Conversion of alkyltrifluoroborates into alkyldichloroboranes with tetrachlorosilane in coordinating solvents

Article abstract of DOI:10.1002/anie.200453690

Organotrifluoroborate salts are converted rapidly by tetrachlorosilane in nucleophilic solvents such as THF or acetonitrile into organodichloroboranes coordinated with the solvent (see scheme). This reaction completes a sequence from alkylboronic esters via the trifluoroborate salts to reactive alkyldichloroboranes under mild conditions.

Full text of DOI:10.1002/anie.200453690

Communications
dichloroborane, RBCl2. This reaction completes a mild and
efficient route from boronic esters to reactive alkyldichloro-
boranes, which are promising intermediates for asymmetric
syntheses.
Previously known chemistry of RBF3K includes the
reaction of PhBF3K with Me3SiCl in acetonitrile to form
PhBF2.[1] Allylic RBF3K derivatives react with aldehydes by
ꢀ
defluoridation with BF3 OEt2 to give the intermediates
RBF2,[2] or by defluoridation with Bu4NI/H2O.[3] Alkylboronic
esters of ꢁ 99% stereopurity are available by the reaction of
boronic esters of chiral diols with LiCHCl2.[4] Conversion of a
stereopure (5-azido-1-phenylbutyl)boronic ester to N3(CH2)3-
CH(Ph)BF3K followed by treatment with Me3SiCl or SiCl4
has been shown to yield 2-phenylpyrrolidine without loss of
stereopurity,[5] but the nature of the alkyldihaloborane inter-
ꢀ
mediates was not investigated. The chemistry of RBF3 salts
has been reviewed in 2003.[6] No previous instance of direct
ꢀ
conversion of an RBF2 to an RBCl2 by means other than B F/
[7]
ꢀ
B Cl exchange has been found in a search of the literature.
The barrier to F/Cl exchange is thermodynamic. Con-
version of BF3 and SiCl4 into BCl3 and SiF4 [Eq (1)] is
endothermic in the gas phase, DH8298 = +74.014 kJmolꢀ1 or
+6.168 kJmolꢀ1 bondꢀ1
;
DG8298 = +64.755 kJ molꢀ1
or
+5.396 kJmolꢀ1 bondꢀ1.[8]
4 BF3þ3 SiCl4 ! 4 BCl3þ3 SiF4
ð1Þ
The 11B NMR spectra of the products from treatment of
potassium (cyclohexyl)trifluoroborate (CyBF3K, 1;
Scheme 1), phenyltrifluoroborate (PhBF3K), or (5-azidopent-
yl)trifluoroborate [N3(CH2)5BF3K] with SiCl4 in THF or
Boranes
Conversion of Alkyltrifluoroborates into
Alkyldichloroboranes with Tetrachlorosilane in
Coordinating Solvents**
Byung Ju Kim and Donald S. Matteson*
Scheme 1. Conversion of potassium (cyclohexyl)trifluoroborate (1) into
cyclohexyl-9-BBN (4).
Addition of tetrachlorosilane to organotrifluoroborates,
RBF3K (R = alkyl or aryl), in THF or acetonitrile at 20–
258C results in immediate evolution of gaseous tetrafluoro-
silane and formation of the corresponding solvated organo-
CH3CN (Table 1) correspond to solvated RBCl2 and show
only minor impurities, including all SiFnCl(4ꢀn) (n = 1–4),[9] in
the 19F NMR spectrum. In CH2Cl2 with a catalytic amount of
[18]crown-6, the reaction stops at CyBF2 or PhBF2, and in
Et2O an unidentified mixture was obtained. The products
from Me3SiCl in all solvents were RBF2. Commercial PhBCl2
and PhBF2 from PhBCl2+NaBF4 showed similar NMR
spectra. The 11B and 19F chemical shifts of RBF2 (Table 1)
are consistent with tetracoordinate boron in THF but also
tricoordinate boron in CH3CN.[10] The RBCl2 derivatives are
tetracoordinate in both solvents. The stronger solvation of
RBCl2 evidently favors its formation.
