Haptotropic Migrations of Cr(CO)3 Complexes
Organometallics, Vol. 25, No. 1, 2006 201
Tricarbonyl[1,2-dihydro-1-lithio-2-phenyl-(η6-1,2-azaborine)]-
chromium (10). In the NMR-scale reaction, a mixture of 7 (12
mg, 41 µmol) and LDA (5 mg, 47 µmol) was dissolved in 0.75
mL of THF-d8. The resulting NMR spectrum showed that 10 was
formed quantitatively. After quenching with CH3I (8 mg, 56 µmol),
it formed the known compound tricarbonyl[1,2-dihydro-1-methyl-
2-phenyl-1,2-azaborine]chromium9 was formed. To isolate 10, the
following large-scale experiment was performed. A 1.7 M solution
of tert-butyllithium (1.5 mL, 2.55 mmol) in pentane was added
slowly to a solution of 7 (582 mg, 2 mmol) in 10 mL of dioxane
at 0 °C. As the mixture was stirred at 0 °C for 1 h and at room
temperature for 4 h, a yellow precipitate was formed. After filtration,
the precipitate was washed with 2 × 10 mL of pentane and dried
in vacuo to give the product as a yellow powder (350 mg, 37%).
The 1H NMR showed that the product contained 2 equiv of dioxane.
1H NMR (500 MHz, THF-d8): δ 4.48 (d, J ) 9.3 Hz, 1H, C(3)H);
5.08 (t, J ) 5.9 Hz, 1H, C(5)H); 5.66 (dd, J ) 9.3, 5.9 Hz, 1H,
C(4)H); 6.48 (br, 1H, C(6)H); 7.16 (t, J ) 7.1 Hz, 1H, Ar H); 7.21
(t, J ) 7.1 Hz, 2H, Ar H); 7.69 (d, J ) 7.1 Hz, 2H, Ar H). 13C
NMR (125.7 MHz, THF-d8): δ 84.8, 88.2 (br), 110.7, 117.4, 127.6,
127.7, 134.1; the signal for Cipso was not observed. 11B NMR (160.4
MHz, THF-d8): δ 21.6.
Tricarbonyl[1,2-dihydro-1-lithio-2-(η6-phenyl)-1,2-azaborine]-
chromium (9). In the NMR-scale reaction, a mixture of 8 (13 mg,
45 µmol) and LTMP (8 mg, 54 µmol) was dissolved in 0.75 mL
of THF-d8. The resulting NMR spectrum showed that 9 was formed
quantitatively. To isolate 9, the following large-scale experiment
was performed. A 1.7 M solution of tert-butyllithium (0.8 mL, 1.36
mmol) in pentane was added slowly to a solution of 8 (305 mg,
1.05 mmol) in 10 mL of dioxane at 0 °C. After the mixture was
stirred at 0 °C for 1 h and at room temperature for 4 h, a yellow
precipitate was formed. After filtration, the precipitate was washed
with 2 × 10 mL of pentane and dried in vacuo to give the product
as a yellow powder (205 mg, 41%). The 1H NMR showed that the
product contained 2 equiv of dioxane. 1H NMR (500 MHz, DMSO-
d6): δ 5.61 (m, 3H, Ar H); 6.07 (t, J ) 5.4 Hz, 1H, C(5)H); 6.24
(m, 2H, Ar H); 6.33 (d, J ) 10.5 Hz, 1H, C(3)H); 7.23 (dd, J )
10.5, 5.4 Hz, 1H, C(4)H); 8.00 (d, J ) 4.1 Hz, 1H, C(6)H). 13C
NMR (125.7 MHz, DMSO-d6): δ 94.7, 95.3, 99.9, 111.4, 118.8
(br), 139.4, 148.3; the signal for Cipso was not observed. 11B NMR
(160.4 MHz, THF-d8): δ 34.4.
investigations of haptotropic migrations of complexes of these
boron-nitrogen heterocycles are in progress.
Experimental Section
General Procedures. Manipulations of air-sensitive compounds
were performed under a nitrogen or argon atmosphere using
standard Schlenk techniques or in a nitrogen-filled drybox. Tetra-
hydrofuran, dioxane, pentane, and hexanes were dried and deoxy-
genated by distillation from sodium/benzophenone ketyl. Tetra-
hydrofuran-d8 was dried over potassium/sodium alloy before use.
t-BuLi (Aldrich), hexamethylbenzene (Aldrich), and cyclohexane-
1
2
d12 (Cambridge) were used without further purification. H, H,
13C, and 11B NMR spectra were recorded on a Varian Inova 400
or 500 NMR spectrometer at ambient temperature. Chemical shifts
are reported in parts per million (δ). Proton and carbon chemical
shifts are relative to respective solvent internal standards shown as
follows: cyclohexane-d12, δ 1.38 (1H), 26.43 (13C); DMSO-d6, δ
2.50 (1H), 39.51 (13C); THF-d8, δ 3.58 (1H), 67.57 (13C). The
coupling constants (J) are reported in Hertz. The following
abbreviations are used to describe peak patterns: br ) broad, s )
singlet, d ) doublet, t ) triplet, q ) quartet, m ) multiplet. All
13C and 11B NMR spectra were determined with complete proton
decoupling. High-resolution mass spectra were recorded on a VG-
250S spectrometer with electron impact at 70 eV. Elemental
analyses were conducted on a Perkin-Elmer 240 CHN analyzer by
the Analytical Service Department of the Chemistry Department
at the University of Michigan, Ann Arbor, MI.
