Rearrangement of cyclopropylborane into boretane

Article abstract of DOI:10.1007/s10593-012-1060-9

Previously unknown boron-containing four-membered unsaturated heterocycle boretane was obtained via novel thermal rearrangement of cyclopropylborane and investigated by NMR and IR spectroscopy. Formation of boretane was also confirmed through its chemical

Full text of DOI:10.1007/s10593-012-1060-9

Chemistry of Heterocyclic Compounds, Vol. 48, No. 5, August, 2012 (Russian Original Vol. 48, No. 5, May, 2012)
REARRANGEMENT OF CYCLOPROPYLBORANE INTO BORETANE
M. Rubina1 and M. Rubin1*
Previously unknown boron-containing four-membered unsaturated heterocycle boretane was obtained
via novel thermal rearrangement of cyclopropylborane and investigated by NMR and IR spectroscopy.
Formation of boretane was also confirmed through its chemical transformation into more stable
derivatives.
Keywords: boretane, cyclopropylborane, ConcIRT-spectra, 19F NMR spectroscopy, real time IR
spectroscopy, ring expansion.
Wagner-Meerwein rearrangement of cyclopropylmethyl cation 1 into cyclobutyl cation 2 [Eq. (1)],
discovered by Demjanow at the beginning of the last century [1], has become a convenient synthetic tool for the
preparation of four-membered carbocycles [2]. This 1,2-alkyl shift proceeds readily, as it is accompanied by
significant strain release, and therefore is thermodynamically favored. However, the corresponding hetero-
analogous reactions are scarce and essentially limited only to the rearrangement of cyclopropanone hemiaminals
3 into -propiolactams 4 [Eq. (2)] [3, 4]. Herein we wish to report the first generation and spectroscopic
identification of boretane 6 generated in the previously unknown strain-driven ring expansion of
cyclopropylborane 5 [Eq. (3)].
Published computations reveal that in striking difference to organoboron heterocycles possessing two or
more boron atoms, which tend to form non-classical aromatic structures [5, 6], boretane molecule was predicted
to remain classical [7, 8]. This molecule has no unusual structural distortions besides a large puckering angle in
the half-envelope conformation, which was explained by significant 1,3-transannular interaction [9]. Therefore,
we were surprised to find that all four-membered boracycles reported to date were unsaturated, possessing at
least one sp2-hybridized carbon atom in the small ring [10-17]. Although intermediacy of boretane species 8 and
12 in the rearrangements of bicyclo[6.1.0]nonatrien-9-ylborane 7 and 9-borabarbaralane 10 was previously
proposed [Eqs. (4) and (5)], to the best of our knowledge, no experimental evidence for the formation of
saturated four-membered heterocycle with a single boron atom existed to date.
We pondered that the molecule of boretane could be obtained via ring expansion of cyclopropylborane.
This hypothesis was based on the argument that such transformation would be isoelectronic to the above-
mentioned rearrangement of cyclopropylmethyl cation [Eq. (1)]. In addition, it should be noticed that a
_______
*To whom correspondence should be addressed, e-mail: mrubin@ku.edu.
1Department of Chemistry, University of Kansas and Center for Environmentally Beneficial Catalysis, 1251
Wescoe Hall Drive, Lawrence KS 66045, USA.
2Department of Chemistry, University of Kansas, 1251 Wescoe Hall Drive, Lawrence KS 66045, USA.
_________________________________________________________________________________________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, 867-882, May, 2012. Original article
submitted June 06, 2011.
0009-3122/12/4805-0807 Springer Science+Business Media, Inc.
807

