Formation and X-ray structure of a seven-membered C4OBN heterocycle by a THF ring-expansion process

Article abstract of DOI:10.1039/b810319d

The reaction of BCl₃ with two equivalents of LiN(H)Dipp (Dipp = 2,6-diisopropylphenyl) in THF produces DippN(H)BO(CH₂) ₄NDipp, which contains a puckered seven-membered C₄OBN ring.

Full text of DOI:10.1039/b810319d

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COMMUNICATION
www.rsc.org/dalton | Dalton Transactions
Formation and X-ray structure of a seven-membered C4OBN heterocycle by a
THF ring-expansion process†
Andrea M. Corrente and Tristram Chivers*
Received 18th June 2008, Accepted 11th July 2008
First published as an Advance Article on the web 29th July 2008
DOI: 10.1039/b810319d
The reaction of BCl3 with two equivalents of LiN(H)Dipp
(Dipp = 2,6-diisopropylphenyl) in THF produces DippN-
(H)BO(CH2)4NDipp, which contains a puckered seven-
membered C4OBN ring.
a concentrated solution of n-hexane and toluene at ca. −30 ◦C
in 37% yield. In addition to the characteristic resonances for the
Dipp substituents, the 1H NMR spectrum of 1a reveals four sets
of equally intense multiplets (at 3.82, 3.14, 1.69 and 1.52 ppm)
with relative intensities that correspond to two protons each and
the 13C NMR spectrum contains resonances at 64.9, 53.5 28.9 and
28.8 ppm. A single broad resonance is observed in the 11B spectrum
at 23.8 ppm indicative of a three-coordinate boron centre. Taken
together, the NMR data indicate the presence of four inequivalent
CH2 groups in 1a, two of which are linked (on the basis of chemical
shifts) to electronegative atoms.
X-Ray quality crystals of 1a§ were obtained by recrys-
tallisation from an n-hexane and toluene solvent mixture.
An X-ray structural determination revealed the formation of
DippN(H)BO(CH2)4NDipp comprised of a seven-membered
C4OBN (1,3,2-oxazaborepane) ring with a Dipp group attached
to nitrogen and a DippNH substituent on the boron atom. The
heterocycle 1a is the first structurally characterised example of a
seven-membered ring containing nitrogen, boron and oxygen, in
which three-coordinate nitrogen and boron atoms are covalently
bonded. A single example of a 1,3,2-oxazaborepane with an alkyl
substituent on boron has been reported. It was obtained from the
reaction of 1-hydroxymethyl-2-aminomethylbenzene with dibutyl
isopropylboronate as a red oil and characterised only by 1H NMR
and IR spectra and by MS.11 Other heterocycles containing the
OBN unit within smaller rings are known, including the well-
studied 1,3,2-oxazaborolidines, which are used as catalysts for
numerous organic transformations,12 and the tricyclic borazine
2 (Chart 1).13 The molecular structure of 1a is shown in Fig. 1 and
selected structural parameters are presented in Table 1.
Examples of THF ring-opening promoted by Lewis acidic
transition-metal,1 lanthanide2 or actinide complexes3 are more
common than those that are initiated by electrophilic main
group element centres. In the latter category, ring-opening occurs
when dimethylethylamine-alane reacts with three equivalents of
2,6-diisopropylphenol in THF.4 The combination of TeBr4 with
triphenylphosphine in THF has also produced a ring-opened
product.5 We have reported that cooperative ring-opening occurs
t
when the tellurium diimide dimer BuNTe(l-NtBu)2TeNtBu is
added to a solution of B(C6F5)3 in THF; the 1 : 1 adduct of
these two reagents also initiates this process.6 More recently,
Stephan and co-workers have demonstrated that sterically bulky
phosphines react with the adduct (THF)·B(C6F5)3 in a ring-
opening fashion to give phosphonium-borate zwitterions.7 In all of
these examples the THF molecule appears as an acyclic fragment
in the final product. The THF molecule can also be incorporated as
part of a ring in the final product, however this behaviour is limited
to a single example. Sekiguchi and co-workers recently reported
that seven-membered rings (1,3,2-oxasila- and germaborepanes)
=
are formed by insertion of in situ >E B– (E = Si, Ge) doubly
bonded species into a C–O bond of THF.8
The cleavage of ethers by boron trihalides BX3 (X = Cl,
Br) occurs readily, and organoboron halides, e.g. Me2BBr, are
known to cleave cyclic ethers such as 2-methyltetrahydrofuran
regioselectively.9 By contrast, the reaction of Ph2BCl with a THF
solution of tetralithiated tetra-tert-butyl calix(4)arene produces
the simple adduct Ph2BCl·THF, indicating that the replacement
of two chlorine atoms in BCl3 by two phenyl groups substantially
reduces the reactivity of the remaining B–Cl bond towards
C–O bond cleavage.10 In this Communication we report that
the in situ generation of DippNH boron halides (Dipp =
2,6-diisopropylphenyl) in THF results in the formation of a
seven-membered C4OBN ring (1,3,2-oxazaborepane), the first
example of THF ring-opening promoted by aminoboron halides
(RNH)3−nBCln (n = 1, 2).
