Chirality transfer in the formation of poly(oxymethylene) helices by anionic polymerization

Article abstract of DOI:10.1002/hlca.201200032

In this article, we present the synthesis of a series of oligo(oxymethylenes) capped with 1-phenylethanol and MeOH. The anionic condition affords enantiomerically pure oligo(oxymethylene) oligomers, while the cationic oligomerization leads to a racemic mixture of the oligo(oxymethylene) chain. Copyright

Full text of DOI:10.1002/hlca.201200032

Helvetica Chimica Acta – Vol. 95 (2012)
845
Chirality Transfer in the Formation of Poly(oxymethylene) Helices by Anionic
Polymerization
by Christian Miculka, Christian R. Noe*, and Simon Eppacher
Department of Medicinal Chemistry, University of Vienna, Althanstrasse 14, A-1090 Wien
(phone: þ 431427755103; fax: þ 43142779551; e-mail: christian.noe@univie.ac.at)
In this article, we present the synthesis of a series of oligo(oxymethylenes) capped with 1-
phenylethanol and MeOH. The anionic condition affords enantiomerically pure oligo(oxymethylene)
oligomers, while the cationic oligomerization leads to a racemic mixture of the oligo(oxymethylene)
chain.
Introduction and Concept. – Formaldehyde was first discovered by A. M. Butlerow
in 1855 and synthesized in 1867 by A. W. Hoffmann by dehydrogenation of MeOH. The
structure of formaldehyde was first investigated by H. Staudinger in 1925 [1 – 3]. It is
well-accepted that paraformaldehyde forms a helix as it was shown by theoretical
calculations [4], by IR spectroscopy [5], and by X-ray analysis [6 – 11]. By insertion of a
poly(oxymethylene) into a diamond lattice, an idealized 21 helix can be found (Fig. 1).
Fig. 1. Idealized oligo(oxymethylene) helices fitted into a diamond lattice. Right handed helix capped
with an A and a left handed helix with a B descriptor, respectively.
By applying the b, pl, H-rule [12][13], a right handed helix is observed when the
starting and the end group has an A descriptor [14], whereas a left-handed helix is
obtained with a B descriptor as start and end group, respectively. Poly(oxymethylene)
serves as the most general model of the anomeric effect, namely the interaction of the
no lone pair of the O-atom and the s* orbital of the CꢀO bond [15]. This interaction
causes the continuous gauche-orientiation of the dihedral angles and a shortening of the
CꢀO bonds in the helix, as it was established by the synthesis and X-ray analysis of 1,13-
diphenyl-2,4,6,8,10,12-hexaoxatridecane [15] and its corresponding heptamer [16]. We
synthesized the enantiomerically pure oligo(oxymethylene) helix by cationic oligome-
ꢀ 2012 Verlag Helvetica Chimica Acta AG, Zꢁrich

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Helvetica Chimica Acta – Vol. 95 (2012)
rization of paraformaldehyde with (1S)-2,2-dimethyl-1-phenylpropan-1-ol (B descrip-
tor) as starter and end group, respectively [17]. By calculations, NMR spectroscopy,
optical rotation, as well by X-ray crystallography, we could show the expected left
handed helical structure. Remarkable is the stability of this carbinol under acidic
condition. Since we isolated only the enantiomerically pure helix, we can conclude that
the formation of the carbocation is slower than the addition to the formaldehyde. The
present work aims at enantiomerically pure oligo(oxymethylene) helices, which are
capped with a chiral inducer at one end and a Me group at the other end.
Results and Discussion. – 1. Cationic Polymerization. In a first experiment, we
investigated the cationic oligomerization to obtain oligo(oxymethylene) helices capped
with (ꢁ)-1-phenylethanol and MeOH by the reaction of 1-phenylethanol, paraformal-
dehyde, and MeOH catalyzed by H2SO4. In this reaction, we found a complex mixture
of products. We found the twofold 1-phenylethanol-capped oligomer, the expected
oligomers in small amounts, and a series of non-identified products. Next, we examined
the insertion reaction of (ꢁ)-methyl 1-phenylethyl ether and paraformaldehyde
(Scheme 1).
