Isolation and characterization of borane complexes of dimethylsulfoxonium methylide

Article abstract of DOI:10.1021/om0208568

Trialkylboranes (R₃B) catalyze the repetitive insertion of methylene from dimethylsul-foxonium methylide (1) to form polymethylene. A proposed intermediate in this reaction is a 1:1 complex between R₃B and ylide 1. Following complexation, an alkyl group (boron-substituted) undergoes a 1,2-migration to the methylide carbon with displacement of a molecule of DMSO. A series of complexes of dimethylsulfoxonium methylide (1) and various organoboranes, X₃B (X = H, Ph, F, C₆F₅), have been prepared and isolated. Molecular structures, obtained by single-crystal X-ray diffraction, for ylide·BF₃ (3) and ylide·B(C₆F₅)₃ (4) were found to contain geometries with potential migrating groups anti-periplanar to the carbon-sulfur bond. The stability of solutions of these complexes ranges considerably. For example, ylide·BPh₃ (6) undergoes reaction at room temperature, while ylide·B(C₆F₅)₃ (3) is stable to temperatures > 100 °C. All complexes can be prepared as solids stable at room temperature. The solid-state stability of ylide·BR₃ complexes was evaluated by differential scanning calorimetry. The decomposition temperature increases across the series for R = H, Ph, C₆F₅, F. The heats of reaction of ylide·BR₃ are R = H (-54.7), Ph (-15.7), C₆F₅ (-21.6), and F (-17.1) kcal mol^-1, respectively.

Full text of DOI:10.1021/om0208568

1124
Organometallics 2003, 22, 1124-1131
Isola tion a n d Ch a r a cter iza tion of Bor a n e Com p lexes of
Dim eth ylsu lfoxon iu m Meth ylid e
J . M. Stoddard and K. J . Shea*
Department of Chemistry, University of California, Irvine, California
Received October 15, 2002
Trialkylboranes (R3B) catalyze the repetitive insertion of methylene from dimethylsul-
foxonium methylide (1) to form polymethylene. A proposed intermediate in this reaction is
a 1:1 complex between R3B and ylide 1. Following complexation, an alkyl group (boron-
substituted) undergoes a 1,2-migration to the methylide carbon with displacement of a
molecule of DMSO. A series of complexes of dimethylsulfoxonium methylide (1) and various
organoboranes, X3B (X ) H, Ph, F, C6F5), have been prepared and isolated. Molecular
structures, obtained by single-crystal X-ray diffraction, for ylide‚BF3 (3) and ylide‚B(C6F5)3
(4) were found to contain geometries with potential migrating groups anti-periplanar to the
carbon-sulfur bond. The stability of solutions of these complexes ranges considerably. For
example, ylide‚BPh3 (6) undergoes reaction at room temperature, while ylide‚B(C6F5)3 (3) is
stable to temperatures > 100 °C. All complexes can be prepared as solids stable at room
temperature. The solid-state stability of ylide‚BR3 complexes was evaluated by differential
scanning calorimetry. The decomposition temperature increases across the series for R )
H, Ph, C6F5, F. The heats of reaction of ylide‚BR3 are R ) H (-54.7), Ph (-15.7), C6F5 (-21.6),
and F (-17.1) kcal mol-1, respectively.
In tr od u ction
Ga, In, Ni, Pd, Cr, and Mo.17-20 These ylide-metal
complexes, however, do not undergo simple rearrange-
ment as do the triorganoboranes. Organoboranes also
form complexes with phosphonium ylides, which un-
dergo subsequent 1,2-migration at elevated reaction
temperatures.21 Recently, trialkylboranes have been
found to react with dimethylsulfoxonium methylide (1)
(Scheme 1),22-28 resulting in multiple insertions of
methylene into the carbon-boron bond. The reaction
produces polymethylene, a linear hydrocarbon polymer
comprised of repeating CH2 units. It has been proposed
that the polyhomologation involves a reaction cycle that
includes the formation of a 1:1 complex between R3B
and ylide 1 (Scheme 2). Subsequent 1,2-migration by
an alkyl group affords a homologated organoborane and
dimethyl sulfoxide. The organoborane product re-enters
the cycle to produce eventually a tris-polymethylene
borane. Subsequent oxidation affords ω-hydroxypoly-
methylene (Scheme 1).
Organoborane chemistry has played an important role
in the construction of carbon-carbon bonds.1,2 Many of
these reactions involve formation of a tetrahedral “ate”
complex followed by transfer of a carbon nucleophile to
an acceptor. The transfer can be intermolecular, such
as in the Suzuki reaction,3,4 or intramolecular.5,6 Ex-
amples of the latter include reactions of trialkylboranes
with nucleophilic reagents such as ethyl bromoacetate,7
R-chloroacetonitrile,7 R-bromo ketones,8 dihalomethyl-
lithium,9 dimethylsulfoxonium methylide (1),10,11 di-
methylsulfonium methylide (2),12 and various diazo
compounds.13-16 These reactions result in formation of
new carbon-carbon bonds, and several have achieved
synthetic utility. Dimethylsulfoxonium methylide (1)
coordinates to a variety of metals including Au, Be, Al,
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Int. Ed. 2001, 40, 4544-4568.
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(16) Seyferth, D. Chem. Rev. 1955, 55, 1155.
(17) Yamamoto, Y. Bull. Chem. Soc. J pn. 1987, 60, 1189-1191.
