A Retro-Staudinger Cycloaddition: Mechanochemical Cycloelimination of a β-Lactam Mechanophore

Article abstract of DOI:10.1021/jacs.5b07345

Mechanical activation of a β-lactam mechanophore using ultrasound induces a formal [2 + 2] cycloelimination reaction producing ketene and imine functional groups - the reverse reaction of the Staudinger cycloaddition. This transformation is predicted by c

Full text of DOI:10.1021/jacs.5b07345

Subscriber access provided by Georgetown University | Lauinger and Blommer Libraries
Communication
A Retro-Staudinger Cycloaddition: Mechanochemical
Cycloelimination of a #-Lactam Mechanophore
Maxwell J. Robb, and Jeffrey S. Moore
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 21 Aug 2015
Downloaded from http://pubs.acs.org on August 21, 2015
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Journal of the American Chemical Society is published by the American Chemical
Society. 1155 Sixteenth Street N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 11
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
A Retro-Staudinger Cycloaddition: Mechanochemical Cy-
cloelimination of a β-Lactam Mechanophore
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Maxwell J. Robb and Jeffrey S. Moore*
Beckman Institute for Advanced Science and Technology and Department of Chemistry, University of Illinois at Urbana–
Champaign, Urbana, Illinois 61801, United States
Supporting Information
Scheme 1. Generation of ketene and imine functional
ABSTRACT: Mechanical activation of a βꢀlactam mechanoꢀ
phore using ultrasound induces a formal [2+2] cycloelimination
reaction producing ketene and imine functional groups—the reꢀ
verse reaction of the Staudinger cycloaddition. This transforꢀ
mation is predicted by computational modeling and verified by
kinetics and UVꢀVis absorption measurements as well as polymer
groups via mechanochemical cycloelimination of
a
β-lactam mechanophore.
1
endꢀgroup analysis using H and 13C NMR spectroscopy. Addiꢀ
tion of the βꢀlactam motif to the current repertoire of covalent
mechanophores coupled with the diverse reactivity of the ketene
functional group provides a promising new platform for achieving
materials capable of autonomic selfꢀhealing behavior in response
to external forces.
tional groups, i.e., a retroꢀStaudinger cycloaddition.13 Staudinger
first reported the cycloaddition reaction between an imine and a
ketene to form a βꢀlactam in 1907.14 Given the mechanochemical
activity of other cycloaddition products (e.g., DielsꢀAlder adꢀ
ducts15) and 4ꢀmembered ring systems,4a,6,16 we reasoned that the
βꢀlactam ring may be capable of mechanicallyꢀmediated cyꢀ
cloelimination to generate ketene and imine functional groups
(Scheme 1).
Polymer mechanochemistry provides a unique and powerful
platform for generating diverse functionality in materials systems.
During the past decade,1 the emerging genre of covalent mechaꢀ
nochemistry has witnessed significant progress, establishing the
activation of latent catalysts,2 mechanoluminescence,3 mechaꢀ
nochromism,4 mechanicallyꢀtriggered polymer degradation,5 and
the generation of a variety of functional groups amenable to selfꢀ
healing materials. In the latter category, cyanoꢀsubstituted cycloꢀ
butane6 and gemꢀdihalocyclopropane7 mechanophores are particuꢀ
larly promising, producing reactive cyanoacrylates and allylic
halides, respectively. Polymer mechanochemistry has even
demonstrated the ability to facilitate formally forbidden chemical
transformations, i.e., the disrotatory electrocyclic ringꢀopening of
cisꢀ1,2ꢀdisubstituted benzocyclobutene8 and the conrotatory ringꢀ
opening of gemꢀdihalocyclopropanes.9
Despite these remarkable achievements, development of new
mechanophores that expand upon the existing repertoire is a key
objective. Mechanochemical generation of ketenes is one particuꢀ
lar target that has remained elusive.10 Ketenes are an incredibly
versatile functional group that participate in diverse chemical
transformations including numerous electrophilic and cycloaddiꢀ
tion reactions.11 Hawker and coworkers have elegantly illustrated
the utility of ketenes in polymeric materials by exploiting the
thermal activation of Meldrum’s acid derivatives for highly effiꢀ
cient functionalization and crosslinking.12 The ability to generate
ketenes mechanochemically would have important implications
for materials systems capable of responding to mechanical forces
and repairing damage autonomically.