The reaction of BCl3 with two equivalents of LiN(H)Dipp in
THF produces a colourless solid 1a‡ and a minor, unidentified oily
product. Compound 1a is isolated as a pure crystalline solid from
Chart 1
Carbon atoms C26 and C27 are disordered over two atomic sites
resulting in two superimposed seven-membered rings with chair-
and boat-like conformations in a 60 : 40 ratio, respectively (Fig. 2),
with dihedral angles of 57.1◦ for C25–C26–C27–C28 and 39.7◦ for
C25A–C26A–C27A–C28A. The C28–N1–B1–O1 torsion angle is
22.4◦, comparable to that of a related seven-membered heterocycle
Department of Chemistry, University of Calgary, Calgary, Alberta, T2N 1N4,
Canada. E-mail: chivers@ucalgary.ca; Fax: +1 (403) 289-9488; Tel: +1
(403) 220-5741
† CCDC reference number 692058. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/b810319d
4840 | Dalton Trans., 2008, 4840–4842
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ꢀ
◦
˚
three-coordinate boron and nitrogen atoms is planar ( ∠ B =
Table 1 Selected bond lengths (A) and angles ( ) for C28H43N2BO
ꢀ
360◦ and
∠ N1 = 358.6◦).
B1–O1
B1–N2
B1–N1
1.381(3) N1–C28
1.421(3) N1–C1
1.428(3) O1–C25
1.478(2)
1.440(2)
1.431(3)
The formation of the C4OBN heterocycle 1a can be envisaged
to involve the initial coordination of THF to a Lewis acidic
chloroborane (DippNH)nBCl3−n (n = 1 or 2; shown for n = 1
in Scheme 1), followed by concomitant N–C bond formation and
ring-opening (C–O bond cleavage) with the elimination of HCl to
give the observed product.
O1–B1–N2
O1–B1–N1
N2–B1–N1
C28–N1–B1–O1
116.5(2)
B1–N1–C28
C1–N1–C28
B1–N1–C1
123.9(2)
113.9(2)
120.8(2)
122.9(2)
120.6(2)
22.4(3)
C28A–C27A–C26A–C25A 39.7(8)
C28–C27–C26–C25 57.1(4)
Scheme 1
In an attempt to obtain evidence for this reaction pathway,
the disubstituted chloroborane ClB(NHDipp)2 (3)17 was stirred in
THF for 18 h; however, the heterocycle 1a was not formed. In the
light of this observation, we considered that the monosubstituted
dichloroborane Cl2B(NHDipp) (4) (a stronger Lewis acid than 3)
may be responsible for initiating the ring-opening. Consequently,
the reaction depicted in Scheme 1 was carried out using a
BCl3 : LiN(H)Dipp molar ratio of 1 : 1 in THF in order to generate
Fig. 1 Molecular structure of one conformation of DippN(H)BO-
(CH2)4NDipp (1a). Hydrogen atoms omitted and only a-carbon atoms
of Dipp groups are shown for clarity.
