Tricarbonylchrommm complexes of 1,2-Dihydro-1,2-benzazaborines

Article abstract of DOI:10.1021/om800383v

l,2-Dihydro-2-phenyl-1,2-benzazaborine reacts with Cr(CO) ₃(CH₃CN)₃ in THF at 140 °C to form complex 18, in which the phenyl is >n⁶-bound to chromium. 1,2-Dihydro-2-methyl-1,2-benzazaborine reacts with Cr(CO)

Full text of DOI:10.1021/om800383v

506
Organometallics 2009, 28, 506–511
Tricarbonylchromium Complexes of 1,2-Dihydro-1,2-benzazaborines
Jun Pan, Jeff W. Kampf, and Arthur J. Ashe III*
Department of Chemistry, UniVersity of Michigan, Ann Arbor, Michigan 48109-1055
ReceiVed May 1, 2008
1,2-Dihydro-2-phenyl-1,2-benzazaborine reacts with Cr(CO)3(CH3CN)3 in THF at 140 °C to form
complex 18, in which the phenyl is η6-bound to chromium. 1,2-Dihydro-2-methyl-1,2-benzazaborine
reacts with Cr(CO)3(CH3CN)3 in THF at 60 °C to form complex 23, in which the benzo ring is η6-bound
to chromium. Treating 23 with potassium bis(trimethylsilyl)amide in THF forms the corresponding
potassium salt 25, which on heating undergoes haptotropic isomerization to complex 26, in which the
anionic heterocyclic ring is η6-bound to the chromium.
be thermally isomerized to 11. Complex 11 can then be
deprotonated to form 12, which on heating is isomerized to 13.
Protonation of 13 forms 10 again (see Scheme 2).9 This cycle
of haptotropic migrations and acid/base steps is unique in that
the Cr(CO)3 groups can be switched to either the phenyl or the
heterocyclic ring. In order to explore this phenomenon further,
we have now examined the corresponding reactions of com-
plexes of 1,2-dihydro-1,2-benzazaborines 15 and 16, which are
ring-fused analogues of 9.
Introduction
Metal complexes of polycyclic aromatic compounds often
undergo haptotropic migrations in which the metal migrates from
one ring to another.1 Chromium tricarbonyl complexes of
naphthalenes (e.g., 1 and 2) in which a Cr(CO)3 group migrates
across a ring fusion have been well studied experimentally and
computationally.1-5 Cr(CO)3 migration across a single bond
between two rings are also known, e.g. 3 f 4.6a In most cases
the migration of an electron withdrawing Cr(CO)3 group from
a neutral to an anionic ring is particularly favorable, e.g. 5 f
6.6,7 See Scheme 1.
Scheme 1. Haptotropic Shifts of Cr(CO)3 Complexes of
Selected Carbocyclic Aromatic Compounds
We have been interested in haptotropic migrations involving
boron-nitrogen anaologues of benzocylic aromatics as shown
in Scheme 2. The isomerization of 7 f 88 is closely analogous
to that of 5 f 6.6b However, reactions of boron-nitrogen
heterocycles with acidic NH groups are potentially more
complex. Thus, the reaction of 1,2-dihydro-2-phenyl-1,2-azabo-
rine (9) with Cr(CO)3(CH3CN)3 gives complex 10, which can
* To whom correspondence should be addressed. E-mail: ajashe@
umich.edu.
(1) For reviews see: (a) Do¨tz, K. H.; Wenzel, B.; Jahr, H. C. Top. Curr.
Chem. 2004, 248, 63. (b) Do¨tz, K. H.; Jahr, H. C. Chem. Recl. 2004, 4, 61.
(c) Oprunenko, Y. F. Russ. Chem. ReV. 2000, 69, 683. (d) Morris, M. J. In
ComprehensiVe Organometallic Chemistry II; Abel, E. W., Stone, F. G. A.,
Wilkinson, G., Eds.; Pergamon Press: New York, 1995; Vol. 5, p 501. (e)
Ustynyuk, N. A. Metalloorg. Khim. 1989, 2, 43.
(2) (a) Kirss, R. U.; Treichel, P. M., Jr. J. Am. Chem. Soc. 1986, 108,
853. (b) Oprunenko, Y. F.; Malugina, S. G.; Ustynyuk, Y. A.; Ustynyuk,
N. A.; Kravtsov, D. N. J. Organomet. Chem. 1988, 338, 357.
(3) (a) Jahr, H. C.; Nieger, M.; Do¨tz, K. H. Chem. Commun. 2003, 2866.
(b) Do¨tz, K. H.; Stendel, J.; Mu¨ller, S.; Nieger, M.; Ketrat, S.; Dolg, M.
Organometallics 2005, 24, 3219.
