Formation of allenes by 1,4-addition of intermolecular phosphane/borane frustrated lewis pairs to a conjugated enyne

Article abstract of DOI:10.1055/s-0033-1341071

The t-Bu₃P/B(C₆F₅)₃ frustrated Lewis pair (6a) undergoes competing acetylene deprotonation (to give the phosphonium-alkenylborate salt 8) and 1,4-P/B FLP addition to the conjugated enyne 2-methylbutenyne to yiel

Full text of DOI:10.1055/s-0033-1341071

CLUSTER
▌1529
cluster
Formation of Allenes by 1,4-Addition of Intermolecular Phosphane/Borane
Frustrated Lewis Pairs to a Conjugated Enyne
Formation of Allenes
Philipp Feldhaus, Birgit Wibbeling, Roland Fröhlich, Constantin G. Daniliuc, Gerald Kehr, Gerhard Erker*
Organisch-Chemisches Institut, Westfälische Wilhelms-Universität Münster, Correnstr. 40, 48149 Münster, Germany
Fax +49(251)8336503; E-mail: erker@uni-muenster.de
Received: 04.02.2014; Accepted: 04.03.2014
jugated enyne 2-methylbutenyne and found some closely
Abstract: The t-Bu3P/B(C6F5)3 frustrated Lewis pair (6a) under-
related behavior.
goes competing acetylene deprotonation (to give the phosphonium–
alkenylborate salt 8) and 1,4-P/B FLP addition to the conjugated
enyne 2-methylbutenyne to yield the zwitterionic allene derivative
9a. The less basic (o-tolyl)3P/B(C6F5)3 system (6b) avoids the acet-
ylene deprotonation pathway. The zwitterionic allene derivative 9b
formed by 1,4-P/B FLP addition to the enyne is again a prominent
reaction product; here competing 1,2-addition is observed to give
the olefinic product 10. The allene derivatives 9a and 9b and their
competing products 8 and 10 were characterized by X-ray diffrac-
tion.
The t-Bu3P/B(C6F5)3 FLP (6a) was treated with a stoichio-
metric amount of 2-methylbutenyne in dichloromethane
solution at ambient temperature (24 h). The NMR spectra
of the crude mixture showed the formation of the deprot-
onation product 8 and the product of 1,4-FLP addition to
the conjugated π-system to give the zwitterionic allene de-
rivative 9a in a ca. 1:1 ratio (Scheme 2). We noted that the
mixture contained more than the expected quantity of the
t-Bu3PH+ cation without any additional detectable anion.
We assume that some hydrolysis had taken place. The
products 8 (33%) and 9a (21%) could be separated by col-
umn chromatography and isolated. Both products were
characterized by spectroscopy and by X-ray diffraction.
Key words: phosphorus, boron, Lewis pairs, allenes
Frustrated Lewis pairs (FLPs) evade strong mutual adduct
formation by steric bulk1 (sometimes by electronic
means).2 The pair of, for example, coexistent phosphane
and borane in solution can then undergo cooperative reac-
tions with a variety of small molecules. Activation of
dihydrogen by heterolytic dissociation to give a phospho-
nium–hydrido borate pair is a most prominent case.3,4 It
has provided the basis for the development of a variety of
metal-free catalytic hydrogenation processes.5 FLPs were
shown to react with CO2,6 SO2,7 nitrogen oxides,8 CO,9
isonitriles,10 etc. Many FLPs add to alkenes or alkynes,
with 1-alkynes sometimes competing deprotonation and
formation of phosphonium–alkynyl borate salts was ob-
served.11
O
R1
Mes2P
R1
B(C6F5)2
O
R2
Mes2P
B(C6F5)3
1
R2
2
H
H
R
R
Mes2P
Mes2P
B(C6F5)2
B(C6F5)2
B(C6F5)2
Mes2P
R
+
We have previously shown that the reactive intramolecu-
lar ethylene-bridged FLP 1 often undergoes 1,4-addition
reactions to unsaturated substrates. This was observed
when 1 was treated with conjugated ynones,12 but also
upon exposure of 1 to a small series of conjugated diynes
and to a conjugated enyne (Scheme 1). In the latter case
some competing acetylene deprotonation was also ob-
served.13 We had begun to investigate some analogous re-
actions of intermolecular FLPs. We had initially found
that the t-Bu3P/B(C6F5)3 pair (6a) undergoes trans-1,4-ad-
dition to 4,6-decadiyne (to yield 7); with the less bulky
2,4-hexadiyne we observed the formation of a mixture of
1,2- (predominant) and 1,4-addition.14
H
5
R
4
3
(1:2)
B(C6F5)3
C3H7
H7C3
H7C3
C3H7
t-Bu3P
+
7
t-Bu3P, B(C6F5)3
6a
Scheme 1
H
(C6F5)3B
We have now treated the intermolecular FLP t-Bu3P/
B(C6F5)3 (6a) and (o-tolyl)3P/B(C6F5)3 (6b) with the con-
(C6F5)3B
t-Bu3PH
+
+
r.t.
