Nicholas Reactions of Alkynyl- and Alkenyltrifluoroborates

Article abstract of DOI:10.1055/s-0036-1588528

The Lewis acid mediated Nicholas reaction of potassium alkynyltrifluoroborates and propargyl acetate-hexacarbonyldicobalt complexes affords 1,4-diyne dicobalt hexacarbonyl complexes in good yields. The analogous Nicholas reactions of potassium alkenyltrif

Full text of DOI:10.1055/s-0036-1588528

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© Georg Thieme Verlag Stuttgart · New York
2017, 28, A–E
letter
A
en
B. St. Onge, J. R. Green
Letter
Synlett
Nicholas Reactions of Alkynyl- and Alkenyltrifluoroborates
R
BF3−K+
R1
Brent St. Onge
OAc
R
R1
James R. Green*
BF3•OEt2, R = Bu, Ph, H
BF3•OEt2
(OC)6Co2
Co2(CO)6
R1 = H, alkyl, Ph, vinyl, CO2Me
Department of Chemistry and Biochemistry, University of
Windsor, 401 Sunset Ave., Windsor, ON, N9B 3P4, Canada
jgreen@uwindsor.ca
BF3•OEt2
60–97%, 12 examples
BF3−K+
R
R1
BF3−K+
R = Me, Ph
R1
R
This paper is dedicated to Prof. Victor Snieckus, in consideration
of his many years of mentorship and friendship, and for his tire-
less and continuing dedication to progress in synthetic aromatic
chemistry
(OC)6Co2
Co2(CO)6
R1 = Bu, SiMe3
R1 = Bu, SiMe3
35–78%, 6 examples
33–57%, 2 examples
Received: 13.06.2017
Accepted after revision: 04.07.2017
Published online: 17.08.2017
OR2
R2
R1
R2
R1
Y
DOI: 10.1055/s-0036-1588528; Art ID: st-2017-r0465-l
SiMe3
Co2(CO)6
R2OH
Y
+
Co2(CO)6
Y = O, CH2
Abstract The Lewis acid mediated Nicholas reaction of potassium
alkynyltrifluoroborates and propargyl acetate-hexacarbonyldicobalt
complexes affords 1,4-diyne dicobalt hexacarbonyl complexes in good
yields. The analogous Nicholas reactions of potassium alkenyltrifluoro-
borates give 1,3-enyne dicobalt hexacarbonyl complexes in most cases,
although the initial site of reaction can vary. Potassium vinyltrifluorobo-
rate itself affords alkynylcyclopropane complexes.
Et3SiH
R1
EDG
R1
Co2(CO)6
Co2(CO)6
1
R22NH
NR2
2
EDG
R1
R1
(CO)6Co2
Co2(CO)6
Key words alkyne complexes, boron, carbocation, coupling, Lewis ac-
ids, alkynes, enynes
Scheme 1 Nicholas reactions of heteroatom, hydride, and carbon nuc-
leophiles
The generation and reaction of hexacarbonyldicobalt
complexes of propargylic carbocations 1, or Nicholas reac-
tion, has achieved a place of prominence in metal-organic
chemistry due to high reaction site selectivity, good cation
stability and reactivity, ease of handling of the alkynedico-
balt complexes, and ready decomplexation of the organic
unit. Many of the requirements for participation of the nuc-
leophile are well-understood,1 and a wide range of hetero-
atom-based nucleophiles, hydride sources, and carbon-
based nucleophiles are compatible (Scheme 1). For carbon–
carbon bond-formation processes, electron-rich arenes,
enol derivatives, and allylmetals are particularly reliable re-
action participants.
Nevertheless, some classes of carbon-based nucleo-
philes remain problematic for the Nicholas reaction.
Alkynyl nucleophiles are known, but solely as the trialky-
nylaluminums;2 there is little further work reported based
on this transformation, perhaps due to reactivity issues
with some of the alanes involved.3 Similarly, there are limit-
ed reports of intermolecular reactions with alkene nucleop-
hiles that are not activated as either enol ethers or allylmet-
als. For simple alkene nucleophiles, modest yields of isomeric
alkene mixtures are the norm, with pendant cation-trap-
ping nucleophiles substituted on the alkene ameliorating
the situation only somewhat.4,5 Simple vinylsilanes have
not been reported to be reliable nucleophiles in this pro-
cess.
