Early main group metal catalysis: How important is the metal?

Article abstract of DOI:10.1002/anie.201408814

Organocalcium compounds have been reported as efficient catalysts for various alkene transformations. In contrast to transition metal catalysis, the alkenes are not activated by metal-alkene orbital interactions. Instead it is proposed that alkene activation proceeds through an electrostatic interaction with a Lewis acidic Ca2+. The role of the metal was evaluated by a study using the metal-free catalysts: [Ph₂N^-Me₄N⁺] and [Ph₃C^-][Me₄N^- ]. These "naked" amides and carbanions can act as catalysts in the conversion of activated double bonds (C=O and C=N) in the hydroamination of Ar-N=C=O and R-N=C=N=R (R=alkyl) by Ph₂NH. For the intramolecular hydroamination of unactivated C=C bonds in H₂C=CHCH₂CPh₂CH₂NH₂ the presence of a metal cation is crucial. A new type of hybrid catalyst consisting of a strong organic Schwesinger base and a simple metal salt can act as catalyst for the intramolecular alkene hydroamination. The influence of the cation in catalysis is further evaluated by a DFT study.

Full text of DOI:10.1002/anie.201408814

Angewandte
Chemie
DOI: 10.1002/anie.201408814
Hydroamination
Early Main Group Metal Catalysis: How Important is the Metal?**
Johanne Penafiel, Laurent Maron, and Sjoerd Harder*
Abstract: Organocalcium compounds have been reported as
efficient catalysts for various alkene transformations. In
contrast to transition metal catalysis, the alkenes are not
activated by metal–alkene orbital interactions. Instead it is
proposed that alkene activation proceeds through an electro-
static interaction with a Lewis acidic Ca2+. The role of the metal
was evaluated by a study using the metal-free catalysts:
[Ph2Nꢀ][Me4N+] and [Ph3Cꢀ][Me4N+]. These “naked”
amides and carbanions can act as catalysts in the conversion
=
=
of activated double bonds (C O and C N) in the hydro-
ꢀ = =
ꢀ = = ꢀ
amination of Ar N C O and R N C N R (R = alkyl) by
Ph2NH. For the intramolecular hydroamination of unactivated
=
=
C C bonds in H2C CHCH2CPh2CH2NH2 the presence of
a metal cation is crucial. A new type of hybrid catalyst
consisting of a strong organic Schwesinger base and a simple
metal salt can act as catalyst for the intramolecular alkene
hydroamination. The influence of the cation in catalysis is
further evaluated by a DFT study.
Scheme 1. Lewis acidic alkene activation by Ca2+
.
T
he field of early main group metal (EMGM) catalysis is
horizontal polarization of the p-electron density thus giving
an incentive for nucleophilic attack (Scheme 1). Clark and co-
workers showed already at an early stage that such electro-
static activation of double bonds by an EMGM cation like Li+
drastically lowers activation energies for epoxidation or
radical addition/polymerization.[7] In fact, also TM activation
of alkenes contains an electrostatic component which can be
modeled by the Li+–alkene interaction. Electrostatic double
bond activation decreases with Lewis acidity, that is, along the
row Li+ > Na+ > K+ > Rb+ > Cs+.[7c] Similar models of cation
activation have been proposed for the stronger Lewis acid
Ca2+.[8]
There is some experimental evidence for an important
role of the metal in EMGM-catalyzed reactions. For example:
the regioselectivity of alkene hydrosilylation can be con-
trolled by metal or solvent choice (Scheme 2).[4] Increasing
the polarity of the system (metal: Ca!K; solvent: C6H6!
