Facile syntheses of dissymmetric ferrocene-functionalized Lewis acids and acid-base pairs

Article abstract of DOI:10.1039/b917304h

A facile synthetic approach is reported for the synthesis of dissymmetric 1,2-ferrocenediyl Lewis acids and mixed acid-base pairs including the first example of a 1-phosphino-2-borylferrocene; the use of non-racemic electrophiles allows for the isolation

Full text of DOI:10.1039/b917304h

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Facile syntheses of dissymmetric ferrocene-functionalized Lewis acids and
acid–base pairsw
Ian R. Morgan,ab Angela Di Paolo,a Dragoslav Vidovic,a Ian A. Fallisb and Simon Aldridge*a
Received (in Cambridge, UK) 21st August 2009, Accepted 26th October 2009
First published as an Advance Article on the web 4th November 2009
DOI: 10.1039/b917304h
A facile synthetic approach is reported for the synthesis of
dissymmetric 1,2-ferrocenediyl Lewis acids and mixed acid–base
pairs including the first example of a 1-phosphino-2-borylferrocene;
the use of non-racemic electrophiles allows for the isolation of
single diastereomer products.
1,2-fc(BMes2)[S(O)tol-p] (5) is outlined in Scheme 1. The
BMes2 function (Mes = 2,4,6-Me3C6H2) was chosen as the
model system with which to probe synthetic methodology, as
boranes of the type (aryl)BMes2 are sufficiently bulky to
prevent coordination of phosphine donors, and tend to be
conveniently air and moisture stable. Moreover, although such
prototype systems are unlikely to be Lewis acidic enough to
heterolytically cleave E–H bonds as they stand,2f oxidation to give
the corresponding ferrocenium species is known to lead to marked
enhancement in the Lewis acidity of ferrocenylboranes.10b,13
Synthetically, the key step involves isomerization of
1,10-fc(Li)Br to its 1,2 isomer in the presence of tetramethyl-
piperidine (tmp);9b the intermediate 1,2-fc(BMes2)Br (2) can
thus be isolated in yields of ca. 76% on a 1–4 g scale from a
one-pot reaction involving 1 and the commercially available
reagents nBuLi, tmp and FBMes2 (see ESIw for synthetic,
spectroscopic and crystallographic details of 2).14
Molecular systems containing non-interacting Lewis acid and
Lewis base components (so-called frustrated Lewis pairs,
FLPs) have attracted considerable recent attention, both from
a structural perspective and due to their activation of E–H
(E = H, B, C, N) bonds.1–4 Such systems typically comprise
Lewis acidic triarylborane and Lewis basic tertiary phosphine/
amine or N-heterocyclic carbene components, which are
sterically encumbered in order to frustrate the formation of
a classical donor–acceptor bond.2–5 An exciting extension of
this approach would be the development of synthetic routes to
dissymmetric FLPs, which have the potential to be exploited in
asymmetric transformations.
2 can subsequently undergo a second lithiation step and
quenching with a range of electrophiles to give 1,2-bifunc-
tional products. In the case of B(OEt)3 and ClPPh2 the
resulting 1,2-bis(boryl) and 1,2-(phosphino)boryl products
(3 and 4) are obtained as racemic mixtures in overall yields
of 65–70% from 1; the use of commercially available
(1R,2S,5R)-(ꢀ)-menthyl(S)-p-toluenesulfinate,6b,15 however,
followed by recrystallization from chloroform–hexanes allows
for the isolation of the related 1,2-(sulfinyl)boryl system 5 as a
single (S,SP) diastereomer. 3–5 have been characterized by
The planar chirality exhibited by 1,2-disubstituted
ferrocenes bearing non-identical substituents has long been
exploited in asymmetric induction.6 Non-racemic systems of
this type have typically been accessed by ortho-lithiation
chemistry directed by a pendant chiral substituent.6 With this
in mind, we were surprised to find that there were no literature
reports of 1-phosphino-2-borylferrocenes,7 despite the fact
that such systems (i) have the ability—in the presence of a
suitable degree of steric shielding—to function as FLPs; and
(ii) are intrinsically dissymmetric. As part of an ongoing
program in borylferrocene chemistry,8 we therefore report a
simple two-step synthesis of compounds of this type from the
readily available precursor 1,10-dibromoferrocene (1).9 In
addition, we demonstrate that the key (racemic) intermediate
1,2-fc(BMes2)Br (2) can be converted into a single diastereomer
Lewis acid–base product by employing a chiral electrophile.
