Pd-Boron-Catalyzed One Carbon Isomerization of Olefins: Water Assisted Process at Room Temperature

Article abstract of DOI:10.1021/acs.joc.7b00580

A palladium-boronate/borane-system -catalyzed isomerization of olefins has been uncovered. An efficient catalytic combination of [Pd(OAc)₂]₃-boronate-PCy₃-enabled olefin isomerization at 80 °C has been investigated. Addition of water to the reaction showed a remarkable improvement and the isomerization occurred at ambient temperature. These catalytic systems function efficiently for the isomerization of functionalized as well as unfunctionalized olefins. The catalytic conditions demonstrate the involvement of both nonhydride and metal-hydride medium and can be switchable with water as an additive.

Full text of DOI:10.1021/acs.joc.7b00580

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Article
Pd-Boron Catalyzed One Carbon Isomerization of
Olefins: Water Assisted Process at Room Temperature
Devi Prasan Ojha, Karthik Gadde, and Kandikere Ramaiah Prabhu
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.7b00580 • Publication Date (Web): 12 Apr 2017
Downloaded from http://pubs.acs.org on April 12, 2017
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PdꢀBoron Catalyzed One Carbon Isomerization of Oleꢀ
fins: Water Assisted Process at Room Temperature
Devi Prasan Ojha, KarthikGadde and KandikereRamaiahPrabhu*
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Department of Organic Chemistry, Indian Institute of Science, Bangalore 560012, Karnataka, India
*Eꢀmail: prabhu@orgchem.iisc.ernet.in.
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ABSTRACT
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A palladiumꢀboronate/boraneꢀsystem catalyzed isomerization of olefins has been uncovered. An effiꢀ
cient catalytic combination of [Pd(OAc)2]3ꢀboronateꢀPCy3,enabled olefin isomerization at 80 °C has
been investigated. Addition of water to the reaction showed a remarkable improvement and the isomeriꢀ
zation occurred at ambient temperature. These catalytic systems function efficiently for the isomerizaꢀ
tion of functionalized as well as unfunctionalized olefins. The catalytic conditions demonstrate the inꢀ
volvement of both nonꢀhydride and metalꢀhydride medium and can be switchable with water as an addiꢀ
tive.
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Introduction
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Alkenes are highly useful precursors for a variety of organic transformations that are present in a numꢀ
ber of organic compounds.1,2 Transition metal catalyzed isomerization of alkenes, for obtaining highly
substituted olefins, is a highly atomꢀ, redoxꢀ, and stepꢀeconomical process.3ꢀ5 Isomerization of terminal
olefins to internal olefins is an important process that leads to more complex molecular scaffolds.6
Isomerization of functionalized olefins, to obtain heterofunctionalized olefins, with a defined stereoseꢀ
lectivity, has been achieved using transition metal catalysts such as, Pd, Rh, Ru, Ir, Fe, Co etc.7 Palladiꢀ
um complexes have been known as excellent catalysts for isomerization of allyl groups and there are
number of reports on the palladium catalyzed isomerization of olefin that involve movement of double
bond over one position. PdꢀCatalyzed isomerization of double bonds can be broadly classified into two
categories viz., isomerization by preꢀmade or in situꢀgenerated palladiumꢀhydride complexes8 and isomꢀ
erization by nonꢀhydride palladium complexes (Scheme 1).4a,9,10 The scope of the isomerization reaction
catalyzed by palladiumꢀhydride complexes is limited, which demonstrates the transformation of (i) allyl
alcohols to the corresponding carbonyl compounds,7a,10 (ii) allyl ethers to their enol ethers,11(iii) allyl
amines to their enamines.12 The isomerization of olefins, using palladium hydride complexes, has been
extensively studied, whereas the isomerization using nonꢀhydride palladium catalytic systems have been
underexplored.13 To date, to the best of our knowledge, there is only one report, on nonꢀhydride based
palladium catalyst for the isomerization of terminal alkenes to 2ꢀalkenes by RajanBabu and coworkers.13
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Scheme 1. General mechanism
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πꢀAcceptor ligands such as boranes or boronates have emerged as efficient ancillary ligands with
metal catalysts for activation of olefins, carbon monoxide, hydrogen, etc.14 This characteristic property
of boranes or boronates can be attributed to their potential to engage in Lewis acidꢀcoligandꢀmetalꢀ
substrate bridging interaction. In light of this, we report isomerization of olefins over one position using
a palladiumꢀboronate/borane catalytic system. Addition of water into the reaction, surprisingly, showed
a remarkable improvement and isomerization occurs at ambient temperature. Catalytic systems, palladiꢀ
umꢀboronate and palladiumꢀborane, function efficiently for the isomerization of functionalized as well
as unfunctionalized olefins. The catalytic composition of B2Pin2 (10 mol%), [Pd(OAc)2]3 (1.6 mol%),
and PCy3 is expected to form a (phosphine)nꢀPdꢀBoronate transient complex, which is evident from litꢀ
erature reports.15 These isomerization reactions catalyzed by Pdꢀborane or Pdꢀborate systems illustrate
the olefin activation concept by coordination of olefins with Pdꢀborane complex. These complexes facilꢀ
itate a facile 1,nꢀhydride shifts at 80 °C. Moreover, a single catalytic combination with simple variation
of conditions, in this case addition of water, provides access to both nonꢀhydride as well as hydride
pathways for olefin isomerization.
