Synthesis of Core-Modified Third-Generation Light-Driven Molecular Motors

Article abstract of DOI:10.1021/acs.joc.0c01235

The synthesis and characterization of a series of light-driven third-generation molecular motors featuring various structural modifications at the central aromatic core are presented. We explore a number of substitution patterns, such as 1,2-dimethoxybenzene, naphthyl, 1,2-dichlorobenzene, 1,1′:2′,1″-terphenyl, 4,4″-dimethoxy-1,1':2′,1″-terphenyl, and 1,2-dicarbomethoxybenzene, considered essential for designing future responsive systems. In many cases, the synthetic routes for both synthetic intermediates and motors reported here are modular, allowing for their post-functionalization. The structural modifications introduced in the core of the motors result in improved solubility and a bathochromic shift of the absorption maxima. These features, in combination with a structural design that presents remote functionalization of the stator with respect to the fluorene rotors, make these novel motors particularly promising as light-responsive actuators in covalent and supramolecular materials.

Full text of DOI:10.1021/acs.joc.0c01235

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Article
Synthesis of Core-Modified Third-Generation Light-Driven Molecular Motors
Jose Augusto Berrocal, Lukas Pfeifer, Dorus Heijnen, and Ben L. Feringa
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.0c01235 • Publication Date (Web): 21 Jul 2020
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Synthesis of Core-Modified Third-Generation Light-Driven Molecular Motors
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José Augusto Berrocal,a Lukas Pfeifer,a Dorus Heijnena and Ben L. Feringaa*
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Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG
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Groningen, The Netherlands.
*to whom correspondence should be addressed: b.l.feringa@rug.nl
KEYWORDS: light-driven rotary motors, molecular machines, core-substituted motors, third-
generation molecular motors.
ToC GRAPHIC:
ABSTRACT:
The synthesis and characterization of a series of light-driven third-generation molecular motors
featuring various structural modifications at the central aromatic core are presented. We
explore a number of substitution patterns, such as 1,2-dimethoxybenzene, naphthyl, 1,2-
dichlorobenzene, 1,1':2',1''-terphenyl, 4,4''-dimethoxy-1,1':2',1''-terphenyl, and 1,2-
dicarbomethoxybenzene, considered essential for designing future responsive systems. In
many cases, the synthetic routes for both synthetic intermediates and motors reported here are
modular, allowing for their post-functionalization. The structural modifications introduced in
the core of the motors result in improved solubility and a bathochromic shift of the absorption
maxima. These features, in combination with a structural design that presents remote
functionalization of the stator with respect to the fluorene rotors, make these novel motors
particularly promising as light-responsive actuators in covalent and supramolecular materials.
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INTRODUCTION
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The field of molecular machines and motors has experienced amazing developments
enabling a transition from molecules to dynamic molecular systems.1,2,11–15,3–10 One reason for
this development relies on the promise that the integration of these artificial molecular tools
into functional materials provides the opportunity for dynamic, responsive and adaptive
properties.16–18 Moreover, the possibility to perform work upon applying an external stimulus
in the form of chemical energy or light7,19 represents an appealing perspective for material
science. In this context, light-driven rotary molecular motors represent promising candidates
for further investigation due to their ability to undergo 360˚ unidirectional rotary motion upon
irradiation.20–25 Recent examples of molecular motors embedded in light-responsive polymer
networks,26,27 surfaces,28 and metal organic frameworks29–31 (MOFs) have shown some of the
future directions of stimuli-responsive materials. In order to advance the field and envision
further opportunities to explore, fundamental work on the design and synthesis of novel
molecular motor structures is paramount.
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Figure 1. General chemical structure of first- (left), second- (center) and third-generation
(right) molecular motors.
The light-driven molecular motors developed in our group are based on overcrowded
alkenes that undergo 360˚ unidirectional rotary motion thanks to a unique interplay between
point and helical chirality.32–34 These molecular systems are classified into three generations,
depending on the number of stereogenic centers, which control the unidirectionality of the
rotary motion.34 First- and second-generation molecular motors possess two35 and one
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stereogenic center,36 respectively, while the recently developed third-generation molecular
motors37,38 are meso-structures. General chemical structures of first-, second- and third-
generation motors are shown in Figure 1.
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Third-generation molecular motors can be considered as a combination of two second-
generation motors with opposite helicity.37,38 They feature one center of pseudo-asymmetry
(denoted with the letter a in Figure 1) that allows for the unidirectional rotary motion of the
parallel rotors with respect to the central aromatic core, meaning that one rotor rotates
clockwise and the other anti-clockwise.37,38 Drawing a parallel with the macroscopic world, the
fluorene rotors rotate similarly to the left and right wheels of a car from the perspective of the
driver. This peculiar feature makes this generation of motors particularly appealing for
locomotion and cargo transport applications, since the combination of the unidirectional rotary
motions of the two rotors should result in unidirectional translation, as conceptually
demonstrated with the nanocar.39
Previous investigations on light-driven third-generation motors carried out in our group
focused on i) the introduction of the concept of unidirectional rotary motion in achiral light-
driven molecular motors,37 and ii) the consequences of structural variations on speed and
unidirectionality.38 Concerning ii, particular attention was dedicated to the role of the
substituents at the pseudo-asymmetric carbon atom, while no synthetic work was carried out
on the central aromatic core that presented either a benzene or p-xylene moiety. Reliable
procedures for the modification of the core of these motors are highly desirable, as it would
greatly facilitate their incorporation into functional materials. We now report the synthesis and
characterization of a library of core-modified third-generation light-driven molecular motors
(Chart 1). Most of the new motors (2-7 and 9) present either the methyl, phenyl (Me,Ph) or
methyl, iso-propyl (Me,i-Pr) combination of substituents at the pseudo-asymmetric carbon
atom. These combinations of substituents have been reported to lead to 70% and 100%
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unidirectionality, respectively.38 The octadecyl, phenyl (C18,Ph) combination was installed in
motor 8, instead. The replacement of the Me substituent with more sterically hindered C18
should result in an increase in unidirectionality from 70 to 78%.38 Despite this modest effect,
our major motivation for the C18,Ph combination was its potential use to facilitate the deposition
of third generation motors onto solid supports, since long alkyl chains are typically responsible
for more favorable molecule-surface interactions. We focused on a number of motifs for the
central aromatic core, such as 1,2-dimethoxybenzene (2), naphthyl (3), 1,2-dichlorobenzene
(4), 1,1':2',1''-terphenyl (5), 4,4''-dimethoxy-1,1':2',1''-terphenyl (6), and 1,2-
dicarbomethoxybenzene (9). Therefore, what distinguishes these motors from previous
examples is the presence of substituents on the central aromatic cores, which results in 1)
bathochromic shifts of the absorption maxima, and 2) ample possibilities for further
functionalization/integration into light-responsive systems/materials and surface anchoring.
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Chart 1. Chemical structures of the third-generation motors presented here.
RESULTS AND DISCUSSION
The key step in the synthesis of overcrowded alkene-based third-generation molecular
motors is the Barton-Kellogg olefination (B-K reaction).37,38 This approach is shown in Scheme
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1a, where disconnection at the level of the overcrowded alkene moieties results in two building
blocks: an indanedithione and a diazo compound. The high stability of indanedithiones deriving
from the 1,3-indanedione skeleton with a quaternary carbon atom in the α-position to the two
thiocarbonyl moieties, as well as that of 9-diazofluorene, makes this combination of reactants
particularly convenient. Both can be stored as stable synthetic intermediates without the
necessity to generate them in situ or use them immediately after purification. The use of
indanedithiones which can be directly prepared from the diketone precursors shifts the attention
to the synthesis of 1,3-indanedione derivatives. In previous studies,37,38 we have successfully
prepared these compounds using Friedel-Crafts acylation of electron-rich aromatic compounds
(Scheme 1b), or NaH-induced Claisen condensation between a dimethyl phthalate-type
derivative and symmetrical ketones with two methylene groups at the α position (Scheme
1c).40,41 In the present study, we predominantly applied the Claisen condensation approach in
view of its more widely compatible reaction conditions. However, we also adopted a third
synthetic strategy consisting of Friedel-Crafts acylation with malonic acids in polyphosphoric
acid41 (PPA) (Scheme 1d). Although less general than the Claisen condensation,40 the PPA-
strategy can save many synthetic steps with certain core designs.