(18) Poerschke, K. R. Chem. Ber. 1987, 120, 425-427.
(19) Lin, I. J . B.; Lai, H. Y. C.; Wu, S. C.; Hwan, L. J . Organomet.
Chem. 1986, 306, C24-C26.
(20) Weber, L. Z. Naturforsch., B: Anorg. Chem., Org. Chem. 1976,
31B, 780-785.
(21) Bestmann, H. J .; Roeder, T.; Suehs, K. Chem. Ber. 1988, 121,
1509-1517.
(22) Shea, K. J .; Busch, B. B.; Paz, M. M.; Staiger, C. L.; Stoddard,
J . M.; Walker, J . R.; Zhou, X.; Zhu, H. J . Am. Chem. Soc. 2002, 124,
3636.
(23) Shea, K. J .; Wagner, C. E. Org. Lett. 2001, 3, 3063.
(24) Shea, K. J .; Zhou, X.-Z. J . Am. Chem. Soc. 2000, 122, 11515.
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10.1021/om0208568 CCC: $25.00 © 2003 American Chemical Society
Publication on Web 01/25/2003

Borane Complexes
Organometallics, Vol. 22, No. 5, 2003 1125
Sch em e 1
Sch em e 2
F igu r e 1. Molecular structure of 3. Selected bond lengths
(Å) and angles (deg): S1-C1 1.742(1), S1-C3 1.761(2), S1-
C2 1.758(2), S1-O1 1.454(1), C1-B1 1.650(2) F1-B1
1.394(2), F2-B1 1.400(2), F3-B1 1.400(2); C1-S1-C2
108.11(7), C1-S1-C3 106.88(7), C1-S1-O1 113.22(7),
C2-S1-C3 105.43(8), C3-S1-O1 111.06(8), C2-S1-O1
111.72(7), C1-B1-F1 107.97(11), C1-B1-F2 110.38(12),
C1-B1-F3 112.45(11), F1-B1-F2 109.24(12), F1-B1-
F3 108.70(14), F2-B1-F3 108.05(12).
We are interested in the structure, stability, and
activation energy for the subsequent 1,2-migration of
the proposed intermediate, a zwitterionic complex.
Screening studies of Lewis acidic catalysts for the
polyhomologation reaction have revealed significant
differences in reactivity of the ylide‚borane complex.29
In an effort to design more efficient catalysts, we wished
to understand how boron substituents influence the
stability of the complex and the activation energy for
the subsequent reaction. We report the synthesis and
isolation of complexes of ylide 1 and boranes. These
complexes are crystalline solids that exhibit a range of
thermal stabilities. The quantification of their stability
by thermal analysis has provided an opportunity to
evaluate the influence of substituents on boron on the
activation energy for subsequent reactions.
shows a quartet at -23 ppm (J B-F ) 45 Hz). Typical
11B chemical shifts for trigonal boranes range from 90
to 25 ppm, while tetrahedral borates range from 0 to
-20 ppm.31 The observed 11B chemical shift for complex
3 is consistent with a borate complex.
The molecular structure of complex 3, obtained by
single-crystal X-ray analysis, is shown in Figure 1. The
complex has a staggered conformation about the B1-
C1 bond with an alignment one of the fluorine atoms
anti to the sulfoxonium group (F1-B1-C1-S1 )
-161.21(11)°). The anti fluorine has a slightly shorter
B-F distance (1.394 Å) compared to the two remaining
fluorines (1.400 Å). In addition, the F1-B1-C1 angle
(107.97(11)°) is smaller than the two remaining F3-
B1-C1 (112.45(11)°) and F2-B1-C1 (110.38(12)°) angles.
The sulfur-oxygen bond of complex 3 is aligned anti to
the B-C bond (B1-C1-S1-O1 166.42(10)°), which may
be due to the opposing dipole moments of the BF3 and
S-O groups.
The ylide‚B(C6F5)3 (4) complex was prepared by the
addition of a solution of ylide 1 in toluene to a solution
of tris(pentafluorophenyl)borane in toluene. The solvent
was evaporated to afford a white solid, which was
crystallized from CHCl3/hexanes to afford complex 4 in
82% yield. The 11B NMR of tris(pentafluorophenyl)-
borane in toluene-d8 shows a broad singlet at 59.8 ppm.
Complex 4, however, exhibits a sharp singlet at -17.8
ppm consistent with a tetrahedral borane. The 19F NMR
shows three resonances consistent with ortho (133.2
ppm), meta (158.8 ppm), and para (164.3 ppm) fluorines
of complex 4.
Resu lts a n d Discu ssion
At low temperatures, trialkylboranes form 1:1 com-
plexes with ylide 1. Upon warming to room temperature,
they readily undergo 1,2-migration to produce homolo-
gated organoboranes. Alkylboronic and alkylborinic
esters also form 1:1 complexes with ylide 1 (11B NMR).
However these complexes do not decompose to form the
corresponding homologated organoboranes, even at
elevated temperatures.29 Additional studies confirmed
that the stability of ylide‚borane complexes was sensi-
tive to substituents on boron. For example, BH3‚SMe2
and Ph3B were efficient initiators for polyhomologation
at 23 °C, while BF3‚OEt2 and B(C6F5)3 were ineffective,
even at temperatures up to 100 °C. It is assumed that
in all cases coordination of ylide 1 with the Lewis acidic
organoborane takes place, but 1,2-migration does not
occur in the two latter cases. To obtain support for this
proposal, we first investigated the BF3 and B(C6F5)3
complexes of ylide 1 since they are ineffective at
bringing about polyhomologation at room temperature.