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Results and Discussion
Optimization of reaction conditions
In light of the literature precedent,4 and continuation of our work,16 we commenced the screening studies
using 4ꢀallylꢀ1,2ꢀdimethoxybenzene (1a) as the model substrate and performed the reactions with difꢀ
ferent boron sources in toluene at 80 °C (Table 1). The initial quantitative amount of B2Pin2 (100 mol
%) was screened with respect to the palladium catalyst source. The fourꢀfold decrease in the boronate
amount (~12.5 mol %, catalytic) furnished the corresponding product in 92% yield. Further, when the
reactions were carried out in lesser amounts (10 mol %) the reactions outcome was unaffected (entries
1ꢀ4). These preliminary studies revealed that B2Pin2 (10mol %), [Pd(OAc)2]3 (1.6 mol %), and PCy3(10
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mol %), are necessary for the olefin isomerization. The reaction of 1a with B2Pin2 (10 mol %),
[Pd(OAc)2]3 (1.6 mol %), and PCy3 (10 mol %) at 80 °C furnished the corresponding isomerized product
(E)ꢀ1,2ꢀdimethoxyꢀ4ꢀ(propꢀ1ꢀenꢀ1ꢀyl)benzene (2a)in 92% yield (entry 4, Table 1). This isomerization of
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o
alkene was not efficient at lower temperature (10% at 60 C, entry 5). The isomerization of 1a using
other ligands such, Segphos, Xphos, and racꢀBINAP were found to be less effective and provided the
isomerized product 2a in low yields (29, 35, and 30%, respectively, entries 6ꢀ8). The reaction of 1a with
other Pdꢀcatalysts such as Pd2dba3, and Pd(PPh3)4 furnished the isomerized product 2a, albeit, in low
yields (50, and 31%, respectively, entries 9ꢀ10), whereas the similar reaction of 1a with Pd(dppf)Cl2 was
futile (entry 11). Next, it was noticed that the boron
Table 1. Screening Studies for Optimizationa
entry
boron reagent
(mol %)
Pdꢀ source
(1.6 mol %)
[Pd(OAc)2]3
none
ligand
(10 mol %)
PCy3
2 yield (%)
1
2
none
trace
nd
B2Pin2 (100)
B2Pin2 (100)
B2Pin2 (10)
B2Pin2 (10)
B2Pin2 (10)
B2Pin2 (10)
B2Pin2 (10)
B2Pin2 (10)
B2Pin2 (10)
B2Pin2 (10)
PCy3
3
[Pd(OAc)2]3
[Pd(OAc)2]3
[Pd(OAc)2]3
[Pd(OAc)2]3
[Pd(OAc)2]3
[Pd(OAc)2]3
Pd2(dba)3
none
nd
4
PCy3
92
10b
5
PCy3
6
Segphos
Xphos
racꢀBINAP
PCy3
29
7
35
8
30
9
50c
31c
ndc
10
11
Pd(PPh3)4
PCy3
Pd(dppf)Cl2
PCy3
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B(OPh)3 (10)
B(OMe)3 (10)
PhꢀBPin (10)
[Pd(OAc)2]3
[Pd(OAc)2]3
[Pd(OAc)2]3
PCy3
PCy3
PCy3
PCy3
PCy3
PCy3
nd
nd
30
41
90
99
PhꢀvinylꢀBpin (10)
BEt3 (10)
[Pd(OAc)2]3
[Pd(OAc)2]3
[Pd(OAc)2]3
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BPh3 (10)
a Reaction conditions: 1a (0.5 mmol), B2Pin2 (0.1 mmol), [Pd](1.6 mol%), PCy3 (10 mol %), toluene (1
mL), at 80 oC, 12 h. b at 60 oC, c [Pd] 5 mol %, nd = not detected.
reagents such as B(OPh)3 and B(OMe)3 were not useful in promoting the isomerization (entries 12ꢀ13).
Interestingly, the use of phenyl boronic acid pinacol ester (PhꢀBPin) and phenylvinylboronic acid pinaꢀ
col ester (PhꢀvinylꢀBPin) under the reaction conditions furnished the product 2 in 30, and 41% yields,
respectively (entries 14 and 15). The isomerization also proceeded well with boranes such as BEt3 and
BPh3 furnishing the product 2a in excellent yields (90 and 99%, respectively, entries 16ꢀ17). The use of
BPh3 was restricted as it is highly moisture sensitive. Additionally, the room temperature reactions with
water as additive did not proceed well with BPh3. Therefore, further reactions for exploring the scope of
the reactions have been carried out using B2Pin2.
With the optimal conditions in hand, we explored the scope and limitations of the palladiumꢀ
catalyzed isomerization reaction. Subjecting a variety of allylbenzene derivatives for isomerization, the
corresponding isoeugenol derivatives 2aꢀ2e were obtained in good to excellent yields (92, 92, 98, 50
and 97%, respectively) with excellent selectivity (94:6, 96:4, 95:5, 97:3, and 95:5 E/Z ratios, respectiveꢀ
ly, entries 1ꢀ5, Table 2). Notably, 4ꢀallylꢀ2ꢀmethoxyphenol, which contains a phenolic OH group,
showed a good compatibility with the metal catalyst and furnished the isomerized product 2d, in moderꢀ
ate yield (entry 4, Table 2). Further, we tested the efficacy of isomerization by subjecting a few Nꢀallyl
derivatives for isomerization, which provided the corresponding enamines and enamides. As enamines
and enamides are important frameworks for a number of organic transformations, we hope this method
could be an easy alternative for synthesizing these precursors. As expected, the reaction of Nꢀ
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allylbenzamide, under the optimal conditions furnished the expected isomerized product (E)ꢀNꢀ(propꢀ1ꢀ
enꢀ1ꢀyl)benzamide (2f) in 69% yield with moderate selectivity (60:40; E:Z ratios, entry 6). Further, the
same isomerization reaction of Nꢀallylbenzamide was also carried out on a gram scale, affording the
isomerized product 2f in 67% yield (entry 6, Table 2). Interesting molecules such as Nꢀallylisatin and Nꢀ
allyl phthalimide derivatives formed the corresponding Eꢀisomers 2g and 2h, exclusively, in excellent
yields (86 and 96%, respectively, entries 7 and 8). Similarly, the isomerization of Nꢀallyl indole derivaꢀ
tives was also facile and afforded the corresponding enamines 2i, 2j, and 2k in excellent yields (89, 93,
and 99%, respectively, entries 9ꢀ11) with good E-selectivity (82:18, 81:19 and 89:11 ratios, respectiveꢀ
ly).