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Scheme 1. (a) Retrosynthetic approach for third-generation motors. Approaches for the
synthesis of 1,3-indanedione derivatives based on (b) Friedel-Crafts acylation with acyl
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chlorides, (c) NaH-induced Claisen condensation, and (d) PPA-induced Friedel-Crafts
acylation with malonic acids.
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As indicated, in most cases we followed the Claisen condensation approach40 for
preparing the required 1,3-indanediones. Starting from the corresponding dimethyl phthalate-
derivatives 10–18, the NaH-induced cyclization reaction with commercially available 1,3-
diphenyl-2-propanone or 2,6-dimethyl-4-heptanone in toluene at 110 °C was performed
(Scheme 2a, i). The reaction mixtures turned into deep red suspensions at different reaction
times (1–5 d), depending on the dimethyl phthalate starting material used. The red precipitates
corresponded to the sodium salts of the cyclized products (intermediates between brackets in
Scheme 2a), which were not soluble in toluene. Different from our previous reports on third-
generation motors,37,38 in the present procedure we did not isolate the neutral compounds by
treating the Na-salts with hydrochloric acid (HCl). Simple filtration of the red suspensions
allowed for obtaining the crude Na-salts, which were then subjected to alkylation with methyl
iodide (MeI) in acetone at 60 °C (Scheme 2a, ii). These reactions were conducted in the
presence of mixtures of Aliquat 336 and potassium fluoride (KF) on celite, as their combination
had previously been shown to favor C-alkylation of the enolates over the competing O-
alkylation.37,38 Core structures 19–27 were obtained in good yields (from 30% to 76%), with
the only exceptions of 22 and 25 (11% and 18%, respectively), after chromatographic
purification on silica (Scheme 2a).
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Scheme 2. Synthesis of motor cores applying (a) the Claisen condensation approach, followed
by filtration of the crude Na-salts and immediate methylation; (b) the PPA-induced Friedel-
Crafts acylation, followed by alkylation of the isolated intermediate. Reaction conditions: i)
diester (1 eq), 1,3-diphenyl-2-propanone or 2,6-dimethyl-4-heptanone (1.5 eq.), NaH 60 wt%
(2 eq.), Toluene, 110 °C, 1–5 d depending on the starting material; ii) MeI (2 eq.), Aliquat
336 (0.05 eq.), KF on celite (0.05 eq.), acetone, 60 °C, o/n; iii) Phenylmalonic acid (1.5 eq.),
PPA (115% H3PO4 basis), 90 °C, mechanical stirring, o/n; iv) MeI or 1-iodooctadecane (2 eq.),
Aliquat 336 (0.05 eq.), KF on celite (0.05 eq.), acetone, 60 °C, o/n.
Given the laborious synthetic routes to obtain starting diester 11 (4 steps),42,43 we
attempted the direct PPA-induced double Friedel-Crafts acylation of commercially available
1,2-dimethoxybenzene 28 (Scheme 2b). Encouraged by a related functionalization with
iso-propylmalonic acid,44 we slightly adapted the reaction conditions to phenylmalonic acid.
Gratifyingly, we successfully obtained 29 in 31% yield after a simple extraction-
recrystallization sequence. Alkylation of 29 with MeI afforded motor core unit 20 after
chromatographic purification (Scheme 2b, iv). It must be emphasized that applying this route
is particularly convenient to access 20, since it shortens the synthesis from 6 steps (4 steps
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account for the preparation of 11) to only 2 steps. Finally, alkylation of 2-phenyl-1H-indene-
1,3(2H)-dione38 with 1-iodooctadecane afforded 30 in 83% yield (Scheme 2b, iv).
A particularly convenient aspect of some of our synthetic routes to these core structures
is their modularity, as additional substituents can be introduced via both the dimethyl phthalate
as well as core building blocks. Bromo-substituted dimethyl phthalate 14 or 1,3-indanedione
23 could be converted into derivatives 15–17 or 24–26, respectively, by means of Suzuki–
Miyaura cross-coupling reaction with phenyl-boronic acids catalyzed by palladium-
tetrakis(triphenylphosphine) (Pd(PPh3)4). Details on the preparation of 14 can be found in the
experimental section, while here we briefly discuss the post-modification of 23 (Scheme 3).
Applying the Suzuki–Miyaura cross-coupling on this core structure, 24–26 were obtained in
50–75% yield (Scheme 3). While the same reaction on 14 consistently afforded the
corresponding esters 15-17 in higher yields (see experimental section), the post-modification
of 23 has the additional advantage of diversifying these syntheses after the NaH-induced
Claisen condensation, which represents the critical step in the preparation of these core units.
Hence, direct modification of 14 is the best option if the target is one molecular design only;
alternatively, post-modification of 23 is more convenient when the screening of multiple
molecular designs is desired.
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Scheme 3. Suzuki-Miyaura cross coupling reaction on indanedione core 23 with phenyl-
boronic acid, 4-methoxyphenylboronic acid, and 4-(tert-butyldimethylsilyloxy)phenylboronic
acid affording 24, 25, and 26, respectively. Reaction conditions: i) boronic acid (2.2 eq.),
Pd(PPh3)4 (10 mol%), toluene, 110 °C, 18 h.
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Synthesis and characterization of the motors.
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With the 1,3-indanedione derivatives 19–27 and 30 in hand, we then focused on their
conversion into indanedithiones, which in some cases was not straightforward. Previous work
on third-generation motors highlighted the importance of using mixtures of phosphorous
pentasulfide (P2S5) and Lawesson’s reagent (LR) in boiling toluene.37,38 We found that this
mixture was necessary in most cases, while treating 20 with only LR sufficed to achieve full
conversion (Scheme 4a). This behavior was attributed to the electron-donating effect of the
methoxy-substituents. Unfortunately, core unit 23 could not be successfully converted into its
corresponding indanedithione, probably due to the electron-withdrawing effect of the two
bromine substituents. Besides electronic effects, steric hindrance may also play a role in the
1,3-indanedione→indanedithione conversion. For example, when 30 was subjected to the
reaction conditions of Scheme 4a, we obtained a mixture of unreacted 30 as well as mono- and
di-substituted (38) products that could not be fully separated by chromatography. Tert-
butyldimethylsilyl- (TBDMS) protecting groups, which are of interest for potential post-
functionalization of the motors due to their ease of removal, were found to be particularly inert
and survive indanedithione formation. Finally, core 27 could not be converted to
indanedithione 39 with the reaction conditions reported in Scheme 4a. We could, however,
obtain 39 by reacting 27 in the presence of LR under microwave irradiation for 4 h, albeit in
very low yield (8%) (Scheme 4b). These conditions were unsuccessful with 23, as we only
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recovered unreacted starting material. Thioketones 31–39 were characterized by H NMR to
ensure full conversion and were directly (without isolation) used in the subsequent Barton-
Kellogg olefination (B-K reaction).
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Scheme 4. Preparation of the indanedithiones starting from the corresponding substituted 1,3-
indanediones, applying (a) conventional heating and (b) microwave irradiation. Reaction
conditions: i) P2S5 (4 eq.), LR (4 eq.), toluene, 110 °C, o/n; ii) LR (8 eq.), toluene, 110 °C, 4 h,
microwave irradiation.
