1,2-Azaboratabenzene: A heterocyclic π-ligand with an adjustable basicity at nitrogen

Article abstract of DOI:10.1021/om049399g

1,2-Dihydro-2-phenyl-l,2-azaborine (6a) has been prepared by a multistep synthesis from the readily available 2,2-dibutyl-2,5-dihydro-1-trimethylsilyl- 1H1,2-azastannole (7). Deprotonation of 6a affords 1-phenyl-1,2-azaboratabemene la, which on reaction with [Cp*RuCl]₄ gives the sandwich compound 5. 5 has been shown to function as a nucleophilic catalyst for the acylation of benzyl alcohol by phenylethylketene.

Full text of DOI:10.1021/om049399g

5626
Organometallics 2004, 23, 5626-5629
1,2-Azaboratabenzene: A Heterocyclic π-Ligand with an
Adjustable Basicity at Nitrogen
Jun Pan, Jeff W. Kampf, and Arthur J. Ashe III*
Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109-1055
Received August 3, 2004
Scheme 1
Summary: 1,2-Dihydro-2-phenyl-1,2-azaborine (6a) has
been prepared by a multistep synthesis from the readily
available 2,2-dibutyl-2,5-dihydro-1-trimethylsilyl-1H-
1,2-azastannole (7). Deprotonation of 6a affords 1-phen-
yl-1,2-azaboratabenzene 1a, which on reaction with
[Cp*RuCl]4 gives the sandwich compound 5. 5 has been
shown to function as a nucleophilic catalyst for the
acylation of benzyl alcohol by phenylethylketene.
There is considerable interest in the chemistry of
π-coordinated nitrogen-containing heterocycles that re-
tain their N-basicity.1-5 Most noteworthy are the ob-
servations by Fu and co-workers that azaruthenocenes
and azaferrocenes are good nucleophilic catalysts.2 In
this context, we have become interested in the novel
heterocycle 1,2-azaboratabenzene 1. 1,2-Azaborataben-
zene is a potentially aromatic ligand that is closely
related to boratabenzenes 2 and to pyridines 3. Bo-
ratabenzenes have a particularly rich π-coordination
chemistry,6 while pyridine forms rather few π-coordi-
nated metal complexes.3,7 On the other hand, pyridines
usually bind metals via the nitrogen atom in an η1-
manner.8 Pyridine is also a good σ-base/nucleophile, and
electron-rich pyridines such as 4-(dimethylamino)pyri-
dine (DMAP) are widely used as nucleophilic catalysts.9
We felt that 1 might π-coordinate to various metals to
afford complexes 4, which retain their σ-basicity. Indeed,
it is our hope that the complexes 4 might have an
adjustable basicity/nucleophilicity, which might be tun-
able by the judicious choice of the coordinating metal
and its ancillary ligands. Herein we report on the
synthesis of 1-phenyl-1,2-azaboratabenzene 1a and its
Ru(II)Cp* complex 5. The marked decrease in the pKa
values of 5 relative to 1a demonstrates that the basicity
of azaboratabenzene can be modulated by π-complex-
ation.
1-Phenyl-1,2-azaboratabenzene 1a should be available
by deprotonation of its conjugate acid, 1,2-dihydro-2-
phenyl-1,2-azaborine 6a. Although 6a was reported by
White in 1963, the extremely low yield makes this
preparation unsuitable for our purposes.10 Instead, we
find that 6a can be prepared in good yield via a route
involving a ring expansion of a 1,2-azaborolide11 (see
Scheme 2). The B/Sn exchange reaction of the readily
available 2,2-dibutyl-2,5-dihydro-1-trimethylsilyl-1,2-
azastannole12 (7) with PhBCl2 gave a 95% conversion
to the corresponding 2,5-dihydro-1-trimethylsilyl-2-
phenyl-1H-1,2-azaborole (8).13 Deprotonation of 8 with
LDA afforded an 89% yield of the lithium 1,2-azaborolide
9, which can be isolated as a white solid. Treatment of
9 with LDA/CH2Cl2 in ether gave the ring-expanded
product 6b in 41% yield. Desilylation of 6b with Bu4-
NF in THF/H2O gave the desired 6a as a white crystal-
line solid with a mp identical to that reported by White.
An X-ray structure of 6a (in the Supporting Information)
unambiguously identifies the compound. Since 6a is
isostructural with biphenyl,14 the phenyl and 1,2-
dihydro-1,2-azaborine rings are crystallographically
equivalent. Thus accurate bond distances could not be
obtained.