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Table 2. Isomerization for linear allyl substratesa,b,c,d
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O
O
[Pd(OAc)2]3 (1.6 mol %), PCy3 (10 mol %)
N
N
or
B2Pin2 (10 mol %)
Ar
Ar
entry
O
or
1
O
2
toluene (1.5 mL), 80 °C, 12 h
yield (%)b
yield (%)
product
E/Z
entry
product
E/Z
9
O
92
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MeO
MeO
O
1
2
94:6
7
100:0
100:0
86
N
90
(BEt3)
2a
2g
O
96:4
92
8
9
96
89
N
2b
2c
MeO
MeO
2h
O
MeO
3
4
95:5
97:3
98
50
82:18
BnO
MeO
N
2i
NC
HO
2d
10
11
81:19
89:11
93
99
N
MeO
5
6
95:5
97
69
2j
TBSO
O2N
2e
2f
O
N
N
60:40
Br
67
H
2k
(gram scale)
O
MeO
O
unsuccessful substrates
O
N
2n, 0
2m, 0
2l, 0
a Reaction conditions: 1a (0.5 mmol), B2Pin2 (0.1 mmol), [Pd(OAc)2]3 (1.6 mol %), PCy3 (10 mol %),
toluene (1.5 mL), at 80 oC, 12 h. b E/Z ratio are determined from 1HꢀNMR (Jtrans vs Jcis).17
As can be seen in Table 2, the isomerization of olefins that contain nitrile and nitro functional groups
proceeded well furnishing the isomerized products 2j and 2k, indicating that these functional groups are
well tolerated under the reaction conditions. We noticed that bromo substitution on the indole derivative
was detrimental for the reaction as the reaction of 1ꢀallylꢀ5ꢀbromoꢀ1Hꢀindole did not furnish the isomerꢀ
ized product 2l. This outcome can be rationalized on basis of the possibility of oxidative insertion of Pdꢀ
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catalyst at the Csp2ꢀBr center resulting in poisoning of the catalyst. Similarly, allyl benzoate and 4ꢀallylꢀ
1ꢀ(allyloxy)ꢀ2ꢀmethoxybenzene that contain Oꢀallyl group did not undergo isomerization reaction and
failed to furnish isomerized products 2m and 2n. The isomerization of Oꢀallyl of ethers and esters are
considered to be unreactive and hence requires more efficient catalytic systems.
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To expand the substrate scope, olefins with extended and functionalized chains were employed for the
palladium catalyzed isomerization reaction. Thus, the reaction of 2ꢀ methylꢀ1ꢀphenylpentꢀ4ꢀenꢀ1ꢀone 3a
under the isomerization conditions was successful and furnished the corresponding thermodynamically
stable Eꢀisomer 4a, exclusively, in good yield (74%, Table 3). The smooth isomerization of 2ꢀmethylꢀ1ꢀ
phenylpentꢀ4ꢀenꢀ1ꢀone 3a over two positions to obtain 4a supported the idea that the heteroatom coorꢀ
dination of the keto group. Therefore, we expected the transformation would also be consistent with the
homoallylic alcohols to produce the corresponding ketone. Hence, employing 1ꢀphenylbutꢀ3ꢀenꢀ1ꢀol
(3b) under the standard reaction conditions proved successful, affording the product 4b in moderate
yield (32%, Table 3). In the reaction of 3b, a smooth isomerization of homoallylic alcohol (3b) furꢀ
nished corresponding enol, which formed thermodynamically more favored butyrophenone 4b. The corꢀ
responding allylic alcohol ((E)ꢀ1ꢀphenylbutꢀ2ꢀenꢀ1ꢀol), which forms after one carbon isomerization was
the only byproduct observed in this reaction. To test the generality of this observation, we subjected the
Oꢀprotected homoallylic alcohol, tertꢀbutyldimethyl((1ꢀphenylbutꢀ3ꢀenꢀ1ꢀyl)oxy)silane, under optimal
isomerization reaction conditions to obtain the isomerized product 4c in excellent yield (96%) and exꢀ
cellent selectivity (97:3 ratio, Table 3). Further, tertꢀbutyldimethyl((1ꢀ(oꢀtolyl)butꢀ3ꢀenꢀ1ꢀyl)oxy)silane,
a substrate with methyl substitution at the ortho–position, underwent a smooth isomerization to form the
product 4d in excellent yield (91%) with good selectivity (96:4 ratio, Table 3). Other substrates such as
3e, and 3f with pꢀmethyl and mꢀmethoxy substitution on the phenyl ring, formed the corresponding
isomerized products 4e and 4f in 88 and 93%, respectively, with good selectivity (94:6, and 95:5 ratios,
respectively, Table 3). In these examples, the reaction outcome was independent of the presence of subꢀ
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stitutions at orthoꢀ or paraꢀpositions for the aryl ring. Interestingly, the reaction of butꢀ3ꢀenꢀ1ꢀylbenzene
(3g), without any extra activating group, furnished a mixture of isomerized products 4g and 4g′ in good
yields (67%, and 47:53 ratios, Table 3).
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Table 3. Substrate scope for functionalized olefinsa
a
Reaction conditions: 3 (0.5 mmol), B2Pin2 (0.1 mmol), Pd(OAc)2]3 (1.6 mol %), PCy3 (10 mol%),
toluene (1 mL), at 80 oC, 12 h. b E/Z ratio are determined from 1HꢀNMR (Jtrans vs Jcis)17 c 1ꢀphenylbutꢀ3ꢀ
.
enꢀ1ꢀol was used as the starting material.
