A rhodium-catalyzed cascade cyclization: Direct synthesis of N-substituted phthalimides from isocyanates and benzoic acids

Article abstract of DOI:10.1002/adsc.201300834

A rhodium(III)-catalyzed amidation between benzoic acids and isocyanates via direct functionalization of an ortho C-H bond followed by intramolecular cyclization is described. This cascade cyclization affords N-substituted phthalimides in one step in 26-91% yields. The reaction is highly atom-economical, since no theoretical waste except for water is generated in the reaction.

Full text of DOI:10.1002/adsc.201300834

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DOI: 10.1002/adsc.201300834
A Rhodium-Catalyzed Cascade Cyclization: Direct Synthesis of
N-Substituted Phthalimides from Isocyanates and Benzoic Acids
Xian-Ying Shi,a,b Andrea Renzetti,a Soumen Kundu,a and Chao-Jun Lia,*
a
Department of Chemistry, McGill University, Montreal, QC, Canada H3A 0B8
Fax : (+1)-514-398-8457; phone: (+1)-514-398-3797; e-mail: cj.li@mcgill.ca
School of Chemistry and Chemical Engineering, Shaanxi Normal University and Key Laboratory for Macromolecular
b
Science of Shaanxi Province, Xi’an 710062, Peopleꢀs Republic of China
Received: September 12, 2013; Revised: November 18, 2013; Published online: February 20, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300834.
À
Abstract: A rhodium
tween benzoic acids and isocyanates via direct func-
tionalization of an ortho C H bond followed by in-
tramolecular cyclization is described. This cascade
cyclization affords N-substituted phthalimides in
one step in 26–91% yields. The reaction is highly
atom-economical, since no theoretical waste except
for water is generated in the reaction.
G
Chelation-assisted metal-catalyzed C H bond acti-
vation at the ortho position of aromatic rings is
emerging as a powerful method for the formation of
[11]
À
C C bonds. This strategy also provides an efficient
route to the construction of cyclic compounds,[12] in-
cluding phthalimides.[13] For instance, phthalimides
can be synthesized by Ru(II)-[13a] or Rh
ACHTNUTRGNE(NUG III)-cataly-
zed[13b] ortho-carbonylation of benzamides, followed
by intramolecular cyclization.
Keywords: amidation; C H activation; cascade cyc-
lization; isocyanates; phthalimides; rhodium
In this context, we have recently reported the
direct synthesis of 3-substituted phthalides from ben-
zoic acids and aldehydes via rhodium
(III)-catalyzed
[14]
À
ortho C H bond activation.
Because isocyanates
À
are known to undergo C H bond functionalization re-
actions,[15] we reasoned that a cascade cyclization be-
Phthalimides are an important class of compounds tween benzoic acids and isocyanates could afford N-
with various biological activities such as antimicrobial, substituted phthalimides following a similar mecha-
anticonvulsant, antihistaminic, anti-inflammatory, and nism. In this paper, we present an unprecedented
hypolipidemic.[1] They are also used as synthetic inter- rhodium
ACTHNUGRTENUNG(III)-catalyzed direct functionalization of
À
mediates in the preparation of dyes, pesticides, liquid ortho C H bonds of benzoic acids followed by intra-
crystals, functional materials, and polymers.[2]
molecular cyclization to afford N-substituted phthali-
Due to their wide range of applications, the synthe- mides (Scheme 1).
sis of phthalimides has received considerable atten-
We began our study using ortho-toluic acid (1a)
tion. Typically, the synthesis of N-substituted phthali- and phenyl isocyanate (2a) as model substrates. No
mides involves condensation of a phthalic acid and desired product was detected in the presence of
a primary amine.[3] However, this reaction requires 5 mol% of [Cp*RhCl2]2 in toluene at 1008C (Table 1,
high temperature and long reaction time. Alternative
approaches include carbonylative cyclization of ortho-
dihaloarenes[4a–c] or ortho-halobenzoates[4d,e] and pri-
mary amines; cycloaminocarbonylation of ortho-diha-
loarenes, ortho-halobenzoic acids, and ortho-halo
esters with formamides;[5] and reaction between 1,2-
phenylenedimethanol and amines.[6] Condensation of
phthalic acid with N,N’-disubstituted ureas,[7] N-aryla-
tion of phthalimide with arylboronic acids[8] or un-
functionalized arenes,[9] and coupling of isocyanates
with ortho-iodobenzoates[10] have also been reported.
