Ferrocene-based heteronuclear bidentate Lewis acids via highly selective ortho-borylation of 1,1′-bis(trimethylstannyl)ferrocene

Article abstract of DOI:10.1021/om0204890

The borylation of 1,1′-bis(tri-methylstannyl)ferrocene for the preparation of disubstituted ferrocenes was investigated. The reaction provided the route for the formation of a kind of ferrocene-based heteronuclear bidentate Lewis acids. Steric and electronic factors controlled the rate of the reaction. Nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry and elemental analysis were used to characterize the heteronuclear bidentate Lewis acids.

Full text of DOI:10.1021/om0204890

Organometallics 2002, 21, 4169-4181
4169
F er r ocen e-Ba sed Heter on u clea r Bid en ta te Lew is Acid s
via High ly Selective or th o-Bor yla tion of
1,1′-Bis(tr im eth ylsta n n yl)fer r ocen e
J uan A. Gamboa, Anand Sundararaman, Lazaros Kakalis, Alan J . Lough,† and
Frieder J a¨kle*
Department of Chemistry, Rutgers University-Newark, 73 Warren Street,
Newark, New J ersey 07102
Received J une 21, 2002
A new type of ferrocene-based heteronuclear bidentate Lewis acid with both Lewis acidic
centers attached to the same cyclopentadienyl (Cp) ring is reported. Borylation of 1,1′-bis-
(trimethylstannyl)ferrocene (1) with BCl3 in hexanes at -78 °C occurs with high selectivity
at the R-position to give 1-(Me3Sn)-2-(Cl2B)fc (2a -Cl; fc ) ferrocenediyl) as the major product
in ca. 87% spectroscopic yield. Only minor amounts of the two other isomers, the 1-stannyl-
3-boryl- (2b-Cl) and the 1-stannyl-1′-borylferrocene (2c-Cl), are observed. The reaction rate
and selectivity strongly depend on the steric and electronic properties of the electrophile.
With the bulkier electrophile C6F5BCl2 larger amounts of the 1,3-product, 1-(Me3Sn)-3-(C6F5-
ClB)fc (2b-P f; ca. 40%), form in addition to the 1,2-isomer 1-(Me3Sn)-2-(C6F5ClB)fc (2a -P f;
ca. 60%). Use of the weaker electrophile PhBCl2 results in a significantly lower reaction
rate. Treatment of 1 with excess borane in hexanes yields 1-(ClMe2Sn)-2-(Cl2B)fc (3a -Cl)
and 1-(ClMe2Sn)-2-(ClPhB)fc (3a -P h ) as a result of preferred cleavage of an Sn-Me over
the Sn-Cp bond in 2a -Cl and 2a -P h . Facile Sn-Me bond cleavage also occurs during thermal
treatment of 2a -Cl (50 °C) and 2a -P h (80 °C), resulting in the rearranged species 1-(ClMe2-
Sn)-2-(ClMeB)fc (4a -Cl) and 1-(ClMe2Sn)-2-(PhMeB)fc (4a -P h ). The heteronuclear bidentate
1
Lewis acids 3a -Cl, 3a -P h , 4a -Cl, and 4a -P h were fully characterized by H, 13C, 11B, and
119Sn NMR spectroscopy, mass spectrometry, and elemental analysis. Single-crystal X-ray
diffraction analyses of 4a -Cl and 4a -P h reveal a dip angle between the Cp plane and the
Cp-B vector of 13.8(2)° and 12.2(3)°, respectively, indicating a significant iron-boron
interaction. The tin centers display a distorted trigonal bipyramidal arrangement as a result
of short contacts to the boron-bound chloride (4a -Cl) and phenyl (4a -P h ) substituent,
respectively. The 119Sn NMR signals are upfield-shifted by 19-35 ppm relative to FcSnMe2-
Cl, suggesting a similar interaction to be present in solution. The presence of two Lewis
acidic substituents on ferrocene is further reflected in a bathochromic shift and increase in
the intensity of the visible absorption bands relative to ferrocene.
In tr od u ction
Lewis bases through chelating difunctional Lewis acids.
Katz and co-workers showed in 1985 that 1,8-bis-
(dimethylboryl)naphthalene (“hydride sponge”) binds
hydride ions in a chelating fashion.3 At the same time,
Wuest and co-workers found that chloride ions could
bridge two mercury centers in a complex with 1,2-bis-
(chloromercury)benzene.4 The first structural evidence
for chelating complexation between a bifunctional Lewis
acid and a carbonyl functionality was presented in 1992
by Wuest and co-workers.5 In this case, an interaction
Lewis acids play a vital role in many aspects of
modern organic synthesis including polymerization
reactions.1 They are well-established and highly useful
reagents in organic transformations involving carbonyl
compounds such as Diels-Alder reactions, aldol con-
densations, or Michael additions and serve as cocata-
lysts in Ziegler-Natta polymerization. However, the use
of bi- and multifunctional Lewis acids has been studied
only recently.2 This is surprising considering that their
counterparts, the bi- and multidentate Lewis bases, play
a major role as ligands in various transition metal
catalyzed processes.
(2) For recent reviews see: (a) Kuivila, H. G. In Heteroat. Chem.
[Int. Conf.] 1990, 245. (b) Katz, H. E. Inclusion Compd. 1991, 4, 391.
(c) Kaufmann, D. E.; Otten, A. Angew. Chem. 1994, 106, 1917. (d)
Vaugeois, J .; Simard, M.; Wuest, J . D. Coord. Chem. Rev. 1995, 145,
55. (e) Schmidtchen, F. P.; Berger, M. Chem. Rev. 1997, 97, 1609. (f)
Beer, P. D.; Smith, D. K. Prog. Inorg. Chem. 1997, 46, 11. (g) Antonisse,
M. M. G.; Reinhoudt, D. N. Chem. Commun. 1998, 443. (h) Wuest, J .
D. Acc. Chem. Res. 1999, 32, 81. (i) Beer, P. D.; Gale, P. A. Angew.
Chem., Int. Ed. 2001, 40, 486. For reviews on the use of bifunctional
Lewis acids in Ziegler-Natta polymerization see: (j) Chen, E. Y.-X.;
Marks, T. J . Chem. Rev. 2000, 100, 1391. (k) Piers, W. E.; Irvine, G.
J .; Williams, V. C. Eur. J . Inorg. Chem. 2000, 10, 2131.
First investigations of bidentate Lewis acids were
directed at the complexation of anions and neutral
* To whom correspondence should be addressed. E-mail: fjaekle@
andromeda.rutgers.edu.
† Department of Chemistry, X-ray Laboratory, University of Toronto,
Canada.
(1) Yamamoto, E. H. Lewis Acid Reagents: A Practical Approach;
Oxford University Press: New York, 1999.
(3) Katz, H. E. J . Am. Chem. Soc. 1985, 107, 1420.
(4) Wuest, J . D.; Zacharie, B. Organometallics 1985, 4, 410.
10.1021/om0204890 CCC: $22.00 © 2002 American Chemical Society
Publication on Web 08/23/2002

4170 Organometallics, Vol. 21, No. 20, 2002
Gamboa et al.
Ch a r t 1
Ch a r t 2
amines and phosphines.14 The dimethylaminomethyl
group acts as an anchoring group directing the lithiation
to the ortho-position. However, in the absence of this
type of interaction, the ortho-selectivity in electrophilic
aromatic substitutions is typically rather low.15 The
situation is even more complex when ferrocene is used
as the substrate. The presence of the first electron-
withdrawing group (LA1) in D is expected to direct
attack of the second electrophile to the other (more
electron-rich) Cp ring. It is therefore not surprising that
only a few reports on 1,2-dimetalated ferrocenes have
appeared in the literature. The 1,2-bis(chloromercury)-
ferrocene E was first discovered as a byproduct in the
mercuration of ferrocene with mercuric acetate by
Rausch and co-workers in 1974.16 However, the low yield
of only ca. 2% limits the utility of this intriguing species.
Another report on a 1,2-dimetalated ferrocene com-
pound, the dialuminated species F , appeared in the
literature shortly thereafter. Compound F was observed
as one of the products in the reaction of 1,1′-bis-
(chloromercury)ferrocene with Me2AlCl, and the struc-
ture was determined by X-ray crystallography.17 How-
ever, F was not further characterized, nor were the
properties of this unusual dimer studied. Cunningham
and co-workers have recently reported that Friedel-
Crafts acetylation of 1,1′-bis(trimethylsilyl)- and 1,1′-
bis(tributylstannyl)ferrocene using acetyl chloride in the
presence of AlCl3 at -70 °C results in mixtures of three
regioisomers, the 1,1′-, 1,2-, and 1,3-silylacetylferrocenes
(31.0%, 15.5%, 53.0%) and the 1,1′-, 1,2-, and 1,3-
stannylacetylferrocenes (28.5%, 13.0%, 44.0%), respec-
tively.18 This unusual observation encouraged us to
investigate the reactivity of 1,1′-bis(trimethylstannyl)-
ferrocene (1) toward inorganic electrophiles, anticipating
that reaction of 1 with a stoichiometric amount of group
13 halides might lead to similar products including the
desired 1,2-dielement substituted ferrocenes.19
between two aluminum centers and a carbonyl group
is promoted by intramolecular chelation. This discovery
led to a series of studies on the reactivity of bifunctional
Lewis acids in organic transformations. Indeed, im-
proved activation and increased stereoselectivity with
bifunctional Lewis acids relative to their monofunctional
counterparts have been reported.6-8 Until now the
variety of backbones for bidentate Lewis acids has been
limited almost exclusively to Wuest’s 1,2-phenylenediyl
(A),4,6 Katz’s 1,8-naphthalenediyl (B),3,8-10 and an-
thracenediyl (C)11 bridges and closely related entities.
These compounds have in common a very rigid two-
dimensional backbone, in which the Lewis acidic centers
are in well-defined positions within the plane of the
molecule. We were intrigued by the fact that the Lewis
acidic centers in 1,2-dimetalated ferrocenes (D) would
not be part of a mirror plane, but rather at the “edge”
of a cylindrical molecule, providing a unique geometric
environment for the Lewis acidic centers.12 The redox
chemistry of the central iron atom in ferrocenes is an
additional attractive feature, as it may provide an
opportunity to fine-tune the Lewis acidity of these
complexes.
Directed electrophilic aromatic substitution may be
envisioned as a possible route to bidentate Lewis acids
D.13 Indeed, the directed synthesis of 1,2-disubstituted
ferrocenes via lithiation of dimethylaminomethylfer-
rocenes and related species has proven highly useful in
the design and synthesis of planar chiral bifunctional
(5) Sharma, V.; Simard, M.; Wuest, J . D. J . Am. Chem. Soc. 1992,
114, 7931.
(6) Wuest, J . D.; Zacharie, B. J . Am. Chem. Soc. 1985, 107, 6121.
(7) (a) Ooi, T.; Miura, T.; Maruoka, K. Angew. Chem., Int. Ed. 1998,
37, 2347. (b) Lee, H.; Diaz, M.; Hawthorne, M. F. Tetrahedron Lett.
1999, 40, 7651. (c) Kaufmann, D.; Boese, R. Angew. Chem., Int. Ed.
Engl. 1990, 29, 545.
Resu lts a n d Discu ssion
Selective or th o-Bor yla tion of 1,1′-Bis(tr im eth yl-
sta n n yl)fer r ocen e (1). 1,1′-Bis(trimethylstannyl)fer-
(8) Reilly, M.; Oh, T. Tetrahedron Lett. 1995, 36, 221.
(9) (a) Katz, H. E. J . Org. Chem. 1985, 50, 5027. (b) Katz, H. E.
Organometallics 1987, 6, 1134. (c) Reilly, M.; Oh, T. Tetrahedron Lett.
1994, 35, 7209.
(10) Tschinkl, M.; Hoefelmeyer, J . D.; Cocker, T. M.; Bachman, R.
E.; Gabba¨ı, F. P. Organometallics 2000, 19, 1826.
(14) Togni, A., Hayashi, T., Eds. Ferrocenes; VCH: Weinheim, 1995.
(15) Notable exceptions include reactions involving the replacement
of a sterically demanding group by a proton (release of steric strain
leads to steric acceleration) and the formation of an intermediate
complex between the solvent and the substituent, thereby leading to
a strong +I effect of this group. The latter effect is for example believed
to be responsible for the observed ortho-selectivity in the nitration of
phenylboronic acid. See: Taylor, R. Electrophilic Aromatic Substitution;
J ohn Wiley & Sons: Chichester, 1990. For recent studies of regiose-
lective electrophilic aromatic substitution reactions, see for example:
(a) Yonehara, F.; Kido, Y.; Yamaguchi, M. Chem. Commun. 2000, 1189.
(b) Herrlich, M.; Mayr, H.; Faust, R. Org. Lett. 2001, 3, 1633. (c)
Herrlich, M.; Hampel, N.; Mayr, H. Org. Lett. 2001, 3, 1629.
(11) Katz, H. E. J . Org. Chem. 1989, 54, 2179.
(12) For 1,1′-disubstituted metallocene-based Lewis acids see for
example: (a) Herdtweck, E.; J a¨kle, F.; Opromolla, G.; Spiegler, M.;
Wagner, M.; Zanello, P. Organometallics 1996, 15, 5524. (b) Herdtweck,
E.; J a¨kle, F.; Wagner, M. Organometallics 1997, 16, 4737. (c) Herberich,
G. E.; Englert, U.; Fischer, A.; Wiebelhaus, D. Organometallics 1998,
17, 4769. (d) Dinnebier, R. E.; Wagner, M.; Peters, F.; Shankland, K.;
David, W. I. F. Z. Anorg. Allg. Chem. 2000, 626, 1400. (e) Carpenter,
B. E.; Piers, W. E.; McDonald, R. Can. J . Chem. 2001, 79, 291. (f) J utzi,
P.; Lenze, N.; Neumann, B.; Stammler, H.-G. Angew. Chem., Int Ed.
2001, 40, 1423. (g) Aldridge, S.; Bresner, C.; Fallis, I. A.; Coles, S. J .;
Hursthouse, M. B. Chem. Commun. 2002, 740.
(16) Roling, P. V.; Rausch, M. D. J . Org. Chem. 1974, 39, 1420.
(17) Atwood, J . L.; Shoemaker, A. L. Chem. Commun. 1976, 536.
(18) Cunningham, A. F. J . Am. Chem. Soc. 1991, 113, 4864.
(13) For an alternative approach via 1,2-dibromoferrocene see:
Butler, I. R.; Drew, M. G. B. Inorg. Chem. Commun. 1999, 2, 234.
(19) A preliminary account of this work has been communicated:
J a¨kle, F.; Lough, A. J .; Manners, I. Chem. Commun. 1999, 453.

