[4 + 4]-cycloaddition of isoprene for the production of high-performance bio-based jet fuel

Article abstract of DOI:10.1039/c9gc02404b

Isoprene was efficiently converted to 1,6-dimethyl-1,5-cyclooctadiene (DMCOD) by selective [4 + 4]-cycloaddition with a catalyst formed by in situ reduction of [(^MePI)FeCl(μ-Cl)]₂ (^MePI = [2-(2,6-(CH₃)₂-C₆H₃-NC(CH₃))-C₄H₅N]). DMCOD was isolated in 92% yield, at the preparative scale, with a catalyst loading of 0.025 mol%, and a TON of 3680. Catalytic hydrogenation of DMCOD yielded 1,4-dimethylcyclooctane (DMCO). The cyclic structure and ring strain of DMCO afforded gravimetric and volumetric net heats of combustion 2.4 and 9.2% higher, respectively, than conventional jet fuel. In addition, the presence of methyl branches at two sites resulted in a-20 °C kinematic viscosity of 4.17 mm² s^-1, 48% lower than the maximum allowed value for conventional jet fuel. The ability to derive isoprene and related alcohols readily from abundant biomass sources, coupled with the highly efficient [Fe]-catalyzed [4 + 4]-cycloaddition described herein, suggests that this process holds great promise for the economical production of high-performance, bio-based jet fuel blendstocks.

Full text of DOI:10.1039/c9gc02404b

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Page 1 of 19
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DOI: 10.1039/C9GC02404B
[4+4]-Cycloaddition of Isoprene for the Production of High-Performance Bio-
Based Jet Fuel
Kyle E. Rosenkoettera, C. Rose Kennedyb, Paul J. Chirikb*, and Benjamin G. Harveya*
aUS NAVY, NAWCWD, Research Department, Chemistry Division, China Lake, California 93555
bDepartment of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
Abstract
Isoprene was efficiently converted to 1,6-dimethyl-1,5-cyclooctadiene (DMCOD) by selective
[4+4]-cycloaddition with a catalyst formed by in situ reduction of [(MePI)FeCl(μ-Cl)]2 (MePI = [2-
(2,6-(CH3)2-C6H3–N=C(CH3))–C4H5N]). DMCOD was isolated in 92% yield, at the preparative
scale, with a catalyst loading of 0.025 mol%, and a TON of 3680. Catalytic hydrogenation of
DMCOD yielded 1,4-dimethylcyclooctane (DMCO). The cyclic structure and ring strain of
DMCO afforded gravimetric and volumetric net heats of combustion 2.4 and 9.2% higher,
respectively, than conventional jet fuel. In addition, the presence of methyl branches at two sites
resulted in a –20 °C kinematic viscosity of 4.17 mm2 s−1, 48 % lower than the maximum allowed
value for conventional jet fuel. The ability to derive isoprene and related alcohols readily from
abundant biomass sources, coupled with the highly efficient [Fe]-catalyzed [4+4]-cycloaddition
described herein, suggests that this process holds great promise for the economical production of
high-performance, bio-based jet fuel blendstocks.

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Introduction
The development of advanced bio-based fuels provides a unique opportunity to reduce greenhouse
gas emissions and the environmental impact of jet aircraft, while delivering performance properties
that in many cases exceed those of conventional jet fuel.1−22 Nature provides a dizzying array of
substrates that can be efficiently converted into reduced hydrocarbon species suitable for
incorporation into jet fuel blends. Isoprene is a particularly intriguing substrate due to the ability
to readily generate it, or precursor molecules, from biomass sources through fermentation with
metabolically engineered microorganisms. For example, Keasling and Lee have demonstrated the
production of 3-methyl-3-buten-1-ol with metabolically engineered E. coli at moderate titer (2.2
g/L).23 Isoprene can then be readily generated from the alcohol by dehydration.24−26 Isoprene can
also be generated from biomass sources by hybrid biological/chemical routes. For example,
biomass sugars can be efficiently converted to mevalonolactone, followed by catalytic conversion
to isoprene at temperatures above 300 °C in the presence of a heterogenous acid catalyst.27
Alternatively, biomass sugars can be fermented to generate mesaconic acid which can then be
hydrogenated and dehydrated to form isoprene.28,29 The direct production of isoprene, and
subsequent removal from the fermentation broth via low temperature distillation, is a more
straightforward approach that obviates the need for a chemical upgrading step. This route has been
extensively explored,30a with reported isoprene titers of >60 g/L.30b Regardless of the pathway,
additional improvements in conversion efficiency and lignocellulosic biomass utilization will be
required to allow for bio-based isoprene to become competitive with petroleum feedstocks.
Isoprene is an ideal starting material for the synthesis of high-performance jet fuel by virtue of
its high reactivity (arising from the conjugated pi system) and methyl substitution. The latter
feature allows for the introduction of branching motifs useful for decreasing the viscosity and
lowering the freezing point of derivative fuels. To maximize the environmental benefits afforded
by a bio-based fuel, it is important to utilize high-throughput catalytic methods that are selective
for formation of molecules with desirable structures and fuel properties.2,6,13,31 From a design
standpoint, conversion of isoprene to a cyclic paraffin should maximize the density of the fuel
while providing good combustion properties and low viscosity. Selective dimerization of isoprene
to a cyclic product, followed by hydrogenation, would yield a C10 hydrocarbon, which falls within
the range of conventional jet fuel (typically C9−C14).