* To whom correspondence should be addressed. E-mail: ajashe@
umich.edu.
Deprotonation of 6a with potassium bis(trimethylsi-
lyl)amide in toluene gave K-1a, which can be isolated
as a white powder. Alternatively treatment of 6a with
LDA in THF afforded Li-1a, which can be isolated as a
(1) (a) Perera, J. R.; Heeg, M. J.; Schlegel, H. B.; Winter, C. H. J.
Am. Chem. Soc. 1999, 121, 4536. (b) Perera, J. R.; Heeg, M. J.; Winter,
C. H. Organometallics 2000, 19, 5263.
(2) (a) Ruble, J. C.; Fu, G. C. J. Org. Chem. 1996, 61, 7230. (b)
Garrett, C. E.; Fu, G. C. J. Am. Chem. Soc. 1998, 120, 7479. (c) Fu, G.
C. Acc. Chem. Res. 2000, 33, 412.
1
pale yellow powder on removal of the solvent. The H
(3) Elschenbroich, C.; Koch, J.; Kroker, J.; Wu¨nsch, M.; Massa, W.;
Baum, G.; Stork, G. Chem. Ber. 1988, 121, 1983.
NMR spectrum of 1a in either THF-d8 or DMSO-d6
shows a first-order pattern which is consistent with the
assigned structure. The observed 1H, 11B, and 13C NMR
(4) Janiak, C.; Kuhn, N. Adv. Nitrogen Chem. 1996, 2, 179.
(5) (a) Joshi, K. K.; Pauson, P. L.; Qazi, A. R.; Stubbs, W. H. J.
Organomet. Chem. 1964, 1, 471. (b) Kuhn, N.; Schulten, M.; Zauder,
E.; Augart, N.; Boese, R. Chem. Ber. 1989, 122, 1891.
(6) (a) Herberich, G. E.; Ohst, H. Adv. Organomet. Chem. 1986, 25,
199. (b) Fu, G. C. Adv. Organomet. Chem. 2001, 47, 101.
(7) Simons, L. H.; Riley, P. E.; Davis, R. E.; Lagowski, J. J. J. Am.
Chem. Soc. 1976, 98, 1044.
(10) (a) White, D. G. J. Am. Chem. Soc. 1963, 85, 3634. (b) Davies,
K. M.; Dewar, M. J. S.; Rona, P. J. Am. Chem. Soc. 1967, 89, 6294.
(11) Ashe, A. J., III; Fang, X.; Fang, X.; Kampf, J. W. Organome-
tallics 2001, 20, 5413.
(8) Elschenbroich, C.; Salzer, A. Organometallics, 2nd ed.; VCH
Publishers: New York, 1992; p 378.
(9) (a) Scriven, E. F. V. Chem. Soc. Rev. 1983, 12. (b) Hassner, A.;
Krepski, L. R.; Alexanian, V. Tetrahedron 1978, 34, 2069. (c) Ho¨fle,
G.; Steglich, W.; Vorbru¨ggen, H. Angew. Chem., Int. Ed. Engl. 1978,
17, 569.
(12) Corriu, R. J. P.; Geng, B.; Moreau, J. J. E. J. Org. Chem. 1993,
58, 1443.
(13) For a similiar conversion see: Liu, S.-Y.; Lo, M. M.-C.; Fu, G.
C. Angew. Chem., Int. Ed. 2002, 41, 174.
(14) Charbonneau, G.-P.; Delageard, Y. Acta Crystallogr. 1976, B32,
1420; 1977, B33, 1586.
10.1021/om049399g CCC: $27.50 © 2004 American Chemical Society
Publication on Web 10/13/2004

Notes
Organometallics, Vol. 23, No. 23, 2004 5627
Scheme 2. Synthesisa
Figure 2. Molecular structure of 5 (ORTEP). Thermal
ellipsoids are at the 50% probability level. Hydrogen atoms
have been omitted for clarity. Selected bond distances (Å):
B-N, 1.450(2), B-C (4), 1.504(3), B-C(5), 1.594(2), B-Ru,
2.2967(17), C(4)-Ru, 2.1950(19), N-Ru, 2.155(3).