B-K reactions were run starting from toluene solutions of indanedithiones 31–39 at
room temperature, followed by the addition of solid diazofluorene (Scheme 5a). The typical
green-blue toluene solutions of 31–39 turned red immediately upon addition of diazofluorene,
and the progress of these reactions was monitored by TLC. Desulfurizing agent
hexamethylphosphorous triamide (HMPT) was added when no traces of starting
indanedithione were detected and the resulting mixtures were heated up to 90 °C and stirred
overnight (Scheme 5a). Motors 1–9 (1 had previously been reported) were obtained after
chromatographic purification. Following the same strategy of post-functionalization previously
discussed for core 23, we highlight the possibility to also access motor 5 via post-modification
of 4 by direct two-fold catalytic cross-coupling with phenyllithium (Ph-Li) (Scheme 5b).
Attempts at preparing 6 via similar post modification of 4 resulted in negligible conversions as
judged by crude product 1H-NMR analysis. This justified the choice of the higher yielding B-
K reaction to obtain 6 and 7.
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All newly synthesized motors were obtained as powders with colors ranging from
orange to deep red, depending on the nature of the core substitution. The core-modified light-
driven third-generation motors 2–9 are significantly more soluble than reference motor 1 in
halogenated solvents and a number of other organic solvents (acetone, THF, diethyl ether, and
ethyl acetate). The presence of substituents at distal positions in the molecular structures with
respect to the overcrowded alkene moieties probably results in favored solvation.
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Scheme 5. Synthesis of core-modified third-generation molecular motors via (a) Barton-
Kellogg olefination and (b) direct catalytic cross-coupling post-modification of motor 4 with
phenyllithium to yield motor 5. Reaction conditions: i) Diazofluorene (2.5 eq.), toluene, RT,
4–18 h. HMPT (2 eq.), 90 °C, 18 h; ii) Dichloro[1,3-bis(2,6-di-3-pentylphenyl)imidazol-2-
ylidene](3-chloropyridyl)palladium(II) (Pd-PEPPSI-IPent catalyst, 5 mol%), dry toluene,
40 °C, 5 min. Slow addition (1 h) of PhLi (2.5 eq. in 1 mL dry toluene), 40 °C.
Previous work on third-generation molecular motors revealed that the substituents at
the pseudo-asymmetric carbon atom a (Figure 1) define the number of isomers potentially
accessible from one motor structure.38 The steric hindrance of these substituents was found to
be the key structural parameter.38 The two substituents at carbon a are placed one each in a
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pseudo-equatorial and pseudo-axial orientation, and can interconvert their positions through
thermal conformational isomerization. Depending on the steric hindrance exerted by the two
substituents, the two conformational isomers are not degenerate, with one being energetically
preferred. The more stable isomer for reference motor 1 was reported to be the one with the
phenyl ring in the pseudo-axial position (s-isomer), more remote from the two fluorene
moieties.38 This allows the phenyl ring to have more spatial freedom. The ratio between the
two isomers with the phenyl ring placed pseudo-axially (s-isomer) and pseudo-equatorially (r-
isomer), respectively, was determined to be 2:1 at room temperature by integrating the two
different singlets corresponding to the methyl substituent in the 1H NMR spectrum of 1.38 The
singlet of the methyl group of the s-isomer was more downfield shifted compared to that of the
r-isomer.38 The two isomers also possess different local asymmetry with respect to the two
fluorene moieties and, as a result, third-generation motors with this Me,Ph substitution pattern
at carbon a are not fully unidirectional. The 2:1 isomer distribution of reference motor 1 means
that roughly 67% of the motor population rotate in one direction, while the remaining 33%
rotate in the opposite direction, resulting in 34% net unidirectional rotation. The thermal
interconversion between the two isomers was confirmed by the observation of signal
coalescence in temperature-dependent (TD) NMR studies.38 Replacing the phenyl substituent
with the more sterically demanding iso-propyl group resulted in exclusive formation of one
isomer only. As a result, third-generation motors featuring Me,i-Pr substitution on carbon a are
fully unidirectional, the NMR spectra are characterized by much sharper signals and no
coalescence phenomena are observed in TD-NMR studies.37,38 The newly synthesized third-
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generation motors 2–9 perfectly fit into this qualitative description. The H NMR spectra of
motors 2–8, with the Me,Ph substitution pattern, show two almost overlapping singlets for the
methyl substituent at room temperature. Moreover, these motors show signal coalescence upon
heating/cooling of NMR samples (see SI for 1H NMR spectra measured at 90 °C and -45 °C).
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Measuring H NMR spectra at -45 °C for motors 2–8 allows to separate the singlets of the
methyl group of the r- and s-isomers and ultimately determine their degree of unidirectionality
at the specific temperature (Table S1). The s-isomer, with the methyl group in the pseudo-
equatorial position, was the more stable isomer for 2–8, as suggested by the more intense
downfield shifted signal (vide supra).38 Hence, motors 2–8 show preferred directional rotary
motion, in line with previous findings.37,38 Motor 9, with the Me,i-Pr combination, instead,
possesses a 1H NMR spectrum characterized by sharp signals and only one resonance for both
the methyl and iso-propyl group (see SI), and hence should undergo fully unidirectional rotary
motion.
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Third-generation molecular motors possess the highest speed of rotation among the
three generations of artificial molecular motors (ultra-fast rotary motion). The unidirectional
rotary cycle is composed of a photochemical E–Z isomerization (PEZ) followed by subsequent
thermal helix inversion (THI).34 This sequence covers the first 180-degree rotation, while
another PEZ-THI combination completes the rotary cycle.34 Assuming a high enough photon
flux, the THI is considered the rate-limiting step of the rotary cycle, and thus defines the rotary
speed of our molecular motors. The ultra-fast rotary motion of third generation molecular
motors implies very low thermal activation barriers for THI.38 While the unidirectionality of
the rotary motion is controlled by the substituents on carbon a, the speed of rotation is mainly
governed by the central aromatic core. The limited steric hindrance exerted by the central
aromatic core on the fluorene moieties, especially in the case of the unsubstituted benzene core,
is the main reason for the high rotary speed. Earlier studies confirmed the ultra-fast rotary
motion looking at motor structures that featured the p-xylene core and desymmetrized fluorine
rotors.37 The lack of readily available p-xylene derivatives simultaneously functionalized with
two ester moieties in the 1,2-positions and two identical substituents in the 4,5-positions,
prevented direct measurement of the ultra-fast rotary motion. The temperatures required for
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studying the rotation of motors 2–9 (below –110 °C) are instrumentally inaccessible and/or not
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studying the unidirectional rotary motions of motors 2–9 by H NMR at –80 °C via in situ
irradiation with 365 nm, 395 nm, or 405 nm light did not afford any change in their 1H NMR
spectra (see SI). Hence, we anticipate that 2–9 undergo ultra-fast rotary motion by their close
structural resemblance to previous third-generation molecular motors.37,38 On one hand, this
represents a challenge to address in the future for a rigorous physico-chemical understanding
of the behavior of these molecules using transient spectroscopy methods, which is part of an
upcoming study. On the other hand, it also makes this class of photochemically-driven
compounds particularly promising for the preparation of light-responsive materials and
actuators, since their use should result in a much faster actuation of the polymeric or
supramolecular ensembles they will be embedded in.
