2,2′-Disubstituted F12binaphthyl derivatives: Stannanes, boranes, and (R)-F12BINOL

Article abstract of DOI:10.1039/b605716k

2,2′-Distannyl derivatives of dodecafluorobinaphthalene are converted to a novel D₂ symmetric borane dimer, which can be oxidized to (±)-F₁₂BINOL; resolution using (S)-acetoxypropanoyl chloride affords enantiopure material. The Royal

Full text of DOI:10.1039/b605716k

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2,29-Disubstituted F12binaphthyl derivatives: stannanes, boranes, and
(R)-F12BINOL{
Darryl J. Morrison,a Susanne D. Riegel,a Warren E. Piers,*a Masood Parveza and Robert McDonaldb
Received (in Berkeley, CA, USA) 24th April 2006, Accepted 2nd June 2006
First published as an Advance Article on the web 19th June 2006
DOI: 10.1039/b605716k
Our interest in perfluoraryl boranes,7 coupled with the proven
2,29-Distannyl derivatives of dodecafluorobinaphthalene are
track record of the 1,19-binaphthyl scaffold as a chiral auxiliary,
led us to explore routes to 2,29 functionalized F12binaphthyl
derivatives for various applications in catalysis. The starting point
for entry into this family of binaphthyl compounds is the known
1-bromo-3,4,5,6,7,8-hexafluoronaphthalene8 2-Br shown in
Scheme 1. Modification of literature methods9 led to improved
yields of this compound (39%) in multigram quantities over three
steps from commercially available octafluoronaphthalene. Copper
mediated coupling of this substrate with its in situ generated
arylcopper coupling partner 2-Cu gave the F12binaphthyl deriva-
tive 3 in 86% yield.{
converted to a novel D2 symmetric borane dimer, which can be
oxidized to (¡)-F12BINOL; resolution using (S)-acetoxypro-
panoyl chloride affords enantiopure material.
The axially asymmetric 1,19-binaphthyl framework is one of the
most effective and common chiral auxiliaries in chemistry. Donor
functionalized, chiral chelating ligands such as 1,19-bi-2-naphthol
(BINOL)1
and
2,29-bis(diphenylphosphino)-1,19-binaphthyl
(BINAP)2 are powerful catalyst modifiers for a myriad of
asymmetric transformations. When Lewis acid (LA) functions
are installed into the 2,29 positions, effective chiral Lewis acids
result.3
Two strategies for functionalization of the 2,29 positions of 3
were considered: 1) electrophilic halogenation to give 2,29 dihalides
as precursors for metallation reactions, and 2) direct deprotonation
with strong bases. Unfortunately, all attempts at selective
halogenation reactions were unsuccessful. Halogenating reagents
such as Br2/Fe, NBS/CCl4, NBS/BF3?H2O,10 and I(py)2BF4/
TfOH11 resulted in either no reaction and/or partial decomposition
at room temperature. Unselective bromination occurred at
elevated temperatures with Br2/Fe and with stronger brominating
agents (20% AlBr3/Br2/SO3/H2SO4).12 Since the hydrogen atoms of
the 2,29 positions of 3 were expected to be substantially acidic due
to the inductive electron-withdrawing effects of the fluorine atoms,
attention was turned to deprotonative approaches. Use of alkylli-
thium reagents led to rapid reduction of the fluoroaryl substrate
and non-regioselective loss of LiF. Selective functionalization of
Modifications to the basic 1,19-binaphthyl framework generally
involve various substitution patterns around the naphthyl rings.1a
While this inevitably perturbs the electronic environment to a
degree, it is mainly the steric effect of substitution that influences
the ligand’s behavior in catalytic applications. In some cases,
however, increases in activities and enantioselectivities of LA
catalysts derived from partially halogenated or fluoroalkylated
BINOLs have been attributed directly to the electronic perturba-
tion by these electronegative groups.4 Yudin and co-workers
introduced the most substantial electronic modification with the
synthesis, resolution5 and applications6 of F8BINOL, in which the
back aryl rings (the 5,59, 6,69, 7,79, and 8,89 positions) are
fluorinated. While sterically very similar to the unfluorinated
BINOL, partial fluorination results in a more acidic BINOL which
imparts greater Lewis acidity to its metal complexes. We now
report the synthesis and resolution of 2,29-dihydroxy-3,39,4,49,5,59,
6,69,7,79,8,89-dodecafluoro-1,19-binaphthyl (F12BINOL, 1) via
selective manipulation of the 2,29 positions of
a novel
F12binaphthyl scaffold.