Scheme 1
a) Cat. H2SO4 paraformaldehyde, CH2Cl2, 3% of 1/1a, 0.8% of 2/2a, 0.6% of 3/3a, 0.6% of 4/4a, and ca.
14.5% mixed fractions.
In this reaction, we obtained the desired 1-phenylethyl/Me-capped racemic
oligomers up to the tetramer i.e., 1/1a to 4/4a, which could be separated by flash
chromatography. In contrast to our experiment with (1S)-2,2-dimethyl-1-phenyl-
propan-1-ol, the reaction with the methyl ether of (ꢁ)-1-phenylethanol must proceed
via a carbocation, followed by the insertion of formaldehylde (HCHO). In fact, this
should happen regardless whether the starting material is enantiomerically pure or a
racemic mixture. In the NMR spectrum of the tetramer 4/4a, the CH2 H-atoms showed
signals of four AB systems. The NMR spectra in different solvents (CD3OD, CDCl3,
and (D8)toluene) displayed a better resolution in the less polar solvent where all 16
1
signals could be resolved. The assignment of the signals was achieved by H,1H and
1H,13C correlated spectroscopy. For the tetramer 4/4a in (D8)toluene, a temperature-
dependent recording was performed at 295, 315, 335, and 355 K. As expected, the shift
difference of the diastereotopic H-atoms of the CH2 units decreased. The H-atoms
corresponding to the A part are shifted to higher field, while the signals corresponding
to the B part are shifted to lower field.

Helvetica Chimica Acta – Vol. 95 (2012)
847
2. Anionic Polymerization. To obtain a new enantiomerically pure poly(oxy-
methylene) helix, we investigated the anionic polymerization. Therefore, we synthe-
sized enantiomerically pure (R)-1-phenylethanol (7) by reduction of acetophenone (5)
with BH3 and the Corey catalyst (6; Scheme 2) [18 – 20] in 91% ee.
Scheme 2
a) THF, BH3; 95%. b) (MBE)2O, cat. TsOH; 82%. c) MeOH, cat. TsOH; 83%.
The (R)-1-phenylethanol was enantiomerically enriched by mixed acetal formation
with the terpene carbohydrate mimic (MBE)2O (¼ [2S,2’S-(2a,2’a,3aa,3’aa,4b,4’b,7b,
7’b,7aa,7’aa)]-2,2’-oxybis[octahydro-7,8,8-trimethyl-4,7-methanobenzofuran] [21] to 8,
followed by crystallization of 8 from MeOH for two times and then methanolysis [14].
By this protocol, we obtained the required (R)-1-phenylethanol (7) in an overall yield
of 67% and an enantiomeric purity of 99.8% ee.
The experiments by Vogl and co-workers provided the starting point for the anionic
polymerization [22 – 27]. In a first experiment, the racemic carbinol was used. The
oligomerization of (ꢁ)-1-phenylethanol was started by deprotonation with BuLi in
THF at ꢀ 208, followed by treatment at ꢀ 958 with gaseous HCHO, which was
obtained from carefully dried poly(oxymethylene) (Scheme 3).
After removal of the cooling bath, we observed a remarkable increase of the
temperature between ꢀ 40 and ꢀ 308, indicating the start of the reaction. The reaction
was quenched with distilled Me2SO4. By this protocol, we could again obtain oligomers
up to the tetramer, 1/1a – 4/4a. In the case of the reaction with enantiomerically pure
(R)-1-phenylethanol, we investigated the enantiomeric purity of the reaction by
quenching a part of the lithium alkoxide with H2O. This product showed no change of
the enantiomeric excess according to the investigation with a chiral column (Chiralcel
OD).
The resolution of the racemic poly(oxymethylenes) (1/1a – 4/4a) with a chiral
column (Chiralcel OD) led to a good separation of 2/2a and a slight separation of 4/4a,
while 1/1a and 3/3a could not be resolved even when the column was adjusted to 08. The
elution sequence of the racemic mixture showed a faster elution for the (S)- than for
the (R)-product as was shown by the addition of the enantiomerically pure 2. The
elution sequence of the oligomers was opposite to the starting unit 1-phenylethanol.