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Recent work by the Chirik group32 has shown that isoprene can be selectively dimerized to
1,6-dimethyl-1,5-cyclooctadiene (1,6-DMCOD, 2a) through [4+4]-cycloaddition catalyzed by
well-defined, reduced iron iminopyridine complexes. The proposed catalytic cycle proceeds
through coordination of two equivalents of isoprene followed by oxidative cyclization,
rearrangement, and reductive elimination.32 In situ activation protocols were reported that allowed
the dimerization to be conducted using low loadings of bench-stable iron (II) halide precatalysts.
Hydrogenation of 2a would yield 1,4-dimethylcyclooctane (DMCO, 3), a C10H20 hydrocarbon that
would be expected to have favorable fuel properties due to its cyclic structure, and chain branching
(Scheme 1). To explore the potential of DMCO as an alternative jet fuel blendstock, this paper
reports the preparative-scale synthesis of DMCO and describes key fuel properties of DMCO and
blends with a common synthetic paraffinic kerosene.
Scheme 1. Synthesis of 1,4-DMCO from isoprene using iron-catalyzed cycloaddition followed by hydrogenation,.
Aryl = 2,6-dimethylphenyl.
[Pt]
[Fe]
2
+ activator
+ H2
1
2a (1,6-DMCOD)
3 (DMCO)
isoprene
1,6-dimethylcycloocta-1,5-diene
1,4-dimethylcyclooctane
[Fe]
[Pt]
PtO2 • n H2O
N
Fe
Cl
Cl
Fe
N
Aryl
Cl
Cl
N
N
[(MePI)FeCl(2
activated with MeMgCl (3 M in THF)
in HOAc solvent
EXPERIMENTAL SECTION
Materials and Methods. All air- and moisture-sensitive manipulations were performed using
standard Schlenk techniques on a dual-manifold vacuum line under ultra-high purity argon, or in
an M. Braun glove box with an inert atmosphere of purified nitrogen. Isoprene, MeMgCl solution
(3M in THF), platinum (IV) oxide hydrate, and glacial acetic acid were purchased from Millipore
Sigma, while a sample of Jet-A/F-24 was obtained from the fuel depot at the China Lake Naval
Air Warfare Center. A sample of HEFA-Jet was obtained from the Air Force Research Laboratory,
Dayton, OH. Prior to use, isoprene was stirred over calcium hydride for >48 hours, degassed by 3
freeze-pump-thaw cycles, and distilled under vacuum. Without exposure to air, the dried isoprene

