Synthesis and investigation of the configurational stability of some dimethylammonium borate salts

Article abstract of DOI:10.1039/b005954o

Two borate salts, dimethylammonium bis[3-isopropylbenzene-1,2-diolato(2-)-O,O′]borate and dimethylammonium 3,3″-ethylenebis(3′-methylbiphenyl-2,2′-diolato(2-)-O,O′ )borate, were synthesised from the corresponding catechol and tetraphenol respectively. The configurational stability of these salts was determined by variable temperature ¹H NMR spectroscopy; the activation energies for the racemisation process were determined to be 85 and 79 kJ mol^-1 respectively. Mechanisms are proposed to explain the configurational instability observed.

Full text of DOI:10.1039/b005954o

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Synthesis and investigation of the configurational stability of some
dimethylammonium borate salts
Stuart Green, Adam Nelson,* Stuart Warriner and Benjamin Whittaker
1
School of Chemistry, University of Leeds, Leeds, UK LS2 9JT
Received (in Cambridge, UK) 24th July 2000, Accepted 27th September 2000
First published as an Advance Article on the web 21st November 2000
Two borate salts, dimethylammonium bis[3-isopropylbenzene-1,2-diolato(2Ϫ)-O,OЈ]borate and dimethylammonium
3,3Љ-ethylenebis(3Ј-methylbiphenyl-2,2Ј-diolato(2Ϫ)-O,OЈ)borate, were synthesised from the corresponding catechol
and tetraphenol respectively. The configurational stability of these salts was determined by variable temperature 1H
NMR spectroscopy; the activation energies for the racemisation process were determined to be 85 and 79 kJ molϪ1
respectively. Mechanisms are proposed to explain the configurational instability observed.
Introduction
Recently, Lacour and co-workers have reported the prepar-
ation and resolution of the D3 symmetric phosphate anion
TRISPHAT 1.1 The phosphate 1 is configurationally stable
and has been used as a chiral shift reagent for determining the
enantiomeric purity of chiral cationic ruthenium() complexes2
and phosphonium salts.3 TRISPHAT has also been exploited as
a resolving agent for ion pair chromatography.4
We are interested in the preparation of configurationally
stable borate salts. Previously, the brucine salt of the borate 2
has been prepared.5 The mutarotation of a solution of the
brucine salt of 2 in chloroform suggested that the 2 epimerised
slowly over a number of days. In this paper, the design and
synthesis of two chiral borate salts are described; their free
energies of racemisation were determined using variable tem-
perature 1H NMR.
Scheme 1
axes is either the same or different. Molecular modelling7 sug-
gested that the two diastereoisomeric conformers were very close
in energy. Calculations using B3LYP/6-31G* predicted rac-4 to
be more stable than the meso-4 by only 1.7 kJ molϪ1 whilst calcu-
lations using HF/6-31G* suggested virtually no difference in
energy between the isomers. Both methods predicted that the
aryl–aryl bonds would be approximately perpendicular in the
conformer meso-4 (Fig. 1) and approximately parallel in the
conformer rac-4 (Fig. 2) and that the seven membered rings
would adopt characteristic8 twisted conformations. An X-ray
crystal structure of the enantiomerically pure borate 5, which
can be considered to be a model of rac-4 in which rotation
about the biaryl bonds is prevented by restricted rotation,
indicates that 5 also adopts a conformation in which the two
aryl–aryl bonds are approximately parallel.9 This conform-
ational similarity reinforced our confidence in our modelling
techniques.†
Design of the chiral borate salts
At the start of our investigation, we wanted to quantify the
configurational stability of a simple borate salt such as 2 in
which the boron atom was attached to two substituted catechol
ligands. We chose to study the configurational stability of the
borate 3. Racemisation of 3 (Scheme 1) would result in
chemical exchange of MeA and MeB, a process which—if it
occurred—we planned to observe by dynamic NMR spectro-
scopy.6 This approach would allow the configurational stability
of borate 3 to be determined without the need for a resolution
step.
