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A stereoselective synthesis of cis-alkenylboranes
c4 Jownat of Oganometaltic Chemistry. 92 (1975) CI-C6 6 Elsevier Sequoia S.A.. Lausanne - Printed in The Netherlands
Prehminary
communication
A STEREOSELECTlVE
EI-ICHI
NEGISHlf.
Department (Received
SYNTHESiS
ROBERT
of Chemistry.
OF cjs-ALKENYLBORAKES
hl. WILLIAMS,
Syracuse
GEORGE
Unicersrty,
Syracuse,
LEW and TAKAO New
York
13210
YOSHIDA (U.SA.)
March 10th. 1975)
The reaction of 1-halo-1-alkynes
ment with either hydride produces manner.
with dialkylboranes followed by treat-
lithium triethylborohydride or potassium tri-s-butylborocis-alkenylboranes in high yields in a highly stereoselective
The cis nature of hydroboration [I] permits the ready formation of alkenylboranes UI which the boron atom and the vlcinal alkyi group are tmns to each other. Using such stereochemlcally defined alkenylboranes. a number of unique olefin syntheses have been developed which provide stereoselective routes to monoolefins [2], cY,P-unsaturatedcarbonyl derivatives [ 31, halides [2;r,4 1, alkenylmercury derivatives [ 51, conjugated dienes and enynes [ 61 and cumulenes [7]. However, the lack of convenient procedures for the synthesis of crs-alkenylboranes has considerably restricted their synthetic applicability. We have found that the reaction of 1-halo-1-alkenylboranes (I) with either lithium hiethylborohydride or potassium tri-s-butylborohydride* produces the desired cis-alkenylbonnes (Ii) in high yields (eqn. 1).
vinyl
RCZCX
HBR
2
R, R’, R” = alkyl,
R
X
MBR’;H
R
BR’,
\
C=C’ / \
H X = halogen, M = LI or K
(II)
+ BR”3
(1)
H
The experimental procedure is quite simple as esemplified by the following preparation of cis-1-hexenylbis(3-methyl-2-butyl)borane (III). To a dry, nitrogen-flushed 300.ml flask equipped with a septum inlet, a magnetic stirring bar and a mercury-sealed outlet were introduced sequentially borane gAtiable from .Xldncb Chemical Co.. Milwaukee. see ref. 8.
WLSCO~SUI. For a review on lbese compounds.
c5
in THF (2.56 AI, 19.6 ml, 50 mmol), 2-methyl-2-butene (7.0 g, 100 mmol, O”, 1 h), freshly distilled 1-iodo-1-hexyne (10.4 g, 50 mmol, O”, 1 h) and lithium triethylborohydride m THF (1.09 Ai, 46 ml, 50 mmol, -25” then warmed to 25” over 1 h). Triethylboranev and THF were evaporated in vacua, and the residue was estracted with petroleum ether. Evaporation of the petroleum ether produced III: NMR (CCL, TMS) 6 0.7-1.15 (m, ca. BlH), 1.15-2.4 (m, ca. 10H) and 5.85-6.05 (m, ca. 2H) ppm. The yield by NMR using benzene as an internal standard was 91%. The essential absence (< 2-3s) of the trans isomer was established by the comparison of the NMR data with those of the tram isomer obtained by the reaction of 1-hexyne with bis(3-methyl-2-butyl)borane which eshibited an entirely different pattern in the olefinic region [NMR (CCL, TMS) 6 0.7-1.15 (m, 21H), 1.15-2.5 (m, lOH), 6.25 (d, lH, J = 18 Hz), 6.5-7.15 (dt, lH, J = 18 and 6Hz) ppm]. The stereochemical results ?Ie consistent with the intramolecular migration of hydrogen from boron to carbon with inversion of configuration at the sp’ hybridized Q carbon center, analogous to the previously observed alkyl mlgratlon [2]. After osidation of a sample of III with sodium hydroxide and 30% hydrogen peroside, GLC examination”” (95% yield) and only a very
indicated
the formation
of 3-methyl-2-butanol
(2% yield) of 2,3_dimethyl-4nonanone which can be formed via the competitive alkyl transfer. This indicates that the migratory aptitude of the hydridic hydrogen relative to that of the 3-methyl-2-butyi group, after correction for the statistical factor, is ca. 100. Quite analogously, IV-VI were obtained in SO-95% yields. minor amount
E[CiiKHjKH(CHj)q;
t-C,P, ‘\
c=c
2
/
Although potassium tri-s-butylborohydride reacts similarly, the ease of removal of triethylborane makes lithium triethylborohydride the reagent of choice. Saline hydrides such as lithium hydride and potassium hydride are not ‘Tnetb~Iborcne IS ~~ro~honc. u-eared wlrh due cauuon.
