Effects of reaction conditions on quinone/diquinone product ratios in the oxidation of 1,4-dimethoxybenzene derivatives with ceric ammonium nitrate

Article abstract of DOI:10.1016/j.tetlet.2014.02.017

Proper choice of reaction conditions allows formation of either the quinone or corresponding diquinone as the major product upon treatment of 2-alkyl-1,4-dimethoxybenzenes with ceric ammonium nitrate.

Full text of DOI:10.1016/j.tetlet.2014.02.017

Accepted Manuscript
Effects of Reaction Conditions on Quinone/Diquinone Product Ratios in the
Oxidation of 1,4-Dimethoxybenzene Derivatives with Ceric Ammonium Nitrate
Brian E. Love, Brian C. Duffy, Alexander L. Simmons
PII:
S0040-4039(14)00238-X
DOI:
http://dx.doi.org/10.1016/j.tetlet.2014.02.017
TETL 44204
Reference:
Tetrahedron Letters
To appear in:
Received Date:
26 November 2013
Please cite this article as: Love, B.E., Duffy, B.C., Simmons, A.L., Effects of Reaction Conditions on Quinone/
Diquinone Product Ratios in the Oxidation of 1,4-Dimethoxybenzene Derivatives with Ceric Ammonium Nitrate,
Tetrahedron Letters (2014), doi: http://dx.doi.org/10.1016/j.tetlet.2014.02.017
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Graphical Abstract
Reaction conditions can be used to favor either quinone or diquinone formation upon treatment of 2-alkyl-
1,4-dimethoxybenzenes with ceric ammonium nitrate.
1

Effects of Reaction Conditions on Quinone/Diquinone Product Ratios in the Oxidation of 1,4-
Dimethoxybenzene Derivatives with Ceric Ammonium Nitrate
Brian E. Love,* Brian C. Duffy and Alexander L. Simmons
Department of Chemistry, East Carolina University, Greenville, NC, 27858, USA
_______________
* Corresponding author. Tel: +1 252 328 9778; fax: +1 252 328 6210
E-mail address: loveb@ecu.edu
Abstract
Proper choice of reaction conditions allows formation of either the quinone or corresponding diquinone as
the major product upon treatment of 2-alkyl-1,4-dimethoxybenzenes with ceric ammonium nitrate.
Keywords
Quinone, Diquinone, Ceric Ammonium Nitrate, Oxidation
2

Quinones represent a chemically interesting and biologically significant class of compounds.1
Though benzoquinones can be prepared by a variety of means,1,2 they are perhaps most commonly
synthesized through treatment of hydroquinone ethers with ceric ammonium nitrate (CAN).3 This same
reaction, however, is likewise one of the most common means of preparing diquinones3 (though other
methods are also known).4 Treatment of hydroquinone ethers with CAN thus can often lead to mixtures
of quinones and diquinones as products.
We are seeking to determine some of the parameters that influence quinone/diquinone ratios in the
product mixtures of such reactions. Preliminary results5,6 suggest that the nature of the alkoxy groups,
other substituents present on the benzene ring, and reaction conditions such as solvent, concentration, and
order of addition can all affect this ratio. One aim of this project is the development of reaction protocols
that would allow one to selectively produce either the quinone or the corresponding diquinone from the
same 1,4-dimethoxybenzene precursor with minimal contamination by the other possible product. Herein
we report our findings toward that end.
We chose 2-tert-butyl-1,4-dimethoxybenzene and 2,5-dimethoxytoluene as our initial two test
substrates. The former was chosen due to its ease of preparation from inexpensive starting materials7 and
the fact that under “traditional” conditions3a comparable amounts of quinone and diquinone were
produced, yet were readily isolated in fairly pure form. The 2,5-dimethoxytoluene was utilized owing to
its commercial availability. A variety of reaction conditions were investigated (see Tables 1 and 2)
though in all reactions 2.0 mmol of substrate were allowed to react with 7.0 mmol of CAN. Solvents
were dispensed using graduated pipets to ensure greater control over precise solvent volumes. With
respect to the order of addition, we have chosen to use the term “standard” to refer to addition of the CAN
to the substrate, and “inverse” to describe addition of the substrate to the CAN.
Table 1 shows the results obtained using 2-tert-butyl-1,4-dimethoxybenzene as a substrate,
arranged according to the mole ratios of the products obtained.8 One trend that emerges is that quinone
formation is generally favored by the “standard” mode of addition, whereas “inverse” addition tends to
3