The addition of BF3‚OEt2 to a solution of ylide 1 in
toluene at room temperature produced a precipitate that
was filtered and recrystallized from EtOH/H2O to afford
analytically pure ylide‚BF3 complex 330 in 50% yield.
The solution 19F NMR (1:1 MeOH-d4/d2O) shows a
quartet at 134.5 ppm (J F-B ) 45 Hz), and the 11B NMR
The molecular structure of complex 4, obtained by
single-crystal X-ray analysis, is shown in Figure 2. The
substituents on B1 and C19 are staggered, and one of
the aryl rings is aligned anti to the C19-S1 bond (C1-
B1-C19-S1 ) 165.4(2)°). The aligned aryl ring has a
longer B-C bond distance (B1-C1 ) 1.664(3) Å) com-
pared to the remaining aryl groups (1.655(3), 1.648(3)
Å). In contrast to the ylide‚BF3 complex 3, the sulfur-
oxygen bond of complex 4 is aligned gauche to the B-C
(29) Unpublished results.
(30) Corey, E. J .; Chaykovsky, M. J . Am. Chem. Soc. 1965, 87, 1353.
(31) Wrackmeyer, B. Annu. Rep. NMR Spectrosc. 1988, 20, 61.

1126 Organometallics, Vol. 22, No. 5, 2003
Stoddard and Shea
F igu r e 4. (a) Proposed molecular orbital interactions favor
the migrating group (HOMO: C-B σ) to align anti-
periplanar to the sulfoxonium leaving group (LUMO: C-S
σ*). (b) Preferred orientation of aryl ring with aromatic
π-system aligned with C-S σ*.
temperature without apparent decomposition. However,
when samples were stored in screw-cap vials at room
temperature, colorless crystals formed near the cap. A
single-crystal X-ray analysis identified the substance as
methylboronic acid, apparently formed by 1,2-migration
of complex 5 followed by reaction with adventitious
water. To avoid this, samples were stored at -20 °C for
prolonged periods of time without apparent decomposi-
tion. The complex 5 was sparingly soluble in DMF at 0
°C. The 11B NMR indicated a sharp singlet at -31.2
ppm. The IR spectrum shows a characteristic B-H
stretching frequency at 2284 cm-1 consistent with a
monomeric hydridoborate species.34
Triphenylborane is also an efficient initiator for the
polymerization of 1 at room temperature; therefore a
low-temperature synthesis of the ylide complex with
triphenylborane was employed. The addition of an
equimolar amount of ylide 1 in THF to a solution of
triphenylborane in THF at -78 °C afforded a homoge-
neous solution of ylide‚BPh3 (6). The 11B NMR (CDCl3)
of triphenylborane shows a broad singlet at 67.0 ppm,
while ylide‚BPh3 complex 6 in toluene-d8 at -40 °C
shows a sharp singlet at -10.3 ppm. The 11B chemical
shift is consistent with the tetrahedral borate of complex
6.31
F igu r e 2. Molecular structure of 4. Selected bond lengths
(Å) and bond angles (deg): S1-C19 1.756(2), S1-C21
1.755(2), S1-C20 1.753(3), S1-O1 1.438(2), C19-B1
1.674(3), B1-C1 1.664(3), B1-C7 1.655(3), B1-C13 1.648(3);
C19-S1-O1 113.88(11), C19-S1-C21 109.94(11), C19-
S1-C20 105.90(12), C20-S1-C21 104.19(14), C21-S1-
O1 111.33(12), C20-S1-O1 111.04(12), S1-C19-B1
118.65(15),C19-B1-C1 105.33(16),C19-B1-C7 105.69(17),
C19-B1-C13 115.05(18), C1-B1-C7 113.76(18), C1-B1-
C13 104.03(16), C7-B1-C13 112.95(17).
F igu r e 3. Complex ylide‚B(C6F5)3 (4) prefers the gauche
conformation. Diminished steric interactions between the
aryl groups and sulfoxonium methyls may contribute to the
preference.
Efforts to obtain single crystals suitable for X-ray
analysis of complex 6 by slowly removing the solvent
in vacuo at -40 °C resulted in the formation of a
microcrystalline solid. Interestingly, after solvent re-
moval at low temperatures (-40 °C), solid complex 6
was stable at room temperature and could be stored at
room temperature without decomposition for more than
six months. The solid-state stability of this complex
must reflect restricted motion along a reaction coordi-
nate imposed by the crystal lattice.35
A microcrystalline sample of complex 6 was heated
to 90 °C for 10 min, then oxidized with trimethylamine-
N-oxide in THF at room temperature for 3 h. The
product mixture was analyzed by GC/MS and found to
contain the following amounts of phenol (57%), benzyl
alcohol (42%), and phenethyl alcohol (<1%).
bond (B1-C19-S1-O1 ) 69.7(2)°). This may be due to
steric interactions between the two sulfoxonium methyls
and aryl rings (Figure 3). The proximity between the
electron-rich sulfoxonium oxygen and one of the electron-
deficient aromatic rings (3.075 Å) in structure 4 could
also contribute to the observed geometry.