Results and Discussion
(4) Oprunenko, Y. F.; Akhmedov, N. G.; Laikov, D. N.; Malyugina,
S. G.; Mstislavsky, V. I.; Roznyatovsky, V. A.; Ustynyuk, Y. A.; Ustynyuk,
N. A. J. Organomet. Chem. 1999, 583, 136.
(5) (a) Albright, T. A.; Hofmann, P.; Hoffmann, R.; Lillya, C. P.;
Dobosh, P. A. J. Am. Chem. Soc. 1983, 105, 3396. (b) Howell, J. A. S.;
Ashford, N. F.; Dixon, D. T.; Kola, J. C.; Albright, T. A.; Kang, S. K.
Organometallics 1991, 10, 1852.
(6) (a) Ceccon, A.; Gambaro, A.; Gottardi, F.; Santi, S.; Venzo, A.;
Lucchini, V. J. Organomet. Chem. 1989, 379, 67. (b) Ceccon, A.; Gambaro,
A.; Gottardi, F.; Santi, S.; Venzo, A. J. Organomet. Chem. 1991, 412, 85.
(7) (a) Ustynyuk, N. A.; Oprunenko, Y. F.; Malyugina, S. G.; Trifonova,
O. I.; Ustynyuk, Y. A. J. Organomet. Chem. 1984, 270, 185. (b) Thoma,
T.; Pleshakov, V. G.; Prostakov, N. S.; Ustynyuk, Y. A.; Nesmeyanov,
A. N.; Ustynyuk, N. A. J. Organomet. Chem. 1980, 192, 359. (c) Trifonova,
O. I.; Galiullin, R. A.; Ustynyuk, Y. A.; Ustynyuk, N. A.; Petrovsky, P. V.;
Kravtsov, D. N. J. Organomet. Chem. 1987, 328, 321. and other papers in
this series.
2-Chloro-1,2-dihydro-1,2-benzazaborine (14) was originally
prepared in 1959 by Dewar and Dietz by the reaction of BCl3
with 2-aminostyrene.10The boron-bound chloro group of 14 can
be easily replaced by nucleophiles to give 15-17, as illustrated
in eq 1. More recently Paetzold and co-workers have reported
on 14, 16, and several more highly substituted 1,2-dihydro-1,2-
benzazaborines.11 These compounds have now been more
thoroughly characterized but are identical with those originally
reported.10 Furthermore, we have obtained a crystal structure
of the parent compound 17, which establishes its structure.
(9) Pan, J.; Kampf, J. W.; Ashe, A. J. Organometallics 2006, 25, 197.
(10) Dewar, M. J. S.; Dietz, R. J. Chem. Soc. 1959, 2728.
(11) Paetzold, P.; Stanescu, C.; Stubenrauch, J. R.; Bienmu¨ller, M.;
Englert, U. Z. Anorg. Allg. Chem. 2004, 630, 2632.
(8) Pan, J.; Wang, J.; Banazak Holl, M. M.; Kampf, J. W.; Ashe, A. J.
Organometallics 2006, 25, 3463.
10.1021/om800383v CCC: $40.75
2009 American Chemical Society
Publication on Web 12/31/2008

Cr Complexes of 1,2-Dihydro-1,2-benzaborines
Organometallics, Vol. 28, No. 2, 2009 507
Scheme 2. Haptotropic Shifts of Cr(CO)3 Complexes of Boron-Nitrogen Heteroaromatic Compounds
may involve the intermediacy of 20, we have not been able to
detect the formation of any 20.
Complex 18 can be quantitatively deprotonated by LiTMP
1
in THF to form solutions of 21. The H NMR spectrum of 21
in THF-d8 shows a first-order pattern which is distinct from the
spectrum of 18. Quenching 21 with excess D2O afforded 18-d,
in which the deuterium was exclusively at N(1). Heating anion
21 to 110 °C in THF led to slow decomposition without giving
any dectectable 22. This greater thermal lability of 21 relative
to that of 12 prevented observation of Cr(CO)3 migration from
phenyl to the anionic heterocyclic ring which had been found
for 12. See Scheme 3.
Our initial goal had been to examine possible “biphenyl-like”
haptotropic migrations of complexes of 15 which might be
analogous to those illustrated in Scheme 2. Since we were unable
to prepare 20 and 22 and no Cr(CO)3 migrations from 18 or 21
were found, this goal could not be achieved. Therefore, we
decided to examine possible “naphthalene-type” migrations
between the fused rings of the 1,2-dihydro-1,2-benzazaborine
system. In order to limit metal coordination to the fused rings,
we have examined the reactions of the methyl derivative 16.
Treating 16 with Cr(CO)3(CH3CN)3 in THF at 60 °C resulted
in formation of complex 23 mixed with excess 16. On heating
to higher temperatures 23 decomposes to give 16 and unidenti-
fied chromium-containing materials. No complex 24 could be
detected. Pure 23 was isolated in 39% yield using column
chromatography on silica gel. The 1H NMR spectrum of 23 in
THF-d8 is first order and can readily be assigned by inspection.