The assignment of the conformation of the chain was investigated as described by
Anderson et al. [28]. They reported a dependence of the direct C,H coupling constants

848
Helvetica Chimica Acta – Vol. 95 (2012)
Scheme 3
a) THF, BuLi. b) HCHO gas. c) Me2SO4; 0.7% of 1, 15% of 2, 6.1% of 3, 2.8% of 4, and 26% of mixed
fractions.
in fixed R13C1H(OR’)2 fragments in dependence of the H-atoms syn-clinal and anti-
periplanar to the non-bonding orbitals of the O-atom. By consideration of a OꢀCH2ꢀO
section of the oligo(oxymethylene) chain, it is possible to formulate four possible
diastereoisomerically different conformations (Fig. 2).
Fig. 2. Four possible conformations of a OꢀCH2ꢀO section and the syn- and anti-arrangement of the
diastereoisomerically different H-atoms
These four conformations should give rise to different 1J(C,H) coupling constants.
Therefore, the helical þ sc, þ sc as the continuous ap conformation should give only
one coupling constant. Only in conformation A, we find a combination of the
diasterotopic H-atoms, and the lone pairs resulting in three syn- and one anti-
orientations. For this conformation, a 1J(C,H) value of 162 Hz is proposed.
In all of our synthesized oligo(oxymethylene) helices, we obtained only one value
for the coupling constants with an overall value of 164.3 Hz. This value is in accordance
with a continuous syn orientation leading to a helical conformation and excludes the ap
conformation of the oligo(oxymethylene) chain.
4. Conclusion. – In this communication, we presented the synthesis of oligo(oxy-
methylene) helices capped with 1-phenylethyl and Me groups by cationic and anionic
oligomerization. While the cationic oligomerization of the methyl ether of (ꢁ)-1-

Helvetica Chimica Acta – Vol. 95 (2012)
849
phenylethanol yielded racemic oligomers, the anionic oligomerization afforded in an
enantiomerically selective way, as we could demonstrate by the reaction with (R)-1-
1
phenylethanol. By investigation of the J(C,H) coupling constants, we could establish
the gauche-orientation of the chain and exclude the anti-orientation.
Experimental Part
General. THF was distilled on Na/benzophenone, toluene on Na, and petroleum ether (PE; b.p. 50 –
758). TLC: Precoated silica-gel plates (Merck silica gel 60 F254); detection by spraying with ꢂMostainꢃ
soln. (400 ml of 10% aq. H2SO4, 20 g of (NH4)6 Mo7O24 · H2O, and 0.4 g of Ce(SO4)2). Flash
chromatography (FC): silica gel Merck 60 (0.04 – 0.063 mm). 1H- and 13C-NMR spectra: at 300 and
75 MHz, resp. Poly(oxymethylene) was dried in an exsiccator over P2O5. HPLC: with a 25 cm column of
Chiralcel OD, 5 mm; eluent: hexane/10 vol-% t-BuOMe/3.5 vol-% i-PrOH.
[1-(Methoxymethoxy)ethyl]benzene (rac-1/1a), {1-[(Methoxymethoxy)methoxy] ethyl}benzene (rac-
2/2a), 9-Phenyl-2,4,6,8-tetraoxadecane (rac-3/3a), and 11-Phenyl-2,4,6,8,10-pentaoxadodecane (rac-4/
4a). A suspension of paraformaldehyde (6.0 g, 0.2 mol) and (ꢁ)-(1-methoxyethyl)benzene (2.72 g,
0.02 mol) in CH2Cl2 (30 ml) was treated dropwise with H2SO4 (2 ml), stirred for 30 min, treated with
solid NaHCO3, filtered over 15 g of SiO2, and evaporated. FC (PE/Et2O 19 :1 ! 8 :1) gave rac-1/1a
(89 mg, 2.68%), rac-2/2a (31 mg, 0.79%), rac-3/3a (26 mg, 0.57%), rac-4/4a (32 mg (0.62%), and 570 mg
of mixed fractions (corresponding of 62% of the material before the FC).
