Reaction of a stable conjugated primary enamine with Piers' Borane

Article abstract of DOI:10.1002/ejic.200901109

Treatment of l,6-diamino-l,6-diphenyl-l,3,5-hexatriene (3) with 2 mol-equiv. of HB(C₆F₂)₂ results in tautomerization of the stable primary enamine to its bis(imine) tautomer, which is stabilized by twofold imine/borane Lewis base/Lewis acid adduct formation to yield an 1.1:6:1. mixture of E,E-S, E,Z-5 and Z,Z-5. Within several hours at room temperature, these systems undergo subsequent hydroboration of the central remaining C=C double bond followed by internal imine-alkylborane coordination to yield the six-membered N,B heterocycle 6 featuring a pendant -CH ₂-C(Ph)=NH[B(H)(C₆F₅)₂] group. Compound 6 was characterized by X-ray diffraction.

Full text of DOI:10.1002/ejic.200901109

FULL PAPER
DOI: 10.1002/ejic.200901109
Reaction of a Stable Conjugated Primary Enamine with Piers’ Borane
Ilona Peuser,[a] Roland Fröhlich,[a][‡] Gerald Kehr,[a] and Gerhard Erker*[a]
Dedicated to Professor Willi Keim on the occasion of his 75th birthday
Keywords: Lewis acids / Tautomerization / Hydroboration / Boron / Enamine
Treatment of 1,6-diamino-1,6-diphenyl-1,3,5-hexatriene (3)
with 2 mol-equiv. of HB(C6F5)2 results in tautomerization of
the stable primary enamine to its bis(imine) tautomer, which
is stabilized by twofold imine/borane Lewis base/Lewis acid
adduct formation to yield an 11:6:1 mixture of E,E-5, E,Z-5
and Z,Z-5. Within several hours at room temperature, these
systems undergo subsequent hydroboration of the central re-
maining C=C double bond followed by internal imine–alkyl-
borane coordination to yield the six-membered N,B hetero-
cycle 6 featuring a pendant –CH2–C(Ph)=NH[B(H)(C6F5)2]
group. Compound 6 was characterized by X-ray diffraction.
yield. The reaction of 3 with Piers’ borane will be described
in this account.
Introduction
[1]
The strong Lewis acid B(C6F5)3 has been shown to be
able to induce tautomerization reactions and to stabilize
otherwise unstable carbonyl compounds relative to their
favoured enol forms by adduct formation.[2] Piers’ borane
[HB(C6F5)2][3] is also a strongly electrophilic reagent that
might be able to influence tautomerization equilibria in a
similar way. At the same time HB(C6F5)2 is a reactive hy-
droboration agent.[4] We will here describe an example
where these two reactivities in their combination lead to an
interesting product formation. We chose the 1,6-diamino-
1,3,5-hexatriene derivative 3 as the substrate for our study.
This unusual compound[5–7] was obtained starting from
(butadiene)zirconocene (1)[8] as described previously by
us.[5] The reaction of 1 with 2 mol-equiv. of benzonitrile
gave the metallacycle 2 (see Scheme 1). Subsequent con-
trolled hydrolysis removed the zirconocene template to give
the stable conjugated primary enamine derivative 3 in good
Results and Discussion
The diaminohexatriene 3 was mixed with 2 mol-equiv. of
Piers’ borane [HB(C6F5)2] (4) in toluene solution at ambi-
ent temperature. This resulted in a rapid isomerization reac-
tion of the enamine moieties to their imine isomers com-
bined with imine–borane adduct formation.[9] After re-
moval of the solvent, an oil was obtained that contained
three isomeric products: E,E-5, E,Z-5 and Z,Z-5 in an
11:6:1 ratio. The structure of the very minor compound of
this mixture was only tentatively assigned as the third pos-
sible isomer Z,Z-5 (see Scheme 2, Figure 1).
The 1H NMR spectrum clearly shows the presence of
three primary ketimine isomer adducts. The major compo-
nent of the mixture features a pair of symmetry-equivalent
ketimine moieties [=NH resonance at δ = 9.65 (s, 2 H) ppm
(see Figure 1) with a single corresponding 13C NMR reso-
nance at δ = 181.7 ppm]. Consequently, we observe only
1
one H NMR signal of the central –CH=CH– unit at δ =
4.99 ppm (13C NMR: δ = 126.8 ppm) and a single 1H NMR
CH2 resonance at δ = 3.20 ppm (m, 4 H; 13C NMR: δ =
1
34.7 ppm). There is a very broad BH H NMR signal (not
differentiated between the isomers) at δ ≈ 4.25 ppm, and a
single broad 11B NMR resonance at δ = –15.0 ppm, as ex-
pected for a four-coordinate boron centre. The major com-
ponent shows three corresponding 19F NMR “borate” sig-
nals of the symmetry-equivalent pair of C6F5 substituents
at the boron atom [δ = –134.4 (o), –157.7 (p), –163.1 (m)
ppm]. The small separation of the p- and m-C6F5 19F NMR
resonances further supports the presence of four-coordinate
Scheme 1.