In light of our earlier studies16 to generate hydrogen using PdꢀH (generated in situ) with water, we perꢀ
formed isomerization reaction using water (5 equiv) as an additive. Our primary concept involved the
array of metalꢀhydride catalyzed olefin isomerization reactions to be feasible with the PdꢀH species that
is generated from the reaction of diboronꢀH2O with palladium. The process is advantageous in many
aspects as it provides advantages such as site selective metalꢀhydride insertion, functional group tolerꢀ
ance etc. This idea led to a remarkable result and the isomerization proceeded very efficiently at room
temperature (Table 4). Thus, the reaction of 4ꢀallylꢀ1,2ꢀdimethoxybenzene 1a at ambient temperature
under the optimal conditions, in the presence of water at room temperature, furnished the isomerized
product 2a in excellent yield and selectivity (92% yield, and selectivity, 93:7 ratio). With this interesting
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outcome, we carried out a few more isomerization reactions, and as can be seen, various allyl benzene
derivatives were isomerized at ambient temperature to furnish the corresponding isomerized olefins in
good yields and good selectivity. Thus, the isomerization of 1ꢀmethoxyꢀ4ꢀvinylbenzene (1b), 1ꢀ
(benzyloxy)ꢀ2ꢀ methoxyꢀ4ꢀvinylbenzene (1c), 2ꢀmethoxyꢀ4ꢀvinylphenol (1d), and tertꢀbutyl(2ꢀmethoxyꢀ
4ꢀvinylphenoxy) dimthylsilane (1e), in the presence of water, proceeded very well at room temperature
furnishing the corresponding isomerized products 2b, 2c, 2d and 2e in good yields (90, 91, 60, and 98%,
respectively) and good selectivity (95:5, 91:9, 93:7, and 95:5 ratios, respectively). The isomerization
reaction of 1d and 1l, allylbenzene systems containing phenolic OH group, proceeded well furnishing
the corresponding Eꢀisomers, 2d and (E)ꢀ4ꢀ(propꢀ1ꢀenꢀ1ꢀyl)phenol (2l), exclusively, in moderate yields
60 and 51%, respectively (Table 4). Importantly, the palladiumꢀhydride mediated olefin isomerization at
room temperature is rare.10c In this regard, our method demonstrated a process which is feasibility at
room temperature. The isomerization reactions with bromoꢀ substituent and Oꢀallyl substrates were inefꢀ
fective under this catalytic condition as well, nevertheless the subject is under study on improving the
catalyst efficiency.
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Table 4. Isomerization at ambient temperaturea
a Reaction conditions: 1a (0.5 mmol), B2Pin2 (0.2 mmol), Pd(OAc)2]3 (1.6 mol %), PCy3 (10 mol %),
H2O (1 mmol), toluene (1 mL), at RT, 12 h. . b E/Z ratio are determined from 1HꢀNMR (Jtrans vs Jcis).17
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The isomerization reaction is expected to follow two different pathways under variable conditions
(Scheme 2). In the first case, wherein Pd/B2Pin2 is employed, the catalytic cycle possibly involves a
nonꢀhydride activation for the isomerization of olefins. The process commences with the formation of
species I from the reaction of Lewis acidic B2Pin2, Pd(OAc)2]3 and ligand PCy3 (Scheme 2a).15,18 Simiꢀ
larly, the reaction of Lewis acidic B2Pin2, Pd(OAc)2]3 and ligand PCy3 in H2O generates the Pdꢀ I with
olefin facilitate the formation of πꢀallylic intermediate int II which forms int III with a hydride migraꢀ
tion, followed by βꢀH elimination results in the formation of the corresponding isomerized product. On
the other hand, the use of water as an additive completely changes the mechanistic pathway. As evident
from our previous study,16b and concurrent reports18 on the formation of the PdꢀH (species II) from
Pd/B2Pin2/H2O combination,16 we hypothesize that the reaction has a possibility of involving a metal
hydride intermediate for the isomerization of olefins. Thus, the reaction of olefin with species II, initiꢀ
ates hydride migration forming the intermediate int IV which upon βꢀhydride elimination forms the
isomerized product.
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Scheme 2. Plausible mechanism
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In conclusion, a simple and mild catalytic system has been developed for a facile isomerization of unacꢀ
tivated as well as activated olefins. This method provides a simple method for synthesizing highly funcꢀ
tionalized olefinic frameworks. This strategy has been used for synthesizing a variety of enamines, enꢀ
amides and Oꢀprotected allylic alcohol derivatives. The investigation to drive the isomerization at room
temperature using a PdꢀHydride, generated in-situ, was successful which is performed using water as an
additive. Further mechanistic studies to obtain insights into the reaction mechanism are underway in our
laboratory.
EXPERIMENTAL SECTION
General Information. 1H NMR spectra were recorded on a 400 MHz spectrometer in CDCl3or DMSOꢀ
1
d6.Tetramethylsilane (TMS; δ = 0.00 ppm) for H NMR in CDCl3, and residual nonꢀdeuterated solvent
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peak (δ = 2.50 ppm) in DMSOꢀd6, served as an internal standard. The solvent signal (CDCl3, δ = 77.00
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ppm; and DMSOꢀd6, δ = 39.5 ppm) was used as internal standard for C NMR. IR spectra were measꢀ
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ured using an FTꢀIR spectrometer. Mass spectra were obtained with a QꢀTOF Mass Spectrometer
(HRMS). Flash column chromatography was carried out by packing glass columns with commercial silꢀ
ica gel 230ꢀ400 mesh (commercial suppliers) and thinꢀlayer chromatography was carried out using silica
gel GFꢀ254. All catalysts, reagents and reactants were procured from commercial suppliers. All solvents
were distilled under nitrogen atmosphere prior to use. Toluene was dried over Na. Unless otherwise notꢀ
ed, commercially available chemicals were distilled and degassed before use. Palladium acetate (II) triꢀ
mer (CAS: 3375ꢀ31ꢀ3) was used for the reaction.
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.
I. General procedures for isomerizationof olefins at 80 oC
To an ovenꢀdried Teflon capped vial equipped with a magnetic stirring bar were added sequentially
bis(pinacolato)diboron (12 mg, 0.1 mmol), PCy3 (14 mg, 10 mol%), [Pd(OAc)2]3 (5 mg, 1.6 mol %) and
alkene (0.5 mmol) in toluene (1.5 mL) under argon. The mixture was stirred at 80 °C for 12 h (moniꢀ
tored by TLC). Upon completion of the reaction, the resulting mixture was filtered through a short colꢀ
umn of silica gel (eluted with EtOAc) and concentrated. This crude mixture was further purified on a
silica gel column chromatography to obtain the pure product.
