Syntheses with organoboranes. IX. Vinyl- and 1-alkenyldichloroboranes as ethylene and 1-alkene equivalents for the Diels-Alder reaction

Article abstract of DOI:10.1016/S0022-328X(98)01175-9

Vinyl- and 1-alkenyldichloroboranes were used as dienophiles for the Diels-Alder reaction with representative aliphatic and cyclic 1,3-dienes. The organoborane adducts were transformed into the corresponding olefins either by protonolysis or by oxidation-mesylation-reduction. Direct protonolysis of the adducts gave in most cases mixtures of olefins whereas the reduction of mesylates with lithium triethylborohydride produced pure olefins in good yields.

Full text of DOI:10.1016/S0022-328X(98)01175-9

Journal of Organometallic Chemistry 580 (1999) 354–362
Syntheses with organoboranes. IX. Vinyl- and
1-alkenyldichloroboranes as ethylene and 1-alkene equivalents for the
Diels–Alder reaction
*
Marek Zaidlewicz *, Jacek R. Binkul, Wojciech Soko´l
Faculty of Chemistry, Nicolaus Copernicus Uni6ersity, PL-7-100 Torun´, Poland
Received 3 August 1998
Abstract
Vinyl- and 1-alkenyldichloroboranes were used as dienophiles for the Diels–Alder reaction with representative aliphatic and
cyclic 1,3-dienes. The organoborane adducts were transformed into the corresponding olefins either by protonolysis or by
oxidation–mesylation–reduction. Direct protonolysis of the adducts gave in most cases mixtures of olefins whereas the reduction
of mesylates with lithium triethylborohydride produced pure olefins in good yields. © 1999 Elsevier Science S.A. All rights
reserved.
Keywords: Diels–Alder reaction; Vinylic organoborane dienophiles
Protonolysis of trialkylboranes can be achieved by
heating with carboxylic acids, but little is known on the
1. Introduction
reaction course of alkenylboranes with isolated double
The Diels–Alder reaction is one of the most impor-
bonds under these conditions [9]. Racemization due to
tant methods for the construction of six-membered
the double-bond shift has been observed in protonolysis
rings. Among a variety of useful dienophiles the sim-
of the monohydroboration product of (+)-limonene
plest one ethylene and 1-alkenes react sluggishly [1].
Consequently, their equivalents are highly desirable and
[10]. Other methods for the transformation of carbon–
boron bond into carbon–hydrogen bond are also
a few such compounds have been introduced [2].
known [11,12]. However, the scope of these reactions is
Vinylic organoboranes are good candidates for that
purpose, provided they would be sufficiently reactive
and the carbon-boron bond of the adduct could be
not well delineated. Consequently, we undertook a
study on the vinylic dialkyl- and dihalogenoboranes as
ethylene and 1-alkene equivalents for the Diels–Alder
transformed into the carbon–hydrogen bond without
reaction.
isomerization of the double bond. Recently, it has been
shown that in contrast to vinylic boronates, vinylic
dialkyl- and dihalogenoboranes are considerably more
reactive dienophiles [3–8]. They are synthetically attrac-
tive since the organoborane adducts can be functional-
ized by further transformations of the carbon–boron
bond.
2. Results and discussion
The following vinylboranes were used as dienophiles:
dibromovinylborane (1), dichlorovinylborane (2), di-n-
butylvinylborane (3) and B-vinyl-3,6-dimethylborepane
(4) (B-vinyl-DMB). They are readily prepared from
boron tribromide, boron trichloride, di-n-butylbromo-
* Corresponding author. Fax: +48-56-6542-477.
E-mail address: zaidlevi@chem.uni.torun.pl (M. Zaidlewicz)
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(98)01175-9

M. Zaidlewicz et al. / Journal of Organometallic Chemistry 580 (1999) 354–362
355
The reactivity of 1-4 was tested for the reaction with
2,3-dimethyl-1,3-butadiene (8) monitored by 11B-NMR
analysis. The results shown in Table 1 indicate high
reactivity of dibromovinylborane. The reaction was
exothermic and the temperature had to be carefully
controlled. Dichlorovinylborane was less reactive but
borane and B-chloro-3,6-dimethylborepane, respec-
tively, by the reaction with tetravinyltin or tri-n-
butylvinyltin [4,13–15]. In the preparation of
dibromovinylborane, the unreacted boron tribromide is
conveniently removed by complexation with dimethyl
sulfide.
Table 1
The reaction of vinylic organoboranes 1-7 with dienes 8-13 and oxidationa

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the addition proceeded at room temperature. Di-
alkylvinylboranes 3 and 4 were of similar reactivity,
considerably lower as compared to 1 and 2.
However, its oxidation produced 2,4-dimethylpent-3-
en-1-ol and no cyclic alcohol corresponding to the
Diels–Alder adduct was obtained. Most probably the
prolonged heating at high temperature caused dehy-
droboration of 4 and monohydroboration of 11. It is
relevant that similarly substituted 4-methyl-1,3-pentadi-
ene adds sluggishly and in low yields even to highly
reactive dienophiles, e.g. maleic anhydride and tetracya-
noethylene [17].