[a] Organisch-Chemisches Institut der Universität Münster,
Corrensstraße 40, 48149 Münster, Germany
Fax: +49-251-8336503
E-mail: erker@uni-muenster.de
[‡] X-ray crystal structure analysis
Eur. J. Inorg. Chem. 2010, 849–853
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
849

I. Peuser, R. Fröhlich, G. Kehr, G. Erker
FULL PAPER
Scheme 3.
Scheme 2.
boron centres[10] in this major product, to which we assign
the structure of E,E-5. The second isomer, to which we as-
sign the structure of E,Z-5, shows a 1:1 intensity pair of
1
=NH H NMR signals (δ = 9.88, 9.47 ppm; 13C NMR: δ =
1
184.4, 181.5 ppm). It features a pair of H NMR signals of
3
the central (E)–CH=CH– unit (δ = 5.17, 4.92 ppm; JHH
=
15.6 Hz) and a pair of CH2 resonances (δ = 3.53, 2.46 ppm).
Keeping a solution of the mixture of the isomers of 5 in
benzene or toluene at room temperature for several hours
resulted in the observation of a subsequent rearrangement
of all three compounds to give a single new product (see
Figure 2. View of the molecular structure of compound 6.
The result of the X-ray crystal structure analysis shows
Scheme 3). On a preparative scale this was obtained by that one of the HB(C6F5)2 molecules has undergone a hy-
treatment of the diaminohexatriene starting material 3 with droboration reaction of the central carbon–carbon double
2 mol-equiv. of HB(C6F5)2 followed by stirring of the reac- bond of the system. This has resulted in the formation of
tion mixture overnight. After removal of the volatiles in an alkylborane product that is stabilized by intramolecular
vacuo, the obtained solid was washed with pentane and B–N(imine) adduct formation of the strongly electrophilic
dried to finally give the product 6 in approximately 55% three-coordinate boron centre. Consequently, the new prod-
yield as a colourless solid. Compound 6 was characterized uct 6 contains a six-membered heterocyclic subunit that
by X-ray diffraction (single crystals were obtained from a features a strong B–N interaction [B1–N2 1.584(5) Å] to the
dichloromethane/pentane solvent mixture) (see Figure 2).
primary imine functional group. The coordinated imine
1
Figure 1. H NMR spectrum of the mixture of E,E-, E,Z- and Z,Z-5 isomers (500 MHz, [D6]benzene, 298 K; *: toluene).
850
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Eur. J. Inorg. Chem. 2010, 849–853

Reaction of a Stable Conjugated Primary Enamine with Piers’ Borane
function is characterized by an N2–C3 bond length of
1.293(4) Å and bond angles N2–C3–C4 of 120.1(3)°, N2–
C3–C31 of 119.7(3)° and a C3–N2–B1 angle of 129.1(3)°.
The boron atom is four-coordinate featuring bond angles
of 106.8(3)° (N2–B1–C6), 104.6(3)° (N2–B1–C11),
109.8(3)° (N2–B1–C21), 117.5(3)° (C6–B1–C11), 106.4(3)°
(C21–B1–C11) and 111.5(3)° (C6–B1–C21). The six-mem-
bered ring attains a distorted cyclohexene-like half-chair
conformation (see Figure 2).
Scheme 4.