The signals for the benzo-ring protons are shifted upfield in
comparison with those of 16, while the signals for the uncom-
plexed C4BN ring are little changed. Therefore, π-complexation
to the benzo ring is indicated,12 which was later confirmed by
obtaining a crystal structure of 23, shown in Figure 2.
The phenyl derivative 15 was of particular interest to us, since
it is the benzo-fused analogue of 9, which we had previously
investigated in detail. Heating 15 with Cr(CO)3(CH3CN)3 in
THF to 140 °C for 40 h gave complex 18 as the sole product
1
in 50% yield. The H NMR spectrum of 18 in THF-d8 is first
order and can be readily assigned by inspection. The signals
for the phenyl protons are shifted upfield relative to those of
15, while those of the 1,2-dihydro-1,2-benzazaborine group are
little effected by complexation. Therefore, π-complexation to
the phenyl ring is indicated.12 The regioselectivity of the
Cr(CO)3 complexation was confirmed by obtaining an X-ray
structure of 18, which is illustrated in Figure 1.
The reaction of 15 with Cr(CO)3(CH3CN)3 under milder
conditions (25 °C) gave a mixture of 18 and an isomer
tentatively identified as 19 in the ratio of 1:3. We have been
unable to isolate pure 19 from the mixture, while heating it to
140 °C converted it to pure 18. The structure of 19 was assigned
on the basis of the 1H NMR spectrum of the mixture. In
particular signals for the fused benzocyclic ring of 19 were
shifted markedly upfield from those of the uncoordinated ring
of 15. Similar π-coordination shifts are characteristic of Cr(CO)3-
coordinated arene rings.12 Although the conversion of 19 to 18
Neutral complex 23 can be quantitatively deprotonated by
treatment with potassium bis(trimethylsilyl)amide in THF to give
a solution of potassium salt 25. The 1H spectrum of 25 in THF-
d8 shows a first-order pattern, which is easily distinguished from
that of 23. Quenching 25 with excess D2O yielded N-deuterium-
labeled 23-d. Heating 25 in THF-d8 to 90 °C converted it to
26. Decomposition was approximately 25%. The conversion of
25 to 26 was monitored by 1H NMRspectroscopy. In comparison
to that of 25 the 1H NMR spectrum of 26 showed that all of the
signals of the benzo ring are shifted downfield and all of
Figure 1. Solid-state structure of 18 (ORTEP). Thermal ellipsoids
are given at the 50% probability level. Hydrogen atoms have been
omitted for clarity.

508 Organometallics, Vol. 28, No. 2, 2009
Scheme 3. Reactions of 1,2-Dihydro-2-phenyl-1,2-benzazaborines
Pan et al.
the signals for the C4BN ring are shifted upfield. In addition,
the 11B NMR spectrum shows that the signal for 26 (δ 24.0) is
shifted upfield relative to that of 25 (δ 40.8). This is consistent
with a coordination shift of the Cr(CO)3 group from the benzo
ring to the heterocyclic ring.12,13
Quenching 26 with methyl iodide in THF at 25 °C afforded
the uncomplexed N-methyl derivative 28.10 We suspect that the
methyl iodide converted 26 to intermediate 27, which must be
highly labile at room temperature since it rapidly decomplexes
to give 28. See Scheme 4.
The thermal conversion of 25 to 26 and the presumed lability
of 27 suggest that the order of ligand strength toward Cr(CO)3
is the anionic heterocyclic ring of 1,2-benzazaboratabenzene >
the benzo ring of 1,2-benzazaboratabenzene > the neutral
heterocyclic ring of 1,2-dihydro-1,2-benzazaborine. This order
is analogous to that previously found for the monocyclic ring
systems 10-13, in which the order of ligand strength was 1,2-
azaboratabenzene > B-phenyl >1,2-dihydro-1,2-azaborine. In
both sets the anionic heterocyclic ring is the strongest ligand,
while the neutral heterocyclic ring is the weakest ligand toward
the electron-withdrawing Cr(CO)3 group. Computational studies
have indicated that the π-bonding of 1,2-dihydro-1,2-azaborine
is weaker than that of benzene due to localization of electrons
by the B-N bond.14 This is consistent with the observed lower
π-basicity of the heterocyclic ring toward Cr(CO)3. Apparently
deprotonation of NH of 1,2-dihydro-1,2-azaborine greatly
enhances its π-basicity so that 1,2-azaboratabenzene is a stronger
π-base toward Cr(CO)3.
Our inability to prepare stable complexes in which the
Cr(CO)3 group is π-bound to the heterocyclic ring of 1,2-
dihydro-1,2-benzazborines implies that this ring system is a
poorer ligand than the 1,2-dihydro-1,2-azaborine ring of 10.