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Study of the single-crystal X-ray structures of some of the newly synthesized motors
highlighted that the core-modifications introduced in the design of 2–9 resulted in significant
variations compared to the previous systems. We obtained single crystals of motors 2, 4 and 5
by slow diffusion of hexane (anti-solvent) into a concentrated 1,2-dichloroethane (solvent)
solution of 2, 4 and 5.45 The crystal structures of motors 4 and 5 are shown in Figure 2a and
2b, respectively, while that of 2 is shown in Figure S88 due to its similarity to the X-ray
structure of 4. Regardless of the presence of both possible isomers (Me,Ph substituents) in
solution, all motors crystallized as one single isomer. The same behavior had also been
observed in previous investigations,38 with reference 1 crystallizing with the phenyl substituent
in the pseudo-axial orientation. While in the single crystals of 4 the phenyl substituent is placed
pseudo-axial and has more spatial freedom (Figure 2a), in the crystals of 5 we observed the
opposite isomer featuring a pseudo-equatorial phenyl ring (Figure 2b). Interestingly, the two
aromatic rings of the 1,1':2',1''-terphenyl system in 5 do not possess the same dihedral angle
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(45.54(17)° and 58.57(16)°) with respect to the central aromatic core, which removes the
symmetry plane of the molecule in the single crystal.
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Finally, further shifting of the absorption spectra of light-triggered molecular motors
towards the visible and near-IR region is also highly desirable and of particular attention in
recent motor designs.31,46–49 The introduction of two methoxy groups in the core of motor 2
resulted in a bathochromic shift of 32 nm compared to 1 (Figure 2c). This was already well
visible in the appearance of the two motors in the solid state: while 1 is an orange powder, 2 is
deeply red colored. However, while increasing the electron density by means of electron
donating substituents proved to be beneficial in this respect, preliminary studies towards
electron-withdrawing substituents showed only a modest or no effect (see SI).
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Figure 2. (a) Single-crystal X-ray structure of motor 4. (b) Single-crystal X-ray structure of
motor 5. Hydrogen atoms are omitted for clarity in the crystal structures. Carbon atoms are
depicted in grey, chlorine atoms in green. (c) UV-vis spectrum of reference motor 1 (black
trace) and motor 2 (red trace) in CH2Cl2 (25 °C, 5 × 10–6 M, optical path 1 cm).
CONCLUSIONS
In conclusion, the synthesis and characterization of several new light-driven third-
generation molecular motors with different designs of the central aromatic core and
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functionalities is reported. While in previous efforts the focus was exclusively on benzene- and
p-xylene-type cores, in this investigation a number of moieties were explored, such as 1,2-
dimethoxybenzene (2), naphthyl (3), 1,2-dichlorobenzene (4), 1,1':2',1''-terphenyl (5), 4,4''-
dimethoxy-1,1':2',1''-terphenyl (6), and 1,2-dicarbomethoxybenzene (9). We present modular
and scalable synthetic routes, which further offer the possibility to post-functionalize synthetic
intermediates and motors. This aspect is particularly beneficial in case screening of different
molecular designs is required to optimize the structure for a specific task. The structural
modifications introduced in the newly synthesized motors resulted in an improved solubility
compared to reference motor 1, as well as a bathochromic shift of the absorption spectra.
Hence, the core-modifications presented here offer ample opportunity for
application/incorporation of these motors in light-responsive materials. In particular, the
functionalization of the cores at the distal side with respect to the fluorene rotors makes the
design of model motors 2–9 highly promising for ongoing research towards cargo transport
and locomotion along tracks.
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EXPERIMENTAL SECTION
Instrumentation:
Microwave reactions were performed on a Discover SP Microwave Synthesizer.
Column chromatography was performed using a Grace Reveleris instrument.
1H NMR and 13C NMR spectra were recorded either on a Varian Mercury Vx 400 MHz (100
MHz for 13C), Varian Oxford AS 500 MHz (125 MHz for 13C) or a Bruker 600 MHz (150 MHz
for 13C) NMR spectrometers. Chemical shifts are given in ppm (δ) values relative to the solvent
(for CDCl3:1H δ: 7.26 and 13C δ: 77.16; for Cl2DCCDCl2:1H δ: 6.00 and 13C δ: 73.78). Splitting
patterns are labelled as s, singlet; d, doublet; t, triplet; q, quartet; p, pentet; h, heptet; m,
multiplet.
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UV-vis absorption spectra were recorded on a Jasco V-630 or a Hewelett-Packard 8453
spectrometer.
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Infrared spectra were recorded on a Perkin Elmer Spectrum One 1600 FT-IR spectrometer or
a Perkin Elmer Spectrum Two FT-IR spectrometer, equipped with a Perkin Elmer Universal
ATR Sampler Accessory.
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High Resolution Mass Spectra (HR-MS) were recorded on an LTQ Orbitrap XL.
Single-crystals were mounted on a cryoloop and placed in the nitrogen stream (100 K) of a
Bruker-AXS D8 Venture diffractometer. Data collection and processing was carried out using
the Bruker APEX3 software suite.50 A multi-scan absorption correction was applied, based on
the intensities of symmetry-related reflections measured at different angular settings
(SADABS).50 The structure was solved either using SHELXS51 (LP18006) or SHELXT52 (18005,
18011), and refinement was performed using SHELXL.51 The hydrogen atoms were generated
by geometrical considerations, constrained by idealized geometries and allowed to ride on their
carrier atoms with an isotropic displacement parameter related to the equivalent displacement
parameter of their carrier atoms.
Materials:
Compound 10 was purchased from TCI. Compound 28 was purchased from Sigma-Aldrich.
All other commercially available products were purchased from either TCI, Combi Blocks or
Sigma-Aldrich, and used as received.
General method A: procedure for esterification. The desired dicarboxylic acid (1 eq) was
loaded in a round bottom flask and dissolved/suspended MeOH (2.2 mL per mmol of acid) at
0 ˚C. Then, SOCl2 (4 eq) was added dropwise via syringe. The reaction mixture was heated
under to reflux and left overnight. After being cooled to room temperature, the volatiles were
removed. The crude mixture was dissolved in DCM and washed with water (2 × 100 mL). The
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organic phase was then dried over MgSO4, filtered and evaporated to achieve the desired esters.
All products were used in the next step without further purification.
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General method B: NaH-induced procedure for the core preparation. The appropriate
diester (1 eq) and ketone (2 eq) were weighed in a round bottom flask and dissolved in toluene
(2 mL per mmol of diester). Sodium hydride on oil (60 wt%) (2 eq) was added in small portions
with a spatula. The resulting suspension was stirred for 5 minutes at room temperature under
nitrogen, then heated at reflux for 1-5 days. The starting white-grey suspensions turned deep
red upon reacting. The mixtures were cooled down to room temperature. The precipitate was
filtered, washed with pentane, and dried. The dried solid (powder) was loaded with KF on celite
(50 wt%) (0.02 eq) and methyl-N,N,N-trioctan-1-ammonium (0.02 eq) in a round bottom flask.
Acetone (same volume as toluene) was added and the mixture was stirred for 5 minutes at room
temperature under nitrogen. Methyl iodide (2 eq) or 1-iodooctadecane (2 eq) was added and
the reaction mixture was heated at reflux overnight. The reaction mixture changed color from
deep red to yellow upon reacting. The solvent was removed and the crude material was
dissolved in CH2Cl2 and H2O. The organic phase was washed (× 3) with H2O, dried over
MgSO4, filtered. The solvent was removed and the crude material purified by column
chromatography (SiO2, gradient from 100% pentane to pentane/EtOAc mixtures).
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General method D: procedure for indanedithione preparation with conventional heating.
The appropriate indanedione (1 eq) was added to an oven-dried round bottom flask under
nitrogen. Dry toluene (5 mL per mmol) was added, followed by the addition of P4S10 (4 eq)
and Lawesson’s reagent (4 eq). The resulting suspension was heated overnight at reflux and
monitored with TLC. When full conversion was not achieved overnight, additional P4S10 (4 eq)
was added and the reaction kept under reflux for additional 24 hours. The reaction mixture was
cooled down to room temperature. The suspension was filtered over a short pad of celite, and
the filtered solution (green in most cases) was evaporated. The crude product was immediately
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purified with column chromatography (SiO2, gradient from 100% pentane to 90:10
pentane/EtOAc). In many cases the desired indanedithione was not obtained in 100% purity
due to the presence of other products deriving from non-complete conversion or unknown
impurities. Using these slightly impure indanedithiones in the following Barton-Kellogg
olefinations never affected the outcome of the reactions, hence they did not undergo further
purification.
