Dimerization of the allylzinc cation: Selective coupling of allyl anions in a metallo-ene reaction

Article abstract of DOI:10.1002/anie.201203698

Metal assistance: Dimerization of the allylzinc monocation gives the dimetalated coupling product in quantitative yield (see scheme). Kinetic and thermodynamic parameters of this reversible metallo-ene reaction have been determined. This reaction serves as a model system for the alkali-metal catalyzed production of 4-methylpentene. Copyright

Full text of DOI:10.1002/anie.201203698

Angewandte
Chemie
DOI: 10.1002/anie.201203698
Allylzinc Monocation
Dimerization of the Allylzinc Cation: Selective Coupling of Allyl
Anions in a Metallo-Ene Reaction**
Crispin Lichtenberg, Thomas P. Spaniol, and Jun Okuda*
Cationic organometallic compounds of electropositive metals
have been identified as key intermediates in important
catalytic processes, such as (stereo)selective olefin oligome-
rization and (co)polymerization.[1] They also show unique
reactivity patterns in stoichiometric reactions.[2] Thus, catio-
nization as a means of controlling reactivity is well established
for compounds of Lewis acidic tri- and tetravalent Group 3, 4,
and 13 metals, and the lanthanoids.[1–3] In contrast, cationic
organometallic complexes of nontoxic and abundant divalent
Group 2 and 12 metals have only recently been described.[4–7]
In the case of organozinc compounds, few monocations have
been applied as selective catalysts in lactide,[6a,e,7b,d] lactone,[6d]
and epoxide polymerization,[6d] as well as in hydrosilyla-
tions[6f] and hydroaminations.[6f] We report herein the iso-
lation and full characterization of the allylzinc monocation,
which—in contrast to its neutral parent compound bis-
Scheme 1. a) Synthesis of allylzinc monocations 1 and 2; [A]=[B-
(C6F5)4]. b) Molecular structure of cationic part of 2. Thermal ellipsoids
are set at 50% probability. Hydrogen atoms are omitted for clarity.
(allyl)zinc—allows for efficient coupling of allyl anions in
a highly selective metallo-ene reaction.
Selected bond lengths [ꢀ] and angles [8]: Zn1–C1 1.982(3), Zn1–N1
2.113(3), Zn1–N2 2.118(3), Zn1–N3 2.118(3), C1–C2 1.469(5), C2–C3
1.316(5); C1-Zn1-N1 116.13(13), C1-Zn1-N2 120.12(13), C1-Zn1-N3
125.90(13), N1-Zn1-N2 86.91(10), N1-Zn1-N3 111.53(11), N2-Zn1-N3
85.74(10), C1-C2-C3 127.5(4).
Reaction of bis(allyl)zinc with one equivalent of
[PhNMe2H][B(C6F5)4] gave the allylzinc monocation [Zn-
(C3H5)(THF)3][B(C6F5)4] (1) in quantitative yield (Sche-
me 1a).[8] The colorless compound can be stored under inert
gas atmosphere over months without decomposition. In the
1H NMR spectrum of 1 in [D8]THF at ambient temperature,
the allyl ligand gives rise to an A2MNX pattern, which
corresponds to an h1 bonding mode. For the neutral parent
compound bis(allyl)zinc, the h1 bonding mode of the allyl
ligands can only be detected at temperatures below À608C in
THF solution.[9] Thus, the allyl exchange rate in cationic 1 is
significantly decreased compared to bis(allyl)zinc. The allyl
exchange in 1 is expected to proceed intermolecularly.[9,10]
Notably, transformation of bis(allyl)zinc to 1 did not induce
a change in the bonding mode of the allyl ligand from h1 to h3
which is in good agreement with the current understanding of
zinc–allyl interactions.[10] The THF ligands in 1 are labile and
reaction of 1 with pentamethyldiethylenetriamine (PMDTA)
gave [Zn(C3H5)(PMDTA)][B(C6F5)4] (2) in 99% yield (Sche-
me 1a). The solid-state structure of 2 was investigated by
single-crystal X-ray analysis. Complex 2 crystallizes in the
direct interactions with the counterion (Scheme 1b). The allyl
ligand coordinates to the metal center in an h1 bonding mode
À
with the Zn C bond being slightly shortened compared to the
corresponding value in neutral allylzinc complexes, which is
ascribed to the higher Lewis acidity of cationic 2.[11]
Neutral allylzinc species are key components in metallo-
ene reactions between allyl and a vinyl anions (Gaudemar/
Normant (G/N) coupling, Scheme 2a).[12–15] This reaction
À
combines a selective C C bond formation with the formation
of two synthetically useful metal–carbon bonds in the
coupling product. The isolation and characterization of the
dimetalated products or detailed experimental investigations
of kinetic and thermodynamic parameters of this reaction
have not been reported to date. In addition, the scope of the
reaction concerning the metalated ene component is still
limited to vinyl and allenyl metal compounds.
