Rhodium- or iridium-catalyzed trans-hydroboration of terminal alkynes, giving (Z)-1-alkenylboron compounds [6]

Full text of DOI:10.1021/ja0002823

4990
J. Am. Chem. Soc. 2000, 122, 4990-4991
Scheme 1. Catalyzed Hydroboration of Alkynes
Rhodium- or Iridium-Catalyzed trans-Hydroboration
of Terminal Alkynes, Giving (Z)-1-Alkenylboron
Compounds
Toshimichi Ohmura, Yasunori Yamamoto, and
Norio Miyaura*
DiVision of Molecular Chemistry
Graduate School of Engineering
Hokkaido UniVersity, Sapporo 060-8628, Japan
Table 1. Effect of Catalyst in the Hydroboration of 1-Octynea
isomeric ratioc
ReceiVed January 26, 2000
entry
catalyst
borane yield/%b
2
3
4
Hydroboration of alkynes is a practical route for the syntheses
of 1-alkenylboron compounds which are a versatile reagent for
organic synthesis.1,2 Although much attention has been recently
focused on the metal-catalyzed hydroboration with catecholborane
(1a) or pinacolborane (1b), both the uncatalyzed1,3 and the
catalyzed reactions4,5 yield (E)-1-alkenylboronates through the
anti-Markovnikov and syn-addition of borane to terminal alkynes.
Thus, (Z)-1-alkenylboron compounds have been synthesized by
an alternative, two-step method.6,7 We wish to report a formal
trans-hydroboration of terminal alkynes with 1a or 1b to yield
cis-1-alkenylboronates in the presence of a Rh(I)- or Ir(I)-PiPr3
complex and Et3N (Scheme 1). For convenience of the analyses,
all products derived from 1a were converted into the correspond-
ing pinacol esters prior to isolation of 2-4.
1
[Rh(cod)Cl]2-4PiPr3
1a
1a
1a
1a
1a
1a
1a
1a
1b
1b
86(74)
60
99
18
9
48
98
21
58
55
91
70
1
0
17
1
7
0
15
10
0
2
5
2d
3e
4
65
90
45
2
64
32
45
7
94f
[Rh(cod)Cl]2-4PnBu3
[Rh(cod)Cl]2-4PCy3
[Rh(cod)Cl]2-4PtBu3
[Ir(cod)Cl]2-4PiPr3
56
86
34
43
5
6
7
8g RuHCl(CO)(PiPr3)2
9h [Rh(cod)Cl]2-4PiPr3
10h [Ir(cod)Cl]2-4PiPr3
7
81(71)
73
25
a To a solution of [M(cod)Cl]2 (M ) Rh or Ir, 0.015 mmol), a
phosphine ligand (0.06 mmol), Et3N (1 mmol), and a borane (1.0 mmol)
in cyclohexane was added 1-octyne (1.2 mmol). The mixture was then
stirred at rt for 1 h, unless otherwise noted. b GC yields based on a
borane and isolated yields by chromatography over silica gel in
parentheses. c Determined by 1H NMR of crude product. d Reaction
carried out in the absence of Et3N. e alkyne/borane ) 0.85/1. f Based
on 1-octyne. g The reaction in CH2Cl2. h alkyne/borane/Et3N ) 2/1/5.
The selected results for the catalytic hydroboration of 1-octyne
are shown in Table 1.
