The iminoborane tBuB≡NtBu as a dipolarphile in (2+3) cycloadditions

Article abstract of DOI:10.1002/zaac.200500010

The iminoborane tBuB≡NtBu and the diazomethane tBuCH=N₂ give the (2+3) cycloadduct [-HC(tBu)-N=N-N(tBu)=B(tBu)-] in a 1:1 reaction and the seven-membered ring [-C(tBu)=N-NH-N(tBu)=B(tBu)-N(tBu)=B(tBu)-] in a 2:1 reaction. The (2+3) cycloadduct decomposes above 0°C to give the seven-membered ring, N₂, and HC(tBu)=N-N=CH(tBu) in the ratio 2:1:1. The borane tBuB≡NtBu and organic azides R″N₃ yield the (2+3) cycloadducts [-R″N-N=N-N(tBu)=B(tBu)-] (R″ = Me, Et, Pr, Bu, iBu, sBu, C₅H₁₁, c-C₅H₉, c-C ₆H₁₁, Bz1, EtOOC).

Full text of DOI:10.1002/zaac.200500010

The Iminoborane tBuBϵNtBu as a Dipolarphile in (2؉3) Cycloadditions
Bernd Kröckert, Karl-Heinz van Bonn, and Peter Paetzold*
Aachen, Institut für Anorganische Chemie der Technischen Hochschule
Received January 5th, 3005.
Abstract. The iminoborane tBuBϵNtBu and the diazomethane
tBuCHϭN2 give the (2ϩ3) cycloadduct [ϪHC(tBu)ϪNϭ
NϪN(tBu)ϭB(tBu)Ϫ] in a 1:1 reaction and the seven-membered
ring [ϪC(tBu)ϭNϪNHϪN(tBu)ϭB(tBu)ϪN(tBu)ϭB(tBu)Ϫ] in a
2:1 reaction. The (2ϩ3) cycloadduct decomposes above 0 °C to give
the seven-membered ring, N2, and HC(tBu)ϭNϪNϭCH(tBu) in
the ratio 2:1:1. The borane tBuBϵNtBu and organic azides RЉN3
yield the (2ϩ3) cycloadducts [ϪRЉNϪNϭNϪN(tBu)ϭB(tBu)Ϫ]
(RЉ ϭ Me, Et, Pr, Bu, iBu, sBu, C5H11, c-C5H9, c-C6H11, Bzl,
EtOOC).
Keywords: Iminoborane, (2ϩ3) Cycloadducts, Azaboracyclopenta-
dienes, Azaboracycloheptatrienes
We have intensively investigated the synthesis of the short-
lived diorganoiminoboranes RBϵNRЈ [1], their stabiliz-
ation by cyclodi-, -tri-, and -tetramerization or polymeriz-
ation [1, 2], and their reactions at the BN triple bond: 1,2-
addtions of polar and unpolar molecules and (2ϩ1), (2ϩ2),
and (2ϩ3) cycloadditions with appropriate systems [1].
Partners in the (2ϩ3) cycloaddition were predominantly or-
ganic azides RЉN3 [Eq. (1)], explored with 19 different im-
inoboranes and three azides (RЉ ϭ Ph, Bzl, iBu) [3]. Azido-
boranes RЉ2BN3 do not react according to Eq. (1), but give
an azidoboration [Eq. (2)] [4], and the azidosilane Me3SiN3
gives a mixture of cycloaddition and azidosilation products
with more of the latter [3c,d]. The nitrone OϪNMeϭCHPh
was explored as a 1,3 dipolar agent with two different im-
inoboranes [Eq. (3)] [5].
Because of the steric demand of the tert-butyl groups, the
iminoborane tBuBϵNtBu can be more easily handled than
other iminoboranes (t1/2 ca. 3 d at 50 °C), but it is still
rather reactive. It gives the (2ϩ3) cycloadduct with PhN3
and exclusively the azidoboration product with Me3SiN3
[6]. We report here on the (2ϩ3) cycloaddition with tert-
butyldiazomethane and on further cycloadditions with a
series of organic azides.