(R)-[1-(Methoxymethoxy)ethyl]benzene (1), (R)-{1-[(Methoxymethoxy)methoxy]ethyl]benzene
(2), (R)-9-Phenyl-2,4,6,8-tetraoxadecane (3), and (R)-11-Phenyl-2,4,6,8,10-pentaoxadodecane (4). A
soln. of (R)-1-phenylethanol (4.0g, 33 mmol) in abs. THF (70 ml) under N2 was cooled to ꢀ 208 and
treated dropwise with BuLi (2.5m soln. in hexane; 13.6 ml, 34 mmol). After 30 min, the completeness of
the deprotonation was established by quenching the reaction of one drop of the mixture with Ac2O in
Et2O, followed by extraction with NaHCO3 and TLC control. The rest of the mixture was cooled to
ꢀ 1058 and treated with monomeric HCHO (obtained by heating of carefully dried oligo(oxymethylene)
(9.9 g, 330 mmol; dried over P2O5 in the exsiccator)) in a way that the temp. of the reaction mixture did
not exceed ꢀ 958. After the treatment with HCHO, the cooling bath was removed, the mixture was
stirred for 2.5 h at r.t. (at ꢀ 308, an exothermic reaction was observed), treated with Me2SO4 (4.2 g,
33 mmol), and stirred overnight at r.t. The suspension was filtered, concentrated, dissolved in CH2Cl2,
and extracted with NaHCO3. The org. phase was dried (Na2SO4) and concentrated. FC (1:80; PE/Et2O
19 :1 ! 8 :1 gave 1 (37 mg, 0.7%), 2 (950 mg, 15%), 3 (457 mg, 6.1%), 4 (207 mg, 2.8%), and 1.7 g of
mixed fractions (corresponding to 61% of the material before FC).
Data of 1. Colorless liquid. [a]2D0 ¼ þ180 (c ¼ 0.62; Et2O). 1H-NMR (CDCl3): 7.26 – 7.37 (m, 5 arom.
H); 4.76 (q, J ¼ 6.6, PhCHꢀO); 4.56, 4.59 (AB, J ¼ 6.7, OCH2O); 3.38 (s, MeO); 1.50 (d, J ¼ 6.6, Me).
1H-NMR ((D8)toluene): 6.98 – 7.24 (m, 5 arom. H); 4.58 (q, J ¼ 6.6, PhCHꢀO); 4.41, 4.48 (AB, J ¼ 6.7,
OCH2O); 3.17 (s, MeO); 1.38 (d, J ¼ 6.6, Me). 13C-NMR (CDCl3): 143.4 (s, C(1)); 128.3 (d, C(3) and
C(5)); 127.4 (d, C(4)); 126.1 (d, C(2) and C(6)); 94.2 (t, 1J(C,H) ¼ 162.9, OCH2O); 73.7 (d, PhCHꢀO);
55.2 (q, MeO); 23.5 (q, Me). 13C-NMR ((D8)toluene): 144.1 (s, C(1)); 128.5 (d, C(3) and C(5)); 127.6 (d,
1
C(4)); 126.6 (d, C(2) and C(6)); 94.2 (t, J(C,H) ¼ 162.0, OCH2O); 73.9 (d, PhCHꢀO); 55.9 (q, MeO);
24.0 (q, Me). Anal. calc. for C10H14O2: C 72.26, H 8.49; found: C 72.50, H 8.25.
