Chemo-, regio-, and stereoselective cobalt-mediated [2+2+2] cycloaddition of alkynyl boronates to alkenes: 1,3- and 1,4-diboryl-1,3-cyclohexadienes

Article abstract of DOI:10.1002/anie.200502038

(Chemical Equation Presented) Diborylated compounds are obtained by means of CpCo-mediated cocyclization of alkynyl(pinacol)boronic esters to alkenes followed by oxidative demetalation (see scheme). This strategy for the rapid and efficient construction o

Full text of DOI:10.1002/anie.200502038

Communications
Synthetic Methods
DOI: 10.1002/anie.200502038
Chemo-, Regio-, and Stereoselective
Cobalt-Mediated [2+2+2] Cycloaddition of
Alkynyl Boronates to Alkenes:
1,3- and 1,4-Diboryl-1,3-cyclohexadienes**
Vincent Gandon, David Leboeuf, Sabine Amslinger,
K. Peter C. Vollhardt, Max Malacria, and
Corinne Aubert*
Alkenylboranes are useful intermediates for the preparation
of a wide range of important organic compounds.[1] Specifi-
cally, mono- and diborylated 1,3-dienes have found various
applications in dienylations,[2] Diels–Alder reactions,[3] and in
the synthesis of a,b- or g,d-unsaturated ketones.[4] Their cyclic
counterparts, namely boryl-1,3-cyclohexadienes, are as yet
unknown. Considering that the 1,3-cyclohexadiene nucleus is
a key subunit of many natural and/or biologically active
compounds,[5] including those of the didehydroretinol and
-carotene families,[6] borylated 1,3-cyclohexadienes constitute
valuable reagents with which to introduce this synthon
directly. The latter task has been accomplished in the past
by using 1,3-cyclohexadienyl–metal complexes,[7] triflates,[8] or
phosphates,[9] although these reagents have to be generated in
several steps from enolizable cyclohexenone derivatives; this
methodology has not been applied to dimetalated 1,3-cyclo-
hexadienes.[10] We report here the preparation of 1,3- and 1,4-
diboryl-1,3-cyclohexadienes by means of the cobalt-mediated
[2+2+2] cocyclization of alkynyl boronates with alkenes.[11]
Alkynyl boronates have recently been used as substrates in
various transition-metal-mediated reactions[12] that lead to
À
valuable building blocks, because the newly formed Csp2
B
bonds can be subjected to couplings[13] and a plethora of other
functional group transformations.[14] Siebert et al. have shown
[*] Dr. V. Gandon, D. Leboeuf, Prof. Dr. M. Malacria, Dr. C. Aubert
UniversitØ Pierre et Marie Curie
Institut de Chimie MolØculaire-FR2769
Laboratoire de Chimie Organique -UMR 7611
Tour 44–54, 28 Øtage, CC. 229, 4 place Jussieu, 75252 Paris Cedex 05
(France)
Fax: (+33)1-4427-7360
E-mail: aubert@ccr.jussieu.fr
Dr. S. Amslinger, Prof. Dr. K. P. C. Vollhardt
Center for New Directions in Organic Synthesis
Department of Chemistry
University of California at Berkeley
and
ChemicalSciences Division
Lawrence Berkeley National Laboratory
Berkeley, CA 94720-1460 (USA)
[**] This work was supported by CNRS, MRES, and by the NSF (CHE
0451241). The Center for New Directions in Organic Synthesis is
enabled by Bristol–Myers Squibb as a Sponsoring Member and
Novartis as a Supporting Member.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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that catechol-substituted mono- and diborylalkynes undergo
facile catalytic cyclotrimerization in the presence of cobalt(0)
or cobalt(i) complexes to furnish oligoborylarenes.[12g] We
have recently disclosed that alkynyl(pinacol)boronic esters
add to a,w-diynes under similar experimental conditions to
assemble fused arylboronic esters.[12i]
The success of the present study was predicated on the
employment of h5-cyclopentadienylbis(ethene)cobalt (1),[15]
which we have employed previously as an active source of
CpCo in cooligomerizations of alkynes with alkenes.[11c–e]
Preliminary experiments were carried out under an ethene
atmosphere by treating 1 with two equivalents of alkynyl-
(pinacol)boronic esters 2–6 in THF (Table 1).[16] Under these
substituted boronic ester 4 afforded products 9C and 9D in an
excellent 93% yield in the ratio 1.5:1 without side products
(entry 3). Similar yields (84% and 92%, respectively), but
better regioselectivities for 1,3-diborylated products (9:1 and
20:1, respectively) were obtained with 5 or 6 as starting
materials (entries 4 and 5). The less symmetrical structures of
isomers 9D–11D were readily assigned by NMR spectrosco-
py, and the structure of 11D was confirmed by a single-crystal
X-ray analysis (Figure 1).[17] Interestingly, boryl substitution
Table 1: CpCo-mediated cycloaddition of alkynylboronic esters to ethene.