Moreover, this simple synthetic approach is also applicable to
potentially chelating 1,2-diborylferrocenes—systems which
despite their inherent chirality and potential for anion binding
have yet to be realized synthetically.10–12
The synthetic approach utilized in the syntheses of
1,2-fc(BMes2)[B(OH)2] (3), 1,2-fc(BMes2)(PPh2) (4) and
a Inorganic Chemistry Laboratory, South Parks Road, Oxford,
UK OX1 3QR. E-mail: Simon.Aldridge@chem.ox.ac.uk;
Fax: +44 (0)1865 272690; Tel: +44 (0)1865 285201
b Cardiff School of Chemistry, Main Building, Park Place, Cardiff,
UK CF10 3AT. E-mail: fallis@cf.ac.uk; Fax: +44 (0)2920 874030;
Tel: +44 (0)2920 875976
w Electronic supplementary information (ESI) available: Preparative
data for 2; crystallographic data for 2, 4 and 5. CCDC 742657–742659.
For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/b917304h
Scheme 1 Syntheses of bifunctional borane 3, borane–phosphine 4
(both as racemic mixtures) and borane–sulfoxide 5 (as a single
diastereomer). Reagents and conditions: (i) nBuLi, tmp (thf–hexanes),
then FBMes2 (thf), 76%; (ii) nBuLi, (Et2O–hexanes), then B(OEt)3, aq.
work-up, 92%; (iii) nBuLi, tmeda (Et2O–hexanes), then ClPPh2,
90%; (iv) nBuLi, (pentane–thf), then (1R,2S,5R)-(ꢀ)-menthyl(S)-p-
toluenesulfinate, separation of diastereomers by recrystallization from
CHCl3–hexanes, 37% of S,SP-5.
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(1R,2S,5R)-(ꢀ)-menthyl(S)-p-toluenesulfinate, allows for the
isolation of single diastereomer products.
Studies of the reactivity of 1-phosphino-2-borylferrocenes
and ferroceniums in metal-free hydrogenation chemistry and
of 1,2-bifunctional Lewis acids in anion sensing are ongoing
and will be reported in due course. Preliminary results show
that 1,2-diborylferrocenes allow for discrimination between
the environmentally relevant ions fluoride and cyanide on the
basis of the binding modes adopted.
Fig. 1 Crystallographically determined molecular structures of 4 and
S,SP-5; hydrogen atoms omitted for clarity and thermal ellipsoids at
the 30% probability level. Key bond lengths (A) and angles (1): (for 4)
C(2)–B(20) 1.558(4), C(21)–B(20) 1.598(4), C(30)–B(20) 1.584(4),
C(2)–B(20)–C(21) 122.0(2), C(2)–B(20)–C(30) 116.6(2), C(21)–B(20)–C(30)
121.0(2), C(3)–C(2)–B(20) 132.0(2), C(2)–C(3)–P(7) 126.7(2),
C(3)–C(2)–B(20)–C(21) 3.92; (for S,SP-5) B(7)–C(6) 1.564(5),
B(7)–C(8) 1.585(5), B(7)–C(17) 1.589(5), B(7)ꢂ ꢂ ꢂO(27) 3.303,
C(6)–B(7)–C(8) 120.5(3), C(6)–B(7)–C(17) 116.5(3), C(8)–B(7)–C(17)
121.0(3), C(5)–C(6)–B(7) 132.0(3), C(6)–C(5)–S(26) 126.9(3),
C(5)–C(6)–B(7)–C(8) 52.1.
Notes and references
z rac-1,2-fc(BMes2)[B(OH)2] (3): to a solution of 2 (0.330 g,
0.643 mmol) in Et2O (15 mL) at ꢀ78 1C was added nBuLi
(0.402 mL of a 1.6 M solution in hexanes, 0.643 mmol), tmeda
(50 mL, 0.335 mmol) and the reaction mixture stirred for 30 min.