Another class of borates in which we were interested were
those based on structure 4 in which the central boron atom was
coordinated by two biphenol ligands. The borate 4 has two
stereogenic axes; we modelled the two conformers of 4—i.e.
rac-4 and meso-4—in which the stereochemistry of the biaryl
We wanted to synthesise configurationally stable, substituted
versions of 4 in which the stereochemistry of the biaryl axes
† The B–O bond length predicted in meso- and rac-4 using the B3LYP/
631-G* method (1.45 Å) is similar to those observed (1.47–1.49 Å) in
the X-ray crystal structure of the borate 23 (Fig. 3).
DOI: 10.1039/b005954o
J. Chem. Soc., Perkin Trans. 1, 2000, 4403–4408
This journal is © The Royal Society of Chemistry 2000
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Fig. 1 Conformer meso-4.
Fig. 2 Conformer rac-4.
was the same. The borates 6, 7 have three axes of asymmetry;
these structures have three possible diastereoisomers—
(P*,P*,R*), (P*,P*,S*) and (P*,M*)—all of which are chiral.
The distance between the C-3 carbons was predicted to be much
shorter in the conformer rac-4 (CA to CB: 4.16 Å) than in meso-4
(CA to CB or CC: 5.71 Å) (Figs. 1 and 2). Therefore, we reasoned
that the borates 6–8 would have ground states in which the
stereochemistry of the biaryl axes was the same [(P*,P*,R*)-6,
(P*,P*,R*)-7 and (P*,P*)-8] because of their short chains
linking the C-3 atoms. Minimisation of the geometries of the
borates (P*,M*)-6 and (P*,M*)-8 gave only ground state con-
formers in which the stereochemistry of the biaryl axes was the
same.
Racemisation of the borates (P*,P*,R*)-6 and (P*,P*,R*)-7
requires all three of the stereogenic axes to be epimerised. We
argued that since the second most stable diastereoisomeric
borates [(P*,M*)-6 and 7] were much less stable than
(P*,P*,R*)-6 and 7, epimersation of either of the biaryl axes—
a process which is necessary for racemisation—was likely to be
exceptionally difficult. Therefore, we reasoned that the borates
(P*,P*,R*)-6, (P*,P*,R*)-7 and (P*,P*)-8 would be more
configurationally stable than simple borates such as 2 and 3.
Scheme 2
Synthesis of chiral borate salts
ortho-Lithiation of the catechol derivative 9, and quench with
acetone gave the alcohol 10 (Scheme 2).10 Hydrogenation of the
alcohol 10, using a 5% palladium on charcoal catalyst, gave the
diether 11 in 92% yield, which was demethylated by refluxing in
45% hydrogen bromide in acetic acid to give the catechol 12 in
61% yield.
In a similar vein, the biphenol 13 was protected as the diether
14 by reaction of its sodium dianion with methyl iodide
(Scheme 3). Dilithiation of 14,11 using n-butyllithium and
TMEDA in refluxing ether, and reaction with methyl iodide,
gave the biphenol derivative 15 in 59% yield. Radical bromin-
ation of 15, using NBS in carbon tetrachloride with AIBN as a
radical initiator, gave the bromide 16 in 96% yield.
The Würtz coupling12 of the bromide 16 was attempted using
half an equivalent of magnesium in refluxing THF (Scheme 4);
Scheme 3
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J. Chem. Soc., Perkin Trans. 1, 2000, 4403–4408

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Scheme 4
Scheme 5
to our surprise, the required tetraphenol derivative 17 was
obtained in a rather low 30% yield along with the biphenol
derivative 15 (31% yield) and the hexaphenol derivative 18 (15%
yield). We have interpreted this result in terms of the mechan-
ism outlined in Scheme 5. Equilibration of the Grignard
reagent 20 and the initial product 17, by deprotonation of a
benzylic position of 17, would give the diether 15 and the
Grignard reagent 21; coupling of 21 with the bromide 16 would
give the observed hexaphenol derivative 18. The tetraether 17
was demethylated with 45% hydrogen bromide in acetic acid to
give the tetraphenol 19. The structure of the tetraphenol 19 was
confirmed by the observation of an HMBC crosspeak between
HA and CB (see Scheme 4).
