Insertion reactions of diazomethanes and electrophilic boranes

Article abstract of DOI:10.1021/om2011613

The electrophilic pentafluoroarylboranes B-(C₆F ₅)₃, PhB(C₆ F₅)₂, and ClB(C₆ F₅)₂ react with (trimethylsilyl) diazomethane, diphenyldiazomethane, or (pentaf

Full text of DOI:10.1021/om2011613

Communication
pubs.acs.org/Organometallics
Insertion Reactions of Diazomethanes and Electrophilic Boranes
Rebecca C. Neu and Douglas W. Stephan*
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6
S
* Supporting Information
ABSTRACT: The electrophilic pentafluoroarylboranes B-
(C6F5)3, PhB(C6F5)2, and ClB(C6F5)2 react with
(trimethylsilyl)diazomethane, diphenyldiazomethane, or
(pentafluorophenyl)diazomethane, affording insertion into
the one or two B−C bonds in a facile and high-yielding
route to the boranes (Me3SiCH(C6F5))B(C6F5)2, (Me3SiCH-
(C6F5))2B(C6F5), (Me3SiCH(C6F5))(Me3SiCH(Ph))B-
(C6F5), (C6F5)ClB(CPh2(C6F5)), and (C6F5)2B(CH(C6F5)2).
Tris(pentafluorophenyl)borane was first prepared in 1964;1
however, it was not until 1991 that the pioneering work of
Marks2 and Ewen3 independently demonstrated the unique
ability of B(C6F5)3 to effect electrophilic alkyl abstraction from
early-metal metallocene derivatives, affording highly effective
olefin polymerization catalysts. This remarkable evolution from
an intellectual curiosity to these applications has been
extensively chronicled by Piers and Chivers.4 Since that time,
a plethora of applications5 of perfluoroarylboranes have been
developed for organometallic5−10 and main-group chemis-
try,11−14 optoelectronic materials,15−26 frustrated Lewis pair
chemistry,27−31 and sensors for anions and toxic agents.32−38 In
catalysis, B(C6F5)3 has been exploited for a variety of Lewis acid
mediated processes,4,39 while more recently it has been used to
effect in the catalytic synthesis of silicones40 as well as in metal-
free hydrogenations.27,41,42
Despite the general and broad interest in perfluoroarylborane
derivatives, the synthesis and purification of electrophilic
boranes can be technically challenging. Several authors43−46
have developed approaches to generating arylboranes from
precursors such as BCl3 and ClB(C6F5)2, while Piers and others
have exploited hydroboration of alkenes and alkynes with
HB(C6F5)2 to generate related alkyl- or alkenylboranes. The
groups of Marks47−54 and Piers55−65 have also developed
elegant strategies to bulky and electrophilic boranes, employing
methodologies which depend on the use of tin and mercury
intermediates.66,67 While most synthetic routes to perfluoroar-
ylborane derivatives involve reactions of B−halide or B−H
bonds, recent reports by the Erker68−74 and Berke75 groups
have described thermally induced 1,1-carboborations of
terminal and internal alkynes by B(C6F5)3, affording new
electrophilic vinylboranes. In this communication, we describe
the facile and high-yielding insertions of diazomethanes into the
B−C bonds of electrophilic boranes, providing an avenue to
sterically elaborated electrophilic boranes.
generate the new borane 1. Following purification by
recrystallization, 1 was isolated in 65% yield. The 11B NMR
signal of 1 was not discernible between −60 and 85 °C. The 19F
NMR spectrum of 1 showed resonances at −130.9, −149.8 and
−161.3 ppm attributable to the o-, p-, and m-fluorine atoms of
the two equivalent C6F5 groups bound to boron. In addition,
resonances in the 19F NMR spectrum attributable to the o-, p-,
and m-fluorine atoms of a C6F5 ring bound to carbon were
observable at −140.8, −158.9 and −162.9 ppm, respectively.
Integration of these peaks confirmed that the two C6F5
environments in 1 were present in a 2:1 ratio. The
corresponding 1H NMR spectrum displayed resonances at
4.38 and 0.16 ppm attributable to CH and SiMe3 fragments in a
1:9 ratio. The latter fragment also gives rise to a broadened 29Si
resonance at −111.0 ppm. These data support the formulation
of 1 as (Me3SiCH(C6F5))B(C6F5)2 (Scheme 1). This
formulation was later confirmed crystallographically (Figure
1). The B−C bond lengths for the fluoroarene groups were
found to be 1.593(2) and 1.577(2) Å, while the B−C bond
length for the tertiary carbon atom was determined to be
1.545(2) Å. The anticipated pseudo-trigonal-planar geometry
about B is evident from the C−B−C angles of 122.3(1),
117.8(2) and 119.6(1)°. The tertiary carbon derived from
formal insertion of the diazomethane fragment into the B−C
bond is found to be chiral. Interestingly, the BC2 plane of the
B(C6F5)2 fragment is oriented away from the SiMe3 group.