give higher percentages of diquinone formation. This is consistent with earlier results,5 and is supported
by direct comparison of reactions that varied only in their mode of addition. Compare, for example,
entries 3 & 8, 4 & 10, 7 & 22, 14 & 20, 15 & 24, and 18 & 23--in all cases the reaction utilizing
“standard” addition provided a higher proportion of quinone in the product mixture than did the
corresponding reaction using “inverse” addition. It would appear that the nature of the solvent also has
some effect on product ratios, but we have not conducted a sufficient number of experiments to clearly
identify this effect or determine a definitive trend.
When CAN was added directly as a solid to the arene solution (entries 1 and 2), no diquinone
formation was observed, though the product mixtures contained incompletely oxidized compounds that
still possessed methoxy groups (as evidenced by their 1H NMR spectra). These side products included,
but were not limited to, 5-nitro-2-tert-butyl-1,4-dimethoxybenzene, and were also present in the product
mixtures of many of the reactions in which DMSO was used as solvent (Entries 3, 5, 7, 8 and 22).
Initially we thought that DMSO (or perhaps impurities in it, such as dimethylsulfide) was being oxidized
by the CAN, thus reducing the amount of oxidant available to react with the arene substrate. Use of larger
excesses of CAN failed to remove these impurities from the product mixtures, however. Such by-
products were also observed when 1,2-dimethoxyethane (DME) was used as solvent (entry 2).
4

Table 1. Oxidation of 2-tert-butyl-1,4-dimethoxybenzene by CAN
Quinone:
Mode of
Addition
Protocol
Designation
Entry Substrate Solvent
CAN Solvent
Diquinone
Mole Ratio
1
2
10.0 mL DMSO
10.0 mL DME
none
Standard
Standard
Standard
Standard
Standard
Standard
Standard
Inverse
quinone only
quinone only
48 : 1
27 : 1
23 : 1
10 : 1
6 : 1
A
none
3
10.0 mL DMSO
14.0 mL THF
5.0 mL H2O
7.0 mL H2O
7.0 mL H2O
7.0 mL H2O
7.0 mL H2O
5.0 mL H2O
7.0 mL H2O + 7.0 mL THF
7.0 mL H2O
14.0 mL H2O
7.0 mL H2O
3.5 mL H2O
3.5 mL H2O
7.0 mL H2O
7.0 mL H2O
7.0 mL H2O
3.5 mL H2O
7.0 mL H2O
3.5 mL H2O
3.5 mL H2O
7.0 mL H2O
3.5 mL H2O
7.0 mL H2O
B
C
4
5
14.0 mL THF + 1.0 mL DMSO
14.0 mL EtOAc
2.0 mL DMSO
10.0 mL DMSO
7.0 mL THF
6
7
8
6 : 1
9
Inverse
5 : 1
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
14.0 mL THF
Inverse
5 : 1
21.0 mL acetone + 7.0 mL H2O
14.0 mL CH3CN
3.5 mL THF
Standard
Standard
Inverse
4 : 1
2.8 : 1
2.7 : 1
2.5 : 1
2.3 : 1
1.7 : 1
1.7 : 1
1.6 : 1
1.5 : 1
1.2 : 1
1 : 1
1.0 mL THF
Standard
Standard
Inverse
7.0 mL CH3CN
3.5 mL CH3CN
2.0 mL THF
Traditional
Inverse
2.0 mL CH3CN
4.0 mL CH3CN
1.0 mL THF
Standard
Inverse
Inverse
D
3.5 mL CH3CN
2.0 mL DMSO
2.0 mL CH3CN
7.0 mL CH3CN
Inverse
Inverse
1 : 1
Inverse
1 : 1.3
1 : 1.9
E
F
Inverse
Results obtained using 2,5-dimethoxytoluene as the substrate (Table 2) were somewhat similar,
though this compound displayed a much greater tendency toward diquinone formation, as had been
observed earlier by others.3 This increased diquinone formation is clearly seen through comparison of
reactions run under identical conditions on the two test substrates. As was noted with 2-tert-butyl-1,4-
dimethoxybenzene, quinone formation tended to be favored by the “standard” mode of addition.
Compare, for example, entries 3 & 4, 5 & 8, 7 & 12, 10 & 14, and 11 & 15--in each of these comparisons
the reaction utilizing “standard” addition once again provided a higher proportion of quinone in the
5