The reactivity order of vinylboranes 1–4 pointed to
1-alkenyldibromoboranes as the first choice ho-
mologous dienophiles. Complexes of these organobo-
ranes with dimethyl sulfide are readily available by the
monohydroboration of 1-alkynes with dibromoborane-
dimethyl sulfide [18]. However, in contrast to highly
exothermic reaction of dibromovinylborane with 2,3-
dimethyl-1,3-butadiene, no reaction was observed when
dibromo(E-1-hexen-1-yl)borane-dimethyl sulfide (14)
was mixed with the diene at room temperature. Heating
the mixture at 100°C for 12 h followed by oxidation
produced trans-6-butyl-3,4-dimethylcyclohex-3-enol in
low yield. Much lower reactivity of 14 as compared to
uncomplexed 1 can be explained by strong complexa-
tion with dimethyl sulfide, however, it has been re-
ported that such complexes react with 1,3-dienes when
refluxed in benzene solution to give the corresponding
adducts [8]. Carrying out the reaction of 14 with 8
under these conditions a dark reaction mixture was
obtained and oxidation of the adduct produced trans-6-
butyl-3,4-dimethylcyclohex-3-enol only in a moderate
yield.
To avoid these inconveniences, we turned to 1-
alkenyldichloroboranes which can be prepared by the
monohydroboration of 1-alkynes with dichloroborane
diethyl etherate [19]. Although the procedure works
well, the reagent has only limited stability and its
reactivity is too low for the direct reaction. Liberation
of free dichloroborane from the etherate adds an addi-
tional step to the synthesis. Recently, hydroboration of
alkenes with dichloroborane generated in situ by the
reduction of boron trichloride with trialkylsilanes was
reported [20]. The procedure is very simple and does
not require a separate preparation of the reagent. Con-
sequently, we used it for the synthesis of 1-
alkenyldichloroboranes 5-7 from 1-hexyne, 1-octyne
and phenylacetylene, respectively (Eq. (1)).
Other dienes used for the study were 2,4-hexadiene
(9), isoprene (10), 2,4-dimethyl-1,3-pentadiene (11), 1,3-
cyclopentadiene (12) and 1,3-cyclohexadiene (13). The
addition reactions were carried out with neat reagents.
To avoid thermal isomerization of organoborane ad-
ducts the most reactive vinylboranes 1 and 2 were
preferred and the temperature of the addition reactions
was kept as low as possible. However, the reactions
with dibromovinylborane were difficult to control,
oligomerization of dienes was observed and the yields
of adducts varied. The reactions with dichlorovinylbo-
rane (2) were free from these inconveniences. Thus, the
dienes 8–10, 12 and 13 reacted with 2 at temperatures
not exceeding 60°C to give the corresponding adducts.
Oxidation of the adducts with alkaline hydrogen perox-
ide under standard conditions produced unsaturated
alcohols shown in Table 1. Isoprene and 2 gave adducts
oxidized to a mixture of 4-methylcyclohex-3-enol and
3-methylcyclohex-3-enol (3: 2). A mixture of endo- and
exo-5-norbonen-2-ol (84:16) was obtained from 2 and
1,3-cyclopentadiene. The addition of 2 to 1,3-cyclo-
hexadiene was highly stereoselective leading after oxi-
dation to endo-bicyclo[2.2.2]oct-5-en-2-ol. The reaction
is a convenient one pot synthesis of this alcohol which
can be readily converted to the corresponding ketone.
Synthesis of both these compounds by other procedures
is tedious, requires several steps including chromato-
graphic separations and fractional crystallization [16].
The addition of di-n-butylvinylborane to 1,3-cyclohexa-
diene was less stereoselective producing a mixture of
endo- and exo-adducts (7: 3). It was interesting to
examine the addition of the most reactive vinylboranes
1 and 2 to 2,4-dimethyl-1,3-pentadiene (11) which could
afford an adduct with a quaternary carbon atom. The
reactions of 11 with 1 and 2 were exothermic and no
unreacted diene was left after 1 h as indicated by GC
analysis. However, the 11B-NMR spectra of both reac-
tion mixtures showed only signals corresponding to
unreacted 1 or 2. Apparently, the Lewis acidity of 1 and
2 was sufficient to induce oligomerization of 11 which is
sensitive to acids. It dimerized and trimerized when
formed by dehydration of 2,4-dimethylpent-3-en-2-ol
which could not be isolated from the reaction of mesitil
oxide with methylmagnesium iodide. Dehydration of
the alcohol prepared using methylmagnesium chloride
had to be carefully controlled. Less acidic vinylborane 3
did not induce oligomerization of 11 but no other
signals than the starting material was observed in the
11B-NMR spectrum after heating the mixture of 3 and
11 for 12 h at 100°C.