Figure 1. Density functional calculations (CoGEF) for a cisꢀβꢀ
lactam mechanophore. Increasing the distance between methyl
groups ultimately triggers a [2+2] cycloelimination reaction to
generate fragments comprising ketene and imine functional
groups. Calculations were performed at the B3LYP 6ꢀ31G* level
of theory.
In this communication, we report a new mechanophore based
on the βꢀlactam motif that undergoes a formal [2+2] cycloeliminaꢀ
tion under mechanical force to generate ketene and imine funcꢀ
1
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 2 of 11
Scheme 2. Ultrasonication of polymer 1 using isobutanol to trap the mechanochemically generated ketene.
1
2
3
4
5
6
7
8
9
O
N
N
isobutanol
O
O
O
PMA
O
+
O
PMA
THF
-10 oC
H
PMA
O
O
PMA
O
O
O
1
Mn = 70.9 kDa
PDI = 1.08
ketene
intermediate
O
C
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
OH
+
O
PMA
trap
in situ
O
CoGEF calculations17 were initially performed to evaluate the
propensity for mechanochemical reactivity of the βꢀlactam motif
(Figure 1). By artificially constraining the distance between two
atoms and optimizing the relaxed geometry at distinct intervals of
elongation using DFT, the mechanical fragmentation of a moleꢀ
cule can be modeled. The computational results indicate that
mechanical elongation of the cisꢀβꢀlactam molecule causes an
abrupt fragmentation that results unambiguously in the generation
of ketene and imine groups. The energy maximum calculated for
this transformation is 284 kJ/mol with a maximum force at rupꢀ
ture (Fmax) of 3.47 nN. It is worth noting that these values are
comparable to the cyanoꢀsubstituted cyclobutane mechanophores
reported previously (239–275 kJ/mol, Fmax = 3.30–4.72 nN).16b
not susceptible to mechanical forces during ultrasonication and a
chainꢀcentered moiety with similar connectivity (control 3), but
lacking any cyclic structure were also prepared (Chart 1). All
polymers in this study were synthesized with a number average
molecular weight (Mn) of 70–77 kDa and PDI ≤ 1.09.
Chart 1. Structures of control polymers and small-
molecule model compounds.
O
O
PMA
O
PMA
PMA
O
N
H
O
3
O
Chain-Centered Control
Mn = 72.3 kDa, PDI = 1.07
O
BnO
2
Chain-End Control
Mn = 76.3 kDa, PDI = 1.09
O
O
Figure 2. The rate of ultrasonicationꢀinduced chain cleavage,
determined from the slope of the curve, is significantly faster for
polymer 1 containing a chainꢀcentered βꢀlactam unit compared to
control 3. Measurements were performed in triplicate with error
bars representing the standard deviation at each time point. The
shaded regions represent the margin of error of the slope at the
99% confidence interval.
O
O
5
O
N
Model Compounds
N
O
O
H
O
O
O
O
4
6
Mechanochemical activity was evaluated using pulsed ultrasonꢀ
ication (8.8 W/cm2) in THF at ꢀ10 °C. Polymer 1 shows a steady
decrease in molecular weight as monitored by GPC with attenuaꢀ
tion of the initial polymer peak (Mp = 75 kDa) and concomitant
increase of a new, wellꢀdefined peak at approximately oneꢀhalf
the original molecular weight (Mp = 42 kDa) (see Figure S1 in the
Supporting Information). After subjecting 1 to ultrasonication for
120 min, the overall Mn decreases from 70.9 to 49.6 kDa. In diꢀ
rect contrast, ultrasonic irradiation of chainꢀcentered control polꢀ
ymer 3 under identical conditions results in substantially less
chain cleavage, generating a poorly defined, low molecular
weight shoulder in the GPC chromatogram and reducing Mn from
76.3 to 58.9 kDa. The significant difference in mechanochemical
activity between 1 and control 3 is highlighted when the rates of
polymer cleavage are compared (Figure 2). The rate constant of
ultrasonicationꢀinduced cleavage, k′, was measured for both polꢀ
ymers with experiments performed in triplicate to ensure reproꢀ
With the successful CoGEF predictions, we set out to syntheꢀ
size a series of polymers to evaluate the mechanochemical activity
of the βꢀlactam motif experimentally. The βꢀlactam core structure
was synthesized in a single step via a Cuꢀcatalyzed Kinugasa
reaction between propargyl alcohol and pꢀbenzyloxyphenylꢀNꢀ
phenylnitrone (see Supporting Information for details). Imꢀ
portantly, the resulting βꢀlactam is isolated exclusively as the cisꢀ
diastereomer (J3,4 = 5.9 Hz), excluding effects of stereochemical
configuration on the analysis of mechanochemical properties.