In pentane at Ϫ15 °C, the iminoborane tBuBϵNtBu and
the diazomethane tBuCHϭN2, in a 1:1 ratio, give the corre-
sponding 1,2-diaza-3-azonia-4-borata-1,3-cyclopentadiene
as a colourless oily liquid, according to Eq. (4). The pro-
posed structure of the product is in accord with the 1H, 11B,
and 13C NMR data, proving the presence of three different
tBu groups and a CH fragment. The 11B NMR signal of
five-membered rings with a CϪB(R)ϭN fragment is ex-
pected in the range δ ϭ 41Ϫ51 [7], but is observed in our
product low field-shifted at δ ϭ 53.8. This is possibly a
* Prof. Dr. P. Paetzold
Institut für Anorganische Chemie
Technische Hochschule Aachen
D-52056 Aachen, Germany
E-mail: peter.paetzold@ac.rwth-aachen.de
866
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
DOI: 10.1002/zaac.200500010
Z. Anorg. Allg. Chem. 2005, 631, 866Ϫ868

The Iminoborane tBuBϵNtBu as a Dipolarphile in (2ϩ3) Cycloadditions
consequence of a weakened BN π-bond, since the bulky tBu
groups force boron and its three ligands out of planarity.
The 2:1 reaction of the same components at 0 °C yields the
why such a 1,3-proton shift does not occur with the hetero-
cyclopentadiene, though this shift would transform the di-
ene-type ring into an aromatic 6π-electron system. We had
observed a comparable 1,3-shift of a Me3Si group at the
same cyclopentadiene ring system, when we performed a
(2ϩ3) cycloaddition of the methyleneborane MeBϭ
C(SiMe3)2 and Me3SiN3 [Eq. (8)] [9].
corresponding
1,2-diaza-3,5-diazonia-4,6-diborata-3,5,7-
cycloheptatriene, according to Eq (5). This product is also
formed, when a second mole of tBuBϵNtBu is added to
the product of Eq. (4), according to Eq. (6). The colourless,
oily heterocycloheptatriene is identified by two character-
istic 11B NMR peaks in the ratio 1:1 at δ ϭ 29.3 (B4, adjac-
ent to C, N, N) and 40.3 (B6, adjacent to C, C, N) [7], by
The addition of a slight excess of the azides RЉN3 to the
iminoborane tBuBϵNtBu yields the corresponding triazaa-
zoniaboratacyclopentadienes, according to Eq. (1) (RЉ ϭ
Me, Et, Pr, Bu, iBu, sBu, C5H11, c-C5H9, c-C6H11, Bzl,
1
1
a broad H NMR signal at δ ϭ 3.73 (CHring), by the H
and 13C NMR signals for five different tBu groups, and by
EtOOC). The products were characterized by their H and
1
a
13C NMR signal at δ ϭ 145 (C7). As a 10π-electron aro-
11B NMR data and by elemental analysis (Table 1). The
11B NMR signals are found in the typical range of δ ϭ
26Ϫ30 [3].
matic system, planarity of the seven-membered ring would
be expected, if the sterically demanding tBu group did not
press the ring too much out of planarity.
The heterocyclopentadiene is not stable in pentane above
0 °C and decomposes into the heterocycloheptatriene, nitro-
gen, and the 2,3-diaza-1,3-butadiene HtBuCϭNϪNϭ
CHtBu, according to Eq. (7). Obviously, half of the cyclo-
pentadiene is decomposed under reversion of the forma-
tion, Eq. (4), the hereby formed iminoborane reacts with
the undecomposed portion of the cyclopentadiene to give
the cycloheptatriene, Eq. (6), and the diazomethane compo-
nent decomposes in a well known manner [8] to give nitro-
gen and the diazabutadiene. A complete reversion of reac-
tion (4) can be achieved, when the heterocyclopentadiene is
heated up to 50 °C without a solvent, but then a part of the
products, tBuBϵNtBu and HtBuCϭN2, is transformed
into unidentified polymers.