Figure 1. ORTEP view of complex 11D in the solid state (30%
probability thermal ellipsoids). Selected bond lengths [Š] and angles
À
À
À
À
[8]: Co CCp 2.05 (av), C2 B1 1.565(5), C4 B2 1.546(5), B O 1.37 (av),
À
À
À
C Cdiene 1.44 (av), Co C1 2.062(3), Co C2 1.996(3); C1-C2-C3-C(4)
3.9.
does not cause significant structural alteration of the
CpCo(h4-1,3-cyclohexadiene) core of the component frag-
ments.[18] The higher symmetry of complexes 8C–10C was
again evident from their NMR spectra, and a distinction
between the 2,3- and 1,4-diboryl substitution pattern was
possible on the basis of the characteristic chemical shifts of
the complexed diene carbons,[11c,e,18a] in conjunction with the
effect of boron nuclear quadrupole broadening. Hence, the
clearly resolved peaks at d = 110.0, 105.2, and 100.1 ppm are
diagnostic of alkyl-substituted internal quaternary diene
carbons in 8C, 9C, and 10C, respectively, whereas the
terminal carbons are either unobservable (8C, 10C) or
appear as a broad signal at d = 46.7 ppm (9C).
The results in Table 1 are consistent with the notions that
steric effects dominate the chemistry of the putative inter-
mediate CpCo(dialkyne) complexes and the resulting metal-
lacycles, which are the relay points on route to the metalated
cyclobutadiene and cyclohexadiene products.[11,19] When R is
bulky, the route leading to the former is favored[19b] and the
steric hindrance retards alkene incorporation to the latter;
cyclohexadiene formation is dominant for less bulky R
groups. Similarly, the regioselectivity appears to be controlled
by the largest substituent, thus favoring the ostensibly least
sterically demanding 2,4-substitution in the cobaltacyclopen-
tadiene. When R is larger or smaller than B(OR)2 such
control is extensive (7A, 10D, 11D), whereas when R is
comparable to B(OR)2 it is not. Remarkably, an electronic
influence of the boryl group is not evident.[20]
Entry
R
A/B (yield [%])[a]
C/D (yield [%])[a]
1
2
3
4
5
tBu (2)
iPr (3)
C6H13 (4)
CH2OMe (5)
Ph (6)
7A/7B 1:0 (59)
–
8A/8B 3.4:1 (54)[b]
8C/8D 1:0 (18)
–
–
–
9C/9D 1.5:1 (93)[c]
10C/10D 1:9 (84)[b]
11C/11D 1:20 (92)[b,d]
[a] Yields of product isolated after flash chromatography. [b] Unsepa-
rated mixture. [c] The yield is the sum of those for isolated 9C (56%) and
9D (37%). [d] The yield is that for complex 11D, obtained pure by
crystallization from the mixture in hexane. The presence of 11C was
surmised on the basis of the 1H NMR spectroscopic data of the mixture.
conditions, alkyne cyclotrimerization[12g,i] was suppressed in
favour of the formation of mixtures of the regioisomeric 1,2-
and
complexes A and B, respectively, as well as the 1,4- and 1,3-
diboryl(h4-1,3-cyclohexadiene)cyclopentadienylcobalt com-
plexes C and D, respectively. The 2,3-diboryl(h4-1,3-cyclo-
hexadiene) substitution pattern was notably absent.