Ring-carbon atom C6 bears the –CH2–imine substituent
[C6–C7 1.543(5) Å, C7–C8 1.487(5) Å]. It contains the sec-
ond imine functionality [pertinent bonding features: C8–N9
1.291(4) Å; angles: C7–C8–N9 120.1(3)°, C7–C8–C41
120.6(3)°, N9–C8–C41 119.2(3)°], which still has the intact
second HB(C6F5)2 unit coordinated to its nitrogen atom
[N9–B10 1.579(5) Å; angle: C8–N9–B10 129.5(3)°]. The bo-
ron atom B10 is also tetracoordinate featuring two C6F5
substituents, the hydride and the imine moiety bonded to it
[B10–C61 1.626(5) Å, B10–C51 1.622(5) Å; angles: N9–
B10–C51 111.0(3)°, N9–B10–C61 110.8(3)°, C51–B10–C61
110.2(3)°]. The 13C NMR spectrum of compound 6 shows
a pair of iminium-type C=N resonances at δ = 189.9 and
183.0 ppm, respectively. The corresponding =NH 1H NMR
resonances show up as a pair of broad signals at δ = 10.08
and 9.69 ppm. The hydroboration reaction has created a
new chiral centre. Therefore, the three CH2 groups feature
pairs of diastereotopic hydrogen atoms each. Also, the pair
of C6F5 substituents inside the heterocyclic six-membered
ring is diastereotopic. This differentiation extends as far as
to the terminal N-bonded –B(H)(C6F5)2 unit. Conse-
quently, we observe a total of four sets of C6F5 19F NMR
resonances [δ(C6F5A) = –134.3 (2 F), –157.7 (1 F), –162.9
(2 F) ppm; δ(C6F5B) = –134.7 (2 F), –157.4 (1 F), –163.0 (2
F) ppm; δ(C6F5C) = –135.3 (2 F), –156.6 (1 F), –162.3 (2
F) ppm; δ(C6F5D) = –135.5 (2 F), –155.8 (1 F), –161.8 (2 F)
ppm]. The small ∆δ(m/p) difference is typical for a situation
involving four-coordinate boron.[10]
of this transformation remains to be studied, but it is clear
that the resulting hydroboration products undergo a rapid
subsequent intramolecular borane–imine adduct formation
that leads to the observed internally stabilized final product
6. It will be seen whether such internal imine adducts might
eventually show reactivities reminiscent of those of re-
motely related frustrated N/B Lewis pairs.[11,12]
Experimental Section
General Procedures: All syntheses involving air- and moisture-sen-
sitive compounds were carried out by using standard Schlenk-type
glassware or in a glove box under argon. Solvents were dried by
the procedure according to Grubbs[13] or were distilled from appro-
priate drying agents and stored under argon. The following instru-
ments were used for physical characterization of the compounds:
NMR spectra: Bruker ARX 300 spectrometer (1H NMR:
300 MHz; 13C NMR: 75 MHz), Bruker AMX 400 (1H NMR:
400 MHz; 13C NMR: 100 MHz), Varian Inova 500 (1H NMR:
500 MHz; 13C NMR: 126 MHz; 19F NMR: 470 MHz; 11B NMR:
160 MHz). 1H NMR and 13C NMR chemical shifts (δ) are given
relative to TMS and referenced to the solvent signal (19F NMR rel.
to external CFCl3; 11B rel. to external BF3·Et2O). NMR spectro-
scopic assignments are supported by additional 1D and 2D NMR
experiments. Elemental analyses were performed with an Ele-
mentar Vario El III. IR spectra were recorded with a Varian 3100
FT-IR (Excalibur Series). Melting points were obtained with a
DSC Q20 (TA Instruments). X-ray crystal-structure analysis: Data
set was collected with a Nonius KappaCCD diffractometer. Pro-
grams used: data collection COLLECT (B. V. Nonius, 1998), data
reduction Denzo-SMN,[14] absorption correction Denzo,[15] struc-
ture solution SHELXS-97,[16] structure refinement SHELXL-97,[17]
graphics SCHAKAL (E. Keller, 1997).
Conclusions
This study shows that Piers’ borane HB(C6F5)2 is a
powerful Lewis acid. Similarly, as it had previously been
shown for its congener B(C6F5)3, the bis(pentafluoro-
phenyl)borane system is able to kinetically effect tauto-
merization and to thermodynamically stabilize an otherwise
unfavourable tautomer by coordination. This had been
E,E-/E,Z-/Z,Z-5: 1,6-Diamino-1,6-diphenyl-1,3,5-hexatriene (3)
(70.0 mg, 0.26 mmol) was dissolved in toluene (20 mL). To this yel-
low solution bis(pentafluorophenyl)borane (184.5 mg, 0.52 mmol)
was added. The reaction mixture was stirred at room temperature
for 7 min. Afterwards, the solvent was removed in vacuo. The pre-
cipitated yellow oil was washed with pentane (2ϫ8 mL) and the
demonstrated, for example, for B(C6F5)3 by the transforma- now beige solid (192.3 mg, 77%) dried in vacuo overnight. E,E-5/
1
E,Z-5/Z,Z-5 = 11:6:1. M.p. 123.3 °C (dec.). H NMR (500 MHz,
C6D6, 298 K): E,E-5: δ = 9.65 (br. s, 2 H, NH), 6.99 (m, 2 H, p-
Ph), 6.98 (m, 4 H, o-Ph), 6.90 (m, 4 H, m-Ph), 4.99 (m, 2 H, 3,4-
H), 4.25 (br., 2 H, BH), 3.20 (m, 4 H, 2,5-H) ppm; E,Z-5: δ = 9.88
(br. s, 1 H, NH1), 9.47 (br. s, 1 H, NH6), 7.21 (m, 2 H, o-Ph1), 7.05
(m, 1 H, p-Ph1), 7.00 (m, 2 H, m-Ph1), 6.94 (m, 1 H, p-Ph6), 6.87
(m, 2 H, m-Ph6), 6.84 (m, 2 H, o-Ph6), 5.17 (dt, 3JH,H = 15.6, 3JH,H
tion of the naphthol (7) to its benzocyclohexadienone tau-
tomer adduct 8 (see Scheme 4);[2] here it is demonstrated
for HB(C6F5)2 by the transformations of the thermo-
dynamically favoured conjugated diaminohexatriene 3 to
the double adduct 5 of its tautomeric bis(imine) form.