Complex 4 contains two geared aryl rings, while a
third aryl ring is synclinal to the ylide carbon-boron
bond (C19-B1-C1-C6 ) 53.7(2)°). The aryl C-B bond
is aligned anti-periplanar to the C-S bond. The twisted
conformation of the aryl ring allows favorable orbital
overlap between the aromatic π-system (HOMO) and
the C-S σ* (LUMO) (Figure 4b). This orientation is also
preferred in the related pinacol and Wagner-Meerwein
shifts of phenyl rings.32
Borane initiates the polymerization of ylide 1 at room
temperature. Therefore, the attempted preparation of
its complex was carried out at -40 °C. A solution of ylide
1 in THF was added to an equimolar amount of BH3‚
SMe2 in THF at -40 °C. A white precipitate formed
within 10 min. The precipitate was filtered cold, washed
with cold hexane, and dried in vacuo to afford ylide‚
BH3 (5) in quantitative yield.33 As a microcrystalline
solid, complex 5 could be handled routinely at room
Th er m a l An a lyses of Ylid e‚Bor a n e Com p lexes
3-6. Although all solid complexes were stable at room
temperature, the onset temperature at which a solid-
state reaction took place varied considerably. It was
(33) Caution, The BH3-ylide complex has been prepared in 3 g
quantities as a white solid substance. The DSC (<1.5 mg) shows a
very exothermic transition above 55 °C, and larger amounts of solid
complex should not be heated.
(34) Golthup, M.; Daly, L.; Wiberey, S. Introduction to Infrared and
Raman Spectroscopy, S. 292; Academic Press: New York/London, 1990.
(35) Boldyrev, V. V. React. Solids 1990, 8, 231-246.
(32) Bachmann, W.; Gerguson, J . J . Am. Chem. Soc. 1934, 56, 2081.

Borane Complexes
Organometallics, Vol. 22, No. 5, 2003 1127
(3). The exothermic peak width for BH3‚ylide (5) is very
sharp34 (<2 °C), whereas the peak widths for B(C6F5)3‚
ylide (4) and BPh3‚ylide (6) are less so (<10 °C). In
contrast the DSC scan for BF3‚ylide (3) shows a slight
endothermic peak preceding two exothermic peaks with
a much broader temperature range (∼30 °C). In an effort
to identify the products of decomposition, complex 3 was
gradually heated to 200 °C and continually monitored
with mass spectroscopy (EI). Evidence for F2BCH2F was
not found. The products of this reaction have not been
characterized. The two reaction exotherms may indicate
1,2-migration, followed by further decomposition. Al-
ternatively, these reactions may occur by completely
different processes other than 1,2-migration. Fluorine
1,2-migration has been demonstrated in one system: the
reaction of diazomethane with BF3 in the gas phase at
-40 °C.38
Reversible (physical) and irreversible (chemical) heat
flows can be deconvoluted using modulated DSC (MDSC)
instrumentation.39 The MDSC heating profile for com-
plex 6, which is representative for the complexes
analyzed in this study, is given in Figure 6. The total
heat flow (top trace) is the sum of the reversible heat
flow (middle trace) and nonreversible heat flow (bottom
trace). The nonreversing heat flow scan shows a large
exothermic peak (∆Hrxn ) -15.7 kcal mol-1), presum-
ably due to 1,2-migration. The heats of 1,2-migration
(∆Hrxn) are listed in Table 1. The most exothermic
rearrangement, ylide‚BH3 (5), has a ∆Hrxn of 54.7 kcal
mol-1 or 517 cal g-1, about 1.5 times that of ammonium
perchlorate by weight (347 cal g-1).40
F igu r e 5. DSC traces of complexes 3-6 at 10 °C min-1
and arbitrary y-offset
Ta ble 1. Th er m od yn a m ic Da ta for Com p lexes 3-6a
Td at
Ton at
compound
∆Hrxn
-11.3
∆Hm
5 °C min-1
5 °C min-1
F3B‚ylide (3)
1.6
0.4
0.1
0.3
157.3-185.2
144.5-153.5
60.9-62.4
157.3
144.5
60.9
(C6F5)3B‚ylide (4) -21.6
BH3‚ylide (5)
Ph3B‚ylide (6)
-54.7
-15.7
88.0-96.5
88.0
a
Heats of reaction in kcal mol-1, temperature in °C. ∆Hrxn and
∆Hm are the nonreversing (1,2-migration) and reversing (melting)
heat flow, respectively, from MDSC. Td is the exothermic range
and Ton is the onset temperature of reaction from DSC.
decided to quantitate these differences using thermal
analyses of the solids. Reactions in the melt or solution
can generally be described by Arrhenius kinetics. In
contrast, physical characteristics (dislocations, crystal
size, nuclei distribution, etc.) play a roll in the kinetic
equations that describe solid-state reactions.36 These
reactions are referred to as topochemical reactions and
occur in the solid state with the least amount of
molecular motion.35 The neighboring molecules in the
crystal lattice largely determine the course and stereo-
chemistry of these reactions. Differences between the
solid-state and solution reactions are well documented.
For example, the kinetics of the thermal cis to trans
isomerization of azobenzene has been studied in the
solid and melt. The Arrhenius parameters in the solid
state (Ea ) 53.3 kcal mol-1, log A ) 31.04) are substan-
tially higher than the melt (Ea ) 24.7 kcal mol-1, log A
) 12.21).37 It is reasonable therefore to expect the solid-
state lattice energy to stabilize otherwise labile, reactive
ylide‚borane complexes.