[*] Dr. B. J. Kim, Prof. D. S. Matteson
Department of Chemistry, Box 644630
Washington State University
Pullman, WA 99164-4630 (USA)
Fax: (+1)509-335-8867
E-mail: dmatteson@wsu.edu
[**] We thank the National Science Foundation for support, grant
number CHE-0072788. The WSU NMR Center equipment was
supported by NIH grants RR0631401 and RR12948, NSF grants
CHE-9115282 and DBI-9604689, and a grant from the Murdock
Charitable Trust.
Tetrachlorosilane in acetonitrile is the preferred reagent
for converting alkyltrifluoroborates and alkyl azides into
secondary amines.[5] It is now apparent that the relevant
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200453690
Angew. Chem. Int. Ed. 2004, 43, 3056 –3058

Angewandte
Chemie
Table 1: NMR spectroscopic data for RBF3K, RBF2, and RBCl2 in various
solvents.
able loss of stereopurity. [(S)-1-Chloropentyl]boronic ester 5,
prepared from the (butyl)boronic ester and LiCHCl2,[4,15] was
converted into the trifluoroborate salt 7a,[5] then treated with
SiCl4 in THF to provide 8a (Scheme 2). Treatment with
methanol and pinacol yielded boronic ester 9 (95% from 7).
Transesterification of separate portions with the enantiomeric
(S)- and (R)-pinanediol esters produced 10 and 11 in high
stereopurity, as shown by their 1H NMR signals in the
Compound
Solvent
11B, d
19F, d
Other
CyBF3K[a]
CyBF2
CyBF2
CyBF2
CyBF2
CyBCl2
CyBCl2
PhBF3K
PhBF2
PhBF2
PhBF2
PhBF2
CD3CN
5.4
9.6
9.5
28
ꢀ145.2[b]
ꢀ149.5
ꢀ149.8
ꢀ79.5
ꢀ79.5
none
J
BF =63 Hz
THF/CDCl3
THF/CD3CN
CH3CN/CDCl3
CH3CN/CD3CN
THF/CDCl3
CH3CN/CD3CN
CD3CN
CDCl3
CD3CN
THF/CD3CN
THF/CDCl3
CDCl3
33
typically
differentiated
region,[15]
d = 1.15–1.20 ppm
15.7
5.7
4.0
24.6
11.6
7.2
7.2
55
none
(Figure 1, Scheme 2).
ꢀ141
JBF =55 Hz
ꢀ92
ꢀ127.9
ꢀ148.6
ꢀ149.8
none
[c]
PhBCl2
PhBCl2
PhBCl2
N3(CH2)5BF3K
N3(CH2)5BF2
N3(CH2)5BCl2
N3(CH2)5BCl2
CD3CN
THF/CD3CN
CDCl3
THF/CDCl3
THF/CDCl3
PhCH3/CDCl3
3.3
12.6
6.1
9.5
14.6
7.5
none
none
ꢀ139.2
ꢀ145.8
none
none
[a] Cy=cyclohexyl. [b] Also measured in D2O, d=ꢀ143 ppm.
[c] Commercial PhBCl2 (Aldrich).
factors are that RBCl2 reacts faster than RBF2 with alkyl
ꢀ
azides and that RBCl2 CH3CN dissociates more easily than
ꢀ
RBCl2 thf. The more weakly solvated primary RBCl2 in
diethyl ether react with alkyl azides at 258C.[11]
The higher reactivity of RBCl2 compared to RBF2 is
significant in hydroboration chemistry. In noncoordinating
solvents, the addition of an alkylsilane to RBCl2 and an alkene
results in rapid hydroboration, with clean formation of
dialkylchloroborane if the molar ratio of reactants is
1:1:1.[12] However, CyBF2 (ꢁ 0.3m in CH2Cl2) with Et2SiH2
does not hydroborate 1-hexene in 24 h at 20–258C. Hydro-
boration with sterically hindered RBCl2 in the presence of
diethyl ether has been reported.[13] Accordingly, we briefly
investigated hydroborations of alkenes with Et2SiH2 and
CyBF3K (1) in THF. Hydroboration is greatly retarded and no
pure single product prior to trialkylborane can be produced.