H
Pt-Bu3
t-Bu3P, B(C6F5)3
9a
6a
(1:1)
8
SYNLETT 2014, 25, 1529–1533
Advanced online publication: 11.04.2014
Scheme 2
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0033-1341071; Art ID: ST-2014-B0098-C
© Georg Thieme Verlag Stuttgart · New York

1530
P. Feldhaus et al.
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The salt 8 shows a 11B NMR resonance at δ = –21.0. It fea-
tures 19F NMR signals at δ = –132.6 (o), –164.0 (p), and
–167.4 (m of C6F5) with a small δ19Fm,p = 3.4 ppm chemi-
–
cal shift difference that is typical for a RB(C6F5)3 borate
situation. The anion of 8 shows 1H NMR signals of the ter-
minal C(CH3)=CH2 group at δ = 4.97/4.92, and 1.82
(CH3) ppm. It shows 13C NMR signals of the conjugated
enyne moiety at δ = 95.4 [≡C; δ(B13C≡) not observed],
130.8 and 116.8 (=CH2) ppm.
The X-ray crystal structure analysis of compound 8 (Fig-
ure 1) has confirmed that a proton had been abstracted by
the phosphane and the enynyl group had become attached
to the (C6F5)3B unit. The central B1 to C3 framework is
linear [B1–C1: 1.582(5) Å, C1–C2: 1.201(4) Å, C2–C3:
1.442(5) Å, angles B1–C1–C2: 174.9(3)°, C1–C2–C3:
175.7(4)°]. It features the –C(CH3)=CH2 moiety at its end
[C3–C4: 1.351(5) Å, angle C2–C3–C4: 121.2(3)°].
Figure 2 A view of the molecular structure of the allene product 9a
(thermal ellipsoids are shown with 30% probability)
We then treated the 2-methylbutenyne reagent with the
(o-tol)3P/B(C6F5)3 FLP. The bulky triarylphosphane is
less basic than the previously used t-Bu3P Lewis base.
Therefore, we did not observe the formation of the acety-
lene deprotonation product any more. After 24 hours at
room temperature the reaction was complete and we mon-
itored the formation of a ca. 3:4 mixture of the 1,2- and
1,4-P/B FLP addition products 10 and 9b (Scheme 3). The
products were separated by chromatography and crystalli-
zation and isolated in 20% (10) and 33% (9b) yield, re-
spectively. Both compounds were characterized by
spectroscopy and by X-ray diffraction.
The 1,2-addition product 10 shows a 11B NMR signal at
δ = –16.2 ppm and a 31P NMR signal at δ = 23.4 ppm. The
19F NMR Δδ19Fm,p = 4.4 ppm chemical shift difference is
Figure 1 Molecular structure of the salt 8 (thermal ellipsoids are
shown with 30% probability)
–
in the typical RB(C6F5)3 borate range. The 13C NMR
spectrum (233 K) of compound 10 shows resonances of
the central [B]–CH=C[P] unit at δ = 181.2 {[B]–CH= 1H
NMR signal at δ = 8.27 ppm} and 124.8 (1JPC = 64.7 Hz)
ppm, respectively. The adjacent –C(CH3)=CH2 substitu-
The allene product 9a shows a typical =C= 13C NMR
3
resonance15 at δ = 207.1 ppm with a small JPC coupling
constant of 6.7 Hz. The adjacent sp2 carbon atoms of the
allene unit give rise to 13C NMR signals at δ = 98.5
(=CHB) and 80.0 ppm, respectively, with corresponding
1H NMR signals at δ = 5.94 (=CHB), 3.13/2.26 {CH2[P]}
and 1.66 (CH3) ppm. The zwitterionic compound 9a
shows heteronuclear magnetic resonance signals at δ =
49.2 (31P) and –15.6 (11B) ppm, respectively.
