Reactions of tris(oxazolinyl)phenylborato rhodium(I) with C-X (X = Cl, Br, OTf) Bonds: Stereoselective intermolecular oxidative addition

Article abstract of DOI:10.1021/om100515u

The achiral and enantiopure chiral compounds To^MRh(CO) ₂ (3) and To^PRh(CO)₂ (4) (To^M = tris(4,4-dimethyl-2-oxazolinyl)phenylborate; To^P = tris(4S-isopropyl-2-oxazolinyl)phenylborate) were p

Full text of DOI:10.1021/om100515u

Organometallics 2010, 29, 4105–4114 4105
DOI: 10.1021/om100515u
Reactions of Tris(oxazolinyl)phenylborato Rhodium(I) with C-X
(X = Cl, Br, OTf) Bonds: Stereoselective Intermolecular Oxidative Addition
Hung-An Ho, James F. Dunne, Arkady Ellern, and Aaron D. Sadow*
Department of Chemistry and U.S. DOE Ames Laboratory, Iowa State University, Ames, Iowa 50011
Received May 25, 2010
The achiral and enantiopure chiral compounds ToMRh(CO)2 (3) and ToPRh(CO)2 (4) (ToM
=
tris(4,4-dimethyl-2-oxazolinyl)phenylborate; ToP = tris(4S-isopropyl-2-oxazolinyl)phenylborate)
were prepared to investigate stereoselective oxidative addition reactions and develop new catalytic
enantioselective bond functionalization and cross-coupling chemistry. Reactivity at the rhodium
center is first shown by the substitution of the carbonyl ligands in 3 and 4 in the presence of the
appropriate ligand; thus treatment of ToMRh(CO)2 with P(OMe)3 provides ToMRh(CO)[P(OMe)3]
(5). However, reaction of ToMRh(CO)2 and MeOTf (Tf = SO2CF3) affords the complex [{N-Me-κ2-
ToM}Rh(CO)2]OTf (6), resulting from N-oxazoline methylation rather than oxidative addition to
rhodium(I). In contrast, ToMRh(CO)2 reacts with allyl bromide and chloroform, forming the
rhodium(III) species (κ3-ToM)Rh(η1-C3H5)Br(CO) (7) and (κ3-ToM)Rh(CHCl2)Cl(CO) (8), respec-
tively. Interestingly, the chiral ToPRh(CO)2 and CHCl3 react to give one diastereomer of (κ3-ToP)-
Rh(CHCl2)Cl(CO) (9; 100:3 dr) almost exclusively. To evaluate the reactivity of these rhodium(I)
compounds, the carbonyl stretching frequencies have been examined. The data for the mono- and
trivalent rhodium oxazolinylborate compounds indicate that the electron-donating ability of [ToM]-
is slightly greater than that of [ToP]-, and both ligands provide electronic environments that can be
compared to the tris(pyrazolyl)borate ligand family.
Introduction
tris(pyrazolyl)borate) compounds are well known to under-
go oxidative addition reactions to give racemic products4-6
or diastereomerically enriched pairs of enantiomers.7 Chiral
auxiliaries on Cp or Tp have had limited success in affecting
stereoselective intermolecular oxidative addition to give
enantio-enriched diastereomeric products,8-10 although
TpmenthRh(CO)2 (Tpmenth = tris(menthylpyrazolyl)borate)
undergoes a stereoselective ligand cyclometalation under
photolytic conditions.10 In another example, a racemic
planar-chiral iridium compound oxidatively adds C-H
bonds to give a stereogenic iridium center with high diaste-
reoselectivity.11 Also, a resolved planar-chiral iridium(I)
complex containing a chiral neomenthyl auxiliary and a
linked bulky phosphine forms one diastereomer upon treat-
ment with [Me3O][BF4].8b
Oxidative addition of polar C-X bonds is fundamental to
organometallic chemistry and is an essential step in homo-
geneous catalytic processes such as rhodium(I)-catalyzed
acetic acid synthesis1 and iridium(I)-catalyzed allylic sub-
stitution.2 Proposed mechanistic pathways, including SN2-
like nucleophilic substitution, concerted oxidative addition,
and radical chain reactions, often depend on conditions,
substrates, and ancillary ligands.3 Meanwhile, the intimate
mechanism (radical, concerted, or SN2) can affect the pro-
ducts’ stereochemistry, the possibility for stereoselectivity,
and application in enantioselective catalytic transformations.
CpML2 and TpML2 (M = Rh, Ir; Cp = η5-C5R5, Tp =
*To whom correspondence should be addressed. E-mail: sadow@
iastate.edu.
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r
2010 American Chemical Society
Published on Web 08/25/2010
pubs.acs.org/Organometallics

4106 Organometallics, Vol. 29, No. 18, 2010
Scheme 1. Reaction Pathways for the Interaction of ToMRh(CO)2 (3) with Strong and Weak Electrophiles
Ho et al.
enantioselective delivery of alkyl groups to organic substrates.
To discover new chemistry along these lines, we prepared the
monoanionic tridentate ligands tris(4,4-dimethyl-2-oxazoli-
nyl)phenylborate [ToM]- and tris(4S-isopropyl-2-oxazolinyl)-
phenylborate [ToP]-. Initially, we synthesized oxazolinyl-
borato iridium(I) compounds with the idea that this ligand
platform would allow straightforward structural and electronic
variation to optimize selectivity and probe mechanistic aspects
of stereochemistry in oxidative addition reactions.12
described for chelating pybox (pybox = 2,6-bis(4,4-dimethyl-
2-oxazolinyl)pyridine) and 2,6-diiminopyridines,16,17 as well as
three monodentate pyridines.18 Several of these reactions are
proposed to proceed via nucleophilic attack by an electron-rich
rhodium(I) center.
An additional complication in rhodium(I) and iridium(I)
scorpionate chemistry involves the multiple configurational
isomersthat can affect reactionpathways, including tridentate
coordination and two bidentate boat conformations.19 Like-
wise, because our {ToM}M complexes contain metal and
oxazoline reactive sites and rapidly equilibrating isomers, it
is difficult to predict which product will be formed. Still,
because Tp*RhL2 and MeI react by oxidative addition,5 we
decided to investigate the reactions of R-X electrophiles and
tris(oxazolinyl)borato rhodium dicarbonyl compounds. Here
we report the synthesis of achiral Tl[ToM] (1) and chiral
Tl[ToP] (2) as effective ligand transfer agents as well as the
corresponding achiral ToMRh(CO)2 (3) and enantiopure
ToPRh(CO)2 (4) compounds. Reactions of these rhodium(I)
complexes with electrophiles have been investigated. We
found that metal-centered oxidative additions are thermally
accessible, N-oxazoline- versus metal-based selectivity is
governed by the nature of the electrophile (Scheme 1), and the
enantiopure oxazolinylborato rhodium(I) complex reacts with
high diastereoselectivity.
Instead, we observed N-methylation and N-protonation of
one oxazoline group upon reaction of these tris(oxazolinyl)-
borato iridium(I) compounds with the strong electrophiles
MeOTf and HOTf. A related N-protonation pathway occurs
upon reaction of TpRh(CO)2 and HBF4 Et2O,13 whereas the
3
corresponding iridium compound undergoes a metal-based
protonation (formal oxidative addition) to give [TpIrH(CO)2]-
BF4. In contrast to N-protonation of TpRh(CO)2, the com-
pounds Tp*Rh(CO)L (Tp* = tris(3,5-dimethylpyrazolyl)bo-
rate; L = CO, PMe3, PMe2Ph, PMePh2, PPh3) and MeI react
by oxidative addition to give [Tp*RhI(COMe)L] through an
SN2 pathway.5 Interestingly, a related cationic tris(oxazolinyl)-
ethane rhodium(I) complex [{tris-ox}Rh(C8H12)]BF4 is oxi-
dized by CsBr3, forming {tris-ox}RhBr3.14 Although that
oxidation reaction does not provide a new stereogenic center,
tris-ox ligands provide high enantioselectivity in a range of
catalytic processes,15 including palladium-catalyzed allylic al-
kylation that involves oxidative addition to {tris-ox}Pd[0].15b
Oxidative addition reactions of chlorocarbons and rhodium(I)
compounds containing N,N,N-donor ligands have also been
Experimental Section
General Procedures. All reactions were performed under an
inert atmosphere using standard Schlenk techniques or in a
glovebox unless otherwise indicated. Dry, oxygen-free solvents
were used throughout. Benzene, toluene, pentane, diethyl ether,
and tetrahydrofuran were degassed by sparging with nitrogen,
filtered through activated alumina columns, and stored under N2.