homologous stereospecific transformation of cyclobutylborane 14 into borolane 15 is known [Eq. (6)] [20]
Thinking along these lines we subjected to thermal conditions a series of different di- and trisubstituted
cyclopropylboranes 17, generated in situ via noncatalytic hydroboration [21-24] of cyclopropenes 16 [Eq. (7)].
However, even prolonged heating at 100oC resulted in no reaction or thermal decomposition of
cyclopropylboranes [Eq. (7)]. We rationalized that increasing steric bulk at the reaction site may potentially
stimulate the desired rearrangement.
R1
H
OR2
O
C+
C+
(1)
N
+R1
N
(2)
1
2
3
4
R
H
B
B
R
(3)
5
6
R
H
H
H
R
R
R
R
~[1,3]B
0°C
B
B
(4)
(5)
B
R
H
7
8
9
R
OMe
B
R
B
R
B
R
B
MeOH
10
11
13
12
Accordingly, cyclopropene 18 bearing two phenyl substituents at atoms C-1 and C-2 was tested next in
the reaction with BH3·THF [Eq. (8)]. Monitoring the reaction course by ReactIR (Fig. 1)) revealed rapid
formation of cyclopropylborane 19 upon addition of compound 18 to a BH3·THF solution at room temperature
(Fig. 1, event 1). Gratifyingly, in contrast to all other cyclopropylboranes tested, heating of borane 19 at 100oC
initiated its gradual conversion into a new component C (Fig. 1, event 2). No visible changes in characteristic
IR stretches were observed at this point, which was expected, as the anticipated rearrangement 1920 should
produce insignificant bond perturbations, detectable in the fingerprint region only. Thus, ConcIRT algorithm
[25] was employed for detailed analysis of changes in the fingerprint region of IR spectra (Fig. 2).
R1

R2
R1
BH2
14
R2
(6)
(7)
B
H
15
R2
R1
R3
R2
R1
R3

BH
2·THF
16
17
R1, R2 = Alk, Ar, COR, R3 = H, Ar
Despite significant similarity between the ConcIRT spectra of B and C, there are two distinct IR bands that
differentiate these components (Fig. 2). Specifically, an absorption line at 1450-1480 cm-1 is attributed to C–H
scissor vibrations of cyclopropyl ring in component B, while the broad band at 1250 cm-1 observed in the
spectrum of component C according to our molecular modeling studies should correspond to rocking vibrations
of methylene group in the boretane ring.
808

Fig. 1. ConcIRT kinetic plot of hydroboration/rearrangement reaction. Event 1 – addition of cyclopropene 18;
event 2 – heating of the mixture at 100oC. Components: A – BH3·THF complex; B – cyclopropylborane 19;
C –boretane 20.
Surprisingly, when analogous reaction was carried out in the presence of BH3·SMe2 complex [Eq. (8)],
no rearrangement of the corresponding cyclopropylborane 21 was observed even upon prolonged heating at
100oC. DFT computations (B3LYP/6-31G*) of Mulliken charges revealed significant difference in electronic
density on boron atoms between structures 19 (+0.338) and 21 (+0.075), suggesting the rearrangement is
strongly dependent on the "cationic" character of the reactive center, and can be completely suppressed by
strong Lewis bases.
Scheme 1
BH
BH3·THF
BH3·SMe2
2·THF

Ph
Ph
Ph
H
Ph
19
B
·THF
(8)
Ph
Ph

18
BH
Ph
2·SMe2
20
Ph
21
To gain additional support for the boretane formation, we treated the reaction mixture with KHF2
hoping to stabilize the four-membered ring in the form of more stable fluoroborate derivatives (Scheme 1).
Analysis of 19F NMR spectrum of the obtained mixture revealed that, along with peaks of
cyclopropyltrifluoroborate 23 and BF4 , the spectrum contained a resonance at -155 ppm (AB system, 2JFBF = 76
-
Hz), which was assigned to Cs-symmetric difluoroborate 25 (Fig. 3, spectrum A). A resonance at -153 ppm was
attributed to a C2-symmetric difluoroborate 27, a putative thermodynamically favored rearrangement product of
the isomerization of compound 25 via acyclic intermediate 26 (Scheme 1). It is important to mention that,
potentially, another meso compound 24, resulting from dicyclopropylborane 22, can be present in the mixture,
809

Fig. 2. ConcIRT spectra for components B (solid line) and C (dashed line), fingerprint region. Bands at 1470
and 1250 cm-1 are discussed in text. Regions highlighted are obscured by solvent (THF).
which is expected to have a similar to compound 25 splitting pattern in 19F NMR spectrum. However,
independently generated compound 22 (Fig. 3, spectrum B) was observed in higher field at -165 ppm, which
allowed us to exclude the possible erroneous assignment.
Having obtained spectroscopic evidences for the formation of boretane species, we attempted to isolate
complexes 25 and 27, albeit as yet unsuccessfully. However, oxidative treatment of the reaction [Eq. (8)]
afforded a mixture of 1,3-diphenylpropan-1-one (29), a product of base-catalyzed ring opening of cyclo-
propanol 28, and meso-1,3-diphenylpropane-1,3-diol (30). The latter was evidently obtained upon oxidative
cleavage of C–B bonds in boretane 20, thus indirectly confirming formation of the four-membered ring under
the reaction conditions. Oxidation with hydrogen peroxide–urea complex afforded syn diastereomer 30 as sole
BH .THF
.
.
BF3 THF
BH2 THF
18
+
Ph
Ph
Ph
Ph
Ph
19
2
22
100 °C
F
F
Ph
BF2–
–
H
BF3
B .
THF
+
Ph
Ph
Ph
Ph
23
24
2
Ph
20
25
F
Ph
Ph
BF2–
Ph
BF2–
Ph
BF2
–
thermodynamically
more favorable
Ph
27
26
810

