiments.
Scheme 3. Scope of ruthenium(II)-catalyzed allenylation: allenes 2.
(Scheme 4a). Competition experiments between allene 2a and
phenyl- or butyl-disubstituted alkynes 4a or b, as well as
alkene 6a reflected the challenges that are associated with CÀ
H functionalizations using allenes 2, such that the alkyne or
alkene reacted preferentially over the allene (Scheme 4b and
c).
allene. Allenes 2b, 2c, 2 f, and 2h reacted to give the desired
products 3ab, 3ac, 3af, and 3ah in high yields, whereas allene
substrates 2 that contained methyl, cyclopropyl, benzyl, or
phenyl groups reacted rather sluggishly. Monosubstituted al-
lenes furnished less satisfactory results.[23]
To further corroborate the reaction mechanism, a set of ex-
periments with isotopically labeled compounds were conduct-
ed (Scheme 5 and 6). In reactions that were carried out in deu-
terated solvent [D4]MeOH or with isotopically labeled substrate
[D5]1a, we did not observe an H/D exchange, which is sugges-
tive of an irreversible CÀH bond metalation step. In agreement
with these findings, intra- and intermolecular kinetic isotope
effects (KIE) were determined to be 2.6 and 2.7, respectively.
KIEs of this magnitude were indicative[24] of a kinetically rele-
vant CÀH ruthenation step.
Mechanistic studies
Given the unique regio- and site-selectivity features of our
ruthenium(II) catalyst, we sought to delineate its mode of
action. To this end, we performed intermolecular competition
experiments. When a mixture of amides 1j and 1m was ex-
posed to the ruthenium-catalyzed allenylation conditions, we
observed that the more electron-poor benzamide 1j reacted
preferentially, which could be rationalized in terms of the
higher kinetic acidity of the CÀH bond in benzamide 1j
On the basis of our mechanistic studies, we propose that
the ruthenium(II)-catalyzed CÀH functionalization with allenes
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Scheme 6. Kinetic isotope effect (KIE) study.
Scheme 5. Ruthenium(II)-catalyzed allenylation with isotopically labeled
compounds.
2 is initiated through the coordi-
nation of acetate complex
A
with the nitrogen of benzamide
1, which is followed by a kineti-
cally relevant, carboxylate-assist-
ed[25] CÀH metalation to form
ruthenacycle B (Scheme 7). After
the isohypsic CÀH activation, in-
termediate B is coordinated by
allene 2 (intermediate C);[26] sub-
sequent
migratory insertion
yields the 7-membered inter-
mediate D, which undergoes
protonation (intermediate E) fol-
lowed by syn-b-H elimination to
give intermediate F. Thereafter,
oxidative insertion of the NÀO
bond in intermediate F leads to
intermediate G, which is proton-
ated to allow liberation of the
product 3 and regenerates the
active ruthenium(II) catalyst A.
Finally, we were pleased to
find that the ruthenium(II) cata-
lyst [RuCl2(p-cymene)]2 was not
limited to oxidative allenylation
reactions, but it also proved ap-
plicable for hydroarylations of
allene 2a with 2-phenoxypyri-
Scheme 7. Proposed catalytic cycle.
dine[27] 8a, which contains a removable directing group
(Scheme 8). Preliminary optimization studies showed that the
use of iPrOH as the solvent was beneficial for a highly regio-
and site-selective allylation of phenol derivative 8a.
Conclusion
We have developed the unprecedented ruthenium(II)-catalyzed
intermolecular direct arene functionalization reaction with al-
Scheme 8. Ruthenium(II)-catalyzed hydroarylation with allene 2a.
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Experimental Section
Representative procedure for ruthenium(II)-catalyzed CÀH
functionalization of benzamides with allenylsilanes
[RuCl2(p-cymene)]2 (15.3 mg, 5.0 mol%), NaOAc (12.3 mg,
30 mol%), N-methoxybenzamide (1a, 75.5 mg, 0.50 mmol), MeOH
(3.0 mL), and allene 2a (89 mg, 0.53 mmol) were placed into
a 25 mL Schlenk tube that was equipped with a septum and
placed under N2. The reaction mixture was stirred at 228C for 22 h.
After evaporation of the solvents and the unreacted allene in
vacuo, the crude product was purified by column chromatography
on silica gel (CH2Cl2/EtOAc, 8:1) to afford the desired product 3aa.
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Acknowledgements
Financial support by the European Research Council under the
European Community’s Seventh Framework Program (EP7
2007-2013)/ERC Grant agreement no. 307535 and the Japanese
Student Service Organization (fellowship to S.N.) is greatly ac-
knowledged.
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Keywords: allenes · allenylation · amides · CÀH Activation ·
ruthenium
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[23] The following monosubstituted allenes were thus far explored:
Received: July 15, 2015
Published online on September 17, 2015
Chem. Eur. J. 2015, 21, 16246 – 16251
www.chemeurj.org
16251
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