Dearomatized BIAN alkaline-earth alkyl catalysts for the intramolecular hydroamination of hindered aminoalkenes

Article abstract of DOI:10.1021/om400955v

Reaction of a sterically encumbered bis(imino)acenapthene (dipp-BIAN) with either potassium alkyl or the heavier alkaline-earth dialkyl [Ae{CH(SiMe ₃)₂}₂(THF)₂] (Ae = Mg, Ca, Sr) reagents results in dearomatizat

Full text of DOI:10.1021/om400955v

Article
pubs.acs.org/Organometallics
Dearomatized BIAN Alkaline-Earth Alkyl Catalysts for the
Intramolecular Hydroamination of Hindered Aminoalkenes
Merle Arrowsmith, Michael S. Hill,* and Gabriele Kociok-Kohn
̈
Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, U.K.
S
* Supporting Information
ABSTRACT: Reaction of a sterically encumbered bis(imino)-
acenapthene (dipp-BIAN) with either potassium alkyl or the
heavier alkaline-earth dialkyl [Ae{CH(SiMe3)2}2(THF)2] (Ae
= Mg, Ca, Sr) reagents results in dearomatization of the
aromatic ligand. The heteroleptic alkaline-earth alkyl species
show enhanced stability toward Schlenk-type redistribution
but undergo solution exchange when the bis(trimethylsilyl)-
methyl substituent is replaced by an anionic ligand of lower
overall steric demands. In contrast, analogous reactions performed with [Ba{CH(SiMe3)2}2(THF)2] evidenced facile solution
redistribution and resulted in an unusual C−C coupling reaction which is suggested to result from a sterically induced reductive
process. An assessment of the Mg, Ca, and Sr alkyl compounds as precatalysts for the intramolecular hydroamination of
aminoalkenes evidenced enhanced reactivity, which is ascribed to the greater solution stability of the catalytically active species.
Most notably the calcium species may even be applied to the high-yielding cyclization of substrates bearing alkyl substitution at
either of the alkenyl positions.
BIAN ligands may also act as electron acceptors in the presence
of strongly reducing metals. For example the addition of 1, 2, 3
or 4 equiv of elemental sodium to I respectively yields the
mono-, di-, tri-, and tetraanionic salts [{I}n−{Nan(S)m}n+] (n =
1−4, S = Et2O, THF).6 This chemistry, extended to s- and p-
block metals7 and, more recently, to f-block elements,8 has
been extensively explored by the group of Fedushkin.
INTRODUCTION
■
In the catalogue of readily available coordinating α-diimines,
bis(imino)acenaphthene (BIAN) ligands such as I have gained
some prominence over the last two decades. Although first
reported in the 1960s,1 the use of this class of ligand only
became popular in the late 1990s.
The redox properties of the BIAN ligand have also led to
some unusual reactivity for main-group and, of most relevance
to the current work, group 2 complexes. The magnesium
species [{I}Mg(THF)3] (II), for example, has been shown to
act as a reducing agent for nonenolizable ketones via single-
electron transfer to form the corresponding magnesium
alkoxide radical anion complex.9 Oxidation of complex II by
halogenated tin, copper, mercury, or silicon reagents led to the
isolation of radical-anion magnesium halides, such as
compound III.10 Furthermore, compound II and its strontium
and barium analogues have also been shown to deprotonate
acidic substrates such as enolizable ketones,11 phenylacety-
lene,12 and α-acidic nitriles13 to give the corresponding enolate,
acetylide, and keteniminate group 2 complexes of the
protonated monoanionic amine-amido ligand I-H−. Alkylation
of the BIAN imine carbon has also been observed in the
reaction between II and ethyl halides, which resulted in the
formation of complex IV via ethylation of the carbon adjacent
to the amide functionality.14 The mechanism proposed by the
authors for the formation of this compound involves a single-
electron transfer from II to the ethyl halide, yielding the
The ability of neutral BIAN ligands to act as strong and rigid
chelators for metals in a variety of oxidation states has given rise
to a rich transition-metal coordination and catalytic chemistry.2
In particular, Elsevier and co-workers have reported the use of
palladium and platinum aryl-BIAN complexes for catalytic C−
C bond formation, hydrogenation, and hydrosilylation
reactions,3 while Brookhart4 and Coates5 have demonstrated
the efficiency of cationic nickel(II), palladium(II), and
platinum(II) aryl-BIAN precatalysts in the polymerization of
olefins. Because of the presence of the naphthalene backbone,
Received: September 26, 2013
Published: December 11, 2013
© 2013 American Chemical Society
206
dx.doi.org/10.1021/om400955v | Organometallics 2014, 33, 206−216

Organometallics
Article
intermediate radical anion magnesium halide complex III, and
Et•, followed by radical attack of Et• to form complex IV. A
similar alkyl transfer has also been observed in the reaction
IX, supported by a meta- or para-alkylated dearomatized
(imine-amido)dihydropyridine ligand.22 A similar strategy had
already been employed to obtain heteroleptic dihydroterpyr-
idide lanthanide alkyl complexes in a selective alkylation/
dearomatization reaction.23
We have previously described, in preliminary form,
dearomatization reactions of dipp-substituted bis(imino)-
acenaphthenes with group 1 and group 2 derivatives of the
sterically demanding bis(trimethylsilyl)methyl anion.24 In this
contribution we provide a complete description of this
reactivity and the application of the resultant heteroleptic
group 2 species for the intramolecular hydroamination of
hindered aminoalkenes.
n
between BuLi or MR3 (M = group 13 metal, R = alkyl
substituent) and I in noncoordinating solvents, leading to the
formation of a imine-amido complex bearing an alkyl
substituent on the carbon adjacent to the amide functionality.15
More recently Cowley and co-workers also reported the double
alkylation of the acenaphthene backbone of I in the C4 and C5
positions by tert-butyllithium, via a radical dearomatization,
two-electron reduction pathway, leading to the formation of
complex V.16
Our studies have focused on the coordination chemistry and
catalytic properties of hitherto understudied alkaline-earth
complexes.17 While rapid progress has been made over the
past decade, one major challenge remains the stabilization of
heteroleptic species of the type [LAeR] (L = monoanionic
ligand, Ae = alkaline-earth dication, R = reactive coligand), as
the heavier alkaline-earth-metal complexes are prone to
Schlenk-type ligand redistribution processes in solution,
which hamper catalytic activity.18 Examples of heteroleptic
group 2 alkyl complexes remain particularly rare, as many
conventional ligand systems, such as the ubiquitous β-
diketiminate ligand [HC{C(Me)Ndipp}2]− (dipp = 2,6-
diisopropylphenyl), are prone to deprotonation reactions by
the highly reactive alkaline-earth-bound alkyl functionality.19
Thus, only the calcium β-diketiminato bis(trimethylsilyl)methyl
derivative VI could be prepared from the reaction between
[{HC{C(Me)Ndipp}2}H] and the homoleptic precursor
[Ca{CH(SiMe3)2)2(THF)2].19b Harder and co-workers have
also reported several heteroleptic calcium, strontium, and
barium benzyl complexes for the catalytic polymerization of
styrene.20 More recently, Carpentier and Sarazin have described
a series of analogous heteroleptic heavier alkaline-earth alkyl
complexes, compounds VIIa−c, supported by a sterically
demanding iminoanilide ligand, and reported their successful
application as precatalysts for intra- and intermolecular
hydroamination reactions.21
EXPERIMENTAL SECTION
■
General Procedures. All glassware was dried for 24 h in an oven
at 150 °C prior to use. Solvents for air- and moisture-sensitive
reactions were provided by an Innovative Technology Solvent
Purification System. Hexane and toluene were used as collected.