The dimethylammonium borate salts 22 and 23 were prepared
from the catechol 12 and the tetraphenol 19 respectively
by treatment with boric acid in DMF at 125 ЊC for 16 h
(Scheme 6). The borate 22 was obtained in 41% yield after two
recrystallisations from ether and the borate 23 was obtained in
>98% yield after removal of the DMF by distillation and
recrystallisation from chloroform. An X-ray crystal structure of
the borate 22 was obtained (Fig. 3).
Scheme 6
Determination of the free energy of activation for the
racemisation of the dimethylammonium borates 22 and 23
Racemisation of 3 and 24, the anions of the salts 22 and 23,
would result in chemical exchange of diastereotopic groups
(Schemes 1 and 7), a property which should allow config-
urational stability to be probed by dynamic NMR spectroscopy.
The anions 3 and 24 are both C2-symmetric; racemisation of 3
would result in chemical exchange of MeA and MeB (Scheme 1)
and racemisation of 24 would result in exchange of HA and HB
(Scheme 7).‡
‡ For a borate similar to 3 with diastereotopic protons on the side
chains of the two catechol units, see ref. 13.
Fig. 3 X-Ray crystal structure of the borate 22.
J. Chem. Soc., Perkin Trans. 1, 2000, 4403–4408
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Summary
The configurational stability of the dimethylammonium borate
salts 22 and 23 were investigated in DMSO-d6 by variable
temperature 1H NMR spectroscopy; the free energies of
racemisation were found to be 85 and 79 kJ molϪ1 respectively.
Mechanisms for the racemisation processes were proposed
which involved protonation of one of the ligands attached to
boron, followed by ring-opening to give a trigonal borate. The
configurational instability of 23 was attributed to ring strain in
its ground state.
Experimental
All solvents were distilled before use. THF and Et2O were
freshly distilled from lithium aluminium hydride whilst CH2Cl2
and toluene were freshly distilled from calcium hydride. Ether
refers to diethyl ether and petrol refers to petroleum spirit
(bp 40–60 ЊC) unless otherwise stated. Diisopropylamine was
purified prior to use by distillation from calcium hydride.
Solvents were removed under reduced pressure using a Büchi
rotary evaporator at water aspirator pressure. Triphenylmeth-
ane was used as indicator for THF. n-Butyllithium was titrated
against diphenylacetic acid before use. All non-aqueous reac-
tions were carried out under argon using oven-dried glassware.
Flash column chromatography was carried out using silica
(35–70 µm particles) according to the method of Still, Kahn
and Mitra.16 Thin layer chromatography was carried out on
commercially available pre-coated plates (Merck silica Kieselgel
60F254). Unless otherwise stated, Rf values were measured with
ethyl acetate as eluant. Proton and carbon NMR spectra were
recorded on a Bruker WM 250, DPX 300, WM 400 or DRX
500 Fourier transform spectrometers using an internal deuter-
ium lock. Chemical shifts are quoted in parts per million down-
field of tetramethylsilane and values of coupling constants (J)
are given in Hz. Carbon NMR spectra were recorded with
broad band proton decoupling.
Melting points were determined on a Reichert hot stage
apparatus and are uncorrected. Infrared spectra were recorded
on a Nicolet Avatar 360 FT-IR ESP infrared spectro-
photometer and signals were referenced to the polystyrene 1601
cmϪ1 absorption. Mass spectra were recorded on a VG autospec
mass spectrometer, operating at 70 eV, using both the electron
impact and fast atom bombardment methods of ionisation.
Accurate molecular weights were obtained by peak matching
using perfluorokerosene as a standard. Electron Impact was
used unless Fast Atom Bombardment (ϩFAB) is indicated.