1
4 in situ. The H NMR spectrum of the product of this reaction
reveals a major species that exhibits equally intense multiplets at
3.69, 3.13, 1.56 and 1.48 ppm; a singlet is observed at 26.8 ppm
in the 11B NMR spectrum. The similarity of the chemical shifts
for –CH2– groups in the 1H NMR spectrum to those observed for
1a together with the small downfield shift of 3.0 ppm in the 11B
NMR resonance with respect to 1a is consistent with the formation
of the ring-opened derivative 1b with an exocyclic Cl substituent
attached to boron (Scheme 1). The reaction mixture also contained
[DippNH3]Cl (1H NMR spectrum), which is presumably formed
by the reaction of eliminated HCl with LiN(H)Dipp in a 2 : 1 molar
ratio. Further evidence that the oily product is 1b was garnered
from the observation that the addition of an excess of LiN(H)Dipp
produces 1a (1H and 11B NMR spectra). As with the synthesis of
1b, crystals of [DippNH3]Cl were also isolated from the reaction
of BCl3 with 2 equivalents of LiN(H)Dipp, which suggests that
the yield could be increased by adding excess amide to scavenge
the HCl by-product. However, carrying out this reaction in a 2 : 5
molar ratio of BCl3 : LiN(H)Dipp produced lower yields of 1a (by
1H NMR).
Fig. 2 Molecular conformations of the C4NBO ring in 1a (hydrogen
atoms omitted for clarity).
containing nitrogen, boron and oxygen where the boron atom is
four-coordinate.14 The boron–oxygen bond length of 1.381(3) A
˚
is in agreement with those in structures containing a nitrogen–
boron–oxygen fragment, which typically range from 1.3515 to
16
˚
1.43 A; specifically, the distance is indistinguishable from that
In summary, the reaction of BCl3 with LiN(H)Dipp in THF
generates the first structurally characterised example of a 1,3,2-
oxazaborepane. Variation of the stoichiometry of this reaction
indicates that this THF ring-opening process is initiated by
the dichloroborane Cl2B(NHDipp).18 Although the formation of
this seven-membered ring formally involves the insertion of the
observed for the tricyclic borazine, 2,13 which provides a compara-
ble cyclic environment to that of 1a. The endocyclic and exocyclic
B–N bond lengths in 1a are equal and intermediate between single
and double bond values. The endocyclic C–N bond is slightly
elongated compared to that of the exocyclic one, however, the
latter is identical, within experimental error, to the N–C bond of
=
iminoborane ClB NDipp into a C–O bond of the THF molecule,
˚
the NHDipp substituent (1.431(2) A). The geometry about the
the generation of an iminoborane under the reaction conditions
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employed here is unlikely as these compounds are typically
obtained from gas phase thermolysis reactions.19 Recently, it has
been shown that iminoboranes can also be formed by induc-
ing HX elimination from sterically bulky aminoboranes using
Na[N(SiMe3)2] in THF,20 demonstrating a contrast in reactivity
to the chloro-aminoboranes presented here.
with anisotropic thermal parameters. The hydrogen atoms were calculated
geometrically and were riding on their respective atoms. Two of the carbon
atoms of the former THF molecule (C26, C27) were disordered over two
atomic sites; the disorder was refined and the final site occupancy was found
to be approximately 60:40. C28H43BN2O, M = 434.45, monoclinic, a =
◦
3
˚
˚
10.839(2), b = 20.360(4), c = 12.049(2) A, b = 97.85(3) , V = 2633.8(9) A ,
T = 173 K, space group P21/n, Z = 4, l = 0.065 mm−1, Rint = 0.0278 (for
26111 measured reflections), R1 = 0.0597 [for 4634 unique reflections with
[I>2r(I)]), wR2 = 0.1671 (for all data).
Acknowledgements
1 (a) A. Mommertz, R. Leo, W. Massa, K. Harms and K. Dehnicke,
Z. Anorg. Allg. Chem., 1998, 624, 1647; (b) M. Polamo, I. Mutikainen
and M. Leskela¨, Acta Crystallogr., Sect. C: Cryst. Struct. Commun.,
1997, 53, 1036; (c) T. L. Breen and D. W. Stephan, Inorg. Chem., 1992,
31, 4019; (d) Z. Y. Guo, P. K. Bradley and R. F. Jordan, Organometallics,
1992, 11, 2690.
The financial support of NSERC (Canada) and the Alberta
Ingenuity Fund is gratefully acknowledged.