Triethylborate (164 mL, 0.965 mmol) was added and the reaction
mixture warmed to 20 1C over 15 h. The reaction was quenched by
degassed water (20 mL) and stirred for 1 h. After dilution with Et2O
(20 mL), the aqueous layer was discarded, and the organic layer
concentrated in vacuo to yield the title compound as a dark red
microcrystalline solid. Yield: 0.283 g, 92%. Spectroscopic data for
3:1H NMR (300 MHz, CDCl3, 20 1C): dH 6.80 (s, 4H, aromatic CH of
Mes), 5.09 (dd, 1H, J = 2.5, 1.3 Hz, C5H3), 4.94 (t, 1H, J = 2.5 Hz,
C5H3), 4.68 (dd, 1H, J = 2.5, 1.3 Hz, C5H3), 4.20 (s, 5H, Cp), 3.95
(s, 2H, OH), 2.27 (s, 6H, o-CH3 of Mes) and 2.26 (s, 12H, p-CH3 of
Mes). 13C NMR (75 MHz, CDCl3, 20 1C): dC 139.6 (ortho-quaternary
of Mes), 138.2 (para-quaternary of Mes), 128.5 (aromatic CH of Mes),
87.3, 82.2, 76.2 (C5H3), 70.7 (Cp), 24.3 (o-CH3 of Mes), 20.1 (p-CH3 of
Mes), B-bound quaternary carbons not observed. 11B (96 MHz,
CDCl3, 20 1C): dB 78 (BMes2), 32 [B(OH)2]. UV-vis (CH2Cl2):
lmax = 486 nm, e = 640 molꢀ1 cmꢀ1 dm3. For MS analysis, the
sample was pre-mixed with ethylene glycol to make the boronic ester
in situ. MS(EI+) 504 (100%) (M + C2H2)+; exact mass (calc. for
standard spectroscopic and analytical techniques and in the
cases of 4 and 5 by single crystal X-ray diffraction (Fig. 1).wz
These compounds represent the first structurally characterized
cyclopentadienyl systems of any type containing ortho-
disposed boron and group 15/16 substituents.
11B NMR data for 3 and 4 are consistent with the presence
of three-coordinate borane functions {dB 78 (BMes2) and 32
[B(OH)2] for 3, 77 for 4; cf. dB 76 for FcBMes2, and 30 for
FcB(OH)2}8e,16 with the 31P chemical shift for 4 also being
within the range expected for a ‘free’ (i.e. uncoordinated)
triarylphosphine.17 These data for 4, together with a Pꢂ ꢂ ꢂB
separation of 3.634 A, imply an ‘unquenched’ FLP formulation
both in solution and in the solid state. By contrast, related
measurements for 5 give some evidence for an Oꢂ ꢂ ꢂB Lewis
acid–base interaction (albeit a very weak one). Thus, the 11B
resonance (dB 48 ppm) is shifted upfield, although not to the
extent observed for strong s-donors.18 Additionally, there is
marginal pyramidalization of the boron centre [S angles at
(M + C2H2)+
,
Microanalysis: calc. (meas.)
11B 10B isotopomer) 503.2131, (meas.) 503.2136.
70.35 (70.42), H, 6.75 (6.68)%.
C
rac-1,2-fc(BMes2)(PPh2) (4): to a solution of 2 (0.218 g, 0.427 mmol)
in Et2O (15 mL) at ꢀ78 1C was added nBuLi (0.267 mL of a 1.6 M
solution in hexanes, 0.427 mmol) and tmeda (64 mL, 0.427 mmol) and
the reaction mixture stirred for 1 h. Chlorodiphenyl phosphine
(118 mL, 0.641 mmol) was then added and the reaction mixture
warmed to 20 1C over 15 h. Volatiles were removed in vacuo to yield
the crude product which was purified by column chromatography
(EtOAc–hexane) to give 4 as a red solid. Yield: 0.237 g, 90%. Single
crystals were obtained by slow evaporation of a solution in EtOAc.