The catalyst in situ generated from [Rh(cod)Cl]2 and PiPr3 (4
equiv) completed the hydroboration of 1-octyne within 1 h at
room temperature (entry 1).8 The presence of more than 1 equiv
of Et3N was critical to achieve high yield and high cis-selectivity
because a similar reaction resulted in a mixture of all isomers
2-4 in the absence of Et3N (entry 2). Another dominant factor
reversing the conventional cis-hydroboration to the trans-hy-
droboration was the use of alkyne in excess of the borane reagent
because (E)-isomer 3 was predominated when using a slightly
excess of 1a (entry 3). The reaction initially yields (Z)-alkenyl-
boronate 2, but an addition/elimination sequence of Rh-H species
isomerizes 2 to a more stable (E)-isomer 3.9a,b The steric and
electronic effects of PiPr3 also play a major role in influencing
the course of the reaction. The PCy3 (Cy ) cyclohexyl) complex
revealed comparable selectivity (entry 5), but other complexes
of PPh3, PMePh2, PMe3, PnPr3, PnBu3 (entry 4), PiBu3, and PtBu3
(entry 6) yielded a mixture of three possible isomers. The Ir and
Ru complexes are not effective for selective hydroboration (entries
7 and 8). The reaction of pinacolborane 1b is shown in entries 9
and 10. The Ir-catalyzed reaction again resulted in a mixture of
products, but high cis-selectivity was achieved by the Rh(I)
complex in the presence of 2 equivalents of 1-octyne.
Both 1a and 1b hydroborate various terminal alkynes in the
presence of a Rh(I)-PiPr3 complex (Table 2).10 There is no large
difference in cis-selectivity for the representative terminal alkynes
(entries 1-13), but the hydroboration with 1b, in general, resulted
in slightly lower selectivity than that of 1a. However, it is
interesting that pinacolborane 1b exceptionally resulted in better
stereoselectivities for tert-butyl acetylene in the presence of a Rh
or Ir catalyst (entries 7-9). All attempts at the trans-hydroboration
of internal alkynes were unsuccessful.
The hydroboration of 1-deuterio-1-octyne (96% d1 incorpora-
tion) with 1a gave mechanistic information for the trans-
hydroboration (Scheme 2). The â-hydrogen in the cis-product
unexpectedly does not derive from the borane reagents because
the deuterium labeled at the terminal carbon selectively shifted
to the â-carbon. Thus, the results do not fit the mechanisms
previously proposed in the catalyzed trans-hydrometalation of
terminal alkynes.9,11 The mechanism isomerizing the trans-product
(1) (a) Smith, K.; Pelter, A.; Brown, H. C. Borane Reagents; Academic
Press: London, 1988. (b) Smith, K.; Pelter, A. In ComprehensiVe Organic
Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon Press: Oxford, 1991;
Vol. 8, p 703.
(2) (a) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457. (b) Suzuki, A.
In Metal-Catalyzed Cross-Coupling Reactions; Diederich, F., Stang, P. J., Eds.;
VCH: Weinheim, 1998; p 49. (c) Suzuki, A. J. Organomet. Chem. 1999,
576, 147.
(3) Tucker, C. E.; Davidson, J.; Knochel, P. J. Org. Chem. 1992, 57, 3482.
(4) For reviews: (a) Burgess, K.; Ohlmeyer, M. J. Chem. ReV. 1991, 91,
1179. (b) Beletskaya, I.; Pelter, A. Tetrahedron 1997, 53, 4957.
(5) Catalytic hydroboration of alkynes: (a) Ma¨nning, D.; No¨th, H. Angew.
Chem., Int. Ed. Engl. 1985, 24, 878. (b) Gridnev, I. D.; Miyaura, N.; Suzuki,
A. Organometallics 1993, 12, 589. (c) Pereira, S.; Srebnik, M. Organometallics
1995, 14, 3127. (d) He, X.; Hartwig, J. F. J. Am. Chem. Soc. 1996, 118, 1696.
(e) Pereira, S.; Srebnik, M. Tetrahedron Lett. 1996, 37, 3283.
(6) Intramolecular substitution of 1-halo-1-alkenylboronates with metal
hydrides. (a) Negishi, E.; Williams, R. M.; Lew, G.; Yoshida, T. J. Organomet.
Chem. 1975, 92, C4. (b) Campbell, J. B. Jr.; Molander, G. A. J. Organomet.
