Facile synthesis of dibenzo-7λ3-phosphanorbornadiene derivatives using magnesium anthracene

Article abstract of DOI:10.1021/ja306902j

Unprotected dibenzo-7λ³-phosphanorbornadiene derivatives RPA (A = C₁₄H₁₀ or anthracene; R = ^tBu, dbabh = NA, HMDS = (Me₃Si)₂N, ⁱPr₂N) are synthesized by treatment of the corresponding phosphorus dichloride RPCl ₂ with MgA·3THF, in cold THF (～20% to 30% isolated yields). Anthracene and the corresponding cyclic phosphane (RP)_n form as coproducts. Characteristic NMR features of the RPA derivatives include a doublet near 4 ppm in their ¹H NMR spectra and a triplet peak in the 175-212 ppm region of the ³¹P NMR spectrum (²J _PH ～14 Hz). The X-ray structures of the AN-PA and (HMDS)PA derivatives are discussed. Thermolysis of RPA benzene-d₆ solutions leads to anthracene extrusion. This process has a unimolecular kinetic profile for the ⁱPr₂NPA derivative. The 7-phosphanorbornene anti-ⁱPr₂NP(C₆H₈) could be synthesized (70% isolated yield) by thermolysis of ⁱPr₂NPA in 1,3-cyclohexadiene.

Full text of DOI:10.1021/ja306902j

Communication
pubs.acs.org/JACS
Facile Synthesis of Dibenzo-7λ3‑phosphanorbornadiene Derivatives
Using Magnesium Anthracene
Alexandra Velian and Christopher C. Cummins*
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139,
United States
S
* Supporting Information
t
= Bu, dbabh (2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-
diene), HMDS ((Me3Si)2N), Pr2N; see Scheme 1). In the
ABSTRACT: Unprotected dibenzo-7λ3-phosphanorbor-
i
nadiene derivatives RPA (A = C14H10 or anthracene; R
t
i
= Bu, dbabh = NA, HMDS = (Me3Si)2N, Pr2N) are
synthesized by treatment of the corresponding phosphorus
dichloride RPCl2 with MgA·3THF, in cold THF (∼20% to
30% isolated yields). Anthracene and the corresponding
cyclic phosphane (RP)n form as coproducts. Characteristic
NMR features of the RPA derivatives include a doublet
near 4 ppm in their 1H NMR spectra and a triplet peak in
the 175−212 ppm region of the 31P NMR spectrum (2JPH
∼14 Hz). The X-ray structures of the AN−PA and
(HMDS)PA derivatives are discussed. Thermolysis of RPA
benzene-d6 solutions leads to anthracene extrusion. This
Scheme 1. General Procedure to Synthesize Unprotected
t
7λ3-Phosphanorbornadiene Derivatives RPA (R = Bu,
dbabh, HMDS, iPr2N) in ∼20% to 30% Isolated Yields from
the Corresponding Phosphorus Dichloride RPCl2 and
a
MgA·3THF
i
process has a unimolecular kinetic profile for the Pr2NPA
derivative. The 7-phosphanorbornene anti-iPr2NP(C6H8)
a
Anthracene and the corresponding cyclic phosphane (RP)n are
could be synthesized (70% isolated yield) by thermolysis
i
formed as coproducts in this reaction.
of Pr2NPA in 1,3-cyclohexadiene.
