Synthesis of a benzodiazepine-derived rhodium NHC complex by C-H bond activation

Article abstract of DOI:10.1021/om8000839

The synthesis and characterization of a Rh(I)-NHC complex generated by C-H activation of a 1,4-benzodiazepine heterocycle are reported. This complex constitutes a rare example of a carbene tautomerofa 1,4-benzodiazepine aldimine stabilized by transition metal coordination and demonstrates the ability of the catalytically relevant RhCl(PCy₃)₂ fragment to induce NHC-forming tautomerization of heterocycles possessing a single carbene-stabilizing heteroatom. Implications for the synthesis of benzodiazepines and related pharmacophores via C-H functionalization are discussed.

Full text of DOI:10.1021/om8000839

2152
Organometallics 2008, 27, 2152–2155
Synthesis of a Benzodiazepine-Derived Rhodium NHC Complex by
C-H Bond Activation
Michael W. Gribble, Jr.,†,‡ Jonathan A. Ellman,*,† and Robert G. Bergman*,†,‡
Department of Chemistry, UniVersity of California, and DiVision of Chemical Sciences, Lawrence Berkeley
National Laboratory, Berkeley, California 94720
ReceiVed January 30, 2008
Scheme 1
Summary: The synthesis and characterization of a Rh(I)-NHC
complex generated by C-H actiVation of a 1,4-benzodiazepine
heterocycle are reported. This complex constitutes a rare
example of a carbene tautomer of a 1,4-benzodiazepine aldimine
stabilized by transition metal coordination and demonstrates
the ability of the catalytically releVant RhCl(PCy3)2 fragment
to induce NHC-forming tautomerization of heterocycles pos-
sessing a single carbene-stabilizing heteroatom. Implications
for the synthesis of benzodiazepines and related pharmacoph-
ores Via C-H functionalization are discussed.
The formation of N-heterocyclic carbene (NHC) complexes
by direct reaction of common nitrogen-containing heterocycles
with transition metal reagents, first discovered by Taube and
co-workers with ruthenium imidazole complexes,1 has become
an important reaction in organometallic chemistry. This process
is significant due to the ubiquity of NHCs as spectator ligands
in homogeneous catalysis,2 the possible existence of NHC
ligands in the chemistry of some metalloenzymes,1,3 the ability
to stabilize uncommon and otherwise unfavorable organic
structures via transition metal coordination, and the promise of
developing atom-economical catalytic reactions that take ad-
vantage of the modified reactivity of the heterocycle NHC
tautomer.4,5 Accordingly, tautomerization reactions involving
interconversion of NHC complexes with either N-coordinated
or free heterocycles have received intense scrutiny in the last
several years.6 Computational studies by Crabtree and Eisenstein
have contributed substantially to elucidating the factors that
control the thermodynamics of metal-induced imidazole tau-
tomerization.3 In addition, more recent work by Ruiz and co-
workers6e has shown that basic additives are capable of
generating kinetically stable NHC complexes from N-bound
imidazoles even when the characteristics of the metal fragment
are predicted to favor N-coordination.3 Mechanistic work in our
laboratories on the Rh(I)-induced tautomerization of N-bound
complex 2 of N-methyl-3,4-dihydroquinazoline (1) to its cor-
responding C2-metalated carbene tautomer 3 (Scheme 1) revealed
a major NHC-forming pathway involving direct participation of
only the N1 nitrogen atom, leading us to conjecture that unsaturated
heterocycles containing only a single nitrogen atom might engage
in similar reactivity.6a
This hypothesis has received subsequent support by the
reports of NHC-forming tautomerization reactions of pyridines
and quinolines induced by complexes of iridium, ruthenium,
and osmium, developed independently by the groups of
Carmona6c,i and Esteruelas.6b,g,h Subsequent to their initial
reports,6b,c our own group found that quinolines and 2-substi-
tuted pyridines undergo alkylation7 by alkenes in the presence
of a Rh(I)/PCy3/HCl-based catalyst system,5c,8 which has been
found to generate NHC tautomers of benzimidazole substrates
* Corresponding authors. E-mail: jellman@uclink.berkeley.edu; bergman@
cchem.berkeley.edu.