Reduction in the π-ligand strength on benzannulation has a
precedent in the observation that naphthalene is a poorer ligand
than benzene toward Cr(CO)3.15
It was of interest to examine the conversion of 25 to 26 in
order to compare it with the formally similar Cr(CO)3 migrations
between the rings of η6-naphthalene complexes 1 and 2. The
reaction of 25 to give 26 at 60 °C in THF-d8 was monitored by
1H NMR spectroscopy. Unfortunately, heating 25 led to
unidentified products as well as 26. When the reaction was
performed in the presence of the good π-donor hexamethyl-
benzene, no crossover products were observed. These data are
consistent with a unimolecular reaction. The first-order rate
constant for the loss of 25 was found to be k ) 2.80 × 10-5
s-1 at 60 °C. For comparison the conversion of 1 to 2, where R
) Me, is 34 times slower with an extrapolated k ) 8.2 × 10-7
s-1 at 60 °C.2a Precise comparison is not warranted because of
differences in charge type, solvent, and ring substituents.
Nevertheless, the rates are qualitatively similar. It has been
proposed that naphthalene-Cr(CO)3 ring exchange occurs via
an intramolecular stepwise decomplexation of the formal double
(12) Elschenbroich, C.; Salzer, A. Organometallics, 2nd ed.; VCH:
Weinheim, Germany, 1991; pp 296-307.
(13) Wrackmeyer, B.; Ko¨ster, R. Methoden der Organischen Chemie;
G. Thieme Verlag: Stuttgart, Germany, 1984; Vol. XIII, Part 3c, p 584.
(14) (a) Kranz, M.; Clark, T. J. Org. Chem. 1992, 57, 5492. (b) Kar,
T.; Elmore, D. E.; Scheiner, S. J. Mol. Struct. 1997, 392, 65.
(15) (a) Mukerjee, S. L.; Lang, R. F.; Ju, T.; Kiss, G.; Hoff, C. D. Inorg.
Chem. 1992, 31, 4885. (b) Zhang, S.; Shen, J. K.; Basolo, F.; Ju, T. D.;
Lang, R. F.; Kiss, G.; Hoff, C. D. Organometallics 1994, 13, 3692.
Figure 2. Solid-state structure of 23 (ORTEP). Thermal ellipsoids
are given at the 50% probability level. Hydrogen atoms have been
omitted for clarity.

Cr Complexes of 1,2-Dihydro-1,2-benzaborines
Organometallics, Vol. 28, No. 2, 2009 509
Scheme 4. Reactions of 1,2-Dihydro-2-methyl-1,2-benzazaborines
Table 1. Comparison of Selected Bond Distances (Å) for 18, 11,a b
,
bonds of one ring concomitant with or followed by complexation
of the double bonds of the second ring.1-5 It seems most
reasonable to propose a similar mechanism for the conversion
of 25 to 26, although a mechanism involving intermediate
complexation to the nucleophilic nitrogen cannot be excluded.9
Crystal Structures. An X-ray structure determination of 17
(in the Supporting Information) unambiguously identifies the
compound. Since 17 is isostructural with naphthalene,16 the
benzocyclic and heterocyclic rings are crystallographically
identical. Thus, accurate bond distances could not be obtained.
It is worth noting that the isomeric 4a,8a-azoniaboratanaphtha-
lene 2917 is also isostructural with naphthalene and that a similar
isostructural relationship has been found for other boron-nitrogen
heterocyclic compounds with their carbocyclic analogues.18 It
has recently been proposed that this close structural similarity
is a general and significant feature of boron-nitrogen hetero-
cycles.19
and 23c
bond
18
11
23
C(1)-N(1)
N(1)-B(1)
B(1)-C(8)
C(8)-C(7)
C(7)-C(6)
C(6)-C(1)
B(1)-C(9)
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(5)-C(6)
1.391(2)
1.422(2)
1.526(2)
1.355(2)
1.439(2)
1.407(2)
1.576(2)
1.405(2)
1.384(2)
1.399(2)
1.379(2)
1.408(2)
1.368(5)
1.430(5)
1.500(5)
1.370(6)
1.413(7)
1.353(6)
1.575(2)
1.376(4)
1.424(3)
1.534(4)
1.358(15)
1.447(6)
1.427(6)
1.583(5)
1.420(4)
1.399(12)
1.412(3)
1.411(11)
1.428(4)
a Reference 9. b The atom-numbering scheme for 11 corresponds to
that of 18 and 23. c Average for two independent molecules.
uncoordinated 1,2-dihydro-1,2-benzazaborine ring which is
bound to a B-phenyl, bearing an η6-bound Cr(CO)3 unit.
Structure 23 consists of a near planar 1,2-dihydro-2-methyl-
1,2-benzazaborine ring which is η6-bound to a Cr(CO)3 unit
through the benzo-ring. In both cases the Cr(CO)3 units are
bound in typical piano-stool fashion.
Compound 18 is particularly notable, since it contains a
noncoordinated 1,2-dihydro-1,2-benzazaborine group. This group
is bound to the B-pendant Ph-Cr(CO)3 unit in the same manner
as is the parent 1,2-dihydro-1,2-azaborine ring of complex 11.