Microanalyses were carried out by the staff of the School of
Chemistry using a Carlo Erba 1106 automatic analysers.
Scheme 7
The borate 22 was dissolved in DMSO-d6, and the coales-
cence of its methyl doublets was observed by variable temper-
ature 400 MHz 1H NMR spectroscopy. The coalescence of the
doublets corresponding to MeA and MeB occurred at 366 K,
indicating that the activation energy for racemisation, ∆G‡, was
85 kJ molϪ1.§ Assuming that ∆G‡ is approximately independent
of temperature, this barrier translates into a half-life of
approximately two minutes at room temperature (293 K). This
process was acid-catalysed: the free energy of racemisation,
∆G‡, was 81 kJ molϪ1 with 16 mM benzoic acid in DMSO-d6.
In a similar vein, the configurational stability of the borate 23
1
was determined by variable temperature 500 MHz H NMR
spectroscopy. In this case coalescence temperature was 383 K,
which indicated that the activation energy for racemisation,
∆G‡, was 79 kJ molϪ1. The borate 23, with its two-carbon chain
linking the biphenol ligands, was, in fact, less configurationally
stable than the borate 22.
Proposed mechanisms of racemisation of the borates 22 and 23
We propose that 22 racemises via the following acid-catalysed
process. Protonation of 3, the anion of 22, and ring-opening
would give the achiral trigonal borate 26 (Scheme 8). The
phenol of 26 can re-attack either face of the boron atom,
leading to either enantiomer of 3.
In the design phase of the borate 23, we argued that racemis-
ation would be difficult because epimerisation of either biaryl
axis would give a highly strained diastereoisomer. However, the
free energy of racemisation of 23 was found to be less than that
of the borate 22. The mechanism which we propose to account
for this configurational instability is shown in Scheme 9. Proton-
ation of 24, the anion of 23, and ring-opening, would give the
borate 28 in which two of 24’s chiral axes have been destroyed:
the boron atom of 28 is trigonal and the transition state for
rotation about the exocyclic biaryl bond only requires the two
ortho substituents to slip past a hydrogen atom.15 Epimerisation
of the remaining chiral axis of 28 (→ent-28) simply requires its
seven-membered ring to flip. The rather low barrier to racemis-
ation of 23, therefore, stems from the strained ring system
which destabilises its ground state.
Computational details
Starting structures for the two isomers were first minimised
at the AM1 level. These structures were then optimised at the
HF/6-31G* and B3LYP/6-31G* level. All calculations used
the Gaussian software7 running on a dual processor Pentium
III-500 machine running redhat 6.0 linux. Default convergence
criteria were used in all minimisations.
Dimethylammonium bis[3-isopropylbenzene-1,2-diolato(2Ϫ)-
O,OЈ]borate 22
Boric acid (66 mg, 1.07 mmol) was added to a stirred solution
of 3-isopropylbenzene-1,2-diol10 12 (320 mg, 2.13 mmol) in dry
DMF (5 ml). The reaction was heated at 125 ЊC for 48 h. The
DMF was removed by evaporation under reduced pressure,
ether (5 ml) was added, the solution was placed in the freezer
for 12 h, filtered and recrystallised from ether to give the borate
22 (154 mg, 43%) as colourless cubes, mp >250 ЊC (Found: C,
67.2; H, 8.00; N, 3.9; C20H28BNO4 requires C, 67.2; H, 7.90; N,
3.9%); νmax/cmϪ1 (Nujol) 2925, 1648, 1557 and 1460; δH (300
§ The relationship ∆G‡ = RTc{23 ϩ ln[Tc√(∆ν2 ϩ 6J2)]} was used to
determine the activation energy, ∆G‡, for a symmetrical coupled
system, from the coalescence temperature, Tc (K), the gas constant,
R, the frequency separation of the peaks, ∆ν (Hz) and the coupling
constant between the nuclei, J (Hz).14
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J. Chem. Soc., Perkin Trans. 1, 2000, 4403–4408

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Scheme 8
Scheme 9
MHz, DMSO-d6) 6.55 (4 H, m), 6.47 (2 H, dd, J 8.5 and 2.0
stirred at 0 ЊC for 1 h, warmed to 40 ЊC and refluxed for 16 h.
