Reductive conjugate addition of haloalkanes to enones to form silyl enol ethers

Article abstract of DOI:10.1021/ol200881v

A new method is presented for tandem reductive conjugate addition and silyl enol ether formation from cyclic and acyclic enones and enals in the presence of a Mn reductant, a Ni(terpyridine) catalyst, and a trialkylchlorosilane. The addition of secondary, tertiary, and hindered primary haloalkanes is demonstrated. Preliminary studies on the mechanism show that the intermediacy of L1(Ni)(ν³-1-triethylsilyloxyalkenyl)X or in-situ-formed RMnX is unlikely.

Full text of DOI:10.1021/ol200881v

ORGANIC
LETTERS
2011
Vol. 13, No. 10
2766–2769
Reductive Conjugate Addition of
Haloalkanes to Enones To Form Silyl
Enol Ethers
Ruja Shrestha and Daniel J. Weix*
Department of Chemistry, University of Rochester, Rochester, New York 14627-0216,
United States
daniel.weix@rochester.edu
Received April 4, 2011
ABSTRACT
A new method is presented for tandem reductive conjugate addition and silyl enol ether formation from cyclic and acyclic enones and enals in the
presence of a Mn reductant, a Ni(terpyridine) catalyst, and a trialkylchlorosilane. The addition of secondary, tertiary, and hindered primary
haloalkanes is demonstrated. Preliminary studies on the mechanism show that the intermediacy of L1(Ni)(η3-1-triethylsilyloxyalkenyl)X or in-situ-
formed RMnX is unlikely.
The conjugate addition of preformed organometallic
reagents to electrophilic olefins is a key bond-forming
reaction in modern synthesis.1 Among the most useful
variants are tandem conjugate additionꢀenolate trapping
reactions to form silyl enol ethers. These products are
versatile intermediates for further CꢀC bond formation
or functionalization.2 Several conditions for tandem
conjugate additionꢀsilyl enol ether formation have been
developed; however, most3 share the requirement of
a preformed organometallic reagent (R__M = R2Zn,
RZnX, RMgBr, R3Al). These organometallic reagents
are generally not commercially available, and their synthe-
sis often requires cryogenic temperatures and rigorous
exclusion of air and moisture, which complicates synthetic
routes.
Although great strides have been made in the synthesis
of functionalized organometallic reagents,4 direct use of
the corresponding haloalkane would eliminate the need for
a preformed organometallic reagent and avoid many of the
associated problems. In contrast to traditional conjugate
addition reactions with selectivity built into the reagents
themselves, the major challenge associated with a direct
reductive approach is selectivity for cross-coupling over
competing dimerization reactions. For example, our envi-
sionedreductiveapproach requires the orderedcoupling of
three different electrophiles: a haloalkane, an R,β-unsatu-
rated ketone, and a trialkylchlorosilane reagent. Mackenzie
reported a stepwise stoichiometric-nickel approach that
tolerates β-substitution on the enone.5a Other methods
which are stoichiometric in nickel or copper have also been
(1) (a) Perlmutter, P. Conjugate Addition Reactions in Organic Synthe-
sis; Pergamon Press: Oxford, 1992. Cu: (b) Kharasch, M. S.; Tawney, P. O. J.
Am. Chem. Soc. 1941, 63, 2308. (c) Taylor, R. J. K. Synthesis 1985, 364. (d)
Nakamura, E. Synlett 1991, 539. (e) Krause, N.; Gerold, A. Angew. Chem.,
Int. Ed. 1997, 36, 186. Ni:(f) Ashby, E. C.; Heinsohn, G. J. Org. Chem.
1974, 39, 3297. (g) Bagnell, L.; Meisters, A.; Mole, T. Aust. J. Chem. 1975,
28, 817. (h) Loots, M. J.; Schwartz, J. J. Am. Chem. Soc. 1977, 99, 8045. (i)
Kobayashi, Y. Reaction of Alkenes and Allyl Alcohol Derivatives. In
Modern Organonickel Chemistry, Tamaru, Y., Ed.; Wiley-VCH: Weinheim,
2005; p 56.
(2) See ref 1a, 1c, and Kobayashi, S.; Manabe, K.; Ishitani, H.;
Matsuo, J.-I. Sci. Synth. 2002, 4, 317.