Despite their advantages, these methods have a draw-
back in common: they require starting materials that
are not readily available.
À
Scheme 1. Rhodium-catalyzed C H activation–cyclization
cascade to access N-substituted phthalimides.
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Xian-Ying Shi et al.
Table 1. Optimization of reaction conditions (selected result-
Dioxane turned out to be the best among the tested
solvents (Table 1, entry 10).
s).[a]
The effects of additive amount and temperature
were investigated as well. A decrease in the amount
of NaOAc from 2.0 to 1.0 equiv. afforded a slight in-
crease in reaction yield from 65 to 72% (Table 1,
compare entries 2 and 15). The use of a higher tem-
perature or one lower than 1008C did not provide
any improvement in reaction performance (Table 1,
entries 17 and 18, respectively). Finally, the use of
3.0 equiv. of 2a realized an increase in yield to 84%
(Table 1, entry 19).
Under our optimized conditions, we investigated
the reaction scope with respect to both acids and iso-
cyanates (Table 2). Most benzoic acids bearing elec-
tron-donating groups in the para position afforded
higher yields than unsubstituted benzoic acid or ben-
zoic acids with electron-withdrawing groups (Table 2,
compare the yields of 3d, 3e, and 3g with those of 3b
and 3c, respectively). No desired product was isolated
using 4-nitrobenzoic acid. The same trend was ob-
served for ortho-substituted benzoic acids, with 2-
Boc-aminobenzoic acid affording the product in 62%
yield and 2-fluorobenzoic acid being unreactive. 2-
Methoxybenzoic acid, although bearing an electron-
donating group on the ring, was not a suitable sub-
strate for the reaction, possibly due to chelation deac-
tivation. Such a substituent effect suggests an aromat-
Entry
Additive (equiv.)
Solvent
Yield [%][b]
1
2
3
4
5
6
7
8
none
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
dioxane
THF
ND[c]
65
36
NaOAc (2.0)
NaHCO3 (2.0)
Na2CO3 (2.0)
AgOAc (2.0)
58
traces
ND[c]
ND[c]
47
16
71
60
68
63
26
CuACHTUNGTRENNUNG(OAc)2 (2)
Cu(OAc)2·H2O (2.0)
ACHTUNGTRENNUNG
K2HPO4 (2.0)
Ag2CO3 (2.0)
NaOAc (2.0)
NaOAc (2.0)
NaOAc (2.0)
NaOAc (2.0)
NaOAc (2.0)
NaOAc (1.0)
NaOAc (0.5)
NaOAc (1.0)
NaOAc (1.0)
NaOAc (1.0)
9
10
11
12
13
14
15
16
17[d]
18[e]
19[f]
PhCl
o-xylene
CH3CN
dioxane
dioxane
dioxane
dioxane
dioxane
72
67
70
25
84
[a]
Reactions conditions: ortho-toluic acid (0.10 mmol),
phenyl isocyanate (0.25 mmol), [Cp*RhCl2]2 (5 mol%),
additive (0.5–2.0 equiv.), solvent (0.5 mL), 1008C, 36 h,
argon.
À
ic electrophilic substitution (SEAr) mechanism for C
H bond activation. In line with this hypothesis, benzo-
ic acids bearing two electron-donating substituents af-
forded higher yields than the corresponding mono-
substituted acids (Table 2, compare the yields of 3m
and 3l with 3e and 3g, respectively). Trialkoxy-substi-
tuted benzoic acids provided the corresponding prod-
ucts 3r and 3t in high (>70%) yields as well. More-
over, the yields of compounds 3c and 3e–j could be
[b]
1
Determined by H NMR analysis of the crude reaction
mixture using mesitylene as an internal standard.
