1-Arsa-3-germaallene tip(t-Bu)Ge=C=AsMes*: The heaviest mixed group 14 and 15 heteroallenic compound

Article abstract of DOI:10.1021/ja110104e

The 1-arsa-3-germaallene Tip(t-Bu)Ge=C=AsMes* (1, Tip = 2,4,6-triisopropylphenyl, Mes* = 2,4,6-tri-tert-butylphenyl), a stable heavier group 14 and 15 congener of allenes, has been synthesized by debromofluorination of Tip(t-Bu)Ge(F)-C(Br)=AsMes. It reacts with methanol and 2,3-dimethyl-1,3-butadiene by the Ge=C double bond. The allenic-type structure of 1, featuring cumulated Ge=C and C=As double bonds, has been evidenced by means of spectroscopic and single-crystal X-ray determination. The electronic properties involved in this new system were obtained from DFT calculations. The mechanism of the reaction between 1 and the dimethylbutadiene is also described to understand the observed regio- and chemoselectivity.

Full text of DOI:10.1021/ja110104e

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1-Arsa-3-germaallene Tip(t-Bu)GedCdAsMes*: The Heaviest
Mixed Group 14 and 15 Heteroallenic Compound
||
Dumitru Ghereg,†, Nathalie Saffon,‡ Jean Escudiꢀe,*,† Karinne Miqueu,§ and Jean-Marc Sotiropoulos§
†Universitꢀe de Toulouse, UPS, LHFA, 118 Route de Narbonne, and CNRS, LHFA, UMR 5069, 31062 Toulouse cedex 09, France
‡Structure Fꢀedꢀerative Toulousaine en Chimie Molꢀeculaire, FR 2599, Universitꢀe Paul Sabatier, 118 Route de Narbonne,
31062 Toulouse cedex 09, France,
§Institut Pluridisciplinaire de Recherche sur l’Environnement et les Matꢀeriaux, UMR-CNRS, 5254,
Universitꢀe de Pau et des Pays de l’Adour, Hꢀelioparc 2 Avenue du Prꢀesident Angot, 64053 Pau cedex 09, France.
S Supporting Information
b
Scheme 1. Synthesis of 1-Arsa-3-germaallene 1
ABSTRACT: The 1-arsa-3-germaallene Tip(t-Bu)GedCd
AsMes* (1, Tip = 2,4,6-triisopropylphenyl, Mes* = 2,4,6-tri-
tert-butylphenyl), a stable heavier group 14 and 15 congener
of allenes, has been synthesized by debromofluorination of
Tip(t-Bu)Ge(F)—C(Br)dAsMes*. It reacts with methanol
and 2,3-dimethyl-1,3-butadiene by the GedC double bond.
The allenic-type structure of 1, featuring cumulated GedC
and CdAs double bonds, has been evidenced by means of
spectroscopic and single-crystal X-ray determination. The
electronic properties involved in this new system were
obtained from DFT calculations. The mechanism of the
reaction between 1 and the dimethylbutadiene is also de-
scribed to understand the observed regio- and chemoselec-
tivity.
a donor-acceptor complex. In view of this recent progress, the
synthesis of stable heavier arsenic analogues of type IV is of great
interest from the standpoint of systematic elucidation of the
llenes (I) are a well-known class of compounds and hold a
central position in organic chemistry, being readily trans-
structure and properties of allenic derivatives containing main-
A
group elements. Herein we report the synthesis, structure, and
characterization of the stable 1-arsa-3-germaallene Tip(t-Bu)-
GedCdAsMes*, the heaviest mixed group 14 and 15 hetero-
allenic compound reported to date, consisting of terminal germa-
nium and arsenic atoms.
formed into new organic functions by various reactions (additions,
oxidations, etc.).1 With regard to analogous cumulative doubly
bonded derivatives containing a heavier group 14 (II) or 15 (III)
element, several examples have also been prepared and isolated.2
Such compounds have received much recent interest, not only
from a structure/bonding viewpoint but also as potential pre-
cursors of heterocyclic systems and polymers. In sharp contrast
to I, II, and III, little is known about the synthesis, structure, and
chemistry of the less common mixed group 14 and 15 allenic
systems, IV.
