Oxidatively Triggered Carbon-Carbon Bond Formation in Ene-amide Complexes

Article abstract of DOI:10.1021/acs.inorgchem.5b02990

Ene-amides have been explored as ligands and substrates for oxidative coupling. Treatment of CrCl₂, Cl₂Fe(PMe₃)₂, and Cl₂Copy₄ with 2 equiv of {(2,6-ⁱPr₂C₆H₃)(1-^cHexenyl)N}Li afforded pseudosquare planar {³-C,C,N-(2,6-ⁱPr₂C₆H₃)(1-^cHexenyl)N}₂Cr (1-Cr, 78%), trigonal {(2,6-ⁱPr₂C₆H₃)(1-^cHexenyl)N}₂Fe(PMe₃) (2-Fe, 80%), and tetrahedral {(2,6-ⁱPr₂C₆H₃)(1-^cHexenyl)N}₂Co(py)₂ (3-Co, 91%) in very good yields. The addition of CrCl₃ to 1-Cr, and FeCl₃ to 2-Fe, afforded oxidatively triggered C-C bond formation as rac-2,2′-di(2,6-ⁱPr₂C₆H₃N - )₂dicyclohexane (EA₂) was produced in modest yields. Various lithium ene-amides were similarly coupled, and the mechanism was assessed via stoichiometric reactions. Some ferrous compounds (e.g., 2-Fe, FeCl₂) were shown to catalyze C-arylation of {(2,6-ⁱPr₂C₆H₃)(1-^cHexenyl)N}Li with PhBr, but the reaction was variable. Structural characterizations of 1-Cr, 2-Fe, and 3-Co are reported.

Full text of DOI:10.1021/acs.inorgchem.5b02990

Article
pubs.acs.org/IC
Oxidatively Triggered Carbon−Carbon Bond Formation in Ene-amide
Complexes
Brian P. Jacobs, Peter T. Wolczanski,* and Emil B. Lobkovsky
Department of Chemistry & Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853, United States
S
* Supporting Information
ABSTRACT: Ene-amides have been explored as ligands and
substrates for oxidative coupling. Treatment of CrCl2,
Cl2Fe(PMe3)2, and Cl2Copy4 with 2 equiv of {(2,6-iPr2C6H3)-
(1-cHexenyl)N}Li afforded pseudosquare planar {η3-C,C,N-
(2,6-iPr2C6H3)(1-cHexenyl)N}2Cr (1-Cr, 78%), trigonal
{(2,6-iPr2C6H3)(1-cHexenyl)N}2Fe(PMe3) (2-Fe, 80%), and
tetrahedral {(2,6-iPr2C6H3)(1-cHexenyl)N}2Co(py)2 (3-Co, 91%) in very good yields. The addition of CrCl3 to 1-Cr, and FeCl3
to 2-Fe, afforded oxidatively triggered C−C bond formation as rac-2,2′-di(2,6-iPr2C6H3N)2dicyclohexane (EA2) was produced
in modest yields. Various lithium ene-amides were similarly coupled, and the mechanism was assessed via stoichiometric
reactions. Some ferrous compounds (e.g., 2-Fe, FeCl2) were shown to catalyze C-arylation of {(2,6-iPr2C6H3)(1-cHexenyl)N}Li
with PhBr, but the reaction was variable. Structural characterizations of 1-Cr, 2-Fe, and 3-Co are reported.
present a logical target as base metal, inexpensive reagents.
Herein are reported ene-amide complexes of Cr, Fe, and Co,
including some that generate CC bonds via oxidation and
disproportionation.