(3) Selected alternative strategies: Radical additions: (a) Srikanth,
G. S. C.; Castle, S. L. Tetrahedron 2005, 61, 10377. (b) Rowlands, G. J.
Tetrahedron 2009, 65, 8603. Alkenes or alkynes in place of organome-
tallic reagents: (c) Trost, B. M.; Surivet, J.-P.; Toste, F. D. J. Am. Chem.
Soc. 2001, 123, 2897. (d) Li, W.; Herath, A.; Montgomery, J. J. Am.
Chem. Soc. 2009, 131, 17024. (e) Ho, C.-Y.; Schleicher, K.; Chan, C.-W.;
Jamison, T. Synlett 2009, 2565.
(4) Handbook of Functionalized Organometallics: Applications in
Synthesis. Knochel, P., Ed.; Wiley-VCH: Weinheim, 2005.
r
10.1021/ol200881v
Published on Web 04/14/2011
2011 American Chemical Society

reported,5 but previous studies on the reductive conjugate
addition with nickel6 and cobalt7 catalysts were limited to
electrophilic olefins without β-substitution or with two
activating groups.8 Montgomery reported a conjugate
additionꢀaldol reaction,6f,g but the analogous silyl enol
ether formation has not been reported. Finally, the use of
haloalkanes as substrates also introduces the additional
challenge of β-hydride elimination,9 and only a few exam-
ples have been published.10
(1a) or the haloalkane (2a) provided similar results (entries
9 and 10respectively). Thus, the lesscostlycomponent may
be used in excess.
Table 1. Reductive Coupling of Cyclohexenone with 2-Bromo-
heptanea
Herein we report an approach to direct reductive con-
jugate addition reactions (Table 1) that addresses these
challenges. Preliminary mechanistic studies suggest a gen-
eral strategy for the development of cross-selective reduc-
tive coupling reactions.
In preliminary studies,11 the best yields of silyl enol
ether 4a were obtained from reactions containing Ni/L1
catalyst, chlorotriethylsilane (3a), and DMF solvent
(Table 1, entry 1).12 Reactions conducted without ligand
L1 (2,20:60,200-tri-tert-butyl-terpyridine) resulted in the
slow consumption of starting materials (entry 2). The use
of pyridine or bipyridine (L2) in reactions instead of L1
resulted in the formation of large amounts of dimer 5
(entries 3 and 4 respectively). Reactions conducted without
TES-Cl (3a) consumed both 2-cyclohexen-1-one (1a) and
2-bromoheptane (2a) but produced only small amounts
of the corresponding ketone product (entry 5). Lastly,
reactions conducted with a slight excess of either the enone
yield 4ab yield 5
entry
change from optimized conditions
None
(%)
(%)
1
76
6c
26
2
No ligand (omit L1)
6
3
4ꢀ12 mol % pyridine in place of L1
4 mol % L2 in place of L1
No chlorotriethylsilane (3a)
No nickel
Ni(acac)2 in place of NiCl2(glyme)e
No Mn powder
5ꢀ7
23
4
15
74
5
4c,d
3c
0
6
0
7
77
38
8
NR
66ꢀ74
65ꢀ68
77f
NR
25ꢀ36
21ꢀ25
16
(5) Stoichiometric: Ni: (a) Johnson, J. R.; Tully, P. S.; Mackenzie,
P. B.; Sabat, M. J. Am. Chem. Soc. 1991, 113, 6172. (b) Manchand, P. S.;
Yiannikouros, G. P.; Belica, P. S.; Madan, P. J. Org. Chem. 1995, 60,
9
1.2ꢀ1.4 equiv of 1a to 1.0 equiv of 2a
1.2ꢀ1.6 equiv of 2a to 1.0 equiv of 1a
1 mol % NiCl2(glyme) and L1
10 mol % NiCl2(glyme) and L1
10
11
12
ꢀ
6574. (c) Bonjoch, J.; Sole, D.; Garcia-Rubio, S.; Bosch, J. J. Am. Chem.
Soc. 1997, 119, 7230. (d) Nicolaou, K. C.; Roecker, A. J.; Follmann, M.;
Baati, R. Angew. Chem., Int. Ed. 2002, 41, 2107. Cu: (e) Petrier, C.;
Dupuy, C.; Luche, J. L. Tetrahedron Lett. 1986, 27, 3149. (f) Shen, Z.-L.;
Cheong, H.-L.; Loh, T.-P. Tetrahedron Lett. 2009, 50, 1051 and refer-
ences cited therein.