II. General procedures for isomerisation of olefins at RT
To an ovenꢀdried Teflon capped vial equipped with a magnetic stirring bar were added sequentially
bis(pinacolato)diboron (24 mg, 0.2 mmol), PCy3 (14 mg, 10 mol %), [Pd(OAc)2]3 (5 mg, 1.6 mol %),
water (1 mmol), and alkene (0.5 mmol) in toluene (1.5 mL) under argon. The mixture was stirred at
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room temperature for 12 h (monitored by TLC). Upon completion of the reaction, the resulting mixture
was filtered through a short column of silica gel (eluted with EtOAc) and concentrated. This crude mixꢀ
ture was further purified on a silica gel column chromatography to obtain the pure product.
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Characterization data for products
(E)ꢀ1,2ꢀDimethoxyꢀ4ꢀ(propꢀ1ꢀenꢀ1ꢀyl)benzene (2a):18 Prepared as described in the general experiꢀ
mental procedure. colourless liquid; yield:92% (82 mg); Rf (5% EtOAc/hexane): 0.6; IR (Neat, cmꢀ1):
1
1513, 1413, 1263, 1232, 1191, 1027, 963, 856; H NMR (400 MHz, CDCl3): δ 6.89 (s, 1H), 6.87ꢀ6.78
(m, 2H), 6.34 (d, J = 15.6 Hz, 1H), 6.15ꢀ6.06 (m, 1H), 3.89 (s, 3H), 3.87 (s, 3H), 1.87 (d, J = 6.4 Hz,
13
3H); C{1H} NMR (100 MHz, CDCl3): δ 148.9, 148.1, 131.1, 130.5, 123.7, 118.6, 111.1, 108.3, 55.9,
55.7, 18.3; HRESIꢀMS (m/z): Calculated for C11H14O2 (M + Na): 201.0891, found (M + Na): 201.0889.
(E)ꢀ1ꢀMethoxyꢀ4ꢀ(propꢀ1ꢀenꢀ1ꢀyl)benzene (2b):4c Prepared as described in the general experimental
procedure. colourless liquid; yield: 92% (68 mg); Rf (5% EtOAc/hexane): 0.9; IR (Neat, cmꢀ1): 1510,
1460, 1282, 1247, 1176, 1112, 1036, 964; 1H NMR (400 MHz, CDCl3): δ 7.25 (d, J = 8.0 Hz, 2H), 6.82
(d, J = 8.4 Hz, 2H), 6.34 (d, J = 15.6 Hz, 1H), 6.13ꢀ6.04 (m, 1H), 3.79 (s, 3H), 1.85 (d, J = 6.4 Hz, 3H);
13C{1H} NMR (100 MHz, CDCl3): δ 158.6, 130.8, 130.3, 126.9, 123.4, 113.9, 55.2, 18.4.
(E)ꢀ1ꢀ(Benzyloxy)ꢀ2ꢀmethoxyꢀ4ꢀ(propꢀ1ꢀenꢀ1ꢀyl)benzene (2c):19Prepared as described in the general
experimental procedure. colourless liquid; yield:98% (124 mg); Rf (5% EtOAc/hexane): 0.7; IR (Neat,
cmꢀ1): 1513, 1413, 1263, 1232, 1191, 1027, 963, 856; H NMR (400 MHz, CDCl3): δ 7.42ꢀ7.41 (m,
1
2H), 7.34 (t, J = 7.2 Hz, 2H), 6.89 (s, 1H), 6.78 (s, 2H), 6.31 (d, J = 15.6 Hz, 1H), 6.13ꢀ6.04 (m, 1H),
3.87 (s, 3H), 1.84 (d, J = 6.4 Hz, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 149.6, 147.2, 137.2, 131.6,
130.5, 128.4, 127.7, 127.2, 123.9, 118.5, 114.1, 109.1, 55.9, 18.3; HRESIꢀMS (m/z): Calculated for
C17H18O2 (M + Na): 277.1204, found (M + Na): 277.1202.
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(E)ꢀ2ꢀMethoxyꢀ4ꢀ(propꢀ1ꢀenꢀ1ꢀyl)phenol (2d):21 Prepared as described in the general experimental
procedure. colourless liquid; yield:50% (41 mg); Rf (5% EtOAc/hexane): 0.1; IR (Neat, cmꢀ1): 1513,
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1413, 1263, 1232, 1191, 1027, 963, 856; H NMR (400 MHz, CDCl3): δ 6.89 (s, 1H), 6.87ꢀ6.78 (m,
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2H), 6.34 (d, J = 15.6 Hz, 1H), 6.15ꢀ6.06 (m, 1H), 3.89 (s, 3H), 3.87 (s, 3H), 1.87 (d, J = 6.4 Hz, 3H);
13C{1H} NMR (100 MHz, CDCl3): δ 148.9, 148.1, 131.1, 130.5, 123.7, 118.6, 111.1, 108.3, 55.9, 55.7,
18.3.
(E)ꢀtertꢀButyl(2ꢀmethoxyꢀ4ꢀ(propꢀ1ꢀenꢀ1ꢀyl)phenoxy)dimethylsilane (2e):20 Prepared as described in
the general experimental procedure. colourless liquid; yield:97% (135 mg); Rf (5% EtOAc/hexane): 0.9;
IR (Neat, cmꢀ1): 1513, 1413, 1263, 1232, 1191, 1027, 963, 856; 1H NMR (400 MHz, CDCl3): δ 6.84 (s,
1H), 6.76 (s, 2H), 6.32 (d, J = 15.6 Hz, 1H), 6.13ꢀ6.04 (m, 1H), 3.80 (s, 3H), 1.85 (d, J = 6.4 Hz, 3H),
13
0.98 (s, 9H), 0.1 (s, 6H); C{1H} NMR (100 MHz, CDCl3): δ 150.9, 144.2, 131.9, 130.7, 123.7, 120.8,
118.6, 109.4, 55.4, 25.7, 18.4, 18,3; HRESIꢀMS (m/z): Calculated for C16H26O2Si (M + Na): 301.1600,
found (M + Na): 301.1601.