ortho-Borylation of Bis(trimethylstannyl)ferrocene
Organometallics, Vol. 21, No. 20, 2002 4171
Sch em e 1. Rea ction of 1 w ith Bor on Tr ich lor id e
the double doublet at δ ) 4.46 may be attributed to the
Cp proton adjacent to a trimethylstannyl group.21,24
A
similar set of signals of much lower intensity (ca. 10%)
consisting of a singlet (5 H), two double doublets, and a
pseudotriplet is detected in the same spectral region.
However, the smaller coupling constant of the pseudo-
triplet at δ ) 4.47 (1.2 Hz) is indicative of a 4J (H, H)
coupling in a 1,3-disubstituted isomer. A third set of
signals of very low intensity (ca. 3%) consists of four
pseudotriplets at δ ) 3.88, 4.18, 4.36, and 4.44 (3/4J )
1.8 Hz) and can be attributed to the 1,1′-disubstituted
isomer. The three isomers give rise to three singlets in
the methyl region of the spectrum (2a -Cl, δ ) 0.28; 2b-
Cl, δ ) 0.12; 2c-Cl, δ ) 0.14) with intensities that
correlate well with those of the Cp resonances. Tin
satellites flank these signals with 2J (Sn, H) coupling
constants identical to the one reported for starting
material 1 (55-56 Hz).21 Accordingly, a single 119Sn
NMR resonance is found for each isomer at a chemical
shift similar to that for 1 (2a -Cl, δ ) -5.1; 2b-Cl, δ )
-10.1; 2c-Cl, δ ) -7.2; 1, δ ) -6.021), providing further
evidence for the presence of unreacted trimethylstannyl
groups in the products. In the 11B NMR spectrum a
broad resonance is observed at δ ) 51. Signals for the
individual isomers could not be resolved as may be
expected considering the broad nature of the resonance
(h1/2 ca. 320 Hz).
The facile and highly selective ortho-borylation of 1
is very unusual considering that Ar-Sn bonds are
typically much more reactive toward boron halides than
Ar-H bonds.26 The 1-stannyl-1′-borylferrocene 2c-Cl
would therefore be expected as the major product.
Moreover, the ortho-attack of the electrophile is remark-
able considering the steric bulk and electronic properties
of a trimethylstannyl group. A similar directing effect
into the ortho-position of stannylated arenes has to our
knowledge not been previously reported. In comparison,
borylation of the less hindered 1,1′-dibromoferrocene
and of alkylated ferrocenes exclusively takes place in
the 3,4-positions, yielding 1,1′-dibromo-3,3′-diborylfer-
rocene and 1-alkyl-3,1′,3′-triborylferrocenes, respec-
tively.27 Similarly, diborylation of ferrocene with boron
tribromide leads to the 1,1′-diborylferrocene and small
amounts of the 1,3-diborylated isomer, whereas a larger
excess of boron tribromide results in the formation of
1,3,1′-triborylated and 1,3,1′,3′-tetraborylated ferrocenes.28
rocene (1)20,21 was reacted with 1 equiv of BCl3 in
hexanes at -78 °C (Scheme 1). When the mixture was
stirred at -78 °C for 1 h, the color of the solution
gradually turned from light orange to red. The reaction
mixture was allowed to slowly warm to 0 °C, and all
volatile material was removed under reduced pressure
(10-3 Torr). When the crude product was analyzed by
1H NMR spectroscopy, we found to our surprise that the
1,2-isomer, 2a -Cl,22 was formed as the major product
in an unusually high spectroscopic yield of ca. 87%
(Scheme 1).
Careful analysis of the NMR data of the crude
reaction mixture revealed formation of small amounts
of the two other regioisomers, the stannylborylfer-
rocenes 2b-Cl and 2c-Cl (Table 1). The 1,3-disubstituted
isomer (2b-Cl) was formed in ca. 10% spectroscopic
yield, whereas only trace amounts of the 1-stannyl-1′-
borylferrocene (2c-Cl) were observed. The oily nature
of the product and the inherent sensitivity toward air
and moisture prevented separation of the isomers by
crystallization or column chromatography. However,
using 2D NMR spectroscopic techniques (dqfCOSY,
HMQC, NOESY), we were able to conclusively assign
the observed NMR signals to the three different isomers
1
and to fully characterize each isomer by H, 13C, and
119Sn NMR spectroscopy.
Three sets of signals were observed in the H NMR
1
spectrum of the isomer mixture 2-Cl (Figure 1). The
most striking feature is a large singlet with the intensity
of five protons in the region typical of unsubstituted
ferrocene Cp rings (δ ) 3.93). This signal can be
attributed to the major isomer 2a -Cl and corresponds
to a set of three Cp resonances (δ ) 4.46, 4.53, 4.71) in
the expected range for downfield-shifted protons at-
tached to a boron-substituted Cp ring.23 Particularly
insightful is the coupling pattern consisting of double
doublets for the resonances at δ ) 4.46 and 4.71 (J )
1.2/2.4 Hz) and a pseudotriplet for the resonance at δ
) 4.53 (J ) 2.4 Hz). The observed coupling constant of
2.4 Hz for the pseudotriplet is consistent with a 3J (H,
H) coupling in ferrocene species16 and clearly indicates
the formation of a 1,2-disubstituted species. A detailed
analysis of the three Cp signals reveals satellites due
to coupling to a tin center. The coupling constant of
J (117/119Sn, H) ) 13 Hz for the resonance at δ ) 4.46 is
considerably larger than the coupling constants for the
other two signals (J ) 5 and J ) 6 Hz). Consequently,
The trimethylstannyl group is characterized by a weak
+
-I and +M effect with a Hammett parameter of σo
)
-0.12 that does not account for the observed high
reactivity of the R-positions.29 Indeed, the R:â ratio for
the acetylation of 1 is relatively low (1:4.3) and compa-
(24) (a) Herberhold, M.; Milius, W.; Steffl, U.; Vitzithum, K.;
Wrackmeyer, B.; Herber, R. H.; Fontani, M.; Zanello, P. Eur. J . Inorg.
Chem. 1999, 145. (b) Ko¨hler, F. H.; Geike, W. A.; Hertkorn, N. J .
Organomet. Chem. 1987, 334, 359.
(25) The pseudotriplet at δ ) 4.46 (J ) 1.2 Hz) for isomer 2b-Cl is
overlapping with the double doublet at δ ) 4.46 for isomer 2a -Cl. The
signal is clearly visible in enriched samples of 2b-Cl, and the
assignment has been further confirmed by 2D NMR spectroscopy
(dqfCOSY). The CpSn-H2,5 protons for 2c-Cl resonate outside the
spectral range shown in Figure 1 at δ ) 3.88.
(26) Eaborn, C. J . Organomet. Chem. 1975, 100, 43.
(27) Wrackmeyer, B.; Do¨rfler, U.; Rinck, J .; Herberhold, M. Z.
Naturforsch. 1994, 49b, 1403.
(20) Dawoodi, Z.; Eaborn, C.; Pidcock, A. J . Organomet. Chem. 1979,
170, 95.
(21) Lenze, N.; Neumann, B.; Salmon, A.; Stammler, A.; Stammler,
H. G.; J utzi, P. J . Organomet. Chem. 2001, 619, 74.
(22) The 1-stannyl-2-borylferrocenes and 1-stannyl-3-borylferrocenes
form as racemic mixtures as a result of their planar chirality. Only
one of the two enantiomers will be shown in the schemes and figures.
(23) See, for example: (a) Ruf, W.; Renk, T.; Siebert, W. Z.
Naturforsch. 1976, 31b, 1028. (b) Wrackmeyer, B.; Do¨rfler, U.; Her-
berhold, M. Z. Naturforsch. 1993, 48b, 121. (c) Appel, A.; No¨th, H.;
Schmidt, M. Chem. Ber. 1995, 128, 621.
(28) (a) Wrackmeyer, B.; Do¨rfler, U.; Milius, W.; Herberhold, M.
Polyhedron 1995, 14, 1425. (b) Appel, A.; No¨th, H.; Schmidt, M. Chem.
Ber. 1995, 128, 621.
(29) Taylor, R. Electrophilic Aromatic Substitution; J ohn Wiley &
Sons: Chichester, 1990.

