Palladium Nanoparticles Supported on Fibrous Silica (KCC- 1-PEI/Pd): A Sustainable Nanocatalyst for Decarbonylation Reactions Pintu K. Kundu, [a] Mahak Dhiman, [b] Atanu Modak, [a] Arindam Chowdhury,* [a] Vivek Polshettiwar,* [b] and Debabrata Maiti* [a] A practical and convenient decarbonylation of a variety of aro- matic, heteroaromatic, and alkenyl aldehydes by using palladi- um nanoparticles supported on novel, fibrous nanosilica, named KCC-1-PEI/Pd, has been developed. Complete conver- sion of aldehyde functionalities into deformylated products was achieved in all cases and in nearly all cycles tested by re- using the catalyst systems. This method eliminates further pu- rification of products after their isolation. Syntheses of at least three different deformylated products have been shown in se- quence with the same catalyst system, which neither requires use of any additives, such as oxidants and bases, nor CO scav- engers. In view of catalysis on solid supports, metal nanoparticles with a support of choice provides a large field for the discovery of new, highly active nanocatalysts for important and challenging reactions, which also offer the additional advantage of recycla- bility. [1–3] These systems have several advantages over conven- tional catalysts, such as superior activity and improved stability. With the innovation of high-surface-area silica with a unique fi- brous morphology (KCC-1), [4] in which the surface area of KCC- 1 is attributable to fibers and not to pores, paved the way to enhanced accessibility of catalytic active sites. Towards sustain- able nanocatalysis, this unique property of silica-supported nanocatalysts [5–9] has been successfully used to demonstrate various catalytic reactions, such as hydrometathesis of olefins by using a KCC-1/TaH catalyst system, [7] Suzuki coupling reac- tions with KCC-1/Pd, [8] and hydrogenolysis of propane and ethane with a KCC-1/Ru nanocatalyst. [9] The removal of functional groups from organic molecules, [10] especially decarbonylation of aldehydes has drawn considera- ble attention over the decades because such processes enable the provisional use of the beneficial features of the ÀCHO functionality, such as regioselectivity in the Diels–Alder reac- tion, [11] in the domino oxa-Michael aldol reaction, [12] and the biosynthesis of alka(e)nes from fatty aldehydes. [13, 14] Metal complexes of transition metals, such as ruthenium, [15] iridi- um, [16] rhodium, [17–22] and palladium, [23–28] have been successful- ly used for deformylation reactions, but they usually require high temperature. Various supported noble-metal-catalyzed re- actions were also employed for the same purpose. [29–31] Recent- ly, we have observed a cost-effective protocol in which com- mercially available Pd(OAc) 2 has been used and shown wide substrate scopes with a catalyst loading of 8–16 mol % under mild conditions. [32–34] On the other hand, there is only a single report of a selective deformylation reaction with metal nano- particles. [35] Decarbonylation of biomass-derived molecules, such as 5-hydroxymethylfurfural and furfural, was achieved with palladium nanoparticles (PdNPs) that were impregnated on mesoporous silica support SBA-15 to provide furfuryl alco- hol and furan, respectively. Although the procedure looked ex- cellent for converting biomass-derived molecules, it suffered from limitations such as substrate scope and recyclability. We have recently observed that the morphology of the support has dramatic effects on its catalytic activity: multifold enhance- ment of nitridated base-catalyzed Knoevenagel condensations and trans-esterification reactions were observed when replac- ing the solid support from conventional silica, such as SBA-15, with fibrous nanosilica, KCC-1. [36, 37] Owing to its immense importance in chemistry, including the synthesis of natural products, [38, 39] there is a need to devel- op an environmentally benign decarbonylation method. The protocol is expected to show substrate scopes similar to transi- tion-metal-catalyzed homogeneous reactions, along with ac- ceptable catalyst recyclability and efficiency similar to hetero- geneous reactions and preferentially easy product separation without extra purification steps. Herein, we report a practical and convenient way of performing a decarbonylation reaction by using PdNPs supported on fibrous silica, KCC-1. Prompted by the efficiency of high-surface-area KCC-1, we