Et2O!THF) results in a regioselectivity switch, which is
likely due to a change in mechanism in which the metal plays
a crucial role.
rapidly growing.[1] This is partially driven by the nontoxicity
and low prices of the metals but also strongly stimulated by
academic interest. In this context, organocalcium compounds
have been reported as efficient precatalysts for catalytic
conversions of alkenes by polymerization,[2] hydroamina-
tion,[3] hydrosilylation,[4] hydrogenation,[5] and hydrophosphi-
nation[6] (Scheme 1). In contrast to transition metal (TM)
catalysis, the alkenes are not activated by metal–alkene (d!
p*) orbital interactions, i.e., backbonding according to
Dewar–Chatt–Duncanson, but instead it is suggested that
alkene activation proceeds through an electrostatic interac-
tion with a Lewis acidic Ca2+. This leads to vertical and
[*] Prof. Dr. S. Harder
Inorganic and Organometallic Chemistry
Friedrich Alexander University Erlangen-Nꢀrnberg
Egerlandstrasse 1, 91058 Erlangen (Germany)
E-mail: sjoerd.harder@fau.de
J. Penafiel
Stratingh Institute for Chemistry, University of Groningen
Nijenborgh 4, 9747 AG Groningen (The Netherlands)
Prof. Dr. L. Maron
Universitꢁ de Toulouse et CNRS INSA, UPS,
CNRS, UMR 5215, LPCNO
135 avenue de Rangueil, 31077 Toulouse (France)
[**] We thank the Dutch National Research School Combination
Catalysis Controlled by Chemical Design (NRSC-Catalysis) for
financial support of this project (J.P.). L.M. is member of the Institut
Universitaire de France and acknowledges the Humboldt founda-
tion for an advanced researcher grant and CalMip for a generous
grant of computing time.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201408814.
Scheme 2. Polarity effects on product distribution in alkene hydro-
silylation.
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Also in catalytic intramolecular hydroamination, metal
cation charge densities can have major consequences for
kinetics, selectivities, or side reactions.[3b] Since the metal also
strongly influences solubility and Schlenk equilibria, it is not
always clear if such differences are related to EMGM–
substrate interactions. Precedence for EMGM–alkene inter-
actions is restricted to intramolecular examples (1 and 2) and
such bonds are long and considered weak (average values:
Ca···C 3.074(2) ꢀ; Na···C 3.246(5) ꢀ).[9] This is especially the
case for the heavier alkali metals (Na+, K+):[9a] the CpNa···(h2-
ethylene) interaction (7.9 kcalmLꢀ1, MP2/6-31 + G*) is half
as strong as a CpNa···OH2 interaction (15.2 kcalmolꢀ1) and
also much weaker than biologically important Na+···(arene)
interactions;[10] for example CpNa···(h6-benzene) (14.0 kcal
molꢀ1).[9a]
less nucleophilic amines (anilines or secondary amines),
however, requires a catalytic protocol. Richeson and co-
workers showed that simple metal amides MN(SiMe3)2-
(THF)2 (M = Li, Na, K) can catalyze this reaction.[14] This
was followed up by Hill and co-workers using similar Group 2
metal catalysts M[N(SiMe3)2]2·(THF)2 (M = Ca, Sr, Ba).[15]
Carrillo-Hermosilla and co-workers explored reactions medi-
ated by MgBn2·(THF)2 or MgnBu2.[16] Similar, the hydro-
=
amination of the highly activated C O bond in isocyanates
yielding ureas can proceed under forced conditions without
catalyst. However, less nucleophilic amines require a catalyst
like M[N(SiMe3)2]2·(THF)2 (M = Ca, Sr, Ba)[17] or LiN-
(SiMe3)2.[1a]
ꢀ =
In our hands, the sterically hindered isocyanate DIPP N
C O (DIPP = 2,6-iPr2-phenyl) does not react with Ph2NH at
=
608C (Table 1, entry 1). Hydroamination is catalyzed by
Ca[N(SiMe3)2]2·(THF)2 (entry 2). Interestingly, the metal-
free catalyst [Ph2Nꢀ][Me4N+] gave a similar conversion,
independent of temperature and solvent (entries 3–5).[18]
Incomplete conversion has earlier been explained by product
inhibition: the urea product acts as a ligand for the catalyti-
cally active calcium species and hinders substrate coordina-
tion and activation at the metal.[17] As incomplete conversion
is also observed with a metal-free catalyst, it likely has to be
explained by an acid–base equilibrium between the product
ꢀ
=
ꢀ
anion O C(NPh2) N(DIPP) and Ph2NH.