(E)ꢀNꢀ(Propꢀ1ꢀenꢀ1ꢀyl)benzamide (2f):22 Prepared as described in the general experimental procedure.
colourless liquid; yield: 69% (56 mg), for gram scale 67 % (675 mg); Rf (10% EtOAc/hexane): 0.2; IR
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(Neat, cmꢀ1): 1513, 1413, 1263, 1232, 1191, 1027, 963, 856; H NMR (400 MHz, CDCl3): δ 7.97 (br s,
1H), 7.79 (d, J = 7.2 Hz, 2H), 7.49ꢀ7.38 (m, 3H), 6.97ꢀ6.91 (m, 1H), 5.38ꢀ5.29 (m, 1H), 1.71 (d, J = 6.4
Hz, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 164.3, 133.8, 131.6, 128.5, 127.0, 123.6, 108.9, 14.9;
HRESIꢀMS (m/z): Calculated for C10H11O (M + Na): 184.0738, found (M + Na): 184.0739.
(E)ꢀNꢀ(Propꢀ1ꢀenꢀ1ꢀyl)benzamide (2f′):22 Prepared as described in the general experimental procedure.
colourless liquid; yield: 55%; R (10% EtOAc/hexane): 0.3; IR (Neat, cmꢀ1): 1513, 1413, 1263, 1232,
f
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1191, 1027, 963, 856; H NMR (400 MHz, CDCl3): δ 7.97 (br s, 1H), 7.79 (d, J = 7.2 Hz, 2H), 7.49ꢀ
7.38 (m, 3H), 6.97ꢀ6.91 (m, 1H), 5.38ꢀ5.29 (m, 1H), 1.71 (d, J = 6.4 Hz, 3H); 13C{1H} NMR (100 MHz,
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CDCl3): δ 164.3, 133.8, 131.6, 128.5, 127.0, 123.6, 108.9, 14.9; HRESIꢀMS (m/z): Calculated for
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C10H11O (M + Na): 184.0738, found (M + Na): 184.0739.
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(E)ꢀ1ꢀ(Propꢀ1ꢀenꢀ1ꢀyl)indolineꢀ2,3ꢀdione (2g): Prepared as described in the general experimental proꢀ
cedure. colourless liquid; yield: 96% (90 mg); Rf (5% EtOAc/hexane): 0.8; IR (Neat, cmꢀ1): 1513, 1413,
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1263, 1232, 1191, 1027, 963, 856; H NMR (400 MHz, CDCl3): δ 7.64ꢀ7.60 (m, 2H), 7.16ꢀ7.10 (m,
2H), 6.44ꢀ6.31 (m, 2H), 1.9 (d, J = 6 Hz, 3H); 13C{1H} NMR (100 MHz, CDCl3): δ 148.9, 148.1, 131.1,
130.5, 123.7, 118.6, 111.1, 108.3, 55.9, 55.7, 18.3; HRESIꢀMS (m/z): Calculated for C11H9NO2 (M +
Na): 210.0531, found (M + Na): 210.0535.
(E)ꢀ2ꢀ(Propꢀ1ꢀenꢀ1ꢀyl)isoindolineꢀ1,3ꢀdione (2h): Prepared as described in the general experimental
procedure. yellow solid; yield:86% (80 mg); Rf (10% EtOAc/hexane): 0.4; IR (Neat, cmꢀ1): 1513, 1413,
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1263, 1232, 1191, 1027, 963, 856; H NMR (400 MHz, CDCl3): δ 7.86ꢀ7.83 (m, 2H), 7.74ꢀ7.71 (m,
2H), 6.59ꢀ6.54 (m, 2H), 1.84 (d, J = 8.4 Hz, 3H); C{1H} NMR (100 MHz, CDCl3): δ 166.6, 134.2,
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131.6, 123.4, 118.3, 118.0, 16.2; HRESIꢀMS (m/z): Calculated for C11H9NO2 (M + H): 188.0712, found
(M + Na): 188.0712.
(E)ꢀ5ꢀMethoxyꢀ1ꢀ(propꢀ1ꢀenꢀ1ꢀyl)ꢀ1Hꢀindole (2i): Prepared as described in the general experimental
procedure. pale yellow liquid; yield:89% (84 mg); Rf (10% EtOAc/hexane): 0.7; IR (Neat, cmꢀ1): 1513,
1
1413, 1263, 1232, 1191, 1027, 963, 856; H NMR (400 MHz, CDCl3): δ 7.28ꢀ7.21 (m, 2H), 7.06 (s,
1H), 6.92ꢀ6.86 (m, 2H), 6.47 (m, 1H), 5.76ꢀ5.70 (m, 1H), 3.83 (s, 3H), 1.85 (d, J = 6.7 Hz, 3H). 13C{1H}
NMR (100 MHz, CDCl3): 136.4, 129.8, 128.3, 127.7, 126.7, 126.3, 126.3, 126.2, 125.0, 124.8, 123.8,
123.7, 121.8, 154.5, 154.4, 131.5, 130.5, 129.1, 128.4, 127.9, 124.9, 124.8, 117.5, 112.2, 112.1, 110.7,
110.4, 103.2, 102.8, 102.4, 55.8, 15.2, 12.9; HRESIꢀMS (m/z): Calculated for C12H13NO (M + H):
188.1075, found (M + Na): 188.1077.
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(E)ꢀ1ꢀ(Propꢀ1ꢀenꢀ1ꢀyl)ꢀ1Hꢀindoleꢀ5ꢀcarbonitrile (2j): Prepared as described in the general experiꢀ
mental procedure. colourless liquid; yield:93% (85 mg); Rf (10% EtOAc/hexane): 0.4; IR (Neat, cmꢀ1):
1513, 1413, 1263, 1232, 1191, 1027, 963, 856; 1H NMR (400 MHz, CDCl3): δ 7.91 (s, 1H), 8.12 (d, 2.4
Hz, 1H), 7.45ꢀ7.41 (m, 3H), 6.91 (d, J = 13.6 Hz, 1H), 6.62 (dd, J = 6.7, 1.4 Hz, 1H), 5.88ꢀ5.80 (m, 1H),
1.90 (d, J = 6.7, 3H). 13C{1H} NMR (100 MHz, CDCl3): 136.4, 129.8, 128.3, 127.7, 126.7, 126.3,
126.3, 126.2, 125.0, 124.8, 123.8, 123.7, 121.8, 120.5, 120.4, 114.1, 110.9, 110.3, 104.2, 103.3, 103.1,
15.1, 12.8; HRESIꢀMS (m/z): Calculated for C12H10N2 (M + Na): 205.0745, found (M + Na): 205.0742.