context of the previously reported cumbersome, multistep
syntheses of protected 7-(heteroatom)norbornadienes, the one-
step procedure illustrated in Scheme 1 is a model of efficiency
despite the fact that competing formation of (RP)n cyclic
oligomers obviates achieving a high yield; we note that some
prior discussion has been devoted to the reduction of aryl-
dichlorophosphines by magnesium including mechanistic
proposals involving phosphinidene intermediates.9
Why was the title reaction not already known? A thorough
search of the chemical literature for clues or precedent revealed
several noteworthy reports of analogous syntheses of group 14
7-(heteroatom)norbornadienes, themselves widely studied as
sources of heavy carbene analogues upon thermolysis.10−13 In
1967 Ramsden, the first to introduce magnesium anthracene as
a reducing agent, reported the direct synthesis of Bu2SnA from
MgA and the corresponding tin dichloride, Bu2SnCl2,14,15 and
also made claims on similar syntheses of the other group 14 (Si
to Pb) dibenzo-7-(heteroatom)norbornadiene derivatives.16
Almost a decade later, Smith and Pounds independently
reported the direct synthesis of (Ph2Si)2A from a mixture of
Ph2SiCl2, Mg, and anthracene,17 and yet another decade later
Appler et al. reported the synthesis of another dibenzo-7-
silanorbornadiene derivative from Li2A and the corresponding
silyl dichloride.11 These early organometallic syntheses of
dibenzo-7-(heteroatom)norbornadiene architectures were later
erein we report a facile synthesis of previously elusive,
3
Hunprotected dibenzo-7λ -phosphanorbornadiene deriva-
tives. Long attractive as synthetic targets, 7-phosphanorborna-
dienes are especially interesting for their potential to serve as
phosphinidene precursors under mild conditions via concerted
loss of an aromatic moiety.1 The early view that “it was quite
obvious that the only reasonable way to prepare 7-
phosphanorbornadienes remained the [2 + 4] cycloadditions
between electrophilic acetylenic dienophiles and phospholes”1
has been unchallenged and not fruitful in terms of delivering
valuable derivatives that bear an unprotected lone pair of
electrons at phosphorus. This status quo fostered a prevailing
sensibility that the lone electron pair at phosphorus and the
strained CPC angle impart inherent instability to the 7-
phosphanorbornadiene architecture.1,2 Protection of the
phosphorus lone pair by either metal complexation3−6 or by
utilizing the corresponding phosphine oxide7,8 allowed the
isolation of stabilized derivatives. In 2000, the first unprotected
7λ3-phosphanorbornadiene derivative was generated (by
decomplexation of a protected 7-phosphanorbornadiene) as a
species not sufficiently stable for isolation in which the
cheletropic loss of the aromatic moiety was disfavored by
incorporation of a highly strained metacyclophane subunit.2
Against the foregoing backdrop, we were surprised to find
that uncoordinated 7λ3-phosphanorbornadiene derivatives can
be synthesized directly from the reaction of MgA·3THF (A =
C14H10 or anthracene) with phosphorus dichlorides, RPCl2 (R
Received: July 18, 2012
Published: August 15, 2012
© 2012 American Chemical Society
13978
dx.doi.org/10.1021/ja306902j | J. Am. Chem. Soc. 2012, 134, 13978−13981

Journal of the American Chemical Society
Communication
overshadowed by the more popular [2 + 4] cycloaddition
synthetic routes3,4,10,11,18−20 and were seemingly forgotten in
the annals of the burgeoning chemical literature.
While exploring chemical reduction of the new dichlor-
ophosphine dbabhPCl2, we found serendipitously that treat-
ment of a cold THF solution of dbabhPCl2 with solid
MgA·3THF led to formation of AN−PA and the cyclic
phosphine [(dbabh)P]4, in an ∼2:1 ratio (based on P),
together with anthracene. The reduction of dichlorophosphines
to cyclophosphines of different ring sizes has been well
documented, and was not surprising to us.22 Positive evidence
of AN−PA formation was signaled in the 1H NMR spectrum of
the crude mixture by a characteristic doublet (2JPH = 14.9 Hz)
at 3.89 ppm. Upon consumption of the bright orange
MgA·3THF, all volatile materials were removed in vacuo from
the reaction mixture, and the residue was extracted with the
minimum amount of n-hexane. The resulting slurry was filtered
through a short charcoal plug, and from the colorless filtrate
AN−PA was selectively crystallized and isolated in 22% yield.
Eager to explore the scope of this synthetic route, we turned
to less exotic dichlorophosphine reagents. Treatment of
(HMDS)PCl2 with MgA·3THF in THF produced an ∼1:1.4
ratio (based on P) of (HMDS)PA and the new cyclo-
triphosphane [(HMDS)P]3, along with anthracene. (HMDS)-
PA was isolated in 20% yield as a white powder. Interestingly,
when the reaction was carried out in diethyl ether only
Figure 1. Solid-state molecular structure of AN−PA (left) and
(HMDS)PA (right) with ellipsoids at the 50% probability level and
rendered using PLATON.21 Hydrogen atoms are omitted for clarity.