4172 Organometallics, Vol. 21, No. 20, 2002
Gamboa et al.
Ta ble 1. 1H NMR Da ta a n d Sp ectr oscop ic Yield s for th e Regioisom er s 2a -Cl, 2b-Cl, a n d 2c-Cl
leading to a high rotational barrier around the Cp-B
bond and the B-C6F5 bond. A similar line broadening,
though at far lower temperature, has been reported for
ferrocenylboranes with two different substituents on
boron.31 For instance, the coalescence temperature for
rotation around the Cp-B bond in FcB(I)Ph was deter-
mined to be Tc ) -42 °C (∆Gq ) 12.2 kcal mol-1).31 The
rotational hindrance of the pentafluorophenyl group in
2b-P f is also reflected in the 19F NMR spectrum, which
shows a broadened resonance for the fluorine atoms in
ortho-position.
We then turned our attention to an evaluation of
electronic factors and their effect on the selectivity of
1
the borylation and the product distribution. We chose
PhBCl2, which is a much weaker electrophile in com-
parison to BCl3 and C6F5BCl2. Upon treatment of 1 with
PhBCl2 in hexanes at -78 °C, we did not observe any
detectable conversion of the starting material. Only
when the mixture was allowed to warm to ambient
temperature, the reaction slowly proceeded as indicated
by a gradual color change of the solution to dark red.
However, complete conversion of PhBCl2 was confirmed
F igu r e 1. Region of substituted Cp rings in the H NMR
spectrum of crude 2-Cl; 2a -Cl (#); 2b-Cl (∧); 2c-Cl (*).25
rable to the reactivity ratio of 1:4.5 for isopropylfer-
rocene and of 1:12.7 for tert-butylferrocene.18 We rea-
soned that an evaluation of the influence of steric and
electronic factors on the selectivity of the borylation of
1 might allow us to gain further insight into the
characteristics of this unusual reaction.30 We therefore
extended our studies to the investigation of organoboron
halides.
1
by H NMR spectroscopy only after extended reaction
time (ca. 5 days) at room temperature. A slightly faster
reaction was observed in toluene leading to full conver-
sion within ca. 2 days. Interestingly, in either toluene
or hexanes, the product distribution for the reaction
with PhBCl2 was more complex than in the case of the
above-described reactions with boron trichloride and
Dep en d en ce of Selectivity on Ster ic a n d Elec-
tr on ic F a ctor s. C6F5BCl2 represents an organoboron
halide of Lewis acidity comparable to that of BCl3, yet
it is a considerably more sterically demanding Lewis
acid. Upon addition of C6F5BCl2 to a solution of 1 in
hexanes at -78 °C, the reaction mixture immediately
turned dark orange-red and a deep red color gradually
1
C6F5BCl2. H NMR spectroscopy revealed that ca. 33%
of 1,3-product 2b-P h and trace amounts of the 1,1′-
isomer 2c-P h were formed in addition to 2a -P h . Fur-
thermore, we detected a large amount of unreacted
starting material 1 (ca. 20%) and formation of a new
1,2-disubstituted species (ca. 30%) containing a chloride
substituent on tin, as confirmed by a downfield-shifted
119Sn NMR signal at δ ) 85.9 (for details on the
synthesis and isolation of this species with excess borane
see vide infra). The amount of byproduct could be
minimized by use of only 0.5 equiv of PhBCl2. With this
procedure, the two isomers 2a -P h and 2b-P h were
formed almost exclusively. Our observations indicate
that the steric bulk of the phenyl or the pentafluorophe-
1
developed upon warming to room temperature. The H
NMR spectrum of the crude product showed in addition
to the expected 1,2-isomer 2a -P f a large amount of the
1,3-isomer, 2b-P f, whereas the 1,1′-isomer was not
detected. The ratio of 2a -P f to 2b-P f was estimated to
be ca. 55:45. The two isomers can readily be distin-
guished through analysis of the coupling patterns in the
1H NMR spectrum as described above for the isomers
2a -Cl and 2b-Cl. Interestingly, the Cp signals adjacent
to the boryl substituent are broadened at ambient
temperature. This effect can be attributed to the pres-
ence of the bulky pentafluorophenyl group on boron,
(30) More detailed mechanistic studies are currently under way and
will be reported in a future contribution.
(31) Renk, T.; Ruf, W.; Siebert, W. J . Organomet. Chem. 1976, 120,
1.

ortho-Borylation of Bis(trimethylstannyl)ferrocene
Organometallics, Vol. 21, No. 20, 2002 4173
Ch a r t 3
F igu r e 2. Product distribution for the reaction of 1 with
varying amounts of PhBCl2 in toluene at 25 °C.
Sch em e 2. Rea ction of 1 w ith 2 Equ iv of RBCl2 (R
Ch a r t 4
) Cl, P h )
under these conditions even when a larger excess of
BCl3 was applied.33 Dark red crystals of the analytically
pure isomer 3a -Cl were isolated after repeated recrys-
tallization from hexanes at -78 °C, and the structure
of 3a -Cl was confirmed by multinuclear and two-
dimensional NMR spectroscopy, mass spectrometry, and
elemental analysis. The 1H NMR spectrum of 3a -Cl
shows a similar pattern in the Cp region as observed
for the 1,2-disubstituted ferrocene 2a -Cl. However, the
signal for the Cp proton adjacent to the tin center is
downfield-shifted in comparison to that for 2a -Cl (3a -
Cl, δ ) 5.24; 2a -Cl, δ ) 4.46). The two methyl groups
on tin are diastereotopic and show two distinct NMR
signals at δ ) 0.52 (J ) 62 Hz) and δ ) 0.69 (J ) 64
Hz) as a result of the planar chirality of 3a -Cl. A
strongly downfield-shifted resonance in the 119Sn NMR
spectrum further confirms the presence of an electron-
withdrawing substituent on tin and indicates an in-
creased Lewis acidity of the stannyl group (3a -Cl, δ )
102.1; 2a -Cl, δ ) -5.1).
The formation of almost equal amounts of the 1,2- and
1,3-isomer in the reaction of 1 with PhBCl2 provided
us with an opportunity to directly compare the reactivity
of a trimethylstannyl group in R-position with that in
â-position to the boron substituent. Hence, we decided
to study the product distribution for the reaction of 1
with PhBCl2 as a function of stoichiometry (Figure 2).
The two isomers 2a -P h and 2b-P h were formed almost
exclusively with 0.5 equiv of PhBCl2 in toluene. As the
amount of PhBCl2 was increased, 2a -P h reacted further
under cleavage of an Sn-Me bond to give 3a -P h
(Scheme 2). However, the 1,3-isomer 3b-P h (Chart 4)
was formed only after extended reaction time when a
2-fold excess of PhBCl2 was applied. The analytically
pure isomer 3a -P h was isolated by repeated recrystal-
lization from hexanes at -37 °C and fully characterized
nyl group directs the attack of the electrophile to some
degree to the more accessible 3,4-positions in 1. The
presence of a relatively electron-rich phenyl group on
boron considerably attenuates the borylation of 1. We
can conclude that steric factors to some extent deter-
mine the position, in which the electrophile attacks,
whereas electronic effects play a major role in determin-
ing the overall reaction rate and the product distribu-
tion. In a continuation of this trend MesBCl2 (Mes )
2,4,6-Me3Ph) entirely fails to react with 1 at room
temperature even after extended reaction time (5 days).
F or m a tion of F er r ocen e-Ba sed Heter on u clea r
Bifu n ction a l Lew is Acid s. Compounds 2a do not meet
the requirements of a ferrocene-based bidentate Lewis
acid of type D (Chart 1) since the trimethylstannyl
group in these species is relatively electron rich. To
develop methods for the functionalization of the remain-
ing trimethylstannyl group and ultimately to increase
the Lewis acidity of the tin center, we decided to further
investigate the reactivity of species 2a .
(i) Rea ction of 1 w ith Excess Bor a n e. The forma-
tion of a byproduct containing an Sn-Cl bond in the
reaction of 1 with PhBCl2 suggested that one of the Sn-
Me bonds of the remaining trimethylstannyl substituent
in 2a can be cleaved with boron halides. Alternatively,
cleavage of the remaining Sn-Cp bond with excess
borane may lead to diborylated compounds. This reac-
tion path was described by Deck and co-workers for the
reaction of 1,1′-bis(trimethylsilyl)ferrocene with excess
BCl3 in dichloromethane, yielding mixtures of 1,1′- and
1,3-diborylated ferrocenes.32 We reacted 1 with 2 equiv
of BCl3 in hexanes at -78 °C (Scheme 2). Interestingly,
not the second Cp-Sn bond but one of the Sn-Me bonds
was cleaved, and the novel heteronuclear bidentate
Lewis acid 3a -Cl was formed as the major product. A
reaction at the second Sn-Cp bond did not take place
(33) For related heteronuclear bifunctional Lewis acids with an
organic backbone, see for example ref 10 and the following: (a) Katz,
H. E. J . Am. Chem. Soc. 1986, 108, 7640. (b) Wrackmeyer, B.; Maisel,
H.; Milius, W.; Badshah, A.; Molla, E.; Mottalib, A. J . Organomet.
Chem. 2000, 602, 45.
(32) Deck, P. A.; Fisher, T. S.; Downey, J . S. Organometallics 1997,
16, 1193.

4174 Organometallics, Vol. 21, No. 20, 2002
Gamboa et al.
Sch em e 3. Th er m a l Rea r r a n gem en t of
1-Sta n n yl-2-bor ylfer r ocen es
Ch a r t 5. P ossible Tr a n sition Sta tes for a n In tr a -
ver su s In ter m olecu la r Th er m a l Rea r r a n gem en t
Rea ction
by multinuclear NMR spectroscopy, mass spectrometry,
and elemental analysis.
(ii) Th er m a l Rea r r a n gem en t of Sta n n ylbor yl-
fer r ocen es 2a -Cl a n d 2a -P h . Initially we kept the
crude products 2 for 3 h at ca. 50 °C under high vacuum
in order to remove any traces of Me3SnCl or Me2SnCl2.
Surprisingly, this procedure led to another rearrange-
ment reaction to give 4a -Cl and 4a -P h (Scheme 3),
providing a simple alternate route for the introduction
of electron-withdrawing substituents on tin. The rear-
rangement of 2a -Cl to form 4a -Cl slowly proceeded even
below room temperature, whereas 2a -P h was stable at
room temperature and required prolonged heating to
ca. 80 °C for complete conversion to 4a -P h . In both
cases, only the 1,2-isomers were isolated after repeated
recrystallization from hexanes, indicating that this
rearrangement reaction may have occurred specifically
for the 1,2-isomers. Indeed, 1H NMR spectroscopic
analysis of the filtrate after crystallization of 4a -P h
confirmed the presence of unchanged 1,3-isomer 2b-P h .
An increased Lewis acidity of the stannyl groups in
the rearranged species 4a -Cl and 4a -P h is reflected in
a downfield shift of the 119Sn NMR signal in comparison
to 2a -Cl and 2a -P h (2a -Cl, δ ) -5.1; 4a -Cl, δ ) 89.8;
2a -P h , δ ) -8.5; 4a -P h , δ ) 94.1). Similarly, introduc-
tion of a methyl group on boron leads to the expected
downfield shift34 in the boron NMR spectra (2a -Cl, δ )
51.1; 4a -Cl, δ ) 61.5; 2a -P h , δ ) 57.2; 4a -P h , δ ) 67.5).
The diastereotopic methyl groups on tin give rise to two
signals with tin satellites in the 1H NMR spectra of both
4a -Cl and 4a -P h . An additional methyl resonance
without tin satellites can be assigned to the methyl
group that was transferred from tin to boron (4a -Cl, δ
) 0.86; 4a -P h , δ ) 1.07).
borylferrocenes. The unusually low reactivity of the Sn-
Cp bonds in 2a -Cl and 2a -P h in comparison to the Sn-
Me bonds suggests a combination of two effects: an
intramolecular activation of the Sn-Me bonds and a
deactivation of the Sn-Cp bond adjacent to an electron-
withdrawing boryl substituent.
The thermal rearrangement of species 2a -Cl and 2a -
P h represents a similar process, except that Sn-Me
bond cleavage is brought about by the chloroborane
functionality in the molecule itself. A mechanism in-
volving a transition state with four-coordinate boron and
five-coordinate tin centers (see “intramolecular rear-
rangement” in Chart 5) has been postulated for the
rearrangement of the analogous 1-stannyl-2-borylben-
zene derivatives.36 Indeed, the formation of stable
pentacoordinate tetraorganostannanes through intramo-
lecular coordination has been reported previously.37
However, we believe that the proposed intramolecular
transition state is less likely considering that the
coordination of nucleophiles is known to lead to elon-
gated tin-carbon bonds trans to the incoming nucleo-
phile, which consequently display enhanced reactivity.38
We thus favor a mechanism involving (i) intramolecular
transfer of a chloride substituent from boron to tin and
(ii) intermolecular transfer of the activated methyl
group in trans-position to the boryl group of another
molecule (Chart 5). This mechanism is further sup-
ported by a study on the concentration dependence of
the rate of rearrangement of 2a -Cl. Neat 2a -Cl is fully
converted to 4a -Cl within 3 h at 60 °C. In contrast, a
0.2 M solution in deuterated benzene shows 70%
conversion within 12 h and a 0.02 M solution gives only
25% conversion over the same period at 60 °C. These
results clearly support an intermolecular mechanism for
Clea va ge of Sn -C(sp 3) ver su s Sn -C(sp 2) Bon d s
in 1-Sta n n yl-2-bor ylfer r ocen es. Preferred cleavage
of an Sn-C(sp3) bond over an Sn-C(sp2) bond as
observed in the formation of species 3a -Cl and 3a -P h
is rare.35 An Sn-C(sp2) bond is generally believed to be
significantly more reactive toward electrophiles.36 At-
tack of an electrophile at the Sn-Cp bond and subse-
quent formation of a cationic Wheland-type intermedi-
ate should be disfavored in the presence of a strongly
electron-withdrawing boryl group at the same Cp ring.
On the other hand, the apparent lower reactivity of the
Sn-Me bonds in the 1,3-isomer, 2b-P h , in comparison
to 2a -P h provides evidence for an activation that is
unique to the Sn-Me bonds in 1-trimethylstannyl-2-
(37) See, for example: (a) Tzschach, A.; J urkschat, K. Pure Appl.
Chem. 1986, 58, 639. (b) J urkschat, K.; Tzschach, A.; Meunier-Piret,
J . J . Organomet. Chem. 1986, 315, 45. (c) Kumar-Das, V. G.; Mun, L.
K.; Wei, C.; Blunden, S. J .; Mak, T. C. W. J . Organomet. Chem. 1987,
322, 163. (d) Kumar-Das, V. G.; Mun, L. K.; Wei, C.; Mak, T. C. W.
Organometallics 1987, 6, 10.
(34) No¨th, H., Wrackmeyer, B., Eds. NMR, Basic Principles and
Progress; Springer-Verlag: Berlin, 1978; Vol. 14.
(38) See, for example: (a) J astrzebski, J . T. B. H.; van Koten, G.
Adv. Organomet. Chem. 1993, 35, 241. (b) Podesta, J . C.; Chopa, A.
B.; Koll, L. C. J . Chem. Res. (S) 1986, 308. (c) J astrzebski, J . T. B. H.;
Boersma, J .; Esch, P. M.; van Koten, G. Organometallics 1991, 10, 930.
(d) Steenwinkel, P.; J astrzebski, J . T. B. H.; Deelman, B.-J .; Grove, D.
M.; Kooijman, H.; Veldman, N.; Smeets, W. J . J .; Spek, A. L.; van
Koten, G. Organometallics 1997, 16, 5486. (e) Yoshida, J .; Izawa, M.
J . Am. Chem. Soc. 1997, 119, 9361.
(35) A similar phenomenon has been observed for the reaction of
stannylated zirconocenes with boron halides and for the borylation of
1,2-bis(trimethylstannyl)benzene: (a) Cheng, X.; Slebodnick, C.; Deck,
P. A.; Billodeaux, D. R.; Fronczek, F. R. Inorg. Chem. 2000, 39, 4921.
(b) Eisch, J . J .; Kotowicz, B. W. Eur. J . Inorg. Chem. 1998, 761.
(36) See ref 35b.