aDepartment of Chemistry, University of Calgary, 2500 University Drive
NW, Calgary, Alberta, Canada T2N 4M5. E-mail: wpiers@ucalgary.ca;
Tel: ++1 403 2205746
bX-ray Structure Laboratory, Department of Chemistry, University of
Alberta, Edmonton, Alberta, Canada T6G 2G2
{ Electronic supplementary information (ESI) available: synthetic proce-
dures and X-ray data for 1?(Et2O)2, 3, 5 and (Rax,S,S)-6 as CIF files. See
DOI: 10.1039/b605716k
Scheme 1
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the 2,29 positions of 3 was achieved by deprotonation using the
poorly reducing, non-nucleophilic base lithium 2,2,6,6-tetramethyl-
piperidide (LTMP) at low temperature and in the presence of a
quenching electrophile. Attempts to deprotonate 3 with LTMP in
the absence of electrophile also led to complex mixtures; thus, it is
necessary to use electrophiles that are compatible with LTMP at
278 uC.13 Tributyltin chloride is one such electrophile, and the
2,29-distannane 4-Bu is available in high yield using this procedure.
Preliminary work has shown that chlorosilanes and -phosphines
are also suitable electrophiles for this chemistry; unfortunately, a
compatible boron-based electrophile was not identified.
Compound 4-Bu, however, can be used to install boron in the
2,29 positions. Treatment with an excess of BBr3 at high
temperatures led to dark solutions from which large, yellow
crystals deposited over the course of several hours. Although the
11B (dB 49.2 ppm)14 and 19F NMR spectroscopic signatures of this
compound were consistent with the C2-symmetric structure of the
anticipated 2,29-bis-dibromoboryl derivative, its low solubility was
not. X-Ray structural analysis of yellow block crystals grown by
cooling a toluene solution to 230 uC revealed the compound to be
the striking D2 symmetric homochiral bromoborane dimer 5
shown in Fig. 1.{ At room temperature, this reaction leads to the
ditin derivative 4-Br; boron–tin transmetallation requires higher
temperature and no other intermediates are observed. Similar
condensations have been observed in other perfluoroaryl bis-
dibromoboryl systems.15 In 5, the boron centers are planar with an
angle of 79.0u between the trigonal planes, suggesting that the two
will behave as non-cooperating Lewis acid centers. Nonetheless, 5
is a promising asymmetric LA and preliminary experiments
suggest that it can be converted to its more hydrolytically stable
bis-C6F5 derivative by treatment with [Cu(C6F5)]4.14
Fig. 2 Thermal ellipsoid diagram of the molecular structure of racemic
1?(Et2O)2 (S enantiomer shown). Selected bond and non-bonded distances
˚
(A): O(1)–C(12), 1.344(2); O(2)–C(22), 1.341(2); O(1)–O(1S), 2.686(2);
…
O(2)–O(2S), 2.691(2). Selected bond angles (u): O(1)–H(1O) O(1S), 157.9;
…
O(2)–H(2O) O(2S), 155.7. Selected dihedral angle (u): C(12)–C(11)–
C(21)–C(22), 80.5(2).
furnishes, after work-up and purification, the bis-diethyl ether
adduct 1?(Et2O)2 which was characterized by X-ray crystal-
lography (Fig. 2). The gross molecular and extended structural
features of 1?(Et2O)2 are analogous to those in the recently
reported crystal structure of BINOL?(Et2O)2.16 However, slightly
…
shorter O Osolvent distances [O(1)–O(1S), 2.686(2); and O(2)–
˚
O(2S), 2.691(2) A] in 1?(Et2O)2 compared to an average
…
˚
Osolvent distance of 2.74 A in BINOL?(Et2O)2 are perhaps
O
indicative of more strongly bound Et2O molecules to the
presumably more acidic fluorobinaphthol 1. Solvent free 1 can
be obtained simply by heating at 50 uC under a dynamic vacuum
to effect removal of the coordinated Et2O.