Experimental
NMR: Bruker WP 80 PFT (1H), Jeol JNM-PS-100 (11B), Bruker
WH 270 (13C); standards: tms (intern; 1H, 13C), BF3(OEt2) (extern;
11B); in CDCl3.
3,4,5-Tri-tert-butyl-1,2-diaza-3-azonia-4-borata-1,3-cyclopentadiene:
An equimolar mixture (10.8 mmol) of tBuBϵNtBu [6] and
tBuCHN2 [10] in pentane (20 ml) is stirred at Ϫ15 °C (3 h). Diethyl
ether is added until the solution is clear. The product crystallizes
over night at Ϫ70 °C. It is filtrated at low temperature (in order to
avoid melting) and again recrystallized from pentane/diethyl ether
(74 %). The solid can be stored at Ϫ70 °C.
1H NMR: δ ϭ 1.07, 1.16, 1.58, 3.83 (4 s; 9:9:9:1). Ϫ 11B NMR (Ϫ30 °C):
δ ϭ 53.8. Ϫ 13C NMR (Ϫ30 °C): δ ϭ 19 (broad) 37.2, 56.3 (3 s; tBu), 30.0,
30.4, 31.2 (3 q; tBu), 91.2 (d; b1/2 ϭ 27 Hz).
The formation of the heterocycloheptatriene includes the
1,3-migration of a proton from carbon to nitrogen, allowing
the aromatization of the seven-membered ring. We wonder,
3,4,5,6,7-Penta-tert-butyl-1,2-diaza-3,5-diazonia-4,6-diborata-3,5,7-
cycloheptatriene: A mixture of tBuBϵNtBu (25.2 mmol) and
Table 1 Synthesis and characterization of the triazaazoniaboratacyclopentadienes [ϪRЉNϪNϭNϪN(tBu)ϭB(tBu)Ϫ]: amounts of
tBuBNtBu (n1) and RЉN3 (n2) (mmol), boiling points (°C) at reduced pressure (Pa) (in the case of R ϭ c-C6H11: sublimation point), yields
1
(%), H and 11B NMR shifts (ppm), and analytical data for C, H, N (%).
RЉ
n1/n2
Me
Et
Pr
Bu
iBu
20.1/30.3
sBua)
10.1/15.1
49/0.3/71
1.25/1.60
0.82 (t, 3H)
1.42 (d, 3H)
1.66Ϫ2.22 (2H)
26.7
27.3/29.8
39/0.5/71
1.25/1.58
3.72 (s, 3H)
18.0/21.1
42/0.5/66
1.26/1.59
1.36 (t, 3H)
4.07 (q, 2H)
10.1/15.3
50/0.8/66
1.25/1.59
0.94 (t, 3H)
1.39Ϫ1.92 (2H)
3.96(t,2H)
27.3
10.1/15.1
53/0.5/79
1.25/1.59
0.95Ϫ1.88 (7H)
4.00 (t, 2H)
b.p./press./yield
δ(1H), tBu (2s,18H)
δ(1H), R
52/0.2/73
1.25/1.60
0.91 (d, 6H)
1.81Ϫ2.38 (1H)
3.83 (d, 2H)
27.4
δ(1H), R
δ(1H), R
δ(11B)
27.5
26.9
26.9
C (calc./found)
H (calc./found)
N (calc./found)
55.12/55.04
10.79/10.56
28.57/28.41
57.16/57.87
11.03/11.47
26.66/26.51
58.94/58.96
11.24/11.56
24.99/25.07
60.51/60.48
11.42/11.44
23.52/23.48
60.51/60.41
11.42/11.59
23.52/23.75
60.51/60.69
11.42/11.24
23.52/23.69
RЉ
n1/n2
C5H11
c-C5H9
10.1/15.3
67/0.5/67
1.26/1.59
1.47Ϫ2.23 (8H)
4.65 (mc, 1H)
27.3
c-C6H11
10.1/15.2
80/0.5/71
1.24/1.59
1.24Ϫ2.00 (10H)