ꢀ
triclinic space group P1 with Z = 2. The zinc center is found in
a distorted tetrahedral coordination geometry without any
[*] Dipl.-Chem. C. Lichtenberg, Dr. T. P. Spaniol, Prof. Dr. J. Okuda
Institut fꢁr Anorganische Chemie, RWTH Aachen
Landoltweg 1, 52056 Aachen (Germany)
E-mail: jun.okuda@ac.rwth-aachen.de
[**] Generous financial support by Fonds der Chemischen Industrie
(Kekulꢂ scholarship to C.L.) is gratefully acknowledged.
Scheme 2. General addition a) of an allyl metal compound to a vinyl
metal compound and b) of an allyl metal compound to an allyl metal
compound.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201203698.
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ꢀ
An exception to this are neutral allylzinc reagents, which
have been reported to dimerize in extremely sluggish
(108 days reaction time) or unselective (10% dimer forma-
tion) reactions (Scheme 2b),[16] the major side reaction being
radical coupling of the allyl ligands with concomitant
formation of Zn0. The products of this reaction have never
been isolated. We therefore set out to investigate the
characteristics of the Lewis acidic allylzinc monocation 1 in
metallo-ene reactions. Compound 1 is stable in THF solutions
at ambient temperature. Addition of an aromatic hydro-
carbon, such as toluene or benzene, to neat 1 gave an
emulsion, from which the dimerization product 3 slowly
precipitated as a colorless solid (Scheme 3). Quenching with
Compound 5 crystallizes in the triclinic space group P1 with
Z = 2 and both zinc centers adopt a distorted tetrahedral
coordination geometry without any direct interactions with
the counterions. Zn C bonds in 5 are slightly shorter than that
in 2 which was ascribed to the stronger electron-donating
character of the alkyl ligands in 5 compared to the allyl group
À
À
in 2. Consequently, Zn N bonds in 5 are slightly elongated
compared to those in 2. Compound 5 is the first product of
a metallo-ene reaction between two metalated reagents that
has been isolated and fully characterized.
To better understand the dimerization of 1, the reaction
was monitored by NMR spectroscopy under homogeneous
conditions using CD2Cl2 as a weakly coordinating solvent. In
an overall slow reaction, 50% conversion was observed after
1 day and equilibrium conditions were reached after 9 days
with 88% conversion (Figure 2, left). This situation suggests
Scheme 3. Dimerization of 1 in a metallo-ene reaction to give 3 and
quenching of the dianionic hydrocarbon fragment with H2O (D2O) to
give 4 (5,5’-[D2]-4).
Figure 2. Time conversion plot for the reversible dimerization of 1 to
give 3 in CD2Cl2 at room temperature; gray bars indicate periods of
heating to 508C.