33
59
a Reactions were run on 0.5 mmol scale in 2 mL of DMF for 14ꢀ24 h.
b Uncorrected GC yield vs dodecane internal standard, mixture of regio-
and stereoisomers. See Supporting Information for details. c Both start-
ing materials remained (76ꢀ87% by GC). d Ketone product obtained
instead of enol ether 4a. e Ni(acac)2 is 17¢/mmol vs $2.88/mmol for
NiCl2(glyme). f Reaction time was 36 h.
ꢀ ꢀ
(6) Nickel-catalyzed: Review: (a) Condon, S.; Nedelec, J.-Y. Synthe-
sis 2004, 3070. Chemical reductants: (b) Boldrini, G. P.; Savoia, D.;
Tagliavini, E.; Trombini, C.; Ronchi, A. U. J. Organomet. Chem. 1986,
301, C62. (c) Lebedev, S. A.; Lopatina, V. S.; Petrov, E. S.; Beletskaya, I. P.
J. Organomet. Chem. 1988, 344, 253. (d) Sustmann, R.; Hopp, P.; Holl, P.
Tetrahedron Lett. 1989, 30, 689. (e) Yu, S.; Berner, O. M.; Cook, J. M. J.
Am. Chem. Soc. 2000, 122, 7827. (f) Subburaj, K.; Montgomery, J. J. Am.
Chem. Soc. 2003, 125, 11210. (g) Chrovian, C. C.; Montgomery, J. Org.
Lett. 2007, 9, 537. (h) Gong, H.; Andrews, R. S.; Zuccarello, J. L.; Lee, S. J.;
The scope of our new reductive coupling process is
summarized in Scheme 1 (next page). Several different
silicon reagents were effective promoters for reactions of
2-bromoheptane (2a) with 2-cyclohexen-1-one (1a), pro-
viding silyl enol ether products 4aꢀc which possess a range
of stabilities.11
ꢀ
Gagne, M. R. Org. Lett. 2009, 11, 879. (i) Kim, H.; Lee, C. Org. Lett. 2011,
13, 2050–2053. Electrochemical methods: (j) Gosden, C.; Pletcher, D. J.
ꢀ
Organomet. Chem. 1980, 186, 401. (k) Condon-Gueugnot, S.; Leonel, E.;
ꢀ ꢀ
ꢀ
Nedelec, J.-Y.; Perichon, J. J. Org. Chem. 1995, 60, 7684.
(7) Co-catalyzed: Chemical: (a) Shukla, P.; Hsu, Y.-C.; Cheng, C.-H.
ꢀ
J. Org. Chem. 2006, 71, 655. (b) Amatore, M.; Gosmini, C.; Perichon, J.
A variety of haloalkanes (2) reacted with enone 1a to
provide silyl enol ethers (4dꢀj) in good yield (Scheme 1).
2-Iodoheptane reacted much faster than 2-bromohep-
tane (2 h vs 18 h), and 2-chloroheptane was unreactive
under these conditions. The conjugate additionꢀenolate
trapping reaction tolerates a variety of cyclic haloalkanes,
including a tetrahydropyran, (4d, e, j), and ester (4h), or
nitrile (4i) functionality. 2-Bromocyclohexenone provides
a high yield of product 4m without loss of the vinylic
bromide.
J. Org. Chem. 2006, 71, 6130. (c) Amatore, M.; Gosmini, C. Synlett 2009,
1073. Electrochemical: (d) Scheffold, R.; Dike, M.; Dike, S.; Herold, T.;
Walder, L. J. Am. Chem. Soc. 1980, 102, 3642. (e) Ozaki, S.; Nakanishi,
T.; Sugiyama, M.; Miyamoto, C.; Ohmori, H. Chem. Pharm. Bull. 1991,
39, 31. (f) Gomes, P.; Gosmini, C.; Nedelec, J.-Y.; Perichon, J. Tetra-
hedron Lett. 2000, 41, 3385.
(8) Generally only acrylates, methyl vinyl ketones, fumarates, or
maleates produce high yields. For a single coupling in 20% yield with
ethyl crotonate, see ref 6k.
(9) Rudolph, A.; Lautens, M. Angew. Chem., Int. Ed. 2009, 48, 2656.