1. INTRODUCTION
Recent investigations in these laboratories have focused on
transition metals that exhibit C−C and C−X bond formations
derived from ligands that have redox noninnocent capability.1−7
In most cases, the azaallyl functionality,8 or a related nacnac
framework,9 was the ligand segment involved in bond
formation, as Figure 1 illustrates. For example, metal−amide
deprotonation of an azaallyl precursor led to the production of
[{Me2C(CHNCHpy)}M]2 (M = Cr, Co, Ni), in which three
new carbon−carbon bonds are formed and six new stereo-
centers are set, encapsulating metal−metal bonds in the
process.10 C−C bonds formed reversibly via the coupling of
the smif ligand of (smif)FeN(TMS)2 (B),11 while ortho-
methylation of smif enabled a single CC bond formation that
linked two pyridine-imine radical anions about iron(II) (C).12
In a related pyridylmethyl-nacnac ligand system, dehydroami-
nations were triggered oxidatively and via carbonylation,
initiating CC coupling (D) and CN bond-forming (E) events,
respectively.13 Finally, bis-ene-amide complexes, {κ-N,N-N-
(2,6-iPr2C6H3)C(CH2)-2-pyridyl)M (M = Cr, Mn, Fe,
Co(py)), were found to cyclize to form indolamide complexes
(F), and corresponding lithium cyclizations suggested anionic
character.14
2. RESULTS
2.1. Ligand Synthesis. Standard condensation procedures
were employed in the synthesis of various imines, as shown in
Scheme 1. In certain instances, TsOH catalysis was employed,
whereas other efforts simply required utilization of 4 Å
molecular sieves. Imine deprotonations were conducted with
either nBuLi, without significant interference from attack at the
CN unit, or LDA, and reasonable yields were typically found.
2.2. {(2,6-iPr2C6H3)(1-cHexenyl)N}2Cr (1-Cr). 2.2.1. Syn-
thesis. Treatment of chromous chloride with 2 equiv of
{(2,6-iPr2C6H3)(1-cHexenyl)N}Li in diethyl ether at −78 °C
led to the generation of dark green {(2,6-iPr2C6H3)-
(1-cHexenyl)N}2Cr (1-Cr) upon warming to 23 °C over 24
h.15−17,20 As Figure 2 illustrates, the complex was isolated in
78% yield, and Evans’ method33 measurements provided a μeff
of 4.7 μB, consistent with an S = 2 center whose spin-only
moment is slightly attenuated due to spin−orbit coupling.34
2.2.2. Structure. Figure 2 illustrates a molecular view of
{(2,6-iPr2C6H3)(1-cHexenyl)N}2Cr (1-Cr), which has a C2 di-
1-azaallyl structure reminiscent of CrII(allyl)2 species.35−37
Pertinent crystallographic information is given in Table 1, while
metric parameters may be found in the caption of Figure 2. The
Cr−N distances average 2.017(6) Å, while the adjacent Cr−C
distances are 2.295(13) Å (av), which is slightly longer than the
end-carbon chromium bond lengths of 2.262(2) Å (av).
Conjugation of the ene-amide is evident in N−C bond lengths
The aforementioned ene-amide cyclizations14 prompted their
possible use in oxidatively triggered CC bond formations, but
the orientation of the CCH2 functionality within these
chelates is not ideal for intramolecular couplings. Bulkier ene-
amides, or 1-azaallyls, were envisaged to promote bond
formation by positioning the relevant carbons via sterics.
There is ample precedent for synthesis of such low-coordinate
amide complexes,15−26−30 whose synthesis and reactivity
constitute a forefront area of first row transition metal research.
In addition, iron species are emerging as targets for catalytic or
stoichiometric applications toward organic synthesis,31,32 and
Received: December 29, 2015
© XXXX American Chemical Society
A
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Figure 1. Previous examples of CC and CX bond formation derived from 2- and 1-azaallyls in smif (di-2,3-(2pyridyl)-2-azapropenyl) and nacnac
fragments, and as ene-amides.
Scheme 1
that average 1.358(8) Å in comparison to those NC(sp2)
distances38 pertaining to the 2,6-iPr2-phenyl of 1.420(6) Å (av).