H2O or D2O gave the expected (deuterated) 4-methyl-1-
pentene 4 and 5,5’-[D2]-4, respectively, which were identified
by NMR spectroscopy and GC/MS. Notably, the dimerization
of 1 proceeds with 100% Markovnikov selectivity and the
dimetalated product 3 was isolated in quantitative yield. This
is the first well-defined metallo-ene reaction in which the
metalated ene is not a vinyl or allenyl metal compound. The
the dimerization of 1 is reversible. The monomer/dimer ratio
was increased at elevated temperature (508C) and reached
the initially observed value again after few days at room
temperature. This process was reversible (Figure 2, right).
The reversibility of the dimerization of 1 was also observed in
THF as a polar solvent at elevated temperatures.[17] Analysis
of the kinetic data of the reaction in CD2Cl2 revealed the
monomer/dimer equilibrium to be a second-order forward
reaction (dimerization) opposed by a second-order reverse
reaction (monomer formation). Rate constants of kf = (4.1 ꢀ
bis(PMDTA)
derivative
of
compound
3,
[{Zn-
=
(PMDTA)CH2)}2CHCH2CH CH2][B(C6F5)4]2 (5), was syn-
thesized to obtain single-crystals for X-ray analysis (Figure 1).
10À4 Æ 1.7 ꢀ 10À6) LmolÀ1 sÀ1
and
kr = (8.7 ꢀ 10À6 Æ 3.6 ꢀ
10À8) LmolÀ1 sÀ1 were determined for these processes (see
the Supporting Information).[18] The reversibility of metallo-
ene reactions plays a crucial role for the a/g-selectivity of such
reactions.[12e] The thermodynamic parameters of this reaction
are
DHA = À(49.3Æ2.0) kJmolÀ1
and
DSA =
Figure 1. Molecular structure of dicationic part of 5. Thermal ellipsoids
are set at 50% probability. Hydrogen atoms are omitted for clarity.
1.5 equivalents of CH2Cl2 and 0.5 equivalents of pentane, that are
present per formula unit, show no direct interactions with 5 and are
omitted for clarity. Selected bond lengths [ꢀ] and angles [8]: Zn1–C1
1.964(4), Zn1–N1 2.133(3), Zn1–N2 2.183(3), Zn1–N3 2.152(3), Zn2–
C3 1.961(4), Zn2–N4 2.117(3), Zn2–N5 2.143(3), Zn2–N6 2.120(3),
C1–C2 1.506(6), C2–C3 1.541(6), C2–C4 1.527(6), C4–C5 1.500(7), C5–
C6 1.311(7); C1-Zn1-N1 128.64(17), C1-Zn1-N2 113.74(16), C1-Zn1-N3
118.90(18), N1-Zn1-N2 82.63(13), N1-Zn1-N3 110.67(14), N2-Zn1-N3
84.32(12), C3-Zn2-N4 122.42(16), C3-Zn2-N5 121.13(16), C3-Zn2-N6
120.68(17), N4-Zn2-N5 84.03(14), N4-Zn2-N6 111.50(13), N5-Zn2-N6
85.30(13).
À(103Æ7) JmolÀ1 KÀ1 as calculated from a vanꢁt Hoff plot in
the range of 23 to 408C (see the Supporting Information).[19]
The methallylzinc monocation, [Zn(h1-CH2(CMe)CH2)-
(THF)3] [B(C6F5)4] (6), was synthesized to investigate
whether the dimerization of allylzinc monocations is limited
to parent compound 1. Compound 6 dimerized smoothly
upon addition of toluene in a slow reaction with 100%
Markovnikov selectivity. The dimer [Zn2(C4H7-2-C4H7)-
(THF)6][B(C6F5)4]2 (7) was isolated in 94% yield. In contrast
to the monomer/dimer pair 1/3, 6 decomposes in CH2Cl2 and 7
can quantitatively be split into monomeric 6 under harsh
2
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conditions in THF. Thus, the dimerization of allylzinc
monocations seems to be a general reaction pathway. Small
alterations in the substitution pattern of the allyl ligand
significantly affect the monomer/dimer equilibrium.
reaction. Allyl alkali-metal species, isoelectronic with 1, are
believed to play a key role.[23] Thus, the dimerization of 1 is
À
a suitable model reaction for the C C bond forming step in
the alkali-metal-catalyzed synthesis of 4-methylpentene. It
shows that a metallo-ene type mechanism is feasible and that
the presence of radical intermediates[23b] is not necessary.