ND=Not detected.
Temperature was 1158C.
Temperature was 808C.
ortho-Toluic acid (0.1 mmol) and phenyl isocyanate
(0.3 mmol) were used.
[c]
[d]
[e]
[f]
increased by using
a
combination of NaOAc
À
entry 1). Because sodium acetate promotes ortho C
(0.75 equiv.) and AgOAc (0.25 equiv.) as the additives.
H bond activation of benzoic acids,[16] we tested the The promoting effect of silver(I) is likely due to the
effect of this salt on our reaction. We were pleased to trapping of chloride ligands, which generates a highly
find that the cyclization product was formed in 65% electrophilic cationic complex of rhodium.[17]
NMR yield in the presence of 2.0 equiv. of NaOAc
meta-Substituted benzoic acids, being non-symmet-
(Table 1, entry 2). Replacement of NaOAc with other rical, afforded a mixture of regioisomers (3o and 3o’,
sodium salts such as NaHCO3 and Na2CO3 led to a de- 3q and 3q’, 3s and 3s’, and 3u and 3u’). In this case,
crease in reaction yield (Table 1, compare entry 2 the major product was that derived from substitution
with entries 3 and 4, respectively). Acetates of metals at the less hindered position, highlighting the impor-
other than sodium such as AgOAc, Cu
ACHTUNGRTEN(NUGN OAc)2, and tance of steric effects of the substituents. meta-Me-
Cu(OAc)2·H2O displayed low or no catalytic activity thoxybenzoic acid and meta-Boc-aminobenzoic acid
ACHTUNGTRENNUNG
(Table 1, entries 5–7). K2HPO4 and Ag2CO3, differing provided higher yields than the corresponding o- or
from NaOAc in both cation and anion, were also p-substituted compounds (Table 2, compare the yield
poorly effective (Table 1, entries 8 and 9, respective- of 3o/3o’ with 3h, and the yield of 3s/3s’ with 3g and
ly). These results indicate that additive selection is 3d, respectively). The high conversion observed for
crucial for the reaction outcome. Screening of sol- meta-substituted benzoic acids can be explained by
vents revealed that common solvents such as dioxane, the fact that meta-substituents donate electrons to the
THF, PhCl, and ortho-xylene can be used in the reac- benzene ring in the ortho-position with respect to the
À
tion in addition to toluene (Table 1, entries 10–13). carboxyl group, promoting C H bond activation.
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A Rhodium-Catalyzed Cascade Cyclization
Table 2. Reaction scope.[a]
[a]
Reactions conditions: acid (0.2 mmol), isocyanate (0.6 mmol), [Cp*RhCl2]2 (5 mol%), NaOAc (0.2 mmol), diox-
ane (0.5 mL), 1008C, 36 h, argon. Yield of purified product is reported.
NaOAc (0.15 mmol) and AgOAc (0.05 mmol) were used. Yield of purified product is reported.
[b]
To further explore the substrate scope and limita- cyanates are less reactive than aliphatic ones due to
tions, we tested the reaction of 3,4,5-trimethoxybenzo- electron donation from the ring, which makes the car-
ic acid and various isocyanates (Table 2). Aromatic bonyl carbon less electrophilic.[18] Notably, the toler-
isocyanates were more reactive than aliphatic isocya- ance of chloro group offers the opportunity for fur-
nates in our reaction (compare the yields of 3v–3z ther transformations through common cross-coupling
with those of 3aa and 3ab). This observation is in con- strategies. However, no desired product was obtained
trast with the reactivity trend of isocyanates in most in the reaction of benzoic acid and 4-nitrophenyl iso-
nucleophilic addition reactions, in which aromatic iso- cyanate.
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Xian-Ying Shi et al.
the basis of the proposed mechanism as well. The
phenyl ring of aromatic isocyanates, being electron-
donating by resonance, increases the nucleophilicity
of C=N double bond, favoring coordination of isocya-
nate to rhodium and the following insertion step.