† Department of Chemistry, University of California.
‡ Division of Chemical Sciences, Lawrence Berkeley National Laboratory.
(1) Sundberg, R. J.; Bryan, R. F.; Taylor, I. F., Jr.; Taube, H. J. Am.
Chem. Soc. 1974, 96, 381.
(6) (a) Wiedemann, S. H.; Lewis, J. C.; Ellman, J. A.; Bergman, R. G.
J. Am. Chem. Soc. 2006, 128, 2452. (b) Esteruelas, M. A.; Ferna´ndez-
Alvarez, F. J.; On˜ate, E. J. Am. Chem. Soc. 2006, 128, 13044. (c) Alvarez,
E.; Conejero, S.; Paneque, M.; Petronilho, A.; Poveda, M. L.; Serrano, O.;
Carmona, E. J. Am. Chem. Soc. 2006, 128, 13060. (d) Burling, S.; Mahon,
M. F.; Powell, R. E.; Wittlesey, M. K.; Williams, J. M. J. J. Am. Chem.
Soc. 2006, 128, 13702. (e) Ruiz, J.; Perandones, B. F. J. Am. Chem. Soc.
2007, 129, 9298. (f) Wang, X.; Chen, H.; Li, X. Organometallics 2007,
26, 4684. (g) Buil, M. L.; Esteruelas, M. A.; Garce´s, K.; Oliva´n, M.; On˜ate,
E. J. Am. Chem. Soc. 2007, 129, 10998. (h) Esteruelas, M. A.; Ferna´ndez-
(2) For recent reviews, see: Hermann, W. A. Angew. Chem., Int. Ed.
2002, 41, 1290, as well as issues 5 and 6 of Coord. Chem. ReV. 2007, 251,
which are devoted to the topic of transition metal NHC chemistry.
(3) Sini, G.; Eisenstein, O.; Crabtree, R. H. Inorg. Chem. 2002, 41, 602.
(4) For reviews on bond-forming reactions involving NHC ligands see:
(a) Cavell, K. J.; McGuiness, D. S. Coord. Chem. ReV. 2004, 248, 671. (b)
Crudden, C. M.; Allen, D. P. Coord. Chem. ReV. 2004, 248, 2247, and
references therein. For a more recent example of insertion chemistry
involving a cationic Ru-NHC complex, see. (c) Becker, E.; Stingl, V.;
Dazinger, G.; Puchberger, M.; Mereiter, K.; Kirchner, K. J. Am. Chem.
Soc. 2006, 128, 6572.
´
Alvarez, F. J.; On˜ate, E. Organometallics 2007, 26, 5239. (i) Alvarez, E.;
Conejero, S.; Lara, P.; Lo´pez, J. A.; Paneque, M.; Petronilho, A.; Poveda,
M. L.; del R´ıo, D.; Serrano, O.; Carmona, E. J. Am. Chem. Soc. 2007, 129,
14130.
(5) (a) Tan, K. L.; Bergman, R. G.; Ellman, J. A. J. Am. Chem. Soc.
2002, 124, 3202. (b) Clement, N. D.; Cavell, K.J. Angew. Chem., Int. Ed.
2004, 43, 3845. (c) Tan, K. L.; Park, S.; Ellman, J. A.; Bergman, R. G. J.
Org. Chem. 2004, 69, 7329. (d) Lewis, J. C.; Wiedemann, S. H.; Bergman,
R. G.; Ellman, J. A. Org. Lett. 2004, 6, 35. (e) Normand, A. T.; Hawkes,
K. J.; Clement, N. D.; Cavell, K. J.; Yates, B. F. Organometallics 2007,
26, 5352.