The corresponding CC bond lengths of the ene-amide
average 1.383(8) Å, a distance that is only slightly longer than
expected for a CCN unit of this type;38 hence, it is difficult
to attribute any significant lengthening to π-backbonding. It
seems reasonable, given the nature of a high-spin chromous
center, that any increase is best construed as a consequence of
σ-donation. It is tempting to argue that steric factors of the
2,6-iPr2-phenyl groups pertaining to the C2-symmetric molecule
cause the N1−Cr−N2 angle to open (124.75(4)°) relative to
∠C6−Cr−C20 = 105.91(5)°, but the difference is mostly due
B
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Figure 2. Synthesis and molecular view of {(2,6-iPr2C6H3)-
(1-cHexenyl)N}2Cr (1-Cr), which crystallizes in a triclinic lattice; a
monoclinic polymorph is reported in Supporting Information.
Interatomic distances (Å) and angles (deg): Cr−N1, 2.0131(11);
Cr−N2, 2.0217(11); Cr−C1, 2.3040(13); Cr−C6, 2.2622(14); Cr−
C19, 2.2859(13); Cr−C20, 2.2619(14); N1−C1, 1.3527(17); C1−C6,
1.3884(19); N2−C19, 1.3637(16); C19−C20, 1.3766(19); N1−C7,
1.4238(16); N2−C25, 1.4156(16); N1−Cr−N2, 124.75(4); N1−C1−
C6, 116.17(12); N1−Cr−C19, 158.52(5); N1−Cr−C20, 165.62(5);
N2−C19−C20, 17.23(12); N2−Cr−C1, 158.14(5); N2−Cr−C6,
165.68(5); C1−Cr−C19, 165.10(5); C1−Cr−C20, 131.46(5); C6−
Cr−C19, 131.11(5); C6−Cr−C20, 105.91(5).
to the disparity in Cr−N versus Cr−C bond lengths, and the
azaallyls are only twisted by ∼19.8°.
2.2.3. Coupling Reactions. Efforts at oxidatively triggered
C−C bond formation13,36 were initiated with {(2,6-iPr2C6H3)-
(1-cHexenyl)N}2Cr (1-Cr), and eqs 1−3 indicate the
stoichiometric probes of the coupling reaction. The exposure
of 1-Cr to CrCl3 (eq 1) led to the oxidative coupling of the ene-
amides, as rac-2,2′-di(2,6-iPr2C6H3N)2-dicyclohexane (EA2)
formed in near quantitative yield, while UV−vis spectra of the
products showed that CrCl2(THF)2 was the byproduct, as
compared with an independently prepared sample. In eq 2,
treatment of CrCl3 with {(2,6-iPr2C6H3)(1-cHexenyl)N}Li
resulted in 1-Cr and half an equivalent of EA2, consistent
with observations that a tris-ene-amide complex of Cr(III)
could not be prepared under these conditions. If
{(2,6-iPr2C6H3)(1-cHexenyl)N}2CrCl is formed under these
Table 1. Select Crystallographic and Refinement Data
1-Cr
2-Fe
3-Co
EA2
formula
formula wt
space group
Z
C36H52N2Cr
564.80
C39H61N2PFe
644.72
C46H62N4Co
729.93
C36H52N2
512.80
P1
P1
P21/n
C2/c
̅
̅
2
4
4
4
a, Å
11.320(2)
11.5351(19)
13.336(3)
87.146(8)
82.848(9)
70.288(7)
1626.5(5)
1.153
10.3180(5)
18.3944(10)
22.0796(11)
68.324(3)
78.952(3)
86.630(3)
3821.7(3)
1.121
10.6653(10)
36.834(4)
21.080(2)
90
15.550(6)
9.151(5)
24.380(11)
90
b, Å
c, Å
α, deg
β, deg
97.136(4)
90
107.24(3)
90
γ, deg
V, Å3
ρcalc, g cm−3
μ, mm−1
temp, K
λ (Å)
8217.1(13)
1.180
3313(3)
1.028
0.377
0.463
0.453
0.059
233(2)
223(2)
223(2)
296(2)
0.710 73
0.710 73
0.710 73
R1 = 0.0575
wR2 = 0.1234
R1 = 0.1316
wR2 = 0.1622
1.068
0.710 73
R1 = 0.0413
wR2 = 0.1019
R1 = 0.0688
wR2 = 0.1181
1.002
R indices
R1 = 0.0388
wR2 = 0.0904
R1 = 0.0569
wR2 = 0.0998
1.034
R1 = 0.0443
wR2 = 0.0950
R1 = 0.0863
wR2 = 0.1169
1.009
ab
,
[I > 2σ(I)]
b
R indices
a
(all data)
c
GOF
a
b
c
2
R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = [∑w(|Fo| − |Fc|)2/∑wFo ]1/2. GOF (all data) = [∑w(|Fo| − |Fc|)2/(n − p)]1/2, n = number of independent
reflections, p = number of parameters.