Although we lack the means to experimentally probe
the charge density of 1a at nitrogen, the nitrogen atom
is nucleophilic. Quenching the solution of K-1a with
excess methyl iodide afforded the known N-methyl
derivative 6c.11 Deprotonation of 6a is reversible in
either THF or DMSO. Bracketing experiments in DMSO
indicate that the acidity of 6a is comparable with
pentamethylcyclopentadiene (pKa ≈ 26).19
The reaction of K-1a with [Cp*RuCl]4 gives adduct 5
1
as amber crystals in 51% yield. The H, 11B, and 13C
NMR signals of the C4BN-ring atoms are shifted upfield
relative to those of K-1a, indicating π-complexation,
which was confirmed by obtaining the X-ray crystal
structure. The molecular structure of 5 illustrated in
Figure 2 shows that the C4BN ring is η6-coordinated to
the Ru atom. On addition of 1 equiv of HOAc to the THF
solution of 5, it forms salt 10. Therefore 10 has an
acidity less than that of common carboxylic acids (pKa
≈ 5). Potentiometric titration of 5 in 80% CH3OH/H2O
gave a pKa of 9.21 ( 0.10. Thus the basicity of 5 is
comparable with that of DMAP (pKa ) 9.0) under the
same conditions.20
a Key: a, PhBCl2; b, LDA; c, LDA/CH2Cl2; d, Bu4NF/THF-
H2O; e, base; f, acid; g, MeI; h, [Cp*RuCl]4.
It was of considerable interest to examine whether
complex 5 could serve as a nucleophilic catalyst. Initally
we have chosen to look for catalysis of the acylation of
benzyl alcohol by phenylethylketene, which had previ-
ously been studied by Fu and Ruble.2a It was found that
5 is an effective catalyst, accelerating the reaction by a
factor of approximately 13. Under identical conditions
the rate of catalysis is 1/26th that of DMAP. Nucleo-
philic pyridine catalysts including DMAP derivatives
show a marked decrease in catalytic activity where
there is steric hindrance R to the nitrogen.21 Thus the
lower activity of 5 relative to DMAP is an expected
consequence of the large phenyl substituent at boron.
We speculate that derivatives of 5 with smaller B
substituents will show much higher catalytic activity.
In summary 2-phenyl-1,2-azaboratabenzene is easily
prepared and can be converted to π-coordinated Ru(II)
complex 5. The marked decrease in basicity between 1a
and 5 is likely to be a general feature on π-coordination
and is apparently due to electron withdrawal from the
Figure 1. Comparison of the 1H NMR, 13C NMR (in
parentheses), and 11B NMR (in circles) chemical shift
values of 1a and 2a in THF-d8.
chemical shift values of K-1a are quite similar to those
shown for Li-2a,15,16 as illustrated in Figure 1. The
spectra exhibit high-field 11B chemical shift values that
are consistent with strong stabilization by π-bonding to
1
boron. The H and 13C NMR chemical shift values of
CH groups that are R and γ to boron are shifted upfield
from the signals for the CH groups that are â to boron.
This is consistent with a greater carbanionic character
of the R- and γ-carbon atoms.17,18 Overall the great
similarity of the spectra imply a similarity in the
electronic structure of 1a and 2a.
(15) (a) Ashe, A. J., III; Shu, P. J. Am. Chem. Soc. 1971, 93, 1804.
(b) Shu, P. Ph.D. Dissertation, University of Michigan, 1973.
(16) Herberich, G. E.; Schmidt, B.; Englert, U.; Wagner, T. Orga-
nometallics 1993, 12, 2891.
(17) Ashe, A. J., III; Fang, X.; Kampf, J. W. Organometallics 1998,
18, 466.
(18) (a) Olah, G. A.; Asensio, G.; Mayr, H.; Schleyer, P.v. R. J. Am.
Chem. Soc. 1978, 100, 4347. (b) Jutzi, P.; Meyer, M.; Rasika Dias, H.
V.; Power, P. P. J. Am. Chem. Soc. 1990, 112, 4841.
(19) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456.
(20) The pKa of DMAP in water is 9.70. It is usually found that
nitrogen heterocycles have a lower pKa in aqueous alcohols. See:
Storey, B. T.; Sullivan, W. W.; Moyer, C. L. J. Org. Chem. 1964, 29,
3118.
(21) (a) Bondarenko, L. I.; Kirichenko, A. I.; Litvinenko, L. M.; L.