Although dimerizations of [M(C3H5)]z (M = alkali metal, z =
0; M = Zn, z =+ 1) proceed more readily, the reaction of
1 with olefins suggest that under the conditions of the alkali-
To compare the reactivity of 1 in the addition to
a metalated hydrocarbon (dimerization of 1) with that in
the addition to non-metalated hydrocarbons, reactions of
1 with olefins were studied (Table 1). Reaction of 1 with an
equimolar amount of styrene as an activated olefin gave
a mixture of 1, 3, and 8-Ph after equilibrium had been reached
(entry 1). Using excess olefin led to formation of 8-Ph with
a selectivity of over 96% (entry 2). Reaction of 1 with excess
styrene at elevated temperature gave polystyrene in quanti-
tative yield (entry 3, blank run: entry 4). Reaction of 1 with
excess propene as a non-activated olefin gave a mixture of 3
and 8-Me at room temperature and 8-Me exclusively at 508C
(entries 5, 6).
metal-catalyzed propene dimerization, carbometalation of
z
À
propene by [M(C3H5)] should be the more probable C C
bond forming step.
In conclusion, we have shown using thermodynamic and
kinetic data that allylzinc cations allow for an efficient and
highly selective metal-assisted coupling of allyl anions. The
dimerization of the allylzinc monocation 1 and reactions of
À
1 with olefins can serve as model systems for the C C bond
forming step in the alkali-metal-catalyzed synthesis
of 4-methylpentene.
Table 1: Reaction of 1 with olefins.
Experimental Section
1: A solution of [PhNMe2H][B(C6F5)4] (200 mg, 0.25 mmol)
in THF (1.0 mL) was added to a solution of bis(allyl)zinc
(37 mg, 0.25 mmol) in THF (0.5 mL) to give a colorless
solution. The volume of the reaction mixture was reduced
in vacuo to 0.5 mL. Addition of pentane (4.0 mL) led to
precipitation of a colorless solid, which was collected by
filtration, washed with pentane (3 ꢀ 2.0 mL), and dried
Entry
R
Conditions
T [8C] t [d]
Product distribution [%]
n
1
3
8-R
Other
1
2
3
4
5
6
7
8
9
Ph
Ph
Ph
Ph
Me
Me
Me
Me
1
23
23
50
50
23
50
5
11
60
29
n.d.[e]
100
100
–
50
50
0.2
4.5
4.5
0.1
0.6
5.0
5.0
5.0
n.d.[e] <4[a]
>96[a] n.d.[e]
n.d.[e] n.d.[e]
n.d.[e]
–
>99(PS)[b]
in vacuo. Yield: 251 mg, 0.25 mmol, quantitative.
3
1H NMR (400.1 MHz, [D8]THF): d = 1.46 (ddd, JHH
=
–
–
n.d.[e]
n.d.[e]
8.8 Hz, 4JHH = 0.8 Hz, 4JHH = 1.3 Hz, 2H, CH2-CH CH2)
1.76–1.79 (m, 12H, b-THF), 3.60–3.63 (m, 12H, a-THF),
4.45 (ddt, 2JHH = 2.3 Hz, 3JHH = 9.8 Hz, 4JHH = 0.8 Hz, 1H,
n.d.[e] 17[a]
83[a]
=
n.d.[e] n.d.[e]
>99[a] n.d.[e]
1000 80
1500 80
n.d.[e] <0.01 n.d.[e]
n.d.[e] <0.01 n.d.[e]
1(OP)[c]
4(OP)[d]
cis
3
CH2-CH CH Htrans), 4.69 (ddt, 2JHH = 2.3 Hz, JHH
=
=
cis
4JHH = 1.3 Hz,
1H,
CH2-CH CH Htrans),
nBu 100
80
n.d.[e] n.d.[e]
>99[a] n.d.[e]
=
16.8 Hz,
3
3
3
6.12 ppm (ddt, JHH = 8.8 Hz, JHH = 9.8 Hz, JHH = 16.8 Hz,
1H, CH2-CH CH2). 1H NMR (400.1 MHz, CD2Cl2): d =
[a] Conversion based on amount of 1 used. [b] PS=polystyrene,
=
Mn =2.23ꢃ103 gmolÀ1, polydispersity index (PDI)=3.30. [c] OP=oligopropene,
Mn =3.26ꢃ102 gmolÀ1, Mw/Mn =2.05. [d] OP=oligopropene,
=
1.47 (br s, 2H, CH2-CH CH2) 2.03–2.07 (m, 12H, b-
THF), 3.93–3.97 (m, 12H, a-THF), 4.52 (br s, 1H, CH2-
Mn =4.14ꢃ102 gmolÀ1, Mw/Mn =14.7. [e] n.d.=not detected.