= =
Given the weakness of the EMGM–alkene interaction,
the question arises whether EMGM-mediated reactions
should be considered organometallic catalysis (crucial role
of metal cation) or organocatalysis (limited or no influence of
metal cation). For further clarification, we replaced the
EMGM cations for organic noncoordinating cations and
tested “naked” anions as catalysts either in the intermolecular
Also iPrN C NiPr does not react with Ph2NH under
forced conditions but this reaction can be catalyzed by
Ca[N(SiMe3)2]2·(THF)2 (entries 6–7). The metal in the cata-
lyst is not a necessity: catalytic quantities of [Ph2Nꢀ][Me4N+]
gave similar results that are independent of temperature and
solvent (entries 8–10).[18] Changing the substrate to CyN C
= =
=
=
hydroamination of activated double bonds (C O, C N) or in
the intramolecular hydroamination of unactivated double
Table 1: Catalytic hydroamination of isocyanate and carbodiimide sub-
strates.
=
bonds (C C).
Metal-free catalysts were prepared according to the early
Catalyst
mol% Solvent T [8C] t [h] Conv. [%][a]
pioneering routes of Wilhelm Schlenk reported nearly
a century ago (Scheme 3).[11] Although Schlenk initially
1
–
–
benzene 60
or THF
benzene 60
benzene 25
48
0
[17]
2
3
4
5
Ca[N(SiMe3)2]2·THF2
[Ph2Nꢀ] [Me4N+]
[Ph2Nꢀ] [Me4N+]
[Ph2Nꢀ] [Me4N+]
6
5
5
5
3
1
1
1
62
74
71
71
Scheme 3. Schlenk’s early syntheses of a “naked” carbanion and amide
and their decomposition in THF solution (208C): t1/2 =5 h for [Ph3Cꢀ]-
[Me4N+] and 3 days for [Ph2Nꢀ][Me4N+].
THF
THF
25
60
believed in the existence of pentavalent N, for example,
ꢀ
+
ꢀ
Ph3C NMe4, the ion pair character in [Ph3C ][Me4N ] is well-
established.[12] As [Ph3Cꢀ][Me4N+] shows limited stability,[13]
we also tested Schlenk’s “naked” amide [Ph2Nꢀ][Me4N+] in
catalysis. The latter is more stable against decomposition
(Scheme 3).
6
–
–
benzene 60
or THF
benzene 25
benzene 25
72
0
[15]
7
8
9
Ca[N(SiMe3)2]2·THF2
[Ph2Nꢀ] [Me4N+]
[Ph2Nꢀ] [Me4N+]
2
5
5
5
1
1
74
82
THF
THF
25
60
0.5 85
1 88
=
Hydroamination of the highly activated C N bond in
10 [Ph2Nꢀ] [Me4N+]
carbodiimides giving guanidines can partially be achieved
under uncatalyzed but harsh reaction conditions. The use of
[a] Conversion determined by 1H NMR spectroscopy.
2
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NCy gave comparable results (see the Supporting Informa-
= =
tion, SI) but no conversion was found for ArN C NAr (Ar=
DIPP or p-tolyl). Problematic catalytic hydroamination of
= =
ArN C NAr is in agreement with the lack of examples for
Group 2 metal-mediated reactions,[15] which is likely caused
=
ꢀ
by the high stability of the product anion ArN C(NPh2)
N(Ar)ꢀ. This resonance-stabilized anion is less basic than an
ꢀ
=
ꢀ
alkylated product anion like iPrN C(NPh2) N(iPr) , thus
inhibiting the deprotonation of Ph2NH. The acid–base
equilibrium constant likely reflects the conversion reached
in catalysis.