1
ent shows H NMR signals at δ = 4.97/4.58 (with corre-
sponding 13C NMR signals at δ = 139.3 and 122.6 ppm)
and 0.88 (CH3) ppm, respectively.
The X-ray crystal structure analysis has confirmed the
trans-1,2-P/B FLP addition to the C≡C triple bond of the
enyne to generate compound 10. It shows typical bonding
features of the central E-configured trisubstituted C=C
double bond [B1–C2: 1.646(3) Å, C2–C1: 1.344(3) Å,
C1–P1: 1.828(2) Å, bond angles B1–C2–C1: 132.5(2)°,
C2–C1–P1: 118.5(2)°]. Both, the boron atom B1 and the
phosphorus atom P1 show pseudotetrahedral coordination
geometries (Figure 3). Carbon atom C1 bears the remain-
The X-ray crystal structure analysis of 9a features the cen-
tral allene unit that was formed by 1,4-addition of the
P/B FLP to the conjugated enyne [C2–C3: 1.315(7) Å,
C3–C4: 1.295(6) Å, angle C2–C3–C4: 174.9(5)°]. It
shows the typical perpendicular arrangements of the sub-
stituent vectors at the allene termini [B1–C4: 1.628(7) Å,
dihedral angle B1–C4···C2–C1: –85.4°] (Figure 2). The
P1–C1 bond length in compound 9a amounts to 1.818(6) Å.16
H
(C6F5)3B
H
P
3
+
+
r.t.
H
P
(C6F5)3B
3
(o-tol)3P, B(C6F5)3
6b
10
9b
Scheme 3
Synlett 2014, 25, 1529–1533
© Georg Thieme Verlag Stuttgart · New York

CLUSTER
Formation of Allenes
1531
ing –C(CH3)=CH2 substituent [C1–C3: 1.499(3) Å, C3– ppm. The 13C NMR signal of the remaining allene termi-
C4: 1.349(3) Å, C3–C5: 1.481(3) Å, angle C1–C3–C4: nus was located at δ = 80.7 ppm.17
118.1(2)°].
The intramolecular P/B FLP 1 has a pronounced tendency
of undergoing 1,4-addition reactions to conjugated π sys-
tems. The cyclic allene 5 is actually the major product of
the reaction of 1 with 2-methylbutenyne, although a minor
product 4 was obtained originating from acetylene depro-
tonation. The intermolecular P/B FLPs 6a and 6b react
similarly albeit with a slightly lower selectivity. In both
cases there is a pronounced tendency of 1,4-P/B addition
to the conjugated enyne 2-methylbutenyne. In both cases
the corresponding allene (9a,b) is a prominent product.
Not unexpectedly, the rather basic t-Bu3P Lewis base
gives rise to a competing formation of the phosphonium–
alkynylborate salt by the deprotonation route. This type of
product seems to be absent in the reaction involving the
less basic (o-tolyl)3P Lewis base. It seems that the bulki-
ness of the Lewis base is a determining feature of the 1,2-
Figure 3 Molecular structure of the 1,2-P/B FLP addition product to
vs. 1,4-addition reaction. The less basic but also sterically
less bulky (o-tolyl)3P/(B(C6F5)3 system avoids the depro-
tonation reaction but allows for the formation of some 1,2-
P/B FLP addition product.14
2-methylbutenyne 10 (thermal ellipsoids are shown with 30% proba-
bility)
The X-ray crystal structure analysis of the 1,4-P/B FLP
addition product 9b shows the typical allene core [C4–C3:
1.294(5) Å, C3–C2: 1.322(5) Å, angle C2–C3–C4:
175.9(3)°] to which the B(C6F5)3 group [B1–C4: 1.641(6)
Å] is bonded at the terminal carbon atom and the methyl
Acknowledgment
Financial support from the Deutsche Forschungsgemeinschaft is
gratefully acknowledged.
