2,5-Bis(diphenylmethylene)-3-cyclopentenone: A solvent-dependent cobalt cluster mediated propargyl radical coupling process

Article abstract of DOI:10.1016/S0040-4039(02)00692-5

Protonation of (1,1-diphenyl-2-propyn-1-ol)Co₂(CO)₆ with HBF₄ in dichloromethane generates the expected metal-stabilized propargyl cation, and also rearranges to give the tricobalt cluster Ph₂C=CH-CCo₃

Full text of DOI:10.1016/S0040-4039(02)00692-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 4049–4053
2
,5-Bis(diphenylmethylene)-3-cyclopentenone: a solvent-dependent
cobalt cluster mediated propargyl radical coupling process
John H. Kaldis and Michael J. McGlinchey*
Department of Chemistry, McMaster University, Hamilton, Ontario, Canada L8S 4M1
Received 26 February 2002; revised 3 April 2002; accepted 8 April 2002
Abstract—Protonation of (1,1-diphenyl-2-propyn-1-ol)Co (CO) with HBF4 in dichloromethane generates the expected metal-
2
6
stabilized propargyl cation, and also rearranges to give the tricobalt cluster Ph CꢀCH-CCo (CO) 9. In contrast, use of THF as
2
3
9
’
solvent affords the radical (Co (CO) )[HCꢁC-CPh ] 2, which dimerizes at the methyne position; subsequent cyclization and
2
6
2
carbonylation yields 2,5-bis-(diphenylmethylene)-3-cyclopentenone 11. The products have been characterized by X-ray crystallog-
raphy, and the scope and mechanism of the reaction are discussed. © 2002 Elsevier Science Ltd. All rights reserved.
1
. Introduction
2. Results and discussion
Treatment of (Co (CO) )[HCꢁC-CPh OH], 6, with
While cobalt-stabilized cations derived from (propargyl
alcohol)Co (CO) clusters, 1, are well-characterized and
have been extensively exploited for synthetic purposes,
the corresponding radical chemistry is less well devel-
oped. In preliminary studies, Nicholas and Melikyan
have generated coupling products via zinc reduction of
the cations, as exemplified in Scheme 1.
2
6
2
HBF4 in SO2 yields the isolable cation (Co (CO) )-
2
6
2
6
+
6
1
[HCꢁC-CPh ] . In contrast, an earlier report on the
2
protonation of (Co (CO) )[HCꢁC-CPh OH] with con-
2
6
2
2
3
centrated sulfuric acid suggests that the major product
is the tricobalt cluster 7. This product was characterized
by microanalysis, but no spectroscopic data were
7,8
given. It is, of course, well established that terminal
alkyne complexes of the type [RCꢁCH](Co (CO) ) rear-
4
2
6
Recently, Melikyan has noted that generation of the
range in acid media to yield the corresponding RCH -
2
cobalt-stabilized cations in certain solvents, notably
THF or ether, results in electron transfer to yield the
same coupling products as were produced by the zinc
reduction procedure. Intrigued by these reports, we
9
CCo (CO) clusters (Scheme 3). One might therefore
3
9
have anticipated the formation of the alcohol 8, or its
elimination product 9, rather than the saturated cluster
7
. Indeed, we find that treatment of (Co (CO) )[HCꢁC-
2
6
’
chose to generate the radical (Co (CO) )[HCꢁC-CPh ]
2
6
2
CPh OH] with HBF in dichloromethane at −78°C does
2 4
2
to see whether it would couple to yield the corre-
yield the alkenyl cluster 9, and its X-ray structure
appears as Fig. 1.
sponding symmetrical ethane 3, or rather mimic the
5
classic behavior of the triphenylmethyl radical 4, which
for steric reasons yields the unsymmetrical dimer 5, as
However, returning to our stated objective of generat-
ing the radical 2, the precursor alcohol was treated with
in Scheme 2.
Scheme 1.
*
Corresponding author. Tel.: 905-525-9140, ext. 27318; fax: 905-522-2509; e-mail: mcglinc@mcmaster.ca
0
040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00692-5

4
050
J. H. Kaldis, M. J. McGlinchey / Tetrahedron Letters 43 (2002) 4049–4053
Scheme 2.