Spectroscopic data for 4: 1H NMR (400 MHz, CDCl3, 20 1C): dH
7.29–7.39 (m, 5H, Ph), 7.15–7.20 (m, 1H, Ph), 7.05–7.10 (m, 2H, Ph),
6.77–6.82 (m, 2H, Ph), 6.60 (s, 4H, Mes), 4.67 (t of d, 1H, J = 2.6,
1.3 Hz, C5H3), 4.64 (t, 1H, J = 2.5 Hz, C5H3), 4.18 (s, 5H, Cp),
4.07–4.09 (m, 1H, C5H3), 2.22 (s, 6H, p-CH3 of Mes) and 2.16
(br s, 12H, o-CH3 of Mes). 13C NMR (125 MHz, CDCl3, 20 1C): dC
139.5 (ortho-quaternary of Mes), 139.0 (d, J = 72.2 Hz, ipso-C of Ph),
138.9 (d, J = 70.3 Hz, ipso-C of Ph), 136.9 (para-quaternary of Mes),
134.0 (d, J = 20.1 Hz, o-CH of Ph), 133.2 (d, J = 21.4 Hz, o-CH of
Ph), 128.1 (d, J = 5.0 Hz, m-CH of Ph), 127.9 (d, J = 6.3 Hz, m-CH of
Ph), 127.8 (aromatic CH of Mes), 127.4 (p-CH of Ph), 127.3 (p-CH
of Ph), 88.2 (d, J = 17.9 Hz, C5H3), 83.2 (d, J = 4.4 Hz, C5H3), 78.5
(d, J = 4.0 Hz, C5H3), 72.7 (s, C5H3), 70.5 (Cp), 24.5 (o-CH3 of Mes),
20.9 (p-CH3 of Mes), B-bound quaternary carbons not observed. 11B
(96 MHz, CDCl3, 20 1C): dB 77. 31P (126 MHz, CDCl3, 20 1C):
dP ꢀ20.6. UV-vis (CH2Cl2): lmax = 515 nm, e = 814 molꢀ1 cmꢀ1 dm3.
MS(EI+) 616 (10%) M+, 603 (100%) (M ꢀ CH3)+; exact mass
B(7)
=
358.0(5)1, cf. 360.01 for FcBMes2],8e and the
O(27)–B(7) distance (3.303 A) lies well within the sum of the
respective van der Waals radii (3.48 A).19 Moreover, unlike all
other related dimesitylboryl ferrocenes,8e,20 the BC2 plane of
the –BMes2 function in 5 does not lie co-planar with that of
the Cp ring (torsion +C(5)–C(6)–B(7)–C(8) = 52.11, cf. 3.91
for FcBMes2),8e and is in fact canted towards O(27) of the
sulfinyl group. These observations imply a weak Lewis
acid–base interaction in the solid state; a similar conformational
minimum for the alternative S,RP stereoisomer is presumably
disfavoured due to steric buttressing between ferrocenyl and
tolyl moieties. Even for the S,SP diastereomer, however, the
significant lability of this interaction in solution on the NMR
timescale is implied by a single set of sharp mesityl resonances
(calc. for M+ 11B isotopomer) 616.2357, (meas.) 616.2373. S,SP-1,2-
,
1
in the H and 13C NMR spectra.
fc(BMes2)[SO(p-tol)] (5): to a solution of 2 (0.088 g, 0.17 mmol) in
thf (20 mL) at ꢀ78 1C was added dropwise tBuLi (0.2 mL of a 1.7 M
solution in pentane, 0.34 mmol) and the reaction mixture stirred for 1 h.