Benzene-d6 and toluene-d8 were vacuum transferred from Na/K
alloy and stored under N2 in the glovebox. All organic reagents
were purchased from Aldrich. Chloroform was distilled from
calcium hydride, and allyl bromide was distilled prior to use.
(12) (a) Dunne, J. F.; Su, J.; Ellern, A.; Sadow, A. D. Organometallics
2008, 27, 2399–2401. (b) Baird, B.; Pawlikowski, A. V.; Su, J.; Wiench,
J. W.; Pruski, M.; Sadow, A. D. Inorg. Chem. 2008, 47, 10208–10210. (c)
Pawlikowski, A. V.; Gray, T. S.; Schoendorff, G.; Baird, B.; Ellern, A.;
Windus, T. L.; Sadow, A. D. Inorg. Chim. Acta 2009, 362, 4517–4525.
(13) Ball, R. G.; Ghosh, C. K.; Hoyano, J. K.; McMaster, A. D.;
Graham, W. A. G. J. Chem. Soc., Chem. Commun. 1989, 341–342.
(14) (a) Gade, L. H.; Bellemin-Laponnaz, S. Chem.;Eur. J. 2008, 14,
4142–4152. (b) Gade, L. H.; Marconi, G.; Dro, C.; Ward, B. D.; Poyatos, M.;
Bellemin-Laponnaz, S.; Wadepohl, H.; Sorace, L.; Poneti, G. Chem.;Eur.
J. 2007, 13, 3058–3075.
20
Li[ToM],12a Li[ToP],12b and [Rh(μ-Cl)(CO)2]2 were prepared
1
by published procedures. H, 11B, and 13C{1H} NMR spectra
(15) (a) Foltz, C.; Stecker, B.; Marconi, G.; Bellemin-Laponnaz, S.;
Wadepohl, H.; Gade, L. H. Chem.;Eur. J. 2007, 13, 9912–9923.
(b) Foltz, C.; Enders, D.; Bellemin-Laponnaz, S.; Wadepohl, H.; Gade,
L. H. Chem.;Eur. J. 2007, 13, 5994–6008. (c) Ward, B. D.; Bellemin-
Laponnaz, S.; Gade, L. H. Angew. Chem., Int. Ed. 2005, 44, 1668–1671.
(d) Dro, C.; Bellemin-Laponnaz, S.; Welter, R.; Gade, L. H. Angew. Chem.,
Int. Ed. 2004, 43, 4479–4482.
(16) Nishiyama, H.; Horihata, M.; Hirai, T.; Wakamatsu, S.; Itoh, K.
Organometallics 1991, 10, 2706–2708.
(17) Haarman, H. F.; Ernsting, J. M.; Kranenburg, M.; Kooijman,
H.; Veldman, N.; Spek, A. L.; van Leeuwen, P. W. N. M.; Vrieze, K.
Organometallics 1997, 16, 887–900.
(18) Bradd, K. J.; Heaton, B. T.; Jacob, C.; Sampanthar, J. T.;
Steiner, A. J. Chem. Soc., Dalton Trans. 1999, 1109–1112.
(19) (a) Bucher, U. E.; Currao, A.; Nesper, R.; Ruegger, H.; Venanzi,
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Chim. Ital. 1995, 125, 181–188. (c) Del Ministro, E.; Renn, O.; R€uegger, H.;
Venanzi, L. M.; Burckhardt, U.; Gramlich, V. Inorg. Chim. Acta 1995, 240,
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(20) McCleverty, J. A.; Wilkinson, G. Inorg. Synth. 1966, 8, 211.

Article
Organometallics, Vol. 29, No. 18, 2010 4107
were collected on Bruker DRX-400 and Avance II-700 spectro-
room temperature for 24 h and then filtered. The solvent was
evaporated under vacuum. The residue was dissolved in diethyl
ether, stirred for 2 h, and then filtered. The filtrate was evapo-
rated to dryness to give ToMRh(CO)2 as a pale green solid in
excellent yield (0.53 g, 0.98 mmol, 92%). X-ray quality crystals
were obtained from slow evaporation of a diethyl ether solution.
1H NMR (benzene-d6, 400 MHz): δ 8.08 (d, 2 H, 3JHH = 7.2 Hz,
ortho-C6H5), 7.51 (t, 2 H, 3JHH = 7.6 Hz, meta-C6H5), 7.34 (t,
1 H, 3JHH = 7.2 Hz, para-C6H5), 3.50 (s, 6 H, CNCMe2CH2O),
1.07 (s, 18 H, CNCMe2CH2O). 13C{1H} NMR (benzene-d6, 150
MHz): δ 188.42 (d, CO, 1JRhC = 66.6 Hz), 135.80 (ortho-C6H5),
127.40 (meta-C6H5), 126.12 (para-C6H5), 79.78 (CNCMe2-
CH2O), 67.60 (CNCMe2CH2O), 28.70 (CNCMe2CH2O). 11B
NMR (benzene-d6, 128 MHz): δ -17.3. 15N{1H} NMR
(benzene-d6, 71 MHz): δ -163.1. ΙR (KBr, cm-1): ν 2966 (s),
2933 (m), 2070 (s, νCO), 2048 (m, νCO), 2010 (s, νCO), 1997 (s,
1
meters. H and 13C{1H} resonances were assigned using stan-
dard 2D methods, including COSY, 1H-13C HMQC, and
1H-13C HMBC experiments. 15N chemical shifts were deter-
mined by 1H-15N HMBC experiments on a Bruker Avance II-
700 spectrometer with a Bruker Z-gradient inverse TXI
1H/13C/15N 5 mm cryoprobe. 15N chemical shifts were originally
referenced to liquid NH3 and recalculated to the CH3NO2 chemical
shift scale by adding -381.9 ppm. 11B NMR spectra were referenced
to an external sample of BF3 Et2O. Elemental analysis was per-
3
formed using a Perkin-Elmer 2400 Series II CHN/S by the Iowa
State Chemical Instrumentation Facility. X-ray diffraction data
were collected on a Bruker APEX2 CCD diffractometer.
Tl[ToM] (1). Thallium nitrate (4.50 g, 16.9 mmol) was placed
in a 500 mL round-bottom flask and dissolved in a water/
methylene chloride mixture (1:1, 200 mL). In a separate flask,
Li[ToM] (5.87 g, 15.1 mmol) was dissolved in a minimal amount
of THF (70 mL). The Li[ToM] solution was added to the Tl[NO3]
solution to produce a white precipitate. This mixture was
vigorously stirred for 30 min, and then the precipitate was
allowed to settle and the liquid separated into aqueous and
organic layers. The aqueous layer was decanted, and the organic
layer was washed with water (3 ꢀ 100 mL) to remove the precipi-
tate. The volatiles from the organic layer were evaporated under
reduced pressure to give a viscous yellow oil. This oil was
triturated with pentane (5 ꢀ 40 mL), precipitating a pure white
product (5.47 g, 9.33 mmol, 61.7%), which was used for further
syntheses. Recrystallization from toluene at -35 °C gave ana-
lytically pure, white crystalline blocks suitable for X-ray diffrac-
tion (4.87 g, 8.32 mmol, 55.0%). 1H NMR (benzene-d6, 400
MHz): δ 8.36 (d, 2 H, 3JHH = 7.6 Hz, ortho-C6H5), 7.58 (t, 2 H,
3JHH = 7.6 Hz, meta-C6H5), 7.37 (t, 1 H, 3JHH = 7.6 Hz, para-
C6H5), 3.41 (s, 6 H, CNCMe2CH2O), 1.03 (s, 18 H, CNC-
Me2CH2O). 13C{1H} NMR (benzene-d6, 100 MHz): δ 190.5
(br, CNCMe2CH2O), 146.7(br, ipso-C6H5), 136.59 (ortho-C6H5),
126.98 (meta-C6H5), 125.56 (para-C6H5), 79.83 (CNCMe2-
CH2O), 66.71 (CNCMe2CH2O), 28.84 (CNCMe2CH2O). 11B
NMR (benzene-d6, 128 MHz): δ -16.0 (br s). 15N{1H} NMR
(benzene-d6, 71 MHz): δ -117.3 (1JTlN = 795 Hz). IR (KBr,
cm-1): ν 3071 (m), 3039 (m), 2963 (s), 2928 (s), 2889 (s), 1600
(s, νCN), 1460 (s), 1430 (m), 1381 (m), 1363 (s), 1348 (m), 1258 (s),
1187 (s), 1133 (s), 1014 (w), 967 (s), 926 (w), 890 (w), 809 (w), 744
(w), 706 (s). Anal. Calcd for C21H29BN3O3Tl: C, 42.99; H, 4.98;
N, 7.16. Found: C, 43.04; H, 4.63; N, 6.74. Mp: 218-220 °C.