Figure 1. Molecular structure of 9.
The bright yellow compound exhibited a strong
−
1
1
13
infrared absorption at 1703 cm , and H and
C
NMR resonances typical of carbonyl, phenyl and
alkene fragments. The parent peak in the mass spec-
trum at m/z 410, together with the observed breakdown
pattern, corresponded to a molecule containing two
Ph C-CꢁCH moieties plus a carbonyl group. Compari-
2
Scheme 3.
10,11
son of these data with those of related systems
suggested that the yellow compound, 11, was 2,5-bis-
(
diphenylmethylene)-3-cyclopentenone, and this assign-
HBF in THF at −78°C and the mixture was allowed to
warm slowly to room temperature before quenching
with water. Chromatographic separation yielded three
4
ment was confirmed by X-ray crystallography (Fig. 2).
One might speculate that, since coupling of the
diphenylmethyl radicals to give 3 is presumably steri-
cally disfavored, coupling of the methyne termini fol-
lowed by a carbonyl insertion process could lead to the
observed diarylidene. The precise nature of the cobalt
center in Scheme 5, which may be mono- or poly-metal-
lic, remains unknown at present, but mechanistic inves-
tigations are continuing.
products: the alkenyl tricobalt cluster
9 (18%),
(
ethynyldiphenylmethane)Co (CO) 10 (8%), and a
2
6
bright yellow material 11 (28%). The cluster 9 has
already been discussed and 10 is readily envisaged as
arising either through hydrogen abstraction by the
intermediate radical, or hydride transfer to the initially
generated cation, as in Scheme 4.
Scheme 4.

J. H. Kaldis, M. J. McGlinchey / Tetrahedron Letters 43 (2002) 4049–4053
4051
Figure 2. Molecular structure of 11.
In seeking precedents for such a process, we note Eaton’s
elegant [4+1] iron-catalyzed cycloaddition of conjugated
diallenes and a carbon monoxide to produce five-mem-
Scheme 5.
1
2
bered rings. However, in those examples, one must first
synthesize the diallene either (a) by dibromocarbene
addition to an appropriate butadiene followed by
In an attempt to explore the generality of this process,
the corresponding deprotected fluorenyl system 14, was
protonated initially in dichloromethane and diethyl ether
and latterly in tetrahydrofuran. The former reaction
yielded only a single product, the tricobalt cluster 15. In
contrast, protonation in diethyl ether, which would be
expected to generate radicals, yields a reduced quantity
of the tricobalt cluster 15, along with the hydrogen
abstraction product 16, while utilizing THF afforded 16
as the major product (Scheme 6). We surmise that 17, the
fluorenyl analogue of 11, would suffer severe steric
problems and that the fluorenyl moieties would need to
be noticeably twisted out of plane, as in the semi-empir-
1
3a
debromination with methyllithium,
or (b) by palla-
dium-catalyzed coupling of a propargyl bromide and an
1
3b
allenyl-zinc reagent. In the present case, it is apparent
that the coupling, cyclization and carbonylation pro-
cesses are all cobalt-mediated in a one-pot synthesis.
Interestingly, protonation of the analogous trimethyl-
silyl protected cobalt-complexed alkynol 12, in THF at
−
78°C, affords only the hydrogen abstraction product
13, shown in Scheme 4. The diarylidene product is
14
presumably not generated because of the steric interfer-
ence introduced by the trimethylsilyl group.
ically derived model (AM1 Hamiltonian) depicted in
Fig. 3.
Scheme 6.
15
Figure 3. Optimized structural model of 17.

4
052
J. H. Kaldis, M. J. McGlinchey / Tetrahedron Letters 43 (2002) 4049–4053
These results contrast markedly with the behavior of the
free ligands 1,1-diphenyl-2-propyn-1-ol and 9-ethynyl-
fluoren-9-ol which undergo the Meyer–Schuster (rather
128.0, 127.6, 126.3 (Ar C’s), 80.6 (COH), 78.3 (CCH).