Deprotection of the phenol followed by esterification with αꢀ
bromoisobutyryl bromide furnished the bifunctional initiator that
was subsequently employed in the living radical polymerization
of methyl acrylate using Cu wire/Me6TREN in DMSO to afford
poly(methyl acrylate) (PMA) polymer 1 containing a chainꢀ
centered βꢀlactam unit (see Scheme 2 for structure). Control polꢀ
ymers containing the βꢀlactam unit at the chainꢀend (control 2)
2
ACS Paragon Plus Environment

Page 3 of 11
Journal of the American Chemical Society
ducibility and meaningful statistics. The rate constant measured
proximately 50% of the βꢀlactam mechanophores are converted
for 1 (k′ = 4.9 x 10ꢀ5 minꢀ1 kDaꢀ1) is nearly 70% larger than that for
control polymer 3 (k′ = 2.9 x 10ꢀ5 minꢀ1 kDaꢀ1), providing outꢀ
standing evidence of the mechanophore character of the βꢀlactam
unit.
1
2
3
4
5
6
7
8
into the corresponding imines after 180 min of ultrasonication
based on integration of the NMR signals. When this sample is
analyzed by GPC using a RI detector, the relative peak areas indiꢀ
cate that approximately 65% of polymer chains are cleaved. The
greater fraction of chain scission by GPC relative to imine forꢀ
mation by NMR likely originates from a distribution in the posiꢀ
tion of the mechanophore along the polymer chains since signifiꢀ
cant deviation from the polymer midpoint has been shown to yield
nonꢀspecific chain scission and results in efficiency less than uniꢀ
ty.16e In addition to imine signals, several new peaks indicative of
the formation of the isobutyl ester endꢀgroup are also apparent in
the spectrum of 1 after ultrasonication including doublets at 3.89
(J = 6.4 Hz) and 0.94 (J = 6.7 Hz) ppm (see Figure S3 in the Supꢀ
porting Information). These new peaks are fully consistent with
the spectrum of model compound 5, which exhibits, among othꢀ
ers, peaks at 3.87 (d, J = 6.6 Hz, 2H) and 0.92 (d, J = 6.7 Hz, 6H)
ppm. Once again, these features are not observed in the 1H NMR
spectrum of chainꢀend control 2 after ultrasonication under identiꢀ
cal conditions (see Figure S4 in the Supporting Information).
UVꢀVis and NMR spectroscopy were used to gain insight into
the mechanochemical cleavage of 1 containing a chainꢀcentered
βꢀlactam mechanophore and provide evidence for the generation
of ketene and imine groups via a cycloelimination mechanism. A
large excess of isobutanol was employed during ultrasonication to
trap the highly reactive ketene in situ according to Scheme 2. To
support these measurements, three wellꢀdefined model comꢀ
pounds were synthesized with the structures displayed in Chart 1.
The structures of the model compounds mimic the chainꢀcentered
βꢀlactam mechanophore (4), the isobutyl ester chainꢀend expected
from the reaction of a ketene with isobutanol (5), as well as the
corresponding imine chainꢀend (6).
