Selective reduction of alkynes by the reagent Sml2 - Transition metal catalysts - proton donors

Article abstract of DOI:10.1016/0040-4039(91)80083-I

Alkynes were selectively reduced to cis-olefins under mild conditions by SmI₂ in admixture with first-row transition metal catalysts and appropriate proton donors, in which the corresponding transition metal hydrides are presumed as the reactive species. The conditions also effected the conversion of 1,6-diynes to the corresponding five-membered carbocycles.

Full text of DOI:10.1016/0040-4039(91)80083-I

Tetrahedron Letters, Vol.32, No 40, pp 555%5562, 1991
Printed m Great Britain
0040-4039/91 $3 00 +
Pergamon Press plc
SELECTIVE REDUCTION OF A L K Y N E S BY THE REAGENT
S m l 2 - TRANSITION METAL CATALYSTS PROTON DONORS
-
Junji Inanaga,* Yasuo Yokoyama, YoshJyasu Baba,t and Masaru Yamaguchi t
Institute for Molecular Science, Myodatji, Okazaki 444, Japan
tDepartment Chemistry, Kyushu Universtty 33, Hakozaki, Fukuoka 812, Japan
Summary: Alkynes were selectively reduced to cis.o/efins under mild conditions by Sml2 in
admixture with first-row transition metal catalysts and appropriate proton donors, in which the
corresponding transition metal hydrides are presumed as the reactive species. The conditions also
effected the conversion of 1,6.diynes to the corresponding five.membered carbocycies.
A number of methods have been reported for selective reduction of alkynes to the corresponding
alkenes. Among them, catalytic hydrogenation1 and transttion metal hydnde reduction2 have been
widely used to obtain cis-olefins.3,4 In both cases, however, the concomitant formation of over-
reduced products and/or undesirable trans-olefins are often inevitable.
Since most transttion metal salts can easily be reduced to the corresponding Iower-valent
species with a Sml2 solution in THF under mild conditions, the combined use of a catalytic amount
of transttion metal complexes and Sml2 seems to have great potential for such reactions that Iow-
valent transition metals are requLred as catalysts.5 As such a react=on, we examined the system,
Sml2-transition metal catalysts-proton donors, for the selective reduction of alkynes (Eq. 1).6
transition metal catalyst ~.-
Sml2, protondonor, rt
R I ' ~ R2
R 1~ ~ - - R
2
R 1 / ~ X R2
+
(1)
In the first instance diphenylacetylene was selected as the substrate for evaluation of different
catalysts. The results are summanzed in Table 1.
Among six transition metals (Fe, Co, Ni, Ru, Rh, and Pd) examined, only first-row elements have
shown catalytic activity and the cobalt complex, CoCI2-4PPh3, was found to be the most effective
catalyst m terms of stereoselectivity (entry 5). It is interesting from the mechanistic point of vtew that
the add=tion of HMPA not only accelerated the reaction but also gave rise to a reversal of the
senese of stereoselection thus affording thermodynamically more stable trans-stilbene as the major
product (entnes 6 and 9).
The reductton did not take place in the absence of proton donors and as shown in Table 2 a
remarkable effect of proton sources on the cobalt(ll)-catalyzed reduction of diphenylacetylene was
realized: Both the reaction rate and product yields were increased in the order of 2-propanol <
ethanol < methanol at the sacrifice of stereoselectMty and when acetic ac=d was employed raptd
and quantitattve formation of cis-stilbene was accomphshed
In Table 3 are shown the results of the reduction of other substrates (1-4).
5559