THF and diethyl ether were dried by distillation over sodium−
benzophenone ketyl. All solvents were stored under argon over
molecular sieves activated at 150 °C in vacuo. C6D6 and d8-toluene
were purchased from Fluorochem and dried over molten potassium
prior to vacuum transfer into a sealed ampule and storage in the
glovebox under argon. Tetrakis(trimethylsilyl)silane (TMSS) was
purchased from Goss Scientific Instruments Ltd. and used as received.
After drying over CaH2 all aminoalkenes were distilled in vacuo and
either vacuum-transferred into sealed ampules or freeze-dried prior to
transfer into the glovebox. The aminoalkene substrates,25 the ligand
precursor I,2g the potassium precursors [KCH2Ph]26 and [KCH-
(SiMe3)2],27 and the homoleptic group 2 alkyls [M{CH-
(SiMe3)2}2(THF)2] (M = Mg, Ca, Sr, Ba)28 were synthesized
following literature procedures. Reactions were carried out under an
argon atmosphere using standard Schlenk line and glovebox
techniques in an MBraun Labmaster glovebox at O2, H2O <0.1
ppm. NMR experiments were conducted in Youngs tap NMR tubes
prepared and sealed in a glovebox under argon. NMR data was
acquired at room temperature on a Bruker 300 Ultrashield (13C, 75.48
MHz) and spectra were referenced using residual solvent resonances.
Elemental analyses were performed by Stephen Boyer at London
Metropolitan Enterprises.
Synthesis of Compounds 1−9. The numbering scheme used for
the NMR assignments of compounds 1−9 follows that in Schemes 1
and 2.
Synthesis of 1. A 25 mg portion of [KCH(SiMe3)2] (99.8 μmol)
and 50 mg of I (99.8 μmol) were dissolved in C6D6. The reaction
mixture instantly turned dark green (NMR yield >99%). Green
crystals suitable for X-ray crystallography were isolated upon
crystallization from toluene at room temperature. The reaction was
later scaled up in toluene with [KCH(SiMe3)2] (396 mg, 2.00 mmol)
and I (1.00 g, 2.00 mmol): 95% isolated yield (1.32 g, 1.90 mmol). 1H
NMR ppm (C6D6): 7.00−7.21 (m, 6H, Ar-H), 6.51 (d, 1H, H-3′, 3J =
3
3
7.5 Hz), 6.03 (t, 1H, H-4′, J = 7.5 Hz), 5.65 (d, 1H, H-5′, J = 7.5
Hz), 5.60 (d, 1H, H-3, 3Jcis = 9.8 Hz), 4.65 (dd, 1H, H-4, 3Jcis = 9.8 Hz,
3J = 4.1 Hz), 4.41 (broad, 1H, H-5), 3.59−3.60 (m, 2H, Pr-CHMe2),
i
3.06 + 2.98 (two sept, 1H each, iPr-CHMe2, 3J = 6.7 Hz), 1.51 + 1.44
i
3
(two d, 3H each, Pr-CH3, J = 6.7 Hz), 1.23 + 1.17 + 1.13 (three d,
9H:3H:6H, iPr-CH3, 3J = 6.7 Hz), 0.55 (s, 1H, CCH(SiMe3)2), 0.18 +
0.04 (two s, 9H each, CCH(SiMe3)2). 13C{1H} NMR ppm (C6D6):
175.7 (C-1′), 153.9, 152.5, 148.7, 148.5, 141.8, 141.6, 136.6, 136.4,
129.9, 128.2, 127.9, 125.4, 124.4, 124.2, 123.7 (C-5′ + C-3), 123.1,
123.0, 122.9, 121.2 (C-4′), 120.4, 116.8 (C-4), 102.7, 38.5 (C-5), 29.1
+ 28.9 + 28.6 + 28.5 (iPr-CHMe2), 25.2 (CCH(SiMe3)2), 24.9 + 24.7
+ 24.6 + 24.4 + 23.9 + 23.8 (iPr-CH3), 2.3 + 2.1 (CCH(SiMe3)2). Anal.
Calcd for C43H59KN2Si2 (699.2): C, 73.86; H, 8.51; N, 4.01. Found: C,
73.79; H, 8.59; N, 3.96. ESI-MS for the protonated dearomatized
ligand [C43H60N2Si2 − Na]+: calcd 683.41928, found 683.4846.
We have reported that bis(imino)pyridine ligands undergo
double deprotonation of the β-imino-methyl substituents in the
presence of the alkaline-earth dialkyls [M{CH-
(SiMe3)2}2(THF)2] (M = Mg, Ca, Sr, Ba). In the case of the
calcium, strontium, and barium precursors this reaction
proceeded via the formation of unstable, intermediate,
heteroleptic alkaline-earth alkyl complexes, such as VIII and
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Article
3
3
Synthesis of 2. [K{CH(SiMe3)(2-NMe2-Ph)}] (25 mg, 83.5 μmol)
and I (42 mg, 83.5 μmol) were dissolved in d8-toluene. The solution
instantly turned dark green. Crystallization after 1 day at room
temperature afforded dark green crystals of the R,S + S,R diastereomer
of complex 2 (51 mg, 68.3 μmol, 82%). 1H NMR ppm (d8-tol): major
R,S + S,R diastereomer (88%), 6.88−7.20 (m, 10H, Ar-H), 6.04 (d,
1H, H-3′, 3J = 7.7 Hz), 5.60 (t, 1H, H-4′, 3J = 7.7 Hz), 5.44−5.50 (m,
2H, H-3/5′), 4.63−4.67 (m, 1H, H-4), 3.81 (broad s, 1H, H-5), 3.49
(dd, 1H, Jcis = 10.2, J = 4.4 Hz), 4.12 (m, 1H, H-5), 3.62 (sept, 2H,
iPr-CHMe2, J = 6.8 Hz), 3.51 (broad m, THF), 3.24 (sept, 2H, Pr-
CHMe2, 3J = 6.7 Hz), 1.49 (d, 6H each, iPr-CH3, 3J = 6.7 Hz), 1.41 +
1.34 (two d, 3H each, iPr-CH3, 3J = 6.8 Hz), 1.18−1.31 (m, 20H, THF
3
i
i
3
+ Pr-CH3), 0.48 (d, 1H, CCH(SiMe3)2, J = 1.6 Hz), 0.21 (s, 18H,
CaCH(SiMe3)2), 0.04 + 0.00 (two s, 9H each, CCH(SiMe3)2), −1.72
(s, 1H, CaCH(SiMe3)2). 13C{1H} NMR ppm (C6D6): 181.7 (C-1′),
150.1, 149.7, 148.2, 146.1, 143.3, 142.7, 138.8, 137.0, 132.5 (C-3′),
131.2, 126.4, 126.2, 124.5, 124.5 (C-3), 124.4, 124.0, 123.8, 123.5 (C-
4), 123.1, 122.7 (C-5′), 122.6 (C-4′), 110.7 (C-2), 69.6 (THF, very
broad), 38.3 (C-5), 29.3 + 29.2 + 28.9 + 28.5 (iPr-CHMe2), 26.0 +
25.6 + 25.3 + 25.2 + 25.2 + 24.7 + 24.4 + 24.3 (iPr-CH3), 25.3 (THF),
23.4 (CCH(SiMe3)2), 14.3 (CaCH(SiMe3)2), 6.1 (CaCH(SiMe3)2),
2.1 + 1.0 (CCH(SiMe3)2). Anal. Calcd for C58H94CaN2O2Si4 (1003.8):
C, 69.40; H, 9.44; N, 2.79. Found: C, 69.34; H, 9.34; N, 2.73.