(1)
Heating 11 with B-vinyl-DMB (4) at 140°C for 24 h
gave a product showing a signal at l 86.0 in the
11B-NMR spectrum, characteristic for trialkylboranes.
The compounds 5–7 isolated by simple distillation
were colorless liquids stable over long periods when

M. Zaidlewicz et al. / Journal of Organometallic Chemistry 580 (1999) 354–362
357
Scheme 1.
stored at 0°C. No exothermic reaction was observed
upon mixing 5 with 2,3-dimethyl-1,3-butadiene at room
temperature. After heating the mixture at 100°C for 12
h a signal at l 53.00 in the 11B-NMR spectrum corre-
sponding to 5 disappeared and a signal of the adduct at
l 63.92 appeared. Oxidation of the adduct gave trans-6-
butyl-3,4-dimethyl-cyclohex-3-enol identified by com-
parison (1H-NMR, 13C-NMR) with a sample prepared
as shown in Scheme 1. No isomeric alcohols were
detected in the oxidation product indicating no thermal
isomerization of the adduct.
nately, the undesired side reaction leading to the elimi-
nation product could be avoided by using lithium
triethylborohydride instead of lithium aluminum hy-
dride. The reaction sequence oxidation–mesylation–re-
duction with lithium triethylborohydride worked well
with all the adducts shown in Table 2 affording the
corresponding olefins in ]80% yield and excellent
purity.
Other methods of transformation of the organobo-
rane adducts into olefins were also tried. Thus, heating
the adduct prepared from 3 and 8 with sodium hydrox-
ide in ethylene glycol at 140–200°C gave a mixture of
1,2- and 1,6-dimethylcyclohexene (Table 2). It is inter-
esting to note that though tri-n-hexylborane and dicy-
clohexylborane, similarly to tri-n-butylborane [12], were
cleanly transformed into n-hexane and cyclohexane,
respectively, primary trialkylboranes branched in b-po-
sition gave preferentially elimination products under
these conditions. Thus, (+)-limonene was monohy-
droborated with dicyclohexylborane and the product
was heated with sodium hydroxide in ethylene glycol at
180–200°C. The distillate was identified as (+)-
limonene showing the same rotation as the starting
material. Under the same conditions trimyrtanylborane
derived from b-pinene gave a mixture of trans-pinane
and b-pinene. A facile protonolysis of tributylborane
with anhydrous hydrogen fluoride has also been re-
ported [11]. However, heating the adduct prepared
from 3 and 8 with a mixture of boron trifluoride
etherate and water (1: 3) or with tetramethylammonium
fluoride tetrahydrate gave a mixture of 1,2-, 1,6-
dimethylcyclohexene and dehydrogenation products
(Table 2).
Initially, protonolysis of the organoborane adducts
was carried out with propionic acid. Adducts prepared
from dichlorovinylborane were treated with two equiv-
alents of sodium methoxide to avoid the formation of
hydrogen chloride and then refluxed with propionic
acid for 2 h. Under these conditions the adducts of 2
with 8, 12 and 13 gave 1,2-dimethylcyclohexene, nor-
bornene and bicyclo[2.2.2]oct-2-ene, respectively (Table
2). However, the yields were low to moderate and
1,2-dimethylcyclohexene partially isomerized in the
acidic reaction medium. At longer reaction times the
yield was slightly higher but 1,2-dimethylcyclohexene
isomerized more extensively and dehydrogenation prod-
ucts were also formed (Table 2). Heating the adduct
with propionic acid in ethylene glycol gave similar
results. Clearly, the yields of olefins obtained by direct
protonolysis of the adducts with propionic acid are not
high and the reaction scope is limited to cases when the
product olefin is relatively stable in acidic medium. To
circumvent these inconveniences the organoborane ad-
ducts were oxidized to the corresponding alcohols,
which in turn were transformed into mesylates or tosy-
lates reduced with lithium aluminum hydride. This
indirect route worked better giving olefins in 40–60%
overall yield. However, 1,2-dimethylcyclohexene ob-
tained in this way was constantly contaminated with
1,2-dimethyl-1,4-cyclohexadiene regardless of the reac-
tion temperature (room temperature, reflux) or the
solvent used (diethyl ether, tetrahydrofuran). Fortu-
3. Conclusion
Vinyl-and 1-alkenyldichloroboranes are highly reac-
tive toward 1,3-dienes and the corresponding adducts
can be prepared at temperatures below 100°C. Only
2,4-dimethyl-1,3-pentadiene disubstituted at the termi-

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M. Zaidlewicz et al. / Journal of Organometallic Chemistry 580 (1999) 354–362
Table 2
Transformation of organoborane adducts into olefins by protonolysis or oxidation–mesylation–reduction
a Isolated yield.
b Crude intermediate alcohol and mesylate were used.
c Products identified by GC–MS comparison with authentic samples.
d The molar ratio organoborane:Me4NF · 4H2O=1:4.
e
The molar ratio organoborane:BF3OEt2:H2O=1:2:6.
nal position was unreactive. The addition of
dichlorovinylborane to 1,3-cyclohexadiene was highly
endo-selective. The adducts were stable for prolonged
periods. They were cleanly transformed into the co-
rresponding olefins by the sequence oxidation–mes-
ylation–reduction with lithium triethylborohydride.
