Tunable differentiation of tertiary C-H bonds in intramolecular transition metal-catalyzed nitrene transfer reactions

Article abstract of DOI:10.1039/c7cc01235g

Metal-catalyzed nitrene transfer reactions are an appealing and efficient strategy for accessing tetrasubstituted amines through the direct amination of tertiary C-H bonds. Traditional catalysts for these reactions rely on substrate control to achieve site-selectivity in the C-H amination event; thus, tunability is challenging when competing C-H bonds have similar steric or electronic features. One consequence of this fact is that the impact of catalyst identity on the selectivity in the competitive amination of tertiary C-H bonds has not been well-explored, despite the potential for progress towards predictable and catalyst-controlled C-N bond formation. In this communication, we report investigations into tunable and site-selective nitrene transfers between tertiary C(sp³)-H bonds using a combination of transition metal catalysts, including complexes based on Ag, Mn, Rh and Ru. Particularly striking was the ability to reverse the selectivity of nitrene transfer by a simple change in the identity of the N-donor ligand supporting the Ag(i) complex. The combination of our Ag(i) catalysts with known Rh₂(ii) complexes expands the scope of successful catalyst-controlled intramolecular nitrene transfer and represents a promising springboard for the future development of intermolecular C-H N-group transfer methods.

Full text of DOI:10.1039/c7cc01235g

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
first methanofullerene derivatives of Sc N@D5h-C80
=
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Tunable Differentiation of Tertiary C–H Bonds in Intramolecular
Transition Metal-Catalyzed Nitrene Transfer Reactions
Joshua R. Corbina and Jennifer. M. Schomakerb
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Metal-catalyzed nitrene transfer reactions are an appealing and between C-H bonds in different steric or electronic
efficient strategy for accessing tetrasubstituted amines through environments can be achieved by exchanging the identity of
the direct amination of tertiary C-H bonds. Traditional catalysts the transition metal; for example, Mn catalysts favour reaction
for these reactions rely on substrate control to achieve site- at benzylic sites, while Rh2Ln catalysts favour a 3° methine
selectivity in the C-H amination event; thus, tunability is (Figure 1B).2h,7a In this work, we expand the utility of metal-
challenging when competing C-H bonds have similar steric or catalyzed nitrene transfer to encompass tunable amination of
electronic features. One consequence of this fact is that the competing 3° C-H bonds in similar steric and/or electronic
impact of catalyst identity on the selectivity in the competititive environments using a combination of ligand-controlled Ag(I)
amination of tertiary C–H bonds has not been well-explored, catalysis and scaffolds based on other metals (Figure 1C). A
despite the potential for progress towards predictable and better understanding of how interactions between substrate
catalyst-controlled C-N bond formation. In this communication, and catalyst selectively differentiate 3° C–H bonds enables
we report investigations into tunable and site-selective nitrene development of 2nd-generation catalysts that install a C-N
transfers between tertiary C(sp3)–H bonds using a combination of bond at a desired site in a complex molecule setting.
transition metal catalysts, including complexes based on Ag, Mn,
A Tunable chemoselectivity
Rh and Ru. Particularly striking was the ability to reverse the
O
selectivity of nitrene transfer by a simple change in the identity of
the N-donor ligand supporting the Ag(I) complex. The combination
of our Ag(I) catalysts with known Rh2(II) complexes expands the
scope of successful catalyst-controlled intramolecular nitrene
transfer and represents a promising springboard for the future
development of intermolecular C–H N-group transfer methods.
O
O
O
X
X
X
( )n
N
O
R
NH
O
NH2
( )n
R
( )n
R
H
X = C, SO
AgL2OTf
Ru, Mn, Fe, Co cat.