Synthesis of 5. A 50 mg portion of [Sr{CH(SiMe3)2}2(THF)2]
(90.8 μmol) and 45 mg of I (90.8 μmol) were dissolved in C6D6. The
i
3
(sept, 2H, Pr-CHMe2, J = 6.7 Hz), 3.35 (s, 1H, CH(SiMe3)), 2.95
3
i
(sept, 1H, iPr-CHMe2, J = 6.6 Hz), 2.85 (broad m, 1H, Pr-CHMe2),
2.33 (s, 6H, NMe2), 1.52 (d, 3H, iPr-CH3, 3J = 6.6 Hz), 1.30−1.44 (m,
i
i
i
3
6H, Pr-CH3), 0.98−0.19 (m, 12H, Pr-CH3), 0.81 (d, 3H, Pr-CH3, J
= 6.7 Hz), −0.03 (s, 9H, SiMe3); minor R,R + S,S diastereomer (12%),
3
6.66 (d, 1H, H-3′, J = 7.7 Hz), 5.85−5.88 (m, 1H, H-4′), 3.46−3.49
(m, 1H, H-3/5′), 4.78 (broad d, 1H, H-4, 3Jcis = 10.1 Hz), 3.81 (broad
i
s, 1H, H-5), 3.53 (m, 2H, Pr-CHMe2 + CH(SiMe3), identified by
i
COSY), 2.95 + 2.85 (two broad sept, 1H each, Pr-CHMe2), 2.45 (s,
i
1
6H, NMe2), 0.99−1.57 (m, 24H, Pr-CH3), 0.00 (s, 9H, SiMe3).
reaction mixture instantly turned dark green. Analysis by H NMR
13C{1H} NMR ppm (d8-tol): mixture of both diastereomers, 174.1 (C-
1), 153.2, 152.6, 148.0, 141.1, 140.8, 140.1, 137.8, 137.7, 137.6, 137.2,
137.1, 136.1, 135.1, 132.1 (C-3′), 128.1, 125.3, 123.8, 123.7, 123.5,
123.0 (C-3/5′), 122.5 (C3/5′), 120.7, 119.3 (C-4′), 111.8 (C-4), 45.1
(NMe2), 42.1 (C-5), 40.1 (CH(SiMe3)), 29.2 +29.1 + 28.8 + 28.4 (iPr-
CHMe2), 25.7 + 25.2 + 25.1 + 24.5 + 24.4 + 24.2 + 23.5 + 23.4 + 23.2
(iPr-CH3), −0.04 (SiMe3). Anal. Calcd for C288H360K6N18Si6
(4477.14): C, 77.26; H, 8.10; N, 5.63. Found: C, 77.22; H, 8.19; N,
5.58.
spectroscopy showed the clean formation of complex 5 as the sole
product of the reaction (NMR yield >99%). The reaction was scaled
up ([Sr{CH(SiMe3)2}2(THF)2], 551 mg, 1 mmol; I, 0.50 g, 1 mmol),
and the mixture was stirred for 10 min at room temperature in toluene
prior to removal of the solvent in vacuo. As the product refused to
crystallize in 0.2 mL of pentane at −30 °C over a period of several
weeks, complex 5 was used as a crude green solid, whose purity was
confirmed by NMR spectroscopy and elemental analysis data. 1H
NMR ppm (C6D6): 6.97−7.16 (m, 6H, Ar-H), 6.45 (d, 1H, H-3′, 3J =
7.7 Hz), 6.06 (t, 1H, H-4′, 3J = 7.7 Hz), 5.41 (dd, 1H, H-3, 3Jcis = 10.2
Synthesis of 3. A 50 mg portion of [Mg{CH(SiMe3)2}2(THF)2]
(103.9 μmol) and 52 mg of I (103.9 μmol) were dissolved in C6D6.
The reaction mixture was heated to 60 °C overnight and slowly turned
dark green. Analysis by 1H NMR spectroscopy showed the clean
formation of complex 3 as the sole product of the reaction (NMR yield
>99%). The reaction was scaled up ([Mg{CH(SiMe3)2}2(THF)2], 271
mg, 0.599 mmol; I, 300 mg, 0.599 mmol), and the mixture was heated
for 24 h at 60 °C in toluene prior to removal of the solvent in vacuo.
As the product refused to crystallize in 0.2 mL of pentane at −30 °C
over a period of several weeks, complex 3 was used as a crude green
solid, whose purity was confirmed by NMR and elemental analysis
4
3
Hz, J = 2.1 Hz), 5.40−5.42 (m, 1H, H-5′), 4.89 (dd, 1H, H-4, Jcis
=
10.2, 3J = 4.6 Hz), 4.15 (m, 1H, H-2), 3.68 (sept, 2H, iPr-CHMe2, 3J =
i
3
6.8 Hz), 3.37 (broad m, THF), 3.27 (sept, 2H, Pr-CHMe2, J = 6.7
i
3
Hz), 1.53 + 1.52 (two d, 3H each, Pr-CH3, J = 6.7 Hz), 1.38 + 1.36
(two d, 3H each, iPr-CH3, 3J = 6.8 Hz), 1.25−1.31 (m, 20H, iPr-CH3 +
THF), 0.49 (d, 1H, CCH(SiMe3)2, 3J = 1.6 Hz), 0.30 (s, 18H,
SrCH(SiMe3)2), 0.06 + −0.01 (two s, 9H each, CCH(SiMe3)2), −1.86
(s, 1H, SrCH(SiMe3)2). 13C{1H} NMR ppm (C6D6): 181.1 (C-1′),
151.1, 150.2, 149.0, 146.9, 143.3, 142.6, 138.6, 138.4, 132.4 (C-3′),
131.0, 126.3, 125.9, 124.5 (C-5′), 124.2, 124.3, 123.7, 123.1, 123.0,
122.9 (C-3), 122.5 (C-4), 122.2 (C-4′), 109.5 (C-2), 69.4 (THF, very
broad), 38.4 (C-5), 29.5 + 29.3 + 29.0 + 28.6 (iPr-CHMe2), 26.0 +
25.7 + 25.4 + 25.3 + 25.2 + 24.9 + 24.4 + 24.3 (iPr-CH3), 25.3 (THF),
23.6 (CCH(SiMe3)2), 19.1 (SrCH(SiMe3)2), 6.4 (SrCH(SiMe3)2), 2.2
+ 1.1 (CCH(SiMe3)2). Anal. Calcd for C58H94N2O2Si4Sr (1051.3): C,
66.26; H, 9.01; N, 2.66. Found: C, 66.33; H, 8.97; N, 2.70.