product, while notable amounts of anti isomer were obtained when the reaction was carried out in the presence
of NaOH. This result can be explained by the ability of strong base to induce epimerization of CS-boretane 20
into С2-epimer, analogous to the epimerization 2527 discussed above (Scheme 1).
Since boretane 20 exists in dynamic equilibrium with diastereomeric dimers 31 and 32, it could be
argued that the latter may undergo the following expansion of the strained four-membered cycles to form the
corresponding 1,5-diboracyclooctanes 33 and 34 (Scheme 2). An alternative mechanism that could be realized
in the presence of excess BH3·THF would involve hydroboration of the boretane cycle affording (3-boryl-
1,3-diphenylpropyl)borane (35). Provided both ring expansions proceed stereospecifically, subsequent oxidation
of the obtained products would also lead to the exclusive formation of syn-diol 30 (Scheme 2). Below we
critically analyze both these hypotheses from the mechanistic standpoint with respect to the obtained
experimental data.
19
Fig. 3. F NMR spectra of the reaction mixture quenched with KHF2. (a) reaction mixture heated at 100oC for
3 h prior to quench; (b) mixture of compound 16 and BH3·THF in a ratio of 2:1, quenched at room temperature.
Ph
H
BH
2·THF
100°C
3 h
B ·THF
BH3·THF
18
Ph
Ph
19
Ph
20
H2O2–aqueous NaOH or
urea complex H2O2
(9)
20%
Ph
Ph
Ph
Ph
OH
Ph
OH
Ph
OH OH
73%
O
29
28
30
Discussion on Putative Routes for Boretane Ring Expansion. Potential reactivity of boretane can be
assessed by analogy with the known higher homolog, borolane 36 (Scheme 3). The dimeric structure of
bisborolane (32) was originally assigned by Köster to a compound obtained by reduction of B-chloroborolane
with lithium aluminum hydride [26]. However, later Brown proposed for the same product an alternative
1,6-diboracyclodecane (41), which is stabilized by transannular hydride bridging [27, 28] (Scheme 3). The
unstable structure of unsubstituted bisborolane 37 remained elusive for a long time before Brown finally
811

Scheme 2
Ph
Ph
Ph
Ph
Ph
Ph
Ph
H
H
H
B
B
B
B
+
20
H
Ph
32
31

Ph
Ph
Ph
Ph
B
OH
OH
Ph
B
[O]
H
H
+
H
H
Ph
B
B
30
Ph
B
Ph
Ph
Ph
B
33
34
H
H
H
H
[O]
BH3·THF

Ph
Ph
35
Brown finally prepared it at low temperatures and intercepted it in reactions with MeOH and 1-octene [29].
However, at ambient temperatures bisborolane 37 was shown to rapidly convert into a thermodynamically more
stable compound 41 and some polymeric products. Product 41 could also be obtained upon hydroboration of
1,3-butadiene with various hydroborating agents at elevated temperatures [30], while at lower temperatures this
reaction provided tetramethylene diborane 43 as a major product along with small quantities of isomeric
trimethylene diborane 44 (Scheme 3) [31]. The rearrangement of bisborolane 37 into diboracyle 41 was
Scheme 3
H
H
H
H
BH
36
B
B
B
B
2
H
37
38
H
H
B
B
B
B
H
H
40
39
HN(SiR3)2
NHSiR3
B
B
H
H
B
41
42
H
H
H
OR
NR2
B
B
B
B
B
B
H
B
B
OR
NR2
Me
45
43
44
46
referred to as "redistribution", and no detailed mechanism was provided [30]. However, the mechanism for
thermal isomerization of organoboranes, proposed by Brown later and now commonly accepted [32-34],
involves the following iterations of dehydroboration-hydroboration steps and presumes unavoidable formation
of intermediate alkene species 38 and 40 (Scheme 3) [35-40].
812