Tl[ToP] (2). A suspension of Li[ToP] (1.38 g, 3.20 mmol) and
Tl[OAc] (1.26 g, 4.78 mmol) was stirred in CH2Cl2 (100 mL) at
room temperature overnight. The reaction mixture was then
filtered to remove excess Tl[OAc] and Li[OAc], and the filtrate
was evaporated under reduced pressure. The residue was crys-
tallized from a concentrated diethyl ether solution at -80 °C to
afford 2 as a white powder (1.26 g, 2.00 mmol, 62.6%). 1H NMR
ν
CO), 1968 (m, νCO), 1616 (m, νCN), 1571 (s, νCN), 1361 (m), 1288
(m), 1204 (m), 965 (m). IR (CH2Cl2, cm-1): ν 2963 (m), 2927 (w),
2069 (s, νCO), 2055 (w sh, νCO) 2010 (w), 1994 (s, νCO), 1967 (w),
1616 (w), 1566 (m, νCN), 1461 (w), 1360 (w), 1289 (w), 1202 (m),
963 (m), 736 (m), 706 (m). Anal. Calcd for C23H29BRhN3O5:
C, 51.04; H, 5.40; N, 7.76. Found: C, 51.51; H, 5.41; N, 7.76. Mp:
204-206 °C, dec.
ToPRh(CO)2 (4). A 20 mL vial was charged with 2 (0.21 g,
0.34 mmol), [Rh(μ-Cl)(CO)2]2 (0.07 g, 0.17 mmol), and benzene
(10 mL). The reaction mixture was allowed to stir at room temp-
erature for 6 h and was then filtered. All volatiles were removed
to afford 4 as a dark green solid (0.19 g, 0.33 mmol, 95%).
1H NMR (benzene-d6, 400 MHz): δ 8.15 (d, 2 H, 3JHH = 7.2 Hz,
ortho-C6H5), 7.44 (t, 2 H, 3JHH = 7.4 Hz, meta-C6H5), 7.25 (t,
3
1 H, JHH = 7.4 Hz, para-C6H5), 3.64 (m, 9 H, overlapping
CNC(CHMe2)HCH2O), 2.02 (m, 3 H, CNC(CHMe2)HCH2O),
0.72 (d, 9 H, JHH = 6.8 Hz, CNC(CHMe2)HCH2O), 0.66
3
(d, 9 H, 3JHH = 6.8 Hz, CNC(CHMe2)HCH2O). 13C{1H} NMR
(benzene-d6, 150 MHz): δ 186.11 (d, CO, 1JRhC = 65.0 Hz), 134.86
(ortho-C6H5), 127.68 (meta-C6H5), 126.28 (para-C6H5), 74.51
(CNC(CHMe2)HCH2O), 67.69 (CNC(CHMe2)HCH2O), 32.00
(CNC(CHMe2)HCH2O), 19.28 (CNC(CHMe2)HCH2O), 15.85
(CNC(CHMe2)HCH2O). 11B NMR (benzene-d6, 128 MHz):
δ -17.3. 15N{1H} NMR (benzene-d6, 71 MHz): δ -182.8. ΙR
(KBr, cm-1): ν 2959 (m), 2898 (w), 2873 (w), 2072 (s, νCO), 2002 (s,
νCO), 1572 (m, νCN), 1480 (w), 1464 (w), 1366 (w), 1210 (m), 1180
(w), 1134 (w), 1118 (w), 983 (m), 961 (m), 737 (w). IR (CH2Cl2,
cm-1): ν 2961 (w), 2077 (s, νCO), 2007 (s, νCO), 1575 (m, νCN), 1481
(w), 1366 (w), 1277 (w), 1209 (w), 1118 (w). Anal. Calcd. for
C26H35BRhN3O5: C, 53.54; H, 6.05; N, 7.20. Found: C, 53.91; H,
5.54; N, 6.99. Mp: 80-82 °C, dec.
ToMRh(CO)[P(OMe)3] (5). P(OMe)3 (22 μL, 0.19 mmol) and
ToMRh(CO)2 (100 mg, 0.185 mmol) were dissolved in benzene
(10 mL) and allowed to stir for 10 min. All volatiles were removed
under vacuum to give ToMRh(CO)[P(OMe)3] as a pale yellow solid
in quantitative yield. X-ray quality crystals were obtained by slow
diffusion of pentane into a concentrated toluene solution at -80 °C.
3
(benzene-d6, 400 MHz): δ 8.28 (d, 2 H, JHH = 7.2 Hz, ortho-
C6H5), 7.57 (t, 2 H, JHH = 7.6 Hz, meta-C6H5), 7.35 (t, 1 H,
3
3JHH = 7.2 Hz, para-C6H5), 3.69 (m, 3 H, CNC(CHMe2)-
HCH2O), 3.51 (m, 6 H, CNC(CHMe2)HCH2O), 1.49 (m, 3 H,
CNC(CHMe2)HCH2O), 0.76 (d, 9 H, JHH = 6.8 Hz, CNC-
1H NMR (dichloromethane-d2, 400 MHz): δ 7.27 (d, 2 H, 3JHH
=
6.8 Hz, ortho-C6H5), 7.13 (t, 2 H, 3JHH = 6.8 Hz, meta-C6H5), 7.03
(t, 1 H, 3JHH = 6.4 Hz, para-C6H5), 3.88 (s, 6 H, CNCMe2CH2O),
3
3
3
(CHMe2)HCH2O), 0.67 (d, 9 H, JHH = 6.8 Hz, CNC-
3.54 (d, 9 H, JPH = 12.4 Hz, P(OMe)3), 1.27 (s, 18 H, CNC-
(CHMe2)HCH2O). 13C{1H} NMR (benzene-d6, 150 MHz): δ
136.46 (ortho-C6H5), 127.03 (meta-C6H5), 125.52 (para-C6H5),
71.51 (CNC(CHMe2)HCH2O), 70.07 (CNC(CHMe2)HCH2O),
33.25 (CNC(CHMe2)HCH2O), 19.42 (CNC(CHMe2)HCH2O),
18.65 (CNC(CHMe2)HCH2O). 11B NMR (benzene-d6, 128
MHz): δ -19.2. IR (KBr, cm-1): ν 3046 (m), 2955 (s), 1590 (s,
Me2CH2O). 13C{1H} NMR (benzene-d6, 175 MHz): δ 189.63 (dd,
CO, 1JRhC =69.8Hz, 2JPC =27.5Hz), 135.68(ortho-C6H5), 127.20
(meta-C6H5), 125.46 (para-C6H5), 78.98 (CNCMe2CH2O), 68.16
(CNCMe2CH2O), 51.82 (P(OMe)3), 28.84 (CNCMe2CH2O). 11B
NMR (benzene-d6, 128 MHz): δ -16.4. 31P{1H} NMR (benzene-
1
d6, 162 MHz): δ -90.0 (d, JRhP = 247.7 Hz). IR (KBr, cm-1):
ν
CN), 1465 (m), 1430 (w), 1387 (w), 1367 (w), 1263 (w), 1166 (m),
ν2966 (s), 1995 (s, νCO), 1965(m, νCO), 1610(m, νCN), 1576(s, νCN),
1462 (m), 1368 (m), 1279 (m), 1120 (s), 1017(s), 970 (m). Anal. Calcd
for C25H38BRhN3O7P: C, 47.12; H, 6.01; N, 6.59. Found: C, 47.17;
H, 5.93; N, 6.52. Mp: 159-163 °C, dec.
