The
arylbis(hydroxymethyl)germanes
Ph(H)Ge(CH2OH)2
(5) and
p-FC6H4(H)Ge(CH2OH)2
(6) as well as the bis(acetoxymethyl)arylgermanes
Ph(H)Ge(CH2OAc)2
(9) and
p-FC6H4(H)Ge(CH2OAc)2
(10) were synthesized, starting from
dichlorobis(chloromethyl)germane
[Cl2Ge(CH2Cl)2 →
Aryl(Cl)Ge(CH2Cl)2 →
Aryl(AcO)Ge(CH2OAc)2 →
Aryl(H)Ge(CH2OH)2 →
Aryl(H)Ge(CH2OAc)2; Aryl
= Ph, p-FC6H4]. Reaction of
the diols 5 and 6 with
Ac2O
and NEt3 (molar ratio 1:1:1) yielded the
(acetoxymethyl)aryl(hydroxymethyl)germanes
rac-Ph(H)Ge(CH2OH)CH2OAc
(rac-1) and
rac-p-FC6H4(H)Ge(CH2OH)CH2OAc
(rac-2), respectively. The (R)- and (S)-enantiomers of
1 and 2 were prepared on a preparative scale
by
enzymatic conversions. (R)-1 and
(R)-2 were obtained by enantioselective
transesterifications
of the prochiral diols 5 and 6, respectively,
with ethyl acetate (acyl donor) using porcine
pancreas lipase (PPL, E.C.3.1.1.3) as the biocatalyst (reaction medium,
ethyl acetate). The
corresponding antipodes (S)-1 and
(S)-2 were prepared by PPL-catalyzed
enantioselective
hydrolyses of the prochiral diacetates 9 and 10,
respectively [reaction medium, Sörensen
phosphate buffer (pH 7)/tetrahydrofuran (25:1, v/v)]. The yields
and enantiomeric purities
of the optically active germanes were as follows:
(R)-1, 76%, 93% ee;
(S)-1, 48%, 84% ee;
(R)-2, 77%, 86/87% ee;
(S)-2, 62%, 94% ee. Alternatively,
(R)-2 and (S)-2 were
obtained by
preparative liquid-chromatographic resolution of
rac-2 using cellulose tribenzoate as
the
chiral stationary phase (yield 80%; enantiomeric purities 97% ee).
For reasons of comparison,
the (R)- and (S)-enantiomers of
Ph(H)C(CH2OH)CH2OAc
(3) and
p-FC6H4(H)C(CH2OH)CH2OAc (4) (carbon analogues of the germanes 1 and
2) were prepared using the same
preparative methods [PPL-catalyzed transesterifications of
Ph(H)C(CH2OH)2 (7)
and
p-FC6H4(H)C(CH2OH)2
(8) with vinyl acetate and ethyl acetate, respectively (→
(R)-3, (R)-4); PPL-catalyzed hydrolyses of
Ph(H)C(CH2OAc)2 (11)
and
p-FC6H4(H)C(CH2OAc)2
(12)
(→ (S)-3, (S)-4);
chromatographic resolution of
rac-p-FC6H4(H)C(CH2OH)CH2OAc
(rac-4)
(→ (R)-4, (S)-4)].
The preparative results were similar to those obtained for the
germanium
compounds. In contrast to the configurationally stable antipodes
of the carbon compounds
3 and 4, the (R)- and
(S)-enantiomers of the corresponding germanium analogues
1 and 2
undergo a slow racemization upon heating (neat compounds). The
chiroptical properties of
the Ge/C analogues
(R)-1/(R)-3,
(S)-1/(S)-3,
(R)-2/(R)-4, and
(S)-2/(S)-4 (dissolved in
acetone)
differ significantly from one another (opposite signs of optical
rotation at various wavelengths). In contrast, the respective optically active Ge/C
analogues (R)- and
(S)-Ph(H)El(CH2OAc)CH2OSiPh2
tBu
[(R)- and (S)-21, El = Ge;
(R)- and (S)-23, El = C],
(R)- and (S)-p-FC6H4(H)El(CH2OAc)CH2OSiPh2
tBu
[(R)- and (S)-22, El = Ge;
(R)- and (S)-24, El =
C],
Ph(H)El(CH2OH)CH2OSiPh2
tBu
[(R)- and (S)-25, El = Ge;
(R)- and (S)-27, El = C], and
(R)-
and
(S)-p-FC6H4(H)El(CH2OH)CH2OSiPh2
tBu
[(R)- and (S)-26, El = Ge;
(R)- and (S)-28, El
= C] di...