As a borane, however, 5 can of course be oxidized to the
corresponding phenol, in this case F12BINOL 1. As seen in
Scheme 1, treatment of a THF solution of 5 with 30% H2O2
Acylation of 1 with (S)-acetoxypropanoyl chloride17 followed by
fractional crystallization gave the diester (Rax,S,S)-6 in > 96% de as
measured by 1H and 19F NMR spectroscopy (Scheme 2). The
molecular structure of (Rax,S,S)-6 was determined crystallographi-
cally;{ due to the low scattering power of the light atoms in the
structure, the absolute configuration of the compound could not
be assigned based on the X-ray data alone, but the stereochemis-
tries of C(2) and C(7) were known from the precursor compound.
Saponification with LiOH in THF/H2O17b gave (R)-1 without any
degradation in optical purity, as evidenced by conversion of (R)-1
into its methyl ether (R)-7 which was > 98% ee as determined by
chiral HPLC analysis.
In summary, we have demonstrated the synthesis of a family
of 2,29-disubstituted F12binaphthyl derivatives including the
Fig. 1 Thermal ellipsoid diagram of the molecular structure of 5 (R,R
enantiomer shown); the two toluene molecules of solvation are omitted for
˚
clarity. Selected bond distances (A): B(1)–C(1), 1.551(9); B(1)–C(40),
1.574(9); B(1)–Br(1), 1.902(7). Selected bond angles (u): C(1)–B(1)–C(40),
123.0(5); C(1)–B(1)–Br(1), 118.3(5); C(40)–B(1)–Br(1), 118.8(5). Selected
dihedral angle (u): C(1)–C(10)–C(11)–C(20), 96.7(7).
Scheme 2
2876 | Chem. Commun., 2006, 2875–2877
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perfluorobinaphthol (R)-1. The key synthetic step involves the
deprotonation of the parent F12binaphthyl 3 with the strong base
LTMP in the presence of a tin electrophile to form the distannane
4-Bu. The introduction of (R)-1 for asymmetric catalysis applica-
tions where high Lewis or Brønsted acidity is desired18 is a
significant new tool that we are currently exploring. Extension of
the methodology to the synthesis of other F12binaphthyl
derivatives, for example diphosphines, is also promising.
Funding for this work was provided by Merck Frosst Canada
Inc. and NSERC of Canada. DJM is an NSERC CGS-D scholar.
WEP thanks Prof. Vic Snieckus (Queen’s University), Prof. Joseph
Sadighi (MIT), and Prof. Brian Keay (University of Calgary) for
helpful synthetic advice.
7 W. E. Piers, Adv. Organomet. Chem., 2005, 52, 1.
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{ Crystal data for 1?(Et2O)2: C28H22F12O4, M = 650.46, T = 193 K, space
group P21/n (No. 14), monoclinic, a = 10.9881(9), b = 20.1333(16), c =
3
˚
12.9050(10) s, b = 103.3641(14)u, V = 2777.6(4) A , Z = 4, Dc
=
1.555 g cm23, m(Mo-Ka) = 0.156 mm21, 21180 reflections measured, 5687
unique (Rint = 0.0448) which were used in all calculations. The final R(F)
was 0.0880.
Crystal data for 3: C20H2F12, M = 470.22, T = 173(2) K, space group
Pbca, orthorhombic, a = 13.1128(2), b = 23.9042(5), c = 10.1940(2) s, V =
3
3295.3(1) A , Z = 8, Dc = 1.955 g cm23, m(Mo-Ka) = 0.213 mm21, 8107
reflections measured, 4604 unique (Rint = 0.0407) which were used in all
calculations. The final R(F) was 0.0649.
˚
Crystal data for 5: C54H16B2Br2F24, M = 1302.11, T = 173(2) K, space
group C2/c, monoclinic, a = 35.880(6), b = 12.916(9), c = 25.835(10) s, b =
3
126.545(16)u, V = 9619(8) A , Z = 8, Dc = 1.798 g cm23, m(Mo-Ka) =
˚
1.820 mm21, 26643 reflections measured, 5869 unique (Rint = 0.0772) which
were used in all calculations. The final R(F) was 0.1143.
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P21, monoclinic, a = 10.1781(12), b = 12.7075(15), c = 11.4937(14) s, b =
3
99.4442(17)u, V = 1466.4(3) A , Z = 2, Dc = 1.654 g cm23, m(Mo-Ka) =
˚
0.168 mm21, 11649 reflections measured, 5994 unique (Rint = 0.0316) which
were used in all calculations. The final R(F) was 0.0378.
CCDC 299722–299725. For crystallographic data in CIF or other
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