‘*Henna4
The U-apped solulron cooLarnm~ lnelhylborane
wb~cb was formed as one of he mam products.
was not analyzed
musx tierefore
quantitatwely
be
by GLC.
C6
satisfactory, producing cis-alkenylboranes m much lower yields at much slower rates over the temperature range of -25 to 65’. The remova: of the h-ialkylborane by-product and the inorganic salt may be omitted in many cases in which the crs-alkenylboranes are used as synthetic intermediates. For example, a lo-mmol ahquot of the solution containing 111 obtained as described above was treated with 6 N sodium hydroxide (6.8 ml, 40 mmol), and iodine dissolved in THF was added until the brown color of iodine began persisting for several minutes (ca. 27 mmol). GLC examination indicated the formation of h-ans-2,3dimethyl-Qnonene* in 82% yield (stereoisomeric purity 2 98%). Unlike the previously developed fmns-olefin synthesis via 1-halo-1-alkenylboranes [ 21, the procedure reported here does not involve protonolysis of organoboranes with carbosyiic acids at high temperatures and should be capable of accommodating various acidsensitive functional groups. A unique synthetic application of cis-alkenylboranes is their conversion to conjugated cis-enynes. In a manner analogous to the synthesis of conjugated Crans-enynes [ 6b]. V was converted in a highly stereospecific manner to cis-5,7-dodecenyne* in 51% yield (eqn. 2). 1. LiC=CC,
V
n-C4 Hg
H, .n
in hexane
*\
“,. I, 3. NaOH
,C=CCJ
Hg -n
(2)
>=C’
\
H >
Use of III or IV in place of V resulted lower yields.
H 98%
CIS
in the formation
of the cis-enyne
in
Acknowledgment We thank support.
Research
Corporation
and Syracuse
University
for financial
References 1 H-C. Brorrn. Hydrobonllon. W.A. Benlamm. Inc. Sew Yorh. 1962. 2 (P) G. Zwelfcl. InLmScience Chem. Rep;.. 7 (1973) 181 and ref. cited (b) C. Kdbnch and M.R. hlerkle. Angew. Cbem. InternaL. Ed. En&. 6 (1967) 74: (c) E J. Corey and T. Rarmdra,,athan. J. Amer. Chem. SIX.. 9-l (1972) 4013:(d) E. Negrsht. J.J. Katz sod H.C. Brown. Synthesrs. (1975)
therem:
555:
(e)
C.
Zwetfel
C C. WtuLney.
3 4 5
and
J. Amer.
R.P.
Fuher.
Chcm.
Sot..
IbId..
(1972)
9-l (1952)
557: 6560:
(f) C. Zwetfel.
(6)
H.C.
R.P.
Broun
and
Fuher. N.
J.T.
Rab-indmn.
Sno,v
and
J. Grgano-
metal. Chsm.. 61 (1973) C5: (b) G. Zwetfel and A. Horug. Syn~besas. (1973) 672; (i) G. Zweirel. A. Homg and J.E. Plamondon. J. Amer. Chem. Sot.. 96 (1974) 316. E Ne&b,. G. Lexr and T. Yosblda. J. Org. Chem .39 (1974) 2311. H.C. Brow-n. T. Hamaoha and N. Ravindnn. J. Amer. Chem. Sot.. 95 (19i3) 5786. 6156. (a1R.C. Larcckand H.C. Brown. J. OrwnomeLaL Cbem.. 36 (1972) 1: (b) R.C Larock. S.K. Gupra and H.C. Brown. J. Amer. Cbem. Sot.. 9-l (1971) 4371.
6
(a) E. Negnh~ and ibfd.. (1973) 874.
7 8
T. Yosbida. R.M. S. Knshnamurths. ‘Idenrifted
by
T. Yoshtda. Chrm. Commun.. (1973) 606; (b) E. Negrshi. G. Lew and (c) H.C. 6rovrn end N. Rarxndran. J. Org. Chem.. 38 (1973) 1617. Wdbamsaod Aldnctica
comparing
specti
E. Negisbt. J. Amer. Chem klh 7 (1974) 55. and
ChromaLoEraPblC
SIX..
beba\Tor
96
(1974)
wtLh
Lhose
T.