Scheme 4. Metal salt effect in catalytic conversion of aminoalkenes by
a strong organic Schwesinger base (4).
Encouraged by these examples of metal-free catalysis of
strongly activated double bonds, we turned to probe the
[19]
=
intramolecular hydroamination of unactivated C C bonds.
double bond activation. We tested this hypothesis by addition
This cyclization reaction is catalyzed by a large variety of
of a metal salt. Simple addition of catalytic quantities of 4 and
heteroleptic or also homoleptic Group 2 metal complexes.[3]
CaI2 (10 mol%) to H2C CHCH2CPh2CH2NH2 gave smooth
=
=
Testing several amino alkenes H2C CHCH2CR2CH2NH2
ring closure to the anticipated five-membered ring (C6H6,
258C, 2 h, > 99%) under mild conditions. Product formation
is clean and favored by polar solvents (THF, 258C, 0.5 h,
> 99%). This polar solvent effect could be explained by
increased CaI2 solubility or decreased CaI2 aggregation. CaI2
alone does not catalyze the intramolecular alkene hydro-
amination, nor does the CaI2/Verkade base combination. The
activating ability of the CaI2/Schwesinger base combination
may be related to bond activation by frustrated Lewis pairs.[23]
A potential route is proposed in Scheme 4: amine–CaI2
coordination acidifies the NH2 protons after which deproto-
nation by the base and cyclization can take place. The cycle is
(CR2 = CMe2, Cy, or CPh2) we found with 5 mol% of the
catalyst [Ph2Nꢀ][Me4N+] under no circumstances any indica-
tion for substrate conversion (T= 25–808C in benzene or
THF over 3–12 days). We reasoned that the anion [Ph2Nꢀ]
might not be basic enough (pKa(Ph2NH) = 25.0)[20] to deprot-
onate a primary alkyl amine sufficiently (estimated pKa value
> 35). Acid–base equilibria with the stronger basic “naked”
anion [Ph3Cꢀ] [pKa(Ph3CH) = 30.6][21] could be more favor-
able for catalysis. However, also [Ph3Cꢀ][Me4N+] gave no
conversion of various aminoalkene substrates at similar
conditions. At this stage it was unclear whether this is due
to: 1) catalyst decomposition (Scheme 3), 2) insufficient
deprotonating abilities of the catalyst, or 3) the lack of
ꢀ
closed by protonation of the product anion either by (P4
tBu)H+ or by the amine substrate.
=
a metal cation for C C bond activation. To exclude catalyst
Preliminary we conclude that for hydroamination of
=
decomposition and catalyst basicity, we tested two strong and
robust organic Brønsted bases: the Verkade base (3, pKa
value of 3–H+ = 29.6) and the Schwesinger P4 base (4, pKa
value of 4–H+ = 42).[22]
unactivated C C bonds the metal plays an important role
=
=
whereas for activated double bonds (C N, C O) metal-free
catalysts with “naked” anions are sufficient. It is questionable,
however, to what extent the amide anion in [Ph2Nꢀ][Me4N+]
is “naked”. The crystal structure of Schlenk’s early “free”
carbanion, [Ph3Cꢀ][Me4N+], already revealed a network of
+
ꢀ
ꢀ
nonclassical C H···C hydrogen bonds between [Me4N ] (C
H donor) and [Ph3Cꢀ] (C acceptor).[12] Although [Ph2Nꢀ]-
[Me4N+] crystallized in thin needle-like crystals that are
unsuitable for a crystal structure determination, the related
[p-tolyl2Nꢀ][Me4N+] crystallized as plates of which we deter-
mined a crystal structure (Figure 1). The crystal structure
clearly shows bonding between anions and cations through
=
ꢀ
ꢀ
The Verkade base
3
did not react with H2C
short C H···N contacts (H···N 2.33–2.46 ꢀ; C H···N 1588–
1748) resulting in a helix. This “metal-like” behavior of
[Me4N+] brings up the question whether it could fulfill an
activating role in double bond hydroamination.