Selected interatomic distances (Å) and angles (deg): (a) for AN−PA
(left) P1−N1 1.691(1) P1−C213 1.927(1), P1−C214 1.906(1),
C113−N1−P1 131.5(1), C114−N1−P1 118.6(1), C113−N1−C114
95.7(1), N1−P1−C214 105.95(6), N1−P1−C213 112.15(6), C214−
P1−C213 79.10(6); (b) for (HMDS)PA (right): P1−N1 1.726(1),
P1−C107 1.917(1), P1−C108 1.912(1), Si1−N1−P1 107.74(6), Si1−
N1−Si2 121.44(7), Si1−N1−P1 129.28(7), N1−P1−C107 107.52(6),
N1−P1−C108 112.75(6), C107−P1−C108 78.97(6).
PA and (HMDS)PA is slightly shorter than 1.82 Å, the sum of
the covalent single-bond radii of the two elements.24 In accord
with Bent’s rule,25 the nitrogen atom is slightly pyramidalized in
AN−PA, with the sum of the angles around N being 345.8(3)°.
In (HMDS)PA, where the electropositive silyl substituents are
more effective at maximizing the involvement of the nitrogen 2s
orbital in the N−Si interactions, the nitrogen atom is practically
planar.
i
[(HMDS)P]3 was formed. Similarly, treatment of Pr2NPCl2
with MgA·3THF in THF produced a mixture of the previously
23
i
characterized cyclic phosphane (iPr2NP)4 and Pr2NPA. We
i
have not yet been able to obtain pure samples of Pr2NPA, as
the anthracene contaminant has proven difficult to separate in
the case of this derivative. Reduction of tBuPCl2 with
MgA·3THF in THF also produced a mixture of the previously
characterized cyclic phosphanes (tBuP)n (n = 3,4) together with
When benzene-d6 solutions of the RPA derivatives were
heated, the RPA species were found to undergo thermal
extrusion of anthracene. Potentially susceptible to loss of 2
equiv of anthracene, the AN−PA derivative converted upon
thermolysis to anthracene as the only soluble species (52%
yield) as measured by 1H NMR spectroscopy and referenced to
tBuCN as an external standard. Also formed in the reaction was
a pale yellow solid that was not soluble in common organic
solvents; characterization of this material is in progress.
t
the desired BuPA, which could be isolated in 20% yield.
Carried out under similar conditions, the reduction of the
aromatic dichlorophosphines PhPCl2 and MesPX2 (X = Cl, Br;
Mes = mesityl) with MgA·3THF led only to the formation of
cyclic phosphane oligomers.
The bridgehead protons in the RPA derivatives are
1
t
characterized by a doublet near 4 ppm in their H NMR
Thermolysis of benzene solutions of (HMDS)PA, BuPA, and
2
iPr2NPA led to formation of anthracene, together with the
corresponding cyclic phosphanes [(HMDS)P]3, (iPr2NP)4, and
(tBuP)n.
spectra, with large JPH coupling constants to phosphorus of
∼14 Hz. Strongly deshielded, the phosphorus bridge gives rise
to a triplet peak in the 175−212 ppm region of the 31P NMR
spectrum, 63 to 100 ppm downfield with respect to the value
previously reported for a 7λ3-phosphanorbornadiene,2 but in
the same region where complexed 7-phosphanorbornadienes
display phosphorus shifts.3,5,6
The thermal release of an aromatic moiety from 7-
phosphanorbornadiene complexes (commonly with W(CO)5)
has been shown to be a unimolecular process that proceeds via
cheletropic elimination of a terminal phosphinidene complex,
e.g. [RPW(CO)5].4,26 The carbene-like chemical reactivity of
the transient phosphinidene complexes so generated has been
extensively studied, in particular with olefins, 1,3-dienes, and
alkynes as the reaction partners.27 In order to determine
whether unprotected 7-phosphanorbornadienes behave sim-
Crystals of suitable quality for X-ray diffraction studies of
AN−PA and (HMDS)PA were grown upon cooling a warm,
saturated n-pentane solution of AN−PA, or from a saturated n-
pentane solution of (HMDS)PA that had been stored at −35
°C (thermal ellipsoid drawings are presented in Figure 1). The
two structures are not remarkable with regard to the observed
metrical parameters; indeed, they compare well to structures
previously reported for lone-pair protected 7-phosphanorbor-
nadiene derivatives.3,5,6 The acute CPC angle, imposed by the
7-phosphanorbornadiene architecture, has a value of 79.10(6)°
in AN−PA and 78.97(6)° in (HMDS)PA. Consistent with
computational predictions,2 the plane defined by phosphorus
and the two bridgehead carbon atoms is inclined away from the
benzo ring situated on the same side as the R substituent on
phosphorus by ca. 5°. The P−N interatomic distance in AN−
i
ilarly upon thermolysis, the disappearance of Pr2NPA was
monitored in benzene-d6 in the temperature range 72−89 °C
1
using H NMR spectroscopy. Indeed, plots of ln[iPr2NPA]
versus time were linear and values of the first-order rate
constant k extracted therefrom were in the range (2−42) ×
10−5 s−1 corresponding to an Arrhenius activation barrier of 40
2 kcal/mol.