Comparison of the boron-nitrogen heterocyclic rings of 18 and
11 shows that the bonds which are R-ꢀ to the ring fusion
(N(1)-B(1) and C(7)-C(8)) are shorter while the other en-
docyclic bonds (C(1)-N(1), B(1)-C(8), C(7)-C(6), and
C(6)-C(1)) are longer than the corresponding bonds of 11.
Similar differentiation of the C-C bond distances is found for
naphthalene15 vs benzene and generally for linearly fused arenes
vs monocyclic arenes.19 It reflects a localization of π-bonding
relative to monocyclic aromatics. However, the B-C bond
lengths of 18 suggest that the 1,2-dihydro-1,2-benzazaborine
ring has strong endocyclic π-bonding. The endocyclic B-C
bond (1.526(2) Å) of 18 is significantly shorter than the
exocyclic B-C bond (1.576(2) Å).
The molecular structures of the 1,2-dihydro-1,2-benzazabo-
rine-Cr(CO)3 complexes 18 and 23 are illustrated in Figures 1
and 2, respectively. Selected bond distances of 18, 11, and 23
are compared in Table 1. Structure 18 consists of a near-planar
(16) (a) Cruickshank, D. W. J.; Sparks, R. A. Proc. R. Soc. London,
Ser. A 1960, 258, 270. (b) Brock, C. P.; Dunitz, J. D. Acta Crystallogr.,
Sect. B 1982, 38, 2218.
(17) Fang, X. D.; Yang, H.; Kampf, J. W.; Banaszak Holl, M. M.; Ashe,
A. J. Organometallics 2006, 25, 513.
(18) Pan, J.; Kampf, J. W.; Ashe, A. J. Organometallics 2004, 23, 5626.
(19) (a) Zhou, H.-B.; Nettles, K. W.; Bruning, J. B.; Kim, Y.;
Joachimiak, A.; Sharma, S.; Carlson, K. E.; Stossi, F.; Katzenellenbogen,
B. S.; Greene, G. L.; Katzenellenbogen, J. A. Chem. Biol. 2007, 14, 659.
(b) Marwitz, A. J. V.; Abbey, E. R.; Jenkins, J. T.; Zakharov, L. N.; Liu,
S.-Y. Org. Lett. 2007, 9, 4905. Badger, G. M. Aromatic Character and
Aromaticity; Cambridge University Press: Cambridge, U.K., 1969; p 38.

510 Organometallics, Vol. 28, No. 2, 2009
Pan et al.
powder. 1H NMR (400 MHz, THF-d8): δ 7.12 (t, J ) 8.1 Hz, 1H,
(Benzo)H); 7.24 (d, J ) 11.4 Hz, 1H, C(3)H); 7.38 (m, 4H,
(Benzo)H, ArH); 7.52 (d, J ) 8.1 Hz, 1H, (Benzo)H); 7.61 (d, J )
8.1 Hz, 1H, (Benzo)H); 7.96 (d, J ) 8.1 Hz, 2H, ArH); 8.10 (d, J
) 11.4 Hz, 1H, C(4)H); 9.62 (br, 1H, NH). 13C NMR (100.6 MHz,
CD2Cl2): δ 118.8, 121.7, 128.8, 129.0, 129.9, 130.2, 133.2, 146.1;
signals for C(ipso), C(3), C(4a), and C(8a) not observed. 11B NMR
(160.4 MHz, CD2Cl2): δ 33.9. HRMS (EI, m/z): calcd for
C14H1211BN (M+), 205.1063; found, 205.1062. Anal. Calcd for
C14H12BN: C, 82.00; H, 5.90; N, 6.83. Found: C, 81.61; H, 5.84;
N, 6.71.
1,2-Dihydro-1,2-benzazaborine (17). The reaction of 14 with
LiAlH4 gave 17 as a white powder.10 1H NMR (500 MHz, DMSO-
d6): δ 5.02 (br, 1H, BH); 6.92 (d, J ) 11.2 Hz, 1H, C(3)H); 7.19
(t, J ) 8.0 Hz, 1H, (Benzo)H); 7.46 (t, J ) 8.0 Hz, 1H, (Benzo)H);
7.54 (d, J ) 8.0 Hz, 1H, (Benzo)H); 7.70 (d, J ) 8.0 Hz, 1H,
(Benzo)H); 8.11 (d, J ) 11.2 Hz, 1H, C(4)H); 10.68 (br, 1H, NH).
13C NMR (100.6 MHz, DMSO-d6): δ 118.4, 120.9, 125.2, 128.2,
129.2, 129.4 (br), 140.5, 144.7. 11B NMR (160.4 MHz, DMSO-
d6): δ 31.5. HRMS (EI, m/z): calcd for C8H811BN (M+), 129.0750;
found, 129.0747.