In this context, our attention has been drawn to the
chemistry of potassium organotrifluoroborates. Arguably
most prominent as boronic acid equivalents for cross-cou-
pling procedures,6,7 the alkynyl- and alkenyltrifluoroborates
recently have been shown to enter into reactions with car-
bocations or related electrophilic carbon centers.8 These tri-
fluoroborates are water-stable and have good long-term
storage capability, and they would appear to be useful in
addressing the limitations with alkynyl and alkenyl nucleo-
philes in Nicholas reaction chemistry. This letter reports
our preliminary work on these reactions.
Our initial experiments involved use of phenylalkynyl-
trifluoroborate 2a and 1-acetoxy-2-pentyne complex 3a
(Figure 1). In the presence of BF3·OEt2 (2.5 equiv) in CH2Cl2at
0 °C (1.5 equiv 3a), a reaction occurred that ceased pro-
gressing after 1.5 hours, giving 4aa (65% yield). Reversing
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

B
B. St. Onge, J. R. Green
Letter
Synlett
Table 1 Lewis Acid Mediated Reactions of Alkynyltrifluoroborates 2
and Propargyl Acetate Hexacarbonyldicobalt Complexes 3
the stoichiometry (1.5 equiv BF3·OEt2, 2.5 equiv 2a) im-
proved the yield significantly (4aa, 86%), while use of 2.5
equivalents each of BF3·OEt2and 2a gave 4aa in 91% yield.9
This latter procedure was adopted as standard conditions
for subsequent experiments.
R
BF3– K+
(2a–c) (2.5 equiv)
R1
OAc
R2
R
R1
BF3•OEt2 (2.5 equiv),
(OC)6Co2
Co2(CO)6
R2
0 °C, 1.5 h
3a, R1 = Et, R2 = H
OAc
3a–j
4
3b, R1 = Bu, R2 = H
BF3–K+
2a, R = Ph
R1
R
3c, R1 = SiMe3, R2 = H
3d, R1 = H2C=CH-, R2 = H
3e, R1 = Ph, R2 = H
3f, R1 = H, R2 = H
3g, R1 = SiMe3, R2 = Me
3h, R1 = SiMe3, R2 = Ph
R2
Co2(CO)6
Entry
1
2
3
4
Product structure
Yield
(%)a
2b, R = Bu
2c, R = H
OAc
OMe
2a
2b
3a
3a
4aa
4ba
91
(OC)6Co2
Et
Me3Si
(CO)6Co2
MeO2C
3j
3i
Ph
Bu
Co2(CO)6
Figure 1 Starting potassium alkynyltrifluoroborates 2 and propargyl
acetate hexacarbonyldicobalt complexes 3
2
3
4
5
6
73
(OC)6Co2
Et
Using these standard conditions, the reactions of three
potassium alkynyltrifluoroborates (2a, butyl- substituted
2b, and unsubstituted 2c) were investigated, with a number
of cobalt hexacarbonyl complexes of propargyl acetates 3
(Table 1).10 The alkynyltrifluoroborates successfully reacted
with ethyl-substituted complex 3a (4aa, 91% yield; 4ba, 73%
yield; 4ca, 67% yield, Table 1, entries 1–3), butyl-substituted
complex 3b (4ab, 73%, Table 1, entry 4), trimethylsilyl-sub-
stituted complex 3c (4ac, 65% yield; 4bc, 77% yield, Table 1,
entries 5 and 6), vinyl-substituted complex 3d (4bd, 65%
yield, Table 1, entry 7), phenyl-substituted complex 3e (4be,
60% yield, Table 1, entry 8), and unsubstituted complex 3f
(4bf, 63% yield, Table 1, entry 9). Monosubstitution at the
propargylic site of the alkynedicobalt complex was well tol-
erated, as methyl-substituted complex 3g (4ag, 90% yield,
Table 1, entry 10) and phenyl-substituted 3h (4ah, 97%
yield, Table 1, entry 11) each gave high yields of condensa-
tion product with 2a. Conversely, tertiary acetate 3i gave
only elimination product 5 (54% yield, Table 1, entry 12)
with 2a to the exclusion of any C–C bond-forming product.