Differential scanning calorimetry (DSC) was em-
ployed to evaluate the thermochemistry of reaction. An
overlay of the DSC scans of the four complexes is shown
in Figure 5. Most crystalline solids display an endo-
therm upon melting; however all the ylide‚borane
complexes exhibit an exothermic peak, presumably due
to the exothermicity associated with reaction superim-
posed on the melting exotherm. The onset temperatures
for the exothermic peak for complexes 3-6 are listed
in Table 1. With a scan rate of 5 °C min-1 these vary
from 60.9 °C for BH3‚ylide (5) to 157.3 °C for BF3‚ylide
The reversing heat flow scan for complex 6 shows a
much smaller endothermic peak (∆Hf ) 0.3 kcal mol-1
)
than the nonreversing heat flow scan. The apparent
heats of fusion for the zwitterionic complexes 3-6 (Table
1) were obtained from integration of the reversing heat
flow curves. The heats of fusion for these are lower than
the heat of fusion of typical organic molecules. For
example, benzene (∆Hf ) 2.4 kcal mol-1 41
acid (∆Hf ) 4.1 kcal mol-1 41
all have higher heats of
)
and benzoic
)
fusion than complexes 3-6. Since the two processes,
reversible and nonreversible heat flow, overlap and the
former dominates the thermal analyses, the uncertain-
ties in the reversing component may be significant
(Figure 3).
The thermal analyses of solid complexes 3-6 all have
strong exothermic signatures. It is reasonable to assume
that the exothermicity of these ylide‚organoborane
complexes is due to the irreversible 1,2-migration from
the “ate”-complex, with the exception of complex 3. In
addition, there may be a component of exothermicity
due to the complexation of the expelled DMSO and
homologated organoborane.
The MDSC (Figure 6) for complex 6 shows a peak
onset for nonreversing heat flow of 71.3 °C and a peak
onset for reversing heat flow of 74.8 °C. All the com-
plexes show an exothermic nonreversing heat flow
(∆Hrxn) that slightly precedes an endothermic reversing
(38) Goubeau, J .; Rohwedder, K. H. Ann. 1957, 604, 168-178.
(39) (a) Brown, M. E. Introduction to thermal analysis: techniques
and applications; Kluwer Academic Publishers: Boston, 2001. (b)
Haines, P. J . Principles of Thermal Analysis and Calorimetry; Royal
Society of Chemistry: Cambridge, 2002.
(36) Davydov, E.; Vorotnikov, A.; Pariyskii, G.; Zalkov, G. Kinetic
Peculiarities of Solid Phase Reactions; J ohn Wiley & Sons: New York,
1998.
(40) Carl, F. Franklin Inst. 1940, 230, 75.
(41) Hodgman, C. D. Handbook of Chemistry and Physics, 31st ed.;
Chemical Rubber Publishing Co.: Cleveland, 1949.
(37) Wolf, E.; Cammenga, H. K. Z. Phys. Chem. Neue Fol. 1977, 107,
21-38.

1128 Organometallics, Vol. 22, No. 5, 2003
Stoddard and Shea
F igu r e 6. MDSC of complex 6 showing total, reversing, and nonreversing heat flow.
Sch em e 3
heat flow (∆Hf). The MDSC data suggest that the solid
complexes undergo exothermic 1,2-migration in the solid
state that induces further reaction and melting. This
implies that the peak onset temperature for the non-
reversing heat flow is determined by the chemical
stability of the ylide‚borane complex and not necessarily
an initiated physical “melting point” of the complex
followed by reaction. The very low heats of fusion
obtained from the reversing heat flows support the
hypothesis of a substantial component of the reaction
in the solid state. The heat of fusion is a reversible
process, but the 1,2-migration is not. If the solid
complexes do not melt, but turn liquid via 1,2-migration,
then the contribution from reversible heat flow would
be expected to be zero. The very low heats of fusion for
these zwitterionic complexes suggest this is at least
Sch em e 4
trimethylamine may irreversibly complex with the
partially oxidized organoborane, thereby shielding it-
toward further oxidation to produce pentafluorophen-
ol.
partially true. The peak onset temperatures from the
DSC qualitatively establish a reaction onset tempera-
ture for 1,2-migration for ylide‚BR3 complexes in the
solid state, R ) C6F5 > Ph > H. Interestingly, this trend
correlates with the measured Lewis acidity of the
corresponding boranes C6F5 (6.38) > Ph (3.85) > H
(3.79).42
An a lysis of th e P r od u cts of Decom p osition of
Com p lex 4. In an effort to relate the exotherm of the
ylide‚borane complex to the 1,2-migration reaction, the
decomposition of solid complex 4 was studied. Solid
ylide‚B(C6F5)3 complex (4) was heated to 150 °C for 10
min under nitrogen. Oxidation was then carried out in
refluxing benzene with 6 equiv of anhydrous trimethyl-
amine-N-oxide (TAO)43 for 15 h (Scheme 5). An analysis
by GC/MS indicated the presence of pentafluorobenzyl
alcohol. Neither pentafluorophenol nor pentafluorophen-
ethyl alcohol were detected. Presumably, the pentafluo-
rophenyl group is less reactive than the pentafluoroben-
zyl group toward oxidation with TAO. This may explain
the preferential formation of pentaflurobenzyl alcohol
over pentafluorophenol. Furthermore, the liberated
In another experiment, complex 4 was heated at 150
°C for 10 min, dissolved in CDCl3, and allowed to
evaporate (Scheme 5). Crystals formed over several
weeks. The molecular structure of (C6F5)2BCH2(C6F5)‚
DMSO complex 7 obtained by single-crystal X-ray
analysis is shown in Figure 7. Since the product orga-
noborane is highly Lewis acidic, complexation with
DMSO in the melt occurs to provide organoborane‚
DMSO complex 7. The B-O bond distance of 1.560(3)
Å indicates a strong coordination between the sulfoxide
oxygen and electrophilic borane. The B-O bond distance
can be compared to a complex with homoboraadaman-
tane (B-O 1.668(3) Å), a highly Lewis acidic borane.24
The isolation of this complex 7 indicates that the
pentafluorophenyl group of complex 4 (Figure 2) has
undergone 1,2-migration. This experiment supports the
hypothesis that complex 4 undergoes 1,2-rearrangment
in the DSC experiments shown earlier. Since complexes
5 and 6 display similar MDSC traces with an exother-
mic nonreversing peak preceding an endothermic re-
versing peak, it is likely that they also undergo 1,2-
rearrangement in the solid state.