A solution of (1R,2S,5R)-(ꢀ)-menthyl(S)-p-toluenesulfinate (0.051 g,
0.17 mmol) in thf (10 mL) was added and the reaction mixture warmed
to 20 1C over 15 h. After quenching with distilled water and extraction
into Et2O, the organic fraction was dried over MgSO4. Volatiles
were removed in vacuo and the crude product purified by column
chromatography (hexane to 25% EtOAc–hexane). 1H NMR monitoring
at this point indicated the presence of both S,SP and S,RP
In summary, a facile synthetic approach has been developed
for the synthesis of dissymmetric 1,2-ferrocenediyl Lewis acids
and mixed acid–base systems. Using this approach we have
been successful in synthesizing the first example of a 1-phosphino-
2-borylferrocene FLP and of a potentially chelating 1,2-diboryl-
ferrocene Lewis acid. Moreover, the use of non-racemic
electrophiles, exemplified by commercially available
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diastereomers in solution. Orange crystals S,SP-5 were obtained by
diffusion of hexane in a CHCl3 solution. Yield (of S,SP-5): 0.036 g,
37%. Spectroscopic data for S,SP-5: 1H NMR (300 MHz, C6D6,
20 1C): d 7.07 (AB m, 2H, C6H4), 6.81 (s, 4H, Mes), 6.61 (AB m,
2H, C6H4), 4.58 (d, J = 1.47 Hz, 1H, CH of C5H3), 4.21 (d, J = 1.17 Hz,
1H, CH of C5H3), 4.08 (t, J = 2.2 Hz, 1H, CH of C5H3), 3.97
(s, 5H, CH of C5H5), 2.53 (s, 12H, o-CH3 of Mes), 2.24 (s, overlapping
signals, 9H, p-CH3 of Mes and CH3 of tol). 13C NMR (125 MHz,
C6D6, 20 1C): d 137.6 (ortho-quaternary of Mes), 137.2 (para-quaternary
of Mes), 128.3 (aromatic CH of Mes), 129.8, 124.5 (CH of C6H4), 79.4,
74.1, 72.8 (C5H3), 71.1 (C5H5), 30.1 (o-CH3 of Mes), 21.3 (CH3 of Ar),
20.9 (p-CH3 of Mes), B-bound quaternary carbons not observed. 11B
6 See for example: (a) T. J. Colacot, Chem. Rev., 2003, 103, 3101;
(b) F. Benoit and H. B. Kagan, Adv. Synth. Catal., 2007, 349, 493;
(c) Ferrocenes: Ligands, Materials and Biomolecules, ed.
P. Stepnicka, Wiley, Chichester, 2008; (d) Chiral Ferrocenes in
Asymmetric Catalysis: Synthesis and Application, ed. L. Dai and
X. L. Hou, Wiley, Chichester, 2009.
7 To our knowledge the only spectroscopic report of a 1,2-derivatized
ferrocene system featuring boron and phosphorus functionalities is
that of an oxazaphospholidene oxide: (a) D. Vinci, N. Mateus,
X. Wu, F. Hancock, A. Steiner and J. Xiao, Org. Lett., 2006, 8,
215. 1,2-phosphino(boryl)benzene systems are known; see, for
example: ; (b) S. Bontemps, G. Bouhadir, D. C. Apperley,
P. W. Dyer, K. Miqueu and D. Bourissou, Chem.–Asian J.,
2009, 4, 428 and references therein.
8 (a) C. Bresner, S. Aldridge, I. A. Fallis, C. Jones and L.-L. Ooi,
Angew. Chem., Int. Ed., 2005, 44, 3606; (b) C. Bresner, J. K. Day,
N. D. Coombs, I. A. Fallis, S. Aldridge, S. J. Coles and
M. B. Hursthouse, Dalton Trans., 2006, 3660; (c) J. K. Day,
C. Bresner, I. A. Fallis, L.-L. Ooi, D. J. Watkin, S. J. Coles,
L. Male, M. B. Hursthouse and S. Aldridge, Dalton Trans., 2007,
3486; (d) J. K. Day, C. Bresner, N. D. Coombs, I. A. Fallis, L.-L. Ooi,
R. W. Harrington, W. Clegg and S. Aldridge, Inorg. Chem., 2008, 47,
793; (e) A. E. J. Broomsgrove, D. Addy, C. Bresner, I. A. Fallis,
A. L. Thompson and S. Aldridge, Chem.–Eur. J., 2008, 14, 7525.
9 (a) A. Shafir, M. P. Power, G. D. Whitener and J. Arnold,
Organometallics, 2000, 19, 3978; (b) I. R. Butler, Inorg. Chem.
Commun., 2008, 11, 15.
10 Reports of ferrocenes with boron substituents in the 1 and 2
positions are confined to the elegant diboryldiferrocenes reported
by Jakle, which by virtue of geometric constraints do not offer the
possibility for anion chelation: (a) K. Venkatasubbaiah,
L. N. Zakharov, W. S. Kassel, A. L. Rheingold and F. Jakle,
Angew. Chem., Int. Ed., 2005, 44, 5428; (b) K. Venkatasubbaiah,
I. Nowik, R. H. Herber and F. Jakle, Chem. Commun., 2007, 2154;
(c) K. Venkatasubbaiah, A. Doshi, I. Nowik, R. H. Herber,
A. L. Rheingold and F. Jakle, Chem.–Eur. J., 2008, 14, 44;
(d) T. Pakkirisamy, K. Venkatasubbaiah, W. S. Kassel,
A. L. Rheingold and F. Jakle, Organometallics, 2008, 27, 3056;
(e) K. Venkatasubbaiah, T. Pakkirisamy, R. A. Lalancette and
F. Jakle, Dalton Trans., 2008, 4507.
NMR (96 MHz, C6D6, 20 1C): d (br) 48. UV-vis (CH2Cl2): lmax
474 nm, e = 307 molꢀ1 cmꢀ1 dm3. MS (EI): M+ 572.2 (17%); exact
mass for (calc. for M+ 10B isotopomer) 571.2038, (meas.) 571.2038.