Finally, a remote ester function on the propargyl ether
complex (3j) was tolerated, giving γ-carbonyl cation reac-
tion product 4aj (65% yield Table 1, entry 13) employing
Bu2BOTf in place of BF3·OEt2.11
2c
2a
2a
2b
3a
3b
3c
3c
4ca
4ab
4ac
4bc
67
(77)
(OC)6Co2
Et
73
65
77
(OC)6Co2
Bu
Ph
(OC)6Co2
Me3Si
Ph
(OC)6Co2
Me3Si
Bu
7
2b
2b
2b
2a
3d
3e
3f
4bd
4be
4bf
65
(72)
(OC)6Co2
Bu
8
60
(67)
(OC)6Co2
Ph
Bu
Bu
9
63
(72)
(OC)6Co2
H
10
3g
4ag
90
97
(OC)6Co2
Me3Si
Ph
BF3− K+
6d
Ph
BF3− K+
BF3− K+
BF3− K+
6c
11
2a
3h
4ah
6a
6b
Ph
(OC)6Co2
Me3Si
Figure 2 Potassium alkenyltrifluoroborates 6
Ph
12
13
2a
2a
3i
3j
5
54
(CO)6Co2
Me3Si
Four potassium alkenyltrifluoroborates were chosen for
investigation with propargyl acetate complexes 3, including
styryltrifluoroborate 6a, n-propenyltrifluoroborate 6b, iso-
propenyltrifluoroborate 6c, and vinyltrifluoroborate 6d
(Figure 2). The styryl-substituted trifluoroborates, reacted
in a quite straightforward manner under the standard con-
ditions, as 6a with 3b afforded 7ab (68% yield, Table 2, entry
1), and 6a with 3c gave 7ac (71% yield, Table 2, entry 2),
4aj
65b
(OC)6Co2
MeO2C
Ph
a Yields in parentheses based on recovered starting 3 (brsm).
b Lewis acid Bu2BOTf.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

C
B. St. Onge, J. R. Green
Letter
Synlett
each as E isomers to the limits of detection by 1H NMR spec-
troscopy.12 n-Propenyltrifluoroborate 6b reacted analo-
gously with 3a to give (E)-7ba (51% yield, 58% brsm, Table 2,
entry 3) and with 3c to give (E)-7bc (35% yield, 69% brsm,
Table 2, entry 4), but the reactions suffered from incom-
plete conversion under the standard conditions. Increasing
the amounts of BF3·OEt2 and 6c to 4.0 equivalents raised the
yield of (E)-7bc to 42% (65% brsm).
BF3− K+
(6c) (2.5 equiv)
OAc
R
Me3Si
Me3Si
BF3•OEt2 (2.5 equiv),
(OC)6Co2
Co2(CO)6
0 °C, 1.5 h
R
3c, R = H
3g, R = Me
7bc, 76% (Z:E = 7.1:1)
7cg, 78% (Z:E = 2.9:1)
BF3− K+
(6d) (2.5 equiv)
OAc
R
R
BF3•OEt2 (2.5 equiv),
Co2(CO)6
Table 2 Lewis Acid Mediated Reactions of Alkenyltrifluoroborates 6
and Propargyl Acetate Hexacarbonyldicobalt Complexes 3
Co2(CO)6
3b, R = Bu
0 °C, 1.5 h
8db, 33% (80% brsm)
8dc, 57% (89% brsm)
3c, R = SiMe3
R
BF3− K+
OAc
Scheme 2 Reactions of isopropenyltrifluoroborate (6c) and vinyltriflu-
oroborate (6d) with propargyl acetate hexacarbonyldicobalt complexes 3
R1
(6a,b) (2.5 equiv)
R1
R
BF3•OEt2 (2.5 equiv),
0 °C, 1.5 h
Co2(CO)6
3a–c
(OC)6Co2
7
R1
R1
(NH4)2Ce(NO3)6
R
R
acetone, –78 to –30 °C
Entry
3
6
7
Product structure
Yield (%)a
68
(OC)6Co2
4bd, R = Bu, R1 = H2C=CH-
4bf, R = Bu, R1 = H
9bd, 86%
9bf, 86%
1
3b
3c
6a
6a
7ab
(OC)6Co2
Bu
Ph
Bu
Bu
(NH4)2Ce(NO3)6
2
7ac
7ba
7bc
71
Ph
Ph
(OC)6Co2
Me3Si
acetone, –78 to –30 °C
(OC)6Co2
Ph
7ab
10ab, 96%
Scheme 3 CAN-mediated decomplexation reactions
3
4
3a
3c
6b
6b
51 (58)
35 (69)
(OC)6Co2
Et
The Nicholas reactions with alkynyltrifluoroborates are
quite successful in the anticipated manner. They give rea-
sonable to excellent chemical yields in all cases except the
elimination-prone tertiary case (3i), with the higher yield-
ing cases generally coincident with the more stabilized
propargyldicobalt cationic species. The modest amounts of
starting propargyl acetate recovered in a few cases likely
stems from a competitive defluorination reaction by
BF3·OEt2 of the alkynyltrifluoroborate salt.15
(OC)6Co2
Me3Si
a Yields in parentheses based on recovered starting 3 (brsm).