Notably, the presence of the B–H bond in the borolane is crucial for successful rearrangement 3641.
Thus, no ring expansion was observed with B-alkyl- [41-45], B-halo- [46-48], B-amino- [49, 50], and B-alkoxy-
substituted [46, 51-53] borolanes. Furthermore, a reverse reaction takes place upon treatment of compound 41
with some amines to form B-aminoborolane derivatives 42 [54-55].
Scheme 4
H
H
Ph
Ph
Ph
H
H
Ph
B
H
H
HB
B
B
Ph
HA
Ph
2
31/32
47
Ph
OH
Ph
H
[O]
B
Ph
OH
49
Ph
48
It should also be pointed out that the described transformation 3641 is both thermodynamically and
kinetically favored only in the case of 2,5-unsubstituted borolanes, since only in the absence of steric hindrance
could species 41 be efficiently stabilized by transannular hydride bridging. In contrast, the presence of
substitutions at the carbon atoms adjacent to boron impedes dehydroboration/re-hydroboration processes,
making 2,5-disubstituted borolanes significantly more stable compounds, which do not undergo the described
rearrangement into 1,6-diboracyclodecanes even at elevated temperatures [51-53].
Accordingly, the resistance of boretane species 20 toward ring expansion into 1,5-diboracyclooctanes
(1,5-diborocanes) 33 or 34 can be rationalized as follows. Firstly, stabilization of 1,5-diborocane molecule by
transannular hydride bridging is expected to be far less efficient due to the angular strain associated with this
interaction. Indeed, while B-alkoxy [41-59] and B-amino [60, 61] derivatives of 1,5-diborocane 45 and 46, in
which no transannular bridging can be realized, are well known, the parent organoborane remains elusive.
Secondly, two bulky substituents at the atoms C-2 and C-4 contribute into stabilization of compound 20 or the
corresponding dimers 30 and 31. Thirdly, dehydroboration of boretane molecule necessary for ring expansion
seems problematic from the following standpoint. According to the principle of microscopic reversibility, the
dehydroboration reaction should proceed via the same transition step as the hydroboration reaction, i.e., a
tetracentered two-electron concerted transition state. This presumes that C(2)–B and C(3)–AH (or C(3)–BH)
bonds should be aligned in the same plane in mandatory syn fashion (Scheme 4). However, such orientation can
hardly be realized in a conformationally restricted four-membered ring, in which the indicated bonds are locked
in a practically orthogonal arrangement (Scheme 4).
Furthermore, our experimental data do not agree with the hypothesis of 1,5-diborocane formation. Thus,
in the unlikely event species 47 is obtained in thermal dehydroboration of boretane dimer 31 (or 32), the issue
of the alternative regioselectivity of the subsequent hydroboration of the newly formed double bond should be
addressed. Although the Baldwin rules were originally formulated for nucleophilic cyclizations, the same
geometric reasons should be generally acceptable for determining the selectivity of synchronous electrophilic
attack as well; i.e., 3-exo-trig direction of the attack should be somewhat preferred over 4-endo-trig cyclization.
Therefore, formation of certain amounts of borirane 48 [62-64] should be unavoidable, and the subsequent
oxidation of this compound should afford 1,2-diol 49 (Scheme 4). The latter product, however, has never been
detected in crude reaction mixtures. Moreover, non-stereoselective re-hydroboration of species 47 would
produce a mixture of five diastereomeric 1,5-diborocanes with different relative arrangements of the phenyl
substituents, which would subsequently give five different difluoroborate species after treatment with fluoride
(Scheme 5). The corresponding 19F NMR spectrum of this mixture would consist of two singlets (corresponding
813

to the molecules with С2h and С2 symmetries), two independent AB systems (corresponding to the molecules
with С2h and С2v symmetries), and two sets of AB systems, belonging to the same C1-symmetric species
(Scheme 5). However, the experimentally obtained spectrum was significantly simpler, consisting of only one
singlet and one AB system (Fig. 3, spectrum A), which rules out the hypothetic formation of 1,5-borocane-type
dimeric structures, at least via the classical dehydroboration–hydroboration mechanism.
Scheme 5
Ph
Ph Ph
Ph Ph
Ph Ph
Ph Ph
Ph Ph
Ph Ph
Ph Ph
Ph Ph
Ph
Ph
B
B
B
B
B
B
B
B
B
B
H
H
H
H
H
H
H
H
H
H
Ph
The point group
of symmetry
C2
C1
C2v
C2h
C2h
FB FA
B
FA FA
B
FA FB
B
FA FA
B
FA FB
B
Ph
Ph
Ph Ph
Ph Ph
Ph Ph
Ph Ph
Ph Ph
Ph Ph
Ph Ph
Ph Ph
Ph
B
B
B
B
B
Ph
FA FB
FA FA
FA FB
FA FA
FD
FC
The point group
of symmetry
C2
C1
C2v
C2h
C2h
Signal
from 19F NMR
spectrum
singlet
AB system
singlet
two AB systems
AB system
Scheme 6