Synthesis of 6. A 50 mg portion of [Ba{CH(SiMe3)2}2(THF)2]
(83.3 μmol) and 42 mg of I (83.3 μmol) were dissolved in C6D6. The
reaction mixture instantly turned dark green. 1H NMR data evidenced
the presence of three components, among which were the heteroleptic
barium alkyl complex 6 (ca. 70%) and the homoleptic barium complex
7 (ca. 20%). Upon recrystallization of the crude mixture from toluene
at −30 °C for 1 week, orange crystals of compound 8 suitable for X-
ray diffraction analysis were isolated from the reaction mixture. The
isolated amount of 8 was, however, insufficient for the acquisition of
either NMR or elemental analysis data. Although attempts to separate
6 and 7 by fractional crystallization failed, preventing the acquisition of
elemental analysis data for compound 6, compound 7 was synthesized
1
data. H NMR ppm (C6D6): 7.15−7.25 (m, 6H, Ar-H), 6.45 (d, 1H,
H-3′, 3J = 7.7 Hz), 6.05 (t, 1H, H-4′, 3J = 7.7 Hz), 5.47 (dd, 1H, H-3,
4
3Jcis = 10.2 Hz, J = 2.3 Hz), 5.44−5.47 (m, 1H, H-5′), 5.05 (dd, 1H,
3
3
H-4, Jcis = 10.2, J = 4.4 Hz), 4.07 (m, 1H, H-5), 3.78 (broad m,
i
3
THF), 3.55 + 3.11 (two sept, 2H each, Pr-CHMe2, J = 6.8 Hz),
1.41−1.44 (m, 14H, iPr-CH3i+ THF), 1.39 + 1.36 + 1.31 + 1.29 + 1.16
+ 1.13 (six d, 3H each, Pr-CH3, 3J = 6.8 Hz)), 0.46 (d, 1H,
3
CCH(SiMe3)2, J = 1.6 Hz), 0.00 (s, 18H, MgCH(SiMe3)2), 0.01 +
−0.03 (two s, 9H each, CCH(SiMe3)2), −1.59 (s, 1H,
MgCH(SiMe3)2). 13C{1H} NMR ppm (C6D6): 181.9 (C-1′), 149.5,
148.8, 148.2, 146.5, 144.5, 139.5, 139.4, 133.1, 133.0 (C-3′), 131.8,
127.2, 126.0, 125.8 (C-4), 125.2, 124.9, 124.8 (C-5′), 124.6, 124.5,
124.0, 123.4 (C-4′), 122.6 (C-3), 122.1, 111.8 (C-2), 68.9 (THF, very
broad), 38.4 (C-5), 29.1 + 28.9 + 28.6 + 28.6 (iPr-CHMe2), 25.9 +
25.33 + 25.29 + 25.27 + 25.1 + 24.9 + 24.8 + 24.7 (iPr-CH3), 23.9
(THF), 22.7 (CCH(SiMe3)2), 5.6 (MgCH(SiMe3)2), 2.1 + 1.0
(CCH(SiMe3)2), −3.1 (MgCH(SiMe3)2). Anal. Calcd for
C54H86MgN2OSi4 (915.9): C, 70.81; H, 9.46; N, 3.06. Found: C,
70.79; H, 9.36; N, 2.95.
1
and analyzed by independent methods (vide infra). H NMR ppm
3
(C6D6) for 6: 7.08−7.23 (m, 6H, Ar-H), 6.48 (d, 1H, H-3′, J = 7.7
Synthesis of 4. A 50 mg portion of [Ca{CH(SiMe3)2}2(THF)2]
3
Hz), 6.08 (t, 1H, H-4′, J = 7.7 Hz), 5.47−5.51 (m, 2H, H-5′ + H-4),
(99.8 μmol) and 50 mg of I (99.8 μmol) were dissolved in C6D6. The
3
3
1
4.88 (dd, 1H, H-4, Jcis = 10.2, J = 4.4 Hz), 4.21 (m, 1H, H-5), 3.69
reaction mixture instantly turned dark green. H NMR data showed
i
3
the clean formation of complex 4 as the sole product of the reaction
(NMR yield >99%). The reaction was scaled up ([Ca{CH-
(SiMe3)2}2(THF)2], 1.00 g, 2 mmol; I ,1.00 g, 2 mmol), and the
mixture was stirred for 10 min at room temperature in toluene prior to
removal of the solvent in vacuo. As the product refused to crystallize in
0.2 mL of pentane at −30 °C over a period of several weeks, complex
4 was used as a crude green solid, whose purity was confirmed by
NMR spectroscopy and elemental analysis data. 1H NMR ppm
(sept, 2H, Pr-CHMe2, J = 6.8 Hz), 3.32 (broad m, 8H, THF), 3.27
(sept, 2H, iPr-CHMe2, 3J = 6.7 Hz), 1.53 + 1.49 (two d, 3H each, iPr-
CH3, 3J = 6.7 Hz), 1.37 + 1.34 (two d, 3H each, iPr-CH3, 3J = 6.8 Hz),
1.22−1.32 (m, 20H, THF + Pr-CH3), 0.50 (d, 1H, CCH(SiMe3)2, J
= 1.6 Hz), 0.29 (s, 18H, BaCH(SiMe3)2), 0.09 + 0.04 (two s, 9H each,
CCH(SiMe3)2), −1.73 (broad s, 1H, BaCH(SiMe3)2). 13C{1H} NMR
ppm (C6D6): 179.9 (C-1′), 150.8, 149.8, 148.8, 147.0, 143.2, 142.7,
138.0, 135.7, 132.2 (C-3′), 131.1, 126.1, 125.6, 124.4 (C-5′), 124.2,
124.0, 123.9, 123.8, 123.6, 123.2, 122.9 (C-3), 122.6 (C-4), 122.0 (C-
4′), 107.5 (C-2), 68.3 (THF, very broad), 38.4 (C-5), 33.8 (Ba−
i
3
3
(C6D6): 7.15−7.26 (m, 6H, Ar-H), 6.46 (d, 1H, H-3′, J = 7.7 Hz),
3
6.06 (t, 1H, H-4′, J = 7.7 Hz), 5.43−5.47 (m, 2H, H-5′ + H-3), 4.96
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Scheme 1
CH(SiMe3)2), 29.6 + 29.5 + 29.0 + 28.8 (iPr-CHMe2), 25.6 + 25.5 +
25.2 + 25.1 + 25.0 + 24.9 + 24.5 + 24.3 (iPr-CH3), 25.4 (THF), 23.8
(CCH(SiMe3)2), 6.1 (BaCH(SiMe3)2), 2.2 + 1.4 (CCH(SiMe3)2).