1111 (m), 1036 (w), 969 (m), 735 (w), 730 (w), 702 (m). Anal.
Calcd for C24H35BN3O3Tl: C, 45.85; H, 5.61; N, 6.68. Found: C,
46.08; H, 5.53; N, 6.61. Mp: 70-73 °C, dec.
ToMRh(CO)2 (3). Benzene (50 mL) was added to a solid
mixture of Tl[ToM] (0.63 g, 1.07 mmol) and [Rh(μ-Cl)(CO)2]2
(0.21 g, 0.54 mmol). The resulting mixture was allowed to stir at
[{N-Me-K2-ToM}Rh(CO)2]OTf (6). A benzene solution of
ToMRh(CO)2 (0.052 g, 0.096 mmol) and MeOTf (0.063 g, 0.35 mmol)
was stirred for 30 min and then evaporated to dryness. The residue

4108 Organometallics, Vol. 29, No. 18, 2010
Ho et al.
was crystallized from a benzene/pentane solution at room tempera-
ture to give green, X-ray quality crystals (0.038 g, 0.053 mmol, 55%).
Trituration of the crystals with pentane provides a white powder
without loss in yield. 1H NMR (dichloromethane-d2, 400 MHz): δ
7.30 (m, 3 H, para- and meta-C6H5), 7.14 (d, 2 H, 3JHH = 6.4 Hz,
ortho-C6H5), 4.48 (s, 2 H, CN(Me)CMe2CH2O), 4.29 (d, 2 H,
3JHH = 9.0 Hz, CN(Rh)CMe2CH2O), 4.25 (d, 2 H, 3JHH = 9.0 Hz,
CN(Rh)CMe2CH2O), 2.90 (s, 3 H, NCH3), 1.51 (s, 6 H, CN-
(Me)CMe2CH2O), 1.44 (s, 6 H, CN(Rh)CMe2CH2O), 1.41 (s, 6 H,
CN(Rh)CMe2CH2O). 13C{1H} NMR (dichloromethane-d2, 175
(d, 1 H, 2JRhH = 3.2 Hz, CHCl2), 7.50 (t, 2 H, 3JHH = 7.6 Hz, meta-
3
C6H5), 7.34 (t, 1 H, JHH = 7.2 Hz, para-C6H5), 3.60 (d, 1 H,
2JHH = 8.4 Hz, cis-CNCMe2CH2O), 3.33 (m, 5 H, CNCMe2-
CH2O), 1.60, (s, 3 H, trans-CNCMe2CH2O), 1.51 (s, 3 H, cis-
CNCMe2CH2O), 1.29 (s, 3 H, cis-CNCMe2CH2O), 1.28 (s, 3 H, cis-
CNCMe2CH2O), 1.11 (s, 3 H, cis-CNCMe2CH2O), 0.63 (s, 3 H,
trans-CNCMe2CH2O). 13C{1H} NMR (benzene-d6, 175 MHz): δ
1
188.42 (d, CO, JRhC = 59.5 Hz), 135.19 (ortho-C6H5), 127.38
(meta-C6H5), 126.70 (para-C6H5), 82.04 (cis-CNCMe2CH2O),
80.77 (cis-CNCMe2CH2O), 79.76 (trans-CNCMe2CH2O), 73.08
(cis-CNCMe2CH2O), 71.41 (cis-CNCMe2CH2O), 68.88 (trans-
1
MHz): δ 183.85 (d, CO, JRhC = 68.3 Hz), 133.35 (ortho-C6H5),
128.77 (meta-C6H5), 127.76 (para-C6H5), 121.54 (q, 1JFC = 319 Hz,
OSO2CF3), 82.19 (CN(Me)CMe2CH2O), 81.25 (CN(Rh)CMe2-
CH2O), 69.21 (CN(Rh)CMe2CH2O), 67.30 (CN(Me)CMe2-
CH2O), 30.16 (NCH3), 28.65 (CN(Rh)CMe2CH2O), 28.25
(CN(Rh)CMe2CH2O), 24.45 (CN(Me)CMe2CH2O). 11B NMR
(dichloromethane-d2, 128 MHz): δ -17.3. 15N{1H} NMR
(dichloromethane-d2, 71 MHz): δ -179.7 (NRh), -211.0 (NMe).
IR (KBr, cm-1): ν 3074 (m), 2971 (s), 2083 (s, νCO), 2012 (s, νCO),
1580 (s, νCN), 1551 (m, νCN), 1462 (s), 1434 (m), 1390 (m), 1372 (m),
1362 (m), 1325 (m), 1262 (s), 1224 (s), 1205 (s), 1157 (s), 1031 (s),
998 (m), 964 (s), 748 (m), 736 (m), 710 (m). Anal. Calcd for
C24H32N3O8SF3RhB: C, 41.58; H, 4.65; N, 6.06. Found: C, 42.00;
H, 4.56; N, 5.70. Mp: 132-134 °C, dec.
CNCMe2CH2O), 63.74 (d, CHCl2, JRhC = 26.3 Hz), 28.87 (cis-
1
CNCMe2CH2O), 28.71 (cis-CNCMe2CH2O), 27.48 (trans-CNC-
Me2CH2O), 26.96 (cis-CNCMe2CH2O), 23.68 (trans-CNCMe2-
CH2O), 21.08 (cis-CNCMe2CH2O). 11B NMR (benzene-d6, 128
MHz): δ -17.8. 15N{1H} NMR (benzene-d6, 71 MHz): δ -177.6
(cis), -187.8 (cis), -198.0 (trans). IR (KBr, cm-1): ν 3076 (m), 3042
(m), 2985 (s), 2969 (s), 2890 (s), 2088 (s, νCO), 2042 (w), 2037 (w),
1575 (s, νCN), 1548 (m), 1496 (m), 1464 (s), 1433 (m), 1388 (s), 1358
(s), 1293 (s), 1273 (s), 1251 (s), 1199 (s), 1155 (s), 1114 (s), 1036 (m),
1024 (m), 957 (s), 933 (s), 891 (m), 800 (m), 785 (m). Anal. Calcd for
C23H30BN3O4Cl3BRh: C, 43.67; H, 4.78; N, 6.64. Found: C, 43.20;
H, 4.52; N, 6.46. Mp: 156-159 °C, dec.