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 3. UVꢀVis absorption spectra of polymer 1 before and
after ultrasonication and model compounds 4 and 6 (shaded
curves) measured in THF. The spectra of 1 were acquired from
GPC measurements and scaled independently to illustrate their
correlation with the model compounds.
GPC monitored with a UVꢀVis photodiode array detector
shows the appearance of new absorption peaks between approxiꢀ
mately 250 and 400 nm after ultrasonication of 1 while the asꢀ
synthesized polymer exhibits no absorption at wavelengths > 295
nm (Figure 3). Significantly, the absorption profiles of 1 before
and after ultrasonication match the UVꢀVis absorption spectra of
model compounds 4 and 6, respectively, indicating that the chainꢀ
centered βꢀlactam mechanophore is transformed into an imine
group consistent with the proposed mechanism. The UVꢀVis
spectrum of the chainꢀend control polymer 2 is unchanged after
ultrasonic irradiation, indicating that mechanical forces are indeed
responsible for the observed changes in 1 and precluding alternaꢀ
tive pathways including thermal or photochemical activation (see
Figure S2 in the Supporting Information).
1
Figure 4. H NMR spectra (500 MHz, acetoneꢀd6) of 1 before
(top) and after (middle) ultrasonication for 180 min reveal the
appearance of characteristic peaks consistent with a newly formed
imine endꢀgroup, represented by the NMR spectrum of model
compound 6 (bottom).
In order to confirm the proposed mechanism of βꢀlactam cleavꢀ
age, we conducted 13Cꢀlabeling experiments in conjunction with
NMR spectroscopy to support, in particular, the mechanochemical
generation of a transient ketene group. Following a similar proꢀ
cedure to polymer 1, an analogous βꢀlactam mechanophore was
synthesized using 13C3ꢀlabeled propargyl alcohol followed by
installation of αꢀbromoester initiating groups and polymerization
of methyl acrylate to afford polymer 1* (Mn = 74.3 kDa, PDI =
1.07). Similarly, 13Cꢀlabeled chainꢀend control polymer 2* was
also prepared (Mn = 76.7 kDa, PDI = 1.09). The protonꢀ
decoupled 13C NMR spectrum of 1* after ultrasonic irradiation for
180 min in the presence of isobutanol exhibits the appearance of
three new resonances at 34.3 (dd, J = 58.2, 39.5 Hz), 61.1 (d, J =
Evidence for generation of both imine and ketene groups upon
1
mechanochemical cleavage of 1 is provided by H NMR measꢀ
urements, which reveal the appearance of characteristic resonancꢀ
es corresponding to model compounds 5 and 6 after ultrasoniꢀ
cation. A new singlet around 8.6 ppm representative of the imine
methine proton is particularly diagnostic of this transformation in
addition to other distinctive aromatic resonances (Figure 4). Apꢀ
3
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 4 of 11
39.4 Hz), and 171.2 (d, J = 58.1 Hz) ppm (Figure 5). The splitꢀ
Corresponding Author
1
2
3
4
5
6
7
8
ting observed for these new peaks is fully consistent with the
structure of the proposed isobutyl propionate endꢀgroup and also
with the relevant chemical shifts associated with model compound
5, which appear at 34.5, 60.8, and 171.3 ppm. The 13C NMR
spectrum of control polymer 2* was unchanged after ultrasonic
irradiation under identical conditions (see Figure S5 in the Supꢀ
porting Information).
jsmoore@illinois.edu
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENTS
This work was supported in part by the National Science Foundaꢀ
tion (CHE 1300313). MJR gratefully acknowledges the Arnold
and Mabel Beckman Foundation for a Beckman Institute Postdocꢀ
toral Fellowship.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
REFERENCES
(1) (a) Berkowski, K. L.; Potisek, S. L.; Hickenboth, C. R.; Moore, J. S.
Macromolecules 2005, 38, 8975–8978. (b) Li, J.; Nagamani, C.; Moore, J.
S. Acc. Chem. Res. 2015, DOI: 10.1021/acs.accounts.5b00184.
(2) (a) Piermattei, A.; Karthikeyan, S.; Sijbesma, R. P. Nat. Chem.