In contrast, 6c and 6d gave unexpected reaction pat-
terns (Scheme 2). Isopropenylborate 6c afforded 1-prope-
nyl adducts as the sole regioisomers, predominantly as Z
isomers, with 3c giving 7bc (76% yield, Z/E = 7.1:1) and 3g
giving 7cg (78% yield, Z/E = 2.9:1).13 Vinylborate 6d, on the
other hand, gave incomplete conversion into cyclopropane
products with only traces (ca. 7%) of alkene isomers evident
by NMR spectroscopy; 3b afforded 8db (33% yield, 80%
brsm), and 3c provided 8dc (57% yield, 89% brsm). An in-
crease in the amounts of BF3·OEt2 and 6d (to 4.0 equiv each)
with 3b gave somewhat improved yields of 8db, but still in-
complete conversion (49% yield, 78% brsm).
Three Nicholas reaction products were selected for de-
complexation, for illustration of the viability of the process
for these substrates (Scheme 3). Employing ceric ammoni-
um nitrate (CAN) in acetone, under carefully controlled
conditions, skipped diyne complexes 4bd and 4bf and
enyne complex 7ab each afforded their metal-free counter-
parts in good to excellent yields (9bd, 86% yield; 9bf, 86%
yield; 10ab, 96% yield).14
–
R
BF3
+
+
R1
BF3− K+
R1
R
6c, R = Me
6d, R = H
Co2(CO)6
Co2(CO)6
11
R = H
R = Me
BF3
–
BF3
–
+
R1
R1
R
+
Co2(CO)6
13
12
Co2(CO)6
R1
(OC)6Co2
R1
Co2(CO)6
8
7
Scheme 4 Rationale for formation of n-propenyl adducts 7 from isopro-
penyltrifluoroborate (6c) and cyclopropanes 8 from vinyltrifluoroborate
(6d)
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

D
B. St. Onge, J. R. Green
Letter
Synlett
For the alkenyltrifluoroborates, it is our belief that the
reaction is being driven by attack of the propargyldicobalt
cation by the most nucleophilic carbon of the alkene func-
tion, with ultimate alkene re-formation by way of BF3 loss
from a β-boryl cation. This results in straightforward sub-
stitution at boron-substituted site with styryltrifluorobo-
rate 6a, and less efficiently for n-propenyltrifluoroborate
6b. For isopropenylborate 6c, attack at the terminal alkene
carbon results in formation of carbocation 11, followed by
hydride migration to give 12, which in turn undergoes BF3
loss to give 7bc/7cg (Scheme 4).16 There are rare examples
of reactions involving isopropenyltrifluoroborate that af-
ford a 1-propenyl adduct;17 however, they likely are not
proceeding by way of a carbocation.18 Finally, the reaction
of vinyltrifluoroborate 6d is consistent with initial forma-
tion carbocation 11, a 1,3-hydride shift to give propargyl
cation 13, and nucleophilic attack on the cation to give
8db/8dc. It is likely that 6d is a less nucleophilic trifluoro-
borate and that BF3-induced defluorination competes more
substantially in these cases. The reasons for the change in
pattern of 6d from 6c are not clear at this time.
(2) (a) Padmanabhan, S.; Nicholas, K. M. J. Organomet. Chem. 1981,
212, 115. (b) Padmanabhan, S.; Nicholas, K. M. Tetrahedron Lett.