cis
3
CH CH Htrans), 4.73 (br d, JHH = 15.6 Hz, 1H, CH2-CH
=
=
CHcisHtrans), 6.03–6.14 ppm (m, 1H, CH2-CH CH2).
=
1H NMR (400.1 MHz, [D5]pyridine/[H5]pyridine (1:1)):
3
d = 1.60–1.63 (m, 12H, b-THF), 1.96 (br d, JHH = 8.5 Hz, 2H, CH2-
When the reaction temperature was raised to 808C,
oligopropene was obtained in low yield after long reaction
times (entries 7, 8). Using 1-hexene, no oligomerization was
observed, but the remarkably stable addition product 8-nBu is
formed exclusively (entry 9). Overall, dimerization of 1 is
favored over addition of 1 to olefins, but the addition reaction
can be enforced by using excess olefin. Cationic 1 with its
higher Lewis acidity proved superior to neutral bis(allyl)zinc
in reactions with non-activated olefins.[20,21] All the addition
reactions proceed with 100% Markovnikov selectivity, in
good agreement with the literature.[20]
The dimerization of 1 is related to the production of 4-
methylpentene (4), which is used as a monomer for the
synthesis of (co)polymers with excellent optical, thermal, and
electrical properties.[22] Compound 4 is currently synthesized
on industrial scale by the alkali-metal-catalyzed dimerization
of propene under heterogeneous conditions.[23] Little is
known about the mechanism of this industrially relevant
3
=
CH CH2) 3.63–3.67 (m, 12H, a-THF), 4.60 (br d, JHH = 9.3 Hz, 1H,
CH2-CH CH Htrans), 4.88 (br d, 3JHH = 17.1 Hz, 1H, CH2-CH
cis
=
=
CHcisHtrans), 6.50 ppm (m, 1H, CH2-CH CH2). 13C NMR
=
=
(100.6 MHz, [D8]THF): d = 13.35 (s, CH2-CH CH2), 26.48 (s, b-
=
THF), 68.37 (s, a-THF), 106.12 (s, CH2-CH CH2), 125.31 (br s, ipso-
C6F5), 137.25 (dm, 1JCF = 248.0 Hz, m-C6F5), 139.26 (dm, JCF
=
=
1
1
=
243.6 Hz, p-C6F5), 142.25 (s, CH2-CH CH2), 149.29 ppm (dm, JCF
242.8 Hz, o-C6F5). 11B NMR (128.4 MHz, [D8]THF): d = À16.59
(s) ppm. Elemental analysis (%) calcd for C39H29BF20O3Zn
(1001.80 gmolÀ1): Zn 6.53; found: Zn 6.42.
3: Toluene (0.60 mL) was added to 1 (50 mg, 50 mmol) to give an
emulsion. After stirring for 36 h a colorless suspension was obtained.
The volume of the liquid phase was reduced to half under reduced
pressure. After addition of pentane (3.0 mL) the colorless solid was
collected by filration, washed with pentane (3 ꢀ 1.5 mL), and dried
in vacuo. Yield: 47 mg, 25 mmol, quantitative.