5560
Table 1. Reduction of Diphenylacetylene a)
catalyst, Sml2, ROH
P h / ~ P h
A
+
P h ~ P h
B
+
P h / ~ / P h
C
P h - - ~ - - P h
.
.
.
.
additive, THF, rt
Run
1
Catalyst
AddJtwe
ROH
Time(h) Yield(O/o) b)
Ratio(AB:C)c)
none
.
none
i-PrOH
24
NR.
45 • 10 : 45
87. 13
2
3
HM PA
.
16
6
97
46
FeCI3-4PPh3
none
MeOH
78 " 21
1
4
5
6
7
8
9
CoCI2"(H20)x
CoCI2.4PPh3
,,
.
none
I-PrOH
.
2
2
84
59
87
80
68
89
>99 :
1
14 80
74 " 19
.
HMPA
none
"
0 5
6
7
NiCI2°(H20)x
NJCI2*4PPh3
"
none
/-PrOH
"
2
2
91
9
HMPA
0.5
1 "90
9
a) General procedure' A Sml2-THF solution (0,1 mol dm3, 5 ml, 0 5 mmol) was added to a
solution of dlphenylacetylene (35 rag, 0.2 mmol), a THF solution of the catalysts (0 025 tool
dm3, 240/Jl, 3 tool%), and ROH (0.5 mmol) Jn THF (0.5 ml) in the presence or absence of
HMPA (0 4 ml) and the mwxture was stirred at room temperature under argon The crude
products were punfied by chromatography on s~hca gel b) Isolated y~eld of the m~xture
(A+B+C) c) Determined by 1H NMR (400 MHz) analysis
Table 2. Effect of Proton Sources on the Reduction of Diphenylacetylene a)
Run
ROH
Time (h)
Yield (%) b)
A
.
B
1
2
3
4
5
i-PrOH
EtOH
2
59
78
gg
9g
56
>99 • 1
1
9 9 .
96
1
4
MeOH
0.5
05
8
.
AcOH
>9g : 1
96 : 4
t-BuCO2H
a) CoCI~.4PPh3 (3 mol%) was used as the catalyst b) Isolated y~eld

5561
Table 3. Sml2-Promoted Redu~on of Alkynes (1-4) a)
Ph - -
~
--Bu
Ph ~
~
--TMS
C5Hll-- ~ --CsHll
Bu--~--CH(OH)CsH13
(1)
(2)
(3)
(4)
Entry
Alkyne
Catalyst
ROH
Time
o/oYieldb)
Z : Ec)
1
2
1
2
-
CoCI2-4PPh3
MeOH
MeOH
i-PrOH
- - -
3 h
2h
97
>99 :
97 :
1
3
w,
96
3d)
4
-
2 h
92
6 " 94
. . .
3
-
CoCI2o(H20)x
CoCI2°4PPh3
.
3 h
22 e,f)
52e)
88 e)
95 g)
80 h)
5
MeOH
AcOH
MeOH
AcOH
5h
>99 :
1
1
1
1
6
-
5 m=n
15 min
5 min
>99 :
>99 :
>99 :
7
4
-
FeCI3°4PPh3
CoCI2°4PPh3
8
a) The reactions were carried out at room temperature by using3 mol% ofcatalyst, b) Isolated Yield
ofolefins, c)Determined bylH and/or 13C NMR analysis, d) HMPAwasadded, e) GLC yield f)
Hexapentylbenzene (18 %) was also produced, g) 5,6-Tndecadiene (5 %) was also Isolated. h) A
m=xuter fo the corresponding allene and acetylenedenvat=ves was also produced (ca 20 %)
The present method seems superior in suppressing the over-reduction to the conventional semi-
hydrogenation utilizing various types of doping Lmdlar catalysts (e.g., entry 1) la It is noted that the
reduction of 2 afforded the corresponding cis-olefin in appreciablly high purity. In the presence of
HMPA a stereochemical reversal was again realized. When Co(ll) chloride polyhydrates was used
as a catalyst in the reduction of 3, a considerable amount of hexapentylbenzene was produced as
the result of reductive trimerizatlon7 (entry 4). The corresponding dimer was also detected on the
reaction of entry 5. However, such ohgomenzation could be dlmimshed by the use of acetic acid
as a proton donor (entry 6). The reduction of 4 proceeded faster than that of dmryl or dialkyl
acetylenes and the Fe(lll) complex turned out to be most effective in this case (entry 7).
While the mechanistic details remain to be explored, formation of the transition metal hydrides ~s
highly presumable because the reduction did not take place either m the absence of catalysts or
proton donors.8 c/s-Addition of the metal hydride to alkynes followed by further reduction of the
resulting adducts with Sml2 would produce the corresponding vinyl anions9 and Iow-valent
transition metals, from which the c/s-olefins and metal hydrides would be produced, respectively.
Reduction of alkynes w~th transition metal hydrides, prepared from the corresponding metal
halides or clusters by reducing with NaBH4,2t)'e'f LiAIH4,2a NaAIH4,2d or Grignard reagent,2c has
been reported. Few of them are, however, catalytic reactions and the chemo- and stereoselectiwty
is not always h~gh. These problems could be overcome in the present reduction, in which only a