35
50
20
30
An alternative potential route for the boretane ring expansion should also be mentioned, which involves
hydroboration of the initially formed strained boretane ring [53, 54] to give trimethylenediborane entities 42
(Scheme 6), analogous to the known species 44 described above (Scheme 3) this case, due to a significant
puckering angle in boretane 20, a substrate envelope conformation with two pseudo-equatorial phenyl
substituents would be thermodynamically more stable, which would result in the borane addition from the
convex face, leading to the formation of cis-diphenyltrimethylenediborane 35. This transformation is expected
814

to proceed with retention of configuration at the reacting carbon atoms due to a synchronous syn-selective
hydroboration mechanism (Scheme 6). After oxidation, trimethylenediborane 35 would selectively provide syn-
diol 28, which is consistent with experimental observations [Scheme 2, Eq. (9)]. This putative route, however,
does not agree with the obtained spectroscopic data. Indeed, treatment of diborane 35 with fluoride should
provide CS-symmetric syn-bis(trifluoroborate) anion 50, which should appear in 19F NMR as a singlet. At most
one more singlet could be observed in this spectrum, corresponding to the C2-symmetric anti diastereomer.
Formation of the latter species in this transformation, however, is highly unlikely. Nontheless, the AB system
observed in our experiments (Fig. 3) clearly indicates the presence of diorganyl-substituted boron atom, and the
symmetry analysis provided above allowed for elimination of all other possible topologies and geometries
monocyclic besides structure of boretane 20.
In conclusion, we investigated the possibility of cyclopropylborane ring expansion reaction, an
isoelectronic heteroanalog of the known rearrangement of cyclopropylmethyl cation. It was demonstrated that
this process can proceed for sterically hindered substrates at elevated temperatures affording previously
unknown saturated four-membered boron-containing heterocycle (boretane), which was confirmed by spectral
data (IR and NMR). Formation of boretane was also confirmed through transformation of this unstable
compound into 1,3-diol.
EXPERIMENTAL
NMR spectra were recorded on a 400.13 MHz instrument, equipped with quadruple-band gradient
probe (H/C/P/F QNP). 13C and 19F spectra were registered with broad-band decoupling. (+) and (-) Signs
13
represent positive and negative intensities, respectively, of signals in C DEPT-135 experiments. In situ FTIR
spectra were recorded using Mettler Toledo ReactIR iC10 spectrometer with 6 mm silicon probe. Column
chromatography was carried out employing silica gel (Selecto Scientific, 63-200 μm). Pre-coated silica gel
plates (Merck Kieselgel 60 F-254) were used for thin-layer chromatography.
Glassware employed in moisture-free syntheses was additionally flash-dried in vacuum and cooled in a
stream of dry nitrogen prior to use. Water was purified by dual-stage deionization, followed by dual-stage
reverse osmosis. Commercially available anhydrous THF was additionally purified by passing consecutively
through two columns with activated alumina. Anhydrous diisopropylamine was obtained by distillation of a
commercially available material (ACS reagent grade) over calcium hydride and stored in Schlenk vessels under
inert atmosphere. Anhydrous methanol and THF-d8 were obtained by double distillation of commercially
available materials (ACS reagent grade) over sodium metal. All other reagents and solvents used were
purchased from commercial sources and used as received without additional purification.
1,2-Diphenylcyclopropene (18). A solution of BH3·Me2NH complex (300 mg, 5.1 mmol) in MeOH
(1.5 ml), followed by a solution of Me3SiCl (630 μl, 5 mmol) in MeOH (1.5 ml) was added to a solution of
2,3-diphenylcycloprop-2-enone (500 mg, 2.4 mmol) in MeOH stirred at 0oC . The mixture was stirred for 1 h at
0oC and then concentrated in vacuum. The residue was dissolved in a hexane–THF (30:1) mixture, washed with
ice-cold water, and evaporated. Flash column chromatography on silica gel (hexane–CH2Cl2 5:1 as eluent)
afforded cyclopropene 16 as a colorless crystalline material, with properties identical to those previously
published [67]. Yield 354 mg (1.8 mmol, 77%).
A. Typical Procedure for Hydroboration of 1,2-Diphenylcyclopropene (18) with BH3·THF Complex
without Rearrangement (General Method). An oven-dried 3 ml Wheaton V-vial equipped with a magnetic spin
vane and a screw cup with a PTFE faced silicon septum was charged with 1,2-diphenyl-cyclopropene 18 (124 mg,
0.65 mmol). A solution of BH3·THF complex in THF (1 M, 2.0 equiv., 1.3 ml) was added at room temperature.
The mixture was stirred for 20 min and then quenched according to Procedures C or D.
815