AgLOTf
Rh2Ln
B Tunable selectivitybetween electronically/stericallydifferent C-H bonds
Ag(L2)OTf
Ag(L1)OTf
O
O
O
O
O
O
O
S
NH2 Rh2L4
H
MnL
R
Transition metal-catalyzed nitrene transfer is a process
utilizing transient hyperelectrophilic metal-bound nitrenes for
directly converting ubiquitous and classically inert C–H bonds
into valuable C–N bonds. Achieving catalyst control using these
reactive intermediates has been a major goal of research in
homogenous transition-metal catalysis.1-9 In the presence of
multiple competitive reactive sites, such as the situation
presented in a substrate containing an alkene and an allylic C-
H bond, a variety of transition metal catalysts have been
shown to selectively form either aziridine or amine products
by exploiting differences in mechanism, substrate bias or
ligand control (Figure 1A).9a-b Orthogonal site-selectivity
S
S
HN
O
R
O
NH
H
R
activatedC-H
electron-rich C-H
R = Ar, vinyl
C This work: Tunable selectivitybetween similar 3o alkyl C(sp3)-H bonds
O
O
NH2
S
H
(Me4phen)AgOTf
[(Py5Me2)AgOTf]2
O
R2
R1
O
O
O
O
O
H
S
HN
O
R2
S
R1, R2 = alkyl
NH
R2
R1
or
R1
R1= aryl, R2= alkyl, aryl
Fig 1. Catalyst control in intramolecular metal-nitrene transfer.
Examples of differentiating between aminations of competing
3° C–H bonds are scarce and no tunable examples have been
reported.2h,7a,9b The Du Bois group has shown that the treatment of
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1 with either Rh2(OAc)4 (1iPr:1cHex 2.0:1)2h or Rh2(TPA)4 5 (Table 1,
The reasons for the tunability between the iPr and cHex C-H
entry 10) favours reaction at the 3° iPr over the 3° cHex C-H bond, bonds of 1 were not readily apparent. [Mn(tBuPc)]Cl 6 and
presumably due to greater ꢀꢁ–ꢂ → ꢀ∗ at iPr, rendering that site Ru2(hp)4Cl 7 promote step-wise nitrene transfer pathways and also
ꢁ–ꢂ
favour amination of the 3° iPr C–H bond of 1 (entries 11-12).3a This
more electron-rich. Selectivity for 1iPr increases from 2.0:1 to 2.4:1
is intriguing, as Rh2(II) catalysts engage in concerted intramolecular
to 6:1 in moving from OAc to the bulky and less electron-rich
nitrene transfer, yet show the same overall site-selectivity as 6-7.
triphenylacetate (TPA) ligand for the Rh2(II) catalyst.
The mechanism of Ag(I)-catalyzed nitrene transfer is necessarily
Investigations of a series of our Ag(I) catalysts (Table 1, entries
step-wise, due to the lack of a vacant π* orbital on the N of the
1-8) yielded interesting and unexpected results. A control reaction
metal nitrene.9d However, we and others have shown that the
employing AgOTf and no ligand gave <3% of the aminated products
nature of the ligand on Ag can impact whether the amination
in a ~1:1 ratio of 1cHex:1iPr. More electron-rich 2,2’-bipyridine (bipy)
proceeds through HAT involving discrete radical intermediates, or
ligands (compare entries 2-3 with entry 1) gave better selectivity
undergoes a barrierless radical rebound that results in 'concerted-
like' behaviour in the nitrene transfer.8-9 This is reflected in the very
and higher yields for 1cHex. More rigid 1,10-phenanthroline-based
(phen) ligand scaffolds (entries 4-6), especially the electron-rich
different selectivities between catalysts 2 and 3. Thus, rationalizing
Me4phen (entry 6), also exhibited selectivity for 1cHex
.
subtle differences in site-selectivity by invoking a broad picture of
the mechanism of nitrene transfer cannot be applied in the same
way as previous reports of chemoselective nitrene transfer (Figure
1A). Rather, subtle interactions between ligand architecture,
substrate and catalyst electronics all play important, but poorly
understood, roles in fine-tuning the steric and electronic
environment of the metal-nitrene.
To gain further insight into the factors most important in
influencing selectivity of nitrene transfer in these types of systems,
substrates 8 and 9 were explored (Table 2) with catalysts 2-7.
Despite the observation that tunability could be achieved using
ligand-controlled Ag(I) catalysis or by switching the metal identity,
no trends in the selectivity were immediately apparent, requiring a
more careful analysis of the interplay between the features of the
substrate (electron density, C-H bond dissociation energy (BDE) and
A-value of the alkyl group) and the catalyst (KIE, mechanism of
nitrene transfer).
Table 2. Selectivity in the amination of carbocyclic vs. acyclic C-H bonds. aSee
the Supplementary Information for detailed reaction conditions using
catalysts 2-7. bNMR yield, mesitylene internal standard. The reactions were
run to complete conversion of the substrate.