(10) See refs 6c (2 examples), 6d (3 examples), 6h (glycosyl bromides
only), 7a (13 examples), 7d (intramolecular), and 7e (intramolecular).
(11) See Supporting Information for Tables S1 (more detailed opti-
mization table), S2 (solvent effects), and S3 (effect of silicon reagent) as
well as details for the TDAE and stoichiometric studies.
(12) Trialkylchlorosilane reagents accelerate copper-mediated con-
jugate addition reactions: (a) See ref 1d. (b) Lipshutz, B. H.; Dimock,
S. H.; James, B. J. Am. Chem. Soc. 1993, 115, 9283. (c) Frantz, D. E.;
Singleton, D. A. J. Am. Chem. Soc. 2000, 122, 3288.
ꢀ ꢀ
ꢀ
Even a tertiary haloalkane, tert-butyl bromide, coupled
to form product 4g with adjacent tertiary and quaternary
carbons in high yield and without detectable isomerization
to the iso-butyl product.13 Only a few examples of the use
Org. Lett., Vol. 13, No. 10, 2011
2767

Several mechanistic possibilities exist for reactions invol-
ving the reductive coupling of electrophiles. We considered
three general mechanisms for our reductive conjugate addi-
tion reaction (Figure 1): (1) in situ formation of an organo-
metallic reagent (e.g., RMnBr) followed by conventional
conjugate addition (Mechanism A),1fꢀi,16,17 (2) dispropor-
tionation of two different organonickel intermediates
(Mechanism B), and (3) sequential reactions of electrophiles
at a single metal center (Mechanisms C5a and D15).
Scheme 1. Scope of Conjugate Addition Reactiona
a Reactions were run on 1.0 mmol scale in 2 mL of DMF. For
products 4aꢀe, 4i, and 4kꢀo the ratio of reactants 1:2:3 = 1.6:1.0:1.3,
for 4fꢀh and 4j the ratio was 1.0:2.0:1.3. Yields are isolated yields,
average of two runs, mixture of isomers. bSingle run. cSilyl triflate was
used. dYield of deprotected ketone product. Crude product was depro-
tected by treatment with 1 M HCl at 40 °C for 8 h.
of tertiary haloalkanes in metal-catalyzed coupling reac-
tions have been published.14 In addition, neopentyl iodide
coupled in good yield (4f). Less hindered primary haloalk-
anes and benzylic secondary haloalkanes do not form
product 4 because of rapid dimerization.15
A variety of cyclic and acyclic alkenones participate in the
reaction (4kꢀo), including substrates with 2-substitution
(4mꢀo) and an aldehyde substrate (4o). Acyclic alkenones
without 2-substitution provide lower yields under these con-
ditions (33% yield for 3-penten-2-one). Finally, the reaction
to form 4a could be scaled to 10 mmol without complication.
Figure 1. Potential mechanisms.
Our results strongly argue against Mechanism A: (1) a
reaction run in the presence of p-tolualdehyde did not
produce any product from the addition of “RMnX” to
the aldehyde;11,18 (2) reactions conducted using manganese
(66% yield), zinc (52% yield), and tetrakis(dimethylamino)-
ethylene (TDAE)19 (44% yield) as reducing agents provided
similar yields of 4a.11,20 Thus, nickel is the only metal
required, and the mechanism is distinct from that of tradi-
tional conjugate addition reactions.5ꢀ7
To evaluate Mechanisms B and C (Figure 1), we
conducted a series of stoichiometric studies on isolated
(pyridine)Ni(η3-1-triethylsilyloxycyclohexenyl)Cl (10)
(Table 2).21 We disfavor mechanism B at this time for
(13) Isomerization with related complexes: Ni: (a) Breitenfeld, J.;
Vechorkin, O.; Corminboeuf, C.; Scopelliti, R.; Hu, X. Organometallics
2010, 29, 3686. Fe: (b) Vela, J.; Vaddadi, S.; Cundari, T. R.; Smith, J. M.;
Gregory, E. A.; Lachicotte, R. J.; Flaschenriem, C. J.; Holland, P. L.
Organometallics 2004, 23, 5226.