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was transferred to a glovebox, stored over activated 4 Å molecular sieves at –35 °C, and (as
needed) filtered through a plug of activated Al2O3 prior to use. Deuterated chloroform (CDCl3) for
NMR spectroscopy was purchased from Cambridge Isotopes and stored over anhydrous potassium
carbonate. All other reagents were used as received. The iminopyridine iron dihalide precatalyst
used herein [(MePI)FeCl(μ-Cl)]2 was prepared as previously described.32,33
Proton nuclear magnetic resonance (1H NMR) spectra were recorded at 25 °C on a Bruker
Avance III 500 spectrometer operating at 500.46 MHz. Proton-decoupled 13C{1H} NMR, 13C{1H}
APT NMR, and quantitative q13C{1H} NMR nuclear magnetic resonance spectra were recorded at
25 °C on a Bruker Avance III 500 instrument operating at 125.86 MHz. All experiments were
performed at the Princeton University Nuclear Magnetic Resonance Facility or the China Lake
Naval Air Warfare Center. Chemical shifts are reported in parts per million downfield from
tetramethylsilane (SiMe4) and are referenced relative to the NMR solvent according to literature
values: δ(1H) = 7.26, δ(13C) = 77.0 for CDCl3. NMR spectra were processed using the MestReNova
or TopSpin software suites.
Analytical gas chromatography was performed using a Shimadzu GC-2010 instrument
equipped with a Shimadzu AOC-20s autosampler. For samples analyzed on a Shimadzu SHRXI-
5MS achiral stationary phase capillary column (15 m × 250 μm), the instrument was set to an
injection volume of 1.0 µL, an inlet split ratio of 20:1, and inlet and detector temperatures of 250
°C and 275 °C, respectively. UHP-grade S3 helium was used as carrier gas with a flow rate of 1.96
mL/min. The column temperature was varied as follows: 0–2 minutes, isotherm, 30 °C; 2–12
minutes, temperature ramp, +3 °C/min; 12–15 minutes, isotherm, 60 °C; 15–19 minutes,
temperature ramp, +10 °C/min; 19–20 minutes, isotherm, 100 °C. Products were further
characterized via GC-MS (electron-impact ionization). The GC-MS system was equipped with an
RTX-5MS 30-meter column and the analysis was conducted under the following conditions: inlet
temperature, 250 °C; initial column temperature, 40 °C; temperature ramp, 4 °C/min to 100 °C;
2nd temperature ramp, 20 °C/min to 300 °C.
Net heat of combustion (NHOC) measurements were conducted by the Southwest Research
Institute (SwRI) using ASTM D240N. The derived cetane number (DCN) of DMCO was measured
via ignition quality testing (IQT) by SwRI using ASTM D6890. The kinematic viscosity and
density of DMCO were measured with a Stabinger Viscometer, SVM 3001 connected to a TC-502
circulation cooler. A 5 mL disposable syringe was used to inject the sample into the viscometer.

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The flash point of DMCO was measured with a Grabner Instruments/Ametek Miniflash FLP
Touch according to ASTM D7094. For each measurement, 2 mL of fuel were transferred via auto
pipette to a 7 mL stainless steel sample cup. The initial temperature of each run was set to 28 °C
and the final temperature was set to 64 °C.
1,6-dimethyl-1,5-cyclooctadiene (DMCOD)
[Caution: The [4+4]-cycloaddition of 1,3-dienes is highly exothermic. When performing the
reaction on preparative scale, special care should be taken to ensure adequate heat transfer.
Maintaining a controlled temperature throughout the course of the reaction is necessary to prevent
dangerous pressure build-up.]
Procedure A (200 mmol scale):
Scheme 2. Preparation of DMCOD by Procedure A.
The dimerization of isoprene was conducted on a 200 mmol scale as depicted in Scheme
2. In a glovebox, an oven-dried 50-mL round bottom flask was charged with [(MePI)FeCl(μ-Cl)]2
(0.035 g, 0.05 mmol dimer, 0.1 mmol [Fe], 0.05 mol% [Fe]), isoprene (1.0 g, 15 mmol), and an
oven-dried PTFE-coated magnetic stir bar. The reaction was initiated by the addition of MeMgCl
(3 M in THF, 0.080 mL, 0.24 mmol, 0.12 mol%). The reaction flask was covered with a vacuum-
dried virgin rubber septum or an oven-dried ground-glass stopper. The reaction mixture was stirred
vigorously in the glovebox for 10 min or until a blue/green color persisted. Additional isoprene
(12.6 g, 185 mmol; to a total of 200 mmol, 1.0 equiv) was then added in the glovebox. The reaction
flask was sealed, and the reaction mixture was maintained with vigorous stirring at ambient
temperature (~23 °C). Aliquots (50 uL) of the reaction mixture were removed regularly by syringe
to monitor the reaction progress. For each aliquot, the sample was removed from the glovebox,
1
diluted with CDCl3 (0.7 mL), filtered through a plug of Celite, and analyzed by H NMR
spectroscopy (500 MHz, CDCl3, 25 °C). After 20 h, or upon complete consumption of isoprene,
the reaction flask was removed from the glovebox and opened to air in a fume-hood. The flask
was fitted with an oven-dried short-path distillation head, which was connected to a high vacuum
manifold. DMCOD (89–95% isolated yield, 96–97% 2, 10:1 2a:2b) was isolated by vacuum