(42) Farfan, N.; Contreras, R. J . Chem. Soc., Perkin Trans. 2 1987,
771.
(43) Soderquist, J . A.; Najafi, M. R. J . Org. Chem. 1986, 51, 1330-
1336.

Borane Complexes
Organometallics, Vol. 22, No. 5, 2003 1129
Sch em e 5
F igu r e 7. Molecular structure of (C6F5)2BCH2(C6F5)‚
DMSO (7). Selected bond lengths (Å) and angles (deg): S1-
C20 1.783(3), S1-C21 1.780(3), S1-O1 1.561(2), B1-C1
1.647(3), B1-C7 1.647(3), B1-C13 1.634(3), B1-O1 1.560(3),
C13-C14 1.507(3); C21-S1-C21 98.09(14), C20-S1-O1
99.48(11), C21-S1-O1 104.71(11), S1-O1-B1 122.84(13),
C1-B1-O1 107.39(17), C1-B1-C7 110.00(17), C1-B1-
C13 114.36(18), O1-B1-C13 109.77(18), B1-C13-C14
113.19(18).
F igu r e 8. Reaction coordinate diagram for the 1,2-
migration reaction of ylide‚B(C6F5)3 (4) (R ) C6F5) and
ylide‚BPh3 (5) (R ) Ph) and the subsequent coordination
of DMSO.
A proposed reaction coordinate diagram (Figure 8) for
complex 4 shows a high activation energy for the 1,2-
migration. The activation energy for dissociation of
complex 4 to produce tris(pentafluorophenyl)borane and
free ylide may be lower. The 1,2-migration initially
produces a homologated organoborane and DMSO. The
DMSO then coordinates to the organoborane to produce
DMSO complex 7. DMSO has also been shown to weakly
coordinate to the less Lewis acidic triethylborane. The
complexation between DMSO and the homologated
organoboranes in this study is likely.
4, which can lead to a higher activation energy barrier
for 1,2-migration. Both of these effects combine to raise
the activation energy barrier and increase the stability
of the complexes. It is not known which one has a
greater affect on the transition state energy in the
current study.
Con clu sion
A series of complexes of dimethylsulfoxonium meth-
ylide (1) and various organoboranes have been prepared
and isolated. Molecular structures, obtained by single-
crystal X-ray diffraction, for ylide‚BF3 (3) and ylide‚
B(C6F5)3 (4) were found to contain geometries with the
potential migrating group anti-periplanar to the carbon-
sulfur bond. The reaction onset of the solid complexes
ylide‚BF3 (3), ylide‚B(C6F5)3 (4), ylide‚BH3 (5), and ylide‚
BPh3 (6) were evaluated using DSC and found to
correlate to the Lewis acidity of the organoborane. Thus
the onset temperature of reaction in the solid state of
ylide‚BR3 (R ) F > C6F5 > Ph > H) correlated with the
Lewis acidity of the organoborane BR3 (R ) F > C6F5
This study reveals that ylide 1 complexes of strongly
Lewis acidic boranes such as B(C6F5)3 have higher
activation energies toward 1,2-migration. The effects of
electron-withdrawing groups attached to boron are
twofold. First, the Lewis acidity is increased42 and the
resulting ylide complex is more stable (Figure 8).
Second, electron-withdrawing groups on boron lower the
HOMO energy of the migrating group (Figure 4). This
is expected to raise the activation energy for 1,2-
rearrangement. For example, the fluorine substituents
serve to lower the HOMO of the π-system for complex

1130 Organometallics, Vol. 22, No. 5, 2003
Stoddard and Shea
Ta ble 2. Cr ysta l Da ta a n d Str u ctu r e Refin em en t P a r a m eter s for Com p lexes 3, 4, a n d 7
3
4
7
empirical formula
fw
C3H8BF3OS
159.96
C21H8BF15OS
604.14
C21H8BF15OS
604.14
temperature (K)
λ (Mo KR) Å
crystal syst
space group
unit cell dimens
a (Å)
178(2)
0.71073
orthorhombic
P212121
158(2)
173(2)
0.71073
triclinic
P ob 1 obx
0.71073
monoclinic
P21/n
5.5467(4)
8.1059(5)
14.4239(10)
90.0
90.0
90.0
648.51(8)
4
1.638
0.472
328
18.8527(15)
13.0080(10)
19.7288(16)
90.0
115.7860(10)
90.0
4356.4(6)
8
1.842
10.3619(4)
10.4008(4)
12.3825(5)
102.1170(10)
101.5670(10)
116.5960(10)
1098.51(7)
2
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
F
calc (g cm-3
)
1.826
0.290
596
µ (mm-1
F(000)
)
0.292
2384
cryst size (mm)
diffractometer
θ range (deg)
0.33 × 0.29 × 0.15
Bruker CCD
2.82-28.26
4197/1533
1533
116
0.0232
0.0633
1.069
0.40 × 0.35 × 0.26
Bruker CCD
1.25-28.30
27 920/10 304
10 304
768
0.0420
0.1156
1.039
0.29 × 0.21 × 0.17
Bruker CCD
2.31-28.29
11 880/5195
3954
384
0.0418
0.0976
1.035
no. of total/unique data [I>2σ(I)]
no. of obsd data
no. of params
final R1a
final wR2b
goodness-of-fitc
difference Fourier (e Å-3
)
0.252/-0.255
0.417/-0.404
0.470/-0.357
∑||Fo| - |Fc||/∑|Fo|. [∑[w(Fo - Fo2)2]/∑[w(Fo2)2]1/2
.
