A supramolecular protecting group strategy introduced to the organic solid state: Enhanced reactivity through molecular pedal motion

Article abstract of DOI:10.1002/anie.201106842

A supramolecular protecting group strategy has been applied to achieve solid-state photodimerizations of olefins lined with a combination of hydrogen-bond-donor and -acceptor groups. Esters were used as protecting groups to generate head-to-head photodime

Full text of DOI:10.1002/anie.201106842

Angewandte
Chemie
DOI: 10.1002/anie.201106842
Supramolecular Chemistry
A Supramolecular Protecting Group Strategy Introduced to the
Organic Solid State: Enhanced Reactivity through Molecular Pedal
Motion**
Elizabeth Elacqua, Poonam Kaushik, Ryan H. Groeneman, Joseph C. Sumrak, Dejan-
ˇ
ˇ
Kresimir Bucar, and Leonard R. MacGillivray*
A major impediment when planning reactions in the organic
solid state[1,2] is unpredictable structure effects of crystal
packing. For bimolecular reactions, reactive centers must
generally lie in proximity, being separated by distances on the
order of 4 ꢀ.[3] To achieve this goal, chemists often function-
alize reactants with groups that participate in molecular
recognition processes that drive the solid-state assembly
process to a prerequisite geometry.[4] The idea is to identify
supramolecular synthons,[2] akin to molecular synthons, that
are able to overcome effects of crystal packing and, ulti-
mately, enable molecular synthesis by design.
In this context, the use of protecting groups is a
functionalization strategy replete in solution-phase synthetic
organic chemistry.[5] A protecting group involves temporarily
derivatizing a reactant so as to mask an organic group and to
hinder it from participating in a chemical reaction and
forming an unwanted covalent bond. In principle, the concept
of a protecting group can be applied to an organic group that
is likely to participate in an unwanted noncovalent bond.[6] In
such a setting, it may be necessary to mask an organic group
from participating in an intermolecular force that disrupts an
assembly process aimed to afford a covalent-bond-forming
supramolecular structure.[6] The field of metal–organic frame-
works (MOFs) has recently benefited from protecting group
strategies that involve noncovalent bonds whereby organic
groups are removed in a postsynthetic step to generate MOFs
of controlled dimensionalities.[6] Efforts to understand and
exploit such interplay between noncovalent and covalent
bonds, however, remain in a stage of infancy, yet can equip
chemists with powerful tools for molecular and supramolec-
ular design. Developing such interplay is especially important
in the organic solid state where structural effects of non-
covalent bonds are accentuated in the closely packed environ-
ment.[2]
Scheme 1. Solid-state supramolecular protecting group strategy.
molecular chemistry to achieve the targeted hydrogen-bond-
À
mediated formation of carbon–carbon single (C C) bonds
and concomitant installation of carboxylic acid (-CO2H)
groups.
Herein, we introduce the concept of a protecting group
strategy applied to molecular syntheses in the organic solid
state (Scheme 1). The strategy employs principles of supra-
Our interests lie in developing cocrystals based on
resorcinol (res) to direct [2+2] photodimerizations in
solids.[4] Res acts as a ditopic hydrogen bond donor template
that assembles and stacks olefins lined with acceptor pyridyl
groups for photoreaction. During studies to use res templates
to direct the [2+2] cycloaddition, we developed an interest to
generate head-to-head 1a (Scheme 1).[7] The diacid is attrac-
tive as a building block for MOFs and related porous
solids.[4,8] Moreover, the presence of the 4-pyridyl groups
suggested that 1a could be generated from a photodimeriza-
[*] E. Elacqua, Dr. P. Kaushik, Prof. R. H. Groeneman, Dr. J. C. Sumrak,
ˇ
Dr. D.-K. Bucar, Prof. L. R. MacGillivray
Department of Chemistry, University of Iowa
Iowa City, IA 52242-1290 (USA)
E-mail: len-macgillivray@uiowa.edu
[**] We are grateful to the National Science Foundation (L. R. M., DMR-
0801329) for funding.
À
tion of the acrylic acid 1b, wherein a res assembles 1b by O
H···N hydrogen bonds in a head-to-head geometry for
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201106842.
À
photoreaction. To be realized, the O H groups of a res
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À
would be required to participate in O H···N interactions and,
stand. Within two days, all samples contained a precipitate
that was dried and subjected to UV irradiation. In contrast to
the acid 1b, an examination of PXRD patterns revealed a new
solid phase in each case, which supported cocrystal formation.