The reaction of n-butyllithium at -100 °C with Mes*Asd
8
5
CBr2 (2) followed by addition of Tip(t-Bu)GeF2 (4) in dry
diethyl ether resulted in the formation of 1-arsa-3-germapropene
5, which was isolated in good yield and fully characterized by
NMR spectroscopy [δ13CCdAs = 176.18 ppm (d, 2JCF = 7.2 Hz),
δ19F = -175.04 ppm] and X-ray analysis (Scheme 1, Figure 1).
The As1-C1 [1.796(2) Å] and As1-C21 [1.968(2) Å] dis-
tances were found to be typical for double and single bonds,
respectively; the three Ge-C bonds [Ge1-C1 1.947(3) Å,
Ge1-C2 1.981(2) Å, and Ge1-C6 1.968(2) Å] also lie within
standard values.9 The C1As1C21 bond angle [101.42(10)°] is
characteristic for an sp2-hybridized arsenic atom in arsaalkenes.
The CdAs double bond adopts a slightly twisted geometry with
torsion angles C21As1C1Br1 of 1.58° and C21As1C1Ge1
of -164.65°. This study also revealed the formation of only the
Z-isomer of 5, probably because the bromine-lithium exchange
Within this field, we have succeeded in the synthesis of the
transient 1-phospha-3-silaallene Tip(Ph)SidCdPMes* 3 and
1-phospha-3-germaallene Mes2GedCdPMes* 4 and thereafter
in the isolation of the stable Tip(t-Bu)GedCdPMes*.5 By con-
trast, >E14dCdN— (E14 = Si,6 Sn7) species are generally better
described as silylene- or stannylene-isocyanide complexes,
with the exception of the derivative reported by Kira,6c which
represents allenic silaketenimine (azasilaallene) rather than such
Received: November 10, 2010
Published: February 7, 2011
r
2011 American Chemical Society
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Figure 1. Molecular structure of 5. Thermal ellipsoids are drawn at the
50% probability level. All hydrogen atoms are omitted for clarity. Relevant
bond lengths (Å) and angles (°): Ge1-C1 1.947(3), Ge1-C2 1.981(2),
Ge1-C6 1.968(2), Ge1-F1 1.771(1), C1-As1 1.796(2), C1-Br1
1.905(2), As1-C21 1.968(2), Ge1-C1-As1 119.73(12), Ge1-C1-
Br1 114.38(12), As1-C1-Br1 124.59(14), C1-As1-C21 101.42(10),
F1-Ge1-C1-Br1 -81.09.
Figure 2. Molecular structure of 1. Thermal ellipsoids are drawn at the
50% probability level. All hydrogen atoms are omitted for clarity.
Relevant bond lengths (Å) and angles (°): Ge1-C1 1.759(2), Ge1-C2
1.973(2), Ge1-C6 1.948(2), C1-As1 1.746(2), C21-As1 2.006(2),
C2-Ge1-C6 122.73(10), C2-Ge1-C1 124.58(11), C6-Ge1-C1
112.33(10), Ge1-C1-As1 158.29(15), C1-As1-C21 100.33(10).
R2CdCdAsMes* 14 169.72(19)°, Mes*AsdCdAsMes* 11
175.6(6)°]. As expected, the bond angle is smaller than in its
phosphorus analogue Tip(t-Bu)GedCdPMes* 5b [166.57(14)°]
in agreement with the fact that the bending generally increases
going down the periodic table.17 The bending is the result of an
interaction between two singlet fragments (>Ge and CdAs—)
leading to a double π-donor-acceptor interaction.17 The angle
between the mean planes C1Ge1C2C6 and C1As1C21 (81.8°)
is not far from the ideal 90°, and this value is in good agreement
with those reported for other heteroallenes. The C21As1C1
bond angle [100.33(10)°] is 2.82° smaller that the corresponding
C21P1C1 angle in Tip(t-Bu)GedCdPMes* 5b [103.15(11)°],
which is consistent with the expected change as one moves to
a heavier pnictogen.
in 2 occurred from the less hindered side, to afford the inter-
mediate Z-3. Treatment of 5 with 1 equiv of tert-butyllithium
at -100 °C led to the formation of lithiated intermediate 6
(δ19F -164.93 ppm), which gave 1-arsa-3-germaallene 1 in nearly
quantitative yield by elimination of lithium fluoride (Scheme 1).