Yosbtdr.
3688.
of an aulheoctc
sample.
Prehminary
communication
A STEREOSELECTlVE
EI-ICHI
NEGISHlf.
Department (Received
SYNTHESiS
ROBERT
of Chemistry.
OF cjs-ALKENYLBORAKES
hl. WILLIAMS,
Syracuse
GEORGE
Unicersrty,
Syracuse,
LEW and TAKAO New
York
13210
YOSHIDA (U.SA.)
March 10th. 1975)
The reaction of 1-halo-1-alkynes
ment with either hydride produces manner.
with dialkylboranes followed by treat-
lithium triethylborohydride or potassium tri-s-butylborocis-alkenylboranes in high yields in a highly stereoselective
The cis nature of hydroboration [I] permits the ready formation of alkenylboranes UI which the boron atom and the vlcinal alkyi group are tmns to each other. Using such stereochemlcally defined alkenylboranes. a number of unique olefin syntheses have been developed which provide stereoselective routes to monoolefins [2], cY,P-unsaturatedcarbonyl derivatives [ 31, halides [2;r,4 1, alkenylmercury derivatives [ 51, conjugated dienes and enynes [ 61 and cumulenes [7]. However, the lack of convenient procedures for the synthesis of crs-alkenylboranes has considerably restricted their synthetic applicability. We have found that the reaction of 1-halo-1-alkenylboranes (I) with either lithium hiethylborohydride or potassium tri-s-butylborohydride* produces the desired cis-alkenylbonnes (Ii) in high yields (eqn. 1).
vinyl
RCZCX
HBR
2
R, R’, R” = alkyl,
R
X
MBR’;H
R
BR’,
\
C=C’ / \
H X = halogen, M = LI or K
(II)
+ BR”3
(1)
H
The experimental procedure is quite simple as esemplified by the following preparation of cis-1-hexenylbis(3-methyl-2-butyl)borane (III). To a dry, nitrogen-flushed 300.ml flask equipped with a septum inlet, a magnetic stirring bar and a mercury-sealed outlet were introduced sequentially borane gAtiable from .Xldncb Chemical Co.. Milwaukee. see ref. 8.
WLSCO~SUI. For a review on lbese compounds.
c5
in THF (2.56 AI, 19.6 ml, 50 mmol), 2-methyl-2-butene (7.0 g, 100 mmol, O”, 1 h), freshly distilled 1-iodo-1-hexyne (10.4 g, 50 mmol, O”, 1 h) and lithium triethylborohydride m THF (1.09 Ai, 46 ml, 50 mmol, -25” then warmed to 25” over 1 h). Triethylboranev and THF were evaporated in vacua, and the residue was estracted with petroleum ether. Evaporation of the petroleum ether produced III: NMR (CCL, TMS) 6 0.7-1.15 (m, ca. BlH), 1.15-2.4 (m, ca. 10H) and 5.85-6.05 (m, ca. 2H) ppm. The yield by NMR using benzene as an internal standard was 91%. The essential absence (< 2-3s) of the trans isomer was established by the comparison of the NMR data with those of the tram isomer obtained by the reaction of 1-hexyne with bis(3-methyl-2-butyl)borane which eshibited an entirely different pattern in the olefinic region [NMR (CCL, TMS) 6 0.7-1.15 (m, 21H), 1.15-2.5 (m, lOH), 6.25 (d, lH, J = 18 Hz), 6.5-7.15 (dt, lH, J = 18 and 6Hz) ppm]. The stereochemical results ?Ie consistent with the intramolecular migration of hydrogen from boron to carbon with inversion of configuration at the sp’ hybridized Q carbon center, analogous to the previously observed alkyl mlgratlon [2]. After osidation of a sample of III with sodium hydroxide and 30% hydrogen peroside, GLC examination”” (95% yield) and only a very
indicated
the formation
of 3-methyl-2-butanol
(2% yield) of 2,3_dimethyl-4nonanone which can be formed via the competitive alkyl transfer. This indicates that the migratory aptitude of the hydridic hydrogen relative to that of the 3-methyl-2-butyi group, after correction for the statistical factor, is ca. 100. Quite analogously, IV-VI were obtained in SO-95% yields. minor amount
E[CiiKHjKH(CHj)q;
t-C,P, ‘\
c=c
2
/
Although potassium tri-s-butylborohydride reacts similarly, the ease of removal of triethylborane makes lithium triethylborohydride the reagent of choice. Saline hydrides such as lithium hydride and potassium hydride are not ‘Tnetb~Iborcne IS ~~ro~honc. u-eared wlrh due cauuon.