CHCH2CR2CH2NH2 even in stoichiometric ratio (C6H6,
808C, 5 days). Stoichiometric addition of the stronger Schwe-
singer P4 tBu base 4, however, gave only in the case of R =
Ph slow but full conversion (toluene, 908C, 5 days, 98%),
which could be accelerated in THF (558C, 60 h, > 99%).
However, instead of the ring closure product we found clean
formation of 1,1-diphenyl-2-ethylethene (Scheme 4). We
propose deprotonation of the aminoalkene followed by
ꢀ
To evaluate the role of the Me4N+ cation in the reaction
mechanism, we chose the hydroamination of iPrN C NiPr as
a model for a set of DFT calculations at the level B3PW91/6-
311G** including solvent correction (THF) at the PCM level.
The reaction of iPrN C NiPr with the “naked” anion
= =
= =
=
decomposition to methanimine H2C NH and a carbanion
(Figure 2) proceeds through a transition state with a small
barrier of 13.9 kcalmolꢀ1 and is slightly endergonic (4.3 kcal
molꢀ1), however, the full reaction profile with subsequent
H transfer from Ph2NH to the product is nearly thermoneu-
tral (see SI for more detailed reaction profiles and geometry
information). The product anion shows a delocalized struc-
stabilized by resonance through Ph substituents. The final
product is formed after double bond isomerization/protona-
tion. Despite the deprotonation of the substrate, no ring
closure is observed. This experiment suggests that in the case
=
of unactivated C C bonds the role of the metal is crucial for
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Figure 1. a) Crystal structure of [p-tolyl2Nꢀ][Me4N+]; only hydrogen
+
ꢀ
atoms for [Me4N ] are shown (short C H···N contacts indicated by
dashed lines).
ꢀ
ture with similar C NiPr distances. Hydroamination with
Ph2NLi starts from a carbodiimide···LiNPh2 complex, in
Figure 2. DFT calculations (B3PW91/6-311G**) for the hydroamination
=
which one of the C N bonds is activated by Li coordination.
ꢀ
ꢀ
+
ꢀ
= =
of iPrN C NiPr by Ph2N , Ph2NLi, [Ph2N ][Me4N ] or Ph2N ···HNPh2;
transition states are shown in wireframe models (relative DG8(298 K)
in kcalmolꢀ1).
= =
This gives rise to asymmetry in the N C N skeleton: the
activated bond is longer (1.238 ꢀ), whereas the other bond is
= =
shorter (1.202 ꢀ) than that in free iPrN C NiPr (1.218 ꢀ).
Subsequent reaction over a cyclic transition state (16.8 kcal
ꢀ1
ꢀ
mol ) gives the product in which unequal C NiPr bonds
and carbodiimide. Especially at the start of the reaction the
reagent could be present as [Ph2Nꢀ···HNPh2], an anion with
indicate electron localization by the Li+ ion. The reaction is
even more endergonic (7.2 kcalmolꢀ1) which originates from
the poor Ph···Li coordination (versus the N···Li coordination
in the starting complex). Hydroamination with the ion pair
[Ph2Nꢀ][Me4N+] starts from a complex in which substrate and
ꢀ
a strong classical N H···N bridge. The starting complex shows
= =
less asymmetry in the N C N skeleton which means that the
Ph2NH···carbodiimide is less activating than the
Me4N+···carbodiimide interaction. The product is formed
through a low-energy transition state (11.5 kcalmolꢀ1) and the
reaction is slightly exergonic (3.5 kcalmolꢀ1). The presence of
ꢀ
ion pair form a network of short nonclassical C H···N bridges.