+
substituent and the CH2P(o-tolyl)3 group is found at-
tached at the other allene terminus [C2–C5: 1.516(5) Å,
C2–C1: 1.521(5) Å, C1–P1 1.820(3) Å, angle C1–C2–C5: Supporting Information for this article is available online
at
http://www.thieme-connect.com/products/ejournals/journal/
113.9(3)°, dihedral angle B1–C4···C2–C1: 98.1°]. Both,
the boron atom B1 and the phosphorus atom P1 show
pseudotetrahedral coordination geometries in the zwitter-
ionic borate–phosphonium product 9b (Figure 4).
10.1055/s-00000083.SunogIpimrfiantoSuIpg
n
fonirtat
ori
References and Notes
(1) Stephan, D. W.; Erker, G. Angew. Chem. Int. Ed. 2010, 49,
46.
(2) (a) Stute, A.; Kehr, G.; Daniliuc, C. G.; Fröhlich, R.; Erker,
G. Dalton Trans. 2013, 42, 4487. (b) Rosorius, C.; Kehr, G.;
Fröhlich, R.; Grimme, S.; Erker, G. Organometallics 2011,
30, 4211.
(3) (a) Welch, G. C.; San Juan, R. R.; Masuda, J. D.; Stephan, D.
W. Science 2006, 314, 1124. (b) Geier, S. J.; Gilbert, T. M.;
Stephan, D. W. J. Am. Chem. Soc. 2008, 130, 12632.
(4) (a) Spies, P.; Erker, G.; Kehr, G.; Bergander, K.; Fröhlich,
R.; Grimme, S.; Stephan, D. W. Chem. Commun. 2007,
5072. (b) Spies, P.; Schwendemann, S.; Lange, S.; Kehr, G.;
Fröhlich, R.; Erker, G. Angew. Chem. Int. Ed. 2008, 47,
7543.
(5) Stephan, D. W.; Erker, G. Topics Curr. Chem. 2013, 332, 85.
(6) (a) Mömming, C. M.; Otten, E.; Kehr, G.; Fröhlich, R.;
Grimme, S.; Stephan, D. W.; Erker, G. Angew. Chem. Int.
Ed. 2009, 48, 6643. (b) Peuser, I.; Neu, R. C.; Zhao, X.;
Ulrich, M.; Schirmer, B.; Tannert, J. A.; Kehr, G.; Fröhlich,
R.; Grimme, S.; Erker, G.; Stephan, D. W. Chem. Eur. J.
2011, 17, 9640. (c) Zhao, X.; Stephan, D. W. Chem.
Commun. 2011, 47, 1833. (d) Takeuchi, K.; Stephan, D. W.
Chem. Commun. 2012, 48, 11304.
(7) (a) Sajid, M.; Klose, A.; Birkmann, B.; Liang, L.; Schirmer,
B.; Wiegand, T.; Eckert, H.; Lough, A. J.; Fröhlich, R.;
Daniliuc, C. G.; Grimme, S.; Stephan, D. W.; Kehr, G.;
Erker, G. Chem. Sci. 2013, 4, 213. (b) Sajid, M.; Kehr, G.;
Wiegand, T.; Eckert, H.; Schwickert, C.; Pöttgen, R.;
Figure 4 Molecular structure of compound 9b (thermal ellipsoids
are shown with 30% probability)
In solution compound 9b shows 11B and 31P NMR reso-
nances at δ = –15.7 and 25.4 ppm, respectively. The com-
pound shows a typical central =C= allene C(sp) 13C NMR
signal at δ = 206.7 ppm (with 3JPC = 9.2 Hz coupling con-
stant). The 13C NMR signal of the adjacent allene [B]–
C(sp2)(H)= unit appears at δ = 99.2 ppm as a typical
1:1:1:1 intensity quartet (1JBC = ca. 50 Hz). The corre-
1
sponding allenic H NMR resonance occurs at δ = 5.55
© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 1529–1533

1532
P. Feldhaus et al.
CLUSTER
Cardenas, A. J. P.; Warren, T. H.; Fröhlich, R.; Daniliuc, C.
Analytical Data of Compound 8
G.; Erker, G. J. Am. Chem. Soc. 2013, 135, 8882.
Anal. Calcd for C35H33BF15P: C, 53.87; H, 4.26. Found: C,
53.04; H, 4.22. Mp 141 °C (DSC). 1H NMR (600 MHz, 299
K, CD2Cl2): δ = 5.05 (d, 1JPH = 428.6 Hz, 1 H, PH), 4.97 (dq,
(8) (a) Otten, E.; Neu, R. C.; Stephan, D. W. J. Am. Chem. Soc.