(14) Representative examples: Pd: (a) Ishiyama, T.; Abe, S.;
Miyaura, N.; Suzuki, A. Chem. Lett. 1992, 21, 691. Co:(b) Tsuji, T.;
Yorimitsu, H.; Oshima, K. Angew. Chem., Int. Ed. 2002, 41, 4137. (c)
Ohmiya, H.; Tsuji, T.; Yorimitsu, H.; Oshima, K. Chem.ꢀEur. J. 2004,
10, 5640. (d) Reference 7a. Ag: (e) Someya, H.; Ohmiya, H.; Yorimitsu,
H.; Oshima, K. Org. Lett. 2008, 10, 969. (f) Someya, H.; Yorimitsu, H.;
Oshima, K. Tetrahedron Lett. 2009, 50, 3270. (g) Mitamura, Y.; Someya,
H.; Yorimitsu, H.; Oshima, K. Synlett 2010, 309. Cu:(h) Sai, M.;
Yorimitsu, H.; Oshima, K. Bull. Chem. Soc. Jpn. 2009, 82, 1194. Ni:
(i) Reference 6d.
(15) Bromooctane yields only 5 and hexadecane. Observation of
rapid formation of alkyl dimers from [(L1)Ni(R)]I: (a) Jones, G. D.;
McFarland, C.; Anderson, T. J.; Vicic, D. A. Chem. Commun. 2005,
4211. Reductivedimerization of alkyl halides catalyzedby (L1)NiX2: (b)
Goldup, S. M.; Leigh, D. A.; McBurney, R. T.; McGonigal, P. R.; Plant,
A. Chem. Sci. 2010, 383. (c) Prinsell, M. R.; Everson, D. A.; Weix, D. J.
Chem. Commun. 2010, 46, 5743.
(16) Organomanganese reagents in conjugate addition reactions: (a)
Cahiez, G.; Alami, M. Tetrahedron Lett. 1986, 27, 569. (b) Cahiez, G.;
Alami, M. Tetrahedron Lett. 1989, 30, 3541.
(17) For a nickel-catalyzed reductive conjugate addition that is
proposed to proceed by in situ formation of organozinc reagents, see
ref 6h.
(18) Organomanganese reagents add to aldehydes rapidly at rt:
Cahiez, G.; Normant, J. F. Tetrahedron Lett. 1977, 18, 3383.
(19) (a) Kuroboshi, M.; Tanaka, M.; Kishimoto, S.; Goto, K.;
Mochizuki, M.; Tanaka, H. Tetrahedron Lett. 2000, 41, 81. (b) Wiberg,
N. Angew. Chem., Int. Ed. 1968, 7, 766.
(20) We previously used a similar experiment to establish a similar
point: Everson, D. A.; Shrestha, R.; Weix, D. J. J. Am. Chem. Soc. 2010,
132, 920.
2768
Org. Lett., Vol. 13, No. 10, 2011

two reasons: (1) attempts to form product 4a by combining
isolated complex 10 with a mixture of Ni(cod)2, L1, and
2-bromoheptane (2a) provided only cyclohexenone dimer
5 and heptenes (entry 1, below);22 (2) selectivity for cross-
coupled product 4a is improved at lower catalyst loadings
(Table 1, entries 1, 11, and 12).
results from these stoichiometric studies suggest that com-
plex 8 is not an intermediate and Mechanism C leads to the
formation of dimer 5, but not product 4a.
These stoichiometric studies and our optimization data
suggest that the cross-selectivity originates from a catalyst
that (1) primarily reacts with one of the two electrophiles
first (inthiscase, the haloalkane) and (2) isslow todimerize
the electrophile that reacts first (dimerization of 2° haloalka-
nes is slow with L1/Ni at 40 °C15). In this specific case, we
propose that the tridentate terpyridine ligand L1 favors the
formation of the 4-coordinate [(L1)Ni(2-heptyl)]Br23
(7 in Figure 1) over the 6-coordinate (L1)Ni(η3-1-
triethylsilyloxycyclohexenyl)Cl (8).24 Under these mild con-
ditions, [(L1)Ni(2-heptyl)]Br (7) is slow to react further with
more haloalkane but can react with 2-cyclohexen-1-one (1a)
and chlorotriethylsilane (3a) to form product (4a). Further
studies on the mechanism of this transformation and the
application of these design principles to the development of
other reductive coupling reactions are in progress.
In conclusion, we report here a strategy to form the
products of conjugate addition and enolate trapping by the
reductive coupling of a haloalkane, a trialkylchlorosilane,
and an electrophilic olefin. The reaction conditions are
mild, do not require specialized techniques (assembled on
benchtop, no special precautions to exclude air), and
proceed in good yield. Future work will elaborate this
method further, including expanding substrate scope and
investigating stereoselective methods.