Table 1. Differentiation between similar tertiary C(sp3)-H bonds. aNMR yield,
mesitylene internal standard. bSee the SI for conditions using catalysts 4-7.
The reactions were run to complete conversion of the substrate 1.
Catalysts with reported KIE values in the range of ~0-4, such as the
dinuclear Rh2(II) paddlewheel catalysts 4-5, are thought to proceed
through concerted nitrene transfer and favor the most electron-rich
C–H bond (Figure 2A, reactivity trend: 3° > ethereal ≈ benzylic > 2°
≫ 1°).2 Catalysts with KIEs > 4, as represented by 6 (KIE = 4.2) and 7
(KIE = 4.9), favour reaction at the weakest C–H bond (Figure 2A,
Switching to a tris(2-pyridylmethyl)amine (tpa) ligand (entry 7)
resulted in a modest switch in site-selectivity to favour 1iPr
. A
dimeric Ag catalyst based on 3 (entry 8) further improved selectivity
for 1iPr, accomplishing tunable, catalyst-controlled nitrene transfer
using a single metal (compare entries 6 and 8).
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reactivity trend: allylic > benzylic > ethereal > 3° > 2° ≫ 1°) due to Ag(Me4phen)OTf 2 (Table 1, entry 6 and Table 2, entries 1 and 7)
the preference for a step-wise nitrene transfer pathway involving increasingly favoured amination of the Cy C-H as the carbocycle
radical intermediates with significant lifetimes. 3a,4c,7a The two 3° C- increased in size from cPent to cHept, perhaps due to the fluxional
H bonds of 1, 8 and 9 have essentially the same BDEs, but the nature of cHept and the greater rate of pseudo-equatorial C-H bond
isopropyl C-H appears to be more electron-rich than the Cy C-H oxidation by strain release during the amination event.1
bonds, according to comparisons of the 13C chemical shifts (Figure
Previous work in intramolecular nitrene transfer competition
2B). An additional factor to consider is the conformational flexibility experiments generally attributes the preference for the amination
of the Cy ring; while cHex is likely to favour a distinct chair of benzylic methylene C-H over 3° alkyl C(sp3)-H bonds to lower
conformation, the flexibility of cPent and cHept may make the cyclic BDEs and stereoelectronic stabilization of the amination transition
C-H bond more difficult to differentiate from an acyclic C-H bond.
state (concerted or stepwise) by relatively large ꢃꢁꢄꢁ → ꢀꢁ∗–ꢂ
interactions.1,3a,4c,7a,9c We were curious how a more electronically
activated benzylic 3° C-H bond in 10-15 might compete with less
activated, but more electron-rich, 3° alkyl C(sp3)-H bonds (Table 3).
We explored three catalysts, 2, 3 and 5, that display very different
reactivity profiles as determined by our results (Tables 1-2.
O
O
O
O
S
S
HBn
HN
R1
O
H2N
O
HBn
R2
Fig 2. Traditional substrate control explanations of site-selectivity fail when
similar logic is applied to tertiary C(sp3)-H differentiation.
catalyst
oxidant
Halk
R1
R1
10a-15a
R2
R1
R3
R3
10-15
+ Bn C-H insertion 10b-15b
If we consider the selectivity exhibited for 1 using 2-7, it is
apparent that Rh2(II) complexes 4-5 favour the more electron-rich
iPr C-H, as expected. Concerted C-H insertion reactions catalysed by
Rh2(II) complexes are proposed to proceed through triangular,
three-membered transition states that show a steric preference for
approaching an equatorial C-H over one in the axial position in the
cHex ring.1-2 In addition, equatorial C-H bonds are more deshielded
than axial ones due to anisotropic effects, an effect that further
exacerbates the tendency for amination at the iPr C-H (Table 1,
entries 9-10). In contrast, the conformational flexibility of the cPent
and cHept groups in 8-9 may lower the site-selectivity, although
Rh2(TPA)4 5 (Table 2, entries 4 and 10) does display a greater
sensitivity to electronics as compared to Rh2(esp)2 4 (entries 3 and
9). This may be due to the relatively electron-poor TPA carboxylate
resulting in a Rh-nitrene with increased electrophilicity.