=
,
Microanalysis: calc. (meas.) C 73.41 (73.22), H 6.52 (6.33)%.
1 H. C. Brown, H. I. Schlesinger and S. Z. Caron, J. Am. Chem. Soc.,
1942, 64, 325.
2 Recent papers on FLPs: (a) G. C. Welch, R. R. San Juan,
J. D. Masuda and D. W. Stephan, Science, 2006, 314, 1124;
(b) G. D. Frey, V. Lavallo, B. Donnadieu, W. W. Schoeller and
G. Bertrand, Science, 2007, 316, 439; (c) G. C. Welch, L. Cabrera,
P. A. Chase, E. Hollink, J. D. Masuda, P. Wei and D. W. Stephan,
Dalton Trans., 2007, 3407; (d) J. S. J. MacCahill, G. C. Welch and
D. W. Stephan, Angew. Chem., Int. Ed., 2007, 46, 4968;
(e) P. A. Chase, G. C. Welch, T. Jurca and D. W. Stephan, Angew.
Chem., Int. Ed., 2007, 46, 8050; (f) G. C. Welch and D. W. Stephan,
J. Am. Chem. Soc., 2007, 129, 1880; (g) P. Spies, G. Erker,
G. Kehr, K. Bergander, R. Frohlich, S. Grimme and
D. W. Stephan, Chem. Commun., 2007, 5072; (h) S. J. Geier,
T. M. Gilbert and D. W. Stephan, J. Am. Chem. Soc., 2008, 130,
12632; (i) P. A. Chase, T. Jurca and D. W. Stephan, Chem.
Commun., 2008, 1701; (j) D. Chen and J. Klankermayer, Chem.
Commun., 2008, 2130; (k) M. A. Dureen, A. Lough, T. M. Gilbert
and D. W. Stephan, Chem. Commun., 2008, 4303; (l) H. Wang,
R. Frohlich, G. Kehr and G. Erker, Chem. Commun., 2008, 5966;
(m) V. Sumerin, F. Schulz, M. Nieger, M. Leskela, T. Repo and
B. Rieger, Angew. Chem., Int. Ed., 2008, 47, 6001;
(n) D. Holschumacher, T. Bannenberg, C. G. Hrib, P. G. Jones
and M. Tamm, Angew. Chem., Int. Ed., 2008, 47, 7428;
(o) P. A. Chase and D. W. Stephan, Angew. Chem., Int. Ed.,
2008, 47, 7433; (p) P. Spies, S. Schwendemann, S. Lange, G. Kehr,
R. Frohlich and G. Erker, Angew. Chem., Int. Ed., 2008, 47, 7543;
(q) G. C. Welch, T. Holtrichter-Roessmann and D. W. Stephan,
Inorg. Chem., 2008, 47, 1904; (r) V. Sumerin, F. Schulz, M. Atsumi,
C. Wang, M. Nieger, M. Leskela, T. Repo, P. Pyykko and
B. Rieger, J. Am. Chem. Soc., 2008, 130, 14117; (s) M. Ullrich,
A. J. Lough and D. W. Stephan, J. Am. Chem. Soc., 2009, 131, 52;
(t) S. J. Geier and D. W. Stephan, J. Am. Chem. Soc., 2009, 131,
3476; (u) M. A. Dureen and D. W. Stephan, J. Am. Chem. Soc.,
2009, 131, 8396; (v) E. Otten, R. C. Neu and D. W. Stephan, J. Am.
Chem. Soc., 2009, 131, 9918; (w) M. Ullrich, K. S.-H. Seto,
A. J. Lough and D. W. Stephan, Chem. Commun., 2009, 2335;
(x) G. C. Welch, R. Prieto, M. A. Dureen, A. J. Lough,
O. A. Labeodan, T. Holtrichter-Rossmann and D. W. Stephan,
Dalton Trans., 2009, 1559.