C
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circumstances, it is likely that disproportionation occurs. In
concert with these arguments, the treatment of
{(2,6-iPr2C6H3)(1-cHexenyl)N}Li with CrCl3 leads to the
coupled product, EA2, in 55% isolated yield. Various oxidants
were explored in an attempt to render the coupling catalytic in
Cr, but most were ineffective, and CrCl3 is quite inexpensive
and worked best.
NMR spectra of EA2 were consistent with either the rac- or
meso-form of coupled ene-amide, and due to hydrolysis
problems (vide infra), correlation with the corresponding
diketone could not be accomplished. The C2-stereochemistry
was proven via X-ray crystal structure, and its metrical data can
be found in the Supporting Information. No indication of the
Ci isomer was found by NMR analysis in any of the coupling
reactions studied.
Some additional couplings were tested to probe the
generality of the process. In each case, after an appropriate
time period, the solvent was removed, and the products were
taken up in CH2Cl2 and washed with water to remove
chromous byproduct(s). The products were simply assayed by
NMR spectroscopy, typically generated in >90% purity, and not
purified further. As eq 4 illustrates, Cr(III) oxidation of
process of pyrroleformation, as eq 8 reports. These cyclizations
to give pyrrole are simple variants of Paal−Knorr pyrrole
syntheses,39−43 and are difficult to circumvent.
{(2,6-iPr2C6H3)(1-cPentenyl)N}Li afforded the coupled prod-
uct as a tan powder in 68% yield, similar to the cyclohexenyl
species (EA2) illustrated in eq 3. Certain acyclic species could
be coupled, as the 2,6-iPr-phenyl-ene-amide from acetophenone
in eq 5 reveals, but extensions to other substrates caused
hydrolysis issues. When a less hindered imine was used in
coupling of a cyclohexenyl ene-amide, the aqueous workup
afforded the dicyclohexyl-phenyl pyrrole shown in eq 6 as a
yellow oil in 54% yield. Similarly, the coupling of the phenyl-
ene-amide derived from ethyl-phenyl-ketone shown in eq 7
afforded its corresponding pyrrole upon workup in the
presence of water. For the protected ene-amides, typically
those derived from 2,6-iPr2-phenylamine, hydrolysis could not
be accomplished unless forcing conditions were used, and the
stereochemical regularity gained via the coupling was lost in the
2.3. {(2,6-iPr2C6H3)(1-cHexenyl)N}2FePMe3 (2-Fe).
2.3.1. Synthesis. Treatment of (Me3P)2FeCl2 with 2 equiv
44
of {(2,6-iPr2C6H3)(1-cHexenyl)N}Li in benzene at room
temperature afforded red crystals of {(2,6-iPr2C6H3)-
(1-cHexenyl)N}2FePMe3 (2-Fe) in 80% yield as, Figure 3
illustrates.15−17,22−25 Evans’ method33 measurements on 2-Fe
generated a μeff of 5.1 μB, slightly greater than the spin-only
value, which is expected for a system with some spin−orbit
coupling.34
2.3.2. Structure. The structure of {(2,6-iPr2C6H3)-
(1-cHexenyl)N}2FePMe3 (2-Fe) is illustrated in Figure 3, and
pertinent metric data is listed in the caption. Select data
collection and refinement information is presented in Table 1.