Independent Synthesis of 7. (a) A 15 mg portion of [Ba{CH-
(SiMe3)2}2(THF)2] (25.0 μmol) and 25 mg of I (50 μmol) were
dissolved in d8-toluene at room temperature. After 10 min the solution
had turned a homogeneous dark green. Analysis by 1H NMR
spectroscopy showed the clean formation of 7 as the sole product of
the reaction.
cocrystallized with one solvent molecule of toluene. Compound 2: the
asymmetric unit contains one-sixth of a hexameric complex. It also
contains one toluene solvent molecule with 50% occupation,
disordered over a 2-fold axis. Another toluene molecule is disordered
about a 3-fold axis with 33% occupation. Traces of a further solvent
molecule were located over a 3-fold axis but could not be properly
solved. The very large crystal diffracted very well at low angle but
presented a falloff in intensity above 2θ = 35°. As a consequence, the
solvent disorders could not be solved satisfactorily. The PLATON
program SQUEEZE was employed to account for the solvent-
containing voids. Checkcif detected a potential disorder in one of the
isopropyl groups (C39−C41). Because of the lack of high-angle
intensity data this could not be properly solved. Compound : the
crystal was small and only weakly diffracting, hence the high R(int)
value. The asymmetric unit contained the main complex and three
solvent molecules: two half-occupied THF molecules (bond lengths to
O3 had to be restrained) and one benzene molecule disordered over
two sites in a 63:37 ratio. Atoms of the minor part of the disordered
benzene were refined isotropically. ADP for C122 had been restrained.
All methyl groups of the bis(trimethylsilyl)methyl substituent
containing Si1 and Si2 displayed rotational disorder in a 42:58 ratio.
Si−C bond lengths herein have been restrained as well as ADPs of
C39 and C43.
(b) [KCH(SiMe3)2] (300 mg, 1.50 mmol) and [Ba{CH-
(SiMe3)2}2(THF)2] (455 mg, 0.75 mmol) were stirred in THF at
room temperature for 24 h prior to removal of the solvent in vacuo
and extraction into toluene. Removal of the solvent yielded complex 7
as a crude dark green solid whose 1H NMR spectrum was reminiscent
of that obtained from the reaction described in (a). 1H NMR ppm (d8-
tol): 6.84−7.32 (m, 12H, Ar-H), 6.46 (d, 2H, H-3′, 3J = 7.7 Hz), 6.00
3
3
4
(t, 2H, H-4′, J = 7.7 Hz), 5.34 (dd, 2H, H-5′, J = 7.7 Hz, J = 4.2
Hz), 5.17 (dd, 2H, H-3, 3Jcis = 10.2 Hz, 4J = 1.5 Hz), 4.77 (dd, 2H, H-4,
3Jcis = 10.2, 3J = 4.3 Hz), 4.16 (broad s, 2H, H-5), 3.46−3.62 (m, 12H,
i
i
3
THF + Pr-CHMe2), 3.09 (sept, 4H, Pr-CHMe2, J = 6.6 Hz), 1.45
i
3
(m, 8H, THF), 1.38 + 1.35 (two d, 6H each, Pr-CH3, J = 6.7 Hz),
1.24 (d, 6H, iPr-CH3, 3J = 6.8 Hz), 1.03−1.14 (m, 30H, iPr-CH3), 0.49
3
(broad s, 2H, CH(SiMe3)2, J = 1.6 Hz), 0.02 + −0.04 (two s, 9H
each, SiMe3). 13C{1H} NMR ppm (d8-tol): 180.3 (C-1′), 151.0,
150.70, 150.69, 149.9, 147.69, 147.64, 143.7, 143.25, 143.22, 138.73,
138.70, 138.61, 138.56, 138.15, 138.23, 137.47, 137.57, 132.5, 131.1,
126.7, 126.0, 124.9, 124.65, 124.71, 124.6, 123.77, 123.73, 123.5,
122.3, 122.1, 109.22, 109.19, 68.0 (THF, broad), 38.8 (C-5), 29.9 +
29.7 + 29.5 + 29.1 +28.8 (iPr-CHMe2), 26.24 + 26.18 + 26.11 + 25.9 +
25.5 + 24.4 + 23.8 (iPr-CH3), 25.1 (THF), 23.4 + 23.2 (CH(SiMe3)2),
2.8 + 1.3 (SiMe3). Repeated attempts at obtaining elemental analysis
data for this complex failed due to its extreme air and moisture
sensitivity.
RESULTS AND DISCUSSION
■
Addition of the potassium alkyl precursors [KCH(SiMe3)2] and
[KCH(SiMe3){2-(NMe2)C6H4}] to an orange suspension of I
in C6D6 resulted in a rapid color change to form a clear, dark
green solution (Scheme 1). In both cases analysis by 1H NMR
spectroscopy showed quantitative conversion of the starting
materials to a single potassium complex, compounds 1 and 2,
respectively, displaying six distinct multiplets in the region from
4.4 to 6.5 ppm, characteristic of selective dearomatization of the
acenaphthene backbone and alkyl functionality transfer. The
alkylation site of the ligand backbone was identified by a COSY
experiment as the C5 position (Scheme 1), which was
confirmed by two subsequent X-ray diffraction experiments
(vide infra). Alkylation at this position generated a chiral center
which caused the two magnetically inequivalent (Si(CH3)3)
groups in both complexes to appear as two separate (9H)
singlets. The asymmetric nature of the new imine-amido ligand
system also caused splitting of the isopropyl methine and
Independent Synthesis of 9. Complex 1 (1.05 g, 1.50 mmol) and
CaI2 (220 mg, 0.75 mmol) were stirred in THF at room temperature
for 24 h prior to removal of the solvent in vacuo and extraction into
hexane. Removal of the solvent yielded complex 9 as a crude dark
green solid which was recrystallized from a 10/1 toluene/THF mixture
at −30 °C over several days, yielding complex 9 as a dark green solid
(265 mg, 25%) in a first fraction and green single crystals in a second
fraction (227 mg, 21%). Because of apparent fluxional behavior in
solution 1H NMR resonances were extremely broad and 13C{1H}
NMR signals were too weak for analysis. 1H NMR (C6D6): 6.99−7.25
(m, 12H, Ar-H), 6.45 (broad d, 2H, H-3′, 3J = 7.7 Hz), 6.05 (broad t,
2H, H-4′, 3J = 7.7 Hz), 5.23−5.38 (broad m, 4H, H-3/4), 4.99 (broad
d, 2H, H-5′, 3J = 7.7 Hz), 4.18 (s, 2H, H-5), 3.60 (broad m, 4H, THF),
1
methyl resonances, in both the H and 13C{1H} NMR spectra.