R1, R2, R3
R1, R2, R3
R1, R2, R3
-(CH2)5-, Me, H 10
-(CH2)5-, Ph, H 12
Me, Me, OMe 14
-(CH2)5-, Me, OMe 11
Me, Me, H
13
Me, Ph, H
15
Alk:Bn
(a:b)
Alk:Bn
(a:b)
substrate cata yieldb
substrate cata yieldb
entry
entry
1
2
3
4
5
6
10
11
12
2
2
2
95% 3.8 : 1
90% 2.2 : 1
83% 1.2 : 1
10
13
14
15
2
2
2
89% 1 : 1.3
73% 1 : 2.6
89% 1 : 4.2
11
12
13
14
15
16
17
10
3
84% 1 : 2.2
13
14
15
3
3
3
91% 1.0 : 1
90% 1 : 1.4
83% 1 : 2.7
11
12
10
3
3
5
89% 1 : 3.4
81% 1 : 8.4
93% 1 : 1.1
92% 1 : 1.8
70% >19 : 1
7
8
9
13
15
5
5
94%
8.3 : 1
11
12
5
5
93% >19 : 1
Table 3. Competitive amination of activated C-H vs. 3° C(sp3)-H bonds. See
the SI for conditions using catalysts 2-5. bNMR yield, mesitylene internal
standard. The reactions were run to complete conversion of the substrate.
a
In contrast to catalysts that promote concerted nitrene transfer,
stepwise reactions proceed through H-atom transfer (HAT)
transition states that lead to radical intermediates displaying a
linear NꢀꢀꢀHꢀꢀꢀC geometry, resulting from three-center/three-
electron or three-center/four-electron bonds.1,9d As shown in Figure
2A, these catalysts rely in differences in BDE to enable predictable
preference for one C-H bond over another. Indeed, catalysts 6-7
showed poor selectivity in differentiating conformationally fluxional
tertiary C-H bonds in 8-9 (Table 2, entries 5-6 and 11-12). Use of 6
resulted in the more electron-rich carbon radical leading to 8iPr
(entry 11), but this is not always a selectivity-determining feature,
as 7 led to formation of 9Cy (entry 12). Using the trends in Rh, Ru
and Mn catalysis to understand the behaviour of Ag catalysts 2-3
was instructive. It is worth noting that 3 (Table 1, entry 3 and Table
2, entries 2 and 8) displays similar behaviour to 6-7, implying that
this Ag(I) complex proceeds through a nitrene transfer involving
HAT and formation of long-lived radical intermediates.3a,7a Indeed,
the KIE for 3 was 5.7, in line with values obtained for 6 (4.2) and 7
(4.9).3a,4c,7a The most intriguing results were the observation that
Substrates 10-12 were designed to compare the activation of a
conformationally accessible cHex C-H bond with a series of different
benzylic methane C-H bonds of decreasing BDE in moving from 10
to 12 (Table 3, entries 1-9). The substrates were utilized as ~1:1
mixtures of diastereomers; however, evidence of tunable selectivity
in the amination can still be assessed. Further studies to determine
how the stereochemistry impacts relative rates of C-H amination
are underway and will be reported in due course. The preference of
Ag(Me4phen)OTf 2 for the most conformationally accessible cHex C-
H bond (entries 1-3) decreased as the Bn C-H bond became weaker.
AgOTf supported by Py5Me2 in 3 (entries 4-6) showed an increase in
selectivity for the Bn C-H bond from 2.2:1 to 8.4:1 as the BDE was
decreased, a trend that fits our mechanistic picture of step-wise
nitrene transfer promoted by 3.9c Finally, Rh2(TPA)4 5 (entries 7-9)
responded to decreased electron density at the C-H bond by heavily
favouring the cHex C-H bond in 12 (entry 9). Somewhat surprising
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Zhang, X. P. Chem. Soc. Rev. 2011, 40, 1899. (h) Collet, F.;
Dodd, R. H.; Dauban, P. Chem. Commun. 2009, 34, 5061.
Selected references on Rh-catalyzed nitrene transfer: (a)
was the lack of selectivity in 10 with 5 (entry 7), reflecting either
similar electron density around the two 3° C-H bonds or a steric
component to the site-selectivity.