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(E)ꢀ5ꢀNitroꢀ1ꢀ(propꢀ1ꢀenꢀ1ꢀyl)ꢀ1Hꢀindole (2k): Prepared as described in the general experimental proꢀ
cedure. pale yellow liquid; yield:98% (99 mg); Rf (10% EtOAc/hexane): 0.5; IR (Neat, cmꢀ1): 1579,
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1505, 1451, 1395, 1332, 1221, 1064, 926; H NMR (400 MHz, CDCl3): δ 8.5 (s, 1H), 8.12 (d, J = 2.4
Hz, 1H), 7.46ꢀ7.42 (m, 2H), 6.95 (d, J = 16 Hz, 1H), 6.73 (dd, J = 6.7 Hz, 1.4 Hz, 1H), 5.93ꢀ5.85 (m,
1H), 1.92 (dd, J = 6.7, 1.4 Hz, 3H). 13C{1H} NMR (100 MHz, CDCl3): 142.0, 137.7, 130.7, 127.9,
127.7, 124.0, 122.5, 118.0, 117.9, 117.8, 117.6, 114.8, 110.1, 109.4, 105.7, 104.9, 15.2, 12.9;HRESIꢀ
MS (m/z): Calculated for C11H10N2O2 (M + H): 203.0821, found (M + Na): 203.0821.
(E)ꢀ2ꢀMethylꢀ1ꢀphenylpentꢀ2ꢀenꢀ1ꢀone & (E)ꢀ2ꢀmethylꢀ1ꢀphenylpentꢀ3ꢀenꢀ1ꢀone (4a & 4a׳): Preꢀ
pared as described in the general experimental procedure. colourless liquid; yield:74% (64 mg); Rf (10%
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EtOAc/hexane): 0.55; IR (Neat, cmꢀ1): 1513, 1413, 1263, 1232, 1191, 1027, 963, 856;(3a) H NMR
(400 MHz, CDCl3): δ 7.39ꢀ7.66 (m, 5H), 6.27ꢀ6.32 (m, 1H), 2.25ꢀ2.35 (m, 2H), 1.97ꢀ1.98 (m, 3H), 1.05
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(t, J = 7.5 Hz, 3H); C NMR (100 MHz, CDCl3): δ = 199.1, 148.2, 138.8, 135.9, 132.9, 131.3, 129.3,
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128.0, 31.8, 22.4, 13.0. (3a׳) H NMR (500 MHz, CDCl3) δ 7.99 (d, J = 7.1 Hz, 2H), 7.55 (t, J = 7.4
Hz, 1H), 7.46 (t, J = 7.5 Hz, 2H), 5.61 – 5.58 (m, J = 5.6, 3.3 Hz, 2H), 4.14 – 4.09 (m, 1H), 1.66 (dd, J
= 3.6, 1.1 Hz, 3H), 1.30 (d, J = 6.8 Hz, 3H); 13C{1H} NMR (100 MHz, CDCl3) δ 201.91, 136.78,
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(M + Na): 197.0942, found (M + Na): 197.0947.
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1ꢀPhenylbutanꢀ1ꢀone (4b):23 Prepared as described in the general experimental procedure. pale yellow
liquid; yield:32% (24 mg); Rf (5% EtOAc/hexane): 0.8; IR (Neat, cmꢀ1): 1513, 1413, 1263, 1232, 1191,
1027, 963, 856; 1H NMR (400 MHz, CDCl3) δ: 7.88 (d, J = 7.5 Hz, 2H), 7.47 (t, J = 7.5 Hz, 1H), 7.39ꢀ
7.36 (m, 2H), 2.87 (t, J = 7.5 Hz, 2H), 1.74–1.66 (m, 2H), 0.93 (t, J = 7.4 Hz, 3H); 13C{1H} NMR (100
MHz, CDCl3) δ: 200.5, 137.2, 132.9, 128.6, 128.0, 40.5, 17.8, 13.9.
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(E)ꢀtertꢀButyldimethyl((1ꢀphenylbutꢀ2ꢀenꢀ1ꢀyl)oxy)silane (4c):18 Prepared as described in the general
experimental procedure. colourless liquid; yield:96% (126 mg); Rf (5% EtOAc/hexane): 0.9; IR (Neat,
cmꢀ1): 1513, 1413, 1263, 1232, 1191, 1027, 963, 856;1H NMR (400 MHz, CHCl3) δ(ppm) 7.35 (m
,5H), 5.65 (m, 1H), 5.60 (m, 1H), 5.15 (, d, 1H, J = 6.4 Hz), 1.83 (d, 1/10 of 3H, J = 5.4 Hz), 1.71 (d,
9/10 of 3H, J = 6.1 Hz), 0.94 (s, 9H), 0.09 (s, 3H), 0.03 (s, 3H); ); 13C{1H} NMR (100 MHz, CDCl3) δ
(ppm) 144.1, 134.6, 127.7, 126.4, 125.4, 125.1, 124.7, 75.2, 69.4(min), 25.5, 17.9, 17.2, ꢀ4.8, ꢀ5.1;
HRESIꢀMS (m/z): Calculated for C16H26OSi (M + Na): 285.1650, found (M + Na): 285.1651.