ortho-Borylation of Bis(trimethylstannyl)ferrocene
Organometallics, Vol. 21, No. 20, 2002 4175
transition state involved in their formation. An interac-
tion between the tin-bound chlorine atom and the Lewis
acidic boron center is not observed. In both structures,
the chloride substituent on tin points away from the
boron center. However, the boron-bound chloride sub-
stituent in 4a -Cl points toward the Lewis acidic tin
center (Cl(2)‚‚‚Sn(1) ) 3.1380(12) Å), leading to a
pyramidalization on tin as reflected in the large equato-
rial angles C(1)-Sn(1)-C(12) of 121.7(2)°, C(1)-Sn(1)-
C(13) of 113.1(2)°, and C(12)-Sn(1)-C(13) of 115.8(2)°
(sum of equatorial angles: 350.6°). A similar effect is
observed for the phenyl group in 4a -P h , which exhibits
two relatively short contacts with the tin center (Sn(1)‚
‚‚C(11) ) 3.516(4) Å; Sn(1)‚‚‚C(12) ) 3.216(4) Å).39 The
resultant pyramidalization of the tin center in 4a -P h
is reflected in a sum of equatorial angles of 351.9°. In
both structures the chloride substituent on tin is placed
in the axial position trans to the chloro and phenyl group
on boron, respectively. The angles Cl(1)-Sn(1)‚‚‚Cl(2)
of 172.54(4)° in 4a -Cl and Cl(1)-Sn(1)‚‚‚C(12) of 175.26-
(7)° in 4a -P h are close to the expected angle of 180° in
an ideal trigonal bipyramidal arrangement. Accordingly,
the Sn(1)-Cl(1) bonds in 4a -Cl (2.4026(12) Å) and 4a -
P h (2.4036(12) Å) are slightly elongated in comparison
to a typical Sn-Cl bond in a tetrahedral environment
(Me3SnCl monomer: 2.351 Å40). It is worth noting that
our structural results correlate nicely with the data
reported for the structure of Me3SnCl, which forms
polymer chains in the solid state as a result of Sn‚‚‚Cl
bridges (Sn-Cl 2.430(2); 3.269 Å; Cl-Sn‚‚‚Cl 176.85-
(6)°).40
F igu r e 3. Molecular structure (ORTEP drawing) of 4a -
Cl.
Interestingly, in both structures the boron atom is
bent out of the Cp plane toward the iron atom in a
fashion similar to that observed in the crystal structures
of dibromoborylferrocene41 and bis(pentafluorophenyl)-
borylferrocene42 (4a -Cl, R* ) 13.8(2)°; 4a -P h , R* ) 12.2-
(3)°; FcBBr2, R* ) 17.7° and 18.9°; FcB(C6F5)2, 16°; R*
) 180° - centroid(C1-C5)-C(2)-B angle), suggesting
the presence of a weak, but significant iron-boron
interaction.41,43 A slightly lower degree of iron-boron
interaction in 4a -Cl and 4a -P h in comparison to FcBBr2
and FcB(C6F5)2 is reflected in a larger Fe-B distance
(4a -Cl, 2.952(5) Å; 4a -P h , 3.087(5) Å; FcBBr2, 2.840 Å;
FcB(C6F5)2, 2.924 Å). Similarly, the B-C(Cp) bonds (4a -
Cl, 1.523(7) Å; 4a -P h , 1.536(6) Å) are slightly longer
than in FcBBr2 (1.474 and 1.482 Å) and FcB(C6F5)2
(1.501(4) Å). The presence of both a ferrocenyl and a
phenyl group in 4a -P h allows for a direct comparison
of the B-Cp bond with the B-Ph bond. The B-Cp bond
(1.536(6) Å) is qualitatively shorter than the B-Ph bond
(1.571(5) Å), indicating a larger degree of π-interaction
with the ferrocenyl group. This effect may be attributed
to the contribution of a boratafulvene-like resonance
structure (I, Chart 6), as further supported by the
F igu r e 4. Molecular structure (ORTEP drawing) of 4a -
P h .
the rearrangement of species 2a . The lower reactivity
of 2a -P h in comparison to 2a -Cl may be traced back to
the increased steric bulk and decreased Lewis acidity
of the boryl group, which should disfavor the transfer
of a methyl group to the boron center.
P r op er ties of F er r ocen e-Ba sed Heter on u clea r
Bifu n ction a l Lew is Acid s. (i) Solid Sta te Str u ctu r e
of 1-Sta n n yl-2-bor ylfer r ocen es. Single-crystal struc-
ture determinations were performed on dark red crys-
tals of 4a -Cl and 4a -P h obtained from hexanes at -37
°C. The structures in Figure 3 and Figure 4 unambigu-
ously show that the proposed rearrangement reactions
had taken place leading to an unsubstituted Cp ring and
a 1-stannyl-2-borylcyclopentadienyl ligand on iron. A
methyl group is attached to the boryl groups in 4a -Cl
and 4a -P h as a result of the substituent rearrangement
reaction between the stannyl and the boryl group
outlined in Scheme 3. Accordingly, a chloride substitu-
ent is found next to two methyl groups on the tin center.
We anticipated that the solid state structure of the
rearranged stannylborylferrocenes, 4a -Cl and 4a -P h ,
may show an interaction between the Lewis acidic boron
center and the chloride substituent on tin, thereby
allowing us to gain insight into the nature of the
(39) For examples of intramolecular π-coordination of an alkynyl
group to an organotin cation see: (a) Wrackmeyer, B.; Kundler, S.;
Milius, W.; Boese, R. Chem. Ber. 1994, 127, 333. (b) Wrackmeyer, B.;
Kundler, S.; Boese, R. Chem. Ber. 1993, 126, 1361. (c) Wrackmeyer,
B.; Kehr, G.; Boese, R. Angew. Chem., Int. Ed. Engl. 1991, 30, 1370.
(40) Lefferts, J . L.; Molloy, K. C.; Hossain, M. B.; van der Helm, D.;
Zuckerman, J . J . J . Organomet. Chem. 1982, 240, 349.
(41) Appel, A.; J a¨kle, F.; Priermeier, T.; Schmid, R.; Wagner, M.
Organometallics 1996, 15, 1188.
(42) Carpenter, B. E.; Piers, W. E.; Parvez, M.; Yap, G. P. A.; Rettig,
S. J . Can. J . Chem. 2001, 79, 857.
(43) Silver, J .; Davies, D. A.; Roberts, R. M. G.; Herberhold, M.;
Do¨rfler, U.; Wrackmeyer, B. J . Organomet. Chem. 1999, 590, 71.