1H NMR analysis showed each solid to consist of 1c and a res
(2:1 ratio).
Of the 10 res R1–R10 screened with 1c, to our surprise,
only the cocrystal 2(1c)·(R2) was photoactive. A single-
crystal-structure analysis revealed a discrete, three-compo-
thereby, successfully compete[9] with the O H acid group of
À
the olefin that directs the self-assembly of pyridine-carboxylic
acids such as 1b in solids.[10] We reveal how an inability to
utilize res templates involving R1–R10 to assemble 1b to
form 1a can be overcome by using a supramolecular
protecting group strategy. The strategy involves masking the
acid group[5] of 1b as an ester in 1c[11] that remains dormant in
the assembly process and can be easily removed post synthesis
to generate the acid groups of 1a. The protecting strategy
enables res templates to afford 1a, and a lengthened congener
2a, stereospecifically and in quantitative yield. In addition to
the solid state, we are unaware of a supramolecular protecting
group strategy having been applied to related hydrogen-
bond-mediated syntheses developed in solution.[12] We also
show how integrating a stilbene unit into a lengthened
protected olefin results in strikingly enhanced reactivity that
enables the generation of lengthened cyclobutane 2a, which
we attribute to often overlooked pedal motion[13] in stilbene-
based solids.
À
nent hydrogen-bonded assembly sustained by two O H···N
hydrogen bonds (O···N distances [ꢀ]: O1···N1 2.763(2),
=
O2···N2 2.741(2); Figure 1a). The stacked C C bonds were
ordered, organized in parallel, and separated by 3.73 ꢀ,
geometries that conform to the criteria for photoreaction.[3]
Olefins between nearest-neighbor assemblies were antipar-
allel and separated by 4.87 ꢀ (Figure 1b).
In our initial studies to synthesize the target 1a, we
attempted to achieve a reactive hydrogen-bonded assembly
involving 1b and a res. The olefin 1b was insoluble in most
organic solvents, which is attributed to 1b participating in
À
intermolecular O H(acid)···N(pyridyl) hydrogen bonds in the
pure solid.[10] For a res template, we employed our cocrystal
screening strategy termed “template switching.”[14] The strat-
egy involves screening a pyridine-based olefin with res
derivatives by solvent precipitation and by exposing the
resulting cocrystals to UV irradiation. The method allows us
to assemble the same olefin into similar yet different packing
environments to improve the probability of obtaining a
photoreactive solid.[14] From our studies, the application of
template switching to 1b using R1–R10 in organic solvents
(e.g. EtOH, DMF) afforded pure 1b alone as a precipitate, as
evidenced by powder X-ray diffraction (PXRD) analyses
(Figure S-25). We attribute the inability of each res to form a
cocrystal with 1b to the marked insolubility of the olefin and,
corresponding, intermolecular hydrogen bonds present in the
pure solid.
Figure 1. Top: X-ray structure of reactive 2(1c)·(R2): a) assembly with
=
parallel C C bonds and b) antiparallel packing. Bottom: X-ray structure
=
of unreactive 2(1c)·(R1): c) assembly with criss-crossed C C bonds
and d) antiparallel packing.
UV irradiation of a powdered sample of 2(1c)·(R2)
revealed 1c to react quantitatively to give 1d, as evidenced by
1H NMR spectroscopy. The product was characterized by the
disappearance of the olefinic signals at 7.65 and 6.95 ppm and
the appearance of cyclobutane signals at 4.38 and 4.15 ppm.
Basic extraction and a subsequent hydrolysis afforded the
To achieve a reactive cocrystal that furnishes 1a, we
designed a protecting group strategy.[5,11] In particular, we
expected that the hydrogen bond donor abilities of the acid
group of 1b could be effectively rendered inactive by
converting the acid group into the corresponding methyl
ester 1c that masks the proton. Conversion of 1b into 1c was
expected to result in increased solubility and, at the same
time, enable a res to form O-H···N hydrogen bonds to the
alkene (Scheme 1). Although the sp2-hybridized O atom of an
ester can act as a hydrogen bond acceptor group, the basicity
of a pyridine (pKa = 5.2) versus an ester (pKa = À6) suggested
that the pyridyl group would selectively participate in a
hydrogen bond with a res.[15] Upon photoreaction, the
resulting diester 1d would be deprotected by hydrolysis to
generate 1a.
