200
J.M. Burke et al. / Journal of Organometallic Chemistry 649 (2002) 199–203
a catalyst precursor in its own right [2] and as an
extremely efficient means by which to prepare
[Rh(PR3)2(acac)] systems which we have shown can be
converted cleanly to zwitterionic [Rh(PR3)2(h6-catBcat)]
(cat=1,2-O2C6H4) catalyst systems by treatment with
B2cat3 or excess HBcat [1–5,16].
dried and deoxygenated by passage through columns of
activated alumina and BASF-R311 catalyst under Ar
pressure using a locally modified version of the Innova-
tive Technology, Inc. SPS-400 solvent purification sys-
tem [21]. C6D6 and hexane were dried over potassium
and sodium, respectively, and were distilled under ni-
trogen before use.
Quite recently, Ueda and Miyaura demonstrated [10]
the efficiency of a catalyst for the addition of ArB(OH)2
to RCHO, which was prepared in situ by addition of
2.2. Synthesis of Na(acac)
t
one equivalent of Bu3P to 1.
Addition of excess phosphine inhibits the reaction,
and thus the active species is presumably a mono-phos-
phine rhodium complex suggesting the importance of
having monodentate labile ligands such as COE, in
contrast to COD.
A suspension of NaH, 60% in mineral oil, (1.63 g,
0.041 mol) was degassed and then added to hexane (150
ml) and cooled to −78 °C with stirring. A solution of
acetylacetone (4.10 g, 0.041 mol) in hexane (50 ml) was
added dropwise over a period of 10 min, to allow for
the evolution of H2, giving a white precipitate. The
reaction mixture was then allowed to warm to room
temperature and was stirred for 1 h after which the
reaction mixture was filtered, washed with hexane, and
the precipitate then dried in vacuo to yield 4.98 g
In 1985, Bennett and Mitchell reported [17] the use of
1 in a reaction with a secondary phosphite yet in this
paper they reported only an outline of the synthesis and
provided no characterisation of 1 itself. In 1996, Esteru-
elas et al. [13] used 1 synthesised by the published
procedure [18] for [Ir(h2-COE)2(acac)], which uses [Ir(-
COE)2(m-Cl)]2 and Tl(acac), and yet again no details
were provided. In fact, a procedure [19] for synthesising
1 using Tl(acac) was reported by Bennet and Patmore
in 1971, along with some characterisation data. It
would appear, however, that this original report is not
widely known, and an alternative procedure avoiding
the use of thallium salts would certainly be desirable for
pharmaceutical applications. Indeed, we received sev-
eral requests for information on our synthetic protocol
for 1. This prompted us to report herein a detailed and
reliable procedure for preparing 1 without the use of
thallium salts, along with its full spectroscopic and
structural characterisation.
1
(99.6%) of Na(acac) as a fine white powder. H-NMR
(CD3CN): l 4.99 (1H, CH), 1.69 (6H, CH3). IR (Nujol,
cm−1): w(acac) 1582, 1518. Found: C=48.97, H=
5.94%. C5H7O2Na requires C=49.18, H=5.78%. N.B.
This simple procedure yields rigorously dry Na(acac).
Although hydrated Na(acac) is commercially available,
and may be suitable for use in the preparation of 1, a
source of any moisture is undesirable for many catalytic
reactions.
2.3. Synthesis of [Rh(p2-COE)2(acac)] (1)
To a mixture of [Rh(COE)2(m-Cl)]2 (0.276 g, 0.385
mmol) and Na(acac) (0.094 g, 0.770 mmol) was added
toluene (20 ml) and the reaction was warmed to 40 °C
with stirring under N2 for 3 h. The reaction mixture
was filtered via filter cannula, to remove NaCl, and the
toluene was removed in vacuo. The product was ex-
tracted into hexane, filtered through a thin pad of
Celite®, and isolated by removal of the hexane in
vacuo, yielding 0.263 g (81%) of 1 as a yellow powder.
Single crystals suitable for X-ray diffraction were
2. Experimental
2.1. General procedures
All reactions were carried out in a nitrogen atmo-
sphere using Schlenk techniques or an Innovative Tech-
nology, Inc. System 1 glove box. Glassware was oven
dried before transfer into the glove box. NMR spectra
were recorded on Varian Inova 500 (1H, HSQC) and
Varian VXR 400 (13C, DEPT) instruments. Proton and
13C-NMR spectra were referenced to external SiMe4 via
residual protons in the deuterated solvents or solvent
resonances, respectively. Elemental analyses were con-
ducted in the Department of Chemistry at the Univer-
sity of Durham using an Exeter Analytical Inc. CE-440
Elemental Analyzer. The [Rh(COE)2(m-Cl)]2 was pre-
pared using published procedures [20], whereas the
NaH (60% dispersion in mineral oil), and acetylacetone
were used as purchased from Lancaster Synthesis and
Aldrich Chemical Company, respectively. Toluene was
1
grown from hexane at −30 °C. H-NMR (C6D6): l
5.04 (s, acac CH), 2.51 (4H, olefin COE), 2.47 (4H,
COE), 2.42 (4H, COE), 1.69 (6H, acac CH3+4H,
COE), 1.56 (4H, COE), 1.41 (8H, COE); 13C{1H}-
NMR: l 185.3 (s, acac CꢀO), 99.1 (s, acac CH), 78.3 (d,
JC–Rh=13 Hz, olefinic COE), 30.5 (s, COE), 28.2 (s,
COE), 27.3 (s, acac CH3), 27.0 (s, COE). IR (Nujol,
cm−1): w(acac) 1620, 1510. Found: C=59.10, H=
8.34%. RhC21H35O2 requires C=59.71, H=8.35%.
2.4. Crystal structure determination
C21H35O2Rh, M=442.40, orthorhombic, a=
,
,
,
17.736(3) A, b=11.041(2) A, c=20.520(3) A, V=