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distillation into a tared, 50- or 100-mL receiving flask cooled by dry ice/acetone. The combined
yield of product isomers was determined from the mass of the isolated material. The composition
of [4+4]-cycloadducts 1,6-DMCOD (2a) and 1,5-DMCOD (2b) relative to other dimeric species
was assessed by gas-chromatography and corroborated by 1H NMR and 13C NMR spectroscopy.
The regioisomer ratio (1,6-DMCOD:1,5-DMCOD; 2a:2b) of the [4+4]-cycloadducts was
13
determined from the relative integration of diagnostic resonances in the quantitative C NMR
spectrum. Spectral data were consistent with values reported previously.32,34 1H NMR (500 MHz,
CDCl3): δ 5.44–5.15 (m, 2H, (=CH-)), 2.34 (s, 4H, CH2), 2.32–2.24 (m, 4H, CH2), 1.70 (s, 6H,
1,6-DMCOD, CH3), 1.68 (s, 6H, 1,5-DMCOD, CH3). 13C{1H} NMR (126 MHz, CDCl3): δ 136.0
(=C(CH3)-), 122.5 (=CH-), 32.4 (CH2), 27.6 (CH2), 26.5 (CH3) for 1,6-DMCOD; 135.9 (=C(CH3)-
), 122.7 (=CH-), 33.5 (CH2), 26.5 (CH2), 26.4 (CH3) for 1,5-DMCOD. GC
(30TO60TO100_20MIN): Rt = 14.0 min for both 1,6- and 1,5-DMCOD
Procedure B (2 mol scale):
Scheme 3. Preparation of DMCOD by Procedure B.
The dimerization of isoprene was conducted on the 2 mol scale as depicted in Scheme 3. In a
glovebox, an oven-dried 500-mL round-bottom Schlenk flask was charged with [(MePI)FeCl(μ-
Cl)]2 (0.175 g, 0.25 mmol dimer, 0.50 mmol [Fe], 0.025 mol% [Fe]), an oven-dried football-shaped
stir bar, and isoprene (10 g, 0.15 mol). A separate, oven-dried 250-mL round-bottom flask was
charged with additional isoprene (126 g, 1.85 mol; to a total of 2.0 mol, 1.0 equiv). Both flasks
were sealed with virgin rubber septa and removed from the glovebox. The Schlenk flask side-arm
was fit with a gas-inlet hose, and the hose was evacuated and back-filled with argon three times
before the stopcock was opened, placing the flask under a positive pressure of argon. The Schlenk
flask was then lowered into a cold-water bath (10–15 °C; maintained with ice chips), and stirring
was initiated. While stirring vigorously under argon, the catalyst was activated with the addition
of MeMgCl solution (3 M in THF, 0.4 mL, 1.2 mmol, 0.6 mol%) injected through the septum. The
reaction was maintained for 10 minutes as the mixture changed colors from purple to green to
blue-green. After 10 minutes, the flask containing the remaining isoprene was placed under a