2 W. J. Evans, J. T. Leman, J. W. Ziller and S. I. Khan, Inorg. Chem., 1996,
35, 4283.
3 (a) L. R. Avens, D. M. Barnhart, C. J. Burns and S. D. McKee, Inorg.
Chem., 1996, 35, 537; (b) M. P. C. Campello, A. Domingos and I.
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4 J. P. Campbell and W. L. Gladfelter, Inorg. Chem., 1997, 36,
4094.
5 S. M. Kunnari, R. Oilunkaniemi, R. S. Laitinen and M. Ahlgre´n,
J. Chem. Soc., Dalton Trans., 2001, 3417.
6 G. Schatte and T. Chivers, Eur. J. Inorg. Chem., 2003, 3314.
7 G. C. Welch, J. D. Masuda and D. W. Stephan, Inorg. Chem., 2006, 45,
478.
8 N. Nakata, R. Izumi, V. Y. Lee, M. Ichinohe and A. Sekiguchi, Chem.
Lett., 2005, 34, 582.
9 (a) Y. Guindon, C. Yoakim and H. E. Morton, Tetrahedron Lett., 1983,
24, 2969; (b) Y. Guindon, M. Therien, Y. Girard and C. Yoakim, J. Org.
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10 W. I. Cross, M. P. Lightfoot, F. S. Mair and R. G. Pritchard, Inorg.
Chem., 2000, 39, 2690.
11 V. V. Kuznetsov, Y. E. Brusilovskii and A. V. Mazepa, Russ. J. Gen.
Chem., 2001, 71, 817.
Notes and references
‡ Synthesis of 1a: A solution of LiN(H)Dipp (1.10 g, 6.0 mmol) in THF
(15 mL) was added to a solution of BCl3 (3 mL, 1 M, 3.0 mmol) in n-
hexane at ca. −80 ◦C. The reaction mixture was allowed to warm to room
temperature and stirred for 18 h. Volatiles were removed in vacuo and the
oily product was extracted with n-hexane and filtered. After removal of
solvent, a sticky solid was isolated and colourless, X-ray quality crystals
of 1a were grown from an n-hexane–toluene solution (0.488 g, 1.12 mmol,
37%) (Found: C, 76.68; H, 10.02; N, 6.28. Calc. for C28H43N2BO: C, 77.41;
H, 9.98; N, 6.45%). 1H: d (399.45 MHz; C6D6; 25 ◦C): 7.18–7.09 (m, 6 H,
Dipp groups), 3.82 (m, 2 H, ring –CH2–O), 3.66 (sept, 2 H, –CH(CH3)2,
3JH-H = 6.9 Hz), 3.40 (sept, 2 H, –CH(CH3)2, 3JH-H = 6.9 Hz), 3.14 (m, 2 H,
–CH2–N), 2.92 (1 H, br s, –NH), 1.69 (m, 2 H, O–CH2–CH2–), 1.52 (m,
2 H, –CH2–CH2–N), 1.35 (d, 6 H, –CH(CH3)2, 3JH-H = 6.8 Hz), 1.23◦and
1.22 (overlapping d, 18 H,–CH(CH3)2). 11B: d (128.16 MHz; C6D6; 25 C):
23.8 (s). 13C: d (100.45 MHz; C6D6; 25 ◦C): 147.7 (Dipp), 145.8 (Dipp),
144.6 (Dipp), 138.1 (Dipp), 127.5 (Dipp), 125.6 (Dipp), 124.9 (Dipp), 123.3
(Dipp), 64.9 (–CH2–O), 53.5 (–CH2–N), 30.8 (–CH(CH3)2), 28.9 (ring –
CH2–), 28.8 (ring –CH2–), 28.5 (–CH(CH3)2), 26.5 (–CH(CH3)2), 24.5
(–CH(CH3)2), 24.4 (–CH(CH3)2). Synthesis of 1b:
A solution of
12 For examples, see: (a) A. Hirao, S. Itsuno, S. Nakahama and N.
Yamazaki, J. Chem. Soc., Chem. Commun., 1981, 315; (b) E. J. Corey,
Angew. Chem., Int. Ed., 2002, 41, 1650.