B. Typical Procedure for Hydroboration of 1,2-Diphenylcyclopropene (18) with BH3·THF
Complex and Rearrangement (General Method). An oven-dried 3 ml Wheaton V-vial equipped with a
magnetic spin vane and Mininert valve was charged with 1,2-diphenylcyclopropene 18 (124 mg, 0.65 mmol). A
solution of BH3·THF complex in THF (1 M, 2 equiv., 1.3 ml) was added at room temperature. The mixture was
heated at 100oC for 3 h, then cooled down and quenched according to Procedures C or D.
C. Typical Procedure for Oxidative Quench with NaOH/H2O2 Mixture (General Method).The
reaction mixture was poured into a mixture of 3 N aqueous NaOH (1.5 ml), and a 30% aqueous H2O2 solution
(1.5 ml) was added, followed by methanol (1.5 ml). The mixture was stirred for 2 h at room temperature and
then extracted with EtOAc (2×10 ml). The combined organic phases were washed with brine, dried with
MgSO4, filtered, and concentrated. The residue was purified by preparative column chromatography on silica
gel, eluent hexane–EtOAc 2:1.
D. Typical Procedure for Oxidative Quench with Urea–H2O2 Complex (General Method). Urea–
H2O2 complex (3.5-4 equiv.) was added by portions to a reaction mixture at 0oC. The resulting mixture was
stirred overnight and then worked up as described in procedure C.
Combination of procedure A with oxidative quench C afforded 1,3-diphenylpropan-1-one (29) as a sole
1
product in 90% yield. H NMR (CDCl3),  7.98, ppm (J, Hz): (2H, d, J = 7.3, ortho-CH); 7.57 (1H, t, J = 7.7,
para-CH); 7.47 (2H, t, J = 7.8, meta-CH); 7.34-7.21 (5H, m, arom.); 3.32 (2H, t, J = 7.7, PhCH2); 3.09 (2H, t,
J = 7.7, COCH2). 13C NMR (CDCl3), , ppm: 199.7; 141.7; 137.2; 133.5 (+); 129.05 (+, 2C); 128.97 (+, 2C);
128.9 (+, 2C); 128.5 (+, 2C); 126.6 (+); 40.9 (-); 30.5 (-).
Combination of procedure B and oxidative quench C afforded 1,3-diphenylpropan-1-one 29 (73%) with
a mixture of meso- and dl-1,3-diphenylpropane-1,3-diols 30 in a ratio of 5:1 to 2:1 (20%).
Combination of procedure A and oxidative quench D produced 1,2-diphenylcyclopropanol 26 as a sole
1
product in 55% yield (compound notably decomposes on column and upon storage). H NMR (acetone-d6), ,
ppm (J, Hz): 7.25 (2H, d, J = 7.8, ortho-CH); 7.12 (2H, t, J = 7.2, meta-CH); 7.07-7.04 (3H, m, para- and meta-
CH); 6.98 (1H, t, J = 7.1, para-CH); 6.94 (2H, d, J = 7.6, ortho-CH); 2.69 (1H, dd, J = 10.0, J = 7.3, СН–ОН); 1.76
(1H, dd, J = 7.3, J = 6.1, cyclopropane); 1.56 (1H, dd, J = 10.0, J = 6.1, cyclopropane). 13C NMR (acetone-d6),
δ, ppm: 141.2; 139.2; 129.0 (+, 2C); 128.8 (+, 2C); 128.3 (+, 2C); 128.2 (+, 2C); 127.2 (+); 126.1 (+); 62.9;
33.7 (+); 19.5 (-).
Combination of procedure B and oxidative quench D afforded a mixture of cyclopropanol 28 (50%) and
meso-1,3-diphenylpropane-1,3-diol 30 (16%), identical to the independently prepared sample (see procedure E).
E. Independent Synthesis of meso-1,3-Diphenylpropane-1,3-diol (30) [68, 69]. A solution of n-BuLi
in hexane (2.5 M, 4.8 ml, 12.0 mmol) was added dropwise to a stirred at -78oC solution of dry i-Pr2NH (1.72 ml,
12.2 mmol) in anhydrous THF (20 ml). The resulting mixture was stirred for 10 min, and a solution of
acetophenone (1.22 g, 10 mmol) in THF (5 ml) was added dropwise. The mixture was stirred for 10 min at
-78oC. Neat benzaldehyde (1.28 ml, 12 mmol) was added, and stirring was continued for 30 min at -78oC. The
cold reaction mixture was poured into a separatory funnel with EtOAc and saturated aqueous NH4Cl. The
organic layer was separated; the aqueous layer was extracted with EtOAc. The combined organic phases were
washed with brine, dried over MgSO4, filtered, and concentrated. Preparative column chromatography on silica
gel (hexane–EtOAc 5:1 as eluent) afforded 3-hydroxy-1,3-diphenylpropan-1-one as a yellowish oil. Yield
1.96 g (8.7 mmol, 87%). A solution of TiCl4 in CH2Cl2 (1 M, 2.8 ml, 2.8 mmol) was added to a solution of the
obtained aldol (530 mg, 2.34 mmol) stirred at -78oC in anhydrous CH2Cl2 (5 ml) . The mixture was stirred at
-78oC for 10 min. BH3-Py complex (0.7 ml, 7 mmol) was added dropwise, and stirring was continued for 2 h.
The cold mixture was quenched by addition of aqueous HCl (1 M) and extracted with CH2Cl2. The combined
organic phases were washed with brine and concentrated. The residue was dissolved in MeOH–Et2O (1:1) and
treated with aqueous NaOH (3 N) and 30% H2O2 overnight. The resulting mixture was extracted with EtOAc,
washed with brine, dried with MgSO4, and concentrated. Preparative column chromatography on silica gel
(hexane–EtOAc 3:2 as eluent) afforded meso-1,3-diphenylpropane-1,3-diol 28 as a white solid. Yield 0.34 g
816