In order to intercept the proposed transient [iPr2NP]
phosphinidene intermediate and compete with its oligomeriza-
i
tion channel, the thermolysis of Pr2NPA was carried out in
13979
dx.doi.org/10.1021/ja306902j | J. Am. Chem. Soc. 2012, 134, 13978−13981

Journal of the American Chemical Society
Communication
neat 1,3-cyclohexadiene (1,3-CHD). When a solution of
iPr2NPA in neat 1,3-CHD was heated at 80 °C for 12 h, only
anti-iPr2N-7-phosphanorbornene and anthracene were ob-
served to form (Scheme 2), as measured by 1H and 31P
sources. A further implication of the results reported herein is
the new-found ability to introduce the −PA substituent, making
it possible to envision thermal access to unsaturated P-
containing intermediates well beyond the phosphinidene
subset.
i
Scheme 2. (i) Thermolysis of Pr2NPA in 1,3-CHD Leading
to Quantitative Formation of anti-iPr2NPC6H8; (ii) A Model
of anti-iPr2NPC6H8 Built and Optimized Using DFT
ASSOCIATED CONTENT
* Supporting Information
■
S
a
Methods28,29 and Visualized Using PLATON21
Provided are full details of experimental procedures for the
synthesis of all new substances together with characterization
data including details of X-ray diffraction studies and Cartesian
coordinates for structures optimized by computational
methods. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
ccummins@mit.edu
■
Notes
The authors declare no competing financial interest.
a
i
Hydrogen atoms on the Pr2N group have been omitted for clarity.
ACKNOWLEDGMENTS
This material is based upon work supported by the National
Science Foundation under CHE-1111357.
■
Selected interatomic distances (Å) and angles (deg): P−N 1.714, P−
C4 1.952, C5−C6 1.555, C2−C6 1.356, C1−P−C4 77.18.
NMR spectroscopy. Upon complete consumption of the
iPr2NPA starting material, all volatile materials were removed
in vacuo from the reaction mixture. The resulting oily residue
was suspended in a minimum amount of cold pentane,
whereupon the mixture was filtered to remove all precipitated
solids. The filtrate was brought to constant mass and was
analyzed as spectroscopically pure anti-iPr2NP(C6H8) (yield
69%). The stereochemistry of the product was established
based on the large 2JPC coupling constant of the olefinic carbon
atoms of 26.6 Hz, diagnostic for a proximal arrangement of the
REFERENCES
■
(1) Mathey, F. Angew. Chem., Int. Ed. Engl. 1987, 26, 275−286.
(2) van Eis, M. J.; Zappey, H.; de Kanter, F. J. J.; de Wolf, W. H.;
Lammertsma, K.; Bickelhaupt, F. J. Am. Chem. Soc. 2000, 122, 3386−
3390.
(3) Marinetti, A.; Mathey, F.; Fischer, J.; Mitschler, A. J. Am. Chem.
Soc. 1982, 104, 4484−4485.
(4) Marinetti, A.; Charrier, C.; Mathey, F.; Fischer, J. Organometallics
1985, 4, 2134−2138.