planned to modify the fibers with PdNPs to use the final mate- rials towards deformylation reactions. The modification of KCC- 1 with PdNPs was achieved as follows: KCC-1 was first func- tionalized with 3-glycidoxypropyltrimethoxysilane (GTMS), which acted as a linker between the silanol groups on the silica surface and amine functionalities of polyethylenimine (PEI). Epoxy group of GTMS underwent ring opening during [a] Dr. P. K. Kundu, A. Modak, Prof. A. Chowdhury, Prof. D. Maiti Department of Chemistry Indian Institute of Technology Bombay Powai, Mumbai 400076 (India) E-mail : [email protected][email protected][b] M. Dhiman, Prof. V. Polshettiwar Department of Chemical Sciences Tata Institute of Fundamental Research (TIFR) Mumbai 400005 (India) E-mail : [email protected]Supporting information for this article can be found under http:// dx.doi.org/10.1002/cplu.201600245. This article is part of the “Early Career Series”. To view the complete series, visit: http://chempluschem.org/earlycareer. ChemPlusChem 2016, 81, 1142 – 1146 # 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1142 Communications DOI: 10.1002/cplu.201600245
Transcript
Palladium Nanoparticles Supported on Fibrous Silica (KCC-1-PEI/Pd): A Sustainable Nanocatalyst for DecarbonylationReactionsPintu K. Kundu,[a] Mahak Dhiman,[b] Atanu Modak,[a] Arindam Chowdhury,*[a]
Vivek Polshettiwar,*[b] and Debabrata Maiti*[a]
A practical and convenient decarbonylation of a variety of aro-matic, heteroaromatic, and alkenyl aldehydes by using palladi-
um nanoparticles supported on novel, fibrous nanosilica,named KCC-1-PEI/Pd, has been developed. Complete conver-
sion of aldehyde functionalities into deformylated products
was achieved in all cases and in nearly all cycles tested by re-using the catalyst systems. This method eliminates further pu-
rification of products after their isolation. Syntheses of at leastthree different deformylated products have been shown in se-
quence with the same catalyst system, which neither requiresuse of any additives, such as oxidants and bases, nor CO scav-
engers.
In view of catalysis on solid supports, metal nanoparticles witha support of choice provides a large field for the discovery of
new, highly active nanocatalysts for important and challengingreactions, which also offer the additional advantage of recycla-
bility.[1–3] These systems have several advantages over conven-
tional catalysts, such as superior activity and improved stability.With the innovation of high-surface-area silica with a unique fi-
brous morphology (KCC-1),[4] in which the surface area of KCC-1 is attributable to fibers and not to pores, paved the way to
enhanced accessibility of catalytic active sites. Towards sustain-able nanocatalysis, this unique property of silica-supported
nanocatalysts[5–9] has been successfully used to demonstrate
various catalytic reactions, such as hydrometathesis of olefinsby using a KCC-1/TaH catalyst system,[7] Suzuki coupling reac-
tions with KCC-1/Pd,[8] and hydrogenolysis of propane andethane with a KCC-1/Ru nanocatalyst.[9]
The removal of functional groups from organic molecules,[10]
especially decarbonylation of aldehydes has drawn considera-
ble attention over the decades because such processes enablethe provisional use of the beneficial features of the @CHO
functionality, such as regioselectivity in the Diels–Alder reac-tion,[11] in the domino oxa-Michael aldol reaction,[12] and the
biosynthesis of alka(e)nes from fatty aldehydes.[13, 14] Metal
complexes of transition metals, such as ruthenium,[15] iridi-um,[16] rhodium,[17–22] and palladium,[23–28] have been successful-
ly used for deformylation reactions, but they usually requirehigh temperature. Various supported noble-metal-catalyzed re-
actions were also employed for the same purpose.[29–31] Recent-ly, we have observed a cost-effective protocol in which com-
mercially available Pd(OAc)2 has been used and shown wide
substrate scopes with a catalyst loading of 8–16 mol % undermild conditions.[32–34] On the other hand, there is only a single
report of a selective deformylation reaction with metal nano-particles.[35] Decarbonylation of biomass-derived molecules,
such as 5-hydroxymethylfurfural and furfural, was achievedwith palladium nanoparticles (PdNPs) that were impregnated
on mesoporous silica support SBA-15 to provide furfuryl alco-
hol and furan, respectively. Although the procedure looked ex-cellent for converting biomass-derived molecules, it suffered