1983, 24, 2239.
(3) For the use of alkynylaluminums, see: (a) Zweifel, G.; Miller, J. A.
Org. React. 1984, 32, 375. (b) Oishi, M. In Science of Synthesis;
Yamamoto, H., Ed.; Thieme: Stuttgart, 2004, 261. (c) Kessabi, J.;
Beaudegnies, R.; Jung, P. M. J.; Martin, B.; Montel, F.;
Wendeborn, S. Synthesis 2008, 655.
7
V
o.l
(4) (a) Krafft, M. E.; Cheung, Y. Y.; Wright, C.; Cali, R. J. Org. Chem.
1996, 61, 3912. (b) Amiralaei, S.; Gauld, J.; Green, J. R. Chem. Eur.
J. 2011, 17, 4157.
(5) There is greater consistent success in intramolecular reactions
with alkenes, see: (a) Nakamura, T.; Matsui, T.; Tanino, K.;
Kuwajima, I. J. Org. Chem. 1997, 62, 3032. (b) Mann, A.; Muller,
C.; Tyrrell, E. J. Chem. Soc., Perkin Trans. 1 1998, 1427. (c) Lu, Y.;
Green, J. R. Synlett 2001, 243.
(6) (a) Molander, G. A.; Jean-Gérard, L. Org. React. 2013, 79, 1.
(b) Molander, G. A.; Jean-Gérard, L. In Boronic Acids, Hall D. G;
Wiley-VCH: Weinheim, 2011, 2nd ed., Vol. 2, 507. (c) Molander,
G. A.; Canturk, B. Angew. Chem. Int. Ed. 2009, 48, 9240.
(d) Darses, S.; Genet, J.-P. Chem. Rev. 2008, 108, 288.
(e) Molander, G. A.; Ellis, N. Acc. Chem. Res. 2007, 40, 275.
(f) Stefani, H. A.; Cella, R.; Vieira, A. S. Tetrahedron 2007, 63,
3623.
Future work will involve investigating other protocols19
for generation of the propargyldicobalt cation for the pur-
pose of obtaining complete conversion in the more difficult
cases, determining the borderline between divergent reac-
tion processes with the alkenyltrifluoroborates, and explo-
ration of aryl- and alkytrifluoroborates as potential Nicho-
las reaction partner. These are in progress and will be re-
ported in due course.
(7) For use in conjugate additions, see: (a) Sanchez-Sancho, F.;
Csaky, A. G. Synthesis 2016, 48, 2165. (b) Koike, T.; Akita, M. Org.
Biomol. Chem. 2016, 14, 6886.
(8) (a) Mitchell, T. A.; Bode, J. W. J. Am. Chem. Soc. 2009, 131, 18057.
(b) Zeng, J.; Vedachalam, S.; Xiang, S.; Liu, X.-W. Org. Lett. 2011,
13, 42. (c) Vo, C.-V. V.; Mitchell, T. A.; Bode, J. W. J. Am. Chem.
Soc. 2011, 133, 14082. (d) Taylor, C.; Bolshan, Y. Org. Lett. 2014,
16, 488. (e) Fisher, K. M.; Bolshan, Y. J. Org. Chem. 2015, 80,
12676. (f) Tatina, M. B.; Hussain, A.; Dhas, A. K.; Mukherjee, D.
RSC Adv. 2016, 6, 75960. (g) Batey, R. A.; Thadani, A. N.; Smil, D.
V.; Lough, A. J. Synthesis 2000, 990. (h) Nakamura, H.;
Yamamoto, H. WO 2005043630, 2005, Chem. Abstr. 2005, 142,
440277.
Funding Information
(9) Experimental Procedure
We are grateful to NSERC (Canada) Discovery Grants programme (RG-
To a solution of 3a (89.7 mg, 0.218 mmol) and 2a (0.113 g, 0.544
mmol) in CH2Cl2 (8 mL) at 0 °C was added BF3·OEt2 (67 μL, 0.54
mmol). After 1.5 h at 0 °C, sat. NH4Cl (aq) was added and the
mixture subjected to conventional extractive workup (CH2Cl2).
Flash chromatography (PE–Et2O, 50:1) afforded 4aa (90.3 mg,
91%) as a viscous red oil.