(E)ꢀtertꢀButyldimethyl((1ꢀ(oꢀtolyl)butꢀ2ꢀenꢀ1ꢀyl)oxy)silane (4d): Prepared as described in the general
experimental procedure. colourless liquid; yield:91% (126 mg); Rf (5% EtOAc/hexane): 0.9; IR (Neat,
1
cmꢀ1): 1513, 1413, 1263, 1232, 1191, 1027, 963, 856; H NMR (400 MHz, CDCl3): δ 7.49 (d, J = 7.6
Hz, 1H), 7.20 (t, J = 7.2 Hz, 1H), 7.14 (t, J = 7.2 Hz, 1H), 7.09 (s, 1H), 5.60ꢀ5.49 (m, 2H), 5.27 (d, J =
5.6 Hz, 1H), 2.29 (s, 3H), 1.60 (d, J = 5.6 Hz, 3H), 0.9 (s, 9H), 0.05 (s, 3H), 0.01 (s, 3H); 13C{1H} NMR
(100 MHz, CDCl3): δ 142.3, 134.0, 133.6, 126.7, 126.1, 125.9, 125.0, 25.9, 19.1, 18.3, 17.5, ꢀ4.6, ꢀ4.8;
HRESIꢀMS (m/z): Calculated for C17H28OSi (M + Na): 299.1807, found (M + Na): 299.1811.
(E)ꢀtertꢀButyldimethyl((1ꢀ(pꢀtolyl)butꢀ2ꢀenꢀ1ꢀyl)oxy)silane (4e): Prepared as described in the general
experimental procedure. colourless liquid; yield:88% (122 mg); Rf (5% EtOAc/hexane): 0.9; IR (Neat,
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cmꢀ1): 1513, 1413, 1263, 1232, 1191, 1027, 963, 856; H NMR (400 MHz, CDCl3): δ 7.24ꢀ7.20 (m,
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1H), 6.93ꢀ6.89 (m, 2H), 6.78ꢀ6.75 (m, 1H), 5.7ꢀ5.61 (m, 1H), 5.57ꢀ5.51 (m, 1H), 5.10 (d, J = 2 Hz, 1H),
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3.8 (s, 3H), 1.70 (d, J = 5.6 Hz, 3H), 0.9 (s, 9H), 0.06 (s, 3H), 0.02 (s, 3H); C{1H} NMR (100 MHz,
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CDCl3): δ 159.5, 146.3, 134.8, 129.0, 125.1, 118.2, 112.2, 111.2, 55.12, 25.9, 25.8, 18.3, 17.5, ꢀ4.5, ꢀ
4.8; HRESIꢀMS (m/z): Calculated for C17H28OSi (M + Na): 299.1807, found (M + Na): 299.1810.
(E)ꢀtertꢀButyl((1ꢀ(3ꢀmethoxyphenyl)butꢀ2ꢀenꢀ1ꢀyl)oxy) dimethylsilane (4f): Prepared as described in
the general experimental procedure. colourless liquid; yield:93% (136 mg); Rf (5% EtOAc/hexane): 0.8;
IR (Neat, cmꢀ1): 1513, 1413, 1263, 1232, 1191, 1027, 963, 856; 1H NMR (400 MHz, CDCl3): δ 6.89 (s,
1H), 6.87ꢀ6.78 (m, 2H), 6.34 (d, J = 15.6 Hz, 1H), 6.15ꢀ6.06 (m, 1H), 3.89 (s, 3H), 3.87 (s, 3H), 1.87 (d,
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J = 6.4 Hz, 3H); C{1H} NMR (100 MHz, CDCl3): δ 148.9, 148.1, 131.1, 130.5, 123.7, 118.6, 111.1,
108.3, 55.9, 55.7, 18.3; HRESIꢀMS (m/z): Calculated for C17H28O2Si (M + Na): 315.1756, found (M +
Na): 315.1753.
(E)ꢀButꢀ1ꢀenꢀ1ꢀylbenzene & (E)ꢀbutꢀ2ꢀenꢀ1ꢀylbenzene (4g & 4g׳):24 Prepared as described in the genꢀ
eral experimental procedure. colourless liquid; yield:67% (45 mg); Rf (10% EtOAc/hexane): 0.4; IR
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(Neat, cmꢀ1): 1513, 1413, 1263, 1232, 1191, 1027, 963, 856; H NMR (400 MHz, CDCl3): δ 7.3ꢀ7.25
(m, 6H), 7.19ꢀ7.15 (m, 4H), 6.37 (d, J = 16 Hz, 0.8 H), 6.29ꢀ6.22 (m, 0.8 H), 5.6ꢀ5.47 (m, 2 H), 3.31 (d,
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J = 6.3 Hz, 2H), 2.2 (m, 1.6 H), 1.68 (m, 3H), 1.08 (t, J = 7.5 Hx, 2.6 H). C{1H} NMR (100 MHz,
CDCl3): 141.0, 137.9, 132.6, 130.8, 128.9, 128.7, 128.4,128.38, 128.32, 126.7, 126.3, 125.88, 125.85,
125.79,124.8, 39.0, 33.1, 29.7, 26.0, 17.8, 14.11, 13.6, 12.8.
(E)ꢀ4ꢀ(Propꢀ1ꢀenꢀ1ꢀyl)phenol (2l):25 Prepared as described in the general experimental procedure. colꢀ
ourless liquid; yield:51% (34 mg); Rf (20% ethyl acetate/hexane): 0.46; IR (Neat, cmꢀ1): 1513, 1413,
1
1263, 1232, 1191, 1027, 963, 856; H NMR (CDCl3, 400 MHz) δ 1.85 (d, J = 7.2 Hz, 3H), 4.9 (s, 1H)
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6.09 (dq, J = 18.0, 7.2 Hz, 1H), 6.34 (d, J = 18.0 Hz,1H), 6.74 (d, J = 8.8 Hz, 2H), 7.21 (d, J = 8.8, 2H);
13C{1H} NMR (CDCl3, 100 MHz) δ 18.3, 115.3, 123.5, 127.0, 130.2, 132.9, 154.4.
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AUTHOR INFORMATION
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Corresponding Author
* Eꢀmail: prabhu@orgchem.iisc.ernet.in
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by ISROꢀIISc Space Technology Cell (ISTC/CO/KRP/371), Indian Institute of
Scienec, Bangalore, the Indian Institute of Science and RL Fine Chem. We are thankful to Dr. A. R.
Ramesha (RL Fine Chem) for useful discussion. DPO thanks UGC, NewꢀDelhi for a senior research felꢀ
lowship.
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SUPPORTING INFORMATION: The optimization data, H, and C NMR spectral data of all comꢀ
pounds.
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