from limitations such as substrate scope and recyclability. Wehave recently observed that the morphology of the support
has dramatic effects on its catalytic activity: multifold enhance-ment of nitridated base-catalyzed Knoevenagel condensations
and trans-esterification reactions were observed when replac-
ing the solid support from conventional silica, such as SBA-15,with fibrous nanosilica, KCC-1.[36, 37]
Owing to its immense importance in chemistry, includingthe synthesis of natural products,[38, 39] there is a need to devel-op an environmentally benign decarbonylation method. Theprotocol is expected to show substrate scopes similar to transi-
tion-metal-catalyzed homogeneous reactions, along with ac-ceptable catalyst recyclability and efficiency similar to hetero-geneous reactions and preferentially easy product separationwithout extra purification steps. Herein, we report a practicaland convenient way of performing a decarbonylation reaction
by using PdNPs supported on fibrous silica, KCC-1.Prompted by the efficiency of high-surface-area KCC-1, we
planned to modify the fibers with PdNPs to use the final mate-rials towards deformylation reactions. The modification of KCC-1 with PdNPs was achieved as follows: KCC-1 was first func-
tionalized with 3-glycidoxypropyltrimethoxysilane (GTMS),which acted as a linker between the silanol groups on the
silica surface and amine functionalities of polyethylenimine(PEI). Epoxy group of GTMS underwent ring opening during
[a] Dr. P. K. Kundu, A. Modak, Prof. A. Chowdhury, Prof. D. MaitiDepartment of ChemistryIndian Institute of Technology BombayPowai, Mumbai 400076 (India)E-mail : [email protected]
[b] M. Dhiman, Prof. V. PolshettiwarDepartment of Chemical SciencesTata Institute of Fundamental Research (TIFR)Mumbai 400005 (India)E-mail : [email protected]
Supporting information for this article can be found under http://dx.doi.org/10.1002/cplu.201600245.
This article is part of the “Early Career Series”. To view the completeseries, visit : http://chempluschem.org/earlycareer.
the SN2 reaction to produce a highly branched network of poly-
meric amines on KCC-1 fibers, denoted as KCC-1-PEI. Subse-
quently, metalation was performed between sodium tetrachlor-opalladate(II) and KCC-1-PEI, which, upon reduction by NaBH4,
yielded highly monodispersed PdNPs. TEM analysis of the syn-thesized catalyst showed a large number of PdNPs supported
on KCC-1 with very narrow particle size distribution rangingfrom 1.8 to 3.5 nm (Figure 1). Energy-dispersive X-ray spectros-
copy (EDS) elemental mapping indicated that the fibers of
KCC-1 were fully loaded with well-dispersed PdNPs, which re-established the advantage of using KCC-1 with unique fibrous
morphology. Palladium loading in the KCC-1-PEI/Pd catalystwas found to be 9.7 wt % by carrying out EDS at 10 different
well-separated positions.[40]
Nitrogen-sorption measurements were performed to assess
the textural properties and surface area of the synthesized cat-
alyst. On investigation, KCC-1, KCC-1-PEI, and KCC-1-PEI/Pdshowed type IV isotherms with an H1-type hysteresis (Fig-
ure 2 a). The BET surface area of synthesized KCC-1 was530 m2 g@1, which was reduced to 271 m2 g@1 upon PEI func-
tionalization. This decrease in surface area was accompaniedby a decrease in pore volume from 0.69 to 0.42 cm3 g@1. Ther-
mogravimetric analysis was performed in air to estimate the
loading of PEI on KCC-1. After initial weight loss owing towater, pure KCC-1 showed no further weight loss, which indi-
cated high thermal stability. The 19 and 18 % weight losses ob-served for KCC-1-PEI and KCC-1-PEI/Pd, respectively, were at-
tributed to the loss of covalently bonded PEI (Figure 2 c). Cova-
lent attachment between KCC-1 and PEI was also validated by29Si CP-MAS NMR spectroscopy analysis. Signals at [email protected],
@97.8, and @106.8 ppm are characteristics of Q2, Q3, and Q4
Figure 1. TEM images (a–c), particle size distributions (d), and EDS mapping(e–h) results of KCC-1-PEI/Pd.
Figure 2. a) Nitrogen adsorption–desorption isotherms, b) pore size distributions, and c) thermogravimetric profiles of KCC-1, KCC-1-PEI, and KCC-1-PEI/Pd,and d) 29Si CP-MAS NMR spectrum of KCC-1-PEI.
sites, and signals at [email protected] and @63.1 ppm are due to theT2 and T3 sites of silica, respectively, which indicates the forma-
tion of silica–carbon bonds through covalent attachment ofPEI with KCC-1 (Figure 2 d).