(7) Lewis, J. C.; Bergman, R. G.; Ellman, J. A. J. Am. Chem. Soc. 2007,
129, 5332.
(8) (a) Tan, K. L.; Bergman, R. G.; Ellman, J. A. J. Am. Chem. Soc.
2002, 124, 13964. (b) Wiedemann, S. H.; Bergman, R. G.; Ellman, J. A.
Org. Lett. 2004, 6, 1685. (c) Wiedemann, S. H.; Ellman, J. A.; Bergman,
R. G. J. Org. Chem. 2006, 71, 1969.
10.1021/om8000839 CCC: $40.75
2008 American Chemical Society
Publication on Web 04/22/2008

Communications
Organometallics, Vol. 27, No. 10, 2008 2153
Figure 1. Numbering scheme employed in reference to 1,4-
benzodiazepine heterocycles.
Scheme 2
Figure 2. DFT geometry-optimized structure of 7′ illustrating the
preagostic interaction between the C6-H bond and the rhodium
center.
under catalytically relevant conditions.9 Consequently, we have
become interested in elucidating the scope of heterocycles
capable of NHC-forming tautomerization reactions induced by
the rhodium catalysts employed in our earlier development of
C-H functionalization chemistry.
Here we wish to report the synthesis and characterization of
the Rh(I)-(amino)(aryl)carbene complex 7 (Scheme 2, Figures
2 and 5), which is a rare example of an NHC tautomer of a
1,4-benzodiazepine ring system (Figure 1) stabilized via transi-
tion metal coordination. The formation of this material also
demonstrates the ability of the strongly π-basic RhCl(PCy3)2
fragment to induce NHC-forming tautomerization of hetero-
cycles possessing only a single carbene-stabilizing heteroatom,
a reaction we believe may have significance to the recently
disclosed Rh-catalyzed alkylation of quinolines and 2-substituted
pyridines.7 The stability of 7 with respect to its N-bound
tautomer 8 (Figure 3) and the mild conditions of its formation
suggest that transition metal NHC complexes may be synthe-
sized by direct reactions with a wider array of heterocycles than
previously thought, including stabilized10 cyclic aldimines of
varying ring sizes as well as six-membered aromatic hetero-
cycles such as pyridines and quinolines.
Figure 3. DFT geometry-optimized structure for 8′.
biological activity.12 Most notably, the 1,4-benzodiazepine sedative/
hypnotics are among the most widely prescribed of all psychoactive
(10) In an effort to determine whether aryl substitution at the C atom
of the imine function is important to promoting the tautomerization reaction,
an analogous transformation of 3,4,5,6-tetrahydropyridine generated in situ
from the stable trimer ꢀ-tripiperidein was attempted. This reaction failed
to provide significant conversion to either an N-bound or NHC complex
by 1H NMR spectroscopy, yielding instead a lilac-colored precipitate
attributed to [RhCl2(PCy3)2]2 within several minutes. Substantial equilibrium
concentrations of the monomeric imine were not detected under the
experimental conditions. The NHC-forming tautomerization is evidently
applicable only to heterocycles capable of efficiently ligating the active
RhCl(PCy3)2 fragment, and consequently only to imines that exist in solution
primarily in the monomeric form. As crystallographic data for 7 imply
minimal interaction between C5 and the aryl π system, it is plausible that
the importance of the aryl substituent consists in stabilizing the reactive
form of the heterocycle. Crude tripiperidein was prepared in a modification
of De Kimpe’s procedure using Rapoport’s method to generate N-
chloropiperidine and was purified to give ꢀ-tripiperidein according to the
recyrstallization protocol described by Claxton et al. (a) Bender, D. R.;
Bjeldanes, L. F.; Knapp, D. R.; Rapoport, H. J. J. Org. Chem. 1975, 40,
1264. (b) Claxton, G. P.; Allen, L.; Grissar, J. M. Org. Synth. 1977, 56,
118, and references therein. (c) De Kimpe, N.; Stevens, C. J. Org. Chem.