This causes a similar but slightly less pronounced asymmetry
= =
ꢀ
in the N C N skeleton as found in the carbodiimide···LiNPh2
the NH···N hydrogen bridge also causes asymmetry in the C
complex. The reaction proceeds through a rather low-energy
transition state (12.8 kcalmolꢀ1), in which the Me4N+ cation
bridges those N atoms that show a decreasing negative charge
(Ph2Nꢀ) or increasing negative charge (carbodiimide). The
Me4N+–anion interaction is still observed in the product ion
NiPr bonds of the product anion but again, less pronounced
than for interaction of this anion with Li+ or Me4N+. Route 4
in Figure 2 seems to be the most competitive pathway,
however, it should be noticed that disrupting the ion pair
[Ph2Nꢀ][Me4N+] costs 68.0 kcalmolꢀ1 (gas phase). Although
the anion–cation dissociation energy drops dramatically to
5.0 kcalmolꢀ1 when a solvent correction for THF solution is
included (PCM), full energy profiles including solvent
correction (Figure 3) show that route 4 is not feasible. In
metal-free catalysis the “naked” anion (Ph2Nꢀ) or ion-pair
routes ([Ph2Nꢀ][Me4N+]) are preferred.
ꢀ
pair and gives rise to unequal C NiPr bonds. Although this
=
ꢀ
asymmetry is less pronounced than that in iPrN C(NPh2)
N(iPr)Li, it reflects a similar charge localization. The
resemblance between the function of Li+ and Me4N+ is
striking. The latter behaves as a very large cation which is
ꢀ
bound to anion and substrate through bifurcating C H···N
bridges. The last pathway evaluates a possible reaction
mechanism in which no cation is involved. Instead the
Ph2NH substrate fulfills the role of a bridge between Ph2Nꢀ
Our combined experimental and calculation studies led us
to conclude the following: 1) Intermolecular hydroamination
of strongly activated double bonds (C O in isocyanates, C N
=
=
4
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ꢀ
= =
Figure 3. Full energy profiles for the hydroamination of iPrN C NiPr by Ph2N , Ph2NLi,
[Ph2Nꢀ][Me4N+] or Ph2Nꢀ···HNPh2; relative DG8(298 K) in kcalmolꢀ1 with solvent correction at
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reveals numerous C H···N bonds between cation and anion
which suggests that the cation is weakly coordinating and
behaves like a larger metal. 3) Calculations on the hydro-
ꢀ
amination of carbodiimides confirm this metal-like behavior
+
=
of Me4N in the reaction mechanism. The C N double bond
in a carbodiimide is activated by interaction with Me4N+.
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carbodiimide···Li+ complex and under polar conditions
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[18] Metal-free catalysts were obtained in the form of well-defined
single crystals. Metal content was analyzed by AAS and found to
vary between 0.0149% (149 ppm K+ in [Ph2Nꢀ][Me4N+]) and
0.0839% (839 ppm Na+ in [Ph3Cꢀ][Me4N+]). It is generally
impossible to rule out the contribution of trace elements in
catalytic reactions. However, Ph2NK and Ph3CNa gave both
a slower conversion of substrates than the corresponding metal-
free catalysts (see SI for details). Therefore we believe that trace
metal content is not responsible for catalysis.
=
intramolecular hydroamination of unactivated C C double
bonds needs a more Lewis acidic metal cation for substrate
activation and should therefore be defined as organometallic
catalysis.[24] A simple hybrid catalyst consisting of a strong
organic Schwesinger base and the salt CaI2 gave smooth and
quantitative conversion already at room temperature. We are
currently exploring the validity of this concept by extending it
to a variety of bases, metal complexes, and substrates.
Received: September 4, 2014
Revised: October 14, 2014
Published online: && &&, &&&&
Keywords: homogeneous catalysis · main group metals ·
.
hydroamination · Schwesinger base
[1] For reviews on Ca in catalysis, see: a) S. Harder, Chem. Rev.
2010, 110, 3852; b) A. G. M. Barrett, M. R. Crimmin, M. S. Hill,
P. A. Procopiou, Proc. R. Soc. London Ser. A 2010, 466, 927;
c) M. R. Crimmin, M. S. Hill, Topics in Organometallic Chemis-
try, Vol. 45 (Ed.: S. Harder), Springer, Berlin, 2013, p. 191 – 241;
d) M. Westerhausen, Z. Anorg. Allg. Chem. 2009, 635, 13 – 32.