4.00 (mc, 1H)
29.0
Bzl
EtOOC
10.1/15.0
66/0.4/82
1.25/1.59
0.77Ϫ1.93 (9H)
3.99 (t, 2H)
26.8
10.1/15.0
100/0.3/62
1.14/1.62
5.29 (s, 2H)
7.20 (mc, 5H)
26.7
10.1/40.0
85/0.5/51
1.30/1.62
1.43 (t, 3H)
4.44 (q, 2H)
30.3
b.p./press./yield
δ(1H), tBu (2s,18H)
δ(1H), R
δ(1H), R
δ(11B)
C (calc./found)
H (calc./found)
N (calc./found)
61.91/61.63
11.59/11.89
22.21/22.08
62.41/62.36
10.88/10.52
22.39/22.25
63.64/63.48
11.06/11.31
21.20/20.95
66.19/66.34
9.26/9.36
20.58/20.25
51.99/51.93
9.12/9.12
22.05/22.20
a) Additional: δ(1H) ϭ 4.21 (pseudo-sext., 1H).
Z. Anorg. Allg. Chem. 2005, 631, 866Ϫ868
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© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
867

B. Kröckert, K.-H. van Bonn, P. Paetzold
tBuCHN2 (12.6 mmol) in hexane (20 ml) is stirred at 0 °C (12 h).
The yellow colour finally disappears. The product is obtained at
87 °C/0.3 Pa as a colourless oil (71 %).
C21H46B2N4 (376.2): C (found/calc.): 66.85/67.04, H 12.55/12.32, N
14.23/14.89 %.
1H NMR: δ ϭ 0.85, 1.01, 1.30, 1.38, 1.60, 3.73 (6 s; 9:9:9:9:9:1). Ϫ 11B NMR:
δ ϭ 29.3, 40.3 (2 s; 1:1). Ϫ 13C NMR: δ ϭ17.9, 21.4, 37.7, 51.2, 58.7 (5 s;
tBu), 29.0, 31.0, 33.3, 34.8, 42.4 (5 q; tBu), 145 (s; b1/2 ϭ 81 Hz). Ϫ MS
(MAT-CH5; 70 eV): m/e ϭ 376 (25 %; Mϩ), 361 (3; MϪMe), 319 (6;
MϪC4H9), 305 (13; MϪMeϪC4H9), 57 (100; C4H9), etc.
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4,5-Di-tert-butyl-1-organyl-1,2,3-triaza-4-azonia-5-borata-2,4-
cyclopentadienes [ϪRЉNϪNϭNϪN(tBu)ϭB(tBu)Ϫ]: The imino-
borane tBuBϵNtBu is added to the azide RЉN3 [11], generally
without a solvent; in the case of RЉ ϭ Me, Et, however, each of the
components is dissolved in pentane (10 ml). The general starting
temperature is Ϫ20 °C, but is Ϫ78 °C in the case of RЉ ϭ Me, Et
and is 22 °C in the case of RЉ ϭ EtOOC. The solutions are brought
to ambient temperature and stirred for 2 h; in the case RЉ ϭ Me,
Et, stirring is not necessary; in the case of RЉ ϭ EtOOC, 6 h stirring
at 50 °C is necessary. The colourless products are liquids, but are
solids in the case of RЉ ϭ c-C6H11, Bzl (m.p. 98 and 58 °C, respec-
tively). Further data are presented in Table 1.
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868
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Z. Anorg. Allg. Chem. 2005, 631, 866Ϫ868