A symmetric X2YM situation results in a nonorbitally
5
degenerate A2 state in C2v symmetry, but deviations can still
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Figure 3. Synthesis and view of one of two inequivalent
{(2,6-iPr2C6H3)(1-cHexenyl)N}2FePMe3 (2-Fe) molecules in the
asymmetric unit, chosen because of its lack of disorder. Interatomic
distances (Å) and angles (deg): Fe2−N3, 1.920(2); Fe2−N4,
1.927(2); Fe2−P2, 2.4490(9); N3−C40, 1.402(4); N3−C51,
1.427(3); C40−C41, 1.502(4); C40−C45, 1.329(4); N4−C58,
1.401(3); N4−C64, 1.423(3); C58−C59, 1.334(4); C58−C63,
1.508(4); N3−Fe2−N4, 136.53(10); N3−Fe2−P2, 110.59(7); N4−
Fe2−P2, 112.84(7); Fe2−N3−C40, 124.23(18); Fe2−N3−C51,
118.06(18); C40−N3−C51, 116.9(2); Fe2−N4−C58, 127.19(18);
Fe2−N4−C64, 115.26(16); C58−N4−C64, 117.5(2).
precursor, it seemed plausible that solubility issues could be
affecting the process. Addition of 5 mol % FeCl2 to the product
mixture of eq 10 failed to elicit much more material after
another day (44%), but the addition of 5 mol % 2-Fe, as shown
in eq 11, shortened the process to roughly 1 day, consistent
provide additional stability if the rotational symmetry is lost, or
if a plane of symmetry is removed. In this instance, the angular
distortion is minor as two of the three core angles are roughly
equivalent: ∠N3−Fe−N4 = 136.52(10)° > ∠N4−Fe−P2 =
112.84(7)° > ∠N3−Fe−P2 = 110.60(8)°. The ene-amides are
twisted relative to one another by 39.4°, a distortion from C2v
symmetry that is quite significant. The d(Fe−N) values average
1.924(3) Å, and unlike 1-Cr, the iron−carbon distances of the
ene-amide are beyond (>2.95 Å) a reasonable bonding limit.
The C−C bond lengths in the ene-amides average 1.331(4) Å,
a value quite close to that of a double bond,38 while the C−N
distances of 1.402(4) and 1.401(4) Å indicate a significant
attenuation of the delocalization observed in the chromium
case (1-Cr), as they approach the 1.426(4) average of the
d(N−CAr).
2.3.3. Ene-amide Couplings. As in the prior chromium case,
C−C bond formation36 was initially probed via reactions with
{(2,6-iPr2C6H3)(1-cHexenyl)N}2FePMe3 (2-Fe), as eq 9
with the greater availability of iron for amidation. Conversions
seemed to be generally limited to 40−65%, and were less well-
behaved than the Cr system; hence, this study was limited to
the ene-amide shown. Since an outer sphere oxidant,
[Cp2Fe]PF6, caused the formation of EA2 in 38% yield, it is
plausible that C−C bond formation is occurring via reduction
of Fe(III) to Fe(I), followed by disproportionation. Various
oxidants were again explored in an attempt to render the
coupling catalytic, but stoichiometric ferric chloride proved to
be the best and cheapest, although some unusual chemistry was
discovered in the survey of oxidants.
2.3.4. Catalytic Ene-amide β-Arylation. In testing oxidants
for the triggered coupling of the ene-amides on
{(2,6-iPr2C6H3)(1-cHexenyl)N}2FePMe3 (2-Fe), arylation at
the β-carbon of the {(2,6-iPr2C6H3)(1-cHexenyl)N} ligand was
observed as illustrated in eq 13. This time the product was
indicates. Analogous to the chromium oxidations, treatment
45−50
of 2-Fe with FeCl3
affected a 65% conversion to rac-2,2′-
di(2,6-iPr2C6H3N)2-dicyclohexane (EA2) after only 2 h at
room temperature, according to monitoring by 1H NMR
spectroscopy. When {(2,6-iPr2C6H3)(1-cHexenyl)N}Li was
treated with FeCl3, as shown in eq 10, the yield of coupled
product was attenuated (42%), and the process required 2 days
in refluxing THF. Since an Fe(III) intermediate of the type
{(2,6-iPr2C6H3)(1-cHexenyl)N}2FeCl was anticipated as a
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Different metal sources were employed in attempted catalytic
44
arylations, with no positive results. A switch to FeBr2(PMe3)2
elicited no conversion, and neither did FeBr3, nor the pseudo-
Fe(0) species, “Fe(PMe3)4”,51 although in the latter case
conversion to the parent imine was indicated. The addition of
excess PMe3 proved deleterious to the catalysis of eq 16, as did
the use of excess PhBr, although the iron was shown to be
necessary, as a control experiment without iron catalyst gave no
product. PhI gave arylation product, but the yield was half
(∼22% at 10 mol % 2-Fe) that of the bromide.