[2] a) S. Harder, F. Feil, K. Knoll, Angew. Chem. Int. Ed. 2001, 40,
4261 – 4264; Angew. Chem. 2001, 113, 4391 – 4394; b) K. Knoll,
H.-H. Brintzinger, S. Harder, A. Weeber, F. Feil, German Patent
DE 19908079, 1999; US Patent 6399727, 2000.
[19] Recent general reviews on intramolecular alkene hydroamina-
tion, see: a) T. E. Mꢁller, K. C. Hultzsch, M. Yus, F. Foubelo, M.
Tada, Chem. Rev. 2008, 108, 3795 – 3892; b) J. G. Taylor, L. A.
Adrio, K. K. Hii, Dalton Trans. 2010, 39, 1171 – 1175; c) J.
Hannedouche, E. Schulz, Chem. Eur. J. 2013, 19, 4972 – 4985;
d) A. L. Reznichenko, K. C. Hultzsch, Topics in Organometallic
[3] a) M. R. Crimmin, I. J. Casely, M. S. Hill, J. Am. Chem. Soc.
2005, 127, 2042 – 2043; b) S. Datta, P. W. Roesky, S. Blechert,
Organometallics 2007, 26, 4392 – 4394; c) S. Datta, M. T. Gamer,
P. W. Roesky, Organometallics 2008, 27, 1207 – 1213; d) F. Buch,
Angew. Chem. Int. Ed. 2014, 53, 1 – 7
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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Angewandte
Communications
Chemistry (Eds.: V. Ananikov, M. Tanaka), Springer, Berlin,
2013, p. 51 – 114.
[20] pKa in DMSO: F. G. Bordwell, J. C. Branca, D. L. Hughes, W. N.
Olmstead, J. Org. Chem. 1980, 45, 3305 – 3313.
[21] W. S. Matthews, J. E. Bares, J. E. Bartmess, F. G. Bordwell, F. J.
Cornforth, G. E. Drucker, Z. Margolin, R. J. McCallum, G. J.
McCollum, N. R. Vanier, J. Am. Chem. Soc. 1975, 97, 7006 –
7014.
[22] T. Ishikawa, Superbases for Organic Synthesis: Guanidines,
Amidines, Phosphazenes and Related Organocatalysts, 2009,
Wiley, New York, p. 32 – 33 (Schwesinger), T. Ishikawa, Super-
bases for Organic Synthesis: Guanidines, Amidines, Phospha-
zenes and Related Organocatalysts, 2009, Wiley, New York, p. 39
(Verkade).
[23] a) D. W. Stephan, G. Erker, Angew. Chem. Int. Ed. 2010, 49, 46 –
76; Angew. Chem. 2010, 122, 50 – 81; b) L. J. Hounjet, D. W.
Stephan, Org. Process Res. Dev. 2014, 18, 385 – 391.
[24] A first example of a true organocatalytic intramolecular alkene
hydroamination through activation by a protic catalyst was
recently published by Jacobsen et al.: A. R. Brown, C. Uyeda,
C. A. Brotherton, E. N. Jacobsen, J. Am. Chem. Soc. 2013, 135,
6747 – 6749.
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 7
These are not the final page numbers!

Angewandte
Chemie
Communications
Hydroamination
J. Penafiel, L. Maron,
S. Harder*
&&&& — &&&&
Early Main Group Metal Catalysis: How
Important is the Metal?
Organometallic catalysis or organo-
catalysis? Although alkene activation by
transition metals is well-established,
electrostatic activation by early main
group metals is less effective. In some
catalytic transformations the main group
metal does not play a role, whereas in
other processes the presence of a highly
Lewis acidic metal is crucial.
Angew. Chem. Int. Ed. 2014, 53, 1 – 7
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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These are not the final page numbers!