Hz), 3.55 (2 H, Me2NH2), 3.12 (2 H, sept, J 7.1 Hz, CHMe2),
2.40 (6H, s, Me2NH2), 1.28 (6 H, d, J 7.1, CHMeAMeB) and
1.27 (6 H, d, J 7.1, CHMeAMeB).
Single crystals of 22 were obtained by slow evaporation from
ether–dichloromethane.
The reaction mixture was cooled to 0 ЊC, methyl iodide (1.55
ml, 25 mmol) added dropwise, stirred for 1.5 h, dilute aqueous
hydrochloric acid (4 M, 20 ml) added and stirred for 30 min.
The layers were separated and the organic layer was washed
with dilute aqueous hydrochloric acid (0.5 M, 100 ml), satur-
ated sodium bicarbonate solution (100 ml) and brine (80 ml),
dried (Na2SO4), filtered and evaporated under reduced pressure
to give a crude product which was purified by flash chrom-
atography, eluting with 1:7 ethyl acetate–petrol to give the
diether 15 (800 mg, 77%) as a yellow oil, Rf 0.30 (12% EtOAc in
petrol) (Found: C, 79.2; H, 7.60; C16H18O2 requires C, 79.3; H,
7.50%); νmax/cmϪ1 (Nujol) 2927, 2855, 1460, 1413, 1255, 1223
and 1170; δH (300 MHz, CDCl3) 7.19 (4 H, m), 7.04 (2 H, t,
J 7.4 Hz, 5-H and 5Ј-H), 3.40 (6 H, s, Me) and 2.34 (6 H, s, Me);
δC (75 MHz, CDCl3) 156.6, 132.6, 131.6, 130.8, 129.5, 123.8,
60.4 and 16.8; m/z (EI) 242 (100%, Mϩ) and 212 (30).
Crystal structure determination of 22
Molecular formula C20H28BNO4 (Mr = 357.24), orthorhombic,
a = 12.9858(4), b = 11.7576(3), c = 25.6292(8) Å, V = 3913.1(2)
Å3, T = 180 K, space group Pbca, Z = 8, µ(Cu-Kα) = 0.664
mmϪ1, 5132 reflections collected, 3170 unique (merging with
R = 0.099) and all were retained in calculations. Refinement,
based on F2 (SHELXL93) converged at rR2 = 0.1098 for all
data, R1 = 0.0415 for relections with F2 > 2.0σ(F2). CCDC
reference number 207/489. See http://www.rsc.org/suppdata/p1/
b0/b005954o/ for crystallographic files in .cif format.
3-Bromomethyl-2,2Ј-dimethoxy-3Ј-methylbiphenyl 16
2,2Ј-Dimethoxy-3,3Ј-dimethylbiphenyl 15
A solution of the diether 15 (1.35 g, 5.6 mmol), N-bromo-
succinimide (1.98 g, 11 mmol) and AIBN (15 mg) in carbon
tetrachloride (80 ml) was refluxed for 2 days. The reaction was
filtered and evaporated under reduced pressure to give a crude
product which was redissolved in dichloromethane (100 ml),
Butyllithium (1.6 M solution in hexanes, 12.5 ml, 20.0 mmol)
was added dropwise to a stirred solution of the diether11 14 (1.0
g, 4.6 mmol) and N,N,NЈ,NЈ-tetramethylethylenediamine (3.4
ml, 23.0 mmol) in dry ether (60 ml) at 0 ЊC. The reaction was
J. Chem. Soc., Perkin Trans. 1, 2000, 4403–4408
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washed with saturated aqueous sodium bicarbonate solution
(20 ml) and water (20 ml), dried (MgSO4), filtered and evap-
orated under reduced pressure to give a crude product which
was purified by flash chromatography, eluting with 1:7→1:4
ethyl acetate–petrol to give the bromide 16 (2.12 g, 96%) as a
yellow oil, Rf 0.30 (12% EtOAc in petrol) (Found: Mϩ Ϫ H,
319.0340; C16H17BrO2 requires M Ϫ H, 319.0342); νmax/cmϪ1
(Nujol) 2927, 2855, 1460, 1233 and 1008; δH (300 MHz, CDCl3)
7.43–7.04 (6H, m), 4.65 (2 H, d, J 0.4 Hz, CH2Br), 3.49 (3H, s,
OMe), 3.40 (3H, s, OMe) and 2.35 (3 H, d, J 0.7 Hz, Me); m/z
(EI) 319 (100%, Mϩ Ϫ H), 239 (35), 225 (50) and 209 (45).
