2 Preparation of 7-Oxabicyclo[2.2.1]heptane
The parent 7-oxanorbornane, 1, is commercially available. Its preparation starts
with the catalytic hydrogenation of hydroquinone to generate a mixture of transand cis-cyclohexane-1,4-diol [15–18]. cis-Cyclohexane-1,4-diol can be isomerized
into the more stable trans isomer with metallic sodium [17]. Dehydratation of the
latter on A4 zeolites, on alumina [19], or over nickel–kieselguhr catalyst [20]
provides 1. This reaction is exergonic (Δ r G
o
¼ Δ r H
o
À TΔ r S
o
¼ À3.1 Æ 2.5
kcal/mol) at room temperature as its standard gas phase heat of reaction amounts
to Δ r H
o
¼ +7.3 Æ 2.5 kcal/mol. A variation of entropy of reaction of ca. +35 eu is
assumed for this fragmentation, what leads to ÀTΔ r S
o
¼ 298(0.035) ¼
À10.4 kcal/mol. The standard gas phase heat of formation of trans-cyclohexane1,4-diol (Scheme 1) is estimated from that of cyclohexanol (À69.0 Æ 2.0 kcal/mol)
and the standard heat of oxidation of cyclohexane into cyclohexanol (À39.5 kcal/
mol) [21]. Chickos and Acree [22] give Δ f H
o (1) ¼ À43.4 Æ 0.5 kcal/mol (see also
[23–25]).
Ring strain of 1 corresponds to the ring strain of two tetrahydrofuran moieties.
Their annulation into the bicyclo[2.2.1]heptane system does not introduce extra
ring strain as demonstrated by the thermochemical analysis reported in Scheme 1.
Double dehydrogenation (twice +16.4 Æ 0.7 kcal/mol, the heat of dehydrogenation
of diethyl ether into tetrahydrofuran) of diisopropyl ether into 1 gives a standard gas
phase heat of formation for 1 that amounts to À43.3 Æ 2.0 kcal/mol, practically the
same value as experimental Δ f H
o (1) ¼ À43.4 Æ 0.5 kcal/mol. The same analysis
(double dehydrogenation of 2,3-dimethylpentane) for bicyclo[2.2.1]heptane
(norbornane) gives an estimated gas phase standard heat of formation of À14.4
O
O
1
2
Fig. 1 7-Oxanorbornane and 2-methylidene-7-oxanorbornane
Scheme 1 Preparation of 7-oxabicyclo[2.2.1]heptane and gas phase thermochemical data
Synthesis of 7-Oxabicyclo[2.2.1]heptane and Derivatives
143
The parent 7-oxanorbornane, 1, is commercially available. Its preparation starts
with the catalytic hydrogenation of hydroquinone to generate a mixture of transand cis-cyclohexane-1,4-diol [15–18]. cis-Cyclohexane-1,4-diol can be isomerized
into the more stable trans isomer with metallic sodium [17]. Dehydratation of the
latter on A4 zeolites, on alumina [19], or over nickel–kieselguhr catalyst [20]
provides 1. This reaction is exergonic (Δ r G
o
¼ Δ r H
o
À TΔ r S
o
¼ À3.1 Æ 2.5
kcal/mol) at room temperature as its standard gas phase heat of reaction amounts
to Δ r H
o
¼ +7.3 Æ 2.5 kcal/mol. A variation of entropy of reaction of ca. +35 eu is
assumed for this fragmentation, what leads to ÀTΔ r S
o
¼ 298(0.035) ¼
À10.4 kcal/mol. The standard gas phase heat of formation of trans-cyclohexane1,4-diol (Scheme 1) is estimated from that of cyclohexanol (À69.0 Æ 2.0 kcal/mol)
and the standard heat of oxidation of cyclohexane into cyclohexanol (À39.5 kcal/
mol) [21]. Chickos and Acree [22] give Δ f H
o (1) ¼ À43.4 Æ 0.5 kcal/mol (see also
[23–25]).
Ring strain of 1 corresponds to the ring strain of two tetrahydrofuran moieties.
Their annulation into the bicyclo[2.2.1]heptane system does not introduce extra
ring strain as demonstrated by the thermochemical analysis reported in Scheme 1.
Double dehydrogenation (twice +16.4 Æ 0.7 kcal/mol, the heat of dehydrogenation
of diethyl ether into tetrahydrofuran) of diisopropyl ether into 1 gives a standard gas
phase heat of formation for 1 that amounts to À43.3 Æ 2.0 kcal/mol, practically the
same value as experimental Δ f H
o (1) ¼ À43.4 Æ 0.5 kcal/mol. The same analysis
(double dehydrogenation of 2,3-dimethylpentane) for bicyclo[2.2.1]heptane
(norbornane) gives an estimated gas phase standard heat of formation of À14.4
O
O
1
2
Fig. 1 7-Oxanorbornane and 2-methylidene-7-oxanorbornane
Scheme 1 Preparation of 7-oxabicyclo[2.2.1]heptane and gas phase thermochemical data
Synthesis of 7-Oxabicyclo[2.2.1]heptane and Derivatives
143