Since excess PMe3 appeared to slow the reaction, anhydrous
ferrous chloride was tested, and the arylated imine was
produced in 54% (∼5 TO) yield, as indicated by eq 17. Use
correlated with the known 2-phenylcyclohexanone via hydrol-
ysis (eq 14) to ensure that arylation did not occur at the
nitrogen. Since the arylation is likely to be accompanied by iron
reduction, and PhBr is a potential oxidant, catalysis of the
cyclohexane ene-amide arylation was attempted with 10 mol %
2-Fe, and 44% (∼4 TO) conversion to arylation product was
observed (eq 15) in THF-d8. Since 2-Fe was prepared from
of FeCl2(OH2)4 as a potential catalyst showed no arylation
product, but hydrolysis of the lithium ene-amide was noted, as
the parent imine was produced. In all successful cases, ∼50%
conversion was noted, suggesting limited turnover by the iron
catalyst, perhaps independent of loading.Since the catalysis was
modest, no further investigations were conducted. Any
mechanistic discussion was deemed speculative, but there are
ample investigations of iron cross-coupling reactions52−64 that
mostly invoke radical paths related to those initially suggested
by Kochi.65 Furthermore, Pd-catalyzed arylations of ketones are
well-established,66−70 and recent, related catalytic vinylations of
ketones have been reported.71
2.4. {(2,6-iPr2C6H3)(1-cHexenyl)N}2Copy2 (3-Co).
2.4.1. Synthesis. CoCl2 or phosphine-ligated equivalents failed
to elicit a clean product when treated with {(2,6-iPr2C6H3)-
(1-cHexenyl)N}Li, but use of CoCl2py472 enabled the synthesis
of {(2,6-iPr2C6H3)(1-cHexenyl)N}2Copy2 (3-Co) in 91% yield,
as shown in Scheme 2.15−17,26,28 The violet compound was
assessed by Evans’ method measurements33 and was shown to
have a μeff of 4.1 μB, consistent with an S = 3/2 center that has a
substantial spin−orbit contribution, as is typical for Co(II)
pseudotetrahedral species.34
FeCl2(PMe3)2, it was utilized in 5 mol %, and catalysis was
noted, as 47% (∼9 TO) of the arylated ene-amine was
produced (eq 16). While the arylation was reproducible, the
Scheme 2
yields were variable, and an increase in the catalyst
concentration to 10% had little effect, affording 53% (∼5
TO) of product. Extended reaction times did not affect
production of arylated product.
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Figure 4. Views of two inequivalent {(2,6-iPr2C6H3)(1-cHexenyl)N}2Copy2 (3-Co) molecules in the asymmetric unit. Interatomic distances (Å) and
angles (deg): Co1−N1, 1.954(5); Co1−N2, 2.145(5); Co1−N3, 1.941(5); Co1−N4, 2.108(6); N1−C1, 1.427(7); N3−C24, 1.421(8); N1−C13,
1.429(7); N3−C36, 1.422(7); C13−C14, 1.332(8); C13−C18, 1.486(8); C36−C37, 1.346; C36−C41, 1.495(8); N1−Co1−N2, 102.3(3); N1−
Co1−N3, 135.2(2); N1−Co1−N4, 107.2(2); N2−Co1−N3, 106.6(2); N2−Co1−N4, 95.5(2); N3−Co1−N4, 103.3(2); Co1−N1−C1, 114.7(4);
Co1−N1−C13, 132.2(4); C1−N1−C13, 113.0(5); Co1−N3−C24, 116.9(4); Co1−N3−C36, 130.5(4); C24−N3−C36, 112.3(5); Co2−N5,
1.963(5); Co2−N6, 1.949(4); Co2−N7, 2.093(5); Co2−N8, 2.122(5); N5−C59, 1.391(7); N5−C47, 1.436(7); C59−C60, 1.340(8); C59−C64,
1.491(8); N6−C65, 1.397; N6−C71, 1.422(7); C65−C66, 1.494(8); C65−C70, 1.338(8); N5−Co2−N6, 131.5(2); N5−Co2−N7, 113.1(2); N5−
Co2−N8, 104.4(2); N6−Co2−N7, 99.03(19); N6−Co2−N8, 106.4(2); N7−Co2−N8, 97.6(2); Co2−N5−C47, 115.1(4); Co2−N5−C59,
131.7(4); C47−N5−C59, 113.0(5); Co2−N6−C65, 124.6(4); Co2−N6−C71, 120.5(4); C65−N6−C71, 114.9(5).