(K3-ToP)Rh(CHCl2)Cl(CO) (9). A solution ToPRh(CO)2
(0.102 g, 0.175 mmol) in CHCl3 (20 mL) was degassed by three
freeze-pump-thaw cycles and then allowed to stir overnight at
room temperature. All volatiles were evaporated and the residue
was purified by silica gel column chromatography, eluting with
hexane/ethyl acetate (85:15) to yield a yellow solid (0.060 g,
0.089 mmol, 51%). 1H NMR (CDCl3, 400 MHz, cis and trans
designations for oxazoline rings are given with respect to CHCl2
moiety): δ 7.65 (d, 2 H, 3JHH = 7.6 Hz, ortho-C6H5), 7.31 (t, 2 H,
(K3-ToM)Rh(η1-C3H5)Br(CO) (7). Excess allyl bromide (1.9 mL)
and ToMRh(CO)2 (0.114 g, 0.211 mmol) were allowed to react in
benzene for 5 h at room temperature. The solution was filtered, the
volatiles were evaporated, and the solid residue was crystallized from
a concentrated acetonitrile solution at -30 °C overnight to give
a yellow solid (0.063 g, 0.098 mmol, 47%). 1H NMR (benzene-d6,
400 MHz, cis and trans designations for oxazoline rings are given
3
with respect to η1-C3H5 moiety): δ 8.25 (d, 2 H, JHH = 7.2 Hz,
3
3JHH = 7.2 Hz, meta-C6H5), 7.25 (t, 1 H, JHH = 7.2 Hz,
ortho-C6H5), 7.53 (t, 2 H, 3JHH = 7.2 Hz, meta-C6H5), 7.35 (t, 1 H,
3JHH = 7.2 Hz, para-C6H5), 6.99 (m, 1 H, RhCH2CHCH2), 5.58
para-C6H5), 7.11 (d, 1 H, 2JRhH = 3.2 Hz, CHCl2), 4.95 (m, 1 H,
cis-CNC(CHMe2)HCH2O), 4.84 (m, 1 H, cis-CNC(CHMe2)H-
CH2O), 4.38 (m, 4 H, overlapping cis- and trans-CNC(CHMe2)-
(d, 1 H, 3JHH = 16.8 Hz, RhCH2CHCH2), 5.24 (d, 1 H, 3JHH
=
9.6 Hz, RhCH2CHCH2), 4.57 (br, 1 H, RhCH2CHCH2), 3.66 (d,
1 H, 2JHH = 8.4 Hz, cis-CNCMe2CH2O), 3.64 (br, 1 H, RhCH2-
CHCH2), 3.40 (d, 1 H, 2JHH = 8.0 Hz, cis-CNCMe2CH2O), 3.35
2
HCH2O), 4.28 (t, 1 H, JHH = 9.6 Hz, trans-CNC(CHMe2)-
HCH2O), 4.09 (t, 1 H, 2JHH = 9.6 Hz, cis-CNC(CHMe2)HCH2O),
4.07 (m, 1 H, trans-CNC(CHMe2)HCH2O), 2.84 (m, 1 H, trans-
CNC(CHMe2)HCH2O), 2.37(m, 1H, cis-CNC(CHMe2)HCH2O),
2.17 (m, 1 H, cis-CNC(CHMe2)HCH2O), 1.02 (d, 3 H, 3JHH = 6.8
(d, 1 H, 2JHH = 8.4 Hz, cis-CNCMe2CH2O), 3.24 (d, 1 H, 2JHH
=
8.0 Hz, cis-CNCMe2CH2O), 3.21 (s, 2 H, trans-CNCMe2CH2O),
1.50 (s, 3 H, cis-CNCMe2CH2O), 1.44 (s, 3 H, cis-CNCMe2CH2O),
1.33 (s, 3 H, cis-CNCMe2CH2O), 1.31 (s, 3 H, cis-CNCMe2CH2O),
1.04 (s, 3 H, trans-CNCMe2CH2O), 0.81 (s, 3 H, trans-CNC-
Me2CH2O). 13C{1H} NMR (benzene-d6, 175 MHz, cis and trans
designations for oxazoline groups are given with respect to η1-C3H5
3
Hz, cis-CNC(CHMe2)HCH2O), 0.92 (d, 3 H, JHH = 6.8 Hz,
cis-CNC(CHMe2)HCH2O), 0.86 (d, 3 H, JHH = 7.2 Hz, trans-
3
3
CNC(CHMe2)HCH2O), 0.75 (d, 3 H, JHH = 6.4 Hz, cis-CNC-
(CHMe2)HCH2O), 0.65 (d, 3 H, 3JHH = 6.8 Hz, cis-CNC(CHMe2)-
3
1
HCH2O), 0.56 (d, 3 H, JHH = 6.8 Hz, trans-CNC(CHMe2)-
moiety): δ 186.45 (d, CO, JRhC = 59.5 Hz), 148.03 (RhCH2-
HCH2O). 13C{1H} NMR (CDCl3, 175 MHz): δ 179.71 (d, CO,
1JRhC = 59.5 Hz), 134.84 (ortho-C6H5), 127.22 (meta-C6H5), 126.63
(para-C6H5), 71.73 (trans-CNC(CHMe2)HCH2O), 70.71 (cis-
CNC(CHMe2)HCH2O), 70.25 (trans-CNC(CHMe2)HCH2O),
69.69 (cis-CNC(CHMe2)HCH2O), 68.42 (cis-CNC(CHMe2)-
CHCH2), 136.34 (ortho-C6H5), 127.32 (meta-C6H5), 126.44 (para-
C6H5), 112.97 (RhCH2CHCH2), 81.94 (cis-CNCMe2CH2O), 81.22
(cis-CNCMe2CH2O), 80.62 (trans-CNCMe2CH2O), 73.06 (cis-CN-
CMe2CH2O), 70.65 (cis-CNCMe2CH2O), 69.30 (trans-CNCMe2-
CH2O), 29.06 (cis-CNCMe2CH2O), 28.88 (cis-CNCMe2CH2O),
27.19 (cis-CNCMe2CH2O), 27.10 (trans-CNCMe2CH2O), 26.62
(trans-CNCMe2CH2O), 25.92 (cis-CNCMe2CH2O), 20.53 (d, Rh-
CH2CHCH2,1JRhC = 15.8Hz). 11B NMR (benzene-d6, 128 MHz):δ
-18.0. 15N{1H} NMR (benzene-d6, 71MHz):δ-163.5 (cis),-187.8
(cis), -203.5 (trans). IR (KBr, cm-1): ν 2967 (m), 2930 (m), 2058
(s, νCO), 1582 (s, νCN), 1462 (m), 1387 (w), 1367 (m), 1290 (m), 1203
(m), 968 (m). Anal. Calcd for C25H34N3O4RhBBr: C, 47.35; H, 5.40;
N, 6.63. Found: C, 47.09; H, 5.39; N, 6.58. Mp: 174-178 °C, dec.
(K3-ToM)Rh(CHCl2)Cl(CO) (8). A solution of ToMRh(CO)2
(0.107 g, 0.20 mmol) in CHCl3 (30 mL) was degassed by three
freeze-pump-thaw cycles and then heated at 60 °C for 18 h.
The reaction mixture was allow to cool to ambient temperature,
and then it was filtered. The solvent was removed from the
filtrate under reduced pressure, and the residue was extracted with
toluene (10 mL) and evaporated to dryness. The resulting solid was
washed with CH3CN (ca. 1 mL) at -30 °C to give an off-white solid
(0.054 g, 0.080 mmol, 43%). 1H NMR (benzene-d6, 400 MHz, cis
and trans designations for oxazoline rings are given with respect
to CHCl2 moiety): δ 8.18 (d, 2 H, 3JHH = 7.2 Hz, ortho-C6H5), 7.81
1
HCH2O), 65.67 (d, CHCl2, JRhC = 28.0 Hz), 65.14 (cis-CNC-
(CHMe2)HCH2O), 29.17 (cis-CNC(CHMe2)HCH2O), 29.01 (cis-
CNC(CHMe2)HCH2O), 28.64 (trans-CNC(CHMe2)HCH2O),
20.03 (cis-CNC(CHMe2)HCH2O), 19.17 (cis-CNC(CHMe2)-
HCH2O), 18.81 (trans-CNC(CHMe2)HCH2O), 14.71 (cis-CNC-
(CHMe2)HCH2O), 14.00 (cis-CNC(CHMe2)HCH2O), 13.82
(trans-CNC(CHMe2)HCH2O). 11B NMR (CDCl3, 128 MHz):
δ -18.1. 15N{1H} NMR (CDCl3, 71 MHz): δ -194.2 (trans-
CNC(CHMe2)HCH2O), -204.3 (cis-CNC(CHMe2)HCH2O),
-217.7 (cis-CNC(CHMe2)HCH2O). IR (KBr, cm-1): ν 3001 (w),
2962 (m), 2929 (m), 2872 (m), 2092 (s, νCO), 1590 (s, νCN), 1479
(w), 1463 (w), 1372 (w), 1364 (w), 1223 (m). Anal. Calcd for
C26H36BN3O4RhCl3: C, 46.29; H, 5.38; N, 6.23; Found: C, 46.11;
H, 5.03; N, 5.99. Mp: 116-118 °C, dec.
Results and Discussion
Synthesis and Characterization of Tl[ToM] (1), Tl[ToP] (2),
ToMRh(CO)2 (3),andToPRh(CO)2 (4).Following an adaptation

Article
Organometallics, Vol. 29, No. 18, 2010 4109
complexes of [ToM]-, where the Tl1-N1 and Tl1-N2 dis-
˚
tances are approximately 0.05 A longer than the Tl1-N3
distance. This distortion is apparently related to the relative
orientation of the phenyl group that is approximately per-
pendicular to the N3-containing oxazoline ring (torsion angle
C17-C16-B1-C11 = 91.72°; C21-C16-B1-C11 = 81.23°)
and pointing at the other two oxazoline rings (C17-C16-
B1-C1 = 30.54°; C21-C16-B1-C6 = 38.42°). This orienta-
tion breaks the C3v symmetry in the crystalline state and results
in longer Tl1-N1 and Tl1-N2 distances versus Tl1-N3.