PIN-2016-04946) for support of this research.
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esearch
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Acknowledgment
We are indebted to Prof. Yuri Bolshan (University of Ontario Institute
of Technology, UOIT) for helpful discussions on the chemistry of po-
tassium organotrifluoroborates.
(10) Representative Characterization Data
Compound 4aa: 1H NMR (CDCl3, 300 MHz): δ = 7.30–7.44 (m, 5
H), 4.00 (s, 2 H), 2.92 (q, J = 7.3 Hz, 2 H), 1.35 (t, J = 7.3 Hz, 3 H).
13C NMR (CDCl3, 75 MHz): δ = 199.7, 131.5, 128.3, 128.0, 123.3,
101.6, 93.7, 86.5, 82.3, 26.9, 25.0, 15.6. IR: νmax = 2971, 2088,
2043, 1989, 1600, 1490 cm–1. HRMS: m/e calcd for C19H12Co2O6
[M+ – CO + H]: 454.9376; found: 454.9367.
Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0036-1588528.
S
u
p
p
ortiInfogrmoaitn
S
u
p
p
ortioInfgrmoaitn
Compound 4ba: 1H NMR (CDCl3, 300 MHz): δ = 3.72 (t, J = 2.3
Hz, 2 H), 2.87 (q, J = 7.4 Hz, 2 H), 2.17 (m, 2 H), 1.31 (t, J = 7.4 Hz,
3 H), 0.90 (t, J = 7.1 Hz, 3 H). 13C NMR (CDCl3, 75 MHz): δ = 200.0,
101.5, 95.3, 82.6, 76.7, 30.6, 26.8, 24.3, 21.9, 18.3, 15.5, 13.5. IR:
References and Notes
ν
C
max = 2964, 2874, 2087, 2043, 1990, 1457. HRMS: m/e calcd for
(1) (a) Kann, N. Curr. Org. Chem. 2012, 16, 322. (b) Shea, K. M. In
Name Reactions for Homologations; Li, J. J., Ed.; Wiley: Hoboken,
2009, Part 1 284. (c) Diaz, D. D.; Betancort, J. M.; Martín, V. S.
Synlett 2007, 343. (d) Teobald, B. J. Tetrahedron 2002, 58, 4133.
(e) Green, J. R. Curr. Org. Chem. 2001, 5, 809. (f) Green, J. R.;
Nicholas, K. M. Org. React., submitted for publication.
17H16Co2O6 [M+ – CO + H]: 406.9740; found: 406.9733.
Compound 4ca: 1H NMR (CDCl3, 300 MHz): δ = 3.76 (d, J = 2.6
Hz, 2 H), 2.88 (q, J = 7.4 Hz, 2 H), 2.19 (t, J = 2.6 Hz, 1 H), 1.32 (t,
J = 7.4 Hz, 3 H). 13C NMR (CDCl3, 75 MHz): δ = 200.2, 128.7,
128.0, 101.3, 98.9, 37.0, 27.1, 17.5, 15.6. IR: νmax = 3314, 2972,
2877, 2089, 2044, 1994 cm–1. HRMS: m/e calcd for C13H8Co2O6
[M+ + H]: 378.9063; found: 378.9063.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

E
B. St. Onge, J. R. Green
Letter
Synlett
(11) Nicholas reactions for γ-carbonyl cation equivalents routinely
give more rapid reaction with Bu2BOTf as Lewis acid relative to
BF3·OEt2. See: Taj, R.; Green, J. R. J. Org. Chem. 2010, 75, 8258.
(12) Representative Characterization Data for Compound 7ab
1H NMR (CDCl3, 300 MHz): δ = 7.29–7.44 (m, 5 H), 6.61 (d, J =
15.7 Hz, 1 H), 6.31–6.41 (dt, J = 15.7, 7.2 Hz, 1 H), 3.77–3.79 (d,
J = 7.2 Hz, 2 H), 2.92 (t, J = 7.6 Hz, 2 H), 1.48–1.77 (m, 4 H), 1.00
(t, J = 7.2 Hz, 3 H). 13C NMR (CDCl3, 75 MHz): δ = 200.3, 137.0,
132.3, 128.6, 127.5, 127.4, 126.2, 99.4, 97.7, 37.4, 33.8, 22.8,
13.9. IR: νmax = 3084, 3062, 3028, 2960, 2874, 2085, 2040, 1994,
1495 cm–1. HRMS: m/e calcd for C21H18Co2O6 [M+ – CO + H]:
456.9896; found: 456.9907.
mmol, 5 equiv) was added and the solution allowed to warm to
–30 °C (2 h), with monitoring by TLC. A sat. NaCl solution was
added and the mixture subjected to a conventional extractive
workup (Et2O). The product was filtered through a silica plug
using Et2O and concentrated under reduced pressure to give
9bd (13.9 mg, 86%).