product mixture than did the corresponding reaction using “inverse” addition. In only one pair of
reactions was this not the case (entries 13 & 16).9
Incompletely oxidized products were once again observed in many of the reactions that tended to
favor quinone formation (entries 2 through 6), though were not observed in every reaction involving
DMSO. Products of reactions listed in entries 1, 7 and 12, for example, contained very little if any such
impurities.
Table 2. Oxidation of 2,5-dimethoxytoluene by CAN
Quinone:
Protocol
Designation
Entry Substrate Solvent
CAN Solvent
Mode of Addition
Diquinone
Mole Ratio
1
2
10.0 mL DMSO
10.0 mL DME
10.0 mL DMSO
10.0 mL DMSO
14.0 mL THF
10.0 mL THF
2.0 mL DMSO
14.0 mL THF
14.0 mL EtOAc
1.0 mL THF
none
Standard
Standard
Standard
Inverse
20 : 1
10 : 1
4 : 1
A
none
3
5.0 mL H2O
5.0 mL H2O
7.0 mL H2O
14.0 mL H2O
7.0 mL H2O
7.0 mL H2O
7.0 mL H2O
3.5 mL H2O
7.0 mL H2O
7.0 mL H2O
3.5 mL H2O
3.5 mL H2O
7.0 mL H2O
3.5 mL H2O
B
C
4
1.2 : 1
1 : 2.4
1 : 4
5
Standard
Standard
Standard
Inverse
6
7
1 : 6
8
1 : 7
9
Standard
Standard
Standard
Inverse
1 : 9
10
11
12
13
14
15
16
1 : 12
1 : 14
1 : 14
1 : 18
1 : 20
1 : 22
1 : 58
7.0 mL CH3CN
2.0 mL DMSO
2.0 mL CH3CN
1.0 mL THF
Traditional
Inverse
E
D
F
Inverse
7.0 mL CH3CN
2.0 mL CH3CN
Inverse
Standard
Based on these preliminary results, some “favored protocols” emerged. Addition of CAN (both
neat and as an aqueous solution) to DMSO solutions of the arene (entries 1 and 3 in both Tables 1 and 2)
reliably gave high percentages of quinone as the product. Replacement of DMSO with THF (entry 4 in
Table 1 and entry 5 in Table 2) also gave reasonable quantities of quinone product (moreso with 2-tert-
butyl-1,4-dimethoxybenzene than 2,5-dimethoxytoluene) and offered the advantage of reduced
6