The allenic structure of 1 was first suggested by 13C NMR
spectroscopy, which shows a very low-field shift (305.2 ppm) for
the central sp carbon atom, close to those reported for 1-arsa-3-
phosphaallene Mes*PdCdAsMes* 10 (299.5 ppm) and 1,3-
diarsaallene Mes*AsdCdAsMes* 11 (297.5 ppm). This is, to
the best of our knowledge, the highest value ever reported for an
allenic structure of type EdCdE0 (E,E0 = main group elements).
Less deshielded sp carbon atoms were observed for 1-phospha-3-
germaallenes Tip(t-Bu)GedCdPMes* 5a (280.8 ppm) and
Mes2GedCdPMes* 4 (280.9 ppm), 1-germaallenes Tip2Ged
Bond lengths calculated using DFT at the B3LYP/6-31G**
level of theory (real molecule) are in good agreement with
experimental data. However, the GeCAs bond angle in the gas
phase (143°) is found to be underestimated in relation to that in
the solid state (-15°). The electronic properties of 1 are rather
close to those of the phosphorus analogue Tip(t-Bu)GedCd
PMes* previously described.5b The HOMO is found to be the
antibonding combination between the πGedC and the arsenic
lone pair orbital, while the HOMO-1 can be described as
13
CdC(t-Bu)Ph12 (235.1 ppm) and Tbt(Mes)GedCdCR2
(243.5 ppm), or arsaallene R2CdCdAsMes* 14 (255.8 ppm).
1
The H NMR spectrum of 1 at room temperature displays
broad signals for each aliphatic o-CHMe2 (2.86 and 3.43 ppm)
and aromatic m-CH (7.04 and 7.07 ppm) protons of the Tip group,
suggesting a hindered rotation around the Ge-Cipso single bond
on the NMR time scale due to a significant overcrowding around
the germanium atom. Thus, the important steric congestion in 1
explains its kinetic stabilization.
the πAsdC - π1
combination and the HOMO-2 as the
Mes*
πAsdC þ π1Mes* one. In fact, the π Mes* MO plane is not perfectly
perpendicular to the πAsdC one (∼80°), allowing a small over-
The molecular structure of 1 was undoubtedly determined by
single-crystal X-ray analysis15 (Figure 2). The Ge1-C1 and the
C1-As1 double bonds [1.759(2) and 1.746(2) Å, respectively]
are the shortest reported to date.16 These bond lengths are about
10-12% shorter than corresponding standard single bonds
[1.94-1.98 Å for Ge-C9 and 1.97-2.00 Å for C-As]. Other
Ge-C [Ge1-C2 1.973(2) Å and Ge1-C6 1.948(2) Å] or
C-As [C21-As1 2.006(2) Å] single bonds lie in the normal
range and correspond to typical values. The shortening of the
C1-As1 bond in comparison with that of 5 [1.796(2) Å] is
partly due to the higher s-character of the orbital used to form the
C-As bond because of the sp-type hybridization of the central C
lap. The π2
is found to be the HOMO-3. From the
Mes*
energetics point of view, the HOMO is found to be quasi
energetic compared to the HOMO calculated for the phosphorus
analogous, consistent with the compensation between the lower
electronegativity and the higher s character of arsenic in relation
to that of the phosphorus atom. Finally, the πAsdC orbital in 1
is destabilized compared to the πPdC one (0.4 eV) in Tip-
(t-Bu)GedCdPMes*, which is coherent with a more diffuse
character in the case of arsenic derivatives.
1-Arsa-3-germaallene 1 was found to undergo addition reac-
tions only on the GedC double bond (Scheme 2). Thus, the
reaction of 1 with methanol at ambient temperature afforded the
corresponding adduct 7 in 69% yield, in accordance with the ex-
pected GeδþdCδ- polarity. 1 reacted smoothly with 2,3-
dimethyl-1,3-butadiene (DMB) as a dienophile to give the
[2þ4] cycloadduct 8.
atom in the allenic structure of 1. The geometry around the
P
germanium atom is trigonal planar ( θGe = 359.64°). The
1-arsa-3-germaallene unit deviates somewhat from linearity, but
the bond angle Ge1C1As1 [158.29(15)°] remains reasonable
for a heteroallenic structure and comparable with literature values
13
for germa- or arsaallenes [Tbt(Mes)GedCdCR2 168.0°,
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COMMUNICATION
Scheme 2. Reactions of 1 with MeOH and DMB
Figure 4. Two possible pathways in the reaction between 1 and DMB.