Direct protonolysis with propionic acid is limited to
adducts giving olefins stable in acidic medium. Alk-
ylcyclohexenes substituted at the homoallylic po-
sition, bicyclo[2.2.1]hept-2-enes and bicyclo[2.2.2]
oct-2-enes can be conveniently prepared by this
method.

M. Zaidlewicz et al. / Journal of Organometallic Chemistry 580 (1999) 354–362
359
4. Experimental
4.3.2. Dichloro(E-1-octen-1-yl)borane (6)
Prepared as described above from 1-octyne using
triethylsilane instead of trimethylsilane, 70% yield, b.p.
52–54°C 1 mm−1 Hg; 11B-NMR (neat), l, 52.93;
1H-NMR (CDCl3), l 0.88 (t, J=6 Hz, 3H, CH3), 1.33
(m, 6H, CH2), 1.48 (quintet, J=6 Hz, 2H, CH2), 2.29
(qd, J=7 Hz, J=2 Hz, 2H, CH2), 6.07 (dt, J=7 Hz,
J=2 Hz, 1H, CH), 7.20 (dt, J=17 Hz, J=6 Hz, 1H,
CH).
All glassware used for work with organoboranes was
kept in an oven at 150°C overnight, assembled hot and
cooled in a steam of dry nitrogen. 1H, 13C and 11B-NMR
spectra were recorded on a Varian Gemini 200
spectrometer. GC–MS analyses were obtained with
Finnigan MAT ITD. 800 instrument (EI, 70 eV). GC
analyses were performed on a Hewlett Packard HP-5890
gas chromatograph equipped with a 30 m×0.32 mm
Supelcowax 10 capillary column. IR spectra were
recorded on a Specord 75, Carl Zeiss, Jena, instrument.
4.3.3. Dichloro(E-2-phenylethen-1-yl)borane (7)
Prepared as described above from phenylacetylene,
using triethylsilane instead of trimethylsilane, 57% yield,
b.p. 70–72°C 1 mm−1 Hg; 11B-NMR (neat), l 52.79;
1H-NMR (CDCl3), l 6.74 (d, J=18 Hz, 1H, CH), 7.45
(m, 3H, CH), 7.63 (m, 2H, CH), 7.83 (d, J=18 Hz, 1H,
CH).
4.1. Materials
Tetravinyltin
[21],
dichlorovinylborane
[14],
di-n-butylvinylborane [15] and B-vinyl-DMB [4] were
prepared according to the literature. Tetrahydrofuran
was distilled from benzophenone ketyl prior to use.
Isoprene, 2,3-dimethyl-1,3-butadiene, 1,3-cyclopenta-
diene, 1,3-cyclohexadiene, 2,4-hexadiene (mixture of
isomers) were commercial products. 2,4-Dimethyl-
1,3-pentadiene was prepared by dehydration of
2,4-dimethylpent-3-en-2-ol with potassium hydrogen
sulfate. The alcohol was prepared by the reaction of
mesitil oxide with methylmagnesium chloride.
4.4. trans-6-Butyl-3,4-dimethylcyclohex-3-enol
A mixture of dichloro(E-1-hexen-1-yl)borane (2.16 g,
13 mmol) and 2,3-dimethyl-1,3-butadiene (2.18 g, 26.5
mmol) was sealed in an ampule and kept at 100°C for
12 h. 11B-NMR (neat), l 63.92. The crude reaction
product was dissolved in tetrahydrofuran (13 ml), the
solution was cooled to 0°C and 3 M sodium hydroxide
(15 ml, 45 mmol) followed with 30% hydrogen peroxide
(1.5 ml, 15 mmol) was added keeping the temperature
below 20°C. The mixture was stirred for 1 h at room
temperature and then for 1 h at 50°C. The organic layer
was separated and the aqueous layer was extracted with
diethyl ether (3×15 ml). Extracts were combined with
the organic layer, washed with saturated brine (10 ml)
and dried over magnesium sulfate. The product was
isolated by distillation, 1.46 g, 61%, b.p. 90–93°C 1
mm−1 Hg, identified by comparison (1H and 13C-NMR)
with an authentic sample prepared by the monohydrob-
oration-oxidation of 4-butyl-1,2-dimethyl-1,4-cyclohexa-
diene.