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General method E: procedure for preparation of indanedithione 39 with microwave
heating. Indanedione 27 (400 mg; 1.26 mmol) was loaded into a 25 mL microwave vial and
dissolved in toluene (10 mL). Lawesson’s reagent (8 eq; 4.06 g; 10.05 mmol) was added and
the vial was sealed. The resulting suspension was sonicated for 5 min. The reaction mixture
was heated at 110 ºC for 4 hours under microwave irradiation. The suspension was filtered over
a short pad of celite, and the filtered solution was evaporated. The crude product was
immediately purified with column chromatography (SiO2, gradient from 100% pentane to 95:5
pentane/EtOAc). Indanedithione 39 was almost always obtained in mixtures with unreacted 27
and product of single-conversion. Prolonged reaction times resulted in degradation of 39 and/or
no further conversion.
General method F: procedure for the Barton-Kellogg olefination. The appropriate
indanedithione (1 eq) was dissolved in toluene (5 mL per mmol of indanedithione). Next, 9-
diazo-9H-fluorenone53 (2.5 eq) was added and the resulting mixture was stirred at room
temperature under a nitrogen atmosphere for 8 hours. Hexamethylphosphanetriamine (2 eq)
was added. The mixture was heated at 90 ºC overnight. The volatiles were evaporated under
reduced pressure. The residue was adsorbed on celite and purified by column chromatography
(SiO2, gradient pentane/CH2Cl2; 0–100%).
General method G: procedure for the Suzuki couplings. The appropriate di-bromo
derivative (1 eq) and boronic acid (2.5 eq) were weighed in a round bottom flask and
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dissolved/suspended in toluene (30 mL per mmol of di-bromo derivative) was added. An 8 M
aqueous solution of K2CO3 (21 eq) was added to the organic phase. The mixture was degassed
for 15 minutes by bubbling nitrogen. Palladium(0) tetrakis(triphenylphospine) (0.1 eq) was
added. The reaction mixture was heated at reflux overnight. The reaction mixture was cooled
down and loaded into a separatory funnel. The toluene layer was separated, and the aqueous
layer washed with CH2Cl2 (× 3). All the organic phases were dried over MgSO4 and filtered.
After solvents removal, the crude materials were purified by column chromatography (SiO2,
gradient pentane/EtOAc; 0–50%).
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dimethyl 4,5-dimethoxyphthalate (11). Synthesized according to literature procedures.42,43
1H-NMR (CDCl3, 600 MHz, δ): 7.15 (s, 2H), 3.90 (s, 6H), 3.84 (s, 6H). 13C-NMR {1H} (CDCl3,
150 MHz, δ): 167.9, 150.7, 125.2, 111.4, 56.2, 52.6. FT-IR (dry powder) (cm-1): 3018 (C-H),
2955 (C-H), 1709 (C=O). HR-MS (m/z): [M+Na]+ calcd for C12H14O6Na 276.0683; found
276.0680 (0.8 ppm). Mp 86.2-87.1 ºC.
dimethyl 4,5-dichlorophthalate (13). Synthesized with general method A. (white solid; 15.8
g; 60.3 mmol; 91% yield). 1H-NMR (CDCl3, 600 MHz, δ): 7.78 (s, 2H), 3.88 (s, 6H). 13C-NMR
{1H} (CDCl3, 150 MHz, δ): 166.0, 135.8, 131.4, 131.0, 53.1. FT-IR (dry powder) (cm-1): 3096
(C-H), 3037 (C-H), 2956 (C-H), 1720 (C=O). HR-MS (m/z): [M+H]+ calcd for C10H9Cl2O4
262.9872; found 262.9873 (0.2 ppm). Mp 44.5-46 ºC.
dimethyl 4,5-dibromophthalate (14). 1,2-dibromo-4,5-dimethyl-benzene (5 g; 18.94 mmol)
was weighed in a custom-made teflon becker equipped with a teflon cap. The compound was
suspended in 35 mL of a 30 wt% HNO3 aqueous solution. A stirring bar was added and the
teflon beaker was sealed with the teflon cap. The teflon reactor was inserted into an autoclave.
The autoclave was place on top of a hot plate previously set at 170 ºC. The reaction was left
overnight, after which it was cooled down to room temperature. The teflon reactor was
extracted from the autoclave and the reaction mixture was poured into a pH 14 aqueous solution
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(200 mL). The resulting solution was filtered (folded paper filter). The filtered solution was
acidified with concentrated (37 wt %) HCl to induce precipitation (addition of HCl significantly
developed heat; cool down the mixture during acidification). The precipitate was filtered
(Buchner filter) and dried. The obtained white solid (1,2-dibromophthalic acid) was then
subjected to esterification following general method A. (white solid; 4.3 g; 12.3 mmol; 65%
yield). 1H-NMR (CDCl3, 600 MHz, δ): 7.96 (s, 2H), 3.91 (s, 6H). 13C-NMR {1H} (CDCl3, 150
MHz, δ): 166.1, 134.1, 132.0, 128.4, 53.2. FT-IR (dry powder) (cm-1): 2953 (C-H), 2923 (C-
H), 2852 (C-H), 1730 (C=O). HR-MS (m/z): [M+H]+ calcd for C10H9Br2O4 352.8842; found
352.8841 (0.2 ppm). Mp 72.6-74.3 ºC.
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dimethyl [1,1':2',1''-terphenyl]-4',5'-dicarboxylate (15). Synthesized with general method
G and purified by column chromatography (SiO2, gradient pentane/EtOAc; 0–50%). (off-white
solid; 2.12 g; 6.12 mmol; 63% yield). 1H-NMR (CDCl3, 600 MHz, δ): 7.80 (s, 2H), 7.24-7.23
(m, 6H), 7.15-7.13 (m, 4H), 3.94 (s, 6H). 13C-NMR {1H} (CDCl3, 150 MHz, δ): 168.0, 143.6,
139.7, 131.4, 130.9, 129.8, 128.3, 127.5, 52.8. FT-IR (dry powder) (cm-1): 2955 (C-H), 1721
(C=O). HR-MS (m/z): [M+Na]+ calcd for C22H18O4Na 369.1097; found 369.1094 (0.9 ppm).
Mp 108.8-110.1 ºC.
dimethyl 4,4''-dimethoxy-[1,1':2',1''-terphenyl]-4',5'-dicarboxylate (16). Synthesized with
general method G and purified by column chromatography (SiO2, gradient pentane/EtOAc; 0–
50%). (off-white solid; 7.05 g; 17.3 mmol; 81% yield). 1H-NMR (CDCl3, 600 MHz, δ): 7.74
(s, 2H), 7.07-7.06 (AA’BB’ system, 4H), 6.79-6.77 (AA’BB’ system, 4H), 3.93 (s, 6H), 3.78
(s, 6H). 13C-NMR {1H} (CDCl3, 150 MHz, δ): 168.1, 159.1, 143.0, 132.2, 131.3, 130.9, 130.5,
113.8, 55.3, 52.8. FT-IR (dry powder) (cm-1): 2955 (C-H), 2841 (C-H), 1722 (C=O). HR-MS
(m/z): [M+Na]+ calcd for C24H22O6Na 429.1309; found 429.1303 (1.4 ppm). Mp 113.6-115.1
ºC.