1H NMR (400.1 MHz, [D8]THF): d = 0.51 (dd, 2JHH = 12.8 Hz,
3JHH = 8.0 Hz, 2H, CH1H2(CHR)CH1H2), 0.66 (dd, 2JHH = 12.8 Hz,
3JHH = 4.5 Hz, 2H, CH1H2(CHR)CH1H2), 1.76–1.79 (m, 16H, b-
THF), 1.93 (dd, 3JHH = 6.5 Hz, 3JHH = 6.8 Hz, 2H, CH2-CH CH2),
=
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2.12 (m, 1H, CH2(CHR)CH2), 3.60–3.63 (m, 16H, a-THF), 4.94–5.01
1079 – 1084; e) A. Jaenschke, J. Paap, U. Behrens, Z. Anorg. Allg.
Chem. 2008, 634, 461 – 469.
3
(m, 2H, CH2-CH CH2), 5.83 ppm (ddt, JHH = 6.8 Hz, 3JHH = 10.2 Hz,
=
3JHH = 16.9 Hz, 1H, CH2-CH CH2). H NMR (400.1 MHz, CD2Cl2):
1
=
[5] Fully characterized organocalcium monocations: a) K. C. Jayar-
atne, L. S. Fitts, T. P. Hanusa, V. G. Young, Organometallics 2001,
20, 3638 – 3640; b) K. Yan, B. M. Upton, A. Ellern, A. D. Sadow,
J. Am. Chem. Soc. 2009, 131, 15110 – 15111; c) C. Lichtenberg, P.
Jochmann, T. P. Spaniol, J. Okuda, Angew. Chem. 2011, 123,
5872 – 5875; Angew. Chem. Int. Ed. 2011, 50, 5753 – 5756.
[6] Fully characterized organozinc monocations: a) C. A. Wheaton,
P. G. Hayes, Chem. Commun. 2010, 46, 8404 – 8406; b) D. A.
Walker, T. J. Woodman, D. L. Hughes, M. Bochmann, Organo-
metallics 2001, 20, 3772 – 3776; c) M. A. Chilleck, T. Braun, B.
Braun, Chem. Eur. J. 2011, 17, 12902 – 12905; d) M. D. Hannant,
M. Schormann, M. Bochmann, J. Chem. Soc. Dalton Trans. 2002,
4071 – 4073; e) C. A. Wheaton, P. G. Hayes, Dalton Trans. 2010,
39, 3861 – 3869; f) R. J. Wehmschulte, L. Wojtas, Inorg. Chem.
2011, 50, 11300 – 11302; g) M. Haufe, R. D. Kçhn, G. Kociok-
Kçhn, A. C. Filippou, Inorg. Chem. Commun. 1998, 1, 263 – 266.
[7] Organozinc monocations or organozinc cations in contact ion
pairs: a) H. Tang, M. Parvez, H. G. Richey, Jr., Organometallics
2000, 19, 4810 – 4819; b) C. A. Wheaton, B. J. Ireland, P. G.
Hayes, Organometallics 2009, 28, 1282 – 1285; c) R. M. Fabicon,
A. D. Pajerski, H. G. Richey, Jr., J. Am. Chem. Soc. 1991, 113,
6680 – 6681; d) H. Sun, J. S. Ritch, P. G. Hayes, Inorg. Chem.
2011, 50, 8063 – 8072; e) E. Wissing, M. Kaupp, J. Boersma, A. L.
Spek, G. van Koten, Organometallics 1994, 13, 2349 – 2356.