5562
catalytic amount of metal hydndes and metal alkene intermed=ates might be present to mm=mJze the
overoreducbon and ~somerfzabon of the products Isomenzat=on of the wnyhc anion intermediates
seems fast in HMPA and th~s m=ght be the reason why E-~somer became major when d=phenyl-
acetylene or 2 was reduced ~n the presence of HMPA
R 1
When the method was apphed
to the reaction of 1,6-d~ynes,
R 1
cat CoCi2.4PPh 3
Sml2-THF, MeOH
~
,=,...-
mtramolecular carbon-carbon bond
formabon took place to gwve five-
membered carbocychc compounds
as a mIxture of stereolsomers lO 11
2
Y
RI=R2=Ph (62 %) ; Rl=Ph, R2=TMS (37 %)
References and Notes
1 For recent works, see a) J. Rajaram, A. P. S. Narula, H S P. Chawla, and S Dev, Tetrahedron,
39, 2315 (1983), b) M Sodeoka and M. ShibasakJ, J. Org. Chem, 50, 1147 (1985)
2 a) E C Ashby and J J Lm, J. Org. Chem., 43, 2567 (1978), b) T. Ito, T. Nagano, and M. Hirobe,
Tetrahedron Lett., 21, 1343 (1980), c) F Sato, H Ishikawa, and M Sato, Ibid., 22, 85 (1981); d)
M Sato and K OshJma, Chem. Lett., 157 (1982); e) N SuzukL Y. Kaneko, T Tsukanaka, T
Nomoto, Y Ayaguch~, and Y. Izawa, Tetrahedron, 41, 2387 (1985), f) N. Satyanarayana and M
Penasamy, Tetrahedron Lett., 27, 6253 (1986), g) J. F Daeuble, C McGettigan, and J M
Stryker, ibid., 31, 2397 (1990) and references cated thereto.
3 For the reduction with zinc, see a) B L Sondengam, G. Charles, and T M Akam, Tetrahedron
Lett, 21, 1069 (1980), b) W Chou, D L Clark, and J. B White, Ibid., 32,299 (1991)
4 For the reductIon wh=ch proceeds through the formabon of mataloalkyne complexes, see Y
Kataoka, K Taka=, and K Ut~moto, Tetrahedron Lett., 31,365 (1990) and references crated therein
5. Actually, the concept has been successfully applied to some palladium-catalyzed reactions. T
Tabuchl, J. Inanaga, and M YamagucN, Tetrahedron Lett, 27, 601, 1195, 5237 (1986); ibid., 28,
215 (1987), Chem Lett, 2275 (1987)
6. A part of this work was presented at the 57th Meebng of the Chem=cal Socrety of Japan, Senda=,
September (1988)
7. J P Collman and L S. Hegedus, Pnnctples and Applications of Organotransitlon Metal
Chemistry, ed by A Kelly, Un=versity Science Books, Mill Valley (1980), p. 523
8 The poss~bflity of alkyne-samanum or alkyne-transltJon metal complex as an mtermedrate may be
excluded by the fact that alkynes did not react with electrophlles such as alkyl hahdes,
aldehydes, ketones, or chlorotnmethylsdane under the present cond=t=ons even in the absence of
proton donor wh~le ~t has been reported that such alkyne-metal complexes were produced by
Iow-valent zJrcomum, mobfum, or tantalum reagent, and reacted w~th electrophwles, see, for
example, K TakaL Y Kataoka, and K Utimoto, J. Org. Chem, 55, 1707 (1990) and references
cfted therem
9. In order to clarify the ongm of hydrogen atom incorporated m the products, dsphenylacetylene
was reduced ~n the presence of CD3OD or CH3COOD as a 'proton' source. The products
obtained were subjected to catalytic hydrogenat=on to g~ve 1,2-d~phenylethane, 13C NMR
(100 MHz) analys=s of whuch revealed that totally ca. 80 % of D atom was incorporated Jn a
m~xture of mono- and dideuterated stilbene On the other hand, the reaction ~n THF-d8 without
proton donor or m the presence of Bu3SnD d~d not produce any trace of deuterated stJlbene,
md=catmg that hydrogen atom-transfer process Js not ~nvolved
10. For similar transformations, see a) T. V RajanBabu, W A Nugent, D F Taber, and P J. Fagan,
J. Am. Chem. Soc., 110, 7128 (1988), b) B M. Trost and D C Lee, ibid., 110, 7255 (1988); c) E
Negisht, S J. Holms, J M Tour, J A Miller, F E Cederbaum, D. R Swanson, and T Takahashl,
Ibid., 111, 3336 (1989).
11 This work was supported by a Grant-in-Aid for Scientific Research (NO 01540431) from the
Ministry of Educatfon, Science and Culture, Japan.
(Received m Japan 3 July 1991)