(5) Compain, C.; Donnadieu, B.; Mathey, F. Organometallics 2005,
24, 1762−1765.
phosphorus lone pair with respect to the CC bond.30
A
cationic P-chloro derivative of syn,anti-iPr2NP(C6H8) has been
reported previously; it was obtained by reaction of the highly
electrophilic phosphenium salt [iPr2NPCl][AlCl4] with 1,3-
CHD.31 In contrast to its 7λ3 relative reported herein,
[iPr2NP(Cl)C6H8][AlCl4] was produced as a mixture of syn
and anti isomers.31
(6) (a) Compain, C.; Donnadieu, B.; Mathey, F. Organometallics
2006, 25, 540−543. (b) Compain, C.; Huy, N. H. T.; Mathey, F.
Heteroat. Chem. 2004, 15, 258−262. (c) Compain, C.; Mathey, F. Z.
Anorg. Allg. Chem. 2006, 632, 421−424.
(7) Stille, J. K.; Eichelberger, J. L.; Higgins, J.; Freeburger, M. E. J.
Am. Chem. Soc. 1972, 94, 4761−4763.
(8) Gottardo, C.; Fratpietro, S.; Hughes, A. N.; Stradiotto, M.
Heteroat. Chem. 2000, 11, 182−186.
P-trivalent 7-phosphanorbornene derivatives previously have
been synthesized from the [2 + 4] Diels−Alder cycloaddition of
a phosphole to a dienophilic alkene,30,32 and enantiopure chiral
7-phosphanorbornenes can also be synthesized using metal
templation.33 Recently, 7-phosphanorbornanes and 7-phospha-
norbornenes have been used as ligands in rhodium-catalyzed
asymmetric hydrogenations.34
In conclusion, we have discovered a simple, direct synthesis
of several dibenzo-7λ3-phosphanorbornadienes that consists of
a magnesium anthracene reaction with dichlorophosphine,
RPCl2; within the scope of our limited survey the reaction was
successful except for the case of an aryl substituent. The new
dibenzo-7λ3-phosphanorbornadiene derivatives so obtained
were found to extrude anthracene smoothly upon heating,
and as such, they very likely give rise thermally to transient
phosphinidene intermediates consistent with the observed
product mixtures. Along with phosphanylidene-σ4-phosphor-
anes,35 protected 7-phosphanorbornadienes, and terminal
phosphinidene complexes,1 the class of dibenzo-7-phospha-
norbornadiene molecules introduced herein adds to the
growing list of species that may serve as facile phosphinidene
(9) (a) Twamley, B.; Sofield, C. D.; Olmstead, M. M.; Power, P. P. J.
Am. Chem. Soc. 1999, 121, 3357−3367. (b) Smith, R. C.; Shah, S.;
Protasiewicz, J. D. J. Organomet. Chem. 2002, 646, 255−261.
(10) Neumann, W. P.; Schriewer, M. Tetrahedron Lett. 1980, 21,
3273−3276.
(11) Appler, H.; Gross, L. W.; Mayer, B.; Neumann, W. P. J.
Organomet. Chem. 1985, 291, 9−23.
(12) Koecher, J.; Lehnig, M.; Neumann, W. P. Organometallics 1988,
7, 1201−1207.
(13) Shusterman, A. J.; Landrum, B. E.; Miller, R. L. Organometallics
1989, 8, 1851−1855.
(14) Ramsden, H. E. Magnesium and Tin Derivatives of Fused-ring
Hydrocarbons and the Preparation Thereof, U.S. Patent 3354190, 1967.
̀
(15) Bogdanovic, B. Acc. Chem. Res. 1988, 21, 261−267.
(16) Ramsden, H. E. Organometallic compounds, U.S. Patent 3240795,
1966.
(17) Smith, C. L.; Pounds, J. J. Chem. Soc., Chem. Commun. 1975,
910−911.
(18) Gilman, H.; Cottis, S. G.; Atwell, W. H. J. Am. Chem. Soc. 1964,
86, 1596−1599.
(19) Bleckmann, P.; Minkwitz, R.; Neumann, W. P.; Schriewer, M.;
Thibud, M.; Watta, B. Tetrahedron Lett. 1984, 25, 2467−2470.
13980
dx.doi.org/10.1021/ja306902j | J. Am. Chem. Soc. 2012, 134, 13978−13981

Journal of the American Chemical Society
Communication
(20) Egorov, M. P.; Ezhova, M. B.; Antipin, M. Y.; Struchkov, Y. T.
Main Group Met. Chem. 1991, 14, 19−25.