1
unmasked diacid 1a, as confirmed by H NMR spectroscopy
(95% yield).[7]
To gain insight into the photostabilities of the remaining
solids, single crystals of 2(1c)·(R1) were obtained after slow
solvent evaporation from MeCN.[17] An X-ray analysis
revealed, similar to 2(1c)·(R2), a discrete assembly sustained
À
by two O H···N hydrogen bonds (O···N distances [ꢀ]:
=
O1···N1 2.787(3), O2···N2 2.800(3)). The C C bonds were
determined to be ordered and separated by 3.63 ꢀ but, in
contrast to 2(1c)·(R2), adopted a criss-cross conformation
(Figure 1c,d).[13] The photostability of the cocrystal 2-
(1c)·(R1) can likely be attributed to the criss-cross arrange-
The methyl ester 1c was, thus, prepared.[16] Our template
switching method was applied to screen for reactive cocrystals
of 1c and R1–R10. Cocrystals were generated by mixing 1c
and each res (0.5 equiv) in MeCN and allowing the solution to
ment of the stacked C C bonds in the solid state.
[13]
=
Given that organization of molecules in solids is
extremely sensitive to subtle changes to the molecular
structure, we investigated the generality of the protecting
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group strategy to generate the extended cyclobutane con-
gener 2a. As for 1b, each attempt to screen 2b for a cocrystal
involving R1–R10 afforded the pure solid olefin, as confirmed
by PXRD analyses (Figure S-26). The corresponding pro-
tected lengthened phenyl ester 2c was, thus, prepared by using
a modified procedure.[18]
In contrast to 1c, the application of template switching to
2c, remarkably, afforded a photoactive solid in each case
(Table 1). As for 1c, each solid was formed by mixing 2c and a
res (0.5 equiv) in EtOH and allowing the solution to stand.
Within two days, each sample contained a solid that was dried
and subjected to UV irradiation. PXRD and 1H NMR
analyses supported cocrystal formation. Of the 10 reactive
solids, four (res = R3, R5, R6, R7) afforded 2d stereospecifi-
cally and in quantitative yield.
Table 1: Template switching applied to 2c.
Entry
Res Rx
t [h]
Conv. [%]
Yield of 2d [%]
Figure 2. X-ray structures of reactive (2c)·(res): 2(2c)·(R3): a) antipar-
=
=
allel C C bonds and b) packing, 2(2c)·(R5): c) parallel C C bonds and
1
2
3
4
5
6
7
8
9
10
R1
R2
R3
R4
R5
R6
R7
R8
R9
R10
175
175
175
75
175
75
130
175
175
175
66
85
44[a]
55[a]
100
90
=
d) packing, 2(2c)·(R6): e) parallel C C bonds and f) packing, and
2(2c)·(R7): g) antiparallel C C bonds and h) packing. Lower occupan-
cies of each C C bond in full gray.
=
100
90
100
100
100
52
=
100
100
100
52
R6, R7) revealed 2c to react stereospecifically and quantita-
tively in each cocrystal. The generation of the cyclobutane
photoproduct was evidenced by the disappearance of the
signals at 7.65 and 7.43 ppm and the appearance of signals at
4.75 and 4.69 ppm.
96
75
64[a]
75
[a] Mixture of 2d and unidentified minor product.
The photoproduct 2d was isolated from the res template
by liquid-phase extraction using CHCl3 and NaOH. Depro-
tection of 2d was achieved through basic hydrolysis with
aqueous NaOH. A subsequent neutralization with aqueous
HCl afforded the target cyclobutane 2a, as revealed by
1H NMR spectroscopy (97% yield). Moreover, the stereo-
chemistry of 2a was confirmed by an X-ray analysis of a
deprotected intermediate in the form of the sodium salt
[Na4(m2-OH2)8(C28H20N2O4)2(OH2)6(OH2)]·H2O (Figure 3).