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positive pressure of argon. An oven-dried stainless-steel cannula was connected between the two-
flasks, and the Schlenk flask was fit with an N2-flushed, but unpressurized balloon. The stopcock
on the Schlenk flask side-arm was closed, and the isoprene was added over the span of 20 minutes
by an argon-driven cannula transfer, releasing pressure into the balloon. After the addition was
complete, the stopcock was reopened, and the cannula and needle were removed. The puncture
marks in the septum were covered with grease and PTFE-tape. The reaction was maintained with
stirring under gentle argon pressure for 24 hours, and the water bath was allowed to warm
gradually to ambient temperature (~23 °C). After 24 hours, the reaction vessel was then opened to
air, and DMCOD (92% conversion; 97% 2; 10:1 2a:2b) was isolated by vacuum distillation
directly from the reaction vessel. Characterization data were consistent with those above.
Dimethylcyclooctane (DMCO).
Scheme 4. Hydrogenation of DMCOD to afford DMCO (3).
The hydrogenation of DMCOD was conducted on the 1.6 mol scale as depicted in Scheme 4.
A 2.5 L reinforced glass Parr reaction vessel was charged with 2 (220 g, 1.62 mol), platinum(IV)
oxide hydrate (2.05 g, ~80 wt% [Pt], 0.008 mol [Pt], 0.5 mol% [Pt] ), and acetic acid (50 mL). The
flask was sealed with a rubber septum and the headspace was filled with nitrogen. The vessel was
evacuated and then placed within the hydrogenation apparatus. The flask was charged with
hydrogen, evacuated, and recharged with hydrogen four times. The vessel was finally charged with
hydrogen (0.34 MPa) and mechanically shook. Rapid consumption of hydrogen was immediately
observed, and the vessel was recharged multiple times with 50 psi of hydrogen until the pressure
stabilized. At this point the flask was recharged with hydrogen again and shook overnight. The
vessel was then removed from the hydrogenation apparatus and quickly put under nitrogen. The
contents were filtered through Celite and the filter cake was washed with pentane. The filtrate was
transferred to a 500 mL separatory funnel and washed with DI water (300 mL), saturated sodium
bicarbonate solution (300 mL), and lastly additional DI water (300 mL). The organic layer was
collected and dried with magnesium sulfate, filtered, and the volatiles were carefully removed on
a rotary evaporator to give a pale yellow solution. The solution was then fractionally distilled under

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reduced pressure. Residual pentane was removed with the first fraction at ambient temperature
under a static vacuum induced by opening the distillation apparatus to a standard high vacuum line
for several seconds. A second distillate fraction was collected under dynamic vacuum (0.01 Torr)
between 25 °C and 40 °C to yield DMCO (equimolar mixture of cis and trans isomers) as a clear,
colorless liquid (192.06 g, 85% yield, 97% 3, 10:1 3a:3b). Anal. Calcd for C10H20: C, 85.63; H,
14.37. Found: C, 85.48; H, 14.54. 1H NMR (500 MHz, CDCl3): δ 1.760 – 1.210 (alkyl-H), 0.939
– 0.906 (alkyl-H). 13C{1H} NMR (126 MHz, CDCl3): δ 34.46 (s), 33.87 (s), 33.85 (s), 33.74 (s),
33.05 (s), 32.64 (s), 26.14 (s), 26.12 (s), 24.79 (s), 23.91 (s). IR (neat, cm-1): 2950, 2914, 2868,
1457, 1379. GC-MS (DCM): 8.58-8.64 min (140 m/z).
RESULTS AND DISCUSSION
Isoprene was readily converted to DMCO on a 100+ gram scale by a straightforward, high-
throughput, two-step method. Prior to conducting the cycloaddition reaction, commercial isoprene
was carefully purified to remove traces of water, oxygen, oligomers, and inhibitor. With
precatalyst loadings as low as 0.025 mol% [Fe], the dimerization reaction proceeded with 92%
conversion and 97% selectivity to [4+4] products (91% 1,6-dimethyl-2,5-cyclooctadiene; Figure
S6) over 24 hours, with complete conversion of substrate achieved over the same time frame with
modestly higher catalyst loadings (0.05 mol% [Fe]). At the lower catalyst loading, a relatively high
turnover number (TON) of 3680 was achieved. This TON is more than 42-times higher than that
obtained for the pyridine(diimine) iron-catalyzed [2+2]-cycloaddition of unactivated α-olefins,2,31
and on the same order as TONs achieved with the Cp2ZrCl2/MAO-catalyzed dimerization of 1-
hexene.6 Hydrogenation was conducted with PtO2 under mild conditions (ambient temperature, 50
psi H2) using acetic acid as a heterogeneous co-solvent. Complete reduction of 2 yielded DMCO
(3) as a 50:50 mixture of the cis- and trans-isomer. The structure and purity of 3 was confirmed
by 1H and 13C{1H} NMR spectroscopy, as well as GC-MS/FID (Figures S8–S11). Traces of other
isomers (~3%), likely formed by [4+2] cycloaddition of isoprene prior to hydrogenation, were
observed in the gas chromatogram of DMCO, but were not distinguishable in the NMR spectra.
With significant quantities of DMCO available for further study, key fuel properties including
density, NHOC, kinematic viscosity, flashpoint, and derived cetane number were measured (Table
1). DMCO exhibited a density of 0.827 g/mL at 15 °C. This value is 6.7% higher than the lower
limit for conventional jet fuel (>0.775 g/mL) and 12.8% higher than the acyclic hydrogenated