M.; Dmitrenko, I. N.; Kobets, V. D. Zh. Org. Khim. 1981, 2588. (b)
Sammakia, T.; Hurley, T. B. J. Org. Chem. 1999, 64, 4652.

5628 Organometallics, Vol. 23, No. 23, 2004
Notes
h. After removal of the solvent under reduced pressure, the
residue was washed with 3 × 20 mL of pentane and then dried
under vacuum, leaving lithium salt 9 as a white solid (43.2 g,
ligand by the metal. This modulation of basicity on
π-complexation is complementary to the adjustment of
the basicity of pyridines on ring substitution. The
observation that 5 is a nucleophilic catalyst is signifi-
cant. We suggest that coordinated azaboratabenzenes
may find a role as catalysts with a tunable nucleophi-
licity.
1
89%). H NMR (500 MHz, DMSO-d6): δ 0.12 (s, 9H, SiMe3);
4.30 (dd, J ) 4.8, 1.7 Hz, 1H, CHB); 5.75 (t, J ) 2.2 Hz, 1H,
CHN); 5.83 (dd, J ) 4.8, 2.7 Hz, 1H, CH); 6.80 (t, J ) 7.3 Hz,
1H, ArH); 6.99 (t, J ) 7.3 Hz, 2H, ArH); 7.40 (d, J ) 7.3 Hz,
2H, ArH). 13C NMR (100.6 MHz, DMSO-d6): δ 2.9 (SiMe3); 88
(br, CHB); 111.0 (CHN), 117.5 (CH), 121.8 (Ar), 125.8 (Ar),
133.3 (Ar). Signal for Cipso, not observed. 11B NMR (160.4 MHz,
DMSO-d6): δ 30.3.
Experimental Section
General Procedures. Manipulations of air-sensitive com-
pounds were performed under a nitrogen or argon atmosphere
using standard Schlenk techniques or in a nitrogen-filled
drybox. Tetrahydrofuran, diethyl ether, hexanes, and pentane
were dried and deoxygenated by distillation from sodium/
benzophenone ketyl. Dichloromethane was dried by distillation
from calcium hydride. Dichlorophenylborane (Aldrich), diiso-
propylamine (Aldrich), n-BuLi (Aldrich), tetrabutylammonium
fluoride (Aldrich), iodomethane (Aldrich), dichloro(pentam-
ethylcyclopentadienyl)ruthenium(III) polymer (Strem), and
super-hydride (Aldrich) were used without further purification.
LDA was prepared by reaction of diisopropylamine and n-BuLi
in pentane followed by washing with pentane and drying
under vacuum. 2,2-Dibutyl-2,5-dihydro-1-trimethylsilyl-1H-
1,2-azastannole (7)12 and chloro(pentamethylcyclopentadienyl)-
ruthenium(II) tetramer22 were prepared according to the
literature procedures. 1H, 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 below: CDCl3
δ 7.26 (1H), 77.23 (13C); C6D6 δ 7.16 (1H), 128.39 (13C); DMSO-
d6 δ 2.50 (1H), 39.57 (13C); THF-d8 δ 3.58 (1H), 67.40 (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, and m )
multiplet. All 13C and 11B NMR spectra were determined with
complete proton decoupling. UV-visible spectra were recorded
on a Shimadzu UV-160U spectrometer. High-resolution mass
spectra were recorded on a VG-250S spectrometer with an
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. Melting points were recorded on a
MEL-TEMP Laboratory Devices melting point apparatus with
an uncorrected thermometer.
1,2-Dihydro-1-trimethylsilyl-2-phenyl-1,2-azaborine
(6b). A solution of LDA (8.82 g, 82.3 mmol) in 100 mL of
diethyl ether was added gradually to a suspension of 9 (18.20
g, 82.3 mmol) in 70 mL of CH2Cl2 at -78 °C. The mixture was
stirred at -78 °C for 2 h and at room temperature for 10 h.