contamination by incompletely oxidized by-products. These “quinone favoring protocols” are designated
A, B and C as indicated in the tables.10
Diquinone formation, on the other hand, appeared to be favored by “inverse” addition of the arene,
dissolved in acetonitrile, to an aqueous solution of CAN (entries 23 and 24 in Table 1, and entries 13 and
15 in Table 2). Replacement of acetonitrile with a minimal amount of THF (entries 20 and 14 in Tables 1
and 2, respectively) also produced significant amounts of diquinone. These “diquinone favoring
protocols” are designated D, E and F as indicated in the tables.
We next sought to test the generality of these protocols on a wider range of substrates. Since the
electronic nature of substituents also appears to play a role in determining quinone/diquinone product
ratios, we restricted our substrates to 2-alkyl-1,4-dimethoxybenzene derivatives in an attempt to reduce
the influence of this variable. Each substrate was subjected to “traditional” reactions conditions (defined
as addition of an aqueous CAN solution (1.0 M) to the arene dissolved in an equal volume of
acetonitrile)11 as well as one or more of our “favored protocols” for quinone and diquinone formation. In
all reactions, the addition of one reagent to the other was allowed to occur over a period of 10-15 minutes.
Once the addition was complete, the reaction mixture was allowed to stir for 1 hour, after which time it
was diluted with water, and any precipitate that formed was collected by filtration--this was usually the
diquinone, which was typically obtained in a high state of purity. Extraction of the filtrate with ether
allowed isolation of the corresponding quinones, though they were often contaminated by impurities from
which they were difficult to separate. In our hands, neither recrystallization nor column chromatography
provided the quinones in analytically pure form. Quinone yields reported are thus approximate yields
based on product weights coupled with purity estimates taken from NMR data. The results of these
studies are summarized in Table 3.
7

Table 3. Oxidation of 2-alkyl-1,4-dimethoxybenzene derivatives by CAN
Quinone-Favoring Protocols
Traditional Protocol
Diquinone-Favoring Protocols
Yield
of 2
(%)
Yield
of 3
(%)
Protocol
Used
2 : 3 Mole
Ratio
Yield
of 2 (%)
2 : 3 Mole
Ratio
2 : 3 Mole
Ratio
Yield
of 3 (%)
Protocol
Used
R
Me
Me
Me
A
B
C
72
42
12
20 : 1
4 : 1
1 : 20
1 : 18
1 : 22
82
90
93
D
E
F
3
8
1 : 14
1 : 4.7
1 : 1.7
89
73
67
1 : 2.4
diquinone
only
Et
B
63
17 : 1
84
75
E
F
Et
n-Pr
C
A
B
C
B
C
B
C
A
B
C
A
B
C
B
C
B
C
C
18
80
63
20
58
35
68
36
70
73
82
70
63
25
51
33
28
32
39
1 : 1
> 50 : 1
19 : 1
1 : 1.2
13 : 1
2.6 : 1
30 : 1
2 : 1
1 : 20
n-Pr
20
1 : 3.9
77
E
n-Pr
n-Bu
17
17
1 : 1.9
1 : 1.9
64
67
1 : 6.1
1 : 3.1
78
76
F
F
n-Bu
i-Bu
i-Bu
t-Bu
> 50 : 1
48 : 1
27 : 1
> 50 : 1
12 : 1
1 : 1
1.2 : 1
1 : 1.3
1 : 1.9
55
55
46
D
E
F
t-Bu
42
2.3 : 1
37
t-Bu
n-pentyl
n-pentyl
n-pentyl
n-heptyl
n-heptyl
n-nonyl
n-nonyl
n-undecyl
14
12
1 : 2.4
1 : 2.4
65
59
1 : 12
80
64
E
14 : 1
2.3 : 1
26 : 1
3.3 : 1
2.3 : 1
1 : 4.4
E
F
1 : 19
93
73
1 : 3.8
E
F
22
21
1: 1.3
57
75
1 : 2.9
1 : 63
69
96
1 : 1.8
D
In all cases, mole ratio enhancements were observed by using one of the “favored” protocols in
place of the “traditional” protocol. In most reactions, use of “diquinone-favoring protocols” improved
yields of the diquinone product by 10% or more over the “traditional” protocol. The exception is when
8