Table 2. Crystal and Data Collection Parameters for 18 and 23
18
23
empirical formula
fw
C17H12BCrNO3
341.09
C12H10BCrNO3
279.02
temp, K
123(2)
123(2)
wavelength, Å
cryst syst
space group
a, Å
b, Å
c, Å
0.710 73
monoclinic
P21/n
11.8163(16)
10.9188(14)
12.0946(16)
104.151(2)
1513.1(3); 4
1.497
0.710 73
monoclinic
P21/n
15.212(4)
10.730(3)
16.259(4)
112.855(4)
2445.6(11); 8
1.516
ꢀ, deg
V, Å3; Z
calcd density, Mg/m3
abs coeff, mm-1
F(000)
0.768
696
0.932
1136
cryst size, mm
limiting indices
0.58 × 0.22 × 0.20 0.36 × 0.32 × 0.22
-15 e h e 15
-14 e k e 14
-16 e l e 16
15 374/3762
-20 e h e 20
-14 e k e 14
-21 e l e 21
39 806/26 502
no. of rflns collected/unique
abs cor
refinement method
no. of data/restraints/params
GOF on F2
semiempirical from equivalents
full-matrix least squares on F2
3762/0/208
Tricarbonyl[1,2-dihydro-2-(η6-phenyl)-1,2-benzazaborine]chro-
mium (18). 1,2-Dihydro-2-phenyl-1,2-benzazaborine (15) (600 mg,
2.93 mmol) and Cr(CH3CN)3(CO)3 (800 mg, 3.09 mmol) were
dissolved in 25 mL of THF. The resulting solution was heated to
140 °C with stirring for 60 h in a Teflon screw-stoppered thick-
walled glass reaction tube. After the solvent was removed in vacuo,
the residue was washed with hexanes (3 × 30 mL) at 0 °C. The
residue was purified by column chromatography on silica gel (25%
ethyl acetate in hexanes elution) to give a pure sample of the product
(494 mg, 49%) as a yellow powder. IR (benzene film): 1968, 1895
1.050
1.099
final R indices (I > 2σ(I))
R1 ) 0.0304
wR2 ) 0.0865
R1 ) 0.0351
wR2 ) 0.0900
R1 ) 0.0583
wR2 ) 0.1559
R1 ) 0.0716
wR2 ) 0.1627
R indices (all data)
largest diff peak and hole, e/Å3 0.392 and -0.291 0.697 and -0.720
It is also interesting to compare the structure of the hetero-
cyclic ring of 18 with that of B-O-B ether 30, which was
reported by Paetzold and co-workers.11 The intra-ring distances
of 18 and 30 were virtually identical, which indicates that the
pendant oxygen makes only a small perturbation to the
π-bonding of the ring. Finally, comparison of the structure of
23 with that of 18 shows that the C-C bonds of the benzocyclic
ring expand by an average of 0.02 Å on complexation. This is
a normal effect which is a consequence of partial removal of
the π electrons by the Cr(CO)3 group. On the other hand, bond
distances in the heterocyclic rings of 23 and 18 differ by an
average of only 0.01 Å, indicating that the coordination in 23
has little effect on the adjacent ring. It should also be noted
that the corresponding bond distances in the more highly
substituted 3111 are not appreciably different from those of 23.
1
cm-1. H NMR (500 MHz, THF-d8): δ 5.50 (t, J ) 6.3 Hz, 2H,
ArH); 5.70 (t, J ) 6.3 Hz, 1H, ArH); 6.10 (d, J ) 6.3 Hz, 2H,
ArH); 7.05 (d, J ) 11.5 Hz, 1H, C(3)H); 7.15 (t, J ) 8.1 Hz, 1H,
(Benzo)H); 7.40 (t, J ) 8.1 Hz, 1H, (Benzo)H); 7.46 (d, J ) 8.1
Hz, 1H, (Benzo)H); 7.63 (d, J ) 8.1 Hz, 1H, (Benzo)H); 8.11 (d,
J ) 11.5 Hz, 1H, C(4)H); 9.60 (br, 1H, NH). 13C NMR (100.6
MHz, THF-d8): δ 93.5, 96.1, 99.8, 119.4, 122.1, 127.0 (br), 129.4,
130.3, 146.9; signals for C(ipso), C(4a), C(8a), and CO not
observed. 11B NMR (160.4 MHz, THF-d8): δ 32.9. HRMS (EI, m/z):
calcd for C17H1211BNO352Cr (M+), 341.0315; found, 341.0327. Anal.
Calcd for C17H12BNO3Cr: C, 59.86; H, 3.55; N, 4.11. Found: C,
60.23; H, 3.49; N, 4.14.
When the starting materials were stirred at 25 °C for 18 h, the
1H spectrum indicated a mixture of 15, 18, and another product
tentatively identified as 19. H NMR for 19 (500 MHz, THF-d8):
δ 5.06 (t, J ) 7 Hz); 5.72 (t, J ) 7 Hz); 5.88 (d, J ) 7 Hz); 6.19
(d, J ) 7 Hz) for the coordinated benzocyclic ring protons. Other
peaks were obscured by peaks from 15 and 18. Heating the mixture
to 140 °C converted it to 18.