2009, 131, 9918. (b) Cardenas, A. J. P.; Culotta, B. J.;
Warren, T. H.; Grimme, S.; Stute, A.; Fröhlich, R.; Kehr, G.;
Erker, G. Angew. Chem. Int. Ed. 2011, 50, 7567. (c) Sajid,
M.; Stute, A.; Cardenas, A. J. P.; Culotta, B. J.; Hepperle, J.
A. M.; Warren, T. H.; Schirmer, B.; Grimme, S.; Studer, A.;
Daniliuc, C. G.; Fröhlich, R.; Petersen, J. L.; Kehr, G.; Erker,
G. J. Am. Chem. Soc. 2012, 134, 10156. (d) Menard, G.;
Hatnean, J. A.; Cowley, H. J.; Lough, A. J.; Rawson, J. M.;
Stephan, D. W. J. Am. Chem. Soc. 2013, 135, 6446.
(e) Pereira, J. C. M.; Sajid, M.; Kehr, G.; Wright, A. M.;
Schirmer, B.; Qu, Z.-W.; Grimme, S.; Erker, G.; Ford, P. C.
J. Am. Chem. Soc. 2014, 136, 513.
(9) (a) Dobrovetsky, R.; Stephan, D. W. J. Am. Chem. Soc.
2013, 135, 4974. (b) Sajid, M.; Elmer, L.-M.; Rosorius, C.;
Daniliuc, C. G.; Grimme, S.; Kehr, G.; Erker, G. Angew.
Chem. Int. Ed. 2013, 52, 2243. (c) Sajid, M.; Lawzer, A.;
Dong, W.; Rosorius, C.; Sander, W.; Schirmer, B.; Grimme,
S.; Daniliuc, C. G.; Kehr, G.; Erker, G. J. Am. Chem. Soc.
2013, 135, 18567. (d) Sajid, M.; Kehr, G.; Daniliuc, C. G.;
Erker, G. Angew. Chem. Int. Ed. 2014, 53, 1118.
Z
2JPH = 2.7 Hz, 4JHH = 1.0 Hz, 1 H, =CH2 ), 4.92 (dq,
E
2JPH = 2.7 Hz, 4JHH = 1.5 Hz, 1 H, =CH2 ), 1.82 (dd,
4JHH = 1.5 Hz, 4JHH = 1.0 Hz, 3 H, CH3), 1.63 (d, 3JPH = 15.9
Hz, 27 H, t-Bu). 13C{1H} NMR (151 MHz, 299 K, CD2Cl2):
δ = 148.6 (dm, 1JFC = ca. 240 Hz, C6F5), 138.5 (dm, 1JFC = ca.
250 Hz, C6F5), 136.9 (dm, 1JFC = ca. 240 Hz, C6F5), 130.8
(=C), 124.8 (br, ipso-C6F5), 116.8 (=CH2), 95.4 (br, ≡C),
38.1 (d, 1JPC = 26.8 Hz, t-Bu), 30.4 (t-Bu), 24.4 (CH3);
resonance for ≡CB was not observed. 31P NMR (243 MHz,
299 K, CD2Cl2): δ = 60.4 (dm, 1JPH = 428.6 Hz). 11B{1H}
NMR (192 MHz, 299 K, CD2Cl2): δ = –21.0 (ν1/2 = ca. 25
Hz). 19F NMR (564 MHz, 299 K, CD2Cl2): δ = –132.6 (m,
2 F, o-C6F5), –164.0 (t, 3JFF = 20.3 Hz, 1 F, p-C6F5), –167.4
(m, 2 F, m-C6F5), [Δδ19Fm,p = 3.4].