2
Espino, C. G.; Du Bois, J. Angew. Chem., Int. Ed. 2001, 40
,
The effect of the conformational accessibility of a cHex C–H
bond was removed in 13-15, where amination of a series of 3°
benzylic methine C-H bonds were compared to an iPr C-H bond
(Table 3, entries 10-17). Ag(Me4phen)OTf 2 showed increased
selectivity for the Bn C-H bond in moving from 13 to 15, rationalized
by adoption of a preferred conformation about the hindered and
activated Bn C–H to maximize π-C-H* interactions. This suggests 2
favours kinetically accessible C-H bonds to a greater extent than the
other catalysts tested in Table 3, an observation also made using 2
(Table 1). Catalyst 3 displayed less selectivity for Bn in 13-15
(entries 13-15) as compared to 10-12 (entries 5-7), suggesting 3
may be more sensitive to the electronics of the C-H bond than the
kinetic accessibility. Finally, Rh2(TPA)4 5 (entries 16-17) responded
to the increased electron density at the iPr C-H.
598. (b) Espino, C. G.; Fiori, K. W.; Kim, M.; Du Bois, J. J. Am.
Chem. Soc. 2004, 126, 15378. (c) Zalatan, D. N.; Du Bois, J. J.
Am. Chem. Soc. 2008, 130, 9220. (d) Liang, C.; Robert-
Peillard, F.; Fruit, C.; Müller, P.; Dodd, R. H.; Dauban, P.
Angew. Chem., Int. Ed. 2006, 45, 4641.(e) Fiori, K. W.; Du
Bois, J. J. Am. Chem. Soc. 2007, 129, 562. (f) Lebel, H.; Spitz,
C.; Leogane, O.; Trudel, C.; Parmentier, M. Org. Lett. 2011,
13, 5460. (g) Breslow, R.; Gellman, S. H. J. Am. Chem. Soc.
1983, 105, 6728. (h) Fiori, K. W.; Espino, C. G.; Brodsky, B. H.;
Du Bois, J. Tetrahedron 2009, 65, 3042.
Aelected references on Ru-catalyzed nitrene transfer: (a)
Harvey, M. E.; Musaev, D. J.; Du Bois, J. J. Am. Chem. Soc.
2011, 133, 17207. (b) Milczek, E.; Boudet, N.; Blakey, S.
Angew. Chem., Int. Ed. 2008, 47, 6825.
Examples of Fe-catalyzed nitrene transfer: (a) Nakanishi, M.;
Salit, A.; Bolm, C. Adv. Synth. Catal. 2008, 350, 1835. (b)
3
4
Paradine, S. M.; White, M. C. J. Am. Chem. Soc. 2012, 134
,
2036. (c) Cramer, S. A.; Jenkins, D. M. J. Am. Chem. Soc.
2011, 133, 19342. (d) Hennessy, E. T.; Liu, R. Y.; Iovan, D. A.;
Duncan, R. A.; Betley, T. A. Chem. Sci. 2014, 5, 1526.
Selected examples of Co-catalyzed nitrene transfer: (a) Lu, H.
J.; Subbarayan, V.; Tao, J. R.; Zhang, X. P. Organometallics
2010, 29, 389. (b) Lu, H.-J.; Jiang, H.-L.; Hu, Y.; Wojtas, L.;
Zhang, X. P. Org. Lett. 2012, 14, 5158. (c) Lu, H.-J.; Jiang, H.-
It is worth noting that both Ag(I) catalysts 2 and 3 favor
amination of the diphenyl methine C-H over the iPr C-H bond of 15,
even though it is more encumbered (A-value of Ph = 3 vs. Me = 1.7).
It appears increasing the stereoelectronic bias against an acyclic 3°
alkyl C–H bond improves the selectivity of Ag(I) catalysts for the
activated C–H bond, due to the inherent stepwise nature of nitrene
transfer in all Ag(I) catalysts. Thus, Ag(I) complexes are promising
scaffolds for developing catalysts that select for any 'activated' 3° C-
H bond over a typical 3° alkyl C(sp3)-H bond. Although Rh2(II)
catalysts rely heavily on substrate control, they often favour 3° alkyl
C(sp3)-H bonds, leading to our demonstration that catalyst-
controlled tunability between 3° C-H bonds (Table 3, entries 6 vs. 9
and 12 vs. 17) can be achieved.