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Table 1. Key Fuel Properties of DMCO and Conventional Jet Fuel.
Property
DMCO
Jet-A/F-24*
Gravimetric Net Heat of Combustion (NHOC)
Volumetric NHOC
Density (15 °C)
Kinematic Viscosity (−20 °C)
Freezing Point
43.822 MJ/kg
36.222 MJ/L
0.827 g/mL
4.17 mm2s−1
<−78 °C
>42.8 MJ/kg
>33.17 MJ/L
>0.775 g/mL
<8.0 mm2s−1
<−40 °C
Flash Point
50 °C
>38 °C
*The values listed here are based on the specification for Jet-A/F-24 (ASTM D1655) and
should not be confused with those obtained for the authentic sample used as a benchmark
below.
isoprene dimer 2,6-dimethyloctane (0.73 g/mL). As expected, the higher density of DMCO
resulted in a volumetric net heat of combustion (NHOC) of 36.222 MJ/L, 9.2% higher than the
lower limit of conventional jet fuel (33.17 MJ/L). Similarly, due to the lack of aromatic
compounds, the gravimetric NHOC of DMCO was 43.822 MJ/kg, 2.4% higher than that of
conventional jet fuel.
DMCO also compares favorably to recently reported alkyl cyclobutane fuels that can be
prepared by [2+2]-cycloaddition of unactivated alkenes.2,31 For example, 1,2-dipropylcyclobutane
(DPC), a C10 cyclic hydrocarbon prepared from 1-pentene, has a similar gravimetric heat of
combustion (43.74 MJ/kg) to DMCO. One would expect to observe similar values considering
both molecules have the same hydrogen content and molecular weight. The cyclobutane ring in
DPC imparts greater strain energy (~26 kcal for cyclobutane vs. ~10 kcal for cyclooctane), 35,36
which should increase the gravimetric NHOC of DPC by ~1.1% compared to DMCO. However,
other factors, including differences in heat of vaporization, and the presence of acyclic isomers in
the DPC samples, result in comparable gravimetric NHOCs. In contrast, the volumetric NHOC of
DMCO is nearly 8% higher than DPC (36.222 MJ/L compared to 33.55 MJ/L) due to the higher
density of DMCO (0.827 g/mL compared to 0.767 g/mL). This higher density is a result of having
more carbon atoms constrained within the ring system.
To further evaluate DMCO, the kinematic viscosity was measured from −40 to 20 °C (Figure
2). Maintaining low viscosity at low temperature is critical for jet fuels, both for safety, as well as
to ensure complete combustion. A viscosity of less than 12 mm2s−1 at −40 °C is required to ensure
that a jet engine can be relit at altitude if the engine has been extinguished.37 Low viscosity is also
important for the rapid atomization of fuels. Higher viscosity fuels can lead to reduced combustion
efficiency and the production of soot which can reduce engine life. DMCO exhibited a −20 °C

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Figure 1. Kinematic viscosity of DMCO and Jet-A from 20 to −40 °C.
kinematic viscosity of 4.17 mm2s−1, which is about half of the upper viscosity limit for Jet-A (8.0
mm2s−1). The −40 °C kinematic viscosity of DMCO was 7.95 mm2s−1, 34% lower than the
specification limit. To provide a comparison, the kinematic viscosity of a sample of Jet-A/F-24
was measured over a comparable temperature range (Figure 1). The viscosity curves of DMCO
and the Jet-A sample were virtually identical, with DMCO exhibiting a slightly lower viscosity at
−40 °C.
The freezing point of DMCO was first studied by differential scanning calorimetry (DSC). The
DSC trace did not reveal an observable freezing point down to −80 °C. Similarly, no crystallization
or gel formation was observed after placing a sample of DMCO in a dry ice/acetone bath (−78 °C)
for up to one hour. Thus, the freezing point of DMCO is at least 40 °C lower than the requirement
for Jet-A/F-24 (< −40 °C). This extremely low freezing point may be due, in part, to the presence
of two diastereomers in the fuel mixture. In addition to an upper limit on freezing point, the
specification for Jet-A/F-24 has a strict flashpoint requirement (>38 °C). This requirement is an
important safety consideration, with more volatile fuels increasing the likelihood of a catastrophic
fire. DMCO has a flashpoint of 50 °C, allowing for its use as a drop-in blendstock.
Although the focus of the current work was the development of a high-performance, bio-based
jet fuel, it was also of interest to explore the potential of DMCO as a diesel fuel. Ignition Quality
Testing (IQT) was conducted and a derived cetane number (DCN) of 18 was measured for DMCO.
DCN directly relates to the ignition delay of the fuel, with higher values representing rapid
combustion under compression ignition conditions, and lower values representing slow
combustion.4,38 The lower cetane number limit for conventional diesel fuel (Diesel #2) is 40, with