We started our investigations by using 2-naphthaldehyde asa substrate and reacted it with KCC-1-PEI/Pd (5 mol % Pd) as
a suspension in cyclohexane at 130 8C (Scheme 1). Notably, the
reaction can be performed in air, along with molecular sieves,and thus, the methodology is user-friendly. We observed com-plete consumption of starting aldehyde after 20 h (3A in theSupporting Information) to produce a single product : naphtha-
lene (Scheme 1). The decarbonylated product thus formed wasseparated simply by decantation/filtration after centrifugation.
The filtrate and washings were evaporated to give the pure
product directly in excellent yield (96 %). The remaining solidwas activated by heating under reduced pressure and reused
without further purification in subsequent cycles. The catalystsystem is as effective as that in the initial reaction cycle, even
after using the same catalyst eight times, without loss in effi-ciency of conversion, yield, and purity of the product (Table 1,
left). In a similar manner, decarbonylation of 9-anthracenecar-
boxaldehyde with KCC-1-PEI/Pd proceeded smoothly to pro-vide solid anthracene, 1 b, in more than 95 % yield (up to six
cycles) and excellent purity of solid anthracene simply afterisolation and without further purification (Table 1, right).
Encouraged by the successful deformylation of unfunctional-ized fused-ring aryl aldehydes, benzaldehyde derivatives with
different functionalities have been reacted under the opti-
under the above PdNP-catalyzed reaction, provided completedeformylation with excellent yield of nitrobenzene (2 a and
2 b ; 96 and 94 %, respectively), and also the yield of the reac-tions was unaffected by recycling the catalyst systems
a number of times. Similar conversion and yield of decarbony-lated product (2 c ; 99 %) was found with p-methoxybenzalde-
hyde; thus promised to have huge functional group tolerance,including electron-withdrawing and -donating groups, at-
tached to aromatic aldehydes. The acid- and base-sensitive (to-
wards hydrolysis) cyano functionality was unaltered in our pro-tocol and provided excellent yield of m/p-benzonitrile (2 d and
2 e ; 99–95 %) in all cycles studied. The reactions of benzalde-hydes containing other functional groups, such as 4-chloro-, 4-
carboxy-, 3,4-dimethoxy-, and densely functionalized 3,4,5-tri-methoxybenzaldehyde, as well as methyl 4-formylbenzoate,
proceed well with excellent yield (2 f–j) and recyclability. On
the other hand, bis-formylated benzaldehyde, such as tereph-thalaldehyde, with a slightly higher catalyst loading of KCC-1-
PEI/Pd (7.5 mol % Pd) undergoes double deformylation to yieldbenzene (2 k) with 100 % conversion. However, ortho-substitut-
ed napthaldehyde, particularly 2-methoxy-1-naphthaldehyde,proceeded well with the deformylation reaction to yield 2-me-thoxynaphthalene in excellent yield (1 c ; 95 %), but the conver-
sion and product yield diminished with recycling (yield insecond and third cycles : 93 and 83 %, respectively).
Next, we looked into more challenging substrate, such asheteroaromatic and aliphatic aldehydes, and it was found that
for most of the heteroaromatic (except 3 a and 3 b ; Table 3)and aliphatic aldehydes a higher catalyst loading was required
(7.5 mol % Pd) for desired conversion of the substrates
(Table 3). Both, N-heterocyclic carbaldehydes, such as N-meth-ylindole-3-carboxaldehyde and 4-pyridinecarboxaldehyde, and
an O-heterocylic carbaldehyde, namely, furan-2-carbaldehyde,were completely consumed to yield 1-methylindole pyridine
and furan, respectively, with excellent yields (3 a : 94 %, 3 b :99 %, 3 c : 100 %) and catalyst recyclability. Notably, unlike furan
carboxaldehyde, N-heteroaromatic aldehyde, under similar re-
action conditions with Pd(OAc)2, one of the best known meth-odologies for deformylation,[32] provided a lower yield of the
desired product. On the other hand, 3-thiophenecarboxalde-hyde, with KCC-1-PEI/Pd (7.5 mol % Pd), transformed complete-
ly to thiophene (3 d, 93 %) without any side product (16 mol %Pd loading of Pd(OAc)2 provided, at most, 66 % yield of the
product[32]). However, both the conversion of 3-thiophenecar-boxaldehyde and yield of the product is significantly reducedupon recycling.
We have also explored the scope of the decarbonylation re-action with alkenylaldehydes by using our protocol (Table 3).