Chem. 1978, 156, 71. (c) Brown, H. C.; Imai, T. Organometallics 1984, 3,
1392.
(10) A typical procedure: To a solution of [Rh(cod)Cl]2 (0.015 mmol),
PiPr3 (0.06 mmol), and Et3N (1 mmol) in cyclohexane (3 mL) was added 1a
(1.0 mmol). After being stirred for 30 min, 1-decyne (1.2 mmol) was added
and the mixture was stirred at room temperature for 1 h. A solution of pinacol
(1.5 mmol) in cyclohexane (1 mL) was added and the resulting mixture was
then stirred at rt for 12 h to convert the catechol ester to the pinacol ester.
The chromatography over silica gel with hexane/ether ) 40/1 afforded 2-[(Z)-
1-octenyl]-4,4,5,5-tetramethyl-1,3,2-dioxaborolane in 79% yield.
(11) (a) Asao, N.; Liu, J.-X.; Sudoh, T.; Yamamoto, Y. J. Org. Chem.
1996, 61, 4568. (b) Sudo, T.; Asao, N.; Gevorgyan, V.; Yamamoto, Y. J.
Org. Chem. 1999, 64, 2494.
(7) Other synthesis of (Z)-1-alkenylboranes. (a) Srebnik, M.; Bhat, N. G.;
Brown, H. C. Tetrahedron Lett. 1988, 29, 2635. (b) Deloux, L.; Srebnik, M.
J. Org. Chem. 1994, 59, 6871. (c) Takahashi, K.; Takagi, J.; Ishiyama, T.;
Miyaura, N. Chem. Lett. 2000, 126.
(8) The Z-geometry was assigned by the coupling constant (J ) 13.4 Hz).7b
(9) (a) Ojima, I.; Clos, N.; Donovan, R. J.; Ingallina, P. Organometallics
1990, 9, 3127. (b) Tanke, R. S.; Crabtree, R. H. J. Am. Chem. Soc. 1990,
112, 7984. (c) Jun, C.-H.; Crabtree, R. H. J. Organomet. Chem. 1993, 447,
177. (d) Maruyama, Y.; Yamamura, K.; Nakayama, I.; Yoshiuchi, K.; Ozawa,
F. J. Am. Chem. Soc. 1998, 120, 1421.
10.1021/ja0002823 CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/04/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 20, 2000 4991
Scheme 3. Catalytic Cycle (M ) RhCl, IrCl)
Table 2. Hydroboration of Terminal Alkynes with 1a or 1ba
isomeric ratioc
entry
alkyne
borane yield/%b
2
3
4
1
2
3
4
5
6
7
8
9d
CH3(CH2)7CtCH
1a
1b
1a
1a
1a
1b
1a
1b
1b
1a
1b
1a
1b
79
81
72
71
70
59
62
69
71
70
59
60
67
99
91
98
1
7
2
0
2
0
0
0
1
0
0
0
0
0
0
1
TBDMSO(CH2)3CtCH
TBDMSOCH2CtCH
CH3(TBDMSO)CHCtCH
>99 <1
98
89 10
89 11
95
97
98
97
99
97
2
t-BuCtCH
5
3
2
3
1
2
10e Me3SiCtCH
11e
metals.12,15 This process will be followed by oxidative addition
of borane and the 1,2-migration of the boryl group to the
R-carbon.16 The key step leading to the cis-product is derived
from the stereospecific formation of a thermodynamically stable
(E)-9 which is observed in the related migratory insertion of an
alkynyl14a or a phenyl17 group into the vinylidene complexes.