Experimental Section
1
General Considerations. Manipulations of air-sensitive com-
pounds were performed under a nitrogen or argon atmosphere using
standard Schlenk techniques or in a nitrogen-filled MBraun drybox.
THF, ether, pentanes, and hexanes were dried and deoxygenated
by distillation from sodium/benzophenone ketyl. THF-d8 was dried
Tricarbonyl[1,2-dihydro-1-lithio-2-(η6-phenyl)-1,2-benzazabo-
rine]chromium (21). A mixture of LiTMP (6 mg, 0.041 mmol)
and 18 (13 mg, 0.038 mmol) was dissolved in 0.75 mL of THF-d8.
The 1H NMR spectrum showed that 21 was formed quantitatively.
Quenching the mixture with 10 µL of D2O resulted in the formation
of 18-d, which was confirmed by 1H spectroscopy (absence of the
NH signal) and HRMS (HRMS (EI, m/z): calcd for
C17H112H11BNO352Cr (M+), 342.0378; found, 342.0376). Heating
a sample of 21 in THF-d8 resulted in slow decomposition to
1
over K/Na alloy before use. H, 11B, and 13C NMR spectra were
recorded on a Varian Inova 400 or 500 NMR spectrometer operating
at ambient temperature. The 1H and 13C NMR chemical shifts were
determined relative to internal solvent standards and are referenced
to tetramethylsilane. The 11B NMR chemical shifts were referenced
to external boron trifluoride diethyl etherate. Combustion analyses
were determined using a Perkin-Elmer 240 CHN analyzer by the
analytical services department of the Department of Chemistry of
the University of Michigan, Ann Arbor, MI.
1,2-Dihydro-2-methyl-1,2-benzazaborine (16) and 2-chloro-1,2-
dihydro-1,2-benzazaborine (14) were prepared by the route of Dietz
and Dewar.10 NMR spectra of 14 and 16 were identical with those
reported by Paetzold and co-workers.11
1
unidentified products. Data for 21 are as follows. H NMR (500
MHz, THF-d8): δ 5.38 (t, J ) 6.3 Hz, 2H, ArH); 5.44 (t, J ) 6.3
Hz, 1H, ArH); 6.05 (d, J ) 6.3 Hz, 2H, ArH); 6.76 (t, J ) 8.0 Hz,
1H, (Benzo)H); 6.88 (d, J ) 11.1 Hz, 1H, C(3)H); 7.10 (t, J ) 8.0
Hz, 1H, (Benzo)H); 7.31 (d, J ) 8.0 Hz, 1H, (Benzo)H); 7.40 (d,
1,2-Dihydro-2-phenyl-1,2-benzazborine (15). Compound 15
was prepared by the reaction of phenylmagnesium bromide with
14 as described by Dewar and Dietz10 and was isolated as a white
J ) 8.0 Hz, 1H, (Benzo)H); 7.86 (d, J ) 11.1 Hz, 1H, C(4)H). 13
C
NMR (100.6 MHz, THF-d8): δ 93.9, 94.8, 100.8, 117.0, 125.9,

Cr Complexes of 1,2-Dihydro-1,2-benzaborines
Organometallics, Vol. 28, No. 2, 2009 511
126.4, 128.0 (br), 130.0, 142.9; signals for C(ipso), C(4a), and C(8a)
not observed. 11B NMR (160.4 MHz, THF-d8): δ 34.8.
for 4 h, the 1H NMR spectrum indicated the formation of 26 along
with 25% of unidentified products. After the reaction mixture was
cooled to room temperature, it was quenched with excess MeI to
Tricarbonyl[(4a,5,6,7,8,8a-η)-1,2-dihydro-2-methyl-1,2-ben-
zazaborine]chromium (23). 1,2-Dihydro-2-methyl-1,2-benzazabo-
rine (16; 1.70 g, 11.89 mmol) and Cr(CH3CN)3(CO)3 (5.80 g, 22.4
mmol) were dissolved in 80 mL of THF. The resulting solution
was heated to 60 °C with stirring for 14 h. After the solvent was
1
give 28 and other unidentified materials. The H NMR and mass
spectra showed that 28 was identical with previously obtained
material.10 Data for 26 are as follows. H NMR (500 MHz, THF-
1
d8): δ 0.56 (s, 3H, CH3); 4.36 (d, J ) 9.5 Hz, 1H, C(3)H); 6.30 (d,
J ) 9.5 Hz, 1H, C(4)H); 6.95 (t, J ) 7.8 Hz, 1H, (Benzo)H); 7.13
(d, J ) 7.8 Hz, 1H, (Benzo)H); 7.22 (t, J ) 7.8 Hz, 1H, (Benzo)H);
7.47 (d, J ) 7.8 Hz, 1H, (Benzo)H). 13C NMR (125.7 MHz, THF-
d8): δ 95.0 (br), 110.3, 129.0, 129.9, 133.1, 133.9; signals for
C(ipso), C(3), C(4a), C(8a), CO, and C(Me) not observed. 11B NMR
(160.4 MHz, THF-d8): δ 24.0.