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typical ranges of about 40–54 found throughout the U.S.39,40 The low DCN of DMCO, along with
its low viscosity would preclude its use as a standalone diesel fuel. In addition, although there is
no DCN requirement in jet fuel specifications, turbine fuels with DCNs below 30 can increase the
likelihood of lean blowout.41–43 High density/high cetane fuel blends were recently prepared by
blending sesquiterpanes with bio-derived synthetic paraffinic kerosenes.4 In a conceptually similar
fashion, blending DMCO with other high cetane fuels is anticipated to remedy this shortcoming.
The high density of DMCO coupled with its high gravimetric NHOC suggest that it is an ideal
candidate to replace aromatic compounds in high performance, bio-based fuel formulations. Most
alternative jet fuels are composed of synthetic paraffinic kerosenes (SPKs) containing acyclic
linear and branched alkanes that have moderate densities (~0.74–0.76 g/mL)44 and do not meet the
minimum density requirement for Jet-A/F-24 (>0.775 g/mL). Blending DMCO with these SPKs
can increase the density and help these fuels meet the specification. DMCO also has a much higher
gravimetric NHOC (43.82 MJ/kg) compared to a typical aromatic compound like toluene (40.59
MJ/kg). This 8% increase in NHOC can increase the range or payload capability of both civilian
and military aircraft. The low viscosity of DMCO suggests that it can be blended in almost any
proportion with conventional SPKs. Alternatively, it could be applied to reduce the viscosity of
higher molecular weight blendstocks.
Minimizing the amount of aromatics in jet fuel can improve the combustion efficiency of the
fuel, while simultaneously reducing the production of soot, and the emission of unburned
hydrocarbons, including carcinogenic polyaromatic hydrocarbons.45,46 Aromatics have
traditionally been an important component of jet fuel, prized for their ability to effectively swell
nitrile rubber elastomers, ensuring engine integrity. However, with the advent of modern
elastomeric materials, the need for aromatic compounds in jet fuel has greatly diminished.
Moreover, a recent study by Boeing has demonstrated that 30% blends of cyclic alkanes are
capable of swelling nitrile o-rings to the same extent as existing low (8%) aromatic fuel blends,
suggesting the viability of aromatic-free alternative jet fuels in modern aircraft.47,48 To evaluate
the suitability of DMCO as a replacement for aromatic compounds in a full-performance, bio-
based jet fuel, 20–50% (v/v) DMCO blends were prepared with HEFA-Jet (Table 2; Figure 2),
which is a complex mixture of acyclic paraffins derived from a mixture of fatty acids and methyl
esters by hydrotreatment and isomerization.49-52 Selected properties of these blends are listed in
Table 2.

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Figure 2. Kinematic viscosity of DMCO/HEFA-Jet blends (20-50% DMCO) from 20 to −40 °C.
Table 2. Fuel properties of HEFA-Jet/DMCO blends.
Fuel
Density (at 15 °C) η (−40 °C, mm2s−1) η (−20 °C, mm2s−1) NHOC (MJ/kg)
HEFA
0.762
0.773
0.780
0.788
0.793
12.77
11.26
10.54
9.93
5.65
5.25
5.03
4.84
4.68
43.73
43.75
43.76
43.77
43.78
20% DMCO
30% DMCO
40% DMCO
50% DMCO
9.43
η denotes kinematic viscosity. The NHOC values are calculated based on measured values of
pure HEFA and DMCO.
Addition of only 20% DMCO yields a fuel blend with a density close to that required for the
Jet-A/F-24 specifications, while the 30–50% DMCO blends meet the density requirement. The
kinematic viscosity of pure HEFA-Jet at −40 °C is higher than the preferred value of 12 mm2s−1,
while all of the DMCO blends have values below this limit. All of the fuels, including pure HEFA,
have kinematic viscosities at −20 °C well within the specification for Jet-A/F-24. The impact on
the gravimetric NHOC of HEFA is modest due to the similarities in NHOC between the two
blendstocks.
With the benefits of DMCO as a jet fuel blendstock firmly established, some perspective on
the further development of this technology is warranted. A recent study by the US Department of
Energy suggests that up to 30% of domestic transportation fuels could be generated from
sustainable biomass sources.53 Although manufacturers of automobiles and light trucks are
transitioning away from liquid fuels, and toward the use of electric motors, there is currently no
alternative to hydrocarbons for propulsion of large aircraft, over long distances, and at high speed.