Once the solvent was removed under reduced pressure, the
residue was extracted with 3 × 150 mL of pentane. After
filtration and removal of the solvent, the dark red oily residue
was vacuum distilled to give the product 6b as a colorless oil
1
(7.77 g, 41%), bp 70-74 °C at 0.05 Torr. H NMR (500 MHz,
DMSO-d6): δ 0.14 (s, 9H, SiMe3); 6.49 (t, J ) 6.6 Hz, 1H,
C(5)H); 6.57 (d, J ) 10.9 Hz, 1H, C(3)H); 7.33 (m, 5H, ArH);
7.55(d, J ) 6.8 Hz,1H,C(6)H); 7.60 (dd, J ) 10.9, 6.4 Hz, 1H,
C(4)H). 13C NMR (100.6 MHz, CDCl3): δ 2.3, 112.4, 127.2,
127.3, 131.0 (br), 132.5, 137.6, 143.5. Signal for Cipso not
observed. 11B NMR (160.4 MHz, DMSO-d6): δ 38.9. HRMS (EI,
m/z): calcd for C13H1811BNSi (M+), 227.1302; found, 227.1297.
Anal. Calcd for C13H18BNSi: C, 68.73; H, 7.99; N, 6.17.
Found: C, 68.98; H, 8.24; N, 6.01.
1,2-Dihydro-2-phenyl-1,2-azaborine (6a). A 1.0 M solu-
tion of Bu4NF (36 mL, 36 mmol) in THF was added slowly to
a solution of 6b (6.84 g, 30 mmol) in 25 mL of THF at 0 °C.
The mixture was stirred at 0 °C for 4 h and at room
temperature for 10 h. After reducing the volume of THF to 10
mL in vacuo, 50 mL of ice water was added and the mixture
was extracted with 3 × 50 mL of pentane. The pentane
extracts were washed with 3 × 30 mL of H2O and dried over
anhydrous Na2SO4. After filtration and removal of the solvent,
the product 6a was obtained as a white powder (3.82 g, 82%),
1
mp 117-9 °C. H NMR (500 MHz, DMSO-d6): δ 6.37 (t, J )
6.5 Hz, 1H, C(5)H); 7.09 (d, J ) 11.2 Hz, 1H, C(3)H); 7.33-
7.41 (m, 3H, ArH); 7.48 (t, J ) 7.1 Hz, C(6)H); 7.69 (dd, J )
11.2, 6.5 Hz, 1H, C(4)H); 7.88 (d, J ) 6.4 Hz, 2H, ArH); 10.61
(br, 1H, NH). 13C NMR (100.6 MHz, DMSO-d6): δ 110.3, 127.3
(br), 127.8, 128.7, 132.5, 135.4, 144.3. Signal for Cipso not
observed. 11B NMR (160.4 MHz, DMSO-d6): δ 33.4. HRMS (EI,
m/z): calcd for C10H1011BN (M+), 155.0906; found, 155.0908.
Anal. Calcd for C10H10BN: C, 77.49; H, 6.50; N, 9.04. Found:
C, 77.14; H, 6.23; N, 8.92. UV (λmax) in hexanes: 241 nm, 287
nm.
2,5-Dihydro-2-phenyl-1-trimethylsilyl-1H-1,2-azabo-
role (8). A solution of PhBCl2 (97.46 g, 0.61mol) in 200 mL of
hexane was gradually added to a solution of 7 (221.28 g,
0.61mol) in 500 mL of hexane at 0 °C. After stirring at 0 °C
for 15 min, the reaction mixture was allowed to warm to room
temperature and then refluxed for 24 h. Once the solvent had
been removed, the residue was vacuum distilled to give the
product as a colorless liquid (125.66 g, 95%), bp 48-52 °C at
1,2-Dihydro-2-phenyl-1,2-azaborine-1-ylpotassium,K-(1a).
A 0.5 M solution of potassium bis(trimethylsilyl)amide (22 mL,
11 mmol) in toluene was added gradually to a solution of 6a
(1.55 g, 10 mmol) in 20 mL of toluene at 0 °C. The mixture
was stirred at 0 °C for 1 h and at room temperature for 4 h.
After filtration and removal of the solvent, the residue was
washed with 2 × 10 mL of toluene and 2 × 10 mL of pentane
and dried in vacuo to give the product as a white powder (1.83
g, 95%). 1H NMR (500 MHz, DMSO-d6): δ 5.98 (t, J ) 5.5 Hz,
1H, C(5)H); 6.40 (d, J ) 10.5 Hz, 1H, C(3)H); 7.04 (t, J ) 7.2
Hz, 1H, ArH); 7.15-7.21 (m, 3H, C(6)H, ArH); 7.96 (d, J )
7.1 Hz, 2H, ArH); 8.12 (d, J ) 4.2 Hz, 1H, C(6)H). 13C NMR
(100.6 MHz, DMSO-d6): δ 110.0 (C5), 119.3 (br, C3), 125.0 (Ar),
126.6 (Ar), 132.3 (Ar), 138.7 (C4), 149.6 (C6). Signal for Cipso
not observed. 11B NMR (160.4 MHz, DMSO-d6): δ 34.9.