4.2. Dibromo6inylborane (1)
Tetravinyltin (14.80 g, 65 mmol) was added to a
mixture of tribromoborane (65.50 g, 0.26 mol) and
mercury (5.00 g) at 0°C. The mixture was stirred for 1
h at room temperature, and the liquid was decanted from
the solid. Distillation gave 40.00 g, b.p. 28–30°C 50
mm−1 Hg, 11B-NMR (neat), l, 53.85 and 37.70 (3:1), the
signals corresponding to dibromovinylborane and
tribromoborane, respectively. Dimethyl sulfide (2.54 g,
41 mmol) was added to the distillate at 0°C, the mixture
was stirred for 1 h at room temperature and the product
was isolated by distillation, 25.50 g, 50% yield, b.p. 28°C
60 mm−1 Hg; 11B-NMR (pentane), l, 53.85.Lit. [13], b.p.
93–94°C.
1H-NMR (CDCl3), l 0.88 (t, J=7 Hz, 3H, CH3),
1.20–1.40 (m, 6H, CH2), 1.57 (s, 6H, CH3), 1.47–1.77 (m,
2H, CH2), 1.83 (s, 1H, OH), 1.89–2.05 (m, 1H, CH),
2.05–2.60 (m, 2H, CH2), 3.54 (m, 1H, CH); 13C-NMR
(CDCl3), l 13.91 (CH3), 18.43 (CH3), 18.63 (CH3), 22.88
(CH2), 28.86 (CH2), 31.36 (CH2), 36.06 (CH2), 40.08
(CH2), 40.57 (CH), 71.46 (CH), 122.88 (C), 124.68 (C).
Lit. [8], b.p. 65–70°C 0.05 mm−1 Hg.
4.3. Synthesis of 1-alkenyldichloroboranes
4.3.1. Dichloro(E-1-hexen-1-yl)borane (5)
A mixture of 1-hexyne (6.00 g, 73 mmol) and
trimethylsilane (5.30 g, 71 mmol) was slowly added with
stirring to trichloroborane (7.68 g, 65 mmol) at −78 to
−70°C. The mixture was allowed to warm to room
temperature and the product was isolated by distillation,
9.02 g, 84%, b.p. 60–62°C 20 mm−1 Hg; 11B-NMR
4.5. trans-6-Hexyl-3,4-dimethylcyclohex-3-enol
Prepared as described above from dichloro(E-1-octen-
1-yl)borane and 2,3-dimethyl-1,3-butadiene, 80% yield,
1
1
(neat), l, 53.12; H-NMR (CDCl3), l, 0.92 (t, J=7 Hz,
bp 110–112°C 1 mm−1 Hg; H-NMR (CDCl3), l 0.85
3H, CH3), 1.43 (m, 4H, CH2), 2.30 (qd, J=7 Hz, J=1.4
Hz, 2H, CH2), 6.07 (dt, J=17 Hz, J=1.4 Hz, 1H, CH),
7.20 (dt, J=17 Hz, J=6.6 Hz, 1H, CH).
(t, J=7 Hz, 3H, CH3), 1.20–1.40 (m, 10H, CH2), 1.60
(s, 6H, CH3), 1.70 (s, 1H, OH), 1.50–1.80 (m, 2H, CH2),
1.85–2.05 (m, 1H, CH), 2.05–2.35 (m, 2H, CH2), 3.55

360
M. Zaidlewicz et al. / Journal of Organometallic Chemistry 580 (1999) 354–362
(m, 1H, CH); 13C-NMR (CDCl3), l 13.98 (CH3), 18.53
(CH3), 18.74 (CH3), 22.59 (CH2), 26.67 (CH2), 29.61
(CH2), 31.75 (CH2), 31.82 (CH2), 36.08 (CH2), 40.16
(CH2), 40.67 (CH), 71.58 (CH), 122.67 (C), 124.80 (C);
Anal. Calc. for C14H26O C 79.92, H 12.48. Found: C
80.02, H 12.41.
hydroxide solution (20 ml), water (20 ml) and dried over
magnesium sulfate. The product was isolated by distilla-
1
tion, 9.51 g, 59%, b.p. 64–65°C 1 mm−1 Hg; H-NMR
(CDCl3), l 1.00 (t, J=7 Hz, 3H, CH3), 1.43 (sextet, J=7
Hz, 2H, CH2), 1.66 (quintet, J=7 Hz, 2H, CH2), 2.30
(s, 3H, CH3), 2.32 (s, 3H, CH3), 2.62 (t, J=7 Hz, 2H,
CH2), 7.05 (m, 3H, CH); 13C-NMR (CDCl3), l 13.87
(CH3), 19.18 (CH3), 19.64 (CH3), 22.37 (CH2), 33.81
(CH2), 35.15 (CH2), 125.77 (CH), 129.52 (CH), 129.83
(CH), 133.58 (C), 136.25 (C), 140.40 (C); Anal. Calc. for
C12H18 C 88.82, H 11.18. Found: C 88.63, H 11.12.