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In 2018, jet fuel accounted for 12% of transportation fuels used in the US,54 thus it is conceivable
that petroleum jet fuel could be replaced with high performance, zero-aromatic bio-based fuels as
these fuels transition to industrial scale production. Despite the promise of this approach, there
are many barriers to the implementation of alternative jet fuels, with price representing the largest
hurdle. A recent study by Scown estimates that biosynthetic limonane (a cyclic, C10H20
hydrogenated isoprene dimer) can potentially be sold for as low as $0.73/L, or $2.77/gal, assuming
significant improvements in biomass deconstruction, sugar conversion, and host optimization.55
This value is similar to the 10-year average for petroleum-derived jet fuel ($0.66/L or $2.50/gal).
Using $0.73/L as the lower limit for the cost of DMCO, it is plausible that DMCO could eventually
be a cost-competitive blendstock for bio-based and conventional jet fuels, particularly as
governments around the world, and the commercial aviation industry, strive to reduce greenhouse
gas emissions.
CONCLUSION
Iron-catalyzed [4+4]-cycloaddition of isoprene, followed by hydrogenation, allowed for a
selective, high-throughput process for the synthesis of 1,4-dimethylcyclooctane (DMCO). DMCO
has a higher gravimetric and volumetric NHOC compared to conventional jet fuel, along with
higher density and lower viscosity. The results suggest that DMCO can be blended with a wide
variety of jet fuel blendstocks, including bio-based SPKs, to create high-performance jet fuel
blends with enhanced properties. Improved catalyst stability and a transition to robust
heterogeneous systems for the [4+4]-cycloaddition step are expected to further enhance the already
impressive TONs achieved by the first-generation homogeneous system described here. These
tractable challenges can likely be addressed by ligand design and immobilization of the catalysts
on solid supports to enable development of a viable commercial process. Studies along these lines
are currently being pursued by our groups.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge via the internet at http://pubs.acs.org.
Characterization data for DMCOD and DMCO including 1H and 13C{1H}NMR spectra and GC
traces (PDF)

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AUTHOR INFORMATION
Corresponding Authors
B.G.H. (Email: benjamin.g.harvey@navy.mil)
P.J.C. (Email: pchirik@princeton.edu)
ORCID
Benjamin G. Harvey: 0000-0003-2091-3539
Kyle E. Rosenkoetter: 0000-0002-6365-3506
Paul J. Chirik: 0000-0001-8473-2898
C. Rose Kennedy: 0000-0003-3681-819X
Author Contributions
All authors contributed to conception of the work, K.E.R. executed the hydrogenation of
DMCOD and characterized the resulting DMCO, C.R.K. developed methods for and executed
the synthesis of DMCOD, B.G.H. and P.J.C. directed the research. All authors have given
approval to the final version of the manuscript.
Notes
A patent application has been filed for the methods and materials described in this manuscript.
ACKNOWLEDGMENT
B.G.H. and K.E.R. thank the Naval Air Warfare Center, Weapons Division PL-219 program for
financial support of this work. We also thank Dr. Josanne Woodroffe for conducting viscosity
studies of DMCO/HEFA blends, Ms. Alicia Hughes for conducting flash point studies, Dr.
Patrick Fedick for GC-MS analysis of DMCOD and DMCO, and Dr. Tim Edwards (AFRL) for
supplying HEFA-Jet. C.R.K. thanks the NIH for a Ruth L. Kirschstein National Research Service
Award (F32 GM126640).

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Table of Contents Graphic
Bio-based isoprene is converted to a high performance jet fuel blendstock by Fe-catalyzed [4+4]
cycloaddition and hydrogenation.