cycloaddition involving the AsdC double bond of the allene is
also a concerted reaction, clearly supra-supra facial (C1CAsC2
dihedral angle ∼7°) and more synchronous than the latter
approach. The difference in synchronicity can probably be
explained by the electronegativity (EN) differences between
the atoms involved. In front of the carbon atom (2.5), the EN of
the germanium is lower than that of the arsenic atom (2.0 vs 2.2). The
two calculated barriers have roughly the same magnitude, slightly in
favor of the GedC cycloaddition (10.9 versus 12.1 kcal mol-1).
3
However, both reactions are exergonic: 8H (cycloaddition on
GedC) is 15 kcal mol-1 more thermodynamically stable than 8H0
3
(cycloaddition on AsdC), in agreement with the experimental
result. It is noteworthy that calculations show higher repulsion
energies for both TSAsdC and 8H0 than for TSGedC and 8H,
respectively, which point out the importance of steric effects.
Taking into account the steric hindrance of the substituent on
the germanium and arsenic atoms for the real molecule, it is clear
that the approach in the πAsdC plane to obtain 8H0 is not favored.
This is probably why, experimentally, the attack occurs only on
the GedC bond.
In summary, we have succeeded in the synthesis and char-
acterization of the first 1-arsa-3-germaallene, a heavy allenic
compound containing arsenic and germanium, both elements
from the fourth row of the periodic table. Spectroscopic (low-
field shift for the central carbon atom in 13C NMR) and structural
data (very short GedC and CdAs bonds, trigonal planar ger-
manium atom, wide GedCdAs bond angle, and almost mutually
orthogonal CGeC and CAsC planes) are strongly consistent with
an allenic structure. The electronic structure is comparable with
that of the phosphorus analogue. The πGedC-nAs MO corre-
sponds to the HOMO and the πAsdC to the HOMO-1. The
GedC double bond of 1 appears (on first investigation) to
dominate its chemistry with the conservation of the GeCAs unit.
A first study on the pathway of this reaction seems to indicate the
importance of the steric effect on the regio- and chemoselectivity.
Further theoretical and experimental investigations on the
reactivity of this new species are currently in progress.
Figure 3. Nature (main contribution) and energetic positions
(Khon-Sham energies) of the main molecular orbitals for 1 and DMB.
Figure 3 presents the main MOs of 1 and of DMB reagent.
From a simple molecular orbital consideration, it is clear that if
we consider the heteroallene as a nucleophile, the reaction with
the DMB must proceed via the πGedC, which is the HOMO, and
not via the πAsdC (HOMO-1). In contrast, if we consider 1 as
an electrophile, the difference between the πDMBCdC (HOMO)
and the π*AsdC (LUMO) is found to be the same as in the
previous case (4.92 and 4.87 eV, respectively). Thus, to gain
more insight into the regioselectivity of this reaction, we calcu-
lated the two pathways involving the GedC or AsdC double
bonds of the arsagermaallene parent compound 1H and the
DMB π system (see Supporting Information).
The [2þ4] cycloaddition with the GedC moiety is supra-
supra facial (C1GeCC2 dihedral angle ∼20°), concerted, and
asynchronous. The geometry around the germanium atom deviates
’ ASSOCIATED CONTENT
P
from planarity ( θGe = 346°), indicating an interaction between
the germanium atom and the carbon C1 of the DMB, while the
new C-C2 bond is not yet been formed (see Figure 4). The
S
Supporting Information. Theoretical results (Z-matrix
b
files); experimental procedures and spectral data for 1, 5, 7, and 8,
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COMMUNICATION
and structural data for 1 and 5. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
escudie@chimie.ups-tlse.fr
Notes
Deceased.
’ ACKNOWLEDGMENT
We are grateful to the CNRS and to the Agence Nationale
pour la Recherche (contract ANR-08-BLAN-0105-01) for finan-
cial support of this work. The theoretical work was granted access
to HPC resources of Idris under allocation 2011 (i2011080045)
made by GENCI (Grand Equipement National de Calcul
Intensif). This paper is dedicated to the memory of Dumitru
Ghereg, a brilliant young chemist who prematurely died.
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