Tricarbonyl[1,2-dihydro-2-phenyl-(η6-1,2-azaborine)]chro-
mium (7). 1,2-Dihydro-2-phenyl-1,2-azaborine (313 mg, 2.02
mmol) in 15 mL of THF was added to Cr(CO)3(CH3CN)3 (522
mg, 2.01 mmol). The resulting red solution was heated to 50 °C
for 24 h. The solvent was removed under reduced pressure. The
residue was extracted by 3 × 15 mL of hot hexanes. After removal
of the solvents, the crude product was purified by column
chromatography on silica gel (20% ethyl acetate in hexanes elution)
to yield a pure sample of the product as an orange powder (470
1
mg, 80%). IR (benzene film): 1973, 1904, 1882 cm-1. H NMR
(500 MHz, THF-d8): δ 5.00 (d, J ) 9.5 Hz, 1H, C(3)H); 5.40 (t,
J ) 5.8 Hz, 1H, C(5)H); 6.02 (dd, J ) 9.5, 5.8 Hz, 1H, C(4)H);
6.43 (t, J ) 5.8 Hz, 1H, C(6)H); 7.34 (m, 3H, Ar H); 7.67 (m, 2H,
Ar H); 7.95 (br, 1H, NH). 13C NMR (100.6 MHz, THF-d8): δ 82.9,
85.0 (br), 100.9, 109.6, 128.7, 130.7, 133.6; the signal for Cipso
was not observed. 11B NMR (160.4 MHz, THF-d8): δ 19.9. HRMS
(EI, m/z): calcd for C13H1011B52CrNO3(M+), 291.0159; found,
291.0150. Anal. Calcd for C13H10BCrNO3: C, 53.65; H, 3.46; N,
4.81. Found: C, 53.83; H, 3.62; N, 4.77.
1,2-Dihydro-1-deuterio-2-phenyl-1,2-azaborine (5-d). To a
solution of (1,2-dihydro-2-phenyl-1,2-azaborin-1-yl)potassium (295
mg, 1.53 mmol) in 10 mL of THF at -78 °C was slowly added 2
mL of CD3COOD. The mixture was warmed to room temperature
and stirred for 2 h. The volatiles were removed in vacuo, and the
residue was extracted with 3 × 10 mL of hexanes. The solution
was concentrated and crystallized at -30 °C to give the product as
white crystals (200 mg, 84%). The isotopic purity determined by
the mass spectroscopy was 80%.
Tricarbonyl[1,2-dihydro-2-(η6-phenyl)-1,2-azaborine]chro-
mium (8). 1,2-Dihydro-2-phenyl-1,2-azaborine (1.55 g, 10 mmol)
in 50 mL of THF was added to Cr(CO)3(CH3CN)3 (2.59 g, 10
mmol). The resulting red solution was heated at 140 °C for 24 h.
The solvent was removed under reduced pressure. The residue was
extracted by 4 × 60 mL of hot hexanes. After removal of the
solvents, the crude product was purified by column chromatography
on silica gel (20% ethyl acetate in hexanes elution) to yield a pure
sample of the product as a yellow powder (2.10 g, 72%). IR
(1,2-Dihydro-2-phenyl-3-deuterio-1,2-azaborin-1-yl)lithium (6b).
The procedure for preparing 6b was identical with that for (1,2-
dihydro-2-phenyl-1,2-azaborin-1-yl)lithium, except CD2Cl2 was
9
1
used in place of CH2Cl2 (see Scheme 3). H NMR (500 MHz,
THF-d8): δ 6.02 (dd, J ) 6.5, 5.0 Hz, 1H, C(5)H); 7.00 (t, J ) 7.3
Hz, 1H, Ar H); 7.13 (t, J ) 7.8 Hz, 2H, Ar H); 7.32 (d, J ) 6.5
Hz, 1H, C(4)H); 7.70 (d, J ) 7.8 Hz, 2H, Ar H); 7.95 (d, J ) 5.0
Hz, 1H, C(6)H). 2H NMR (76.7 MHz, THF-d8): δ 6.55 (br, C(3)D).
1
(benzene film): 1968, 1894 cm-1. H NMR (500 MHz, DMSO-
d6): δ 5.73 (t, J ) 6.3 Hz, 2H, Ar H); 5.86 (t, J ) 6.3 Hz, 1H, Ar
H); 6.18 (d, J ) 6.3 Hz, 2H, Ar H); 6.42 (t, J ) 6.4 Hz, 1H, C(5)H);
6.92 (d, J ) 11.2 Hz, 1H, C(3)H); 7.48 (t, J ) 6.9 Hz, 1H, C(6)H);
7.71 (dd, J ) 11.2, 6.4 Hz, 1H, C(4)H); 10.70 (br, 1H, NH). 13C
NMR (125.7 MHz, DMSO-d6): δ 94.6, 96.4, 99.5, 111.2, 125.9
(br), 135.3, 144.8; s the signal for Cipso was not observed. 11B NMR
(160.4 MHz, THF-d8): δ 32.7. HRMS (EI, m/z): calcd for
C13H1011B52CrNO3 (M+), 291.0159; found, 291.0148. Anal. Calcd
for C13H10BCrNO3: C, 53.65; H, 3.46; N, 4.81. Found: C, 53.64;
H, 3.21; N, 4.62.
1
The isotopic purity determined by H NMR was 90%.
Haptotropic Migration from 7 to 8. 1. In THF. In a sealed
NMR tube, a solution of 7 (13 mg, 45 µmol) in 0.75 mL of THF-
1
d8 was heated to 101 °C and the migration was monitored by H
NMR spectroscopy. The percent conversion was determined by
monitoring the signal of C(3)H of 7 and that of the corresponding
proton of 8. The resulting rate constant k is 8.0 × 10-5 s-1. The
decomplexed product 1,2-dihydro-2-phenyl-1,2-azaborine was less
than 10% of the amount of product.