4176 Organometallics, Vol. 21, No. 20, 2002
Gamboa et al.
Ta ble 3. 1H, 119Sn , a n d 11B NMR Da ta of
Ch a r t 6
Com p ou n d s 3a a n d 4a a
δ1H
CpH H3
3.87 4.55 4.53 5.24 0.52
(10) (7) (15) (62)
3a -P h 3.86 4.57 4.63 5.46 0.66
(11) (8) (15) (63)
3.83 4.22 4.54 5.33 0.65
(11) (7) (15) (63)
H4
H5
Sn-Me
δ
119Sn
δ
11B
3a -Cl
0.69
(64)
0.82
(66)
0.82
(66)
102.1 49.8
85.9 56.4
89.8 61.5
94.1 67.5
4a -Cl
4a -P h 4.02 4.70 4.72 5.20 -0.16 0.54
(13) (6)
(14) (64)
(60)
Ta ble 2. Selected Bon d Len gth s (Å), In ter a tom ic
Dista n ces (Å), a n d An gles (d eg) for 4a -Cl a n d
4a -P h a
CpH H2,5 H3,4
Sn-Me
δ
119Sn
121.1
δ
11B
FcSnMe2Cl 3.94 4.00
4.17
(8)
4.28
4.41
0.48
(60)
4a -Cl
4a -P h
(11)
3.86 4.38
3.86 4.62
Sn(1)-Cl(1)
Sn(1)-C(1)
Sn(1)-C(12)
Sn(1)-C(13)
B(1)-Cl(2)
B(1)-C(2)
2.4026(12) Sn(1)-Cl(1)
2.4036(12)
2.121(4)
2.130(4)
2.121(4)
1.623(5)
1.536(6)
1.571(5)
122.7(2)
113.1(2)
31
FcBCl2
49.8
55.9
2.130(4)
2.123(5)
2.139(5)
1.808(5)
1.523(7)
1.562(7)
Sn(1)-C(1)
Sn(1)-C(18)
Sn(1)-C(19)
B(1)-C(17)
B(1)-C(2)
FcBClPh31
In C6D6 at 20 °C, coupling constants J (119Sn, H) are given in
a
1
parentheses.
Ta ble 4. 13C NMR Da ta of Com p ou n d s 3a a n d 4a a
B(1)-C(11)
B(1)-C(11)
C(1)-Sn(1)-C(12)
C(1)-Sn(1)-C(13)
C(12)-Sn(1)-C(13)
C(12)-Sn(1)-Cl(1)
C(1)-Sn(1)-Cl(1)
C(13)-Sn(1)-Cl(1)
C(2)-B(1)-Cl(2)
C(2)-B(1)-C(11)
C(11)-B(1)-Cl(2)
121.71(19) C(1)-Sn(1)-C(19)
113.10(18) C(1)-Sn(1)-C(18)
CpH
71.7 82.1 80.7 81.1 86.2 1.1
(43) (49) (48) (420) (462)
3a -P h 70.3 83.8 80.9 80.6 85.4 1.4 2.6
(47) (52) (53) (430) (473)
C1
C3
C4
C5
Sn-Me
B-Me
115.8(2)
100.37(15) C(18)-Sn(1)-Cl(1)
99.17(12) C(1)-Sn(1)-Cl(1)
101.41(15) C(19)-Sn(1)-Cl(1)
C(18)-Sn(1)-C(19) 116.1(2)
3a -Cl
2.5
100.65(13)
98.52(10)
99.54(12)
121.2(3)
120.4(3)
118.3(3)
117.1(4)
127.4(4)
115.5(4)
C(2)-B(1)-C(17)
C(2)-B(1)-C(11)
C(11)-B(1)-C(17)
4a -Cl
70.3 81.8 80.0 80.0 84.8 1.3
2.7
9.3
(nr) (nr) (52) (427) (473)
4a -P h 69.9 77.9 83.0 79.1 83.6 -0.6 2.4
10.7
(C1-C5) // (C6-C10) 1.1(2)
(C1-C5)//(C6-C10) 2.5(2)
(57) (54) (58) (454) (411)
Cent-C(2)-B(2)
Cent-C(1)-Sn(1)
166.2(2)
182.2(4)
Cp-staggering angle 5.9(5)
Cent-C(2)-B(1)
Cent-C(1)-Sn(1)
167.8(3)
182.7(2)
Cp-staggering angle 0.7(5)
CpH
68.6
C1
C2,5
C3,4
Sn-Me
Fe(1)‚‚‚B(1)
Sn(1)‚‚‚Cl(2)
2.952(5)
Fe(1)‚‚‚B(1)
3.1380(12) Sn(1)‚‚‚C(11)
Sn(1)‚‚‚C(12)
3.087(5)
3.516(4)
3.216(4)
FcSnMe2Cl
69.0
74.0
(69)
76.2
76.8
71.9
(52)
77.3
77.4
-1.7
(415)
31
FcBCl2
71.0
70.4
FcBClPh31
a
Cent ) centroid(C1-C5)
a
In C6D6 at 20 °C, coupling constants J (119Sn, 13C) are given
in parentheses; nr ) not resolved.
observation of a bond length alternation within the
substituted Cp rings of both 4a -Cl and 4a -P h . The tin
atom does not show an interaction with the iron atom
in 4a -Cl or 4a -P h , but is slightly bent away from the
iron center (4a -Cl, â* ) -2.2(4)°; 4a -P h , â* ) -2.7-
(2)°; â* ) 180° - centroid(C1-C5)-C(1)-Sn angle).
(ii) NMR Sp ectr oscop ic Stu d ies of 1-Sta n n yl-2-
bor ylfer r ocen es. A comparison between the NMR
spectroscopic properties of the bifunctional Lewis acids
3a and 4a and their monofunctional counterparts
FcBCl2, FcBClPh, and FcSnMe2Cl was expected to give
us some insight as to whether the two Lewis acidic
centers influence each other in solution (Table 3 and
Table 4). The 11B NMR resonances of 3a -Cl (δ ) 49.8)
and 3a -P h (δ ) 56.4) are very similar to those observed
for FcBCl2 (δ ) 49.8) and FcBClPh (δ ) 55.9), respec-
tively, and therefore do not seem to be very sensitive to
the substitution pattern of the ferrocene moiety. In
contrast, the 119Sn NMR signals vary from 85.9 to 102.1
ppm for species 3a and 4a and are strongly upfield-
shifted in comparison to FcSnMe2Cl (δ ) 121.1). Inter-
estingly, this effect seems to be unique for the 1,2-
isomers, as the chemical shift of δ ) 118.9 for the 1,3-
isomer 3b-P h is very close to that of FcSnMe2Cl. An
influence of the boryl on the stannyl group is also
evident in the proton NMR spectra of 3a and 4a , all of
which show two distinct signals for the methyl groups
on tin. With the exception of those for 4a -P h , the methyl
groups are downfield-shifted (δ ranges from 0.52 to 0.82)
relative to those in FcSnMe2Cl (δ ) 0.48) and the
coupling constants of 60-66 Hz are slightly larger than
in FcSnMe2Cl (2J (119Sn, H) ) 60 Hz), indicating an
increased s-character of the Sn-C bonds.21,24 A similar
yet more pronounced effect is observed in the 13C NMR
spectra. Two methyl resonances with tin satellites are
observed for each compound. The coupling constant for
one of these signals ranges from 411 to 430 Hz, which
is very close to the one for FcSnMe2Cl (J (119Sn, C) )
415 Hz). The second methyl resonance exhibits an
unusually large coupling of 454-473 Hz to tin.
The strong dependence of the 119Sn NMR shift on the
substitution pattern on boron in combination with a
significant upfield shift indicates that an interaction
between the boron substituents and the Lewis acidic tin
center as discussed above for the solid state structures
of 4a -Cl and 4a -P h also plays a role in solution. This
conclusion is further supported by the observed strong
coupling of the tin center to the methyl groups in the
1-stannyl-2-borylferrocenes, which is typical of equato-
rial ligands in pentacoordinate tin compounds.38
The Cp-H5 proton adjacent to the stannyl group is
strongly downfield-shifted (δ ranges from 5.20 to 5.46)
relative to the chemical shift of δ ) 4.00 observed for
FcSnMe2Cl.44 Furthermore, an unusually strong cou-
pling of the CpH3 and CpH5 protons to the tin center
is observed (3J (Sn, H5) ranges from 14 to 15 Hz; 4J (Sn,
3
H3) ranges from 10 to 13 Hz; for FcSnMe2Cl, J ) 11
4
Hz, J ) 8 Hz). This suggests that the boratafulvene-

ortho-Borylation of Bis(trimethylstannyl)ferrocene
Organometallics, Vol. 21, No. 20, 2002 4177
relative to those for FcBCl2 (λmax ) 454 nm) and
FcBClPh (λmax ) 464 nm), respectively. A similar effect
is also observed for the lowest energy absorption in the
UV region of the spectrum, which for the stannylboryl-
ferrocenes ranges from 332 to 361 nm with shoulders
reaching into the visible region. Again, the extent of the
bathochromic shift of this band increases in the order
Me < Cl < Ph and is more pronounced than in the
parent ferrocenylboranes (3a -Cl, λmax ) 342 nm; FcBCl2,
λmax ) 331 nm; 3a -P h , λmax ) 361 nm; FcBClPh, λmax
)
353 nm). The strong dependence of λmax on the substitu-
tion pattern on boron indicates a significant charge-
transfer contribution to these d-d transitions. Three
strong absorption bands between 200 and 267 nm for
3a -Cl and 4a -Cl can be assigned to charge-transfer
bands. The UV region of the spectra for 3a -P h and 4a -
P h are further complicated by absorption bands due to
the phenyl group.
F igu r e 5. Absorption spectra of 1-stannyl-2-borylfer-
rocenes (region from 300 to 570 nm shown).
like resonance structure (I, Chart 6) significantly con-
tributes not only in the solid state but also to the
structure in solution. An enhanced Sn-H coupling is
not observed for the 1,3-isomer 3b-P h (δ (Cp-H5) ) 4.51;
3J (Sn, H) ) 11 Hz). A possible explanation is that
coupling between tin and the Cp protons is not promoted
by extended delocalization in the boratafulvene-like
resonance structure of the 1,3-disubstituted species (II,
Chart 6).
A direct iron-boron interaction in the stannylboryl-
ferrocenes 4a -Cl and 4a -P h has been deduced from
strong bending of the boryl substituent out of the Cp
plane toward the iron atom. Furthermore, charge
transfer from iron to boron has been substantiated by
theoretical calculations on the parent molecule, FcBH2
(Fc ) ferrocenyl), which indicate orbital overlap between
2
2
2
the dx -y (HOMO-1) and dz (HOMO-2) orbitals on iron
and the empty p-orbital on boron.41 This interaction may
in part be responsible for the unusual absorption
characteristics of the described stannylborylferrocenes,
including the strong absorptions in the visible part of
the electronic spectra and their dependence on the
substitution pattern on boron.
(iii) UV-Visible Absor p tion Ch a r a cter istics of
1-Sta n n yl-2-bor ylfer r ocen es. The 1-stannyl-2-boryl-
ferrocenes 3a -Cl, 3a -P h , 4a -Cl, and 4a -P h are dark
orange-red in the solid state as well as in solutions of
noncoordinating solvents. Hence, their visible absorp-
tion spectrum is markedly different from that of fer-
rocene, which gives a light yellow solution as a result
of two weak absorption bands at λmax ) 441 (ꢀ ) 49)
and λmax ) 325 (ꢀ ) 91).45 In contrast, for the 1-stannyl-
2-borylferrocenes strong absorption bands are observed
in the range 454-471 nm (ꢀ ) 400-1000) and 332-
361 (ꢀ ) 1600-2000) (Figure 5; see also Supporting
Information). For the parent molecule, ferrocene, these
bands have been assigned to symmetry-forbidden d-d
transitions.45 When substituents are introduced at the
Cp rings, the transitions become formally allowed and
the intensity consequently increases. For instance, the
bands observed for phenylferrocene at 446 nm (310) and
339 nm (850) are significantly more intense than those
for ferrocene.45 It has also been noted that introduction
of electron-withdrawing groups on ferrocene leads to a
bathochromic shift of the visible absorption maxima.46
This effect is pronounced for the stannylborylferrocenes,
which show a red-shift of 13 to 30 nm relative to
ferrocene for the band at 454-471 nm. The extent of
the bathochromic shift depends on the substituents on
boron and increases in the order Me < Cl < Ph, with
3a -P h displaying the strongest and lowest energy band
(Figure 5). Interestingly, the bathochromic shift not only
is due to the presence of a boryl group but can in part
be traced back to the presence of the Lewis acidic
stannyl group. The maxima for 3a -Cl at λmax ) 458 nm
and 3a -P h at λmax ) 471 nm are slightly red-shifted
Con clu sion s
We have investigated the borylation of 1,1′-bis(tri-
methylstannyl)ferrocene (1) as a potential route to 1,2-
disubstituted ferrocenes. Borylation of 1 does not involve
direct tin-boron exchange, as is typically observed with
arylstannanes, but occurs with unprecedented selectiv-
ity at the R-position. Steric and electronic factors
determine the selectivity and overall rate of the reaction.
With more sterically demanding substituents on boron,
increased amounts of the 1,3-product are formed. A
decrease in Lewis acidity of the electrophile on the other
hand leads to a considerably lower reaction rate and,
in the case of PhBCl2, results in side-reactions involving
the Sn-Me bonds. A further increase of steric bulk and
decrease of Lewis acidity as in the case of MesBCl2
prevent the reaction entirely.
Whereas compounds 2a are examples for 1,2-diele-
ment-substituted ferrocenes, only the boryl group is
truly Lewis acidic. To increase the Lewis acidity of the
tin center, functionalization of the remaining trimeth-
ylstannyl group was necessary. We found that the
presence of a boryl group adjacent to a trimethylstannyl
group in 2a decreases the ability of an electrophile to
attack at the Sn-Cp bond. However, selective cleavage
of an Sn-Me bond can be brought about either with
excess borane or via an unusual thermal rearrange-
ment, in which a methyl group is transferred from tin
to boron and a chloride substituent from boron to tin.
The resulting heteronuclear bidentate Lewis acids 3a
and 4a consist of two Lewis acidic groups, a dimethyl-
chlorostannyl and a boryl group, both of which are
attached to the same Cp ring of ferrocene.
(44) The assignments of the Cp protons have been further confirmed
through the presence of a cross-peak between the Cp-H2 protons and
the ortho-protons on the phenyl group in the NOESY spectrum of 3a -
P h .
(45) Sohn, Y. S.; Hendrickson, D. N.; Gray, H. B. J . Am. Chem. Soc.
1971, 93, 3603.
(46) Lundquist, R. T.; Cais, M. J . Org. Chem. 1962, 27, 1167.