Indeed, the SiMe3 fragment is oriented so as to block one face
of the B plane with the closest C−H···B approach being a
methyl group of the SiMe3 fragment at 3.10 Å from the
electron-deficient boron. It is noteworthy that there was no
evidence of this interaction in solution by 1H NMR
spectroscopy, even down to −85 °C.
In an analogous manner, reaction of B(C6F5)3 with 2 equiv of
Me3SiCH(N2) proceeds smoothly and rapidly at −78 °C to
The reaction of B(C6F5)3 with a slight stoichiometric excess
of Me3SiCH(N2) proceeds smoothly and rapidly at −78 °C to
Received: November 21, 2011
Published: December 7, 2011
© 2011 American Chemical Society
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Organometallics
Communication
Scheme 1. Synthesis of Compounds 1−4 and 6
Figure 2. Pov-ray depiction of 2: C, black; B, yellow-green; F, pink; Si,
red.
Indeed, 2 is easily and cleanly prepared via addition of excess
Me3SiCH(N2) to B(C6F5)3 with simple separation from the
unreacted Me3SiCH(N2) via product recrystallization from cold
pentane.
The analogous reaction of Me3SiCH(N2) and PhB(C6F5)2 in
a 1:1 ratio was undertaken. Preliminary 19F NMR spectroscopy
of the reaction mixture was consistent with the formation of
two products, resulting from single and double insertions of
Me3SiCH(N2) into B−C bonds. While these species could not
be separated, a single product formulated as (Me3SiCH-
(C6F5))(Me3SiCH(Ph))B(C6F5) (3) was prepared in a clean
fashion via addition of 2 equiv of Me3SiCH(N2) at −78 °C to
PhB(C6F5)2. The crude product, obtained as a clear and
colorless oil, was recovered in 64% isolated yield. Recrystalliza-
tion from pentane over a number of days at −35 °C resulted in
large rectangular crystals which were crystallographically
analyzed, confirming the proposed formulation of 3 (Figure
Figure 1. Pov-ray depiction of 1: C, black; B, yellow-green; F, pink; Si,
red.
generate the new borane 2. Following recrystallization from
pentane at −35 °C, 2 was isolated as colorless crystals in 67%
yield. This species exhibits a broad 11B NMR resonance at 75.4
1
ppm. The corresponding 19F and H NMR spectral data show
signals attributable to two products in a ratio of 72:28. The
major product gives rise to 8 19F resonances corresponding to 3
distinct C6F5 rings in a 1:1:1 ratio, while the minor product
gave rise to 15 distinct resonances at −85 °C. The
1
corresponding H NMR data for the methine and methyl
protons were observed at 3.79 and 0.22 ppm for the major
isomer and 3.58 and −0.17 ppm for the minor isomer. These
data were consistent with the formulation of the product 2 as
the RR/SS and RS/SR diastereomers of 2 (Me3SiCH(C6F5))2B-
(C6F5) (Scheme 1). Crystals of the racemic diastereomer of 2
were characterized crystallographically, thereby confirming the
proposed “double insertion” of the diazomethane into two B−
C bonds of B(C6F5)3 (Figure 2). The B−C bonds of the
fluoroarene and tertiary carbons at 1.600(3), 1.556(3) and
1.566(3) Å are slightly longer than those noted for compound
1. The geometry about B in 2 is similar to that in 1, although
the steric crowding resulting from the adjacent tertiary C
centers gives rise to a cog-wheel type meshing of the
substituents. The impact of this steric congestion is also
demonstrated as addition of further equivalents of Me3SiCH-
(N2) (2, 5, and 10 equiv) to 2 resulted in no further reaction.
Figure 3. Pov-ray depiction of 3: C, black; B, yellow-green; F, pink; Si,
red.