c [∑[w(Fo - Fc2)2]/(Nref - Nvar)]1/2
.
a
b
2
2
were processed using SAINT44 and SADABS45 to yield the
reflection data file. Subsequent calculations were carried out
using the SHELXTL45 program. The structure was solved by
direct methods and refined on F2 by full-matrix least-squares
techniques. The analytical scattering factors46 for neutral
atoms were used throughout the analysis. Hydrogen atoms
were located from a difference Fourier map and refined (x,y,z
and Uiso). All details of the crystal, data collection, and
refinement parameters are summarized in Table 2.
> Ph > H). Thermal analysis using DSC was also used
to obtain the ∆Hrxn for 1,2-migration for complexes 4-6.
The heats of reaction for the ylide‚BR3 complexes (R )
H > C6F5 > Ph > F) correlate with heats of reaction
estimated with average bond dissociation energies (R
) H > C6F5 ≈ Ph > F).
Exp er im en ta l Section
Dim eth ylsu lfoxon iu m Meth ylid e‚Tr iflu or obor a n e (3).
To a solution of ylide 1 (0.4 M in toluene, 10 mL, 4 mmol) was
gradually added freshly distilled BF3‚OEt2 (0.5 mL, 4 mmol,
1 equiv). The reaction mixture was stirred for 12 h at room
temperature. The precipitate was filtered and recrystallized
three times from 1:1 EtOH/H2O to yield pale brown crystals
(0.24 g, 37% yield): mp 167.4-168.2 °C (lit31 mp 245-270 °C);
Gen er a l Con sid er a tion s. All reactions were carried out
in anhydrous solvents under N2. Toluene was dried over
calcium hydride, and tetrahydrofuran (THF) was obtained by
passing through an alumina column under Ar. Solutions of
ylide 1 in THF30 and toluene22 were synthesized as previously
reported. Triphenylborane (Aldrich) was sublimed (0.05 mmHg,
temp) and BF3‚OEt2 (Aldrich, 99% redistilled) was distilled
before use. Anhydrous trimethylamine-N-oxide was prepared
from trimethylamine-N-oxide dihydrate (Aldrich).43 THF was
dried by passing through an alumina column under Ar. Tris-
(pentafluorophenyl)borane (95% Aldrich) was used as received.
Th er m a l An a lysis: MDSC a n d DSC. Differential scan-
ning calorimetry (DSC) was recorded on a Du Pont 910 DSC
instrument with Al2O3 as an internal reference standard and
a heating rate of 5 °C min-1. MDSC was obtained on a DSC
2920 instrument with a heating rate of 0.2 °C min-1, MDSC
heating profile of 0.313 °C and 60 s period, and under N2 flow
(50 cm3 min-1). Samples were weighed (5-8 mg) into crimped
aluminum pans for complexes 3, 4, and 6. Complex 5 was
weighed out in smaller amounts (1.5 mg) due to its violent
decomposition.
IR (KBr) νmax 3037.7, 2939.7, 1225.3, 1039.4, 989.1, 575.8 cm-1
;
1H NMR (500 MHz, MeOH-d4/d2O, 1:1) δ 3.59 (s, 6H), 2.97 (s,
2H); 19F NMR (376 MHz, MeOH-d4/d2O, 1:1) δ 134.5 (q, J )
44.7 Hz); 13C NMR (125 MHz, MeOH-d4/d2O, 1:1) δ 40.7 (q, J
) 1.9 Hz); 11B NMR (160 MHz, MeOH-d4/d2O, 1:1) δ -23.0 (q,
J ) 45.0 Hz); HRMS (CI/isobutane) m/z calcd for BC3F3H8OS
(M - F)+ 141.0358, found 141.0357. Anal. Calcd for C3H8-
SBF3O: C, 22.50; H, 5.04. Found: C, 22.62; H, 4.97.