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dimethyl 4,4''-bis((tert-butyldimethylsilyl)oxy)-[1,1':2',1''-terphenyl]-4',5'-dicarboxylate
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(17). Synthesized with general method G and purified by column chromatography (SiO2,
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gradient pentane/EtOAc; 0–30%). (transparent oil; 6.7 g; 11.03 mmol; 97% yield). H-NMR
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(CDCl3, 600 MHz, δ): 7.75 (s, 2H), 6.99-6.98 (AA’BB’ system, 4H), 6.71-6.69 (AA’BB’
system, 4H), 3.93 (s, 6H), 0.97 (s, 18H), 0.18 (s, 12H). 13C-NMR {1H} (CDCl3, 150 MHz, δ):
168.2, 155.3, 143.2, 132.9, 131.2, 130.9, 130.4, 120.8, 120.0, 116.0, 52.8, 25.8, 25.8, 18.4, -
4.28. FT-IR (dry liquid) (cm-1): 2954 (C-H), 2930 (C-H), 2858 (C-H), 1728 (C=O), 1244 (Si-
O). HR-MS (m/z): [M+H]+ calcd for HR-MS C34H47O6Si2 607.2906; found 607.2887 (3.1
ppm).
tetramethyl benzene-1,2,4,5-tetracarboxylate (18). Synthesized with general method A.
(white solid; 25 g; 75.1 mmol; 95% yield). 1H-NMR (CDCl3, 600 MHz, δ): 8.07 (s, 2H), 3.94
(s, 12H). 13C-NMR {1H} (CDCl3, 150 MHz, δ): 166.5, 134.4, 129.8, 53.2. FT-IR (dry powder)
(cm-1): 2956 (C-H), 1720 (C=O). HR-MS (m/z): [M+Na]+ calcd for C14H14O8Na 333.0581;
found 333.0579 (0.6 ppm). Mp 137.1-138.9 ºC.
5,6-dimethoxy-2-methyl-2-phenyl-1H-indene-1,3(2H)-dione (20). Synthesized with general
method B and purified by column chromatography (SiO2, gradient from 100% pentane to 7:3
pentane/EtOAc mixture). (off-white solid; 882 mg; 2.98 mmol; 76% yield). 1H-NMR (CDCl3,
600 MHz, δ): 7.32 (s, 2H), 7.28-7.15 (m, 5H), 3.96 (s, 6H), 1.63 (s, 3H). 13C-NMR {1H}
(CDCl3, 150 MHz, δ): 201.0, 156.4, 138.4, 136.5, 129.4, 128.8, 128.8, 128.8, 127.6, 126.7,
103.9, 57.6, 56.8, 20.0. FT-IR (dry powder) (cm-1): 3012 (C-H), 2979 (C-H), 2959 (C-H), 1688
(C=O). HR-MS (m/z): [M+H]+ calcd for C18H18O4 297.1121; found 297.1120 (0.5 ppm). Mp
168.2-170.7 ºC.
2-methyl-2-phenyl-1H-cyclopenta[b]naphthalene-1,3(2H)-dione (21). Synthesized with
general method B and purified by column chromatography (SiO2, gradient from 100% pentane
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to 7:3 pentane/EtOAc mixture). (off-white solid; 1.24 g; 4.33 mmol; 40% yield). H-NMR
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7.31-7.23 (m, 3H), 1.78 (s, 3H). C-NMR {1H} (CDCl3, 150 MHz, δ): 202.3, 138.2, 136.9,
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136.3, 130.7, 129.8, 128.9, 127.7, 126.9, 125.2, 59.6, 20.3. FT-IR (dry powder) (cm-1): 2979
(C-H), 2934 (C-H), 2867 (C-H), 1688 (C=O). HR-MS (m/z): [M+H]+ calcd for C20H16O2
287.1067; found 287.1065 (0.5 ppm). Mp 113.1-115.6 ºC.
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5,6-dichloro-2-methyl-2-phenyl-1H-indene-1,3(2H)-dione (22). Synthesized with general
method B and purified by column chromatography (SiO2, gradient from 100% pentane to 7:3
pentane/EtOAc mixture). (off-white solid; 1.01 g; 3.31 mmol; 11% yield). 1H-NMR (CDCl3,
600 MHz, δ): 8.07 (s, 2H), 7.28-7.21 (m, 5H), 1.66 (s, 3H). 13C-NMR {1H} (CDCl3, 150 MHz,
δ): 199.7, 141.6, 140.1, 137.2, 129.1, 128.1, 126.7, 125.7, 58.5, 20.3. FT-IR (dry powder) (cm-
1): 2929 (C-H), 1709 (C=O). HR-MS (m/z): [M+H]+ calcd for C16H12Cl2O2 305.0131; found
305.0129 (0.4 ppm). Mp 104.9-106.8 ºC.
5,6-dibromo-2-methyl-2-phenyl-1H-indene-1,3(2H)-dione (23). Synthesized with general
method B and purified by column chromatography (SiO2, gradient from 100% pentane to 7:3
pentane/EtOAc mixture). (off-white solid; 4.0 g; 10.15 mmol; 62% yield). 1H-NMR (CDCl3,
600 MHz, δ): 8.25 (s, 2H), 7.27-7.21 (m, 5H), 1.66 (s, 3H). 13C-NMR {1H} (CDCl3, 150 MHz,
δ): 199.8, 140.5, 137.1, 134.5, 129.1, 129.1, 129.0, 128.1, 126.7, 58.4, 20.2. FT-IR (dry
powder) (cm-1): 3072 (C-H), 2934 (C-H), 1707 (C=O). HR-MS (m/z): [M+H]+ calcd for
C16H12Br2O2 392.9120; found 392.9189 (0.4 ppm). Mp 145.6-147.1 ºC.
2-methyl-2,5,6-triphenyl-1H-indene-1,3(2H)-dione (24). Synthesized with general method
B and purified by column chromatography (SiO2, gradient from 100% pentane to 7:3
pentane/EtOAc mixture). (off-white solid; 1.26 g; 3.25 mmol; 61% yield). Synthesized also
with general method G and purified by column chromatography (SiO2, gradient from 100%
pentane to 7:3 pentane/EtOAc mixture). (off-white solid; 900 mg; 2.32 mmol; 50 % yield). 1H-
NMR (CDCl3, 600 MHz, δ): 8.07 (s, 2H), 7.41-7.39 (m, 2H), 7.34-7.31 (m, 2H), 7.27-7.23 (m,
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7H), 7.16-7.14 (m, 4H), 1.76 (s, 3H). C-NMR {1H} (CDCl3, 150 MHz, δ): 201.8, 149.2,
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140.3, 139.7, 138.1, 129.7, 129.0, 128.4, 128.1, 127.8, 126.9, 125.9, 58.6, 20.3. FT-IR (dry
powder) (cm-1): 3060 (C-H), 1707 (C=O). HR-MS (m/z): [M+H]+ calcd for C22H22O2
389.1536; found 389.1534 (0.4 ppm). Mp 195.3-197.2 ºC.
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5,6-bis(4-methoxyphenyl)-2-methyl-2-phenyl-1H-indene-1,3(2H)-dione (25). Synthesized
with general method B (off-white solid; 2.07 g; 0.53 mmol; 18% yield) and purified by column
chromatography (SiO2, gradient from 100% pentane to 7:3 pentane/EtOAc mixture).