+
Mass spec. (DEI, m/z (%)): 538 (3) ([M−CO] ), 398 (17)
+
+
([M−6CO] ), 280 (14) ([M−Co (CO) ] ), 263 (91) ([M−
2
6
1
5
+
than the Rupe) rearrangement to 3,3-diphenylacrolein
OH−Co (CO) ] ). (CI, NH , m/z (%)): 522 (4) ([M+H−
2 6 3
+ +
and 9H-fluoren-9-ylideneacetaldehyde 18, respectively,
OH−CO] ), 455 (4) ([M+H−4CO] ), 398 (5)
+ +
1
6
under acidic conditions (Scheme 6).
([M−6CO] ), 263 (100) ([M−OH−Co (CO) ] ).
2 6
To conclude, we describe the novel cobalt carbonyl
mediated transformation of two propargyl units and a
carbon monoxide into a bis-methylene-3-cyclopen-
tenone, which had previously been available only via a
multi-step route. We are presently investigating the
scope and generality of this reaction, and hope to
elucidate the mechanism in some detail.
Treatment of 12, (1.55 g, 2.74 mmol) with HBF (0.4
4
mL, 2.94 mmol) in tetrahydrofuran (40 mL) afforded,
(trimethylsilylethynyldiphenylmethane)Co (CO) 13, as
2
6
1
a black solid, (65%, 0.98 g, 1.8 mmol), mp 91–93°C. H
1
1
NMR (300.13 MHz, CDCl ): l 7.58–7.55 (4H, m,
3
[Ph’s]), 7.41–7.30 (6H, m, [Ph’s]), 5.46 (1H, s, [Ph CH]),
2
13
0.47 (9H, s, [Si(CH ) ]). C NMR (75.48 MHz, CDCl3):
3
3
l 200.0 (Co-CO’s), 143.9, 128.6, 128.2, 127.3 (Ar C’s),
15.4 (CCH), 78.7 (CCH), 59.8 (Ph CH). Mass spec.
1
2
+
3
. Experimental
(DEI, m/z (%)): 466 (10) ([M−3CO] ), 382 (13) ([M−
+
+
6
CO] ), 264 (4) ([M−Co (CO) ] ). (CI, NH , m/z (%)):
2 6 3
+ +
6
,7
Treatment of 6 (1.00 g, 2.02 mmol) in CH Cl (40 mL)
383 (3) ([M+H−6CO] ), 265 (23) ([M+H−Co (CO) ] ).
2
2
2 6
with HBF (0.4 mL, 2.71 mmol) at −78°C in a dry
4
1
8
ice/isopropanol bath, afforded 9 (41%, 0.34 g, 0.55
Treatment of 14, (1.00 g, 2.03 mmol) with HBF (0.4
4
mmol). The analogous reaction in Et O (40 mL) gave 9
mL, 2.71 mmol) in dichloromethane (40 mL) yielded the
2
(
34%, 0.28 g, 0.45 mmol) and (ethynyldiphenyl-
fluorenylidene–tricobalt cluster 15, as a black solid
1
methane)Co (CO) 10 (2%, 0.02 g, 0.04 mmol), while in
THF (40 mL), the products were 9 (18%, 0.15 g, 0.24
mmol) 10 (8%, 0.08 g, 0.16 mmol) and 2,5-bis(diphenyl-
methylene)-3-cyclopentenone 11, (28%, 0.12 g, 0.28
mmol). Each reaction was monitored to completion
using thin-layer chromatography, before the solution
was allowed to warm to room temperature and
quenched with H O. All products were extracted with
Et O, dried over MgSO , and flash chromatographed
with a 2:1 mixture of hexanes/CH Cl . 9: dark red
crystals, mp 132–133°C. H NMR (200.20 MHz,
(38%, 0.32 g, 0.51 mmol), mp 141–142°C. H NMR
2
6
(200.20 MHz, CDCl ): l 8.94 (1H, s, [CꢀCH]), 8.11 (1H,
3
d, [Ar]), 7.77–7.70 (3H, m, [Ar]), 7.45–7.29 (4H, m [Ar]).