1
removed in vacuo, a H NMR spectrum of the residue indicated
that it was a mixture of 16 and 23 in a 1:3 ratio. Heating the reaction
mixture to higher temperature resulted in the destruction of 23, but
16 survived. The residue was extracted with ethyl ether (4 × 20
mL). Ethyl ether was removed in vacuo, and the residue was washed
with pentane (3 × 15 mL). The residue was purified by column
chromatography on silica gel (30% CH2Cl2 in hexanes elution) to
give a pure sample of the product (1.30 g, 39%) as an orange
powder. IR (hexanes film): 1975, 1908 cm-1. 1H NMR (500 MHz,
benzene-d6): δ 0.45 (s, 3H, CH3); 4.09 (t, J ) 6.3 Hz, 1H,
(Benzo)H); 4.35 (d, J ) 6.3 Hz, 1H, (Benzo)H); 4.76 (t, J ) 6.3
Hz, 1H, (Benzo)H); 5.14 (d, J ) 6.3 Hz, 1H, (Benzo)H); 5.86 (br,
1H, NH); 6.37 (d, J ) 11.5 Hz, 1H, C(3)H); 6.96 (d, J ) 11.5 Hz,
1H, C(4)H). 13C NMR (125.7 MHz, benzene-d6): δ 80.5, 85.9, 94.5,
95.6, 131.6 (br), 144.9; signals for C(ipso), C(4a), C(8a), Me, and
CO not observed. 11B NMR (160.4 MHz, benzene-d6): δ 39.9.
HRMS (EI, m/z): calcd for C12H1011BNO352Cr (M+), 279.0159;
found, 279.0152. Anal. Calcd for C12H10BNO3Cr: C, 51.66; H, 3.61;
N, 5.02. Found: C, 51.84; H, 3.73; N, 4.96.
Kinetics of the Haptotropic Migration of 25. The reaction was
carried out on solutions of 25 prepared as described above and
1
sealed in NMR tubes. The reaction was monitored by H NMR
spectroscopy. The integrals of various proton multiplets of 25 were
normalized against that of the signal of cyclohexane used as an
internal standard. Use of the first-order rate equation gave satisfac-
tory plots (R2 ) 0.998) for up to 2 half-lives. The rate constant
was found to be 2.80 × 10-5 s-1 at 333 K. No crossover products
were found when the reaction was performed in the presence of
hexamethylbenzene.
Single-Crystal X-ray Crystallography. Crystals of 17, 18, and
23 suitable for X-ray diffraction were obtained by recrystallization
at room temperature from pentane/CH2Cl2, benzene/ethyl acetate,
and ethyl acetate, respectively. All crystallographic data for 17 are
available in the Supporting Information. Crystallographic and data
collection parameters for 18 and 23 are collected in Table 2. ORTEP
drawings of 18 and 23 showing the atom-numbering schemes are
given in Figures 1 and 2, respectively. Selected bond distances are
collected in Table 1. Additional crystallographic data are available
in the Supporting Information.
Conversion of 23 to 25. A mixture of KHMDS (8 mg, 0.040
mmol) and 23 (10 mg, 0.036 mmol) was dissolved in 0.75 mL of
THF-d8. The resulting NMR showed that 25 was formed quanti-
tatively. Quenching the mixture with 10 µL of D2O resulted in the
formation of 23-d, which was confirmed by HRMS (HRMS (EI,
m/z): calcd for C12H92H11BNO352Cr (M+), 280.0222; found, 280.0229).
1
Data for 25 are as follows. H NMR (500 MHz, THF-d8): δ 0.54
(s, 3H, CH3); 4.92 (t, J ) 6.3 Hz, 1H, (Benzo)H); 5.51 (t, J ) 6.3
Hz, 1H, (Benzo)H); 5.59 (d, J ) 6.3 Hz, 1H, (Benzo)H); 6.05 (d,
J ) 6.3 Hz, 1H, (Benzo)H); 6.48 (d, J ) 11.1 Hz, 1H, C(3)H);
7.34 (d, J ) 11.1 Hz, 1H, C(4)H). 13C NMR (100.6 MHz, THF-
d8): δ 84.0, 92.1, 97.4, 100.7, 133.8(br), 142.7; signals for C(ipso),
C(4a), C(8a), CO, and Me not observed. 11B NMR (160.4 MHz,
THF-d8): δ 40.8.
Conversion of 25 to 26. A solution of 25 was prepared from 23
(12 mg, 0.043 mmol) and KHMDS (10 mg, 0.05 mmol) in 0.75
mL of THF-d8. After the resulting solution was heated to 90 °C
Acknowledgment. Partial support of this research has
been provided by the NSF.
Supporting Information Available: CIF files giving X-ray
characterization data for 17, 18, and 23 and figures giving 1H NMR
spectra. This material is available free of charge via the Internet at
http://pubs.acs.org.
OM800383V