2009, 1, 133–137. (b) Diesendruck, C. E.; Steinberg, B. D.; Sugai, N.;
Silberstein, M. N.; Sottos, N. R.; White, S. R.; Braun, P. V.; Moore, J. S.
J. Am. Chem. Soc. 2012, 134, 12446–12449.
(3) Chen, Y.; Spiering, A. J. H.; Karthikeyan, S.; Peters, G. W. M.;
Meijer, E. W.; Sijbesma, R. P. Nat. Chem. 2012, 4, 559–562.
(4) (a) Potisek, S. L.; Davis, D. A.; Sottos, N. R.; White, S. R.; Moore,
J. S. J. Am. Chem. Soc. 2007, 129, 13808–13809. (b) Davis, D. A.; Hamꢀ
ilton, A.; Yang, J.; Cremar, L. D.; Van Gough, D.; Potisek, S. L.; Ong, M.
T.; Braun, P. V.; Martínez, T. J.; White, S. R.; Moore, J. S.; Sottos, N. R.
Nature 2009, 459, 68–72. (c) Imato, K.; Irie, A.; Kosuge, T.; Ohishi, T.;
Nishihara, M.; Takahara, A.; Otsuka, H. Angew. Chem. Int. Ed. 2015, 54,
6168–6172.
(5) Diesendruck, C. E.; Peterson, G. I.; Kulik, H. J.; Kaitz, J. A.; Mar,
B. D.; May, P. A.; White, S. R.; Martínez, T. J.; Boydston, A. J.; Moore, J.
S. Nat. Chem. 2014, 6, 623–628.
(6) Kryger, M. J.; Ong, M. T.; Odom, S. A.; Sottos, N. R.; White, S. R.;
Martinez, T. J.; Moore, J. S. J. Am. Chem. Soc. 2010, 132, 4558–4559.
(7) (a) Black, A. L.; Orlicki, J. A.; Craig, S. L. J. Mater. Chem. 2011,
21, 8460–8465. (b) Ramirez, A. L. B.; Kean, Z. S.; Orlicki, J. A.; Chamꢀ
phekar, M.; Elsakr, S. M.; Krause, W. E.; Craig, S. L. Nat. Chem. 2013,
5, 757–761.
(8) (a) Hickenboth, C. R.; Moore, J. S.; White, S. R.; Sottos, N. R.;
Baudry, J.; Wilson, S. R. Nature 2007, 446, 423–427. (b) RibasꢀArino, J.;
Shiga, M.; Marx, D. Chem. Eur. J. 2009, 15, 13331–13335.
(9) (a) Wang, J.; Kouznetsova, T. B.; Niu, Z.; Ong, M. T.; Klukovich,
H. M.; Rheingold, A. L.; Martinez, T. J.; Craig, S. L. Nat. Chem. 2015, 7,
323–327. (b) Wollenhaupt, M.; Krupička, M.; Marx, D. ChemPhysChem
2015, 16, 1593–1597.
(10) Groote, R.; Szyja, B. M.; Leibfarth, F. A.; Hawker, C. J.;
Doltsinis, N. L.; Sijbesma, R. P. Macromolecules 2014, 47, 1187–1192.
(11) Hyatt, J. A.; Raynolds, P. W. In Organic Reactions; John Wiley
& Sons, 2004; Vol. 45, pp. 159–646.
(12) (a) Leibfarth, F. A.; Kang, M.; Ham, M.; Kim, J.; Campos, L. M.;
Gupta, N.; Moon, B.; Hawker, C. J. Nat. Chem. 2010, 2, 207–212. (b)
Leibfarth, F. A.; Schneider, Y.; Lynd, N. A.; Schultz, A.; Moon, B.; Kraꢀ
mer, E. J.; Bazan, G. C.; Hawker, C. J. J. Am. Chem. Soc. 2010, 132,
14706–14709. (c) Leibfarth, F. A.; Hawker, C. J. J. Polym. Sci. Part A:
Polym. Chem. 2013, 51, 3769–3782.
(13) Wright, S. W. In Name Reactions for Carbocyclic Ring For-
mations; Li, J. J., Ed.; John Wiley & Sons, 2010; pp. 45–69.