2,5-dimethoxytoluene was used as the substrate, owing to the high diquinone yield obtained using the
traditional protocol. Even larger yield improvements were observed using “quinone-favoring protocols,”
again in part due to the low yields of such compounds obtained following the traditional protocol.
Though yield improvements were often significant, total yields were low when substrates containing large
lipophilic sidechains were employed. Additionally, for the most lipophilic substrate tested (the undecyl-
substituted compound), reduced solubility in acetonitrile and DMSO restricted protocols to those utilizing
THF as solvent if substrate molar concentrations were to be held constant. Owing to the pronounced
biological activity of quinones possessing long nonpolar sidechains12 we are continuing to work on the
development of methodologies that would allow the preparation of such compounds in greater yield and
higher states of purity. It is gratifying, however, that the current methods do allow easy preparation of the
corresponding diquinones, which are of interest in their own right.13
In summary, quinone:diquinone product ratios from the CAN oxidation of 1,4-dimethoxybenzene
derivatives can be shifted to favor either product by proper choice of reaction conditions. Diquinones
were generally obtained in a high state of purity, though most of the quinones were contaminated with
incompletely oxidized products from which they were difficult to separate.
Supplementary data
Supplementary data (experimental procedures and characterization data) associated with this
article can be found in the online version at doi: xxx
References and notes
1) Patai, S.; Rappoport, Z. (editors), The Chemistry of the Quinonoid Compounds, Vol. 2, Pt. 1 & 2,
Wiley, Chichester (1988).
2a) Akai, S.; Kita, Y. Recent Progress in the Synthesis of p-Quinones and p-Dihydroquinones Through
Oxidation of Phenol Derivatives. A Review. Organic Preparations and Procedures International, 1998,
30, 603-629. b) Tomatsu, A.; Takemura, S.; Hashimoto, K. Nakata, M. Synthesis of Quinones from
9

Hydroquinone Dimethyl Ethers. Oxidative Demethylation with Cobalt(III) Fluoride. Synlett 1999, 1474-
1476.
3a) Jacob III, P.; Callery, P. S.; Shulgin, A. T.; Castagnoli, Jr., N. A Convenient Synthesis of Quinones
from Hydroquinone Methyl Ethers. Oxidative Demethylation with Ceric Ammonium Nitrate. Journal of
Organic Chemistry 1976, 41, 3627-3629; b) Ali, M. H.; Niedbalski, M.; Bohnert, G.; Bryant, D. Silica-
Gel Supported Ceric Ammonium Nitrate (CAN): A Simple and Efficient Solid-Supported Reagent for
Oxidation of Oxygenated Aromatic Compounds to Quinones. Synthetic Communications 2006, 36, 1751-
1759.
4a) Paraskevas, S. M.; Konstantinidis, D.; Vassilara, G. Oxidation of hydroquinones with oxygen in the
presence of bis(1,3-propanediaminato)copper(II) chloride. Synthesis 1988, 897-899. b) Rao, D. V.;
Ulrich, H.; Sayigh, A. A. R. Synthesis of benzoquinone-1,4-aldehyde diacetate. Journal of Organic
Chemistry 1975, 40, 2548-2549. c) Posternak, T.; Alcalay, W.; Luzzati, R.; Tardent, A. Biphenylquinone
derivatives formed by oxidation of substituted hydroquinone ethers. Helvetica Chimica Acta 1948, 31,
525-535. d) Liebeskind, L. S.; Riesinger, S. W. Palladium-catalyzed oxidative dimerization of
stannylquinones. A simple method for the synthesis of symmetrical 2,2'-bisquinones. Tetrahedron
Letters 1991, 32, 5681-5682. e) Musgrave, O. C. The Oxidation of Alkyl Aryl Ethers. Chemical Reviews
1969, 69, 499-531; f) Giza, Y.-H. C.; Kun, K. A.; Cassidy, H. G. Electron Exchange Polymers. XVI. The
Oxidation Behavior of Dimethyl Ethers of Hydroquinone and Methyl-Substituted Hydroquinones.
Journal of Organic Chemistry 1962, 27, 679-682.
5) Love, B. E.; Bonner-Stewart, J.; Forrest, L. A. Improved Synthesis of Diquinones. Synlett 2009, 813-
817.
10