Although this effect was observed in other {κ3-ToM}M com-
pounds, competing steric effects of the substituents on the metal
center also affect M-N bond distances, obscuring the structur-
al impact of the phenyl group.21a This posterior phenyl effect is
most pronounced in 1 because the Tl center is three coordinate.
Attempts to prepare 2 from Li[ToP] and Tl[NO3], using a
modification of the procedure that provides Tl[ToM], were
unsuccessful. Additionally, Tl[PF6] and Li[ToP] or K[ToP]
did not afford isolable 2. The most efficient synthetic proce-
dure involves reaction of Li[ToP] and Tl[OAc] in methylene
chloride to provide 2 (eq 2).
Figure 1. ORTEP diagram of Tl[ToM] (1). H atoms are not
shown. Relevant bond distances (A): Tl1-N1, 2.550(4);
Tl1-N2, 2.543(5); Tl1-N3, 2.498(5).
˚
of Venanzi0s preparation of tris(pyrazolyl)borato rhodium(I)
dicarbonyl,19a,c reaction of Li[ToM] and [Rh(μ-Cl)(CO)2]2 in
acetonitrile at -40 °C yields ToMRh(CO)2 (3), but several
impurities also are formed. We previously found that reaction
of 2 equiv of Li[ToM] and [Ir(μ-Cl)(C8H12)]2 in benzene or
THF provides a dimeric LiCl adduct, (LiCl)2[(κ2-ToMIr-
(η4-C8H12)]2.12c We suspected that LiCl was interfering in the
synthesis of the corresponding rhodium(I) compounds. There-
fore, 1 was prepared as an alternative [ToM]-transfer agent by
reaction of Li[ToM] and Tl[NO3] (eq 1).
We were not able to detect a cross-peak in a 1H-15N HMBC
experiment, nor could an X-ray quality single crystal of 2 be
obtained. However, a single νCN band in the infrared spec-
trum of 2 (1590 cm-1) suggests that all three oxazolines are
coordinated to the thallium center as in 1. Additionally, the
1
4S-isopropyl-2-oxazoline groups are equivalent in the H
NMR spectrum (benzene-d6). These data also indicate that the
compound has not epimerized and is enantiomerically pure.
Reaction of 1 and 0.5 equiv of [Rh(μ-Cl)(CO)2]2 provides 3
in excellent yield (eq 3).
The 1H NMR spectrum of 1 in benzene-d6 contained singlet
resonances for methyl and methylene groups. This spectrum is
consistent with a freely rotating phenyl group and a pseudo C3v-
symmetric structure. Cross-peaks were detected between the
oxazoline nitrogen and the ring methyl and methylene groups
1
in a H-15N HMBC experiment (performed with 15N natural
abundance) that provided the 15N NMR chemical shift (-117
ppm) and 1JTlN = 795 Hz. These data are consistent with N,N,N-
coordination to thallium. For comparison, the 15N NMR chemi-
cal shift of 2H-4,4-dimethyl-2-oxazoline is -127.9.12c The down-
field 15N NMR chemical shift for 1 contrasts the upfield chemical
shifts of all of the transition-metal compounds containing the
[ToM]- ligand that we have prepared, measured, and
reported.12,21 Only one νCN band (1600 cm-1) for the three
oxazoline groups was observed in the infrared spectrum of 1.
These IR data also provide support for the coordination of all
three oxazolines to thallium. An X-ray crystal structure confirmed
the tridentate coordination mode in the solid state (Figure 1).
In addition to verifying the connectivity of 1, the X-ray
structure shows a structural effect of the posterior phenyl
group on the bond lengths and angles in coordination
1
The oxazoline groups in 3 are equivalent on the H NMR
time scale, as in the previously reported ToMIrL2 and
ToPIrL2 compounds.12b,c Cross-peaks in a 1H-15N HMBC
experiment provided the 15N NMR chemical shift for 3
(-163 ppm), which is upfield of noncoordinated oxazoline.
Five carbonyl bands were observed in the infrared spec-
trum of 3 (see Table 1). The IR bands indicate that 3 is a
mixture of three isomers in which the tris(oxazolinyl)borate
ligands are bi- and tridentate, and these species interconvert
(21) (a) Pawlikowski, A. V.; Ellern, A.; Sadow, A. D. Inorg. Chem.
2009, 48, 8020–8029. (b) Dunne, J. F.; Manna, K.; Wiench, J. W.; Ellern, A.;
Pruski, M.; Sadow, A. D. Dalton Trans. 2010, 39, 641–653.

4110 Organometallics, Vol. 29, No. 18, 2010
Ho et al.
Table 1. Infrared Spectroscopic Data for Tris(oxazolinyl)borato Rhodium and Related Tris(pyrazolyl)borato
Rhodium Carbonyls
compound
ToMRh(CO)2 (3)
ν
CO (KBr, cm-1
)
ν )
CN (KBr, cm-1
κ2: 2070, 2010, 1997
κ3: 2048, 1968
κ2: 2072, 2002
κ2: 1995
noncoordinated: 1616
Rh coordinated: 1571
Rh coordinated: 1575
noncoordinated: 1610
Rh coordinated: 1576
1644, 1643, 162314
n.a.
ToPRh(CO)2 (4)
ToMRh(CO)[P(OMe)3] (5)
κ3: 1965
n.a.
[Rh{iPr-trisox}(η4-C8H12)][BF4]14
Tp*Rh(CO)2
κ2: 2108, 207319c
κ2: 2083, 201219c
κ3: 2051, 197219c
κ3: 2052, 197419a
κ3: 2061, 198119a
κ2: 2083, 2017
κ3: 2059, 1984
2083, 2012
2058
19
TpMeRh(CO)2
n.a.
n.a.
19
TpMe2ClRh(CO)2
26
[{N-Me-κ2-ToM}Rh(CO)2]OTf (6)
(κ3-ToM)Rh(η1-C3H5)Cl(CO) (7)
(κ3-ToM)Rh(CHCl2)Cl(CO) (8)
(κ3-ToP)Rh(CHCl2)Cl(CO) (9)
TpMe2ClRh(CHCl2)Cl(CO)6
1580, 1551
1582
1575
1590
n.a.
2088
2092
21106
rapidly on the 1H NMR time scale. Similar mixtures of
bidentate and tridentate tris(pyrazolyl)borate coordinate
modes were reported for Tp*Rh(CO)2 based on infrared
analysis,19 and these data allow a comparison of the electron-
donating properties of ToM and Tp* ligands in this rhodium
system. The symmetric normal modes for {κ2-ToM}Rh(CO)2
(2070 cm-1) and {κ2-Tp*}Rh(CO)2 (2083 cm-1 19c
suggest
)
that the oxazolinylborate ligand is more electron donating
than Tp*. Likewise, the bands for the tridentate isomer {κ3-
ToM}Rh(CO)2 (2048 and 1968 cm-1) suggest a rhodium(I)
center more electron rich than in {κ3-Tp*}Rh(CO)2 (2052 and
1974 cm-1). These comparisons are complicated by conforma-
tional changes in coordination modes; however the trend that
[ToM] is more electron donating than Tp* is also consistent
with the νCO obtained from the solution-phase IR spectra.
The presence of {κ2-ToM}Rh(CO)2 in the mixture is
also supported by the νCN of the oxazoline moiety. Two
νCN bands were observed in the IR spectrum of 3 corre-
sponding to coordinated (1571 cm-1) and noncoordinated
(1616 cm-1) oxazoline rings. For comparison, the νCN band
for 2H-4,4-dimethyl-2-oxazoline is 1630 cm-1 22
.