1993, 58, 2904. See also. (d) Wolckenhauer, S. A.; Rychnovsky, S. D. Org.
Lett. 2004, 6, 2745.
Benzodiazepine 6, prepared from 2-methylaminobenzalde-
hyde (4), as illustrated in Scheme 2, was identified as a
promising substrate for investigating NHC formation on the
basis of DFT calculations using PMe3 as a computational model
for PCy3. These predicted the model NHC complex 7′ (Figure
2) to be close in energy to its N-bound tautomer 8′ (Figure 3,
also see Supporting Information).11
Further, we anticipated that transformations involving the
“activated” NHC ligand could provide new strategies for modular
syntheses of 5-aryl and alkyl 1,4-benzodiazepines, which have been
categorized as privileged structures due to their wide-ranging
(11) Gas-phase calculations at the B3LYP/LACVP** level of theory
place the model N-bound adduct 8′ 4.17 kcal/mol lower in free energy than
the NHC model complex 7′, at variance with experimental observation of
the greater stability of 7 with respect to 8 in THF solution at ambient
temperature.
(12) Evans, B. E.; Rittle, K. E.; Bock, M. G.; DiPardo, R. M.; Freidinger,
R. M.; Whitter, W. L.; Lundell, G. F.; Veber, D. F.; Anderson, P. S.; et al.
J. Med. Chem. 1988, 31 (12), 2235–2246.
(9) Gribble, M. W., Jr.; Ellman, J. A.; Bergman, R. G. Unpublished
results.

2154 Organometallics, Vol. 27, No. 10, 2008
Communications
Figure 4. NMR spectra of a representative tautomerization reaction, showing selected diagnostic resonances of compounds 6, 7, and 8.
drugs due to their efficacy and relative safety in the treatment of
related Rh(I) (alkyl)(amino)carbene complex,17 although sub-
stantially shorter than those found for square-planar Rh(I)
(amino)(aryl) and (alkyl)(amino)carbene complexes reported by
anxiety disorders, insomnia, and epilepsy.13
Addition of 6 to a THF solution containing RhCl(PCy3)2
generated in situ from the reaction of [RhCl(coe)2]2 with 2.6 equiv
of PCy3 under an inert atmosphere15 resulted in the formation of
a brown solution that within hours developed an intense yellow-
green hue. Monitoring the progress of the reaction by NMR
spectroscopy16 (Figure 4) led to the observation of clean formation
of a new complex exhibiting two singlets in the 1H NMR spectrum
at δH ) 3.35 (3 H) and δH ) 9.35 (1 H), accompanied by a broad
doublet in 31P NMR spectrum at δP ) 20.75 with JRhP ) 146 Hz.
After one day, these signals began to decay with concomitant
formation of 7, as determined by the appearance of its diagnostic
resonances at δH ) 9.6, 8.55, 7.1, 4.1, and 3.7. Although we have
been unable to isolate the transient species, its spectroscopic features
are consistent with its assignment as 8, because N-binding is
expected to induce a downfield shift of the resonance corresponding
to the C5-H proton of 6, as is observed for the C2-H resonance
of 2. Additional support for this assignment arises from the
established intermediacy of 2 in the analogous tautomerization of
N-methyl-3,4-dihyroquinazoline,6a the comparable energies pre-
dicted for the model structures 7′ and 8′,11 and the well-known
propensity of the 14-electron RhCl(PCy3)2 fragment to accept a
fourth ligand to achieve a stable square-planar d8 coordination
geometry.