Data of 2. Colorless liquid. [a]D20 ¼ þ188 (c ¼ 0.69, CH2Cl2). 1H-NMR (CDCl3): 7.26 – 7.34 (m, 5
arom. H); 4.60, 4.82 (AB, J ¼ 6.9, CH2(1)); 4.78 (q, J ¼ 6.6, PhCHꢀO); 4.66, 4.78 (AB, J ¼ 6.5, CH2(2));
3.37 (s, MeO); 1.48 (d, J ¼ 6.6, Me). 1H-NMR ((D8)toluene): 6.98 – 7.23 (m, 5 arom. H); 4.50, 4.73 (AB,
J ¼ 6.6, CH2(1)); 4.52, 4.70 (AB, J ¼ 6.3, CH2(2)); 4.68 (q, J ¼ 6.5, PhCHꢀO); 3.13 (s, MeO); 1.35 (d, J ¼
6.5, Me). 13C-NMR (CDCl3): 143.1 (s, C(1)); 128.3 (d, C(3) and C(5)); 127.4 (d, C(4)); 126.2 (d, C(2) and
1
1
C(6)); 93.3 (t, J(C,H) ¼ 163.5, CH2(2)); 89.7 (t, J(C,H) ¼ 164.0, CH2(1)); 74.6 (d, PhCHꢀO); 55.6 (q,
MeO); 23.5 (q, Me). 13C-NMR ((D8)toluene): 144.0 (s, C(1)); 128.6 (d, C(3) and C(5)); 127.6 (d, C(4));
126.7 (d, C(2) and C(6)); 93.0 (t, J(C,H) ¼ 163.2, CH2(2)); 89.3 (t, J(C,H) ¼ 163.6, CH2(1)); 74.6 (d,
PhCHꢀO); 55.3 (q, MeO); 24.0 (q, Me). Anal. calc. for C11H16O3 (196.24): C 67.32, H 8.22; found: C
67.10, H 8.15.
1
1

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Helvetica Chimica Acta – Vol. 95 (2012)
Data of 3. Colorless liquid. [a]D20 ¼ þ160 (c ¼ 0.72, CH2Cl2). 1H-NMR (CDCl3): 7.26 – 7.35 (m, 5
arom. H); 4.84, 4.89 (AB, J ¼ 6.5, CH2(2)); 4.61, 4.83 (AB, J ¼ 6.6, CH2(1)); 4.78 (q, J ¼ 6.6, PhCHꢀO);
4.68, 4.73 (AB, J ¼ 6.5, CH2(3)); 3.37 (s, MeO); 1.47 (d, J ¼ 6.6, Me). 1H-NMR ((D8)toluene): 6.98 – 7.22
(m, 5 arom. H); 4.78, 4.85 (AB, J ¼ 6.5, CH2(2)); 4.54, 4.75 (AB, J ¼ 6.9, CH2(1)); 4.67 (q, J ¼ 6.5,
PhCHꢀO); 4.51, 4.61 (AB, J ¼ 6.8, CH2(3)); 3.12 (s, MeO); 1.31 (d, J ¼ 6.5, Me). 13C-NMR (CDCl3):
143.0 (s, arom C(1)); 128.4 (d, C(3) and C(5)); 127.5 (d, C(4)); 126.2 (d, arom. C(2) and C(6)); 93.6 (t,
1
1
1J(C,H) ¼ 164.6, CH2(3)); 90.0 (t, J(C,H) ¼ 164.6, CH2(1)); 88.6 (t, J(C,H) ¼ 165.7, CH2(2)); 74.6 (d,
PhCHꢀO); 55.6 (q, MeO); 23.5 (q, Me). 13C-NMR ((D8)toluene): 143.8 (s, arom C(1)); 128.6 (d, C(3)
and C(5)); 127.6 (d, C(4)); 126.7 (d, C(2) and C(6)); 93.4 (t, 1J(C,H) ¼ 163.4, CH2(3)); 89.8 (t, 1J(C,H) ¼
163.9, CH2(1)); 88.1 (t, 1J(C,H) ¼ 164.4, CH2(2)); 74.7 (d, PhCHꢀO); 55.3 (q, MeO); 23.9 (q, Me). Anal.
calc. for. C12H18O4 (226.27): C 63.70, H 8.02; found: C 63.99, H 7.85.
Data of 4. Colorless liquid. [a]D20 ¼ þ142 (c ¼ 0.38, CH2Cl2). 1H-NMR (CDCl3): 7.26 – 7.34 (m, 5
arom. H); 4.84, 4.91 (AB, J ¼ 6.7, CH2(2)); 4.86, 4.86 (AB, J ¼ 6.6, CH2(3)); 4.60, 4.83 (AB, J ¼ 6.9,
CH2(1)); 4.77 (q, J ¼ 6.5, PhCHꢀO); 4.71, 4.72 (AB, J ¼ 6.5, CH2(4)); 3.37 (s, MeO); 1.46 (d, J ¼ 6.6, Me).