Synthesized also with general method G and purified by column chromatography (SiO2,
gradient from 100% pentane to 7:3 pentane/EtOAc mixture). (off-white solid; 950 mg; 2.12
mmol; 55 % yield). 1H-NMR (CDCl3, 600 MHz, δ): 8.02 (s, 2H), 7.40-7.39 (m, 2H), 7.34-7.31
(m, 2H), 7.27-7.25 (m, 1H), 7.10-7.09 (AA’BB’ system, 4H), 6.82-6.80 (AA’BB’ system, 4H),
3.80 (s, 6H), 1.75 (s, 3H). 13C-NMR {1H} (CDCl3, 150 MHz, δ): 201.9, 159.5, 148.7, 140.0,
138.2, 132.2, 131.0, 129.0, 127.7, 126.9, 125.7, 114.0, 58.5, 55.4, 20.2. FT-IR (dry powder)
(cm-1): 3059 (C-H), 2961 (C-H), 1701 (C=O). HR-MS (m/z): [M+H]+ calcd for C30H25O4
449.1747; found 449.1738 (2.0 ppm). Mp 196.2-198.7 ºC.
5,6-bis(4-((tert-butyldimethylsilyl)oxy)phenyl)-2-methyl-2-phenyl-1H-indene-1,3(2H)-
dione (26). Synthesized with general method G and purified by column chromatography (SiO2,
gradient from 100% pentane to 8:2 pentane/EtOAc mixture). (off-white solid; 622 mg; 0.96
mmol; 75% yield). 1H-NMR (CDCl3, 600 MHz, δ): 8.02 (s, 2H), 7.40-7.39 (m, 2H), 7.34-7.31
(m, 2H), 7.27-7.25 (m, 1H), 7.02-7.01 (AA’BB’ system, 4H), 6.74-6.73 (AA’BB’ system, 4H),
1.75 (s, 3H), 0.98 (s, 18H), 0.19 (s, 12H). 13C-NMR {1H} (CDCl3, 150 MHz, δ): 201.9, 155.8,
148.9, 140.0, 138.2, 132.8, 131.0, 129.6, 129.0, 128.9, 127.7, 126.9, 125.6, 120.2, 58.5, 25.8,
20.2, 18.4, -4.3. FT-IR (dry powder) (cm-1): 2931 (C-H), 2858 (C-H), 1703 (C=O), 1266 (Si-
O). HR-MS (m/z): [M+H]+ calcd for C40H49O4Si2 649.3164; found 649.3147 (2.6 ppm). Mp
134.2-136.5 ºC.
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dimethyl 2-isopropyl-2-methyl-1,3-dioxo-2,3-dihydro-1H-indene-5,6-dicarboxylate (27).
Synthesized with general method B and purified by column chromatography (SiO2, gradient
from 100% pentane to 7:3 pentane/EtOAc mixture). (yellow solid; 3.07 g; 9.65 mmol; 30%
yield). 1H-NMR (CDCl3, 600 MHz, δ): 8.25 (s, 2H), 3.96 (s, 6H), 2.16 (h, J = 6 Hz, 1H), 1.27
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(s, 3H), 0.91 (d, J = 6 Hz, 6H). C-NMR {1H} (CDCl3, 150 MHz, δ): 203.5, 166.4, 142.7,
138.6, 123.9, 57.5, 53.4, 34.7, 18.1, 17.4. FT-IR (dry powder) (cm-1): 2957 (C-H), 2876 (C-H),
1729 (C=O), 1710 (C=O). HR-MS (m/z): [M+H]+ calcd for C17H19O6 319.1176; found
319.1175 (0.4 ppm). Mp 92.2-94.6 ºC.
5,6-dimethoxy-2-phenyl-1H-indene-1,3(2H)-dione (29). Synthesized adapting a literature
procedure.44 (off-white solid; 7.78 g; 27.5 mmol; 31% yield).1H-NMR (CDCl3, 600 MHz, δ):
7.40 (s, 2H), 7.34-7.28 (m, 3H), 7.18-7.16 (m, 2H), 4.20 (s, 1H), 4.04 (s, 6H). 13C-NMR {1H}
(CDCl3, 150 MHz, δ): 197.4, 156.3, 138.0, 133.9, 129.1, 128.8, 127.9, 103.8, 59.6, 56.9. FT-
IR (dry powder) (cm-1): 3006 (C-H), 2946 (C-H), 1686 (C=O). HR-MS (m/z): [M+H]+ calcd
for C17H15O4 283.0965; found 283.0961 (1.3 ppm). Mp 101.2-103.5 ºC.
2-octadecyl-2-phenyl-1H-indene-1,3(2H)-dione (30). Synthesized with general method B
applying only the alkylation part on 2-phenyl-1H-indene-1,3(2H)-dione38 and purified by
column chromatography (SiO2, gradient from 100% pentane to 9:1 pentane/EtOAc mixture).
(off-white solid; 2.89 g; 6.09 mmol; 83% yield). 1H-NMR (CDCl3, 600 MHz, δ): 8.03-8.02 (m,
2H), 7.86-7.85 (m, 2H), 7.42-7.40 (m, 2H), 7.30-7.28 (m, 2H), 7.24-7.22 (m, 1H), 2.25 (t, J =
9 Hz, 2H), 1.30-1.13 (m, 35H), 0.88 (t, J = 6 Hz, 3H). 13C-NMR {1H} (CDCl3, 150 MHz, δ):
202.2, 142.2, 137.4, 136.0, 128.9, 127.7, 127.0, 123.7, 62.5, 36.5, 32.1, 30.1, 29.8, 29.8, 29.8,
29.7, 29.7, 29.6, 29.5, 29.3, 25.4, 22.8, 14.3. FT-IR (dry powder) (cm-1): 2949 (C-H), 2916 (C-
H), 2850 (C-H), 1705 (C=O). HR-MS (m/z): [M+H]+ calcd for C33H47O2 475.3571; found
475.3557 (2.9 ppm). Mp 63.2-64.3 ºC.
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In some cases, indanedithiones 31-39 were not obtained in 100% purity. Hence, we only report
their 1H NMR spectra. However, the impurities present in 31-39 did not affect the subsequent
B-K olefinations to obtain motors 1-9.
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5,6-dimethoxy-2-methyl-2-phenyl-1H-indene-1,3(2H)-dithione (32). Synthesized with
general method D and purified by column chromatography (SiO2, gradient from 100% pentane
to 9:1 pentane/EtOAc). (green solid; 180 mg; 0.55 mmol; 33% yield). 1H-NMR (CDCl3, 300
MHz, δ): 7.40 (s, 2H), 7.21-7.17 (m, 5H), 4.09 (s, 6H), 1.92 (s, 3H).
2-methyl-2-phenyl-1H-cyclopenta[b]naphthalene-1,3(2H)-dithione (33). Synthesized with
general method D and purified by column chromatography (SiO2, gradient from 100% pentane
to 90:10 pentane/EtOAc). (green solid; 576 mg; 1.81 mmol; 52% yield). 1H-NMR (CDCl3, 300
MHz, δ): 8.63 (s, 2H), 8.15-8.12 (m, 2H), 7.72-7.69 (m, 2H), 7.22-7.12 (m, 5H), 2.00 (s, 3H).
5,6-dichloro-2-methyl-2-phenyl-1H-indene-1,3(2H)-dithione (34). Synthesized with
general method D and purified by column chromatography (SiO2, gradient from 100% pentane
to 9:1 pentane/EtOAc). (green solid; 275 mg; 0.81 mmol; 32% yield). 1H-NMR (CDCl3, 400
MHz, δ): 8.15 (s, 2H), 7.24-7.15 (m, 5H), 1.91 (s, 3H).
2-methyl-2,5,6-triphenyl-1H-indene-1,3(2H)-dithione (35). Synthesized with general
method D and purified by column chromatography (SiO2, gradient from 100% pentane to 9:1
pentane/EtOAc). The compound was not obtained in high purity. (green solid; 120 mg; 2.85
mmol; 35% yield). 1H-NMR (CDCl3, 400 MHz, δ): 8.14 (s, 2H), 7.31-7.29 (m, 10H), 7.25-7.22
(m, 5H), 2.01 (s, 3H).
5,6-bis(4-methoxyphenyl)-2-methyl-2-phenyl-1H-indene-1,3(2H)-dithione
(36).