Preliminary results suggested that the presence of Et3N suppress
the cis-hydroboration starting from 10 because the treatment of
the key intermediate [RhH(Cl)(Bcat)(PiPr3)2] (10) with Et3N led
to the complete reductive elimination of 1a/Et3N complex.18
12 PhCtCH
13
a To a solution of [Rh(cod)Cl]2 (0.015 mmol), P(i-Pr)3 (0.06 mmol),
Et3N (1 mmol for 1a and 5 mmol for 1b), and 1a or 1b (1.0 mmol) in
cyclohexane was added an alkyne (1.2 mmol for 1a and 2.0 mmol for
1b) at rt. The mixture was then stirred for 1-2 h, unless otherwise
noted. b Isolate yields based on a borane after chromatography over
1
silica gel. c Determined by H NMR of crude product. d [Ir(cod)Cl]2
(0.015 mmol), P(i-Pr)3 (0.06 mmol), Et3N (5 mmol), 1b (1.0 mmol),
and t-BuCtCH (1.2 mmol) were used. e At rt for 4 h.
Scheme 2. Deuterium-Labeling Experiment
Acknowledgment. We are indebted to Professor F. Ozawa for many
helpful discussions. The research is supported by JSPS Research
Fellowship for Young Scientists.
Supporting Information Available: Experimental procedures and
characterization data for of all compounds (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
to a cis-isomer should be ruled out because the isomerization of
(Z)-2 selectively led to a thermally stable (E)-3 (entry 3 in Table
1). The mechanism directly producing the cis-product via the trans
to cis isomerization of a vinyl-metal intermediate9 and the
mechanism proceeding through the attack of metal hydride to the
alkyne coordinated to a Lewis acid11 do not result in the migration
of an acetylenic hydrogen. A possible mechanism which might
account for both the acetylenic hydrogen migration and the anti-
addition of the B-H bond is one proceeding through a vinylidene
complex 7 as shown in Scheme 3.12
JA0002823
(13) (a) Wolf, J.; Werner, H.; Serhadli, O.; Ziegler, M. L. Angew. Chem.,
Int. Ed. Engl. 1983, 22, 414. (b) Alonso, F. J. G.; Ho¨hn, A.; Wolf, J.; Otto,
H.; Werner, H. Angew. Chem., Int. Ed. Engl. 1985, 24, 406.
(14) (a) Wakatsuki, Y.; Yamazaki, H.; Kumegawa, N.; Satoh, T.; Satoh, J.
Y. J. Am. Chem. Soc. 1991, 113, 9604. (b) Gru¨nwald, C.; Gevert, O.; Wolf,
J.; Gonza´lez-Herrero, P.; Werner, H. Organometallics 1996, 15, 1960. (c)
Katayama, H.; Ozawa, F. Organometallics 1998, 17, 5190.
(15) Ohmura, T.; Yorozuya, S.-i.; Yamamoto, Y.; Miyaura, N. Organo-
metallics 2000, 19, 365.
(16) A sequence of oxidative addition and migratory insertion of R3M (R
) Me, Et; M ) Al, Ga) to the iridium(I) vinylidene complex has been
reported: Fryzuk, M. D.; Huang, L.; McManus, N. T.; Paglia, P.; Rettig, S.
J.; White, G. S. Organometallics 1992, 11, 2979.
(17) Huang, D.; Streib, W. E.; Bollinger, J. C.; Caulton, K. G.; Winter, R.
F.; Scheiring, T. J. Am. Chem. Soc. 1999, 121, 8087.
The high electron-donating PiPr3 favoring oxidative addition
of the terminal C-H bond and stabilizing the vinylidene complex
(6f7) has been amply demonstrated in the Rh,13 Ru,14 and Ir13b
complexes and in the catalytic reactions induced by those
(12) For reviews of vinylidene complexes, see: (a) Bruce, M. I. Chem.
ReV. 1991, 91, 197. (b) Bruneau, C.; Dixneuf, P. H. Acc. Chem. Res. 1999,
32, 311.
(18) A private communication from F. Ozawa. The synthesis of 10:
Westcott, S. A.; Taylor, N. J., Marder, T. B.; Baker, R. T.; Jones, N. J.;
Calabrese, J. C. J. Chem. Soc., Chem. Commun. 1991, 304.