Although the carbonyl region of the infrared spectrum
suggests that two or three isomers are present in 3,19 an X-ray
diffraction study showed one {κ2-ToM}Rh(CO)2 isomer
(Figure 2). This {κ2-ToM}Rh(CO)2 is the first crystallo-
graphically characterized neutral (i.e., nonmethylated and
nonprotonated), monovalent group 9 tris(oxazolinyl)borate
compound. For comparison, tris(pyrazolyl)borato rhodium(I)
compounds crystallize with tridentate and/or bidentate coordi-
nation geometries.19,23-27 The crystallized structure contains
Figure 2. ORTEP diagram of ToMRh(CO)2 (3) drawn at 35% pro-
˚
bability. Hydrogen atoms are not included. Bond distances (A):
Rh1-C22, 1.851(2); Rh1-C23, 1.854(2); Rh1-N1, 2.087(1);
Rh1-N2, 2.096(2). Bond angles (deg): C22-Rh1-C23,
86.14(8); N1-Rh1-N2, 92.28(7); N1-Rh1-C23, 178.37(7);
˚
N2-Rh1-C22, 178.52(7). Nonbonding distances (A): Rh1-
N3, 3.43; Rh1-O3, 3.82; Rh1-C15, 3.20.
P
L-Rh-L0
a square-planar rhodium center (
= 359.99°). The
(22) Meyers, A. I.; Collington, E. W. J. Am. Chem. Soc. 1970, 92,
6676–6678.
(23) (a) Hessell, E. T.; Jones, W. D. Organometallics 1992, 11, 1496–
1505. (b) Jones, W. D.; Hessell, E. T. J. Am. Chem. Soc. 1993, 115, 554–562.
(c) Northcutt, T. O.; Lachicotte, R. J.; Jones, W. D. Organometallics 1998,
17, 5148–5152. (d) Wick, D. D.; Northcutt, T. O.; Lachicotte, R. J.; Jones,
W. D. Organometallics 1998, 17, 4484–4492.
(24) (a) Akita, M.; Ohta, K.; Takahashi, Y.; Hikichi, S.; Moro-oka,
Y. Organometallics 1997, 16, 4121–4128. (b) Akita, M.; Hashinoto, M.;
Hikichi, S.; Moro-oka, Y. Organometallics 2000, 19, 3744–3747.
(25) Rheingold, A. L.; Ostrander, R. L.; Haggerty, B. S.; Trofimenko, S.
Inorg. Chem. 1994, 33, 3666–3676.
(26) Malbosc, F.; Chauby, V.; Berre, C. S.-L.; Etienne, M.; Daran,
J.-C.; Kalck, P. Eur. J. Inorg. Chem. 2001, 2689–2697.
(27) (a) Ruman, T.; Ciunik, Z.; Wolowiec, S. Polyhedron 2004, 23,
219–223. (b) Criado, R.; Cano, M.; Campo, J. A.; Heras, J. V.; Pinilla, E.;
Torres, M. R. Polyhedron 2004, 23, 301–309.
pendant, noncoordinated oxazoline blocks one face of the
square-planar complex. That oxazoline is oriented such that its
oxygen and nitrogen atoms have roughly similar distances to the
˚
˚
rhodium center (Rh O: 3.82 A; Rh N: 3.42 A). Chelation
3 3 3
3 3 3
forms a six-membered ring, and the B1-C5-N1-Rh1-
N2-C10 atoms in the ring adopt a boat conformation. The
pendant oxazoline is pseudoaxial, and the phenyl group is
pseudoequatorial.
Reaction of 2 and 0.5 equiv of [Rh(μ-Cl)(CO)2]2 provides
4. As observed in 3, the oxazoline groups are equivalent in
the 1H NMR spectrum at room temperature. The 15N NMR
chemical shift (-182.8 ppm) was similar to that observed for
ToPIr(CO)2 (-184 ppm). In contrast to 3, only two carbonyl

Article
Organometallics, Vol. 29, No. 18, 2010 4111
bands were observed in the infrared spectrum with relative
energies that suggest a κ2-ToPRh interaction.19 Strangely,
only one νCN band at 1575 cm-1 due to coordinated oxazo-
line was detected; a band consistent with noncoordinated
oxazoline (ca. 1630-1610 cm-1) was not evident.
A related cationic chiral compound [{κ2-iPr-trisox}Rh-
(η4-C8H12)][BF4] was reported by Gade and co-workers, in
which the neutral tris(oxazolinyl)ethane ligand forms a
bidentate chelate with rhodium(I).14 This bidentate coordi-
nation mode is observed in an X-ray crystal structure, but
several νCN bands in the infrared spectrum (1664, 1643, and
1623 cm-1) and equivalent oxazoline groups in the solution-
phase 1H NMR spectrum suggest that additional conforma-
tions are present. It is interesting as well that the νCN bands in
the cationic iPr-trisox rhodium(I) are 48-89 cm-1 higher in
energy than the zwitterionic 4. Thus, the zwitterionic char-
acter in compound 4 likely significantly perturbs the electro-
nic interaction between the oxazoline and the metal center.
Reactivity of ToMRh(CO)2 (3) and ToPRh(CO)2 (4). Ox-
azoline N-Methylation and Substitution Reactions. TpRh-
(CO)2 reacts with phosphines to give mixed TpRh(CO)-
(PR3).5,28 In the [ToM]-rhodium system, attempts to substitute
one carbonyl ligand with a phosphine were not successful
(PPh3: no reaction up to 80 °C for 5 h; PMe3: multiple
products formed immediately after addition, as determined
Figure 3. ORTEP diagram of ToMRh(CO)[P(OMe)3] (5).
Hydrogen atoms are not depicted.
1
by H NMR spectroscopy). However treatment of 3 with
the phosphite P(OMe)3 rapidly produces ToMRh(CO)-
[P(OMe)3] (5), showing that one of the carbonyl ligands
can be substituted (eq 4). The three oxazolines are equi-
Specifically, the Rh1-P1 bond is coplanar with the C2-C22
bond in the oxazoline cis to the phosphite (torsion angle
P1-Rh1-C2-C22: 1.74°); the Rh1-C26 bond is coplanar
with the C9-C11 bond in the oxazoline cis to the carbonyl
ligand (torsion angle C26-Rh1-C9-C11: 0.24°). These two
oxazoline methyl groups (C22 and C11) are disposed syn to
each other. The other two oxazoline methyl groups (C3 and
C10) are pseudoaxial and are both directed toward the
opposite face of the rhodium center with respect to the
noncoordinated oxazoline.
1
valent in the H NMR spectrum at room temperature due
to rapid exchange (as in 3). Both bidentate and tridentate
tris(oxazolinyl)borate binding modes are present in 5 on
the basis of its observed infrared spectrum (νCO: 1995 and
1965 cm-1; νCN: 1610 and 1576 cm-1).
Compound 3 reacts with MeOTf (3.5 equiv, methylene
chloride solvent, room temperature) to give an N-methylated
oxazoline rather than a rhodium(III) methyl complex (eq 5).
The connectivity of the N-methylated product was un-
ambiguously identified by a 1H-15N HMBC experiment
that showed a cross-peak between a nitrogen signal at
-211.0 ppm and a methyl resonance at 2.90 ppm. A cross-
peak between the N-methylated oxazoline nitrogen reso-
nance and the methylene singlet in the 1H-15N HMBC
experiment was also observed, as expected for a Cs-sym-
metric compound. The chemical shift of the two equivalent
rhodium-coordinated oxazolines was observed as a cross-
peak at -176.7 ppm. In the 1H NMR spectrum, three
methylene signals from the oxazolines were evident as one
singlet and two diastereotopic doublets. Symmetric and
asymmetric νCO bands were apparent in the product’s infra-
red spectrum, and no acyl signals were visible. Furthermore,
the νCO frequencies do not change significantly from 3 after
methylation, arguing against a Rh(I) to Rh(III) oxidation. A
single-crystal X-ray structure, shown in Figure 4, confirmed
As in the single-crystal structure of 3, the X-ray structure
of 5 shown in Figure 3 contains a square-planar rhodium
P
L-Rh-L0
center (
= 360.2°). In the latter compound, the
Rh-N distances are slightly longer (Rh1-N1, 2.117(2);
˚
Rh1-N3, 2.118(2) A) than the distances in the former (Rh1-
˚
N1, 2.087(1); Rh1-N2, 2.096(2) A). The nonbonding Rh-N
˚
˚
distance is longer as well (3.69 A versus 3.43 A), and these
longer distances are most likely due to steric effects. As in 3,
the six-membered chelate rings in the structure of compound
5 form the boat configuration. The relative disposition of the
oxazoline methyl groups are best described by their spatial
relationship with the phosphite and the carbonyl ligands.