(21) Spek, A. L. J. Appl. Crystallogr. 2003, 7−13.
(22) (a) Smith, L. R.; Mills, J. L. J. Am. Chem. Soc. 1976, 98, 3852−
3857. (b) Baudler, M.; Glinka, K.; Cowley, A. H.; Pakulski, M.
Organocyclophosphanes. In Inorganic Syntheses; John Wiley & Sons,
Inc.: 1989; pp 1−5. (c) Baudler, M.; Glinka, K. Chem. Rev. 1993, 93,
1623−1667.
(23) King, R. B.; Sadanani, N. D. J. Org. Chem. 1985, 50, 1719−1722.
(24) Pyykko, P.; Atsumi, M. Chem.Eur. J. 2009, 15, 12770−12779.
̈
(25) Bent, H. A. Chem. Rev. 1961, 61, 275−311.
(26) Mathey, F.; Huy, N. H. T.; Marinetti, A. Helv. Chim. Acta 2001,
84, 2938−2957.
(27) (a) Lammertsma, K.; Vlaar, M. J. Eur. J. Org. Chem. 2002, 2002,
1127−1138. (b) Jansen, H.; Samuels, M. C.; Couzijn, E. P. A.;
Slootweg, J. C.; Ehlers, A. W.; Chen, P.; Lammertsma, K. Chem.Eur.
J. 2010, 16, 1454−1458.
(28) Neese, F. ORCA − an ab initio, Density Functional and
Semiempirical program package, Version 2.8.0; University of Bonn: 2009.
(29) Schaftenaar, G.; Noordik, J. J. Comput.-Aided Mol. Des. 2000, 14,
123−134.
(30) (a) Szewczyk, J.; Quin, L. D. J. Org. Chem. 1987, 52, 1190−
1196. (b) Quin, L. D.; Caster, K. C.; Kisalus, J. C.; Mesch, K. A. J. Am.
Chem. Soc. 1984, 106, 7021−7032. (c) Quin, L. D.; Bernhardt, F. C.
Magn. Reson. Chem. 1985, 23, 929−934. (d) Hung, J.-T.; Lammertsma,
K. J. Organomet. Chem. 1995, 489, 1−4.
(31) Cowley, A. H.; Kemp, R. A.; Lasch, J. G.; Norman, N. C.;
Stewart, C. A.; Whittlesey, B. R.; Wright, T. C. Inorg. Chem. 1986, 25,
740−749.
(32) (a) Mattmann, E.; Simonutti, D.; Ricard, L.; Mercier, F.;
Mathey, F. J. Org. Chem. 2001, 66, 755−758.
(33) (a) He, G.; Qin, Y.; Mok, K. F.; Leung, P.-H. Chem. Commun.
2000, 167−168. (b) Leung, P.-H.; He, G.; Lang, H.; Liu, A.; Loh, S.-
K.; Selvaratnam, S.; Mok, K.; White, A. J.; Williams, D. J. Tetrahedron
2000, 56, 7−15.
(34) (a) Chen, Z.; Jiang, Q.; Zhu, G.; Xiao, D.; Cao, P.; Guo, C.;
Zhang, X. J. Org. Chem. 1997, 62, 4521−4523. (b) Jiang, Q.; Jiang, Y.;
Xiao, D.; Cao, P.; Zhang, X. Angew. Chem., Int. Ed. 1998, 37, 1100−
1103. (c) Jiang, Q.; Xiao, D.; Zhang, Z.; Cao, P.; Zhang, X. Angew.
Chem., Int. Ed. 1999, 38, 516−518. (d) Zhang, Z.; Zhu, G.; Jiang, Q.;
Xiao, D.; Zhang, X. J. Org. Chem. 1999, 64, 1774−1775. (e) Clochard,
M.; Mattmann, E.; Mercier, F.; Ricard, L.; Mathey, F. Org. Lett. 2003,
5, 3093−3094.
(35) (a) Shah, S.; Protasiewicz, J. D. Coord. Chem. Rev. 2000, 210,
181−201. (b) Protasiewicz, J. D. Eur. J. Inorg. Chem. 2012,
DOI: 10.1002/ejic.201200273.
13981
dx.doi.org/10.1021/ja306902j | J. Am. Chem. Soc. 2012, 134, 13978−13981