These air-stable products were purified by flash chroma-
tography on silica gel. As shown in Table 1 (entry 1), only the
1,3-diboryl(h4-cyclobutadiene)cyclopentadienylcobalt com-
plex 7A was formed for R = tBu. Its structure was confirmed
unambiguously by an X-ray analysis (see Supporting Infor-
mation). Changing the alkynyl substituent to the less bulky
iPr group (entry 2) resulted in the formation of a mixture of
8A and 8B in 54% yield, accompanied by 18% of the 1,4-
diboryl(h4-1,3-cyclohexadiene)cyclopentadienylcobalt com-
plex 8C. Following this trend, the reaction with the hexyl-
1,3-diboryl(h4-cyclobutadiene)cyclopentadienylcobalt
The cyclization is not restricted to ethene, and with 6 as
the alkyne component and five equivalents of alkene the
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results summarized in Table 2and Table 3 were obtained.
In the case of acyclic, particularly internal, alkenes,
competition with ethene becomes an issue (Table 3). For
example, trans-stilbene failed to undergo cycloaddition, the
reaction mixture rendering only 11D in 70% yield. On the
other hand, cis-stilbene provided 18 as a single diastereomer,
albeit in moderate yield (entry 1), diminished by the gener-
ation of 11D (50% yield). Terminal alkenes proved more
amenable and revealed interesting stereo- and regioselectiv-
ities. For instance, styrene (entry 2) yielded only the endo-
2,4,6-triphenyl derivative 19 (50% yield). A similar, but
attenuated, preference for placing the substituted ethene
carbon adjacent to the boron-substituted terminus was found
with vinyltrimethylsilane and vinyl(tributyl)tin (entries 3 and
4, respectively). However, in sharp contrast to styrene, the
stereochemistry of addition was exclusively exo, an outcome
that is most likely caused by steric factors.
Specifically, the alkene and 1 were stirred initially in THF at
room temperature for 4 h,[21] during which ethene evolution
was visible. Subsequently, 6 was added at À408C, followed by
Table 2: CpCo-mediated cocyclizations of 6 with cyclic alkenes.
Entry
X
exo/endo[a] (yield [%])[b]
1
2
3
O
CH2
(CH2)2
12/13 1:3.6 (88)
14/15 1:1.9 (86)
16/17 1:20 (70)
The scope of the reaction was expanded to the cyclo-
addition of various a,w-diboryldiynes to alkenes (Scheme 1).
These substrates enforce the 1,4-orientation of the boryl
[a] The designations exo and endo refer to the configuration of the
precursor cobaltacyclopentadiene(alkene) complexes and are in analogy
to Diels–Alder cycloaddition terminology. [b] Yields of product isolated
after flash chromatography on SiO2. Compounds 13–15 could be
obtained in pure form, but 12 remained contaminated by some 13,
and 17 by traces of 16.
Table 3: CpCo-mediated cocyclizations of 6 with acyclic alkenes.
Entry
R’
R’’
exo or endo[a] (yield [%])[b]
1
2
3
Ph
H
H
SiMe3
H
Ph
Ph
SiMe3
H
SnBu3
H
18, endo (25)
19, endo (50)
20, exo (50)
21, exo (18)
22, exo (34)
23, exo (11)
Scheme 1. CpCo-mediated cocyclizations of a,w-diboryldiynes with
alkenes leading to bicyclic and tricyclic 1,4-diborylcyclohexadiene com-
plexes.
4
SnBu3
[a] The designations exo and endo refer to the configuration of the
precursor cobaltacyclopentadiene(alkene) complexes and are in analogy
to Diels–Alder cycloaddition terminology. Regio- and stereochemical
assignments were made by chemicalshift, NOE, DEPT, and 2D-NMR
(COSY, HMBC) analyses. [b] Yields of product isolated after flash
chromatography on SiO2, except for 22 and 23, for which basic alumina
was used.
substituents in the resulting diene and grant access to the first
polycyclic 1,4-diborylcyclohexa-1,3-diene derivatives. The
reactions proceed in good yield (61–80%) and, in the case
of 26, high stereoselectivity, which bodes well for synthetic
application to more complex structures. The 13C NMR
spectroscopic data confirm the location of the boryl groups
in complexes 8C–11C. The structure of 27, while showing
some disorder of one of the boryl groups, was confirmed by an
X-ray analysis (Figure 2).[25] Again, the effect of the boryl
substituents on the remainder of the molecule appears
minimal.