4178 Organometallics, Vol. 21, No. 20, 2002
Gamboa et al.
The presence of two Lewis acidic centers is reflected
in the solution and solid state characteristics of these
novel bifunctional Lewis acids. Structural studies of the
rearranged species 4a -Cl and 4a -P h reveal a significant
iron-boron interaction, as deduced from the dip angles
of 13.8° and 12.2° between the Cp plane and the Cp-
boron vector. An interaction between the tin-bound
chloride and the Lewis acidic boron center could not be
deduced from the solid state structure of the stannyl-
borylferrocenes 4a -Cl and 4a -P h . Our findings suggest
that the iron-boron interaction is energetically more
favorable than a boron-chlorine interaction in the
ferrocene-based systems 4a -Cl and 4a -P h . However, in
the case of the higher homologues the situation may well
be different. Indeed, Gabba¨ı and co-workers10 have
recently reported a related “iron-free” stannagallacycle
that readily forms a chloride bridge from tin to gallium,
which consequently displays a tetrahedral geometry. We
are currently developing synthetic routes to the analo-
gous aluminum- and gallium-functionalized ferrocenes.
Whereas the boron centers in 4a -Cl and 4a -P h can
draw electron density from the electron-rich iron atom,
a similar interaction is not observed for tin. However,
short contacts between the boron-bound chloride and
the tin center in 4a -Cl as well as the phenyl group and
the tin center in 4a -P h result in a distorted trigonal
bipyramidal geometry around tin in the solid state. A
similar interaction between the boron substituents and
the tin center in solution can be deduced from a
comparison of the NMR data of compounds 3a and 4a
with the “boron-free” compound FcSnMe2Cl. Particu-
larly revealing is the observed strong upfield-shift of
19-35 ppm for the 119Sn NMR signals relative to
FcSnMe2Cl. Interestingly, this hypercoordination of tin
may be responsible for the observed high reactivity of
the Sn-Me bonds in 2a , leading to facile formation of
species 3a with excess borane and of 4a via thermal
rearrangement.
The presence of two strongly electron-withdrawing
substituents in compounds 3a and 4a is reflected in the
UV-visible spectra of the 1-stannyl-2-borylferrocenes.
Intense absorptions are observed in the range 453-470
and 333-362 nm. Absorption bands in this range have
been assigned to weak, symmetry-forbidden d-d transi-
tion in the parent ferrocene molecule. These bands are
red-shifted for the stannylborylferrocenes relative to
ferrocene, and their position strongly depends on the
substitution pattern on boron, indicating a significant
charge-transfer contribution. A comparison with the
absorption spectra of the related ferrocenylboranes
suggests that both the boryl and the stannyl group
contribute to the observed bathochromic shift.
A particularly appealing feature of compounds 3a and
4a is that the chloride substituents on tin and boron
lend themselves to fine-tuning of the Lewis acidity via
substituent exchange reactions. In addition, the redox
chemistry of the ferrocene backbone may prove valuable
for further manipulation of the Lewis acidic centers.
However, probably the most intriguing aspect of the
described stannylborylferrocenes is their planar chiral-
ity.47 Racemic mixtures are obtained for all compounds
2a , 3a , and 4a . A critical step will be to develop methods
for the isolation of enantiomerically pure species. One
route that we are currently exploring is the separation
of the enantiomers via formation of complexes with
chiral amines or phosphines.
Exp er im en ta l Section
Ma t er ia ls a n d Gen er a l Met h od s. The compounds
PhBCl2, BCl3 (1 M in hexanes), and Me3SnCl were purchased
from Acros and BBr3 and N,N,N′,N′-tetramethylethylene-
diamine (TMEDA) were purchased from Aldrich. PhBCl2
and BBr3 were distilled under vacuum prior to use. The
compounds MesBCl2,48 FcBCl2,49 C6F5BCl2,50 and FcSnMe2Cl51
were prepared according to literature procedures. 1,1′-Bis-
(trimethylstannyl)ferrocene was obtained from dilithioferro-
cene‚nTMEDA52 and 2 equiv of Me3SnCl in Et2O according to
a literature procedure20 (purity >97%). All reactions and
manipulations were carried out under an atmosphere of
prepurified nitrogen using either Schlenk techniques or an
inert-atmosphere glovebox (Innovative Technologies). TMEDA
and all ether solvents were distilled from Na prior to use.
Hydrocarbon and chlorinated solvents were purified using a
solvent purification system (Innovative Technologies), and the
chlorinated solvents were subsequently degassed via several
freeze-pump-thaw cycles. All 400 or 500 MHz 1H NMR
spectra and 100.5 or 125.7 MHz 13C NMR spectra were
recorded on a Unity 400 and a Unity 500 spectrometer,
respectively. Two-dimensional NMR data were acquired on a
Unity 500 spectrometer. The 149.1 MHz 119Sn NMR, 376.2
MHz 19F NMR, and 128.3 MHz 11B NMR spectra were recorded
on a Unity 400 spectrometer. All solution 1H and 13C NMR
119
spectra were referenced internally to the solvent signals.
-
Sn and 11B NMR spectra were referenced externally to SnMe4
(δ ) 0) and BF3‚Et2O (δ ) 0), respectively, and the 19F NMR
spectra were referenced to R,R′,R′′-trifluorotoluene (0.05% in
C6D6; δ ) -67.73). UV-visible absorption data were acquired
on a Varian Cary 500 UV-vis/NIR spectrophotometer. Solu-
tions (10-3 and 10-4 M) were prepared using a microbalance
((0.1 mg) and volumetric glassware and then charged into
quartz cuvettes with sealing screw caps (Starna) inside the
glovebox. Mass spectra were obtained with the use of a VG
70-250S mass spectrometer operating in an electron impact
(EI) mode. Elemental analyses were performed by Quantitative
Technologies Inc., Whitehouse, NJ .
Rea ction of 1 w ith BCl3. Syn th esis of 1-Sta n n yl-2-
bor ylfer r ocen es 2a -Cl, 2b-Cl, a n d 2c-Cl. A solution of BCl3
(1.0 mL, 1.0 M in hexanes) was added dropwise to a solution
of 1,1′-fc(SnMe3)2 (0.51 g, 1.00 mmol) in hexanes (10 mL) at
-78 °C. The reaction mixture was stirred at -78 °C for 1 h
and then allowed to slowly warm to 0 °C. All volatile material
was removed under reduced pressure (10-3 Torr), and the
product was subsequently kept under dynamic vacuum at
ambient temperature over a period of 1 h. The crude product
was obtained as a dark red oil and characterized by NMR
spectroscopy without further purification. The isomer ratios
for compounds 2a -Cl (87%), 2b-Cl (10%), and 2c-Cl (3%) were
estimated via integration of the SnMe3 proton NMR signal.
For 2a -Cl: 1H NMR (400 MHz, C6D6, 20 °C): δ 4.71 (dd/ddd,
J ) 1.2 Hz, 2.4 Hz, J (117/119Sn, H) ) 6 Hz, 1 H, Cp-H3), 4.53
(pst/dpst, J ) 2.4 Hz, J (117/119Sn, H) ) 5 Hz, 1 H, Cp-H4), 4.46
(dd/ddd, J ) 1.2 Hz, 2.4 Hz, J (117/119Sn, H) ) 13 Hz, 1 H, Cp-
(48) J a¨kle, F.; Manners, I. Organometallics 1999, 18, 2628.
(49) Ruf, W.; Fueller, M.; Siebert, W. J . Organomet. Chem. 1974,
64, C45.
(50) Chambers, R. D.; Chivers, T. J . Chem. Soc., Chem. Commun.
1965, 3933.
(51) Kabouche, Z.; Nguyen Huu, D. J . Organomet. Chem. 1989, 375,
191.
(52) Wrighton, M. S.; Palazzotto, M. C.; Bocarsly, A. B.; Bolts, J .
M.; Fischer, A. B.; Nadjo, L. J . Am. Chem. Soc. 1978, 100, 7264.
(47) For a rare example of a complex between an enantiomerically
pure planar-chiral Lewis acid and a chiral Lewis base see: Tweddell,
J .; Hoic, D. A.; Fu, G. C. J . Org. Chem. 1997, 62, 8286.