6) Love, B. E., unpublished results.
7) Zaytsev, A. V.; Anderson, R. J.; Bedernjak, A.; Groundwater, P. W.; Huang, Y.; Perry, J. D.; Orenga,
S.; Roger-Dalbert, C.; James, A. Synthesis and testing of chromogenic phenoxazinone substrates for -
alanyl aminopeptidase. Organic & Biomolecular Chemistry 2008, 6, 682-692.
8) Owing to the dimeric nature of the diquinone, the mole ratio of quinone:diquinone is not the same as
either the weight ratio or yield ratio of these same products.
9) In these reactions, the amount of monoquinone formed was so small (less than 0.01 g in both cases)
that small errors in weight determination could easily have had a large effect on calculated mole ratios.
10) Though designated as “quinone-favoring protocols,” in some cases use of such reaction conditions
still provided the diquinone as the major product.
11) See supplementary data for experimental details.
12) Vitamin K and the ubiquinones are perhaps the best-known examples of such compounds, though
embelin, maesaquinone and avarone are also representative members of this class that have biological
activity.
13) Examples include biembelin, burmanin A and cuculoquinone. Additionally, diquinones are potential
precursors to biologically significant dibenzofuran-1,4-diones such as the anticancer agent popolohuanone
E.
11

Table 1. Oxidation of 2-tert-butyl-1,4-dimethoxybenzene by CAN
Quinone:
Mode of
Addition
Protocol
Designation
Entry Substrate Solvent
CAN Solvent
Diquinone
Mole Ratio
1
2
10.0 mL DMSO
10.0 mL DME
none
Standard
Standard
Standard
Standard
Standard
Standard
Standard
Inverse
quinone only
quinone only
48 : 1
27 : 1
23 : 1
10 : 1
6 : 1
A
none
3
10.0 mL DMSO
14.0 mL THF
5.0 mL H2O
7.0 mL H2O
7.0 mL H2O
7.0 mL H2O
7.0 mL H2O
5.0 mL H2O
7.0 mL H2O + 7.0 mL THF
7.0 mL H2O
14.0 mL H2O
7.0 mL H2O
3.5 mL H2O
3.5 mL H2O
7.0 mL H2O
7.0 mL H2O
7.0 mL H2O
3.5 mL H2O
7.0 mL H2O
3.5 mL H2O
3.5 mL H2O
7.0 mL H2O
3.5 mL H2O
7.0 mL H2O
B
C
4
5
14.0 mL THF + 1.0 mL DMSO
14.0 mL EtOAc
2.0 mL DMSO
10.0 mL DMSO
7.0 mL THF
6
7
8
6 : 1
9
Inverse
5 : 1
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
14.0 mL THF
Inverse
5 : 1
21.0 mL acetone + 7.0 mL H2O
14.0 mL CH3CN
3.5 mL THF
Standard
Standard
Inverse
4 : 1
2.8 : 1
2.7 : 1
2.5 : 1
2.3 : 1
1.7 : 1
1.7 : 1
1.6 : 1
1.5 : 1
1.2 : 1
1 : 1
1.0 mL THF
Standard
Standard
Inverse
7.0 mL CH3CN
3.5 mL CH3CN
2.0 mL THF
Traditional
Inverse
2.0 mL CH3CN
4.0 mL CH3CN
1.0 mL THF
Standard
Inverse
Inverse
D
3.5 mL CH3CN
2.0 mL DMSO
2.0 mL CH3CN
7.0 mL CH3CN
Inverse
Inverse
1 : 1
Inverse
1 : 1.3
1 : 1.9
E
F
Inverse
12

