Cycloheptyne-cobalt complexes via allylation of stabilized γ-carbonyl cations

Article abstract of DOI:10.1039/a803316a

The Bu₂BOTf mediated reaction of stannylsilanes (5 and 9) with γ-methoxy-alkynoate and -alkynone hexacarbonyldicobalt complexes (8), followed by conversion of the organic carbonyl into an acetate and a BF₃·OEt₂ mediated intramolecular reaction, affords cycloheptenyne hexacarbonyldicobalt complexes (13 and 15).

Full text of DOI:10.1039/a803316a

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Cycloheptyne–cobalt complexes via allylation of stabilized g-carbonyl cations
James R. Green*†
Chemistry and Biochemistry, School of Physical Sciences, University of Windsor, Windsor, Ontario, N9B 3P4, Canada
Cl
The Bu2BOTf mediated reaction of stannylsilanes (5 and 9)
CO2Me
Co2(CO)6
CO2Me
Co2(CO)6
with g-methoxy-alkynoate and -alkynone hexacarbonyldi-
cobalt complexes (8), followed by conversion of the organic
carbonyl into an acetate and a BF3·OEt2 mediated intra-
molecular reaction, affords cycloheptenyne hexacarbonyldi-
cobalt complexes (13 and 15).
SiMe3
3
4
Bu3Sn
SiR3
SiR3
Cyclic alkynes are compounds of limited stability.1 The
smallest unsubstituted member of the series which can be
isolated under conventional laboratory conditions is cyclooc-
tyne; in the vast majority of cycloheptynes and smaller
cycloalkynes, the strain of bending the sp hybridized carbon
atoms substantially away from 180° has too great an energetic
cost. This situation may be ameliorated by resorting to transition
metal complexes of cycloheptynes, particularly the dicobalt
hexacarbonyl complexes.2Alkyne hexacarbonyldicobalt com-
plexes have bond angles which average ca. 140° at the alkynyl
carbons; the resultant lower angle strain renders the cyclo-
heptyne and cyclohexyne complexes thermally stable.3
In addition to the above reasons, cobalt cycloheptyne
complexes are of interest due to the potential for applying the
rich synthetic utility of cobalt–alkyne complexes4–6 to seven
membered ring systems. This potential is largely unexplored,
however, as these systems have been prepared infrequently,7,8
and their systematic synthesis and study has escaped report.
Notably, the attempt to prepare systems of this type via a double
Nicholas reaction using allyldimetal equivalent 1 and propa-
rgylic ether 2 met with complete failure.9
Bu3Sn
5a R = Me
5b R = Et
5a′ R = Me
5b′ R = Et
O
R
O
R
Co2(CO)6
Co2(CO)6
R′3Si
R′3Si
6a R′ = Me, R = OMe
6b R′ = Et, R = OMe
6c R′ = Et, R = Ph
6d R′ = Et, R = Me
7a R′ = Me, R = OMe
7b R′ = Et, R = OMe
7c R′ = Et, R = Ph
7d R′ = Et, R = Me
Based on the report from Jacobi’s laboratory of the
condensation of boron enolates with g-methoxyalkynoate
hexacarbonyldicobalt complexes,13 we found that Bu2BOTf (0
°C, CH2Cl2) was capable of inducing the condensation between
propargyl ether 8a with 5a to give 6a/7a (84%, 6a : 7a = 78 :
22). While regiochemical impurity 7a was still present, the use