(1.5 mmol, 64%). 1H NMR (400.13 MHz, CDCl3), , ppm (J, Hz): 7.39-7.31 (8H, m, arom.); 7.28-7.26 (2H, m,
arom.); 4.98 (2H, dd, J = 10.1, J = 2.8, PhCHOH); 3.71 (2H, br. s, OH); 2.17 (1H, dt, J = 14.6, J = 10.1, СН2);
1.95 (1H, dt, J = 14.6, J = 2.8, СН2). 13C NMR (100.65 MHz, CDCl3), δ, ppm: 144.1 (2C); 128.4 (+, 4C); 127.6
(+, 2C); 125.7 (+, 4C); 74.9 (+, 2C); 47.6 (-).
PREPARATION OF FLUOROBORATES FOR 19F NMR STUDIES
F. Procedure for Generation of Fluoroborates 25 and 27. The reaction mixture obtained according to
procedure B was treated with anhydrous MeOH (4 equiv.) and the solvents were removed in vacuum. A solution
of KHF2 in MeOH–water (30:1) was added to the dry residue, and the mixture was stirred for 30 min. The
solvents were evaporated, and the residue was dissolved in dry THF-d8. 19F NMR spectrum showed four major
components: cis-(1,2-diphenylcyclopropyl)trifluoroborate 27: 19F NMR (THF-d8) δ -148.3 (3F, s);
tetrafluoroborate (KBF4) resulting from excess BH3-THF: 19F NMR (THF-d8), δ, ppm: -150.1 (4F, s) (used as
chemical shift secondary internal standard); C2-symmetric difluoroborate 27: 19F NMR (THF-d8), δ, ppm: -153.5
2
(2F, s); CS-symmetric difluoroborate 25: 19F NMR (THF-d8), δ, ppm (J, Hz): -154.6 (1F, d, JFF = 76); -155.2
(1F, d, 2JFF = 76).
Procedure for Generation of Difluoroborate 24. A solution of BH3·THF complex in THF (1 M, 0.50
equiv., 125 μl) was added at room temperature to a solution of 1,2-diphenylcyclo-propene 18 (48 mg, 0.25
mmol) in THF (500 μl). The mixture was stirred for 2 h, and then quenched with MeOH (40 μl). The solvents
were removed in vacuum, and a solution of KHF2 in MeOH–water (30:1) was added to the dry residue. The
mixture was stirred for 30 min; then the solvent was evaporated and the residue dissolved in dry THF-d8. 19F
NMR spectrum showed three major components: cis-(1,2-diphenylcyclopropyl)trifluoroborate 23: 19F NMR
(THF-d8), δ, ppm: -148.3 (3F, s); C2-symmetric difluoroborate dl-24: 19F NMR (THF-d8), δ, ppm: -165.2 (2F, s);
CS-symmetric difluoroborate meso-24: 19F NMR (THF-d8), δ, ppm (J, Hz): -164.2 (1F, d, 2JFF = 101); -65.3 (1F, d,
2JFF = 101).
ReactIR STUDIES AND IR SPECTRA INTERPRETATION
Procedure for ReactIR Monitored Reactions. The pressure tube (102 mm L×13 mm OD) with a
Teflon stopcock side arm and magnetic bar was equipped with a 6 mm Silicon ReactIR probe using #7 Ace-
Thread polypropylene adaptor. BH3·THF solution (1 M in THF, 0.5 ml) was added via syringe using a bore of
the stopcock opened for a short time, and the spectra acquisition sequence was initiated (256 scans per
spectrum, one spectrum every 2 min). After 10 min a solution of cyclopropene 18 (48 mg, 0.25 mmol) in
anhydrous THF (0.5 ml) was added (Fig. 1, event 1), and acquisition was continued for 16 min. Then the
reaction tube was placed in a pre-heated to 100oC oil bath (Fig. 1, event 2)) and the reaction was monitored for
another 1.5 h. Acquired spectra were processed using ConcIRT (Concentration in Real Time) [25] algorithm.
Three components were identified (A, B, and C, Fig. 1). A linear kinetic plot ln(CcompB)/Time revealed first-
order kinetics in the rearrangement of B (Fig. 4). An analogous experiment performed in the presence of
BH3·SMe2 complex showed rapid formation of cyclopropylborane 21, which remained unchanged when heated
for 1 h at 100oC.
Theoretical modeling was performed using Gaussian 03 software [70]. Geometries of both
cyclopropylborane 19 and boretane 20 were optimized and the corresponding vibrational spectra were DFT-
modeled (B3LYP/6-31G). The spectra were simulated for the gaseous phase (Fig. 5) without taking into account
temperature and solvation effects and possible conformational changes. Nevertheless, both experimental
ConcIRT (Fig. 2) and simulated spectra (Fig. 5) contain a set of characteristic bands in the fingerprint area,
based on which the structure of cyclopropyl borane 17 was assigned to component B (Fig. 5, group of bands B)
817