Scheme 3
2.4.2. Structure. Two molecules of {(2,6-iPr2C6H3)-
(1-cHexenyl)N}2Copy2 (3-Co) are contained in the asym-
metric unit, and each has a particular conformation, as
illustrated in Figure 4. Pertinent metric parameters may be
found in its caption, and data collection and refinement details
are recorded in Table 1. In the conformer containing Co1, the
CNC fragments of the ene-amides are roughly eclipsed, and the
c
2,6-iPr2C6H3 and Hexenyl substituents are disposed in a C2
arrangement. In the alternate conformer containing Co2, the
CNC portions of the ene-amides are staggered, although the
substituents are also in essentially a C2 relationship. The
eclipsed configuration opens up N1−Co1−N3 to 135.2(2)°
G
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Scheme 4
over the corresponding N5−Co2−N6 angle of 131.5(2)°, and
the related N2−Co1−N4 angle is compressed by a few degrees
relative to N7−Co2−N8 (95.5(2)° vs 97.6(20°)). The
remaining core angles exhibit significant differences between
the conformers, likely due to subtle steric differences. As for the
distances, the configurations are quite similar, with d(Co−Nam)
and d(Co−Npy) averaging 1.952(9) and 2.117(22) Å,
respectively.
Ene-amide CC bond lengths average 1.339(6) Å, but the
related C−N bonds are slightly shorter in the Co2 conformer
(1.394(4) (av) vs 1.426(5) (av) Å). With the eclipsed CNC
orientation in the Co1 conformer, the ene-amide N(p)-orbitals
compete for donation into the same Co(dπ)-orbital(s), whereas
in the Co2 conformer, the staggered ene-amide disposition
permits the related interactions to occur with two different
Co(dπ)-orbitals. While it is tempting to invoke slightly greater
NCC conjugation in Co1 as a consequence, the Co−N bond
lengths do not really support this argument, as the d(Co2−N)
values are slightly longer than those corresponding to Co1, and
all the distances are within 3σ. Localized geometric differences
in the conformers are likely a consequence of subtle electronic
factors, but are too small to support substantive arguments, and
the fact that both conformations exist in the asymmetric unit
reflects their overall similar energies. It must be noted that the
structure is of low resolution, and metric parameters are less
reliable than normal; hence, the arguments are intrinsically
limited.
3. DISCUSSION
3.1. Mechanism of Cr-Based Ene-amide Coupling.
Scheme 3 illustrates a probable mechanism for the formation of
EA2 from {(2,6-iPr2C6H3)(1-cHexenyl)N}2Cr (1-Cr). Note
that NMR spectroscopic monitoring was limited due to the
broad resonances in this system and in the subsequent iron
case. As portrayed, the generation of {(2,6-iPr2C6H3)-
(1-cHexenyl)N}2CrCl from CrCl3 and 2 equiv of
{(2,6-iPr2C6H3)(1-cHexenyl)N} enables a subsequent chlorine
atom transfer to afford {(2,6-iPr2C6H3)(1-cHexenyl)N}CrCl2.