1,2-Dihydro-1-methyl-2-phenyl-1,2-azaborine (6c). In
the NMR tube, 14 mg of CH3I (0.1 mmol) was added to a
solution of K-1a (19 mg, 0.1 mmol) in 1 mL of DMSO-d6. The
resulting NMR spectra were identical with that of the known
compound 6c.
1
0.05 Torr. H NMR (500 MHz, C6D6): δ 0.05 (s, 9H, SiMe3);
3.67 (t, J ) 1.7 Hz, 2H, CH2); 6.57 (d, J ) 7.8 Hz, 1H, CHB);
7.12 (d, J ) 7.8 Hz, 1H, CH); 7.22 (m, 3H, ArH); 7.61 (d, J )
6.6 Hz, 2H, ArH). 13C NMR (100.6 MHz, CDCl3): δ 1.5 (SiMe3);
60.2 (CH2); 127.4 (Ar); 128.1 (Ar); 132.6 (Ar); 136 (br, CHB);
153.9 (CH). Signal for Cipso, not observed. 11B NMR (160.4 MHz,
CDCl3): δ 45.9. HRMS (EI, m/z): calcd for C12H1811BNSi (M+),
215.1302; found, 215.1301.
2,3-Dihydro-1-trimethylsilyl-2-phenyl-1H-1,2-azaborol-
3-yllithium (9). A solution of 8 (47.3 g, 0.22 mol) in 100 mL
of diethyl ether was slowly added to a solution of LDA (25.9 g,
0.24 mol) in 250 mL of diethyl ether at -78 °C. The mixture
was stirred at -78 °C for 2 h and at room temperature for 10
(22) Fagan, P. J.; Ward, M. D.; Calabrese, J. C. J. Am. Chem. Soc.
1989, 111, 1698.

Notes
[η6-1,2-Dihydro-2-phenyl-1,2-azaborine-1-yl][pentameth-
Organometallics, Vol. 23, No. 23, 2004 5629
Table 1. Crystal and Data Collection Parameters
for 5
ylcyclopentadienyl]ruthenium(II) (5). A solution of 1a (785
mg, 4.07 mmol) in 5 mL of THF was slowly added to a
suspension of [Cp*RuCl]4 (1.11 g, 4.08 mmol) in 25 mL of THF
at -78 °C. The mixture was stirred at -78 °C for 2 h and room
temperature for 10 h. The solvent was removed in vacuo, and
the residue was extracted with 3 × 10 mL of hot hexanes. After
filtration and removal of the solvent, the crude product was
obtained as an amber powder, which was washed with 3 × 10
mL of pentane at -78 °C to give the product as a sand color
powder (810 mg, 51%). 1H NMR (500 MHz, DMSO-d6): δ 1.66
(s, 15H, Cp*Me); 4.63 (d, J ) 8.3 Hz, 1H, C(3)H); 5.25 (dd, J
) 5.4, 2.9 Hz, 1H, C(5)H); 5.37 (dd, J ) 8.3, 5.4 Hz, C(4)H);
6.37 (m, 1H, C(6)H); 7.22 (t, J ) 7.3 Hz, 1H, ArH); 7.29 (t, J
) 7.3 Hz, 2H, ArH); 7.86 (d, J ) 7.9 Hz, 1H, ArH). 13C NMR
(100.6 MHz, DMSO-d6): δ 10.4, 76.5 (br), 81.7, 89.8, 93.8,
106.3, 126.9, 127.4, 132.9. 11B NMR (160.4 MHz, DMSO-d6):
δ 15.2. HRMS (EI, m/z): calcd for C20H2311BN102Ru [M - H]+,
390.0967; found, 390.0969. Anal. Calcd for C20H24BNRu: C,
61.55; H, 6.20; N, 3.59. Found: C, 61.42; H, 6.48; N, 3.50.