4.6. trans-3,4-Dimethyl-6-phenylcyclohex-3-enol
Prepared as described above from dichloro(E-2-
phenylethen-1-yl)borane and 2,3-dimethyl-1,3-butadi-
1
ene, 80% yield, b.p. 110–115°C 1 mm−1 Hg; H-NMR
(CDCl3), l 1.58 (s, 1H, OH), 1.60 (s, 3H, CH3), 1.65 (s,
3H, CH3), 2.00–2.45 (m, 4H, CH2), 2.75 (q, J=7 Hz,
1H, CH), 4.05 (m, 1H, CH), 7.25 (m, 5H, CH); 13C-NMR
(CDCl3), l 18.26 (CH3), 18.79 (CH3), 39.93 (CH2), 40.06
(CH2), 49.37 (CH), 71.26 (CH), 123.69 (C), 125.14 (C),
126.94 (CH), 128.04 (2CH), 128.88 (2CH), 142.55 (C);
Anal. Calc. for C14H18O C 83.11, H 8.98. Found: C 83.02,
H 8.90.
4.9. 4-Butyl-1,2-dimethyl-1,4-cyclohexadiene
Lithium (1.74 g, 0.25 mol) was added in pieces to a
stirred mixture of liquid ammonia (150 ml), diethyl ether
(25 ml), anhydrous ethanol (18 ml, 0.3 mol) and 4-butyl-
1,2-dimethylbenzene (8.15 g, 50 mmol). The mixture was
stirred for 3 h and left overnight at room temperature.
Water (50 ml) was added, the organic layer was separated
and the aqueous layer was extracted with petroleum ether
(3×30 ml). Extracts were combined with the organic
layer and dried with magnesium sulfate. The product was
isolated by distillation, 3.04 g, 37%, b.p. 55–56°C 1
4.7. 3,4-Dimethylbutyrophenone
Butyryl chloride (53.27 g, 0.50 mol) was added with
stirring to a mixture of o-xylene (159.25 g, 1.5 mol) and
anhydrous aluminum chloride (80.00 g, 0.6 mol) at
50–60°C and stirring was continued at this temperature
for 1 h. The mixture was poured into 15% hydrochloric
acid (150 ml) and ice (150 g). The organic layer was
separated, washed with water (50 ml), 10% sodium
hydroxide solution (50 ml), water (50 ml) and dried with
magnesium sulfate. Distillation gave a mixture of 2,3-
dimethyl- and 3,4-dimethylbutyrophenone, 55.29 g, 63%,
b.p. 90–110°C 1 mm−1 Hg. 3,4-Dimethylbutyrophenone
was isolated by fractional distillation on a ‘Spaltrohr’
concentric tube column, 21.90 g, 25%, b.p. 95–97°C 1
1
mm−1 Hg; H-NMR (CDCl3), l 0.92 (t, J=7 Hz, 3H,
CH3), 1.20–1.35 (m, 4H, CH2), 1.65 (s, 3H, CH3), 1.66
(s, 3H, CH3), 1.95 (t, J=7 Hz, 2H, CH2), 2.40–2.55 (m,
4H, CH2), 5.40 (m, 1H, CH); 13C-NMR (CDCl3), l 13.84
(CH3), 18.06 (CH3), 18.33 (CH3), 22.34 (CH2), 29.56
(CH2), 33.72 (CH2), 35.93 (CH2), 36.56 (CH2), 118.26
(CH), 123.05 (C), 123.12 (C), 135.57 (C); Anal. Calc. for
C12H20 (164.29), C 87.73, H 12.27. Found: C 87.56, H
12.20.
4.10. exo- and endo-5-Norbornen-2-ol
1
mm−1 Hg; H-NMR (CDCl3), l 1.00 (t, J=7 Hz, 3H,
Dichlorovinylborane (5.44 g, 50 mmol) was cooled to
−20°C and 1,3-cyclopentadiene (3.30 g, 50 mmol) was
slowly added under nitrogen keeping the temperature of
the reaction mixture at −20–0°C. The mixture was left
at 0°C for 1 h and then it was allowed to warm to room
temperature. 11B-NMR (neat), l 63.50. The crude adduct
thus obtained was dissolved in tetrahydrofuran (50 ml).
The solution was cooled to 0°C and 6 M aqueous sodium
hydroxide (30 ml, 180 mmol) was added with stirring
followed with 30% aqueous hydrogen peroxide (6.5 ml,
65 mmol) added dropwise at 0–20°C. After the addition
was completed, the mixture was stirred for 1 h at 50°C
and saturated with sodium chloride. The organic layer
was separated and the aqueous layer was extracted with
diethyl ether (2×20 ml). The organic solutions were
combined, washed with saturated brine (2×20 ml) and
dried over magnesium sulfate. Solvents were removed
and the product was isolated by sublimation, 3.15 g, 57%.
GC analysis showed exo- and endo-5-norbornen-2-ol
(16:84) identified by comparison with an authentic sam-
CH3), 1.75 (sextet, J=7 Hz, 2H, CH2), 2.30 (s, 6H, CH3),
2.90 (t, J=7 Hz, 2H, CH2), 7.20 (d, J=8 Hz, 1H, CH),
7.70 (m, 2H, CH); 13C-NMR (CDCl3), l 13.81 (CH3),
17.85 (CH2), 19.67 (CH3), 19.87 (CH3), 40.33 (CH2),
125.75 (CH), 129.14 (CH), 129.68 (CH), 135.02 (C),
136.78 (C), 142.25 (C), 200.39 (CꢀO).