In the case of complex 2 the presence of a second chiral center
at the methine position of the 1-trimethylsilyl-o-dimethylami-
nobenzyl substituent led to the formation of a ca. 9:1 mixture of
two diastereomers, as determined by integration of the H
NMR spectrum.
Recrystallization at room temperature in toluene afforded
large dark green crystals of compounds 1 and 2 suitable for X-
ray diffraction. Although the structure of compound 1 was
described in our earlier communication24 and will thus not be
described in detail here, selected bond length and angle data for
both compounds are given in Table 1 for purposes of
comparison. While compound 1 comprised a centrosymmetric
dimer in the solid state, compound 2 assembles as a hexameric
array in which coordination between molecules is provided by a
i
2.99−3.44 (broad m, 8H, Pr-CHMe2), 0.68−1.34 (broad m, 52H
i
each, Pr-CH3 + THF), 0.51 (broad s, 1H, CCH(SiMe3)2), 0.05 +
1
−0.04 (two broad s, 9H each, CCH(SiMe3)2). Anal. Calcd for
C90H126CaN4OSi4 (1432.4): C, 75.46; H, 8.87; N, 3.91. Found: C,
75.29; H, 8.74; N, 3.84.
Notes on X-ray Crystallographic Refinements. Single-crystal
X-ray diffraction data were collected at 150 K on a Nonius KappaCCD
diffractometer, equipped with an Oxford Cryosystem, using graphite-
monochromated Mo Kα radiation (λ = 0.71073 Å). Data were
processed using the Nonius software.29 Crystal data and data
collection and refinement parameters for 2, 8, and 9 are presented
in Table S1 (Supporting Information). Structure solution, followed by
full-matrix least-squares refinement, was performed using the WINGX-
1.80 suite of programs throughout.30 Compound 1: the dimer
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of the alkylated imine-amido ligand (K−Nimine = 2.762(4) Å;
K−Namide = 2.702(4) Å).
Table 1. Selected Bond Lengths (Å) and Angles (deg) for
Compounds 1, 2, 8, and 9
In a manner similar to the potassium-based reactions,
addition of [M{CH(SiMe3)2}2(THF)2] (M = Ca, Sr) to a
suspension of I in C6D6 resulted in instant solubilization of the
starting materials and a rapid color change to dark green. The
analogous reaction with [Mg{CH(SiMe3)2}2(THF)2] required
24 h of heating at 60 °C to yield the same result (Scheme 2). In
a
a
b
1
2
8
9
M−N1
M−N2
N1−C25
N2−C36
C34−C36
C30−C31
C31−C32
C32−C33
2.694(3)
2.703(3)
1.281(4)
1.335(4)
1.393(4)
1.533(5)
1.517(5)
1.351(5)
2.866(4)
2.702(4)
2.762(4)
1.267(5)
1.318(5)
1.415(6)
1.524(6)
1.514(7)
1.390(6)
2.884(5)
2.571(4)
2.372(4)
1.286(7)
1.355(7)
1.418(8)
1.519(8)
1.510(9)
1.345(8)
1.263(4)
1.283(5)
1.487(5)
1.523(5)
1.563(5)
1.501(5)
1
all three cases H NMR spectra were reminiscent of that of 1,
showing clean formation of the heteroleptic alkaline-earth
organometallic species 3−5, with the additional 18H and 1H
singlets of the metal-bound monoanionic [CH(SiMe3)2]−
coligand at ca. 0.3 and −1.7 ppm, respectively (Figure 2).
Although an analogous reaction with the barium dialkyl
precursor [Ba{CH(SiMe3)2}2(THF)2] also led to an instant
M−centroid
C32−C32′
1.557(6)
N1−M−N2
M−N1−C25
M−N2−C36
N1−C25−C36
C30−C31−C32
C30−C31−C37
C32−C31−C37
C31−C32−C33
C33−C32−C32′
C31−C32−C32′
63.52(8)
118.2(2)
117.4(2)
121.7(3)
111.6(3)
113.4(3)
111.3(3)
61.95(11)
119.7(3)
118.2(3)
119.0(4)
109.6(4)
110.5(4)
109.7(4)
70.07(15)
109.6(3)
114.7(3)
121.6 (5)
111.7(5)
113.7(5)
111.8(5)
126..3(6)
1
color change to dark green, H NMR data at the first point of
analysis showed the formation of three distinct dearomatized
species, among which the heteroleptic barium alkyl complex 6
(50−70% conversion) was identified by the characteristic
upfield BaCH(SiMe3)2 proton shift at −1.73 ppm (Scheme 3).
A subsequent independent synthesis of the homoleptic barium
complex 7 from the reaction between 2 equiv of I and
[Ba{CH(SiMe3)2}2(THF)2] enabled the identification of the
second component of the reaction mixture as complex 7,
present in 25−40% yield.
122.0(3)
114.4(3)
114.7(3)
110.1(3)
117.9(3)
109.9(2)
110.7(4)
a
b
M = K. M = Ca.
A third product, present in up to 10% yield, was only
identified after storage of the reaction mixture in a 1:2 toluene/
hexane solution at −30 °C over several days provided orange
crystals suitable for X-ray diffraction analysis. The result of this
experiment, displayed in Figure 3, shows two C5-alkylated
dearomatized BIAN ligands coupled at the C4 position.
Selected bond length and angle data are provided in Table 1.
Compound 8 crystallizes as a single 4R,4′R,5S,5′S enantiomer
(absolute structure parameter 0.03(15)) in the chiral space
group P3121. As in the ligand precursor, both N−C bond
η interaction between a N2-coordinated potassium atom and
the C30, C34, and C35 carbon atoms of the alkylated backbone
ring of the adjacent complex, with a K−centroid distance of
2.884(5) Å (Figure 1). Compound 2 crystallizes as a single R,S/
S,R diastereomeric couple which, on the basis of the crystalline
yield (∼80%) and its H NMR spectrum, was identified as the
major diastereomer. In contrast to potassium complex 1 the K−
N bond lengths in compound 2 reflect the asymmetric binding
1
Figure 1. ORTEP representation of compound 2: hexameric array (left); two-molecule binding motif (right). Thermal ellipsoids are drawn at the
30% probability level . Isopropyl methyl groups and hydrogen atoms are omitted except for those attached to chiral centers C31 and C37.
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Scheme 2
1
Figure 2. Annotated H NMR spectrum of the dearomatized calcium complex 4.
Scheme 3
lengths (1.263(4), 1.283(5) Å) are indicative of imine bonds.