(14) Preparation of trans-chloro(1-methyl-1,3,4,5-tetrahydro-benzo[e]-
[1,4]diazepin-2-one-5-ylidene)bis(tricyclohexylphosphine)rhodium(I) (7). An
oven-dried 40 mL glass-walled vessel fitted with a Kontes HI-VAC valve
employing a PTFE Kontes plug (8 mm bore diameter, no O-ring) opening
to a glass sidearm terminating in a 14-20 female ground-glass joint was
taken inside a nitrogen glovebox. This vessel was charged with a solution
of 6 (90.0 mg, 517 µmol), [RhCl(coe)2]2 (185 mg, 258 µmol), and
tricyclohexylphosphine (374 mg, 1.33 mmol) in THF (18 mL). The valve
was stoppered with the Teflon plug, and the reaction mixture was subjected
to four freeze-pump-thaw cycles on a dual manifold. The reaction mixture
was blanketed with an atmosphere of argon, and the reaction vessel was
resealed. The resulting reaction mixture was stored at room temperature in
the dark for 45 days, at which time crystallization of the NHC complex
was apparently complete. The crystals were collected on a glass frit inside
a nitrogen glovebox, washed with pentane, and dried under high vacuum
for ca. 10 min to provide 7 as red, blade-like crystals, confirmed by X-ray
crystallographic analysis and elemental analysis to incorporate two molecules
of THF per molecule of the NHC complex into the crystal lattice (132 mg,
25% yield). The low solubility of this complex in common organic solvents
precluded acquisition of satisfactory 13C NMR data. 1H NMR data were
acquired using crystals grown from THF-d8 solutions, as the downfield non-
deuterated THF resonance partially obscured one of the NHC methylene
resonances; NMR spectra obtained from crystals grown from non-deuterated
THF are otherwise identical. Crystals incorporating THF-d8 were obtained
from NMR-scale experiments of the type described below (ref 16). All other
analytical data pertain to material obtained from preparative-scale reactions.
1H NMR (400 MHz, THF-d8): δ 9.60 (apparent d, 1H, J ) 6.8 Hz, C6-H),
8.55 (br m, 1H, N-H), 7.36 (m, 1H, Ar-H), 7.20 (m, 1H, J ) Ar-H), 7.11
(apparent d, 1 H, J ) 8.4 Hz, C9-H), 4.06 (dd, 1 H, J ) 13.6, 6.4 Hz,
methylene C-H), 3.68 (dd, 1 H, J ) 13.6, 4.8 Hz, methylene C-H), 3.32 (s,
3H, N-Me). 31P NMR (162 MHz, THF-d8): δ 24.18 (ABX dq, J ) 290,
160 Hz). IR (solid): 3371, 2922, 2846, 1669, 1593, 1564, 1469, 1445, 1371,
1344, 1295, 1264, 1230, 1173, 1129, 1066, 1005, 910, 899, 846, 773, 731,
655 cm-1. HR-MS (FAB+): calcd for [M-Cl]+, C46H76N2OP2Rh:
837.448795. Found: 837.450660. Anal. Calcd for 4 · 2THF, C54H92ClN2-
O3P2Rh: C, 63.73; H, 9.11; N, 2.75. Found: C, 63.64; H, 9.50; N, 2.80.
The X-ray crystal structure of 7 (Figure 5) reveals a distorted
square-planar coordination geometry about Rh and a Rh1-C1
bond distance of 1.914(3) Å, comparable to the Rh-CNHC bond
distance reported by Bertrand and co-workers for a somewhat
(13) For a recent review, see: Leonard, B. E. Hum. Psycopharmacol.
Clin. Exp. 1999, 14, 125.

Communications
Organometallics, Vol. 27, No. 10, 2008 2155
the same authors.18 The C5-N4 and C5-C5a bond distances
of 1.330(4) and 1.506(4) Å are typical of sp2-sp2 C-N double
bonds and C-C single bonds, respectively, indicating relatively
little delocalization of electron density from the π system of
the aryl ring over the vacant carbene p orbital.