4.8. 4-Butyl-1,2-dimethylbenzene
A mixture of zinc granules (65.37 g, 1 mol), water (100
ml), mercury(II) chloride (5.00 g) and concentrated
hydrochloric acid (10 ml) was shaken for 5 min and the
liquid was decanted. Water (50 ml) and concentrated
hydrochloric acid (75 ml) was added followed with
3,4-dimethylbutyrophenone (17.63 g, 0.1 mol). The mix-
ture was stirred at reflux for 6 h. The organic layer was
separated and the aqueous layer was extracted with
petroleum ether (50 ml). The organic solutions were
combined, washed with water (20 ml), 5% sodium

M. Zaidlewicz et al. / Journal of Organometallic Chemistry 580 (1999) 354–362
361
ple, (90% exo- and 10% endo) prepared by the monohy-
4.13. 1,2-Dimethylcyclohexene
droboration–oxidation of norbornadiene.
4.13.1. Reduction of 3,4-dimethylcyclohex-3-enol mesy-
late with lithium aluminum hydride
4.11. 2-Norbornene
3,4-Dimethylcyclohex-3-enol (3.79 g, 30 mmol) was
mesylated as described above to give 2.62 g, 93% of
crude mesylate, IR (film) 2900 cm−1 and no absorption
characteristic for the hydroxyl group. The mesylate was
reduced with lithium aluminum hydride (1.90 g, 50
mmol) in diethyl ether (80 ml) as described above to
gave 1.98 g of the product identified by comparison
(GC and GCMS) with authentic samples as a mixture
of 1,2-dimethylcyclohexene and 1,2-dimethyl-1,4-cyclo-
hexadiene (3: 1). The mixture was dissolved in diethyl
ether (5 ml) and borane–dimethyl sulfide (0.3 ml, 3
mmol) was added at 0°C. The solution was left
overnight at room temperature. Distillation under 1
mm Hg and redistillation at normal pressure gave 1.42
g, 43% of 1,2-dimethylcyclohexene, b.p. 135–136°C;
1H-NMR (CDCl3), l 1.45–1.70 (m, 4H, CH2), 1.59 (s,
6H, CH3), 1.80–2.00 (m, 4H, CH2); 13C-NMR (CDCl3),
l 19.14 (CH3), 23.50 (CH2), 31.74 (CH2), 125.63 (CꢀC);
GCMS (EI, 70 eV) z/e=110 (M+,24), 95 (46), 81 (50),
67 (100). Lit. [23], b.p. 135–136°C.
Methanesulfonyl chloride (4.01 g, 35 mmol) was
slowly added with stirring at −5–0°C to a solution of
exo- and endo-5-norbornen-2-ol (16:84, 3.30 g, 30
mmol) and triethylamine (6.07 g, 60 mmol) in
methylene chloride (100 ml). The mixture was left at
0–5°C for 24 h, washed with cold water (3×20 ml),
10% hydrochloric acid (2×20 ml), saturated sodium
bicarbonate solution (2×20 ml) and dried with magne-
sium sulfate. The solvent was removed under vacuum
at room temperature and crude methanesulfonate was
obtained, 5.01 g, 89%, IR (film) 2950 cm−1 and no
absorpion characteristic for the hydroxyl group. The
methanesulfonate was disolved in dry diethyl ether (20
ml) and the solution was added to a mixture of lithium
aluminum hydride (1.14 g, 30 mmol) and diethyl ether
(60 ml). The mixture was stirred overnight at room
temperature and then refluxed for 5 h. A 5 M aqueous
sodium hydroxide (8 ml, 40 mmol) was added and the
mixture was stirred for 0.5 h. The solution was de-
canted and the solid was washed with diethyl ether
(3×30 ml). The combined etheral solution was washed
with saturated brine (2×25 ml) and dried with magne-
sium sulfate. The product was isolated by distillation
under vacuum, 1.16 g, 41% yield, m.p. 44–46°C and
identified by comparison (GC) with a commercial
sample.
4.13.2. Reduction of 3,4-dimethylcyclohex-3-enol
mesylate with lithium triethylborohydride
A 1 M solution of lithium triethylborohydride in
tetrahydrofuran (44 ml, 44 mmol) was added in one
portion to a solution of 3,4-dimethylcyclohex-3-enol
mesylate (4.26 g, 21 mmol) in tetrahydrofuran at room
temperature. The mixture was stirred at 60°C for 4 h
and left at room temperature for 48 h. A 3 M aqueous
sodium hydroxide (15 ml, 45 mmol) was added fol-
lowed slowly with hydrogen peroxide (30%, 20 ml), the
mixture was stirred for 3 h at room temperature and
saturated with potassium carbonate. The organic layer
was separated and the aqueous layer was extracted with
diethyl ether (2×20 ml). The organic solutions were
combined, washed with brine and dried with magne-
sium sulfate. Solvents were removed and 1,2-dimethyl-
cyclohexene was isolated by distillation, 1.88 g, 81%,
b.p. 135–136°C, GC analysis \98%.