ortho-Borylation of Bis(trimethylstannyl)ferrocene
Organometallics, Vol. 21, No. 20, 2002 4179
H5), 3.93 (s, 5 H, Cp), 0.28 (s/d, J (117/119Sn, H) ) 52/55 Hz, 9
H, Sn-Me). 13C NMR (100.5 MHz, C6D6, 20 °C): δ 85.2 (s/d,
J (117/119Sn, C) ) 45 Hz, Cp-C5), 80.8 (s/d, J (117/119Sn, C) ) 31
Hz, Cp-C3), 80.3 (ipso-CpSn), 79.4 (s/d, J (117/119Sn, C) ) 41 Hz,
Cp-C4), 70.7 (Cp), -6.8 (s/d, J (117/119Sn, C) ) 349/366 Hz, Sn-
Me), not observed (ipso-CpB). 119Sn NMR (149.1 MHz, C6D6,
20 °C): δ -5.1. For 2b-Cl: 1H NMR (400 MHz, C6D6, 20 °C):
δ 4.60 (dd, J ) 1.2 Hz, 2.4 Hz, 1 H, Cp-H4), 4.47 (pst, J ) 1.2
Hz, 1 H, Cp-H2), 4.36 (dd, J ) 1.2 Hz, 2.4 Hz, 1 H, Cp-H5),
3.93 (s, 5 H, Cp), 0.12 (s/d, J (117/119Sn, H) ) 53/56 Hz, 9 H,
Sn-Me). 13C NMR (100.5 MHz, C6D6, 20 °C): δ 83.4 (s/d,
J (117/119Sn, C) ) 47 Hz, Cp-C5), 82.7 (s/d, J (117/119Sn, C) ) 46
Hz, Cp-C2), 78.7 (s/d, J (117/119Sn, C) ) 35 Hz, Cp-C4), 76.9 (ipso-
Cp), 70.9 (Cp), -9.0 (s/d, J (117/119Sn, C) ) 346/361 Hz, Sn-Me).
119Sn NMR (149.1 MHz, C6D6, 20 °C): δ -10.1. For 2c-Cl: 1H
NMR (400 MHz, C6D6, 20 °C): δ 4.44 (pst, J ) 1.8 Hz, 2 H,
CpB-H3,4), 4.36 (pst, J ) 1.8 Hz, 2 H, CpB-H2,5), 4.18 (pst, J
) 1.8 Hz, 2 H, CpSn-H3,4), 3.88 (pst, J ) 1.8 Hz, 2 H, CpSn-
H2,5), 0.14 (s/d, J (117/119Sn, H) ) 53/56 Hz, 9 H, Sn-Me). 119Sn
NMR (149.1 MHz, C6D6, 20 °C): δ -7.2. Reliable 13C NMR
data for 2c-Cl could not be obtained, as the amount of the 1,1′-
disubstituted isomer in the reaction mixture is very small. The
11B NMR signals for the individual isomers could not be
resolved. The following assignment is made for the isomer
mixture: 11B NMR (128.3 MHz, C6D6, 20 °C): 51 (h1/2 ) 320
Hz).
solution of PhBCl2 (0.17 g, 1.00 mmol) in toluene (5 mL) was
added dropwise to a solution of 1,1′-fc(SnMe3)2 (1.02 g, 2.00
mmol) in toluene (20 mL) at ambient temperature. The
reaction mixture was allowed to stir for 12 h, and subsequently
all volatile material was removed under reduced pressure at
ambient temperature. A dark red oil was obtained and
characterized by NMR spectroscopy without further purifica-
tion. The ratios for compounds 1 (52), 2a -P h (19), 2b-P h (20),
and 2c-P h (9) were estimated via integration of the SnMe3
proton NMR signal. For 2a -P h : 1H NMR (400 MHz, C6D6, 20
°C): δ 8.01 (d, J ) 8.0 Hz, 2 H, ortho-Ph), 7.25-7.23 (m, 3 H,
para-Ph, meta-Ph), 4.68 (dd/ddd, J ) 1.2 Hz, 2.4 Hz, J (117/119Sn,
H) ) 8 Hz, 1 H, Cp-H3), 4.61 (pst, J ) 2.4 Hz, J (117/119Sn, H)
) nr, 1 H, Cp-H4), 4.61 (dd, J ) 1.2 Hz, 2.4 Hz, J (117/119Sn, H)
) nr, 1 H, Cp-H5), 3.93 (s, 5 H, Cp), 0.34 (s/d, J (117/119Sn, H) )
52/54 Hz, 9 H, Sn-Me). 13C NMR (100.5 MHz, C6D6, 20 °C): δ
140.5 (br, ipso-Ph), 134.9 (ortho-Ph), 131.5 (para-Ph), 128.1
(meta-Ph), 84.5 (s/d, J (117/119Sn, 13C) ) 53 Hz, Cp-C5), 81.6 (s/
d, J (117/119Sn, 13C) ) 36 Hz, Cp-C3), 81.1 (ipso-CpSn), 78.8 (s/
d, J (117/119Sn, 13C) ) 42 Hz, Cp-C4), 70.3 (Cp), -6.4 (s/d,
J (117/119Sn, 13C) ) 348/365 Hz, Sn-Me), not observed (ipso-CpB).
119Sn NMR (149.1 MHz, C6D6, 20 °C): δ -8.5. For 2b-P h : 1H
NMR (400 MHz, C6D6, 20 °C): δ 8.14 (d, J ) 8.0 Hz, 2 H, ortho-
Ph), 7.25-7.23 (m, 3 H, para-Ph, meta-Ph), 4.85 (dd/ddd, J )
1.2 Hz, 2.4 Hz, J (117/119Sn, H) ) 8 Hz, 1 H, Cp-H4), 4.73 (pst/
dpst, J ) 1.2 Hz, J (117/119Sn, H) ) 11 Hz, 1 H, Cp-H2), 4.49
(dd/ddd, J ) 1.2/2.4 Hz, J (117/119Sn, H) ) 10 Hz, 1 H, Cp-H5),
3.93 (s, 5 H, Cp), 0.18 (s/d, J (117/119Sn, H) ) 54/56 Hz, 9 H,
Sn-Me). 13C NMR (100.5 MHz, C6D6, 20 °C): δ 139.8 (br, ipso-
Ph), 135.0 (ortho-Ph), 131.8 (para-Ph), 128.3 (meta-Ph), 83.7
(s/d, J (117/119Sn, 13C) ) 46 Hz, Cp-C2), 82.9 (s/d, J (117/119Sn, 13C)
) 47 Hz, Cp-C5), 79.7 (s/d, J (117/119Sn, 13C) ) 37 Hz, Cp-C4),
79.2 (ipso-CpSn), 70.3 (Cp), -9.0 (s/d, J (117/119Sn, 13C) ) 345/
360 Hz, Sn-Me), not observed (ipso-CpB). 119Sn NMR (149.1
MHz, C6D6, 20 °C): δ -10.6. For 2c-P h : 1H NMR (400 MHz,
C6D6, 20 °C): δ 8.08 (d, J ) 8.0 Hz, 2 H, ortho-Ph), 7.25-7.00
(m, 3 H, para-Ph, meta-Ph), 4.66 (pst, J ) 1.8 Hz, 2 H, CpB-
H3,4), 4.61 (pst, ) 1.8 Hz, 2 H, CpB-H2,5), 4.18 (pst, J ) 1.8
Hz, 2 H, CpSn-H3,4), 3.89 (pst, J ) 1.8 Hz, 2 H, CpSn-H2,5),
0.21 (s/d, J (117/119Sn, H) ) 55/58 Hz, 9 H, Sn-Me). Reliable 13C
and 119Sn NMR data for 2c-P h could not be obtained since
the relative ratio of the 1,1′-disubstituted isomer in the
reaction mixture is very low. The 11B NMR signals for the
individual isomers could not be resolved. The following as-
signment is made for the isomer mixture: 11B NMR (128.3
MHz, C6D6, 20 °C): 57.2 (h1/2 ) 500 Hz).
Rea ction of 1 w ith 2 Equ iv of BCl3. Syn th esis of 3a -
Cl. A solution of BCl3 (7.2 mL, 1.0 M in hexanes) was added
dropwise to a solution of 1,1′-fc(SnMe3)2 (1.84 g, 3.60 mmol)
in hexanes (20 mL) at -78 °C. The reaction mixture was
allowed to slowly warm to ambient temperature and allowed
to stir for 12 h. All volatile material was removed under
reduced pressure (10-3 Torr), and the product was subse-
quently kept under dynamic vacuum at room temperature over
a period of 12 h. The red oily product was redissolved in
hexanes (20 mL) and filtered through a glass frit. After
repeated recrystallization at -78 °C dark red crystals of
1-stannyl-2-borylferrocene 3a -Cl (0.63 g, 35%) were isolated.
For 3a -Cl: 1H NMR (500 MHz, C6D6, 25 °C): δ 5.24 (dd/ddd,
J ) 1.2 Hz, 2.4 Hz, J (117/119Sn, H) ) 14 Hz, 1 H, Cp-H5), 4.55
(dd/ddd, J ) 1.2 Hz, 2.4 Hz, J (117/119Sn, H) ) 10 Hz, 1 H, Cp-
H3), 4.53 (pst/dpst, J ) 2.4 Hz, J (117/119Sn, H) ) 7 Hz, 1 H,
Cp-H4), 3.87 (s, 5 H, Cp), 0.69 (s/d, J (117/119Sn, H) ) 61/64 Hz,
3 H, Sn-Me), 0.52 (s/d, J (117/119Sn, H) ) 60/62 Hz, 3 H, Sn-
Me). 13C NMR (125.7 MHz, C6D6, 25 °C): δ 86.2 (s/d, J (117/119Sn,
C) ) 48 Hz, Cp-C5), 82.1 (ipso-CpSn), 81.1 (s/d, J (117/119Sn, C)
) 49 Hz, Cp-C4), 80.7 (s/d, J (117/119Sn, C) ) 43 Hz, Cp-C3),
77.6 (br, ipso-CpB), 71.7 (Cp), 2.5 (s/d, J (117/119Sn, C) ) 441/
462 Hz, Sn-Me), 1.1 (s/d, J (117/119Sn, C) ) 402/420 Hz, Sn-Me).
11B NMR (128.3 MHz, C6D6, 20 °C): δ 49.8 (h1/2 ) 350 Hz).
119Sn NMR (149.1 MHz, C6D6, 20 °C): δ 102.1. MS (70 eV,
Rea ction of 1 w ith C6F 5BCl2. Neat C6F5BCl2 (122 mg, 0.49
mmol) was added dropwise to a solution of 1,1′-fc(SnMe3)2 (250
mg, 0.50 mmol) in hexanes (10 mL) at -78 °C. The reaction
mixture was stirred at -78 °C for 1 h and then allowed to
slowly warm to 0 °C. All volatile material was removed under
reduced pressure (10-3 Torr), and the product was subse-
quently kept under dynamic vacuum at ambient temperature
over a period of 1 h. The crude product was obtained as a dark
red oil and characterized by NMR spectroscopy without further
purification. The isomer ratios for compounds 2a -P f (55%) and
2b-P f (45%) were estimated via integration of the SnMe3
proton NMR signal. The 1,1′-isomer 2c-P f was not observed.
For 2a -P f: 1H NMR (500 MHz, C6D6, 70 °C): δ 4.70 (dd/ddd,
J ) 1.2 Hz, 2.4 Hz, J (117/119Sn, H) ) 14 Hz, 1 H, Cp-H5), 4.61
(pst/dpst, J ) 2.4 Hz, J (117/119Sn, H) ) 5 Hz, 1 H, Cp-H4), 4.16
(br, 1 H, Cp-H3), 4.05 (s, 5 H, Cp), 0.29 (s/d, J (117/119Sn, H) )
53/56 Hz, 9 H, Sn-Me). 19F NMR (376.2 MHz, C6D6, 20 °C):
-130.3 (dd, J (F, F) ) 26 Hz, J (F, F) ) 8 Hz, 2 F, ortho-F),
-152.5 (t, J (F, F) ) 22 Hz, 1 F, para-F); -162.3 (m, 2 F, meta-
F). 13C NMR (100.5 MHz, C6D6, 20 °C): δ 146.9 (d, J (C, F) )
246 Hz, C6F5), 142.2 (d, J (C, F) ) 255 Hz, C6F5), 137.6 (d, J (C,
F) ) 252 Hz, para-C6F5), 113.5 (br, ipso-C6F5), 86.5 (s/d,
J (117/119Sn, 13C) ) 49 Hz, Cp-C5), 81.8 (ipso-CpSn), 81.0 (s/d,
J (117/119Sn, 13C) ) 33 Hz, Cp-C3), 80.5 (s/d, J (117/119Sn, 13C) )
40 Hz, Cp-C4), 77.5 (br, ipso-CpB), 70.5 (Cp), -6.8 (s/d,
J (117/119Sn, 13C) ) 351/368 Hz, Sn-Me). 119Sn NMR (149.1 MHz,
C6D6, 20 °C): δ -6.0. For 2b-P f: 1H NMR (500 MHz, C6D6,
70 °C): δ 4.53 (dd/ddd, J ) 1.2 Hz, 2.4 Hz, J (117/119Sn, H) ) 10
Hz, 1 H, Cp-H5), 4.46 (br, 1 H, Cp-H4), 4.42 (br, 1 H, Cp-H2),
4.08 (s, 5 H, Cp), 0.15 (s/d, J (117/119Sn, H) ) 53/56 Hz, 9 H,
Sn-Me). 19F NMR (376.2 MHz, C6D6, 20 °C): δ -130.3 (br, 2
F, ortho-F), -151.9 (br, 1 F, para-F), -162.3 (m, 2 F, meta-F).
13C NMR (100.5 MHz, C6D6, 20 °C): δ 146.7 (d, J (C, F) ) 245
Hz, C6F5), 142.1 (d, J (C, F) ) 255 Hz, para-C6F5), 137.6 (d,
J (C, F) ) 252 Hz, C6F5), 113.5 (br, ipso-C6F5), 84.4 (s/d,
J (117/119Sn, 13C) ) 45 Hz, Cp-C2), 82.9 (s/d, J (117/119Sn, 13C) )
46 Hz, Cp-C5), 81.2 (ipso-CpSn), 79.0 (s/d, J (117/119Sn, 13C) )
35 Hz, Cp-C4), 75.2 (br, ipso-CpB), 70.5 (Cp), -9.1 (s/d,
J (117/119Sn, 13C) ) 346/362 Hz, Sn-Me). 119Sn NMR (149.1 MHz,
C6D6, 20 °C): δ -9.9. The 11B NMR signals for the individual
isomers could not be resolved. The following assignment is
made for the isomer mixture: 11B NMR (128.3 MHz, C6D6, 20
°C): 52.0 (h1/2 ) 500 Hz).
Rea ction of 1 w ith 0.5 Equ iv of P h BCl2. Syn th esis of
1-Sta n n yl-2-bor ylfer r ocen es 2a -P h , 2b-P h , a n d 2c-P h . A