Such a resonance effect is absent in aliphatic isocya-
nates, which hence coordinate to metal less effectively
and display lower reactivity.
The lack of reactivity observed for 4-nitrophenyl
isocyanate is likely due to the effect of the nitro
À
group, which increases the delocalization of the C N
p bond onto the phenyl ring. Delocalization makes
À
the C N p bond unavailable for coordination to the
metal, preventing the formation of complex
(Scheme 2) and hence the formation of product.
D
In conclusion, we have reported the synthesis of N-
substituted phthalimides from benzoic acids and iso-
cyanates under rhodiumACTHNUTRGNE(NUG III) catalysis. The reaction
involves a cascade amidation of benzoic acid and iso-
À
cyanate via Rh
(III)-catalyzed C H bond functionali-
Scheme 2. Tentative mechanism for the rhodium-catalyzed
cascade cyclization.
zation, followed by intramolecular cyclization. It pro-
vides a direct access to phthalimides in a simple one-
pot procedure, uses readily available starting materi-
als with wide scope, and is highly atom-economical,
Based on the above results and the knowledge of generating only water as theoretical waste. In view of
À
analogous chelation-assisted C H bond activation re- its advantages, this procedure is expected to find
actions,[19] a plausible mechanism for the rhodium-cat- broad application in organic synthesis.
alyzed reaction of benzoic acids and isocyanates is
outlined in Scheme 2. Dissociation of the dimer pre-
catalyst [Cp*RhCl2]2 provides the coordinatively un- Experimental Section
saturated monomer, which undergoes ligand exchange
with NaOAc to afford the active species A. With the General Procedure
help of NaOAc, A is expected to readily coordinate
An oven-dried reaction vessel was charged with
[Cp*RhCl2]2 (6.18 mg, 5 mol%, 0.01 mmol), NaOAc
to the carboxylic oxygen to form rhodium benzoate
B.[16] Subsequent ortho-C H bond activation leads to
À
(16.4 mg, 0.2 mmol), substituted benzoic acid (0.2 mmol)
and isocyanate (0.6 mmol). The tube was evacuated and
purged with argon three times, and then dioxane (0.5 mL)
was added to the vial by syringe. The mixture was stirred at
1008C (oil bath temperature) for 36 h. After this time, the
resulting mixture was cooled down to room temperature, fil-
tered through a short pad of silica gel, and then concentrat-
ed under vacuum. The residue was purified by preparative
TLC (eluent: dichloromethane) to afford the corresponding
product.
rhodacycle C with concomitant formation of acetic
acid. After coordination of isocyanate, the C=N
À
double bond inserts into the Rh C bond in intermedi-
ate D, with generation of rhodium alkoxide E and for-
À
mation of a new C C bond. Intramolecular dehydra-
tion of intermediate E provides the final product F
and regenerates the active catalytic species A. The ki-
netic isotope effect (KIE) was measured to be 0.4
(see the Supporting Information for details). This in-
À
verse KIE value indicates that C H bond activation
is the rate-limiting step, and is consistent with an elec-
trophilic aromatic substitution (SEAr) mechanism for Acknowledgements
the addition of RhACTHNUTRGNE(NUG III) to the benzene ring of benzoic
acid.[20] Such a mechanism involves a change in hy-
bridization of the carbon with the labelled hydrogen
from sp2 to sp3 in the rate-limiting step. Because the
We are grateful to the Canada Research Chair Foundation
(to C.-J. Li), the CFI, FQRNT Center for Green Chemistry
and Catalysis, NSERC, and McGill University for support of
our research. Dr. Xianying Shi is grateful for the support
from Shaanxi Normal University Program of Training and
Studying Abroad for Teachers.
À
À
difference in zero point energy between C D and C
H bonds is larger in the transition state than in the
ground state, the deuterated substrate reacts faster
than the non-deuterated substrate, leading to an in-
verse KIE for an SEAr mechanism.[21] The reactivity
trend observed for isocyanates can be explained on
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Adv. Synth. Catal. 2014, 356, 723 – 728