In contrast to the B(C6F5)3 adducts, the HB(C6F5)2–
imine adducts contain pairs of active H–[B] reagents. One
of these is eventually used for the hydroboration of the sin-
gle remaining C=C double bond, located in the central po-
3
3
= 6.3 Hz, 1 H, 3-H), 4.92 (dt, JH,H = 15.6, JH,H = 6.8 Hz, 1 H,
3
4-H), 4.25 (br., 2 H, BH), 3.53 (d, JH,H = 6.2 Hz, 2 H, 2-H), 2.46
(d, JH,H = 6.7 Hz, 2 H, 5-H) ppm; Z,Z-5: δ = 9.70 (br. s, 1 H,
3
sition of the adducts 5. The detailed mechanistic pathway NH), 5.04 (m, 2 H, 3,4-H), 2.79 (m, 4 H, 2,5-H), n.o. (Ph) ppm.
Eur. J. Inorg. Chem. 2010, 849–853
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
851

I. Peuser, R. Fröhlich, G. Kehr, G. Erker
FULL PAPER
13C{1H} NMR (126 MHz, C6D6, 298 K): E,E-5: δ = 181.7 (C-1,6),
133.5 (i-Ph), 134.4 (p-Ph), 130.0 (m-Ph), 126.6 (o-Ph), 126.8 (C-
3,4), 34.7 (C-2,5) ppm; E,Z-5: δ = 184.4 (C-6), 181.5 (C-1), 134.8
(p-Ph1), 133.4 (i-Ph1), 132.8 (p-Ph6), 132.8 (C-3), 131.9 (i-Ph6),
130.2 (m-Ph1), 128.5 (m-Ph6), 127.7 (o-Ph6), 126.9 (o-Ph1), 124.1
(C-4), 42.3 (C-5), 34.6 (C-2) ppm; Z,Z-5: δ = 184.1 (C-1,6), 130.3
Acknowledgments
Financial support from the Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen Industrie is gratefully acknowledged.
We thank the BASF for a gift of solvents.
(C-3,4), 42.7 (C-2,5), n.o. (Ph) ppm; no differentiation of the C6F5
1
groups: δ = 148.0 (dm, JF,C = 238.7 Hz, C6F5), 139.9 (dm, 1JF,C
=
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1
249.2 Hz, p-C6F5), 137.5 (dm, JF,C = 252.6 Hz, C6F5), 118.6 (br.,
i-C6F5) ppm. 19F NMR (470 MHz, C6D6, 298 K): E,E-5: δ =
–134.4 (m, 2 F, o-C6F5), –157.7 (t, JF,F = 20.5 Hz, 1 F, p-C6F5);
–163.1 (m, 2 F, m-C6F5) ppm; E,Z-5: δ = –134.4 (m, 2 F, o-C6F5),
–134.5 (m, 2 F, o-C6F5), –157.5 (t, JF,F = 20.3 Hz, 1 F, p-C6F5),
–158.1 (t, JF,F = 20.7 Hz, 1 F, p-C6F5), –163.1 (m, 2 F, m-C6F5),
–163.5 (m, 2 F, m-C6F5) ppm. 11B{1H} NMR (160 MHz, C6D6,
[2]
298 K): δ = –15 (ν1/2 = 420 Hz) ppm. IR (KBr): ν = 3352 (w, NH),
˜
2477 (w, BH) cm–1. C42H20B2F20N2 (954.2): calcd. C 52.87, H 2.11,
N 2.94; found C 52.74, H 2.29, N 2.91.