Unlike previously reported procedures for deformylation, thereactions of trans-cinnamaldehyde and a-methyl-trans-cinna-
maldehyde proceeded with 85–100 % conversion to product in
high yield of styrene (4 a ; 87 %) and b-methylstyrene (4 b ;90 %), respectively. Aliphatic aldehydes were moderately active
under similar reaction conditions; 2-phenyl-acetaldehyde andn-octaldehyde provided moderate to high yield of deformylat-
ed product (4 c : 80 %, 4 d : 68 %) and recyclability.
Scheme 1. KCC-1-PEI/Pd-catalyzed decarbonylation of 2-naphthaldehyde.MS = molecular sieves.
Table 1. Recyclability of catalyst systems for decarbonylation.
cycle 1: 96 % (100 %)[b] cycle 1: 98 % (100 %)[c]
cycle 2: 94 % (100 %)[b] cycle 2: 96 % (100 %)[c]
cycle 3: 97 % (100 %)[b] cycle 3: 97 % (100 %)[c]
cycle 4: 94 % (100 %)[b] cycle 4: 98 % (100 %)[c]
cycle 5: 96 % (100 %)[b] cycle 5: 98 % (100 %)[c]
cycle 6: 95 % (100 %)[b] cycle 6: 97 % (100 %)[c]
cycle 7: 96 % (100 %)[b]
cycle 8: 96 % (100 %)[b]
[a] Yield of product isolated; conversion is given in parentheses. [b] Con-version determined by GC. [c] Conversion determined by 1H NMR spec-troscopy.
To check further the efficiency of the present protocol, sever-
al aldehydes were reacted in a series upon recycling of the cat-alytic systems (Scheme 2). After isolating pure naphthalene
(1 a ; 96 %) from the first cycle(deformylation of 2-naphthal-dehyde) simply by filtering thesupernatant from the centri-fuged reaction tube (also thewashings), the recovered cata-
lyst systems were used directlyafter activation in the second
cycle. Subsequently, deformyla-tion of 9-anthracenecarboxal-dehyde proceeded well withcomplete consumption of thestarting material to yield an-
thracene (1 b ; 98 %) as the soleproduct. In a similar fashion,
the recovered catalyst systems
from the second cycle wereused to treat 4-formylbenzoic
acid, which gave benzoic acid(2 g, 94 %) as the only product.
Based on a report in the lit-erature,[23, 34] a catalytic cycle
for the decarbonylation reac-
tion was proposed (Scheme 3).According to the proposed
mechanism, an acyl–palladiumcomplex was considered to be
a key intermediate in decar-bonylation. Carbon monoxide extrusion and release of decar-
bonylated product regenerated the catalyst.
In summary, we have developed an easy and practical heter-ogeneous catalytic protocol for the decarbonylation of a variety
of aldehydes with complete conversion to products, and thus,easy product purification (without chromatography) by using
the novel nanocatalyst KCC-1-PEI/Pd. This decarbonylation pro-tocol was not only suitable for aromatic aldehydes, but the de-
carbonylation of heteroaromatic, alkane, and alkenylaldehyde
derivatives was also achieved. In addition, nanocatalystsshowed excellent stability (recyclability) and, even after eight
cycles, the used catalyst was as active as a fresh one. Owing tohuge functional group tolerance on the aromatic ring systemsand sustainability towards recycling, there is an emergence tostudy the efficiency of the catalytic systems in other organictransformation reactions, such as C@H activation.
Acknowledgements
This activity is supported by grants from DST Nano Mission (SR/
NM/NS-1065/2015). Financial support received from IIT-Bombayfor postdoctoral research fellowship (P.K.K), CSIR-India (A.M) are
gratefully acknowledged. M.D. and V.P. thank the Department of
Atomic Energy (DAE), Government of India for financial support.
[a] Conversion is given in parentheses. [b] Yield of product isolated. [c] Yield determined by GC (owing to highvolatility of the product). [d] 7.5 mol % Pd loading.
Table 3. Results for the decarbonylation of heteroaromatic, alkane, andalkenylaldehyde derivatives.[a]
[a] Conversion is given in parentheses. [b] For entries 3 a and 3 b, 5 mol %Pd was used and for entries 3 c–4 d 7.5 mol % Pd was used. [c] Yield ofproduct isolated. [d] Yield determined by GC (owing to high volatility ofthe product).
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Manuscript received: May 14, 2016Accepted Article published: August 2, 2016Final Article published: August 24, 2016
Scheme 2. Decarbonylation in a sequence by recycling the catalyst systems (100 % conversions in all cases ; yields of products isolated are given).
Scheme 3. Proposed catalytic cycle for the decarbonylation of aldehydes.