‘*Henna4
The U-apped solulron cooLarnm~ lnelhylborane
wb~cb was formed as one of he mam products.
was not analyzed
musx tierefore
quantitatwely
be
by GLC.
C6
satisfactory, producing cis-alkenylboranes m much lower yields at much slower rates over the temperature range of -25 to 65’. The remova: of the h-ialkylborane by-product and the inorganic salt may be omitted in many cases in which the crs-alkenylboranes are used as synthetic intermediates. For example, a lo-mmol ahquot of the solution containing 111 obtained as described above was treated with 6 N sodium hydroxide (6.8 ml, 40 mmol), and iodine dissolved in THF was added until the brown color of iodine began persisting for several minutes (ca. 27 mmol). GLC examination indicated the formation of h-ans-2,3dimethyl-Qnonene* in 82% yield (stereoisomeric purity 2 98%). Unlike the previously developed fmns-olefin synthesis via 1-halo-1-alkenylboranes [ 21, the procedure reported here does not involve protonolysis of organoboranes with carbosyiic acids at high temperatures and should be capable of accommodating various acidsensitive functional groups. A unique synthetic application of cis-alkenylboranes is their conversion to conjugated cis-enynes. In a manner analogous to the synthesis of conjugated Crans-enynes [ 6b]. V was converted in a highly stereospecific manner to cis-5,7-dodecenyne* in 51% yield (eqn. 2). 1. LiC=CC,
V
n-C4 Hg
H, .n
in hexane
*\
“,. I, 3. NaOH
,C=CCJ
Hg -n
(2)
>=C’
\
H >
Use of III or IV in place of V resulted lower yields.
H 98%
CIS
in the formation
of the cis-enyne
in
Acknowledgment We thank support.
Research
Corporation
and Syracuse
University
for financial
References 1 H-C. Brorrn. Hydrobonllon. W.A. Benlamm. Inc. Sew Yorh. 1962. 2 (P) G. Zwelfcl. InLmScience Chem. Rep;.. 7 (1973) 181 and ref. cited (b) C. Kdbnch and M.R. hlerkle. Angew. Cbem. InternaL. Ed. En&. 6 (1967) 74: (c) E J. Corey and T. Rarmdra,,athan. J. Amer. Chem. SIX.. 9-l (1972) 4013:(d) E. Negrsht. J.J. Katz sod H.C. Brown. Synthesrs. (1975)
therem:
555:
(e)
C.
Zwetfel
C C. WtuLney.
3 4 5
and
J. Amer.
R.P.
Fuher.
Chcm.
Sot..
IbId..
(1972)
9-l (1952)
557: 6560:
(f) C. Zwetfel.
(6)
H.C.
R.P.
Broun
and
Fuher. N.
J.T.
Rab-indmn.
Sno,v
and
J. Grgano-
metal. Chsm.. 61 (1973) C5: (b) G. Zwetfel and A. Horug. Syn~besas. (1973) 672; (i) G. Zweirel. A. Homg and J.E. Plamondon. J. Amer. Chem. Sot.. 96 (1974) 316. E Ne&b,. G. Lexr and T. Yosblda. J. Org. Chem .39 (1974) 2311. H.C. Brow-n. T. Hamaoha and N. Ravindnn. J. Amer. Chem. Sot.. 95 (19i3) 5786. 6156. (a1R.C. Larcckand H.C. Brown. J. OrwnomeLaL Cbem.. 36 (1972) 1: (b) R.C Larock. S.K. Gupra and H.C. Brown. J. Amer. Cbem. Sot.. 9-l (1971) 4371.
6
(a) E. Negnh~ and ibfd.. (1973) 874.
7 8
T. Yosbida. R.M. S. Knshnamurths. ‘Idenrifted
by
T. Yoshtda. Chrm. Commun.. (1973) 606; (b) E. Negrshi. G. Lew and (c) H.C. 6rovrn end N. Rarxndran. J. Org. Chem.. 38 (1973) 1617. Wdbamsaod Aldnctica
comparing
specti
E. Negisbt. J. Amer. Chem klh 7 (1974) 55. and
ChromaLoEraPblC
SIX..
beba\Tor
96
(1974)
wtLh
Lhose
T.
Yosbtdr.
3688.
of an aulheoctc
sample.