The presence of the two pseudo-tetrahedral sp3 carbon centers
C31 (C30−C31−C32 = 114.4(3)°; C30−C31−C37 =
114.7(3)°; C32−C31−C37 = 110.1(3)°) and C32 (C31−
C32−C33 = 117.9(3)°; C33−C32−C32′ = 109.9(2)°; C31−
C32−C32′ = 110.7(4)°) induces a significant distortion of the
alkylated C6H3 rings of the ligand backbones, accompanied by
an increase in the C−C bond lengths (C30−C31 = 1.523(5) Å;
C31−C32 = 1.563(5) Å; C31−C32 = 1.501(5) Å). While we
have not fully rationalized the synthesis of this chiral, dimerized
tetraimine product, we suggest its formation is possibly the
result of a sterically induced radical coupling reminiscent of
well-precedented reactivity observed for similarly redox inactive
4f-element complexes.31 In support of this hypothesis, Evans
and Cowley have observed a radical anion C5-alkylated free
radical as an intermediate in the formation of the dialkylated
dearomatized lithium complex V.16
Multiple attempts to obtain crystals of the alkaline-earth
organometallic complexes 3−6 suitable for X-ray analyses were
thwarted by the extreme solubility of these compounds even
after months of storage at −30 °C in minimal amounts of
hydrocarbon solvents such as hexane and pentane. NMR data
and elemental analyses of the crude magnesium, calcium, and
strontium products, however, left no doubt as to their
formulation. Attempts to recover the alkylated amino-imine
ligand by controlled hydrolysis of the organometallic complexes
initially led to a color change from green to purple, followed by
a gradual change toward dark red over a period of several hours.
1H NMR data and later recrystallization of the crude product
from toluene evidenced quantitative conversion to the
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Figure 3. ORTEP representation of compound 8. Thermal ellipsoids are drawn at 30% probability. Isopropyl groups and hydrogen atoms are
omitted except for those attached to the chiral centers C31 and C32.
Scheme 4
Figure 4. ORTEP representation of compound 9. Thermal ellipsoids are drawn at the 30% probability level. Isopropyl methyl groups and hydrogen
atoms are omitted, except for those attached to the chiral centers C31 and C74.
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Table 2. Scope of Intramolecular Hydroamination with Precatalysts 3−5
a
b
c
NMR yields were measured against an internal TMSS standard in C6D6 or d8-toluene. 11% isomerization to the internal aminoalkene. 21%
isomerization to the internal aminoalkene.
rearomatized ligand precursor I, with formation of the alkane
CH2(SiMe3)2. Although the mechanism of this reaction
remains to be elucidated, it is possible that the high stability
of the aromatic BIAN ligand may be the driving force for this
dealkylation process.
amine, and diphenylphosphine in C6D6 resulted in immediate
protonolysis of the alkyl coligand, as observed in the H NMR
1
spectra by the disappearance of the upfield CaCH(SiMe3)2
singlet at −1.72 ppm and the appearance of the characteristic
alkane resonance at −0.2 ppm (Scheme 4). Given the
comparable steric demands of the bis(trimethylsilyl)amide
and bis(trimethylsilyl)methanide coligands, the resulting
heteroleptic calcium bis(trimethylsilyl)amide complex evi-
denced solution stability similar to that of complex 4. In
contrast, monitoring by 1H NMR spectroscopy of the
heteroleptic calcium complexes bearing the less sterically
demanding diphenylamide and diphenylphosphide coligands
under the same conditions suggested slow Schlenk-type ligand
redistribution toward the homoleptic calcium species 9
characterized by the broadening and shifting of the
dearomatized ligand backbone and isopropyl resonances,
accompanied by precipitation of insoluble colorless products
tentatively attributed to homoleptic calcium amide and
phosphide species (Scheme 4), which are known to be
insoluble in noncoordinating hydrocarbon solvents.32 An
independent synthesis of complex 9 by the reaction between
The kinetic stability of the heteroleptic complexes 3−5 was
assessed by heating a d8-toluene solution of these compounds
at 80 °C for 1 week. Apart from a slight amount of protonolysis
of the alkyl ligand to form the alkane CH2(SiMe3)2 in the cases
of 4 and 5 (<5%), possibly due to reaction with the solvent, the
1H NMR spectra remained virtually unchanged, suggesting an
absence of Schlenk-type ligand redistribution processes.
Addition of 2 equiv of the ligand precursor to 1 equiv of the
magnesium, calcium, or strontium dialkyl species in d8-toluene
did not yield the expected homoleptic complexes at room
temperature. Rather, the heteroleptic complexes 3−5 were
formed while the excess ligand remained unreacted. Heating of
these mixtures at 90 °C over a period of several days also did
not result in reaction with the second equivalent of ligand.
Stoichiometric NMR-scale reactions of calcium complex 4
with protic substrates such as hexamethyldisilazane, diphenyl-
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2 equiv of potassium complex 1 and calcium iodide in THF
confirmed the formation of 9 in both of these reactions.
Crystals of compound 9 suitable for X-ray diffraction analysis
were obtained after storage in a 4/1 toluene/THF mixture at
−30 °C for several weeks. The result of this experiment is
displayed in Figure 4. Selected bond length and angle data are
given in Table 1. Compound 9 crystallizes as a mono-THF
adduct, and the five-coordinate calcium center displays a highly
distorted trigonal bipyramidal geometry with both imine
nitrogen atoms in axial positions. The mean planes formed
by the two almost planar ligand backbones form an angle of
82.8°. While in the case of complex 1 the KN2C2 ring remains
planar, the presence of the large calcium center in 9 causes a
slight distortion in the CaN2C2 rings. The N1−Ca−N2 and
N3−Ca−N4 bite angles of the two alkylated BIAN ligands
(70.07(15) and 69.65(14)°, respectively) are notably smaller
than the N−Ca−N angles observed in calcium complexes
supported by a doubly reduced BIAN ligand (75.76(7)−
78.5(2)°)7f or the homoleptic complex of the radical anion
[{I•}2Ca] (73.58(9)°).7g The Ca−Nimine distances of 2.571(4)
and 2.572(5) Å are substantially longer than the Ca−Namide
bonds of 2.372(4) and 2.354(5) Å, reflecting the stronger
bonding between the metal center and the anionic amide
moiety of the ligand. As in the potassium complex 1 the N2−
C36 and N4−C79 bond lengths of the amide residues
(1.355(7) and 1.347(7) Å) are significantly longer than the
N1C25 and N3C68 bond lengths of the imine moieties
(1.286(7) and 1.278(7) Å), while the ethanediylidyne bridges
are slightly shorter in 9 than in 1 (9, 1.489(8) and 1.509(8) Å;
1, 1.518(4) Å). As previously observed for compound 1, the
dearomatization of the alkylated C6H3 rings only induces a
slight distortion from planarity, despite the presence of the sp3
C31 and C74 carbon centers indicated by the elongated C−C
bond lengths (C30−C31 = 1.519(8) Å; C31−C32 = 1.510(9)
Å; C73−C74 = 1.522(8) Å; C74−C75 = 1.497(8) Å) and the
pseudo-tetrahedral angles (C32−C31−C30 = 111.7(5); C32−
C31−C37 = 111.9(5); C30−C31−C37 = 113.7(5); C75−
C74−C73 = 111.5(5); C75−C74−C80 = 112.0(5); C73−
C74−C80 = 114.7(5)°).