Analytical Data of Compound 9a
HRMS: m/z calcd for C35H33BF15PNa+: 803.2072; found:
803.2031. Decomposition: 247 °C (DSC). 1H NMR (600
MHz, 298 K, CD2Cl2): δ = 5.94 (br m, 1 H, =CH), 3.13 (ddd,
2JPH = 16.2 Hz, 2JHH = 14.8 Hz, 5JHH = 3.1 Hz, 1 H, CH2),
2.26 (ddd, 2JPH = 15.5 Hz, 2JHH = 14.8 Hz, 5JHH = 1.3 Hz,
1 H, CH2), 1.66 (br d, 4JPH = 3.1 Hz, 3 H, CH3), 1.57 (d,
3JPH = 13.9 Hz, 27 H, t-Bu). 13C{1H} NMR (151 MHz, 298
K, CD2Cl2): δ = 207.1 (d, 3JPC = 6.7 Hz, =C=), 148.4 (dm,
1JFC = ca. 238 Hz, C6F5), 138.4 (dm, 1JFC = ca. 231 Hz, C6F5),
136.8 (dm, 1JFC = ca. 233 Hz, C6F5), 125.1 (br, i-C6F5), 98.5
[br q (1:1:1:1), 1JBC = ca. 50 Hz, =CH], 80.0 (br m, =C), 39.7
(d, 1JPC = 27.4 Hz, t-Bu), 30.4 (t-Bu), 23.5 (d, 1JPC = 31.4
Hz, CH2), 23.1 (d, 3JPC = 3.2 Hz, CH3). 31P{1H} NMR (243
MHz, 298 K, CD2Cl2): δ = 49.2 (ν1/2 = ca. 10 Hz). 11B{1H}
NMR (192 MHz, 298 K, CD2Cl2): δ = –15.6 (ν1/2 = ca. 15
Hz). 19F NMR (564 MHz, 298 K, CD2Cl2): δ = –132.2 (m, 2
F o-C6F5), –163.6 (t, 3JFF = 20.4 Hz, 1 F, p-C6F5), –167.4 (m,
2 F, m-C6F5), [Δδ19Fm,p = 3.8].
(10) Ekkert, O.; Miera, G. G.; Wiegand, T.; Eckert, H.; Schirmer,
B.; Petersen, J. L.; Daniliuc, C. G.; Fröhlich, R.; Grimme, S.;
Kehr, G.; Erker, G. Chem. Sci. 2013, 4, 2657.
(11) (a) Ullrich, M.; Seto, K. S.-H.; Lough, A. J.; Stephan, D. W.
Chem. Commun. 2009, 2335. (b) Dureen, M. A.; Stephan, D.
W. J. Am. Chem. Soc. 2009, 131, 8396. (c) Mömming, C.
M.; Frömel, S.; Kehr, G.; Fröhlich, R.; Grimme, S.; Erker, G.
J. Am. Chem. Soc. 2009, 131, 12280. (d) Chen, C.; Eweiner,
F.; Wibbeling, B.; Fröhlich, R.; Senda, S.; Ohki, Y.;
Tatsumi, K.; Grimme, S.; Kehr, G.; Erker, G. Chem. Asian J.
2010, 5, 2199. (e) Dureen, M. A.; Brown, C. C.; Stephan, D.
W. Organometallics 2010, 29, 6594. (f) Voss, T.; Chen, C.;
Kehr, G.; Nauha, E.; Erker, G.; Stephan, D. W. Chem. Eur.
J. 2010, 16, 3005. (g) Voss, T.; Sortais, J.-B.; Fröhlich, R.;
Kehr, G.; Erker, G. Organometallics 2011, 30, 584.
(h) Zhao, X.; Stephan, D. W. Chem. Sci. 2012, 3, 2123.
(12) Xu, B.; Kehr, G.; Fröhlich, R.; Wibbeling, B.; Schirmer, B.;
Grimme, S.; Erker, G. Angew. Chem. Int. Ed. 2011, 50,
7183.
(17) Compounds 10 and 9b
A solution of tri-ortho-tolylphosphane (152 mg, 0.50 mmol)
in CD2Cl2 (4 mL) was added to a solution of tris(pentafluoro-
phenyl)borane (256 mg, 0.50 mmol) in CD2Cl2 (4 mL) at r.t.
Then the reaction mixture was added to 2-methyl-1,3-
butenyne (33 mg, 47.5 μL, 0.50 mmol), and the resulting
mixture was stirred for 10 min. After one day, an aliquot of
the reaction mixture was investigated by NMR experiments
at low temperature. A mixture of compound 10 (1,2-
addition), compound 9b (1,4-addition) and tri-ortho-
tolylphosphane. [10/9b/phosphane = ca. 28:37:35 (1H NMR
at 248 K)] was characterized. Then the volume of the
reaction mixture was reduced to one half followed by
separation of the compounds by column chromatography
(silica gel; eluent CH2Cl2–n-pentane = 2:3). The first
fraction contained compound 10 admixed with tri-ortho-
tolylphosphane, which subsequently was purified by
crystallization from a solution of compound 10 in CH2Cl2
layered by n-pentane to give compound 10 (89 mg, 20%).