13 V. Stepanenko, M. Ortiz-Marciales, C. E. Barnes and C. Garcia,
Tetrahedron Lett., 2006, 47, 7603.
14 H. Ho¨pfl, M. Sa´nchez, N. Farfa´n and V. Barba, Can. J. Chem., 1998,
76, 1352.
15 E. v. Steuber, G. Elter, M. Noltemeyer, H.-G. Schmidt and A. Meller,
Organometallics, 2000, 19, 5083.
LiN(H)Dipp (0.917 g, 5.0 mmol) in THF (15 mL) was added to a solution
of BCl3 (5 mL, 1 M, 5.0 mmol) in n-hexane at ca. −80 ◦C. The reaction
mixture was allowed to warm to room temperature and stirred for 18 h.
Volatiles were removed in vacuo and the product was extracted with n-
hexane and filtered to remove LiCl. After removal of solvent a peach-
coloured oil was isolated. 1H: d (399.45 MHz; C6D6; 25 ◦C): 7.15–6.97 (m,
3H, Dipp), 3.69 (m, 2 H, –CH2–O), 3.36 (sept, 2 H, –CH(CH3)2), 3.13
(m, 2 H, –CH2–N), 1.56 (m, 2 H, O–CH2–CH2–), 1.48 (m, 2 H, –CH2–
CH2–N), 1.13 (overlapping d, –CH(CH3)2). 11B: d (128.16 MHz; C6D6;
25 ◦C): 26.8 (br). The resonances of [DippNH3]Cl were also observed in
the 1H NMR spectrum. The product 1b could not be separated from the
by-product [DippNH3]Cl. Addition of LiN(H)Dipp (0.288 g, 1.57 mmol)
to this oily product (0.258 g) in either hexane or THF gave an oil, which
exhibited multiplet resonances at d 3.82, 3.14, 1.69, 1.52 in the 1H NMR
spectrum attributable to 1a.
16 B. Wrackmeyer, E. V. Klimkina, W. Milius, O. L. Tok and M.
Herberhold, Inorg. Chim. Acta, 2005, 358, 1420.
17 The chloroborane ClB(NHDipp)2 (d 11B 26.4 ppm) is obtained in good
yield from the reaction of DippNH2 with a solution of BCl3 in a 4 : 1
molar ratio in n-hexane. A. M. Corrente and T. Chivers, unpublished
results.
18 The reaction of DippNH2 with a solution of BCl3 in a 2:1 molar
ratio in n-hexane produces Cl2B(NHDipp) (d 11B 32.3 ppm), but it
is contaminated with significant amounts of ClB(NHDipp)2. However,
stirring this mixture in THF for 18 h also produces 1b, identified by 1H
and 11B NMR spectra.
19 P. Paetzold, Adv. Inorg. Chem., 1987, 31, 123 and references therein.
20 E. Rivard, W. A. Merrill, J. C. Fettinger, R. Wolf, G. H. Spikes and P. P.
Power, Inorg. Chem., 2007, 46, 2971.
21 G. M. Sheldrick, SHELXS-97, Program for solution of crystal struc-
tures, University of Go¨ttingen, Germany, 1997.
22 G. M. Sheldrick, SHELXL-97, Program for refinement of crystal
structures, University of Go¨ttingen, Germany, 1997.
§ Crystal data for 1a: A single crystal of 1a suitable for X-ray analysis was
covered with Paratone oil and mounted on a glass fibre. Data were collected
in a stream of N2 at 173 K on a Nonius KappaCCD diffractometer (Mo-Ka
˚
radiation, k = 0.71073 A) using COLLECT (Nonius, B.V. 1998) software.
The unit cell parameters were calculated and refined from the full data set.
All crystal cell refinement and data reduction was carried out using the
Nonius DENZO package. After data reduction, the data were corrected for
absorption based on equivalent reflections using SCALEPACK (Nonius,
B.V. 1998). The structures were solved by direct methods with the
SHELXS-9721 program package and refinement was carried out on F2
against all independent reflections by the full-matrix least-squares method
by using the SHELXL-97 program.22 All non-hydrogen atoms were refined
4842 | Dalton Trans., 2008, 4840–4842
This journal is
The Royal Society of Chemistry 2008
©