While the reported cyclizations furnish the desired
organic products complexed to cobalt, this outcome is
advantageous as it stabilizes the ligands and, at the same
time, potentially activates them towards hydride abstraction
and further substitution.[11,22,24,26] Nevertheless, the air- and
heat-sensitive free dienes could be liberated by rapid
oxidative demetalation with iron(iii) chloride (1.5 equiv) in
acetonitrile (Table 4, entries 1–11). As expected, the respec-
tive endo- and exo-cyclopentadienylcobalt diastereomers
stirring at room temperature for 4 h. With 2,5-dihydrofuran,
cyclopentene, and cyclohexene, the reactions proceeded
completely regioselectively in very good overall yields, with
moderate to excellent stereoselectivity and no competing
ethene insertion (Table 2). Stereochemical assignments were
based on the characteristic chemical shifts of the tertiary
cyclohexadiene hydrogen atoms, which are shielded when
located anti to cobalt and deshielded when located
syn.[11c,e,18a,22] The endo diastereomer was favored in all
cases, in agreement with the stereochemical preference of
cycloadditions of some a,w-diynes to pyrimidines[23] and
indoles.[24]
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highly functionalized 1,3-cyclohexadiene synthons of poten-
tial use in complex molecule synthesis. A mechanistic ration-
ale for the regio- and diastereoselective outcome of this
cycloaddition reaction is currently being sought by means of
density functional computations.
Received: June 13, 2005
Published online: October 11, 2005
Keywords: alkenes · alkynylboronates · cobalt · cycloaddition ·
.
diene ligands
Figure 2. ORTEP view of complex 27 in the solid state (30% proba-
bility thermal ellipsoids). Selected bond lengths [Š] and angles [8]: Co
À
À
À
À
À
C
Cp 2.07 (av), C1 B1 1.547(4), C4 B2 1.543(4), B O 1.37 (av), C
À
À
Cdiene 1.44 (av), Co C1 2.055(3), Co C2 1.980(3); C1-C2-C3-C4 2.1.
[1] a) A. Pelter, K. Amith, H. C. Brown, Borane Reagents, Aca-
demic Press, London, 1988; b) B. Carboni, M. Vaultier, Bull. Soc.
Chim. Fr. 1995, 132, 1003; c) A. Suzuki, Rev. Heteroat. Chem.
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1197; e) M. Vaultier, G. Alcaraz, in Science of Synthesis:
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Houben–Weyl Methods of Molecular Transformations, Vol. 6
(Eds.: D. E. Kaufmann, D. S. Matteson), Thieme, Stuttgart, 2005,
pp. 697.
Table 4: Oxidative demetalation of CpCo(cyclohexadiene) complexes
with FeCl3·6H2O and demetalative aromatizations with ceric ammonium
nitrate.
Entry
Substrate
Product
Yield [%][a]
[2] For examples, see: a) A. B. Smith, G. K. Friestad, J. Barbosa, E.
Bertunesque, J. J.-W. Duan, K. G. Hull, M. Iwashima, Y. Qiu,
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1
2
3
4
5
6
7
8
9
10
11
12[c]
13[d]
9D
9C
32
71
69
74
75
79
80
69
69
76
80
82
45
49
33
10D[b]
11D[b]
14/15
16/17
12/13
25
34
35
37
38
36
40
41
42
43
27
29
30/31
36
30/31
39[e]
44[e]
[a] Yields of product isolated after column chromatography. [b] Reacted
as a mixture with minor isomer C. [c] Treatment with CAN (4ꢀ0.5 equiv.)
in acetone (RT, 2 h). [d] Treatment with CAN (4ꢀ0.75 equiv.) in acetone
(RT, 12 h). [e]
[5] For cyclohexadiene synthesis, see: W. H. Okamura, A. R. De
Lera, in Comprehensive Organic Synthesis, Vol. 5 (Eds.: B. M.
Trost, I. Fleming), Pergamon, New York, 1991, chap. 6.2.
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[10] Mixtures of polylithiated 1,3-cyclohexadienes have been
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Soc. 1975, 97, 5015.
[11] For general reviews of transition-metal-mediated [2+2+2]
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Organometallic Chemistry II, Vol. 12 (Eds.: E. W. Abel, F. G. A.
gave the same diborylcyclohexa-1,3-diene after cobalt
removal, thus providing additional proof for the structure of
the starting complexes. Aromatization was also accomplished,
albeit in only moderate yields so far, using ceric ammonium
nitrate (CAN), either starting from the free ligand (entry 12)
or directly from the complex (entry 13).