Conversion of CyBCl2 (2) and 1,5-cyclooctadiene to
cyclohexyl-9-BBN (4) in refluxing THF requires ꢁ 4 h, or
only ꢁ 1 h if an equimolar amount of 1-(dimethylamino)-
naphthalene is added (Scheme 1). In accord with previous
reports,[14] the borabicyclo[4.2.1]nonane isomer 3 (11B NMR:
d = 90.1 ppm) formed nearly as rapidly as the [3.3.1] isomer 4,
(11B NMR: d = 86.3 ppm), but rearranged into 4. In contrast
to the reaction in the absence of coordinating solvent,[12] 1-
hexene could not be converted to CyBClR, R = n-hexyl,
without generating CyBR2 (11B NMR spectrum: d =
85.3 ppm).
Scheme 2. Reactions of asymmetric alkyldihaloboranes. a) KHF2.[3]
b) For X=Cl, SiCl4 in THF; for X=F, Me3SiCl/THF. c) 1. MeOH, 08C;
2. Pinacol, 258C, 1 h, 95% from 7. d) (S)-Pinanediol yields 10, (R)-
pinanediol 11; filtered through silica. e) Et2Zn. f) 1. MeOH, NaOMe;
2. H2O2, NaOH.
Reaction of CyBF2 with BCl3 in CH2Cl2 liberates gaseous
BF3 and free CyBCl2, opening the way to the previously
described stepwise control of hydroboration,[12] but this
chemistry is predictably restricted to RBCl2 without alkoxy
or other nucleophilic substituents and has not yet been
explored.
Figure 1. Differentiated 1H NMR signals of (S)- and (R)-pinanediol (S)-
1-(chloropentyl)boronates 10 and 11.
An anticipated future synthetic application of asymmetric
(a-chloroalkyl)dihaloboranes such as 8 is for joining two alkyl
groups, especially combinations in which neither is available
from a Grignard reagent. Connection to boron by hydro-
Asymmetric (a-chloroalkyl)boronic esters can be con-
verted into (a-chloroalkyl)dichloroboranes without measur-
Angew. Chem. Int. Ed. 2004, 43, 3056 –3058
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ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3057

Communications
Glaser, H. Nöth, Angew. Chem. 1985, 97, 424 –425; Angew.
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boration is not an option because (a-chloroalkyl)hydrobor-
anes undergo rapid self-reduction in THF,[16] but organo-
metallic reagents should be useful. In view of the wide variety
of organozinc reagents that have become available from the
work of Knochel's group,[17] diethylzinc was chosen for
preliminary tests. All four (a-chloroalkyl)dihaloboranes 8
yielded optically active secondary alcohols 13 after treatment
with excess diethylzinc followed by sodium methoxide and
then alkaline hydrogen peroxide. The enantiomeric purities
of 13 have not been verified, but rotations are in the expected
range. Yields were satisfactory for 13a, excellent for the less
volatile 13b. Intermediate 12 from 8a, X = F, was verified by
the 11B NMR signal at d = 87.5 ppm.
[8] Calculated from NIST-JANAF Thermochemical Tables (Ed.:
M. W. Chase, Jr.), J. Phys. Chem. Ref. Data, Monograph 9,
American Institute of Physics, 1998.
[9] a) K. Hamada, G. A. Ozin, E. A. Robinson, Bull. Chem. Soc.
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[10] B. Wrackmeyer, R. Köster, Houben-Weyl, Vol. 13/3c (Ed.: R.
Köster), Thieme, Stuttgart, 1984, p. 377 –611.
[11] H. C. Brown, M. M. Midland, A. B. Levy, J. Am. Chem. Soc.
1973, 95, 2394 –2396.
[12] a) R. Soundararajan, D. S. Matteson, J. Org. Chem. 1990, 55,
2274 –2275; b) D. S. Matteson, R. Soundararajan, Organometal-
lics 1995, 14, 4157 –4166.