Table 2. Oxidation of 2,5-dimethoxytoluene by CAN
Quinone:
Diquinone
Mole Ratio
Protocol
Designation
Entry Substrate Solvent
CAN Solvent
Mode of Addition
1
2
10.0 mL DMSO
10.0 mL DME
10.0 mL DMSO
10.0 mL DMSO
14.0 mL THF
10.0 mL THF
2.0 mL DMSO
14.0 mL THF
14.0 mL EtOAc
1.0 mL THF
none
Standard
Standard
Standard
Inverse
20 : 1
10 : 1
4 : 1
A
B
C
none
3
5.0 mL H2O
5.0 mL H2O
7.0 mL H2O
14.0 mL H2O
7.0 mL H2O
7.0 mL H2O
7.0 mL H2O
3.5 mL H2O
7.0 mL H2O
7.0 mL H2O
3.5 mL H2O
3.5 mL H2O
7.0 mL H2O
3.5 mL H2O
4
1.2 : 1
1 : 2.4
1 : 4
5
Standard
Standard
Standard
Inverse
6
7
1 : 6
8
1 : 7
9
Standard
Standard
Standard
Inverse
1 : 9
10
11
12
13
14
15
16
1 : 12
1 : 14
1 : 14
1 : 18
1 : 20
1 : 22
1 : 58
7.0 mL CH3CN
2.0 mL DMSO
2.0 mL CH3CN
1.0 mL THF
Traditional
Inverse
E
D
F
Inverse
7.0 mL CH3CN
2.0 mL CH3CN
Inverse
Standard
13

Table 3. Oxidation of 2-alkyl-1,4-dimethoxybenzene derivatives by CAN
Quinone-Favoring Protocols
Traditional Protocol
Diquinone-Favoring Protocols
Yield
of 2
(%)
Yield
of 3
(%)
Protocol
Used
2 : 3 Mole
Ratio
Yield
of 2 (%)
2 : 3 Mole
Ratio
2 : 3 Mole
Ratio
Yield
of 3 (%)
Protocol
Used
R
Me
Me
Me
A
B
C
72
42
12
20 : 1
4 : 1
1 : 20
1 : 18
1 : 22
82
90
93
D
E
F
3
8
1 : 14
1 : 4.7
1 : 1.7
89
73
67
1 : 2.4
diquinone
only
Et
B
63
17 : 1
84
75
E
F
Et
n-Pr
C
A
B
C
B
C
B
C
A
B
C
A
B
C
B
C
B
C
C
18
80
63
20
58
35
68
36
70
73
82
70
63
25
51
33
28
32
39
1 : 1
> 50 : 1
19 : 1
1 : 1.2
13 : 1
2.6 : 1
30 : 1
2 : 1
1 : 20
n-Pr
20
1 : 3.9
77
E
n-Pr
n-Bu
17
17
1 : 1.9
1 : 1.9
64
67
1 : 6.1
1 : 3.1
78
76
F
F
n-Bu
i-Bu
i-Bu
t-Bu
> 50 : 1
48 : 1
27 : 1
> 50 : 1
12 : 1
1 : 1
1.2 : 1
1 : 1.3
1 : 1.9
55
55
46
D
E
F
t-Bu
42
2.3 : 1
37
t-Bu
n-pentyl
n-pentyl
n-pentyl
n-heptyl
n-heptyl
n-nonyl
n-nonyl
n-undecyl
14
12
1 : 2.4
1 : 2.4
65
59
1 : 12
80
64
E
14 : 1
2.3 : 1
26 : 1
3.3 : 1
2.3 : 1
1 : 4.4
E
F
1 : 19
93
73
1 : 3.8
E
F
22
21
1: 1.3
57
75
1 : 2.9
1 : 63
69
96
1 : 1.8
D
14

(Figure for Table 3)
15