11
13
C NMR (50.35 MHz, CDCl ): l 199.8 (Co-CO’s),
3
148.4, 141.3, 140.4, 139.2, 136.3, 129.7, 128.8, 128.4,
128.3, 127.2, 126.0, 120.3, 120.1. Mass spec. (DEI, m/z
+
+
(%)): 590 (1) ([M−CO] ), 450 (7) ([M−6CO] ), 178 (100)
+
([M−C−Co (CO) ] ). (CI, NH , m/z (%)): 619 (5) ([M+
2
3
9
3
+
+
+
H] ), 591 (3) ([M+H−CO] ), 366 (10) ([M−9CO] ), 179
2
4
+
(100) ([M+H−C−Co (CO) ] ). Repetition of this reac-
2
2
3
9
1
tion in 40 mL of diethyl ether afforded 15 (23%, 0.19 g,
0.31 mmol), and (9-ethynylfluorene)Co (CO) 16, as a
CDCl ): l 8.25 (1H, s, [CꢀCH]), 7.40–7.21 (10H, m,
Ph’s]). C NMR (50.35 MHz, CDCl ): l 199.8 (Co-
3
2
6
1
3
1
[
dark red solid (7%, 0.07 g, 0.14 mmol), mp 98°C. H
3
CO’s), 149.1, 142.7, 139.5, 136.6, 130.2, 128.8, 128.5,
27.9, 127.8, 127.6. Mass spec. (DEI, m/z (%)): 536 (23)
NMR (500.13 MHz, CDCl ): l 7.78–7.69 (4H, d of d,
3
1
[Ar]), 7.45–7.34 (4H, m [Ar]), 6.38 (1H, s, [CCH]), 5.13
+
+
+
13
[M−3CO] ), 452 (25) ([M−6CO] ), 368 (42) ([M−9CO] ),
(1H, s, [CHCCH]). C NMR (125.77 MHz, CDCl ): l
3
+
91 (42) ([M−Co (CO) ] ). (CI, NH , m/z (%)): 621 (25)
199.3 (Co-CO’s), 146.7, 140.6, 128.2, 127.4, 125.1, 120.2
(Ar C’s), 98.6 (CCH), 74.9 (CCH), 49.7 (Ar-CH). Mass
3
9
3
+
+
3 9
+
spec. (DEI, m/z (%)): 308 (100) ([M−6CO] ), 249 (25)
2
4
11
9
3
+
4
6.08; H, 1.90%. (Ethynyldiphenylmethane)Co (CO) ,
0, dark red solid, mp 121–122°C. H NMR (200.20
([M−Co−6CO] ). (CI, NH , m/z (%)): 449 (42) ([M+H−
2
6
3
1
+
+
+
1
CO] ), 308 (100) ([M−6CO] ), 249 (13) ([M−Co−6CO] ).
Addition of HBF (0.4 mL, 2.71 mmol) to 14 (1.0 g, 2.03
MHz, CDCl ): l 7.46–7.18 (10H, m, [Ph’s]), 6.40 (1H, s,
3
4
13
[CCH]), 5.32 (1H, s, [Ph CH]). C NMR (50.35 MHz,
mmol) in THF (40 mL), afforded 15 (10%, 0.08 g, 0.13
mmol) and 16 (30%, 0.29 g, 0.61 mmol).
2
CDCl ): l 199.7 (Co-CO’s), 144.0, 128.7, 127.9, 127.2
3
(
Ar C’s), 73.4 (CCH), 57.8 (Ph CH). Mass spec. (DEI,
2
+ +
m/z (%)): 450 (39) ([M−CO] ), 394 (100) ([M−3CO] ),
3
NH , m/z (%)): 451 (47) ([M+H−CO] ), 310 (36) ([M−
X-Ray crystal data for 9: C H O Co , dark red plate
24
11
9
3
+
+
10 (44) ([M−6CO] ), 251 (37) ([M−Co−6CO] ). (CI,
(0.08×0.28×0.34 mm), monoclinic, C2/c, a=16.793(4),
+
b=7.874(2), c=36.834(8)
A
,
,
i=96.923(4)°, V=
3
+
+
3
−3
6
CO] ), 192 (31) ([M−Co (CO) ] ). Anal. calcd for
4834.6(19) A
,
, Z=8, z
1.704 g cm , T=197(2) K,
calcd
2
2
6
−
1
C H O Co ·H O: C, 50.83; H, 2.84. Found: C, 50.82;
v=2.086 mm ; R =0.0612, wR =0.1294 (based on F )
21
12
6
2
2
1 2
H, 2.86%.