and the structure of boretane 18 to component C (Fig. 5, group of bands C), respectively. Analysis of computed
vibration modes revealed that band B corresponds to C–H scissor vibrations of the cyclopropyl ring in structure
19 (Fig. 5), while band C can be interpreted as a combination of symmetric and nonsymmetric rocking
vibrations of the boretane ring (Fig. 7).
Fig. 4. Kinetic plot of the rearrangement of cyclopropylborane 18 (component B) at 100oC, obtained from
ConcIRT spectra of the reaction mixture.
Fig. 5. Theoretical IR spectra of compounds 19 (dashed) and 20 (solid line): fingerprint region calculated for
gas phase. Specific absorption bands discussed in text are marked with arrows.
818

Fig. 6. Displacement vectors and vibrational frequencies corresponding to the absorptions at 1479 cm-1 (left)
and 1447 cm-1 (right) in theoretically modelled IR spectrum of compound 19. These frequencies belong to the
group of absorption bands marked with letter B in Fig 5.
Fig. 7. Displacement vectors and vibrational frequencies corresponding to the absorptions at 1249 (top left),
1233 (top right), 1220 (bottom left), and 1216 cm-1 (bottom right) in theoretically modelled IR spectrum of
compound 20. These frequencies belong to the group of absorption bands marked with letter C in Fig. 5.
This work was supported by the University of Kansas (KU) and the Center for Environmentally
Beneficial Catalysis (CEBC).
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