[13] a) U. P. Dhokte, S. V. Kulkarni, H. C. Brown, J. Org. Chem. 1996,
61, 5140 –5148; b) U. P. Dhokte, H. C. Brown, Organometallics,
1998, 17, 2891 –2896.
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[15] D. S. Matteson, K. M. Sadhu, M. L. Peterson, J. Am. Chem. Soc.
1986, 108, 812 –819.
[16] a) D. J. Pasto, Sr. , R. Snyder, J. Org. Chem. 1966, 31, 2773 –
2777; b) D. J. Pasto, J. Hickman, T. C. Cheng, J. Am. Chem. Soc.
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[17] A. Boudier, L. O. Bromm, M. Lotz, P. Knochel, Angew. Chem.
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4435.
[18] E. Keinan, E. K. Hafeli, K. K. Seth, R. Lamed, J. Am. Chem.
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[19] a) P. A. Levene, H. L. Haller, J. Biol. Chem. 1929, 83, 579 –600;
b) Further confirmation of absolute configuration with partially
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367 –372; R. L. Johnson, J. Kenyon, J. Chem. Soc. 1932, 722.
Experimental Section
13a: Trifluoroborate 7a (1.57 g, 7.4 mmol) was stirred for 4 h with
SiCl4 (1.2 mL, 14.8 mmol) in THF (20 mL) at 20–258C under argon.
Most of the excess SiCl4 was removed by concentration under
vacuum. The residue of 8a (X = Cl) in THF (20 mL) at 08C was
treated with ZnEt2 (15 mL, 1m in hexanes). After 12–16 h MeOH
(5 mL) was added at 08C. When gas evolution ceased, the mixture
was treated with NaOMe (2 g, 37 mmol) at 08C, then stirred for 4 h at
20–258C. After cooling to 08C, aqueous NaOH (5 mL, 3m) and H2O2
(30%, 5 mL) were added. After 3 h, the mixture was worked up with
ether and water and the residue was purified by flash chromatography
(10% ether/pentane) and bulb to bulb distillation of 13a (0.507 g,
1
59%); H NMR (CDCl3): d = 3.52 (m, 1H), 1.73 (bs, 1H), 1.23–1.63
(m, 8H), 0.94 (t, J = 7.2 Hz, 3H), 0.91(t, J = 6.9 Hz, 3H); 13C NMR
(CDCl3): d = 73.3, 36.6, 30.1, 27.8, 22.8, 14.1, 9.9 ppm; [a]2D2 = +7.9
(c = 0.03 in CHCl3), [a]52426 = +8.9 (c = 0.03 in CHCl3); (lit., [a]D =
+5.83 (CHCl3) (“95% ee”));[18] [a]D = +6.7 (neat), [a]D = +8.0
(EtOH), [a]D = +8.33 (Et2O).[19]
13b: Similar treatment of 7b (partially epimerized, d.r. 86:14)
1
yielded 13b (94%); H NMR (CDCl3): d = 7.18–7.34 (m, 5H), 3.73
(m, 1H), 2.82 (AB, dd, J = 13.5, 4.2 Hz, 1H), 2.63 (AB, dd, J = 13.5,
8.4 Hz, 1H), 1.64 (bs, 1H), 1.52 (m, 2H), 0.99 ppm (t, J = 7.8 Hz, 3H);
13C NMR (CDCl3): d = 138.6, 129.4, 128.5, 126.3, 74.0, 43.5, 29.5,
10.0 ppm; [a]D = +15.7 (ee < 72%, c = 0.054 in Et2O); [a]546 = +20.0;
no literature data available.
Received: January 7, 2004 [Z53690]
Keywords: alkyl boranes · asymmetric synthesis · boron ·
.
hydroboration
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ꢀ
ꢀ
[7] Other than B F/B Cl exchange, the only examples found of any
derivation of a chloroborane from a corresponding fluoroborane
involved two steps, elimination of hydrogen fluoride or trime-
thylsilyl fluoride from a suitably substituted sterically hindered
(amino)(fluoro)borane to form an iminoborane, which was
ꢀ
isolated, followed by addition of HCl to make a B Cl bond: a) B.
3058
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2004, 43, 3056 –3058