In this work we have described the extensively chemo-,
regio-, and diastereoselective assembly of mono-, bi-, and
tricyclic 1,3- and 1,4-diboryl-1,3-cyclohexadienes by means of
the CpCo-mediated cycloaddition of alkynyl pinacolboro-
nates to alkenes. This method provides a rapid entry into
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Communications
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2004, p. 41.
[14] For representative examples, see: a) C. Thiebes, G. K. Surya P-
rakash, N. A. Petasis, G. A. Olah, Synlett 1998, 141; b) K. S.
Webb, D. Levy, Tetrahedron Lett. 1995, 36, 5117; c) T. D. Quach,
R. A. Batey, Org. Lett. 2003, 5, 4397.
[21] The use of toluene or hexane led to lower yields due to
competitive ethene insertion.
[22] E. D. Sternberg, K. P. C. Vollhardt, J. Org. Chem. 1984, 49, 1564.
[23] H. Pelissier, J. Rodriguez, K. P. C. Vollhardt, Chem. Eur. J. 1999,
5, 3549.
[24] R. Boese, A. P. Van Sickle, K. P. C. Vollhardt, Synthesis 1994,
1374.
[25] 27: C27H35B2CoO4, Mr = 504.13, crystal size 0.10 ” 0.10 ”
3
¯
0.10 mm , triclinic, space group P1 (no. 1), a = 10.4024(7), b =
12.1415(11), c = 12.1749(8) Š, a = 100.563(6), b = 99.681(6), g =
113.834(6)8, V= 1331.6(2) Š3, Z = 2, 1calcd = 1.257 gcmÀ3, scan
range
23.2 < 2q < 558,
l(MoKa) = 0.71073 Š,
m(MoKa) =
0.674 cmÀ1, T= 200 K, 5589 unique reflections, of which 3033
were taken as observed (I > 3.00s(I)), R = 0.038, Rw = 0.043.
CCDC-272960 (27) 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.
[26] Y.-H. Lai, W. Tam, K. P. C. Vollhardt, J. Organomet. Chem. 1981,
216, 97.
[15] a) K. Jonas, E. Deffense, D. Habermann, Angew. Chem. 1983, 95,
729; Angew. Chem. Int. Ed. Engl. 1983, 22, 716; K. Jonas, E.
Deffense, D. Habermann, Angew. Chem. 1983, Suppl., 1005. See
also: b) J. K. Cammack, S. Jalisatgi, A. J. Matzger, A. Negron,
K. P. C. Vollhardt, J. Org. Chem. 1996, 61, 4798.
[16] For some rare examples of ethene incorporation in CpCo-
mediated cotrimerizations with alkynes, see: a) Y. Wakatsuki, T.
Kuramitsu, H. Yamazaki, Tetrahedron Lett. 1974, 15, 4549; b) Y.
Wakatsuki, H. Yamazaki, J. Organomet. Chem. 1977, 139, 169;
c) R. G. Beevor, S. A. Frith, J. L. Spencer, J. Organomet. Chem.
1981, 221, C25; d) R. Benn, K. Cibura, P. Hofmann, K. Jonas, A.
´
Rufinska, Organometallics 1985, 4, 2214; e) K. Jonas, Angew.
Chem. 1985, 97, 292; Angew. Chem. Int. Ed. Engl. 1985, 24, 295;
f) U. Kölle, B. Fuss, Chem. Ber. 1986, 119, 116.
[17] 11D: C35H43B2CoO4, Mr = 608.28, crystal size 0.10 ” 0.10 ”
3
¯
0.10 mm , triclinic, space group P1 (no. 1), a = 10.584(3), b =
11.347(4), c = 14.593(3) Š, a = 95.72(2), b = 109.511(19), g =
94.51(2)8, V= 1632.0(8) Š3, Z = 2, 1calcd = 1.238 gcmÀ3
,
scan
range
4.00 < 2q < 508,
l(MoKa) = 0.71073 Š,
m(MoKa) =
0.562cm À1, T= 295 K, 5702 unique reflections, of which 3702
were taken as observed (I > 1.50s(I)), R = 0.051, Rw = 0.043.
CCDC-272959 (11D) contains the supplementary crystallo-
graphic 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.
[18] For pertinent examples of CpCo(diene) structures, see: a) D. W.
Macomber, A. G. Verma, R. D. Rogers, Organometallics 1988, 7,
7118
www.angewandte.org
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Angew. Chem. Int. Ed. 2005, 44, 7114 –7118