Compound 6: 1,6-Diamino-1,6-diphenyl-1,3,5-hexatriene (100.0 mg,
0.38 mmol) was dissolved in toluene (30 mL). To this solution bis-
(pentafluorophenyl)borane (263.7 mg, 0.76 mmol) was added. The
yellow solution was stirred at room temperature overnight. After-
wards, the solvent of the now colourless solution was removed in
vacuo. The precipitated white product was washed with pentane
(20 mL) and dried in vacuo overnight (200.0 mg, 55%). M.p.
[3]
[4]
1
183 °C. H NMR (500 MHz, C6D6, 298 K): δ = 10.08 (br. s, 1 H,
NH1), 9.69 (br. s, 1 H, NH6), 7.22 (m, 2 H, o-Ph6), 7.16 (m, 2 H,
o-Ph1), 7.00 (m, 1 H, p-Ph6), 6.98 (m, 1 H, p-Ph1), 6.94 (m, 2 H,
2
m-Ph6), 6.89 (m, 2 H, m-Ph1), 4.34 (br., 1 H, BH), 3.31 (dd, JH,H
3
= 14.4, JH,H = 12.4 Hz, 1 H, 5-H), 2.84, 2.03 (each m, each 1 H,
2
3
2-H), 2.70 (dd, JH,H = 14.4, JH,H = 3.8 Hz, 1 H, 5-H), 2.25 (m,
1 H, 4-H), 1.54, 1.45 (each m, each 1 H, 3-H) ppm. 13C{1H} NMR
(126 MHz, C6D6, 298 K): δ = 189.9 (C-6), 183.0 (C-1), 148.2 (dm,
1JF,C ≈ 238 Hz, C6F5), 140.0 (dm, 1JF,C ≈ 259 Hz, C6F5), 137.6 (dm,
1JF,C ≈ 250 Hz, C6F5), 135.1 (i-Ph6), 134.5 (p-Ph1), 133.8 (p-Ph6),
133.5 (i-Ph1), 129.94, 129.92 (m-Ph1,6), 126.5 (o-Ph6), 125.9 (o-Ph1),
32.8 (C-5), 28.0 (C-2), 22.7 (br., C-4), 21.7 (C-3) ppm. 19F NMR
(470 MHz, C6D6, 298 K): δ = –134.3 (m, 2 F), –157.7 (t, JF,F
=
[5]
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20.6 Hz, 1 F), –162.9 (m, 2 F, C6F5A); –134.7 (m, 2 F), –157.4 (t,
JF,F = 20.5 Hz, 1 F), –163.0 (m, 2 F, C6F5B); –135.3 (m, 2 F), –156.6
(t, JF,F = 20.8 Hz, 1 F), –162.3 (m, 2 F, C6F5C); –135.5 (m, 2 F),
–155.8 (t, JF,F = 20.8 Hz, 1 F), –161.8 (m, 2 F, C6F5D) ppm.
11B{1H} NMR (160 MHz, C6D6, 298 K): δ = –15 (ν1/2 = 200 Hz,
B6), –6 (ν1/2 = 360 Hz, B1) ppm. IR (KBr): ν = 3363 (s, NH), 2393
˜
(w, BH) cm–1. C42H20B2F20N2 (954.2): calcd. C 52.87, H 2.11, N
2.94; found C 52.45, H 2.17, N 2.72.
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X-ray Crystal Structure Analysis of 6: Empirical formula
C42H20B2F20N2, M = 954.22, colourless crystal, 0.25ϫ0.10ϫ
0.03 mm, a = 11.7813(8), b = 13.5712(9), c = 14.3960(9) Å, α =
64.063(4), β = 85.653(4), γ = 86.702(3)°, V = 2063.2(2) Å3, ρcalcd.
=
1.536 gcm–3, µ = 1.374 mm–1, empirical absorption correction
¯
(0.725 Յ T Յ 0.960), Z = 2, triclinic, space group P1 (no. 2), λ =
1.54178 Å, T = 223(2) K, ω and φ scans, 23938 reflections collected
(Ϯh, Ϯk, Ϯl), [(sinθ)/λ] = 0.60 Å–1, 6707 independent (Rint = 0.087)
and 4239 observed reflections [IՆ2σ(I)], 604 refined parameters,
R = 0.056, wR2 = 0.170, max. (min.) residual electron density 0.31
(–0.27) eÅ–3, hydrogen atoms at N and B from difference Fourier
map, others calculated and refined as riding atoms. CCDC-742130
contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
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852
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2010, 849–853

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Received: November 17, 2009
Published Online: January 19, 2010
Eur. J. Inorg. Chem. 2010, 849–853
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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