5
L.; Hu, Y.; Wojtas, L.; Zhang, X. P. Chem. Sci. 2011, 2, 2361.
(d) Lu, H.-J.; Jiang, H.-L.; Wojtas, L.; Zhang, X. P. Angew.
Chem. Int. Ed. 2010, 49, 10192. (e) Lu, H.-J.; Li, C.-Q.; Jiang,
H.-L.; Lizardi, C. L.; Zhang, X. P. Angew. Chem. Int. Ed. 2014,
53, 7028.
Selected examples of Cu-catalyzed nitrene transfer: (a)
Duran, F.; Leman, L.; Ghini, A.; Burton, G.; Dauban, P.; Dodd,
6
R. H. Org. Lett. 2002,
Parmentier, M. Org. Lett. 2007,
Saniere, L.; Tarrade, A.; Dodd, R. H. J. Am. Chem. Soc. 2001,
4, 2481. (b) Lebel, H.; Lectard, S.;
9
, 4797. (c) Dauban, P.;
̀
123, 7707. (d) Srivastava, R. S.; Tarver, N. R.; Nicholas, K. M.
J. Am. Chem. Soc. 2007, 129, 15250. (e) Barman, D. N.;
Nicholas, K. M. Eur. J. Org. Chem. 2011, 2011, 908. (f) Bagchi,
V.; Paraskevopoulou, P.; Das, P.; Chi, L.; Wang, Q.;
Choudhury, A.; Mathieson, J. S.; Cronin, L.; Pardue, D. B.;
Cundari, T. R.; Mitrikas, G.; Sanakis, Y.; Stavropoulos, P. J.
Am. Chem. Soc. 2014, 136, 11362.
Selected examples of Mn-catalyzed nitrene transfer: (a)
Paradine, S. M.; Griffin, J. R.; Zhao, J.; Petronico, A. L.; Miller,
S. M.; White, M. C. Nat. Chem. 2015, 7, 987−994. (b) Liang,
S.; Jensen, M. P. Organometallics. 2012, 31, 8055. (c) Abu-
Omar, M. M. Dalton Trans. 2011, 40, 3435−44. (d) Lai, T.-S.;
In conclusion, we report the first examples of site-selective
C–H aminations of substrates containing reactive 3° C-H bonds
in similar steric and electronic environments. In many cases,
tunability was achieved solely by changing the nature of an N-
donor ligand for the Ag(I) complex; however, in challenging
substrates, the orthogonal reactive site was favoured using
Rh2(TPA)4. Though the underlying mechanistic rationale for
differentiating similar C–H bonds is not yet well-understood,
achieving this level of catalyst control is promising for future
development of site-selective and tunable intermolecular
metal-nitrene transfer; a long-standing challenge in the field.
JMS thanks NSF-CAREER 1254397. The NMR facilities at UW-
Madison are funded by the NSF (CHE-9208463, CHE-9629688)
and NIH (RR08389-01). The National Magnetic Resonance
Facility at Madison is supported by the NIH (P41GM103399,
S10RR08438, S10RR029220) and the NSF (BIR-0214394).
7
8
Kwong, H.-L.; Che, C.-M.; Peng, S. M. Chem. Commun. 1997
2373.
,
Selected examples of Ag-catalyzed nitrene transfer: (a) Diaz-
Requejo, M. M.; Perez, P. J. Chem. Rev. 2008, 108, 3379. (b)
Cui, Y.; He, C. Angew. Chem., Int. Ed. 2004, 43, 4210. (c) Li, Z.;
Capretto, D. A.; Rahaman, R.; He, C. Angew. Chem., Int. Ed.
2007, 46, 5184. (d) Fructos, M. R.; Trofimenko, S.; Diaz-
Requejo, M. M.; Perez, P. J. J. Am. Chem. Soc. 2006, 128
,
11784. (e) Gomez-Emeterio, B. P.; Urbano, J.; Diaz-Requejo,
M. M.; Perez, P. J. Organometallics 2008, 27, 4126.
Notes and references
9
(a) Rigoli, J. W.; Weatherly, C. D.; Alderson, J. M.; Vo, B. T.;
Schomaker, J. M. J. Am. Chem. Soc. 2013, 135, 17238. (b)
Alderson, J. M.; Phelps, A. M.; Scamp, R. J.; Dolan, N. S.;
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