The second fraction contained compound 9b (148.9 mg,
33%). Crystals suitable for the X-ray crystal structure
analysis for both compounds 10 and 9b, respectively, were
obtained from a solution of the respective compound in
CH2Cl2 layered with n-pentane at –40 °C.
(13) Mömming, C. M.; Kehr, G.; Wibbeling, B.; Fröhlich, R.;
Schirmer, B.; Grimme, S.; Erker, G. Angew. Chem. Int. Ed.
2010, 49, 2414.
(14) Feldhaus, P.; Schirmer, B.; Wibbeling, B.; Daniliuc, C. G.;
Fröhlich, R.; Grimme, S.; Kehr, G.; Erker, G. Dalton Trans.
2012, 41, 9135.
(15) Kalinowski, H.-O.; Berger, S.; Braun, S. In 13C NMR-
Spektroskopie; Thieme: Stuttgart, 1984.
(16) Compounds 8 and 9a
Tri-tert-butylphosphane (102 mg, 0.50 mmol) in CH2Cl2
(3 mL) was added to a solution of tris(pentafluorophenyl)-
borane (256 mg, 0.50 mmol) in CH2Cl2 (10 mL). After
addition of 2-methyl-1,3-butenyne (33 mg, 47.5 μL, 0.50
mmol) the reaction mixture was stirred for 24 h at r.t.
Subsequently, all volatiles were removed under reduced
pressure and the obtained residue was dried in vacuo to give
a ca. 1:1 mixture of compounds 8 and 9a (302 mg). The both
products were separated by column chromatography (silica
gel; eluent: n-pentane–CH2Cl2, 3:5). Drying of the
respective fraction in vacuo gave compound 8 (131.9 mg,
33%) and compound 9a (81.6 mg, 21%). Crystals suitable
for the X-ray crystal structure analysis for both compound 8
and compound 9a were obtained from a solution of the
respective compound in CH2Cl2 layered by n-pentane at
–40 °C.
Analytical Data of Compound 10
Anal. Calcd for C44H27BF15P·CH2Cl2: C, 55.87; H, 3.02.
Found: C, 55.50; H, 2.55. Mp 98 °C (DSC). 1H NMR (500
MHz, 233 K, CD2Cl2): δ = 8.27 (br d, 3JPH = 37.0 Hz, 1 H,
=CH), 7.86 (o), 7.63 (p), 7.51 (m), 7.33 (m′) (4 × m, 4 × 1 H,
o-tola), 7.65 (p), 7.60 (o), 7.43 (m), 7.36 (m′) (4 × m, 4 × 1 H,
Synlett 2014, 25, 1529–1533
© Georg Thieme Verlag Stuttgart · New York

CLUSTER
o-tolb), 7.65 (p), 7.52 (m′), 7.27 (m), 7.27 (o) (4 × m, 4 × 1
Formation of Allenes
1533
2 F, o-C6F5), –162.2 (m, 1 F, p-C6F5), –166.6 (m, 2 F, m-
C6F5), [Δδ19Fm,p = 4.4].