The 1H and 31P NMR spectra of 7 in THF-d8 exhibit several
noteworthy features. First, the resonance of the C6-H proton
of the benzodiazepine ligand is shifted dramatically downfield
from ca. 7.4 ppm (in 6) to 9.59 ppm due to a preagostic
interaction with the rhodium center similar to that observed in
the analogous N-bound complex of N-methyl-3,4-dihydro-
quinazoline. The geometry-optimized model structure 7′ (Figure
2) closely reproduces the structure of 7 and places the hydrogen
atom at the 6 position of the benzodiazepine ligand at a distance
of 2.824 Å away from the rhodium center, which is within the
upper range of separations observed for preagostic interactions.20
Second, the resonances of the methylene protons appear as
distinct doublets of doublets, revealing coupling to the N4-H
proton as required by the NHC structural assignment and
indicating that the protons have become diastereotopic. Replace-
ment of the C5 H atom of 6 with the bulky RhCl(PCy3)2
fragment at C5 renders interconversion of the enantiomeric
conformers of the benzodiazepine ring slow on the NMR time
scale, as is well-documented for 1,4-benzodiazepines with
organic substituents at the N1 and C5 positions.21 Consequently,
the phosphine ligands also become diastereotopic and give rise
to an ABX double quartet in the 31P NMR spectrum of 7.
Figure 5. ORTEP drawing of 7.19 Thermal ellipsoids are drawn at
the 50% probability level. Two molecules of THF have been deleted
for clarity. Hydrogens other than H(1) have been omitted for clarity.
Selected bond distances (Å) and angles (deg): Rh-C(5), 1.914(3);
C(5)-N(4), 1.330(4); C(5)-C(5a), 1.506(4); N(1)-H(1), 0.79(3);
Rh-P(1), 2.345(1); Rh-P(2), 2.3333(8); Rh-Cl, 2.4456(8);
N(4)-C(5)-C(5a), 113.1(3); Rh-C(5)-N(4), 129.4(2); Rh-C(5)-
C(5a), 117.4(2); P(1)-Rh-P(2), 159.05(3); Cl-Rh-C(5), 166.15(9);
Cl-Rh-P(1), 88.67(3); Cl-Rh-P(2), 87.66(3); P(1)-Rh-C(5),
94.10(9); P(2)-Rh-C(5), 94.37(9).
(15) NMR scale experiments may be conducted simply by sealing the
tube under high vacuum while the reaction mixture is frozen, followed by
warming the tube to the reaction temperature. However, preparative-scale
experiments must be performed under an atmosphere of argon using de-
nitrogenated solvent in order to prevent competitive formation of the
dinitrogen adduct RhCl(PCy3)2(N2), a reaction first reported in: Van Gaal,
H. L. M.; Moers, F. G.; Steggerda, J. J. J. Organomet. Chem. 1974, 65,
C43. The dinitrogen complex otherwise crystallizes simultaneously with 7.
(16) Representative procedure for monitoring the formation of trans-
chloro(1-methyl-1,3,4,5-tetrahydrobenzo[e][1,4]diazepin-2-one-5-ylidene)bis-
(tricyclohexylphosphine)rhodium(I) (7) by NMR: inside a nitrogen glovebox,
a solution of [RhCl(coe)2]2 (6.2 mg, 8.5 µmol) and tricyclohexylphosphine
(12.5 mg, 45 µmol) in THF-d8 (600 µL) was used to transfer 6 (3.0 mg, 17
µmol) and 2,6-dimethoxytoluene (2.6 mg, 17 µmol) to an oven-dried
medium-walled Wilmad NMR tube. The NMR tube was inserted into a
Cajon adapter and flame-sealed under vacuum while the reaction mixture
was frozen in liquid nitrogen. Upon thawing, the reaction mixture was
monitored by 1H NMR (400 MHz) and 31P NMR (162 MHz) spectroscopy,
allowing the reaction mixture to stand at ambient temperature for the
duration of the experiment. The reaction mixture took on a dark greenish-
brown color shortly after preparation and gradually became reddish-brown.