Dimethylammonium 3,3Љ-ethylenebis(3Ј-methylbiphenyl-2,2Ј-
diolato(2Ϫ)-O,OЈ)borate 23
The tetraphenol 19 (55 mg, 0.12 mmol) was heated in DMF
(1 ml) at 125 ЊC for 48 h. The DMF was removed by evapor-
ation under reduced pressure; recrystallisation from chloroform
gave the borate 23 (62 mg, >98%) as brown plates, mp >250 ЊC,
(Found: C, 74.9; H, 6.00; N, 2.8. C30H30BNO4 requires C, 75.2;
H, 6.30; N, 2.9%); νmax/cmϪ1 (Nujol) 2925, 1645, 1557, 1461,
1377 and 1149; δH (300 MHz, DMSO-d6) 8.23 (2 H, br s,
Me2NH2), 7.28 (2 H, dd, J 7.6 and 1.6), 7.14 (4 H, m), 7.12 (2 H,
dd, J 7.6 and 1.7), 6.94 (2 H, t, J 7.4), 6.90 (2 H, t, J 7.4), 3.41
(6 H, Me2NH2), 2.94 (2 H, d, J 10 Hz, CHAHBCHAЈHBЈ), 2.84
(2 H, d, J 10 Hz, CHAHBCHAЈHBЈ) and 2.63 (6 H, s, Me); δB
(80 MHz, DMSO-d6) 9.1; m/z (EI) 434 (MHϩ, 30%), 322 (35)
and 149 (100).
1,2-Bis(2,2Ј-dimethoxy-3Ј-methylbiphenyl-3-yl)ethane 17
The bromide 16 was dried by azeotropic removal from dry tolu-
ene (3 × 50 ml) and dissolved in dry THF (50 ml). Magnesium
turnings (65 mg, 2.7 mmol) were added and the reaction was
stirred for 30 min and then refluxed for 16 h. The solvent was
removed by evacuation under reduced pressure and the residue
was dissolved in dichloromethane (100 ml), washed with water
(50 ml), dried (MgSO4), filtered and evaporated under reduced
pressure to give a crude product which was purified by flash
chromatography, eluting with 1:9 ethyl acetate–petrol to give
1,2-bis(2,2Ј-dimethoxy-3Ј-methylbiphenyl-3-ylethane 17 (390
mg, 30%) as a yellow oil, Rf 0.25 (10% EtOAc in petrol)
Acknowledgements
Dr Mark Thornton-Pett is thanked for determining the crystal
structure of the salt 22. We thank GlaxoWellcome for support-
ing an undergraduate research bursary (to B. W.), AstraZeneca
and Pfizer for strategic research funding and EPSRC for a
grant (GR/M68428). Dr Martin Jones, Dr Julie Fisher and
John Baker are thanked for useful discussions.