3 For recent quantum chemical studies of FLP reactivity see, for
example: (a) T. A. Rokob, A. Hamza, A. Stirling, T. Soos and
I. Papai, Angew. Chem., Int. Ed., 2008, 47, 2435; (b) T. Privalov,
Eur. J. Inorg. Chem., 2009, 2229; (c) T. A. Rokob, A. Hamza,
A. Stirling and I. Papai, J. Am. Chem. Soc., 2009, 131, 2029;
(d) S. Gao, W. Wu and Y. Mo, J. Phys. Chem., 2009, 113, 8108;
(e) T. A. Rokob, A. Hamza and I. Papai, J. Am. Chem. Soc., 2009,
131, 10701.
4 Recent reviews of FLPs: (a) D. W. Stephan, Org. Biomol. Chem.,
2008, 6, 1535; (b) A. L. Kenward and W. E. Piers, Angew. Chem.,
Int. Ed., 2008, 47, 38; (c) D. W. Stephan, Dalton Trans., 2009, 3129.
5 For reports of phosphino-ferrocenes in FLPs see: (a) D. P. Huber,
G. Kehr, K. Bergander, R. Frohlich, G. Erker, S. Tanino,
Y. Ohki and K. Tatsumi, Organometallics, 2008, 27, 5279;
(b) A. Ramos, A. J. Lough and D. W. Stephan, Chem. Commun.,
2009, 1118.
11 For a chelating ferrocene-based B/Sn Lewis acid see, for example:
R. Boshra, K. Venkatasubbaiah, A. Doshi, R. A. Lalancette,
L. Kakalis and F. Jakle, Inorg. Chem., 2007, 46, 10174.
12 For chelating 1,2-diborylbenzene Lewis acids see, for example:
W. E. Piers, G. J. Irvine and V. C. Williams, Eur. J. Inorg. Chem.,
2000, 2131.
13 C. Dusemund, K. R. A. S. Sandanayake and A. Shinkai, J. Chem.
Soc., Chem. Commun., 1995, 333.
14 Preliminary computational studies indicate that conversion of
1,10-fc(Br)Li to the 1,2 isomer is thermodynamically favourable.
15 For reports of the use of sulfinyl reagents in the enantioselective
synthesis of ferrocene boronic acids via ortho-lithiation chemistry,
see: (a) J. F. Jensen and M. Johannsen, Org. Lett., 2003, 5, 3025;
(b) J. F. Jensen, I. Søtofte, H. Sørensen and M. Johanssen, J. Org.
Chem., 2003, 68, 1258; (c) J. G. Seitzberg, C. Dissing, I. Søtofte,
P.-O. Norrby and M. Johannsen, J. Org. Chem., 2005, 70, 8332;
(d) V. E. Albrow, A. J. Blake, R. Fryatt, C. Wilson and
S. Woodward, Eur. J. Org. Chem., 2006, 2549; (e) A. Novak,
R. Fryatt and S. Woodward, C. R. Chim., 2007, 10, 206.
16 (a) S. McVey, I. G. Morrison and P. L. Pauson, J. Chem. Soc. C,
1967, 1847; (b) C. Bresner, S. Aldridge, I. A. Fallis and L.-L. Ooi,
Acta Crystallogr., Sect. E, 2004, 60, m441.
17 L. D. Quin and J. G. Verkade, Phosphorus-31 NMR Spectral
Properties in Compound Characterization and Structural Analysis,
Wiley-VCH, Chichester, 1994.
18 By means of comparison a 11B shift of ꢀ16.1 ppm has been
measured for
a relatively strongly bound (cyanide) adduct
[FcBMes2CN]ꢀ (DGdiss E 28 kJ molꢀ1), see ref. 8e.
19 J. Emsley, The Elements, Clarendon Press, Oxford, 1989.
20 A. E. J. Broomsgrove, D. Addy, A. Di Paolo, I. R. Morgan,
C. Bresner, V. Chislett, I. A. Fallis, A. L. Thompson, D. Vidovic and
S. Aldridge, Inorg. Chem., 2009, in press (DOI: 10.1021/ic901673u).
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This journal is The Royal Society of Chemistry 2009
7290 | Chem. Commun., 2009, 7288–7290