of silylstannane 5b14 with 8a was now possible, and allylsilane
6b could be prepared in good yield with only a trace of 7b (63%,
6b : 7b = 96 : 4; Table 1). Application of this protocol to phenyl
ketone 8b gave 6c as the major product, with a more substantial
amount of vinylsilane 7c (73%, 6c : 7c = 82 : 18). The methyl
ketone 8c would undergo additional reaction of the alkyl ketone
function in the presence of > 1 equivalents of Bu2BOTf, and
optimum results for the formation of 6d were obtained by
inverting the order of reagent addition, and by conducting the
reaction at 260 °C (39%, 77% based on recovered starting
material, 6d : 7d = 92 : 8). Finally, silylstannane 915 reacted
smoothly with 8a to give 10 in good yield (83%).
The carbonyl functions in 6 and 10 could be converted into a
leaving group by low temperature (278 °C) reduction with
Bui2AlH, and trapping of the resultant alkoxide with freshly
distilled acetic anhydride at room temperature, affording
acetates 11–12 in excellent yields. In the case of phenyl ketone
6c, the acylation step was very slow, and the addition of sodium
acetate with catalytic amounts of DMAP was required to give
useful amounts of 11c.
BnO
OBn
Me3Si
SiMe3
Co2(CO)6
1
2
During our recent work involving silver mediated reactions
of g-chloro-alkynoate and -alkynone hexacarbonyldicobalt
complexes,10 we observed striking effects of the presence of an
additional oxygen based function on the viability of propargyl
alcohol or propargyl ether based Nicholas reactions. As a result,
we believed that a stepwise reaction of 1 at the carbon bearing
the chloride, manipulation of the carbonyl into a leaving group,
and intramolecular allylsilane attack would give a cyclohepte-
nyne complex.‡ Therefore, we tested the reaction of chloride 3
with 1 and AgBF4 (0 °C, CH2Cl2), and to our surprise obtained
a small amount of 4 as the sole condensation product.
Compound 4 most probably results from the preferential loss of
the internal trimethylsilyl group from the b-silyl cation
intermediate, and allyldimetal equivalents with different elec-
trofuges were investigated for their reactivity with 3. Stannylsi-
lane 5a11 gave more satisfactory results, and allylsilane 6a was
obtained as the major product, contaminated with vinylsilane 7a
(55%, 6a : 7a = 78 : 22). In this case the source of the isomeric
impurity is believed to be Lewis acid mediated allylic
rearrangement of the tin moiety in 5a to give isomeric allyltin
5aA;12 despite this, recovered 5a showed no evidence of 5aA.
Attempts to use systems with more bulky silyl groups gave
improved regiochemical ratios in favour of the allylsilane
product, at the expense of a satisfactory chemical yield.
Table 1 Bu2BOTf mediated condensations of allyldimetals 5 and 9 with
8
Substrate
Allyldimetal Product
Ratio
Yield (%)
8a
8a
8b
8c
8a
5a
5b
5b
5b
9
6a + 7a
6b + 7b
6c + 7c
6d + 7d
10
78 : 22
96 : 4
82 : 18
92 : 8
—
84
63
73
39 (77)a
83
a Based on recovered starting material.
Chem. Commun., 1998
1751