(Found: Mϩ, 482.2456; C32H34BrO4 requires M, 482.2456); νmax
/
cmϪ1 (Nujol) 2926, 2855, 1460, 1255, 1223, 1011 and 906; δH
(500 MHz, CDCl3) 7.22–7.06 (12 H, m), 3.41 (6 H, s, OMe),
3.40 (6 H, OMe), 3.05 (4 H, CH2CH2) and 2.36 (6 H, Me);
δC (125 MHz, CDCl3) 156.1, 135.1, 132.3, 132.1, 131.3, 129.7,
129.6, 129.5, 129.1, 123.5, 123.4, 60.7, 60.0, 31.5 and 16.4 (one
peak missing); m/z (EI) 482 (55%, Mϩ) and 241 (100). The
HMBC spectrum showed a crosspeak between CH2CH2 and
CH2CH2.
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Also obtained was the diether 15 (0.41 g, 31%), spectroscopi-
cally identical to that obtained previously.
Also obtained was 2,2Ј-dimethoxy-3,3Ј-bis[2-(2,2Ј-dimeth-
oxy-3Ј-methylbiphenyl-3-yl)ethyl]biphenyl 18 (200 mg, 15%) as
a yellow oil, Rf 0.20 (10% EtOAc in petrol) (Found: MHϩ,
723.3705; C48H51O6 requires MH, 723.3686); νmax/cmϪ1 (Nujol)
2935, 1461, 1255, 1223, 1011 and 906; δH (300 MHz, CDCl3)
7.22–6.98 (18 H, m), 3.42 (12 H, s, OMe), 3.41 (6 H, OMe), 3.05
(8 H, CH2CH2) and 2.35 (6 H, Me); m/z (EI) 722 (90%, Mϩ),
481 (50) and 241 (100); (ESϩ) 740 (100%, MNH4ϩ) and 723 (15,
MHϩ).
1,2-Bis(2,2Ј-dihydroxy-3Ј-methylbiphenyl-3-yl)ethane 19
The tetraether 17 (147 mg, 0.3 mmol) was dissolved in 48%
aqueous hydrogen bromide (4 ml) and acetic acid (4 ml) and
refluxed for 48 h. The reaction was cautiously quenched with
saturated sodium carbonate solution (20 ml), extracted with
ether (2 × 20 ml), dried (MgSO4), filtered and evaporated under
reduced pressure to give a crude product which was purified by
flash chromatography, eluting with 1:3 ethyl acetate–petrol to
give the tetraphenol 19 (55 mg, 38%) as a yellow oil, Rf 0.25
(25% EtOAc in petrol) (Found: MHϩ, 427.1916; C28H27O4
requires MH, 427.1909); νmax/cmϪ1 (Nujol) 3600, 3465, 2984,
2909, 1446, 1373, 1238 and 1045; δH (300 MHz, CDCl3) 7.2–7.1
(6H, m), 7.10 (2 H, d, J 7.5 Hz), 6.97 (2 H, t, J 7.5 Hz), 6.93
(2 H, t, J 7.5 Hz), 6.23 (2 H, br s, OH), 5.60 (2 H, br s, OH), 3.00
(4 H, s, CH2) and 2.31 (6 H, s, Me); δC (75 MHz, CDCl3) 151.7,
151.5, 131.6, 130.8, 130.0, 129.0, 128.8, 126.1, 124.1, 123.5,
121.6, 121.3, 31.9 and 16.7; m/z (ESϩ) 444 (45%, MNH4ϩ), 427
(100, MHϩ). The HMBC spectrum showed a crosspeak
between CH2CH2 and CH2CH2.
13 Y. Kobuke, Y. Sumida, M. Hayashi and H. Ogoshi, Angew. Chem.,
Int. Ed. Engl., 1991, 30, 1496.
—
14 M. Oki, Applications of Dynamic NMR Spectroscopy to Organic
Chemistry, VCH, Weinheim, 1985, pp. 3–11.
15 F. H. Westheimer, in Steric Effects in Organic Chemistry, ed. M. S.
Newman, Wiley, New York, 1956, ch. 12.
16 W. C. Still, M. Kahn and A. Mitra, J. Org. Chem., 1978, 43,
2923.
4408
J. Chem. Soc., Perkin Trans. 1, 2000, 4403–4408