for 326 variables and 16663 reflections (4236 indepen-
dent); R =0.1102, with I>2|(I) and 1.11<q<24.99.
int
Addition of Co (CO) (1.83 g, 5.4 mmol) to trimethylsi-
2
8
17
lyl(1,1-diphenyl-2-propyn-1-ol) (1.50 g, 5.4 mmol) in
0 mL of tetrahydrofuran afforded trimethylsilyl(1,1-
diphenyl-2-propyn-1-ol)Co (CO) 12, as a dark red solid
X-Ray crystal data for 11: C H O, yellow rod (0.04×
31
22
4
0.05×0.40 mm), orthorhombic, P2 2 2 , a=5.9605(15),
1 1 1
3
b=12.804(3), c=28.760(8) A
zcalcd 1.242 g cm , T=197(2) K, v=0.073 mm ; R =
,
, V=2194.9(10) A
,
, Z=4,
2
6
1
−3
−1
(
88%, 2.67 g, 4.7 mmol). H NMR (200.20 MHz,
1
2
CDCl ): l 7.68–7.63 (4H, m, [Ph’s]), 7.35–7.19 (6H, m,
0.0514, wR =0.0909 (based on F ) for 378 variables and
3
2
13
[Ph’s]), 2.71 (1H, s, [OH]), 0.45 (9H, s, [SiCH ) ]).
C
16498 reflections (3849 independent); R =0.0927 with
3
3
int
NMR (50.35 MHz, CDCl ): l 199.8 (Co-CO’s), 146.6,
I>2|(I) and 1.42<q<25.00.
3

J. H. Kaldis, M. J. McGlinchey / Tetrahedron Letters 43 (2002) 4049–4053
4053
Crystallographic data (excluding structure factors) for
the molecules reported in this paper have been
deposited with the Cambridge Crystallographic Data
Centre as supplementary publication nos. CCDC
439; (c) Neumann, W. P.; Uzick, W.; Zarkadis, A. K. J.
Am. Chem. Soc. 1986, 108, 3762.
6. Connor, R. E.; Nicholas, K. M. J. Organomet. Chem.
1977, 125, C45.
1
80385 (9) and 180386 (11). Copies of the data can be
7. Kim, P.-J.; Hagihara, N. Bull. Chem. Soc. Jpn. 1968, 41,
1184.
obtained free of charge on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, UK (fax: (+44)
8. One could envisage initial formation of the alkene 9
which is subsequently hydrogenated to the alkane 7 by
reaction of the acid with cobalt-containing decomposition
products.
1
223-336-033; e-mail: deposit@ccdc.cam.ac.uk).
Acknowledgements
9
. (a) Markle, R.; Wender, I.; Friedel, R. A.; Cotton, F. A.;
Sternberg, H. W. J. Am. Chem. Soc. 1958, 80, 6529; (b)
Sutton, P. W.; Dahl, L. F. J. Am. Chem. Soc. 1967, 89,
Financial Support from the Natural Sciences and Engi-
neering Research Council of Canada (NSERC) is grate-
fully acknowledged. J.H.K. thanks NSERC and the
Province of Ontario for graduate scholarships. Mass
spectra were acquired courtesy of Dr. Kirk Green of
the McMaster Regional Mass Spectrometry Center.
2
61; (c) Khand, I. U.; Knox, G. R.; Pauson, P. L.; Watts,
W. E. J. Organomet. Chem. 1974, 73, 383.
0. (a) Kishan, K. V. R.; Desiraju, G. R. Indian J. Chem.
988, 27B, 953; (b) Desiraju, G. R.; Bernstein, J.; Kishan,
K. V. R.; Sarma, J. A. R. P. Tetrahedron Lett. 1989, 30,
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1. Kawase, T.; Muro, S.; Oda, M. Tetrahedron Lett. 1992,
3, 5961.
1
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