(14) (a) Staudinger, H. Liebigs Ann. Chem. 1907, 356, 51–123. (b)
Staudinger, H. Chem. Ber. 1907, 40, 1145–1148.
Figure 5. 13C NMR spectra (acetoneꢀd6) of 13C3ꢀlabeled polymer
1* before (top, 125 MHz) and after (middle, 150 MHz) ultrasoniꢀ
cation for 180 min reveal the appearance of three new resonances
(labeled d, e, f). These new peaks are consistent with the chemiꢀ
cal shifts of model compound 5 (bottom, 125 MHz) and support
the formation of an isobutyl propionate polymer chainꢀend via
reaction of a ketene intermediate with isobutanol. The three carꢀ
bon atoms derived from propargyl alcohol (labeled a,b,c) were
13Cꢀlabeled.
In summary, we demonstrated that the βꢀlactam motif repreꢀ
sents a new mechanophore capable of generating ketene and
imine groups via a mechanicallyꢀfacilitated formal [2+2] cyꢀ
cloelimination reaction—the reverse transformation of the
Staudinger cycloaddition. This finding is supported by compariꢀ
sons of sonicationꢀinduced chain cleavage rates, UVꢀVis absorpꢀ
tion measurements, and endꢀgroup analysis using 1H and 13C
NMR spectroscopy. In addition, these experimental results valiꢀ
date predictions from density functional calculations (CoGEF).
Given the diverse reactivity of ketene groups and their versatility
in polymer and materials science, the βꢀlactam mechanophore has
outstanding potential for a variety of applications including selfꢀ
healing materials capable of autonomic restoration of mechanical
damage.
(15) (a) Larsen, M. B.; Boydston, A. J. J. Am. Chem. Soc. 2013, 135,
8189–8192. (b) Church, D. C.; Peterson, G. I.; Boydston, A. J. ACS Mac-
ro Lett. 2014, 3, 648–651. (c) Li, J.; Shiraki, T.; Hu, B.; Wright, R. A. E.;
Zhao, B.; Moore, J. S. J. Am. Chem. Soc. 2014, 136, 15925–15928.
(16) (a) Cravotto, G.; Cintas, P. Angew. Chem. Int. Ed. 2007, 46,
5476–5478. (b) Kryger, M. J.; Munaretto, A. M.; Moore, J. S. J. Am.
Chem. Soc. 2011, 133, 18992–18998. (c) Kean, Z. S.; Black Ramirez, A.
L.; Yan, Y.; Craig, S. L. J. Am. Chem. Soc. 2012, 134, 12939–12942. (d)
Kean, Z. S.; Niu, Z.; Hewage, G. B.; Rheingold, A. L.; Craig, S. L. J. Am.
Chem. Soc. 2013, 135, 13598–13604. (e) Kean, Z. S.; Gossweiler, G. R.;
Kouznetsova, T. B.; Hewage, G. B.; Craig, S. L. Chem. Commun. 2015,
51, 9157–9160.
ASSOCIATED CONTENT
Supporting Information
Experimental details, synthetic procedures, NMR and UVꢀvis
spectra, and GPC chromatograms. This material is available free
of charge via the Internet at http://pubs.acs.org.
(17) Beyer, M. K. J. Chem. Phys. 2000, 112, 7307–7312.
AUTHOR INFORMATION
4
ACS Paragon Plus Environment

Page 5 of 11
Journal of the American Chemical Society
For Table of Contents Use Only
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
5
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 6 of 11
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
83x74mm (300 x 300 DPI)
ACS Paragon Plus Environment

Page 7 of 11
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
67x54mm (300 x 300 DPI)
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 11
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
83x71mm (300 x 300 DPI)
ACS Paragon Plus Environment

Page 9 of 11
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
92x101mm (300 x 300 DPI)
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 10 of 11
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
86x87mm (300 x 300 DPI)
ACS Paragon Plus Environment

Page 11 of 11
Journal of the American Chemical Society
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
82x44mm (300 x 300 DPI)
ACS Paragon Plus Environment