4.12. 3,4-Dimethylcyclohex-3-enol
2,3-Dimethyl-1,3-butadiene (5.00 g, 60 mmol) was
added to dichlorovinylborane (6.40 g, 59 mmol) with
stirring at −10°C. The cooling bath was removed and
the mixture was carefully stirred at room temperature
for 0.5 h cooling in a water bath and then tetrahydro-
furan (30 ml) was added. The solution was cooled to
0°C and 3 M aqueous sodium hydroxide (60 ml, 180
mmol) was added followed with 30% aqueous hydrogen
peroxide (6.5 ml, 65 mmol) added dropwise at 0–20°C.
After the addition was completed, the mixture was
stirred for 1 h at 50°C and saturated with sodium
chloride. The organic layer was separated and the
aqueous layer was extracted with diethyl ether (2 x 20
ml). The organic solutions were combined, washed with
saturated brine (2×20 ml) and dried over magnesium
sulfate. Solvents were removed and the product was
isolated by distillation, 5.20 g, 70%, b.p. 65°C 0.1
mm−1 Hg; 1H-NMR (CDCl3), l 1.58 (s, 6H, CH3),
1.7–2.3 (m, 7H, CH), 3.90 (m, 1H, CH); 13C-NMR
(CDCl3), l 18.18 (CH3), 18.72 (CH3), 29.71 (CH2),
31.18 (CH2), 40.19 (CH2), 66.97 (CH), 122.61 (C),
124.81 (C). Lit. [22], b.p. 109–112°C 32 mm−1 Hg.
4.14. endo-Bicyclo[2.2.2]oct-5-en-2-ol
1,3-Cyclohexadiene (4.00 g, 50 mmol) was slowly
added to dichlorovinylborane (4.34 g, 40 mmol) at 0°C
under nitrogen, and the mixture was kept at room
temperature for 1 h. 11B-NMR (neat), l, 63.90 and no
signals of dichlorovinylborane at 52.66. Tetrahydro-
furan (30 ml) was added followed with 3 M aqueous
sodium hydroxide (50 ml, 150 mmol) and 30% aqueous
hydrogen peroxide solution (5 ml, 50 mmol) added
dropwise with stirring at a rate to keep the temperature
of the reaction mixture below 30°C. After the addition

362
M. Zaidlewicz et al. / Journal of Organometallic Chemistry 580 (1999) 354–362
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was completed the mixture was stirred at 50°C for 1 h
to ensure complete oxidation and saturated with
sodium chloride. The organic layer was separated and
the aqueous layer was extracted with diethyl ether
(2×20 ml). The combined organic solution was washed
with brine (2×15 ml) and dried with magnesium sul-
fate. The product was isolated by sublimation, 3.48 g,
1
70% yield, m.p. 166–167°C, 97% GC pure; H-NMR
(CDCl3), l 1.00-1.50 (m, 5H, CH2), 1.48 (s, 1H, OH),
1.96 (ddd, J=12 Hz, 8 Hz, 2 Hz, 1H, CH2), 2.55
(m,1H, CH), 2.70 (m, 1H, CH), 3.90 (dm, J=8 Hz, 1H,
CH), 6.10 (t, J=7 Hz, 1H, CH), 6.45 (t, J=7 Hz, 1H,
CH); 13C-NMR (CDCl3), l 21.60 (CH2), 23.62 (CH2),
29.72 (CH), 37.29 (CH), 38.60 (CH2), 69.89 (CH), 129,
59 (CH), 135.88 (CH). Lit. [24], m.p. 166–168.5°C.
4.15. Bicyclo[2.2.2]oct-2-ene
To the adduct prepared as described above from
dichlorovinylborane (1.09 g, 10 mmol) and 1,3-cyclo-
hexadiene (0.80 g, 10 mmol), a 1.0 M solution of
sodium methoxide in methanol (21 ml, 21 mmol) was
added at −10°C and the mixture was stirred for 0.5 h.
Methanol was removed under vacuum at room temper-
ature, propionic acid (1.56 g, 21 mmol) was added and
the mixture was refluxed for 2 h, cooled to room
temperature and a 2 M sodium hydroxide solution (20
ml) was added. The mixture was extracted with diethyl
ether (3×20 ml), the combined extracts were washed
with brine (2×20 ml) and dried over magnesium sul-
fate. Solvent was removed and the product was isolated
by distillation, 0.55 g, 50%, m.p. 111–112°C; GCMS
(EI, 70 eV) z/e=108 (M+, 15), 80 (30), 79 (100), 66
(80). Lit. [25], m.p. 111–112°C.
Acknowledgements
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Financial support from the State Committee for Sci-
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