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R′
F
F
OMe
O
Et3Si
CO2Me
Co2(CO)6
10
R
R
R
Co2(CO)6
Me3Si
SnBu3
Me3Si
Co2(CO)6
Co2(CO)6
Co2(CO)6
Co2(CO)6
8a R = OMe
8b R = Ph
8c R = Me
9
13a R = H, R′ = H
13b R = Ph, R′ = H
13c R = Me, R′ = H
15b R = H, R′ = CH3
14a R = Ph
14b R = Me
15a
16
OAc
R
OAc
complexes, and the synthetic applications of these compounds
are in progress and will be reported in due course.
Co2(CO)6
Co2(CO)6
R′3Si
Me3Si
11a R′ = Me, R = H
11b R′ = Et, R = H
11c R′ = Et, R = Ph
11d R′ = Et, R = Me
12
Notes and References
† E-mail: jgreen@uwindsor.ca
‡ This represents an endo–trig variant of the Schreiber group ring closure
step.7
With the appropriately attached allylsilane and propargylic
acetate functions in place, the ability of the substrates to form
cycloheptyne complexes was investigated. Slow addition of a
CH2Cl2 solution of 11 to a 0 °C CH2Cl2 solution of excess
1 A. Krebs and J. Wilke, Top. Curr. Chem., 1983, 109, 189; H. Meier,
Adv. Strain Org. Chem., 1991, 1, 215.
2 M. J. Went, Adv. Organomet. Chem., 1997, 41, 69.
3 The stabilization of other strained alkynes also has been accomplished
by complexation to the hexacarbonyldicobalt unit; see Y. Rubin, C. B.
Knobler and F. Diederich, J. Am. Chem. Soc., 1990, 112, 4966; M. H.
Haley and B. L. Langsdorf, Chem. Commun., 1997, 1121.
4 R. S. Dickson and P. J. Fraser, Adv. Organomet. Chem., 1974, 12,
323.
5 A. J. M. Caffyn and K. M. Nicholas, in Comprehensive Organometallic
Chemistry II, ed. E. W. Abel, F. G. A. Stone and G. Wilkinson, ed.
L. S. Hegedus, Pergamon, Oxford, 1995, vol. 12, ch. 7.1; K. M.
Nicholas, Acc. Chem. Res., 1987, 20, 207.
6 N. E. Schore, in Comprehensive Organometallic Chemistry II, ed. E. W.
Abel, F. G. A. Stone and G. Wilkinson, ed. L. S. Hegedus, Pergamon,
Oxford, 1995, vol. 12, ch. 7.2; N. E. Schore, Org. React., 1991, 40, 1;
N. E. Schore, in Comprehensive Organic Synthesis, ed. B. M. Trost, ed.
L. A. Paquette, Pergamon, Oxford, 1991, vol. 5, ch. 9.1; N. E. Schore,
Chem. Rev., 1988, 88, 1081.
7 S. L. Schreiber, M. T. Klimas and T. Sammakia, J. Am. Chem. Soc.,
1986, 108, 3128; T. Nakamura, T. Matsui, K. Tanino and I. Kuwajima,
J. Org. Chem., 1997, 62, 3032.
8 N. E. Schore and S. D. Najdi, J. Org. Chem., 1987, 52, 5296; M. Isobe,
C. Yenjai and S. Tanaka, Synlett, 1994, 916; S. Hosokawa and M. Isobe,
Synlett, 1995, 1179.
9 S. Takano, T. Sugihara and K. Ogasawara, Synlett, 1992, 70.
10 C. S. Vizniowski, J. R. Green, T. L. Breen and A. V. Dalacu, J. Org.
Chem., 1995, 60, 7496.
11 G. E. Keck and D. R. Romer, J. Org. Chem., 1993, 58, 6083.
12 M. Pereyre, J.-P. Quintard and A. Rahm, Tin in Organic Synthesis,
Butterworths, London, 1987, pp. 216–218; A. Yanagisawa, A. Ishiba,
H. Nakashima and H. Yamamoto, Synlett, 1997, 88.
BF3·Et2O (final substrate concentration = 1 m ) rapidly
M
afforded cycloheptenyne complexes 13, as red–violet oils of
good thermal stability, in excellent yields (Table 2). In the
phenyl and methyl substituted cases 11c and 11d, trace amounts
of fluorocycloheptyne complexes 14b (8%) and 14c (6%),
respectively, were also isolated. In the case of substrate 12,
cyclization under these conditions afforded methylenecyclo-
heptyne complex 15a contaminated with a minor amount of the
endo double bond isomer 15b (46%, 87 : 13), along with
desilylated fluorocycloheptyne complex 16 (44%). An alter-
native procedure which employed the slow addition of
BF3·Et2O (5 equiv.) to a solution of 12 (1.5 m ) at 0 °C gave
M
slightly enhanced amounts of 15a + 15b (55%, 90 : 10) and a
small amount of 16 (8%).
The results demonstrate the facility with which the Nicholas
reaction chemistry of cobalt stabilized g-carbonyl cations can be
applied to the preparation of cycloheptyne cobalt complexes.
Further work in this area, including that on superior allyldimetal
equivalents and one pot, [4 + 3] cycloaddition approaches to the
Table 2 Conversion of condensation products 6 and 10 to cycloheptenynes
complexes 13 and 15
Acetate
(Yield [%])
Cycloheptenyne
(Yield [%])
Fluorocycloheptyne
(Yield [%])
Substrate
13 P. A. Jacobi, S. C. Buddhu, D. Fry and S. Rajeswari, J. Org. Chem.,
1997, 62, 2894.
14 Prepared in 90% yield, as adapted from the procedure of J. M.
Muchowski, R. Naef and M. L. Maddox, Tetrahedron Lett., 1985, 26,
5375.
15 Prepared in 60% yield by the stannylation of 2-chloromethyl-
3-trimethylsilylprop-1-ene according to S. Chandrasekhar, S. Latour,
J. D. Wuest and B. Zacharie, J. Org. Chem., 1983, 48, 3810.
6a
6b
6c
6d
10
11a (88)
11b (90)
11c (84a)
11d (88)
12 (90)
13a (89)
13a (87)
13b (84)
13c (85)
15a + b (46)
[87 : 13]b
—
—
14b (8)
14c (6)
16 (44)
a
DMAP (0.2 equiv.) and NaOAc (excess) added during acylation step.
b Numbers in square brackets represent the 15a : b ratio.
Received in Corvallis, OR, USA, 4th May 1998; 8/03316A
1752
Chem. Commun., 1998