[η6-1,2-Dihydro-2-phenyl-1,2-azaborine][pentamethyl-
cyclopentadienyl]ruthenium(II) Acetate (10). Acetic acid
(9 µL) was added to a solution of 5 (20 mg) in 1 mL of benzene-
d6, and the yellow mixture immediately turned amber. No
empirical formula
fw
C20H24BNRu
390.28
temp, K
150(2)
wavelength, Å
cryst syst
space group
unit cell dimens
a, Å
0.71073
monoclinic
P2(1)
7.772(2)
b, Å
13.838(1)
c, Å
8.312(2)
â, deg
93.343(2)
V, Å3, Z
892.5(4), 2
calcd density, Mg/m3
abs coeff, mm-1
F(000)
1.452
0.876
400
cryst size, mm
limiting indices
0.42 × 0.42 × 0.14
-10 e h e 10, -18 e k e 18,
-11 e l e 11
11 677/4372
no. of reflns collected/unique
abs corr
refinement method
no. of data/restraints/params
GOF on F2
semiempirical fr equiv
full-matrix least squares on F2
4372/1/310
1.014
final R indices (T>2σ(J))
R indices (all data)
R1 ) 0.0274, wR2 ) 0.0692
R1 ) 0.0286, wR2 ) 0.0704
1
attempt was made to purify the resulting salt. H NMR (500
largest diff peak and hole, e/A3 0.746 and -0.459
MHz, Benzene-d6): δ 1.42 (s, 15H, Cp*Me); 1.78 (s, 3H,
CH3COO-); 4.71 (d, J ) 8.5 Hz, 1H, C(3)H); 4.94 (dd, J ) 8.4,
5.5 Hz, 1H, C(4)H); 5.37 (t, J ) 4.9 Hz, C(5)H); 7.29 (t, J ) 7.4
Hz,1H, ArH); 7.45 (t, J ) 7.6 Hz, 2H, ArH); 7.82 (d, J ) 4.2
Hz, 1H, C(6)H); 7.86 (d, J ) 6.6 Hz, 2H, ArH). 13C NMR (100.6
MHz, benzene-d6): δ 10.4, 21.45, 78.9 (br), 82.4, 94.7, 97.2,
97.7, 128.7, 130.7, 134.4. 11B NMR (160.4 MHz, benzene-d6):
δ 16.2.
benzyl alcohol with phenylethylketene in C6D6 at 25.0 °C using
1% catalyst was identical to the procedure of Fu and Ruble.2
The experiments were set up in a nitrogen-filled drybox. Two
runs were measured for each catalyst. The results were t1/2
0.084 ( 0.005 h. For DMAP, t1/2 ) 2.2 ( 0.2 h for 5 and t1/2
27.5 ( 0.7 h without catalyst.
)
)
Determination of the pKa Values. 1. The pKa Value of
6a. In the NMR tube, 14 mg of Cp*Li (0.1 mmol) was added
to a solution of 6a (15 mg, 0.1 mmol) in 1 mL of DMSO-d6.
The resulting NMR spectrum showed the peaks of Cp*H as
well as Cp*Li. In the reverse reaction, 19 mg of Li-1a (0.1
mmol), which had been prepared from 6a and LDA, was added
to a solution of Cp*H (14 mg, 0.1 mmol) in 1 mL of DMSO-d6.
The resulting NMR spectrum also showed the peaks of Cp*H
as well as Cp*Li. The above bracketing experiments indicate
that the acidity of 6a is comparable with that of pentameth-
ylcyclopentadiene (pKa ) 26).
X-ray Crystallography. Crystals of 5 suitable for X-ray
diffraction were obtained by recrystallization from hexanes.
Crystallographic and data collection parameters are listed in
Table 1. An ORTEP drawing of 5 showing the atom-numbering
scheme used in the refinement is illustrated in Figure 2,
respectively. Additional crystallographic data are available in
the Supporting Information.
Acknowledgment. We thank the NSF for partial
support of this work.
2. The pKa Value of 5. The pKa value of 5 was determined
by the potentiometric titration of 5 in 80%(v) CH3OH/20%(v)
0.1 N KCl aqueous solution with CF3COOH, monitored by an
Accumet AR20 pH/mV/conductivity meter. The pKa of 5 is 9.21
( 0.10. The pKa of DMAP is 9.00 ( 0.03 under the same
conditions.
Supporting Information Available: X-ray characteriza-
tion of 5 and 6a (CIF). Additional crystallographic information
for 5 and copies of the NMR spectra of 8, 9, and K-1a (PDF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
Catalytic Acylation of Benzyl Alcohol with Phenyl-
ethylketene. The experimental procedure for the reaction of
OM049399G