3). The structure of 3 is in fact very similar to that of 2, bearing
the similar cog-wheel type arrangement of the substituents. The
product is pseudoplanar at boron, as demonstrated by the C−
B−C angles 121.0(1), 121.6(1) and 117.3(1)°, which add up to
nearly 360°. The crowding about the boron center presumably
accounts for the inability to effect further insertion. The
presence of the trimethylsilyl groups gave rise to a broad signal
in the 29Si NMR at −111.5 ppm, while the 11B NMR displayed
a broadened signal at 76.3 ppm significantly downfield as
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Organometallics
Communication
compared to the chemical shift of the parent borane,
PhB(C6F5)2. Only two methine and two methyl signals at 3.7
1
and 3.5 ppm and at 0.2 and 0.7 ppm were observed in the H
NMR spectrum. The 19F NMR spectrum of 3 was pristinely
clean, showing nine resonances attributable to the three C6F5
1
environments. These 19F and H data are consistent with the
presence of only the RR/SS diastereomers. The diastereose-
lectivities seen for the “double insertion” into PhB(C6F5)2 and
B(C6F5)3 suggests that the chirality of the initial insertion
influences the chirality of the second insertion. It is also
interesting to note that the resulting dissymmetric substitution
about the planar borane center of 3 makes this species prochiral
at boron.
The generality of diazomethane insertion reactions with
electrophilic boranes was further probed. While the analogous
reaction of Me3SiCH(N2) with ClB(C6F5)2 at −78 °C gave a
complex mixture of products, the reaction of Ph2C(N2) and
ClB(C6F5)2 resulted in a very fast and clean reaction.
Examination of the 19F NMR spectra revealed the presence
of six distinct resonances, in keeping with two chemically
distinct perfluoroaryl groups, where one C6F5 group has
migrated from the boron center to the adjacent carbon. On the
basis of NMR data this product borane was formulated as
(C6F5)ClB(CPh2(C6F5)) (4) and was isolated as an oil in 97%
yield. Confirmation of the structure of 4 was obtained from a
crystallographic study of the pyridine adduct (C6F5)ClB-
(CPh2(C6F5))(py) (5) (Figure 4). It should be noted that
Figure 5. Pov-ray depiction of 6: C, black; B, yellow-green; F, pink.
of the boron center. The B−C bond length of the boron center
in 6 bound to the tertiary carbon is, however, markedly
lengthened at 1.573(3) Å as compared to 1 with a B−C bond
length of 1.545(2) Å. The C−B−C bond angles at boron were
found to be 120.8(2), 119.3(2) and 119.9(2)°, in keeping with
a pseudo-trigonal-planar geometry about the boron center.
While Jie and Shea77 and Soderquist et al.78 have exploited
diazomethanes to prepare trialkylborane derivatives, the only
related precedent for perfluoroarylboranes involves the rather
slow transformation of the silylene adduct (HCNtBu)2SiB-
(C6F5)3 to (HCNtBu)2Si(C6F5)B(C6F5)2 upon standing at 20
°C for 3 months.79 The present work establishes that
electrophilic boranes react readily with a series of electronically
diverse diazomethanes to provide a facile route to new sterically
encumbered yet electrophilic borane derivatives. In this fashion
a range of electrophilic boranes can easily be designed for
specific applications in Lewis acid catalysis or chemistry.
Indeed, we are currently exploring the applications of such
boranes in “frustrated Lewis pair” chemistry and catalysis. The
results of these studies will be reported in due course.
ASSOCIATED CONTENT
■
S
* Supporting Information
Text, figures, and CIF files giving experimental and crystallo-
graphic data. This material is available free of charge via the
Internet at http://pubs.acs.org.
Figure 4. Pov-ray depiction of 5: C, black; B, yellow-green; F, pink; N,
blue.
that no further reaction was observed upon reaction of 4 with
further equivalents of Ph2C(N2) (1, 2, and 5 equiv).
AUTHOR INFORMATION
■
The diazomethane (C6F5)CH(N2) was prepared from the
corresponding hydrazone with decomposition to the diazo-
methane by Swern oxidation.76 Stoichiometric reaction of the
neat diazomethane with B(C6F5)3 at −78 °C resulted in
immediate consumption of the diazomethane, as evidenced by
the disappearance of the diagnostic yellow color, resulting in
the generation of a slightly beige solution. The resulting
product was isolated as a beige solid in 60% yield that gave rise
to two sets of 19F NMR resonances attributable to two C6F5
environments present in a 1:1 ratio, with a single resonance in
the 1H NMR corresponding to the methane proton. These data
are consistent with the formation of ((C6F5)2CH)B(C6F5)2
(6). This formulation was later confirmed crystallographically
(Figure 5). It is noted that the B−C bond lengths of the
perfluoroarenes at 1.553(4) and 1.568(4) Å are shorter than
those observed in the related compound 1 at 1.577(2) and
1.593(2) Å, which is in keeping with the increased Lewis acidity
Corresponding Author
*E-mail: dstephan@chem.utoronto.ca.
ACKNOWLEDGMENTS
■
D.W.S. gratefully acknowledges the financial support of the
NSERC of Canada, in the award of a Canada Research Chair.
R.C.N. is grateful for the award of an Ontario Graduate
scholarship.
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