[8] Reactions of bis(allyl)zinc with Lewis acids such as BPh3 or
[Al(C3H5)3(THF)] (see Ref. [2d]) or weaker Brønsted acids, such
as [NEt3H][BPh4], did not give the desired allylzinc monocation
irrespective of the presence of neutral chelating ligands, such as
diglyme. Disproportionation of bis(allyl)zinc in the presence of
neutral chelating ligands to give an ion pair (as reported for an
equimolar mixture of ZnEt2 and ZnPh2 (see Ref. [7c])) was also
not observed.
d = 0.27 (dd, 2JHH = 12.9 Hz, 3JHH = 8.8 Hz, 2H, CH1H2-
(CHR)CH1H2), 0.58 (dd, JHH = 12.9 Hz, JHH = 6.3 Hz, 2H, CH1H2-
2
3
(CHR)CH1H2), 1.86–1.97 (m, 1H, CH2(CHR)CH2), 2.03 (dd, JHH
=
3
5.5 Hz, 3JHH = 7.5 Hz, 2H, CH2-CH CH2), 2.06–2.09 (m, 16H, b-
=
THF), 3.97–4.01 (m, 16H, a-THF), 5.18 (ddt, 2JHH = 2.8 Hz, JHH
3
17.8 Hz, JHH = 1.3 Hz, 1H, CH2-CH CH Htrans), 5.55 (br dd, 2JHH
=
4
cis
=
2.8 Hz, 3JHH = 9.8 Hz, 1H, CH2-CH CH Htrans), 5.84 ppm (ddt,
cis
=
3JHH = 7.5 Hz, 3JHH = 9.8 Hz, 3JHH = 17.8 Hz, 1H, CH2-CH CH2).
=
13C NMR (100.6 MHz, [D8]THF): d = 21.66 (s, CH2(CHR)CH2),
=
26.49 (s, b-THF), 37.29 (s, CH2(CHR)CH2), 51.05 (s, CH2-CH
=
CH2), 68.38 (s, a-THF), 115.87 (s, CH2-CH CH2), 125.35 (br s, ipso-
C6F5), 137.20 (dm, 1JCF = 241.0 Hz, m-C6F5), 139.28 (dm, JCF
=
=
1
1
=
245.4 Hz, p-C6F5), 139.87 (s, CH2-CH CH2), 149.28 (dm, JCF
239.3 Hz, o-C6F5) ppm. 13C NMR (100.6 MHz, CD2Cl2): d = 18.85 (s,
CH2(CHR)CH2), 25.85 (s, b-THF), 37.31 (s, CH2(CHR)CH2), 51.17 (s,
=
=
CH2-CH CH2), 72.47 (s, a-THF), 117.76 (s, CH2-CH CH2), 125.43
(br s, ipso-C6F5), 136.87 (dm, 1JCF = 239.3 Hz, m-C6F5), 138.80 (dm,
1JCF = 248.8 Hz, p-C6F5), 148.71 (dm, 1JCF = 237.6 Hz, o-C6F5),
151.60 ppm (s, CH2-CH CH2). 11B NMR (128.4 MHz, [D8]THF):
=
d = À13.00 ppm (s). 11B NMR (128.4 MHz, CD2Cl2): d = À16.67 ppm
(s). Elemental analysis (%) calcd for C70H42B2F40O4Zn2
(1859.40 gmolÀ1): Zn 7.03; found: Zn 6.96.
CCDC 880470 (2) and 880471 (5) contains the supplementary
crystallographic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Received: May 13, 2012
Published online: && &&, &&&&
Keywords: 4-methylpentene · allylzinc monocation ·
.
carbometalation · Lewis acids · metallo-ene reaction
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4
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Angewandte
Chemie
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´
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Angew. Chem. Int. Ed. 2012, 51, 1 – 6
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5
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.
Angewandte
Communications
Communications
Allylzinc Monocation
C. Lichtenberg, T. P. Spaniol,
J. Okuda*
&&&& — &&&&
Dimerization of the Allylzinc Cation:
Selective Coupling of Allyl Anions in
a Metallo-Ene Reaction
Metal assistance: Dimerization of the
allylzinc monocation gives the dimeta-
lated coupling product in quantitative
yield (see scheme). Kinetic and thermo-
dynamic parameters of this reversible
metallo-ene reaction have been deter-
mined. This reaction serves as a model
system for the alkali-metal catalyzed
production of 4-methylpentene.
6
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
These are not the final page numbers!