After 7-10 days, the NHC complex began to crystallize as thin, dark red
blades suitable for X-ray crystallographic analysis. 1H NMR yields of 7
were determined prior to the onset of crystallization by comparison of the
integral corresponding to the C6-H proton resonance of 7 to the integral of
the resonance arising from the two protons meta to the methyl group of the
internal standard (2,6-dimethoxytoluene). Yields of 35% are typical for the
above procedure; higher NMR yields can be observed (up to 50%) by
increasing the volume of solvent to 900 µL and by increasing the number
of equivalents of 6 with respect to rhodium to 1.3.
In conclusion, a rare example of stabilization of the 5-carbene
tautomer of a 1,4-benzodiazepine substrate has been demon-
strated via direct reaction with RhCl(PCy3)2 fragment, suggest-
ing that C-metalation of nitrogen heterocycles via metal-induced
tautomerization may be feasible with a substantially broader
range of substrates than has previously been envisioned.
Investigation of the stoichiometric reactivity of this novel NHC
complex is ongoing in our group. Further, as the Rh(I) complex
employed in this study and closely related complexes have
shown good activity toward catalytic C-H functionalization of
a wide range of substrates, we are currently also pursuing the
development of catalytic transformations of 6 and related
heterocycles (e.g., benzotriazepines), in the interest of providing
new methods for modular, atom-economical syntheses of drugs
and drug analogues based on these pharmacologically very
important structures.
Acknowledgment. This work was supported by NIH Grant
GM069559 to J.A.E. and the Director and Office of Energy
Research, Office of Basic Energy Sciences, Chemical Sciences
Division, U.S. Department of Energy, under Contract DE-
AC03-76SF00098 to R.G.B. Thanks to Dr. Allen G. Oliver at
the UCB X-ray diffraction facility (CHEXRAY) for solving
the structure of 7. M.W.G. thanks Dr. Jared C. Lewis and Dr.
Sean H. Wiedemann for helpful discussions. The authors would
also like to thank an anonymous reviewer for helpful suggestions.
(17) Lavallo, V.; Canac, Y.; DeHope, A.; Donnadieu, B.; Bertrand, G.
Angew. Chem., Int. Ed. 2005, 44, 7236.
(18) (a) Cattoe¨n, X.; Gornitzka, H.; Bourissou, D.; Bertrand, G. J. Am.
Chem. Soc. 2004, 126, 1342. (b) Lavallo, V.; Mafhouz, J.; Canac, Y.;
Donnadieu, B.; Schoeller, W. W.; Bertrand, G. J. Am. Chem. Soc. 2004,
126, 8670.
(19) The numbering scheme used in this figure deviates from that used
in the crystallographic structural solution in order to be compatible with
the numbering scheme (Figure 1) conventionally used for benzodiazepine
heterocycles.
Supporting Information Available: Experimental procedures
for the synthesis of compounds 4, 5, and 6, details regarding
computational methodology, energies for DFT geometry optimized
structures 7′ and 8′, and the X-ray structural solution and crystal
data for 7 are provided. This material is available free of charge
via the Internet at http://pubs.acs.org.
(20) (a) Yao, W.; Eisenstein, O.; Crabtree, R. H. Inorg. Chim. Acta 1997,
254, 105. (b) Lewis, J. C.; Wu, J.; Bergman, R. G.; Ellman, J. A.
Organometallics 2005, 24, 5737. (c) Zhang, Y.; Lewis, J. C.; Bergman,
R. G.; Ellman, J. A.; Oldfield, E. Organometallics 2006, 25, 3515.
(21) See for example: Gilman, N. W.; Rosen, P.; Earley, J. V.; Cook,
C.; Todaro, L. J. J. Am. Chem. Soc. 1990, 112, 3969, and references therein.
OM8000839