disappearance of the distinctive upfield alkyl singlet at ca. −1.7
ppm within the first point of analysis in all these reactions. In
contrast, HN(SiMe3)2 may compete with the substrate for
neutral coordination to the calcium center and reversible σ-
bond metathesis.33c,34 Furthermore, the presence of the
stabilizing dearomatized BIAN ligand leads to increased
reactivity in comparison to the simple homoleptic precursor
[Ca{CH(SiMe3)2}2(THF)2].24 This positive spectator ligand
effect has already been noted when comparing heteroleptic and
homoleptic group 2 amide precatalysts24,33c and is most
pronounced with the iminoanilide species VIIa−c, which
remain the most reactive precatalysts reported for hydro-
amination/cyclization reactions.21 Complex 4 displayed the
usual reactivity trends, depending on the substitution pattern of
the aminoalkene substrate. Substrates providing a favorable
Thorpe−Ingold effecti.e. with larger digeminal β-substitu-
entsrequired lower precatalyst concentrations and shorter
reaction times (Table 2, entries 1−4). It is notable that the 2,2-
diphenyl-substituted substrate provided essentially quantitative
conversion to the corresponding pyrrolidine within the first
point of analysis at room temperature, using only 0.5 mol % of
4 (entry 1), while the simplest substrate, 1-amino-4-pentene,
did not provide any reaction under the conditions tested (entry
4). In line with previous findings, catalytic turnover was also
more efficient for smaller ring sizes, in the order 5 > 6 ≫ 7
(entries 1b, 12b, and 14b) and the calcium precatalyst 4 was
prone to induce alkene isomerization to internal positions for
longer aminoalkene chains (entries 13b and 14b). In contrast to
this latter observation, the magnesium analogue afforded
complete selectivity for the cyclized product (entries 12a and
14a). As has been previously observed with heteroleptic
alkaline-earth β-diketiminato species the calcium precatalyst 4
proved incapable of effecting the cyclization of 1-amino-2,2-
diphenyl-6-heptene to the corresponding hexahydroazepine,
even under forcing conditions (entry 15b).25,33a In contrast, the
magnesium precatalyst 3 (20 mol %) provided clean and near-
quantitative conversion to the desired seven-membered N-
heterocycle over 24 h at 80 °C (entry 15a). In comparison, the
n-butyl magnesium β-diketiminate derivative required 5.5 days
at 80 °C to achieve only 88% conversion.25 Internal
substitution of the alkene moiety only caused a small decrease
in reactivity (entry 6), whereas the activated terminally
substituted substrate 1-amino-2,2,5-triphenyl-4-pentene was
smoothly converted to the corresponding pyrrolidine under
conditions even milder than those for the parent 1-amino-2,2-
diphenyl-4-pentene (entry 1b). Most notable, however, was the
relative ease with which complex 4 catalyzed the intramolecular
hydroamination of terminally substituted, unactivated sub-
strates. While to date only [Ca{CH(SiMe3)2}2(THF)2] has
proved successful for the cyclization of 1-amino-2,2-diphenyl-4-
hexene with modest yields under forcing conditions,33c complex
4 provided near-quantitative conversion to the corresponding
pyrrolidine within 5 h at room temperature and only 5 mol %
catalyst loading (entry 7). A marked Thorpe−Ingold effect was
also apparent for these terminally substituted unactivated
substrates. Although 4-methyl-2,2-diphenylpent-4-en-1-amine
and (E)-2,2-diphenylhex-4-en-1-amine underwent smooth
cyclization (entries 6 and 7), complete loss of catalytic activity
was encountered in the case of 1-amino-2,2-dimethyl-4-hexene
under the reaction conditions assessed (entry 9). The slightly
more challenging terminal ethyl derivative 1-amino-2,2-
diphenyl-4-heptene also underwent clean cyclization to 2-
methyl-4,4-diphenylpyrrolidine, albeit at higher catalyst loading
Catalytic Intramolecular Hydroamination of Hindered
Aminoalkenes. A preliminary comparison of turnover
frequencies for the intramolecular hydroamination of 1-
amino-2,2-diphenyl-4-pentene showed the calcium complex 4
to be superior by far to its magnesium and strontium analogues,
with the order of reactivity being 4 > 5 ≫ 3 (Table 2, entries
1a−c). The same trend has been observed for the majority of
previously described alkaline-earth-catalyzed hydroamination/
cyclization reactions.21,24,25,33 Complex 4 was then assessed for
the intramolecular hydroamination of a wide range of
substituted aminoalkenes on a NMR scale in C6D6 or d8-
1
toluene. In all cases careful inspection of the H NMR spectra
showed no change suggestive of ligand redistribution in the
characteristic shifts of the dearomatized ligand backbone
throughout the reaction times up to 80 °C, above which
signs of Schlenk equilibria started to become apparent. As
already observed in the case of homoleptic alkaline-earth dialkyl
precatalysts and the heteroleptic iminoanilide species VIIa−c,
the reactive bis(trimethylsilyl)methanide coligand of 4 greatly
increases reaction rates in comparison to homoleptic or
heteroleptic calcium bis(trimethylsilyl)amide precatalysts.21,24
This is due to the large pKa difference between CH2(SiMe3)2
and the aminoalkene substrates, leading to rapid and
irreversible catalyst activation, as shown by the complete
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Article
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cyclization to the desired piperidine, even at high catalyst
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amination of aminoalkenes bearing secondary amine function-
alities, with little loss of activity, as shown by the facile
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15). In the cases of substrates which did not undergo
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1
instant protonolysis of the reactive alkyl ligand, and H NMR
́
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CONCLUSION
■
In conclusion, we have shown that reactions of a dipp-
substituted bis(imino)acenaphthene with a range of sterically
demanding potassium alkyl or heavier alkaline-earth dialkyl
reagents result in facile dearomatization through alkyl transfer
to the C5 position of the conjugated acenaphthene system. The
resultant heteroleptic group 2 derivatives display enhanced
kinetic stability to Schlenk-type redistribution equilibria, which
we suggest is a consequence of both the steric demands of the
N-dipp and the rigidity provided by the deraromatized fused
ring acenaphthene ligand backbone. Facile solution exchange is
encountered, however, when the bis(trimethylsilyl)methyl
substituent is replaced by an anionic ligand with lower overall
steric demands. An assessment of the heteroleptic group 2
compounds as precatalysts for the intramolecular hydro-
amination of aminoalkenes evidences enhanced reactivity,
which we ascribe to the greater solution stability of the
catalytically active species. Most notably the calcium species 4
may even be applied to the high-yielding cyclization of
substrates bearing alkyl substitution at either of the alkenyl
positions. We are continuing to study this chemistry and to
elaborate the application of these easily accessed organometallic
species to a broader spectrum of demanding stoichiometric and
catalytic reactions.
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, a table, and CIF files giving full experimental and
instrument details, characterization data for products of all
catalytic cyclization reactions, and details of the X-ray
diffraction analyses of compounds 1, 2, 8, and 9. This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for M.S.H.: msh27@bath.ac.uk.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the Engineering and Physical Sciences Research
Council for funding of this work (EP/I014519/1).
■
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(11) Fedushkin, I. L.; Skatova, A. A.; Fukin, G. K.; Hummert, M.;
Schumann, H. Eur. J. Inorg. Chem. 2005, 2332.
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