Synthesized with general method D and purified by column chromatography (SiO2, gradient
from 100% pentane to 9:1 pentane/EtOAc). (green solid; 113 mg; 0.23 mmol; 45% yield). 1H-
NMR (CDCl3, 400 MHz, δ): 8.05 (s, 2H), 7.28-7.21 (m, 5H), 7.16-7.14 (AA’BB’system, 4H),
6.83-6.80 (AA’BB’system, 4H), 3.81 (s, 6H), 1.96 (s, 3H).
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5,6-bis(4-((tert-butyldimethylsilyl)oxy)phenyl)-2-methyl-2-phenyl-1H-indene-1,3(2H)-
dione (37). Synthesized with general method D and purified by column chromatography (SiO2,
gradient from 100% pentane to 95:5 pentane/EtOAc). (green solid; 200 mg; 0.29 mmol; 48%
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yield). H-NMR (CDCl3, 400 MHz, δ): 8.06 (s, 2H), 7.25-7.20 (m, 5H), 7.08-7.06 (AA’BB’
system, 4H), 6.75-6.73 (AA’BB’ system, 4H), 1.97 (s, 3H), 0.98 (s, 18H), 0.19 (s, 12H).
dimethyl 2-isopropyl-2-methyl-1,3-dioxo-2,3-dihydro-1H-indene-5,6-dicarboxylate (27).
Synthesized with general method E and purified by column chromatography (SiO2, gradient
from 100% pentane to 9:1 pentane/EtOAc). However, this procedure afforded singly converted
product in all attempts, with the only exception of one case in which desired 39 was obtained
1
(green solid; 181 mg; 0.51 mmol; 33% yield). H-NMR (CDCl3, 400 MHz, δ): 8.27 (s, 2H),
3.96 (m, 6H), 2.41 (h, J = 7 Hz, 1H), 1.52 (s, 3H), 0.86 (d, J = 7 Hz, 6H).
Motor 2. Synthesized with general method F and purified by column chromatography (SiO2,
gradient pentane/CH2Cl2; 0–100%). (deep red solid; 200 mg; 0.34 mmol; 61% yield). 1H-NMR
(Cl2DCCDCl2, 500 MHz, 90 ºC, δ) (signals of the main isomer): 8.41 (d, J = 6 Hz, 2H), 7.80
(d, J = 6 Hz, 2H), 7.72 (d, J = 6 Hz, 2H), 7.68 (d, J = 6 Hz, 2H), 7.65 (s, 2H), 7.30 (t, J = 6Hz,
3H), 7.27 (t, J = 6Hz, 3H), 7.18 (t, J = 6Hz, 3H), 7.13 (t, J = 6Hz, 3H), 3.86 (s, 6H), 2.44 (s,
3H). 13C-NMR {1H} (CDCl3, 125 MHz, -45 ºC, δ): 160.7, 157.2, 154.3, 152.2, 150.2, 149.7,
142.2, 140.9, 140.4, 140.1, 139.6, 139.5, 138.9, 137.6, 137.4, 137.3, 132.9, 130.4, 128.7, 128.6,
128.0, 127.5, 127.3, 127.2, 127.0, 126.8, 126.7, 126.7, 126.6, 126.2, 126.0, 125.8, 123.6, 119.9,
119.6, 119.5, 119.4, 110.7, 105.4, 71.4, 68.7, 56.9, 56.7, 56.6, 23.4, 19.4. HR-MS (m/z):
[M+H]+ calcd for C44H33O2 593.2475; found 593.2461 (2.3 ppm). UV-Vis (CH2Cl2) λmax, nm
(ε): 247 (149200), 471 (69400). Mp 260-262 ºC. Single crystals for XRD were obtained from
slow diffusion of hexane (anti-solvent) into a saturated solution of 1,2-dichloroethane (solvent).
Motor 3. Synthesized with general method F and purified by column chromatography (SiO2,
gradient pentane/CH2Cl2; 0–100%). (orange solid; 395 mg; 0.678 mmol; 54% yield). 1H-NMR
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(Cl2DCCDCl2, 500 MHz, 90 ºC, δ): 8.72 (s, 2H), 8.64 (d, J = 6 Hz, 2H), 7.84-7.82 (m, 4H),
7.80-7.78 (m, 2H), 7.71 (d, J = 6 Hz, 2H), 7.68-7.67 (m, 2H), 7.57-7.55 (m, 4H), 7.73 (t, J = 6
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Hz, 3H), 7.29-7.26 (m, 3H), 7.15-7.12 (m, 5H), 2.48 (s, 3H). C-NMR {1H} (CDCl3, 125
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MHz, -45 ºC, δ): 159.5, 156.6, 144.0, 141.8, 141.6, 140.6, 140.2, 140.1, 140.00, 139.6, 139.4,
137.5, 135.6, 134.3, 134.0, 133.5, 133.5, 132.0, 130.9, 129.5, 129.0, 128.9, 128.8, 128.6, 128.0,
127.8, 127.7, 127.5, 127.3, 127.2, 126.5, 126.4, 126.2, 125.2, 125.0, 123.1, 122.9, 119.6, 119.5,
119.4, 118.8, 70.0, 67.6, 20.1, 19.7. HR-MS (m/z): [M+H]+ calcd for C46H31 583.2420; found
583.2410 (1.8 ppm). UV-Vis (CH2Cl2) λmax, nm (ε): 244 (239352), 400 (94064). Mp > 300 ºC.
Motor 4. Synthesized with general method F and purified by column chromatography (SiO2,
gradient pentane/CH2Cl2; 0–100%). (orange solid; 313 mg; 0.52 mmol; 64% yield). 1H-NMR
(Cl2DCCDCl2, 500 MHz, 90 ºC, δ) (signals of the main isomer): 8.35 (d, J = 6 Hz, 2H), 8.32
(s, 2H), 7.73-7.68 (m, 5H), 7.63 (d, J = 6 Hz, 2H), 7.34 (t, J = Hz, 3H), 7.26 (t, J = 6 Hz, 3H),
7.21 (t, J = 6 Hz, 3H), 7.10 (t, J = 6 Hz, 3H), 2.42 (s, 3H). 13C-NMR {1H} (CDCl3, 125 MHz,
-45 ºC, δ): 157.1, 154.4, 154.2, 147.2, 144.6, 140.8, 140.4, 140.3, 140.2, 139.8, 139.6, 138.5,
138.4, 137.1, 135.3, 135.0, 134.4, 134.0, 133.1, 132.7, 130.7, 130.4, 129.6, 128.8, 128.3, 128.2,
128.1, 127.9, 127.7, 127.5, 126.6, 126.4, 125.4, 125.3, 124.4, 123.5, 123.3, 122.8, 120.3, 120.2,
119.8, 119.6, 119.5, 119.0, 70.7, 68.5, 19.7, 18.9. HR-MS (m/z): [M+H]+ calcd for C42H27Cl2
601.1484; found 601.1481 (0.5 ppm). UV-Vis (CH2Cl2) λmax, nm (ε): 241 (156766), 441
(59518). Mp 269-271 ºC. Single crystals for XRD were obtained from slow diffusion of hexane
(antisolvent) into a saturated solution of 1,2-dichloroethane (solvent).
Motor 5. Motor 4 (108 mg; 0.18 mmol) and Pd-PEPPSI-IPent complex (5 mol %) were
dissolved in toluene (4 mL) in a dried Schlenk flask under N2. The mixture was stirred at 40
ºC for 5 min. Subsequently, a toluene solution (1 mL) of PhLi (2,5 eq.) was added over 1 h by
use of a syringe pump. After complete addition, MeOH (1 mL) was added to quench the
remaining PhLi. The reaction mixture was transferred to a round-bottom flask, celite was
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