(15) (a) Bir, G.; Schacht, W.; Kaufmann, D. J. Organomet. Chem. 1988,
340, 267. (b) Vedejs, E.; Chapman, R. W.; Fields, S. C.; Lin, S.;
Schrimpf, M. R. J. Org. Chem. 1995, 60, 3020. (c) For a compari-
son of BF3·OEt2 with other common Lewis acids in aldehyde-all-
yltrifluoroborate reactions, see ref. 8g.
(16) Our operating hypothesis for the predominant Z-isomer forma-
–
(13) Representative Characterization Data
tion comes from the proposed larger size of the BF3 unit rela-
Compound (Z)-7bc: 1H NMR (CDCl3, 300 MHz): δ = 5.50–5.67
(m, 2 H), 3.68 (d, J = 6.7 Hz, 2 H), 1.70 (d, J = 5.9 Hz, 3 H), 0.31 (s,
9 H); resonances from the minor isomer were observed at δ =
3.56 (d, J = 5.9 Hz, 2 H), 0.30 (s, 9 H).13C NMR (CDCl3, 75 MHz):
δ (major isomer only) = 200.4, 128.0, 125.9, 111.2, 78.7, 32.3,
12.9, 0.6. IR: νmax = 3025, 2959, 2984, 2085, 2041, 2000, 1581.
HRMS: m/e calcd for C15H16Co2O6Si [M+ – CO + H]: 410.9509;
found: 410.9501.
tive to a methyl group. Initial reaction consequently gives a
cation 11 with the methyl nearly eclipsed to the homopropar-
gyl-Co2(CO)6 unit (Scheme 4); subsequent hydride migration
then gives a cation 12 where rotation of the C–B bond to a colin-
ear orientation relative to the empty p-orbital of the cation is of
lower energy than a rotation of the methyl group anti to the
homopropargyl-Co2(CO)6 group.
(17) (a) Corey, E. J.; Lallic, G. Tetrahedron Lett. 2008, 49, 4894.
(b) Zhang, Y.; Fu, Z.; Wang, J.; Li, J.; Chen, S.; Wei, Y.; Yu, L.; Tao,
X. WO 2016045587, 2016, Chem. Abstr. 2016, 164, 438142.
(18) For analogous reactivity of vinylsilanes with more electrophilic
carbocations, see: Laub, H. A.; Mayr, H. Chem. Eur. J. 2014, 20,
1103.
Compound 8db: 1H NMR (CDCl3, 300 MHz): δ = 2.78 (t, J = 8.0
Hz, 2 H), 2.15 (tt, J = 7.4, 4.2 Hz, 1 H), 1.43–1.67 (m , 4 H), 1.11
(ddd, J = 7.4, 6.6, 4.3 Hz, 2 H), 0.97 (t, J = 7.2 Hz, 3 H), 0.72–0.77
(ddd, J = 6.6, 4.3, 4.2 Hz, 2 H). 13C NMR (CDCl3, 75 MHz):
δ = 200.1, 103.8, 98.4, 33.9, 33.6, 22.7, 15.2, 13.8, 12.5. IR: νmax
2962, 2876, 2086, 2040, 2005, 1450 cm–1. HRMS: m/e calcd for
16H14Co2O6 [M+ – CO + H]: 380.9583; found: 380.9589.
=
(19) El-Amouri, H.; Gruselle, M.; Jaouen, G.; Daran, J. C.;
Vaissermann, J. Inorg. Chem. 1990, 29, 3238.
C
(14) Experimental Procedure
Complex 4bd (47.6 mg, 0.110 mmol) was dissolved in acetone
(8 mL) and the solution cooled to –78 °C. CAN (0.302 g, 0.551
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E