Table 2. Stoichiometric Studies with Nickel-Allyl 10a
yield 4a
(%)b
ratio
4a:5:11c
entry
1
conditions
Premixed solution of Ni(cod)2, L1,
and 2-bromoheptane (2a)d
nde
00:100:00
2
3
4
5
6
7
8
2a
68
30
8
81:5:14
2a and Mn
35:nd:65
8:56:36
2a and L1
1a, 2a, and L1
6
7:89:4
L1, TES-Cl (3a), and enone 1a
nde
nde
nde
nd:100:nd
nd:100:nd
nd:100:nd
L1, 1a
L1
a Reactions conducted at 0.05 M in DMF at 40 °C for 17ꢀ21 h with
equimolar amounts of all reagents. b Uncorrected GC yield vs dodecane
internal standard. c Ratio of products by GC analysis, uncorrected;
nd = not detected. d Ni(cod)2 and L1 were premixed until homogeneous,
followed by sequential addition of 2a and 10. e Dimer 5 was formed
quantitatively.
Acknowledgment. This work is supported by the Uni-
versity of Rochester. Acknowledgment is made to the
Donors of the American Chemical Society Petroleum Re-
search Fund for partial support of this research. Analytical
data were obtained from the CENTC Elemental Analysis
Facility at the University of Rochester, funded by NSF
CHE-0650456. We thank Dr. William W. Brennessel
(University of Rochester) for X-ray analysis of complex
10 and Dr. Sandip Sur (University of Rochester) for his
assistance with low temperature NMR analysis of complex 10.
The remaining two mechanisms, C and D, differ only in
the order of oxidative additions of the electrophiles,
alkenone5a or haloalkane15 first (Figure 1). As expected
from Mackenzie’s studies, pyridine-ligated complex 10
reacted with 2-bromoheptane (2a) to form product 4a in
the absence of any added ligand or reductant (Table 2,
entry 2).5a However, reactions conducted in the presence of
L1 led primarily to dimeric product 5 and reduction product
11 (entry 4). Finally, competition studies of 10 with both
2-bromoheptane (2a) and 2-cyclohexen-1-one (1a) pro-
duced only small amounts of product 4a and large
amounts of dimer 5 (entry 5).
Supporting Information Available. Full experimental
procedures, copies of NMR spectra, X-ray data (CIF
file), supplementary stoichiometric studies, and supple-
mentary tables of optimization data. This material is
available free of charge via the Internet at http://pubs.
acs.org.
Formation of dimer 5 from the combination of complex
10 and ligand L1, even in the absence of enone 1a and TES-
Cl (Table 2, entries 6ꢀ8), suggests that the dimer product is
derived from disproportionation of 2 equiv of complex 8
(Figure 1). Given that ligand L1 is required for high yields
of 4a (76% with L1, 7% with pyridine; see Table 1), the
(23) A related [(L1)Ni(Me)]I complex has been structurally charac-
terized. Its reduction product, (L1)Ni(Me), is a postulated intermediate
in Ni-catalyzed Negishi coupling reactions: (a) ref 15a; (b) Jones, G. D.;
Martin, J.; McFarland, C.; Allen, O.; Hall, R.; Haley, A.; Brandon, R.;
Konovalova, T.; Desrochers, P.; Pulay, P.; Vicic, D. J. Am. Chem. Soc.
2006, 128, 13175. (c) Lin, X.; Phillips, D. J. Org. Chem. 2008, 73, 3680.
(24) [(L1)Ni(2-heptyl)]Br would be a 16 electron square-planar spe-
cies, while (L1)Ni(η3-1-triethylsilyloxycyclohexenyl)Cl would be either
an 18-electron, 5-coordinate complex or an even less stable 20-electron,
octahedral complex. The CSD contains only 19 examples of 18-electron,
5-coordinate LNi(allyl) out of >900 [Ni](allyl) structures. CSD Version
5.31, accessed Dec. 14th, 2010.
(21) Synthesized in analogy to similar complexes found in ref 5a. See
Supporting Information for procedure and full characterization.
(22) Attempts to isolate nickel complex 7 and study its reactivity have
been hindered by its relative instability. See ref 15a.
Org. Lett., Vol. 13, No. 10, 2011
2769