Dim et h ylsu lfoxon iu m Met h ylid e‚Tr is(P en t a flu or o-
ph en yl)Bor an e (4). A stirred solution of tris(pentafluorophen-
yl)borane (66 mg, 0.13 mmol) in degassed toluene (3.0 mL)
was treated with ylide 1 (0.64 M in toluene, 0.25 mL, 0.16
mmol). After stirring 45 min, the solution was concentrated
under reduced pressure to yield a glassy solid. The material
was dissolved in CHCl3 (10 mL), and hexane (10 mL) was
added. The white cubic crystals formed were filtered (63 mg,
82% yield): mp 140.7-147.9 °C; IR (KBr) νmax 1638.2, 1518.6,
X-r a y Cr ysta llogr a p h y: Da ta Collection a n d Refin e-
m en t. Single crystals of the complexes 3, 4, and 7 were grown
from MeOH/H2O, toluene, and CDCl3, respectively. Diffraction
data were obtained on a Bruker CCD platform diffractometer.
The SMART44 program package was used to determine the
unit-cell parameters and for data collection (30 s/frame scan
time for a hemisphere of diffraction data). The raw frame data
1
1465.0, 1087.9, 979.3 cm-1; H NMR (400 MHz, tol-d8) δ 3.25
(br s, 2H), 1.49 (s, 6H); 19F NMR (376 MHz, tol-d8) δ 133.2 (m,
2F), 158.8 (t, J ) 20.4 Hz, 1F), 164.3 (m, 2F); 11B NMR (160
(45) Sheldrick, G. M. SADABS; Bruker Analytical X-Ray Systems,
Inc.: Madison, WI, 1999.
(44) SMART Software Users Guide, Version 5.1; Bruker Analytical
X-Ray Systems, Inc.: Madison, WI, 1999.
(46) International Tables for X-Ray Crystallography; Kluwer Aca-
demic Publishers: Dordrecht, 1992; Vol. C.

Borane Complexes
Organometallics, Vol. 22, No. 5, 2003 1131
MHz, tol-d8, 100 °C) δ -16.3 (s); DSC (exotherm) 150.0 °C (94.2
J /g); HRMS (FAB/CI) m/z calcd for C15H8BSF10O (M - C6F5)+
437.0224, found 437.0229. Anal. Calcd for C21H8BSF15O: C,
41.75; H, 1.33. Found: C, 41.72; H, 1.38.
added. The solution was allowed to evaporate slowly at room
temperature over several weeks. The colorless crystals were
separated from a yellow oily residue: 1H NMR (500 MHz,
CDCl3) δ 2.89 (s, 6H), 2.58 (s, 2H); 19F NMR (376 MHz, CDCl3)
δ -133.6 (dd, J ) 23.9, 8.7 Hz, 4F), -143.9 (dd, J ) 22.7, 7.2
Hz, 2F), -157.1 (t, J ) 20.3 Hz, 2F), -162.0 (t, J ) 21.2 Hz,
1F), -163.7 (dt, J ) 2.2, 7.8 Hz, 4F), -165.2 (dt, J ) 21.6, 6.4
Hz, 2F); 11B NMR (160 MHz, CDCl3) δ 3.9 (s); HRMS (FAB/
CI) m/z calcd for C19H2BF15 (M - C2H6SO)+ 526.0014, found
526.0008.
Dim eth ylsu lfoxon iu m Meth ylid e‚Bor a n e (5).34 Ylide 1
(0.61 M in THF, 1.4 mL, 1.0 mmol) was added dropwise over
10 min to a suspension of BH3‚SMe2 (1.0 M in THF, 1 mL, 1.0
mmol) in THF (5 mL) at -78°. The white precipitate was
filtered cold, washed with hexane, and dried in vacuo to afford
a white solid (0.14 g, 100% yield): mp 57.8-58.8 °C (dec); DSC
(exotherm) 66.3 °C, 67.4 °C; IR (KBr) νmax 3021.9, 2920.1,
2283.6, 1204.2, 1132.3, 1034.1 cm-1 11B NMR (160 MHz, DMF,
;
Ack n ow led gm en t. We thank the Chemistry Divi-
sion of the National Science Foundation for financial
support of this work, Dr. J oe Ziller for X-ray crystal-
lographic data, and Dr. Marvin Grzys (TA Instruments,
109 Lukens Dr., New Castle, DE 19720) for generously
providing the MDSC data for complexes 3-6.
0 °C) δ -31.2 (s).
Dim eth ylsu lfoxon iu m Meth ylid e‚Tr ip h en ylbor a n e (6).
Ylide 1 (0.61 M in THF, 0.70 mL, 0.49 mmol) was added
dropwise over 10 min to a stirred suspension of Ph3B (0.12 g,
0.49 mmol) in degassed THF (5 mL) at -78 °C. The solvent
was removed in vacuo over 2 days at -40 °C to yield a white
solid (0.14 g, 100% yield): mp 81.0-88.1 °C; DSC (exotherm)
92.0 °C (196.9 J /g); IR (KBr) νmax 3058.3, 3013.2, 2912.4,
Su p p or tin g In for m a tion Ava ila ble: MDSC traces for
complexes 3-6. Tables of atomic coordinates, all bond dis-
tances and angles, and anisotropic thermal parameters for
compounds 3, 4, 7, and MeB(OH)2. This material is available
free of charge via the Internet at http://pubs.acs.org.
1228.3, 710.5 cm-1
(s).
Bis(p en ta flu or op h en yl) 2,3,4,5,6-P en ta flu or oben zyl-
bor a n e‚DMSO Com p lex (7). Ylide‚B(C6F5)3 complex 4 (10
mg) was heated to 150 °C in an NMR tube for 10 min and
cooled to room temperature, and CDCl3 (0.5 mL) was then
;
11B NMR (160 MHz, tol-d8, -20 °C) δ -10.3
OM0208568