1H-NMR ((D8)toluene): 7.26 – 7.34 (m, 5 arom. H); 4.79, 4.86 (AB, J ¼ 6.6, CH2(2)); 4.76, 4.79 (AB, J ¼
6.6, CH2(3)); 4.53, 4.74 (AB, J ¼ 6.8, CH2(1)); 4.67 (q, J ¼ 6.5, PhCHꢀO); 4.53, 4.57 (AB, J ¼ 6.5, CH2(4));
3.12 (s, MeO); 1.35 (d, J ¼ 6.5, Me). 13C-NMR (CDCl3): 143.8 (s, arom C(1)); 128.3 (d, arom. C(3) and
C(5)); 127.4 (d, arom. C(4)); 126.6 (d, arom. C(2), arom. C(6)); 93.6 (t, 1J(C,H) ¼ 164.6, CH2(4)); 90.0 (t,
1J(C,H) ¼ 164.6, CH2(1)); 88.69 (t, 1J(C,H) ¼ 165.7, CH2(2)); 88.62 (t, 1J(C,H) ¼ 165.7, CH2(3)); 74.6 (d,
PhCHꢀO); 55.6 (q, MeO); 23.5 (q, Me). 13C-NMR ((D8)toluene): 143.0 (s, arom. C(1)); 128.6 (d, arom.
C(3) and C(5)); 127.6 (d, arom. C(4)); 126.7 (d, arom. C(2) and C(6)); 93.4 (t, 1J(C,H) ¼ 163.6, CH2(4));
1
1
1
89.8 (t, J(C,H) ¼ 163.9, CH2(1)); 88.56 (t, J(C,H) ¼ 164.5, CH2(2)); 88.45 (t, J(C,H) ¼ 164.5, CH2(4));
74.7 (d, PhCHꢀO); 55.3 (q, MeO); 23.9 (q, Me). Anal. calc. for C13H20O5 (256.30): C 60.92 H, 7.87; found:
C 61.15, H 7.80.
(R)-1-Phenylethanol (7). A soln. of 6 (2.27g, 6.7 mmol) in THF (13 ml) and 1n BH3 · THF (6.5 ml,
6,5 mmol) was cooled to 08 and simultaneously treated with acetophenone (8.0 g, 67 mmol) dissolved in
Et2O (30 ml) and 1n BH3 · THF (33.5 ml, 33.5 mmol) in a way that the temp. did not exceed 2 – 78. After
addition, the mixture was stirred for 3 min and treated dropwise, under cooling, with 10 ml of abs. MeOH
during 10 min. The soln. was treated with 2 ml of HCl sat. in Et2O, stirred for 30 min, concentrated,
dissolved in benzene, again concentrated, and dissolved in Et2O (70 ml). The soln. was stored overnight
in the refrigerator, the precipitated hydrochloride of 6 was filtered (1.64 g; 85% catalyst recovery), and
the Et2O phase was extracted with NaHCO3, dried (NaSO4), and concentrated to yield 7 (7.7 g, 95%)
with 91% ee according to HPLC control (Chiralcel OD, 5 mm).
According to the procedure described in [21], a soln. of 7 (7.4 g, 60.2 mmol) and (MBE)2O (11.8 g,
32 mmol) in CH2Cl2 (70 ml) was treated with 1 g of Na2SO4 and ca. 20 mg of TsOH, stirred for 2 h,
treated with NaHCO3, filtered, concentrated, and crystallized two times with MeOH to yield 8 (14.9 g,
82%) with de > 99.8%.
A soln. of these crystals in MeOH and ca. 20 mg of TsOH was stirred for 40 min, the reaction was
quenched with solid NaHCO3, and the mixture was concentrated. FC (PE/Et2O 20 :1 ! Et2O) gave 7
(5.0 g, 83%), ee > 99.8% according to HPLC (Chiralcel OD 5 mm).
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Received January 24, 2012