The atom transfer is envisaged to occur via a {(2,6-iPr2C6H3)-
(1-cHexenyl)N}2Cr(μ-Cl)2CrCl2 binuclear complex that can
break apart into CrCl2 and the Cr(IV) transient. Oxidative
coupling from the Cr(IV) center ensues, providing EA2 and
Cr2(THF)2. It is plausible that other complexities, such as the
disproportionation of {(2,6-iPr2C6H3)(1-cHexenyl)N}2CrCl to
the Cr(IV) intermediate and 1-Cr, can happen, but the latter
species was not directly observed during the process; hence, its
conversion to Cr(II) and EA2 must be rapid if invoked.
3.2. Mechanism of Fe-Based Ene-amide Coupling. For
the ene-amide coupling derived from {(2,6-iPr2C6H3)-
(1-cHexenyl)N}2FePMe3 (2-Fe), shown in eq 9, a plausible
mechanism is given in Scheme 4. The process could operate in
a pathway directly analogous to that of Cr, but an Fe(I)/Fe(III)
path is favored instead on the basis of the fact that ferrecinium
oxidation of 2-Fe induces coupling, yet this oxidant is unlikely
to afford an Fe(IV) species related to the Cr(IV) complex of
Scheme 4. Ferric chloride oxidation of 2-Fe is likely to produce
{(2,6-iPr2C6H3)(1-cHexenyl)N}2FeCl, or a PMe3/solvent
adduct, and it oxidatively couples its bound ene-amides to
afford EA2 and FeCl, which can redistribute with another
equivalent of FeCl3 to produce 2 equiv of FeCl2. The process
2.4.3. Coupling Attempts. All efforts at inducing coupling
reactions via {(2,6-iPr2C6H3)(1-cHexenyl)N}2Copy2 (3-Co)
and various oxidants failed, with the parent imine observed as
a product with several reagents under varied conditions.
H
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also seems viable in the context of eq 10, in which
{(2,6-iPr2C6H3)(1-cHexenyl)N}Li was treated with FeCl3 to
provide a reasonable amount of EA2. While the path in Scheme
4 could also be used to rationalize the Cr-based ene-amide
coupling, the generation of Cr(I) under the reaction conditions
seems less likely than the generation of Cr(IV).
Wolczanski, P. T.; DeBeer, S.; Santiago-Berrios, M.; Abruna, H. D.;
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Inorg. Chem. 2011, 50, 12414−12436. (c) Frazier, B. A.; Wolczanski, P.
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4. CONCLUSIONS
Low-coordinate ene-amide complexes, {(2,6-iPr2C6H3)-
(1-cHexenyl)N}2Cr (1-Cr) and {(2,6-iPr2C6H3)(1-cHexenyl)-
N}2FePMe3 (2-Fe), exhibit different hapticities, but both
generate rac-{2,2′-di(2,6-iPr2C6H3N)-dicyclohexane} EA2
when oxidatively triggered by CrCl3 and FeCl3, respectively.
Tetrahedral {(2,6-iPr2C6H3)(1-cHexenyl)N}2Copy2 (3-Co), in
contrast, did not generate any coupled product when subjected
to oxidants. An unusual imine arylation occurred when 2-Fe
was treated with PhBr, and while some catalysis of the arylation
of {(2,6-iPr2C6H3)(1-cHexenyl)N}Li was achieved, low turn-
over numbers (4−9) and sporadic reactivity may hamper any
significant application.
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5. EXPERIMENTAL SECTION
Reasonable details have been given in the equations, figures, and
schemes, and a detailed experimental section, including procedural and
spectroscopic details is given as Supporting Information.
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ASSOCIATED CONTENT
* Supporting Information
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The Supporting Information is available free of charge on the
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chem.5b02990.
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Full experimental details, including procedures, and X-ray
structural details, such as metric data pertaining to the
monoclinic polymorph of 1-Cr, and EA2 (PDF)
Crystallographic details (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: ptw2@cornell.edu. Fax: 607 255 4173.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
P.T.W. thanks the NSF (CHE-1402149) and Cornell
University for financial support, and Dr. Samantha N.
MacMillan for crystallographic help.
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