4180 Organometallics, Vol. 21, No. 20, 2002
Gamboa et al.
EI): m/z (%) 450 (100) [M+] 435 (28) [M+ - CH3], 420 (73) [M+
- 2 CH3]; C12H14BCl3FeSn (450.0) calcd C 32.03, H 3.14; found
C 32.40, H 3.33.
(7.34 g, 14.5 mmol) in hexanes (20 mL) at -78 °C. The reaction
mixture was stirred at -78 °C for 1 h and then allowed to
slowly warm to ambient temperature. All volatile material was
removed under reduced pressure (10-3 Torr), and the product
was subsequently kept under dynamic vacuum at 50 °C over
a period of 3 h. The red oily product was redissolved in hexanes
(20 mL) and filtered through a glass frit to remove a small
amount of insoluble material. After repeated recrystallization
at -37 °C dark red crystals of 1-stannyl-2-borylferrocene 4a -
Cl (2.43 g, 39%) were isolated. For 4a -Cl: 1H NMR (400 MHz,
C6D6, 20 °C): δ 5.33 (dd/ddd, J ) 1.2 Hz, 2.4 Hz, J (117/119Sn,
H) ) 15 Hz, 1 H, Cp-H5), 4.54 (pst/dpst, J ) 2.4 Hz, J (117/119Sn,
H) ) 7 Hz, 1 H, Cp-H4), 4.22 (dd/ddd, J ) 1.2 Hz, 2.4 Hz,
J (117/119Sn, H) ) 11 Hz, 1 H, Cp-H3), 3.83 (s, 5 H, Cp), 0.86 (s,
3 H, B-Me), 0.82 (s/d, J (117/119Sn, H) ) 63/66 Hz, 3 H, Sn-Me),
0.65 (s/d, J (117/119Sn, H) ) 60/63 Hz, 3 H, Sn-Me). 13C NMR
(100.5 MHz, C6D6, 20 °C): δ 84.8 (s/d, J (117/119Sn, C) ) 52 Hz,
Cp-C5), 81.8 (ipso-CpSn), 80.3, 80.0 (Cp-C3,4), 70.3 (Cp), 9.3
(br, B-Me), 2.7 (s/d, J (117/119Sn, C) ) 443/473 Hz, Sn-Me), 1.3
(s/d, J (117/119Sn, C) ) 408/429 Hz, Sn-Me), not observed (ipso-
CpB). 11B NMR (128.3 MHz, C6D6, 20 °C): δ 61.5 (h1/2 ) 400
Hz). 119Sn NMR (149.1 MHz, C6D6, 20 °C): δ 89.8. MS (70 eV,
EI): m/z (%) 430 (100) [M+], 415 (27) [M+ - CH3], 400 (78)
[M+ - 2 CH3], 246 (59) [MH+ - SnMe2Cl]; C13H17BCl2FeSn
(429.6) calcd C 36.35, H 3.99; found C 36.35, H 4.10.
Rea ction of 1 w ith a La r ge Excess of BCl3. A solution
of 1 (1.02 g, 2.00 mmol) in hexanes (25 mL) was cooled to -78
°C, and BCl3 (10.0 mL, 1.0 M in hexanes) was slowly added
via syringe. The reaction mixture was allowed to slowly warm
to ambient temperature and allowed to stir for 12 h. All volatile
material was removed under reduced pressure (10-3 Torr), and
the product was subsequently kept under dynamic vacuum at
room temperature over a period of 12 h. The red oily product
was redissolved in hexanes (20 mL) and filtered through a
glass frit. An NMR spectroscopic investigation of the crude
product revealed species 3a -Cl as the major component.
Rea ction of 1 w ith 2 Equ iv of P h BCl2. Syn th esis of 3a -
P h . Neat PhBCl2 (3.34 g, 21.0 mmol) was added dropwise to
a solution of 1,1′-fc(SnMe3)2 (5.12 g, 10.0 mmol) in toluene (50
mL) at -78 °C. The reaction mixture was allowed to slowly
warm to ambient temperature and stirred for 2 days. All
volatile material was removed under high vacuum at ambient
temperature over a period of 12 h. The red oily product was
redissolved in hexanes (20 mL), a small amount of insoluble
green material was removed via filtration through a glass frit,
and the solvent was removed under high vacuum. A dark red
oil was obtained and characterized by NMR spectroscopy. The
isomer ratios were estimated via integration of the SnMe3
proton NMR signal to ca. 60/40 for 1,2-isomers/1,3-isomers.
Repeated recrystallization of the crude product from hexanes
yielded dark red crystals (1.53 g, 31%) of the analytically pure
isomer 3a -P h . For 3a -P h : 1H NMR (400 MHz, C6D6, 20 °C):
δ 7.89 (d, J ) 8.0 Hz, 2 H, ortho-Ph), 7.23 (t, J ) 7.5 Hz, 1 H,
para-Ph), 7.18 (pst, J ) 7.8 Hz, 2 H, meta-Ph), 5.46 (dd/ddd,
J ) 1.2 Hz, 2.4 Hz, J (117/119Sn, H) ) 15 Hz, 1 H, Cp-H5), 4.63
(pst/dpst, J ) 2.4 Hz, J (117/119Sn, H) ) 8 Hz, 1 H, Cp-H4), 4.57
(dd/ddd, J ) 1.2 Hz, 2.4 Hz, J (117/119Sn, H) ) 11 Hz, 1 H, Cp-
H3), 3.86 (s, 5 H, Cp), 0.82 (s/d, J (117/119Sn, H) ) 63/66 Hz, 3
H, Sn-Me), 0.66 (s/d, J (117/119Sn, H) ) 60/63 Hz, 3 H, Sn-Me).
13C NMR (100.5 MHz, C6D6, 20 °C): δ 138.7 (ipso-Ph), 134.7
(ortho-Ph), 132.0 (para-Ph), 128.3 (meta-Ph), 85.4 (s/d, J (117/119Sn,
13C) ) 53 Hz, Cp-C5), 83.8 (ipso-CpSn), 80.9 (s/d, J (117/119Sn,
13C) ) 47 Hz, Cp-C3), 80.6 (s/d, J (117/119Sn, 13C) ) 52 Hz, Cp-
C4), 75.8 (br, ipso-CpB), 70.3 (Cp), 2.6 (s/d, J (117/119Sn, 13C) )
453/473 Hz, Sn-Me), 1.4 (s/d, J (117/119Sn, 13C) ) 409/430 Hz,
Syn th esis of 4a -P h . A solution of PhBCl2 (1.58 g, 9.95
mmol) in hexanes (30 mL) was added dropwise to a solution
of 1,1′-fc(SnMe3)2 (5.12 g, 10.0 mmol) in hexanes (80 mL) at
-78 °C. The reaction mixture was allowed to slowly warm to
ambient temperature and stirred for another 12 h. All volatile
material was removed under reduced pressure (10-3 Torr), and
the product was subsequently kept under dynamic vacuum at
80 °C over a period of 5 h. The red oily product was redissolved
in hexanes (20 mL) and filtered through a glass frit. After
repeated recrystallization at -37 °C dark red crystals of
1-stannyl-2-borylferrocene 4a -P h (1.09 g, 23%) were isolated.
For 4a -P h : 1H NMR (400 MHz, C6D6, 20 °C): δ 7.38 (d, J )
7.6 Hz, 2 H, ortho-Ph), 7.08-7.04 (m, 3 H, para-Ph, meta-Ph),
5.20 (dd/ddd, J ) 1.2 Hz, 2.4 Hz, J (117/119Sn, H) ) 14 Hz, 1 H,
Cp-H5), 4.72 (pst/dpst, J ) 2.4 Hz, J (117/119Sn, H) ) 6 Hz, 1 H,
Cp-H4), 4.70 (dd/ddd, J ) 1.2 Hz, 2.4 Hz, J (117/119Sn, H) ) 13
Hz, 1 H, Cp-H3), 4.02 (s, 5 H, Cp), 1.07 (s, 3 H, B-Me), 0.54
(s/d, J (117/19Sn, H) ) 57/60 Hz, 3 H, Sn-Me), -0.16 (s/d,
J (117/119Sn, H) ) 62/64 Hz, 3 H, Sn-Me). 13C NMR (100.5 MHz,
C6D6, 20 °C): δ 147.3 (ipso-Ph), 132.8 (ortho-Ph), 130.7 (para-
Ph), 129.1 (meta-Ph), 83.6 (s/d, J (117/119Sn, 13C) ) 58 Hz, Cp-
C5), 83.0 (s/d, J (117/119Sn, 13C) ) 57 Hz, Cp-C3), 80.9 (br, ipso-
CpB), 79.1 (s/d, J (117/119Sn, 13C) ) 54 Hz, Cp-C4), 77.9 (ipso-
CpSn), 69.9 (Cp), 10.7 (br, B-Re), 2.4 (s/d, J (117/119Sn, 13C) )
396/411 Hz, Sn-Me), -0.6 (s/d, J (117/119Sn, 13C) ) 438/454 Hz,
Sn-Me). 11B NMR (128.3 MHz, C6D6, 20 °C): δ 56.4 (h1/2
)
500 Hz). 119Sn NMR (149.1 MHz, C6D6, 20 °C): δ 85.9. MS (70
eV, EI): m/z (%) 492 (100) [M+], 477 (15) [M+ - CH3], 462 (53)
[M+ - 2 CH3]; C18H19BCl2FeSn (491.6) calcd C 43.98, H 3.90;
found C 43.55, H 3.90. For 3b-P h : 1H NMR (400 MHz, C6D6,
20 °C): δ 8.08 (d, J ) 8.0 Hz, 2 H, ortho-Ph), 7.3-7.1 (m, 3 H,
meta-Ph, para-Ph), 4.83 (dd/ddd, J ) 1.2 Hz, 2.4 Hz, J (117/119Sn,
H) ) 7 Hz, 1 H, Cp-H4), 4.73 (pst/dpst, J ) 1.2 Hz, J (117/119Sn,
H) ) 12 Hz, 1 H, Cp-H2), 4.51 (dd/ddd, J (H, H) ) 1.2 Hz, 2.4
Hz, J (117/119Sn, H) ) 11 Hz, 1 H, Cp-H5), 3.94 (s, 5 H, Cp),
0.41 (s/d, J (117/119Sn, H) ) 62 Hz, 3 H, Sn-Me), 0.40 (s/d,
J (117/119Sn, H) ) 62 Hz, 3 H, Sn-Me). 13C NMR (100.5 MHz,
C6D6, 20 °C): δ 139.3 (br, ipso-Ph), 135.1 (ortho-Ph), 132.2
(para-Ph), 128.2 (meta-Ph), 82.9 (s/d, J (117/119Sn, 13C) ) 66 Hz,
Cp-C2), 81.8 (J (117/119Sn, 13C) ) 62 Hz, Cp-C5), 80.2 (J (117/119Sn,
13C) ) 49 Hz, Cp-C4), 78.7 (ipso-CpSn), 74.2 (br, ipso-CpB),
70.7 (Cp), -1.8 (s/d, J (117/119Sn, 13C) ) 398/417 Hz, Sn-Me), -1.6
(s/d, J (117/119Sn, 13C) ) 398/417 Hz, Sn-Me). 119Sn NMR (149.1
MHz, C6D6, 20 °C): δ 118.9.
Sn-Me). 11B NMR (128.3 MHz, C6D6, 20 °C): δ 67.5 (h1/2
)
500 Hz). 119Sn NMR (149.1 MHz, C6D6, 20 °C): δ 94.1. MS (70
eV, EI): m/z (%) 472 (100) [M+], 457 (10) [M+ - CH3], 442 (50)
[M+ - 2 CH3]; C19H22BClFeSn (471.2) calcd C 48.43, H 4.71;
found C 48.02, H 4.70.
Cr yst a l St r u ct u r e Det er m in a t ion for 4a -Cl a n d
4a -P h . Data were collected on a Nonius Kappa-CCD dif-
fractometer and were integrated and scaled using the Den-
zo-SMN package.53 The structures were solved by direct
methods (SHELXS97) and refined by full-matrix least squares
(SHELXTL V5.154) based on F2 with all reflections. Non-
hydrogen atoms were refined anisotropically, and hydrogen
atoms were included in calculated positions. Atomic coordi-
nates, bond lengths and angles, and thermal parameters have
been deposited at the Cambridge Crystallographic Data Centre
(CCDC) as supplementary publications nos. 188 727 (4a -Cl)
Attem p ted Rea ction of 1 w ith MesBCl2. A solution of
1,1′-fc(SnMe3)2 (0.51 g, 1.00 mmol) in CH2Cl2 (10 mL) was
treated with neat MesBCl2 (0.20 g, 1.00 mmol) at -78 °C. The
reaction mixture was allowed to slowly warm to ambient
temperature and stirred for 2 days. A proton NMR spectro-
scopic investigation of the reaction mixture revealed only
signals for the two precursors.
(53) Otwinowski, Z.; Minor, W. Methods in Enzymology, Vol. 276,
Macromolecular Crystallography, Part A; Carter, C. W., Sweet, R. M.,
Eds.; Academic Press: London, 1997; pp 307-326.
(54) Sheldrick, G. M. SHELXTL/ PC, Version 5.1, Windows NT
Version; Bruker AXS Inc.: Madison, WI, 1999.
Syn th esis of 4a -Cl. A solution of BCl3 (14.5 mL, 1.0 M in
hexanes) was added dropwise to a solution of 1,1′-fc(SnMe3)2

ortho-Borylation of Bis(trimethylstannyl)ferrocene
Organometallics, Vol. 21, No. 20, 2002 4181
and 182/1154 (4a -P h ). Copies of the data can be obtained free
of charge on application to CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK (fax: (+44) 1223-336-033; e-mail: deposit@
ccdc.cam.ac.uk).
for his support and to Prof. J . B. Sheridan for his
interest in this work and for many useful suggestions.
Su p p or tin g In for m a tion Ava ila ble: Crystallographic
data for compound 4a -Cl, including tables of crystal data,
atomic coordinates, bond lengths and angles, and anisotropic
thermal parameters. UV-visible data and selected 2D NMR
spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
Ack n ow led gm en t is made to the donors of the
Petroleum Research Fund, administered by the ACS,
and the Rutgers University Research Council for sup-
port of this research. We are grateful to Prof. J . Manners
OM0204890