The markedly enhanced reactivity of cocrystals of 2c
Single-crystal X-ray diffraction was employed to study the
enhanced solid-state reactivity of 2c versus 1c.[17] Moreover, a
structure analysis of each solid that afforded 2d in 100% yield
in 2(2c)·(res) (where: res = R3, R5, R6, R7) revealed the
components, similar to 2(1c)·(R2), to form discrete assem-
À
blies sustained by two O H···N hydrogen bonds (O···N
distances [ꢀ]: 2(2c)·(R3) O1···N1 2.750(3), O2···N2
2.706(4); 2(2c)·(R5) O1···N1 2.698(4), O2···N2 2.707(3);
2(2c)·(R6) O1···N1 2.713(4), O2···N2 2.725(5); 2(2c)·(R7)
O1···N1 2.737(4), O2···N2 2.760(5)) (Figures 2a–h). In con-
=
compared to 1c with R1–R10 can be attributed to the C C
units undergoing pedal, or crankshaft, motion in each solid.[13]
=
trast to 2(1c)·(R2), however, the C C bonds in three stacked
Stilbenes such as 2c exhibit dynamic motion in the crystalline
stilbene pairs were disordered (2(2c)·(R5): 0.51/0.49 and 0.91/
0.09; 2(2c)·(R6): 0.53/0.47 and 0.50/0.50; 2(2c)·(R7): 0.71/0.29
state that can interconvert[13] C C bonds from a criss-cross to
=
a parallel conformation suitable for a photodimerization
=
and 0.66/0.34.), with the C C bonds being separated by 3.94,
3.94, and 3.96 ꢀ, respectively. Olefins between the stacked
assemblies were either parallel (R5) or antiparallel (R6 and
R7), being separated by 5.42, 5.11, and 4.01 ꢀ, respectively
=
(Figures 2d,f,h). For 2(2c)·(R3), the C C bonds were ordered
and separated by 4.10 ꢀ, with adjacent assemblies adopting
=
an antiparallel arrangement (Figures 2a,b). The C C bonds
between adjacent assemblies were also parallel and separated
by 4.01 ꢀ. UV irradiation of (2c)·(res) (where: res = R3, R5,
Figure 3. X-ray structure of the sodium carboxylate salt of 2a: a) wire-
frame and b) space-filling model of the dianion of 2a.
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(Scheme 2).[19,20] Herein, the pedal motion of the stilbene acts
to our advantage, compared to the acrylate 1c, to achieve
reactivity within a series of res-based solids. Given that our
goal is to expand the synthetic versatility of reactivity in
organic solids, these observations are important since the
enhanced reactivities suggest that stilbene units and protect-
ing groups, when applied in combination, can provide a route
2(1c)·(R2), 798920 2(2c)·(R3), 836620 2(2c)·(R5), 823834 2(2c)·(R6),
823835 2(2c)·(R7), 798922 2a Na+ salt contain the supplementary
crystallographic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Photoreactions: Photoreactions were conducted using UV irra-
diation from a 500 W medium-pressure mercury lamp in an ACE
Glass photochemistry cabinet. The cocrystals were finely ground
using a mortar and pestle and were placed between a pair of pyrex
glass plates. The samples were irradiated in 10 h increments and were
mixed between consecutive irradiations. Product formation was
À
to highly reactive olefins for the directed formation of C C
bonds in solids.
1
monitored using H NMR spectroscopy. Upon completion of photo-
reaction, products were isolated using basic extraction with CHCl3.
Deprotections of photoproducts: Cyclobutanes 1d and 2d were
stirred in 2m NaOH for 2 h. 10% HCl was added until the solutions
tested neutral by pH paper and the solutions were allowed to stir
overnight. Evaporation gave the product and sodium chloride.
Trituration of the solid with 2:1 CH3OH/CHCl3 solution, followed
by evaporation, afforded diacids 1a and 2a. 1H NMR (1a, 300 MHz,
[D6]DMSO): d = 3.89 (d, 2H), 4.26 (d, 2H), 7.08 (dd, 4H), 8.36 ppm
(dd, 4H). 1H NMR (2a, 400 MHz, [D6]DMSO): d = 4.62 (d, 4H), 7.12
(d, 4H), 7.28 (dd, 4H), 7.66 (d, 4H), 8.36 ppm (dd, 4H).
Scheme 2. Enhanced reactivity of protected olefin in a cocrystal of res
achieved through pedal motion.
In conclusion, we have introduced a protecting group
strategy to the organic solid state used to direct the formation
Received: September 26, 2011
Published online: December 12, 2011
À
of C C bonds mediated by principles of supramolecular
Keywords: photochemical reactions · protecting groups ·
self-assembly · solid-state reactions · supramolecular chemistry
chemistry. An ester masks a carboxylic acid to generate head-
to-head photodimers with restored acid groups from organic
solids. We have also shown how the solid-state reactivity is
enhanced using stilbenes as protected functionalities. We
anticipate the protecting strategy to be amenable to other
protecting group strategies developed in the liquid phase and
applicable to other reactions mediated by templates (e.g.
hydrogen bond acceptors) in both the solid state and in
solution.
.
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