Analytical Data of Compound 9b
Z
E
H, o-tolc), 4.97 (s, 1 H, =CH2 ), 4.58 (s, 1 H, =CH2 ), 2.63
(s, 3 H, CH3 of o-tolc), 1.63 (s, 6 H, CH3 of o-tola,b), 0.88 (s,
3 H, CH3). 13C{1H} NMR (126 MHz, 233 K, CD2Cl2):
δ = 181.2 (br, =CH), 147.7 (dm, 1JFC = ca. 242 Hz, C6F5),
145.1 (d, 2JPC = 8.0 Hz, ortho′), 134.8 (d, 2JPC = 11.4 Hz,
ortho), 134.7 (d, 4JPC = 2.1 Hz, para), 133.3 (d, 3JPC = 10.2
Hz, meta′), 126.8 (d, 3JPC = 12.2 Hz, meta), 116.0 (d,
1JPC = 80.7 Hz, ipso) (of o-tolc), 144.1 (d, 2JPC = 7.7 Hz,
ortho′), 134.5 (d, 2JPC = 12.0 Hz, ortho), 134.2 (d, 4JPC = 2.6
Hz, para), 133.4 (d, 3JPC = 11.4 Hz, meta′), 126.9 (d,
3JPC = 12.8 Hz, meta), 118.1 (d, 1JPC = 83.2 Hz, ipso) (of o-
tolo), 143.3 (d, 2JPC = 6.8 Hz, ortho′), 136.2 (d, 2JPC = 13.5
Hz, ortho), 134.1 (d, 4JPC = 2.4 Hz, para), 132.9 (d,
HRMS: m/z calcd for C44H27BF15PNa+: 905.1604; found:
905.1602. Mp 110 °C (DSC). 1H NMR (500 MHz, 298 K,
CD2Cl2): δ = 7.67 (p), 7.65 (o), 7.45 (m′), 7.37 (m) (4 × m, 4
× 3 H, o-tol), 5.55 (br m, 1 H, =CH), 4.12, 3.24 (2 × m, 2 ×
1 H, CH2), 2.15 (s, 9 H, CH3 of o-tol), 1.27 (m, 3 H, CH3).
13C{1H} NMR (126 MHz, 298 K, CD2Cl2): δ = 206.7 (d,
3JPC = 9.2 Hz, =C=), 148.4 (dm, 1JFC = ca. 240 Hz, C6F5),
143.7 (d, 2JPC = 8.6 Hz, o′ of o-tol), 138.4 (dm, 1JFC = ca. 245
Hz, C6F5), 136.8 (dm, 1JFC = ca. 250 Hz, C6F5), 135.9 (d,
2JPC = 11.7 Hz, o of o-tol), 135.3 (d, 4JPC = 2.9 Hz, p of o-
tol), 133.9 (d, 3JPC = 11.0 Hz, m′ of o-tol), 127.6 (d,
3JPC = 12.7 Hz, m of o-tol), 125.1 (br, i-C6F5), 117.5 (d,
1JPC = 80.6 Hz, i of o-tol), 99.2 [br q (1:1:1:1), 1JBC = ca. 51
Hz, =CH], 80.7 (br m, =C), 31.3 (d, 1JPC = 47.8 Hz, CH2),
23.1 (d, 3JPC = 3.8 Hz, CH3 of o-tol), 20.4 (d, 3JPC = 4.6 Hz,
CH3). 31P{1H} NMR (202 MHz, 298 K, CD2Cl2): δ = 25.4
(ν1/2 = ca. 10 Hz). 11B{1H} NMR (160 MHz, 298 K, CD2Cl2):
δ = –15.7 (ν1/2 = ca. 20 Hz). 19F NMR (470 MHz, 298 K,
CD2Cl2): δ = –132.2 (m, 2 F o-C6F5), –163.6 (t, 3JFF = 20.3
Hz, 1 F, p-C6F5), –167.4 (m, 2 F, m-C6F5), [Δδ19Fm,p = 3.7].
3JPC = 10.3 Hz, meta′), 126.9 (d, 3JPC = 12.8 Hz, meta), 120.7
(d, 1JPC = 80.6 Hz, ipso) (of o-tola), 139.3 (d, 2JPC = 14.6 Hz,
=C), 138.1 (dm, 1JFC = ca. 240 Hz, C6F5), 136.3 (dm, 1JFC
=
ca. 240 Hz, C6F5), 124.0 (br, i-C6F5), 124.8 (d, 1JPC = 64.7
Hz, =CP), 122.6 (d, 3JPC = 10.9 Hz, =CH2), 23.1 (br, CH3 of
o-tolc), 22.9 (CH3), 22.7 (d, 3JPC = 1.8 Hz, CH3 of o-tolb),
22.5 (d, 3JPC = 3.7 Hz, CH3 of o-tola). 31P{1H} NMR (202
MHz, 233 K, CD2Cl2): δ = 23.4 (ν1/2 = ca. 60 Hz). 11B{1H}
NMR (160 MHz, 233 K, CD2Cl2): δ = –16.2 (ν1/2 = ca. 60
Hz). 19F NMR (282 MHz, 295 K, CD2Cl2): δ = –131.6 (m,
© Georg Thieme Verlag Stuttgart · New York
Synlett 2014, 25, 1529–1533

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