(28) (a) Connelly, N. G.; Emslie, D. J. H.; Metz, B.; Orpen, A. G.;
Quayle, M. J. Chem. Commun. 1996, 2289–2290. (b) Connelly, N. G.;
Emslie, D. J. H.; Geiger, W. E.; Hayward, O. D.; Linehan, E. B.; Orpen, A. G.;
Quayle, M. J.; Rieger, P. H. J. Chem. Soc., Dalton Trans. 2001, 670–683.

4112 Organometallics, Vol. 29, No. 18, 2010
Ho et al.
Figure 5. Newman projection of one of two enantiomers of the
racemic mixture of C1-symmetric (κ3-ToM)Rh(η1-C3H5)Br(CO)
(7) viewed along the Rh-B bond. Through-space close contacts
between oxazoline methyl groups and the σ-allyl ligand, de-
tected using a NOESY experiment, are illustrated with arrows.
Through-bond coupling between an oxazoline nitrogen and
1
σ-allyl ligand is observed with a H-15N HMBC experiment
and is highlighted in blue.
Additionally, the 13C{1H} NMR spectrum of 7 contained a
single carbonyl resonance at 186.5 ppm (1JRhC = 60 Hz). In
contrast to the Cs-symmetric, N-alkylation product formed from
MeOTf and 3, compound 7 is C1 symmetric. The 4,4-dimethyl
substituents on the oxazoline rings are inequivalent and provide
Figure 4. ORTEP diagram of the cationic portion of [{N-Me-
κ2-ToM}Rh(CO)2]þ (6). Hydrogen atoms, the OTf- counteran-
ion, and a disordered solvent molecule are not included in the
representation.
1
six singlet resonances in the H NMR spectrum. In a COSY
experiment, the multiplet signal of the internal vinylic proton on
the η1-allyl (6.99 ppm) correlated with the terminal vinylic
resonances (doublets at 5.58 and 5.24 ppm) and with the allylic
that the connectivity established in solution is maintained in
the solid state.
1
Given our previous observations with the tris(oxazolinyl)-
borato iridium(I) compounds ToMIr(CO)2, ToPIr(CO)2, and
ToPIr(η4-C8H12),12 N-alkylation is not a surprising reaction
pathway. However, this reactivity directly contrasts the
oxidative addition reactions of Tp*RhL(CO) and MeI that
give [Tp*RhMe(L)(CO)]I.5 Considering the oxidative addi-
tion chemistry of Tp*Rh(CO)2 and the comparable electron
richness of 3 with the Tp analogues, it is difficult to rationa-
lize the lack of oxidative addition reactivity for either the
tris(oxazolinyl)borato rhodium(I) or iridium(I) compounds.
For this reason, we explored reactions of 3 with other
electrophiles.
Oxidative Addition Reactions of ToMRh(CO)2 (3) and
ToPRh(CO)2 (4). In contrast to the N-methylation reactions
described above, treatment of 3 with a large excess of allyl
bromide (1.9 mL) in benzene gives the oxidative addition
product (κ3-ToM)Rh(η1-C3H5)Br(CO) (7) after 5 h (eq 6). A
large excess is necessary, as 1, 2, and 4 equiv give mixtures of
7 and unidentified species. The crude complex was purified
by recrystallization from a concentrated acetonitrile solution
at -30 °C.
CH2 group (4.57 and 3.64 ppm). A H NOESY experiment
provided evidence that these two diastereotopic allylic protons
and the four methyl groups on the oxazolines are in close
proximity, and therefore those oxazolines are cis to the η1-
C3H5 ligand. The dimethyl of the remaining oxazoline did not
show through-space coupling to the σ-allyl, and this oxazoline
was assigned as trans (as illustrated by the Newman projection in
1
Figure 5). These assignments were supported by a H-15N
HMBC experiment that showed correlations between those
dimethyls (1.04 and 0.81 ppm) and the same oxazoline nitrogen
(-203.5 ppm), proving that the methyl groups are on the same
oxazoline ring. Cross-peaks between the other two oxazoline
nitrogens (-163.5 and -187.8 ppm) and the four cis methyl
groups complete the possible oxazoline assignment since CO and
Br ligands are not NMR active. We also observed weak cross-
peaks in the 1H-15N HMBC experiment between the nitrogen
on the trans oxazoline and the allylic CH2CHdCH2 resonances.
Finally, the infrared spectrum further supports the assigned
structure, as only one carbonyl band (2058 cm-1) and no
RhC(dO)R bands were observed.
Likewise, reaction of 3 and chloroform results in C-Cl
oxidative addition rather than oxazoline alkylation (eq 7). A
related thermal reaction of TpMe2ClRh(CO)2 and chloroform
affords TpMe2ClRh(CHCl2)Cl(CO), but Tp*Rh(CO)2 and
CHCl3 do not provide a C-Cl oxidative addition product under
reported thermal conditions.6 However, Jones has described
oxidative addition reactions of Tp*RhL2 (L = CNCH2CMe3)
and CHCl3 that occur upon photolysis, and presumably a
photon mediates the Rh-L dissociation as the first step.30
Oxidative addition and Rh-C bond formation to give an
η1-C3H5 ligand is supported by the 13C{1H} NMR spectrum
that contained a resonance at 20.5 ppm (1JRhC = 16 Hz) for
the σ-bonded carbon of the η1-allyl moiety. In contrast,
Rh-C coupling was not observed in the N-methylated 6. A
similar spectral signature to that observed for 7 was reported
for Tp*Rh(η1-C3H5)Br(NCMe) (δRhC = 18.0; 1JRhC =18Hz).29
(29) Ikeda, S.; Maruyama, Y.; Ozawa, F. Organometallics 1998, 17,
3770–3774.
(30) Vetter, A. J.; Rieth, R. D.; Brennessel, W. W.; Jones, W. D.
J. Am. Chem. Soc. 2009, 131, 10742–10752.

Article
Like 7, compound 8 is C1-symmetric. This symmetry was
Organometallics, Vol. 29, No. 18, 2010 4113
15N chemical shift to the ligand field of its trans-disposed ligand
can be currently identified. For example, the most downfield 15N
NMR chemical shift in 9 was the oxazoline nitrogen trans to
CHCl2, whereas the oxazoline trans to the dichlorohydrocarbyl
ligand was the farthest upfield in 8.
reflected in the 1H NMR spectrum that contained six singlet
resonances for six inequivalent methyl groups on three
inequivalent oxazolines. Evidence for the structure of 8
was provided by a doublet resonance in the 1H NMR
spectrum at 7.81 ppm (2JRhH = 3.2 Hz) for the RhCHCl2
moiety. A 1H-13C HMQC experiment showed a correlation
between the CHCl2 resonance and the CHCl2 at 63.7 ppm for
the rhodium carbyl (1JRhC = 26 Hz). Through-space cou-
pling between the CHCl2 signal and oxazoline methyl reso-
nances was observed in a NOESY experiment, and these
methyls were thus assigned to the oxazoline groups cis to the
CHCl2 ligand. A 1H-15N HMBC experiment contained
cross-peaks for the three different oxazoline nitrogens at
-198.0, -187.8, and -177.6 ppm, respectively. The most
upfield resonance is correlated to the oxazoline trans to the
CHCl2 group (assigned using a NOESY); in 7, the oxazoline
trans to the allyl group is also most upfield (-203 ppm). The
other two oxazoline nitrogens correlate with the methyl groups
assigned as cis using data from the NOESY experiment.
Unexpectedly, the 1H-15N HMBC experiment revealed
through-bond correlations between one of the cis-oxazoline
nitrogens and the CHCl2 group, while no through-bond inter-
action was detected with the nitrogen on the trans-oxazoline.
The oxidative addition reactions of 3 and C3H5Br or
CHCl3 described above provide chiral racemic products.
We were therefore curious to investigate possible selectivity
for the formation of diastereomers controlled by the enantio-
pure chiral ligand [ToP]-. The reaction of 4 and chloroform
gives (κ3-ToP)Rh(CHCl2)Cl(CO) (9) (eq 8), and the major
product is isolated as analytically and spectroscopically pure
material after column chromatography. As the 1H and 13C-
{1H} NMR spectrum demonstrated for 8, the 1H NMR
Inspection of the 1H NMR spectrum of the crude mixture
obtained from treatment of 4 in chloroform indicated that
two products were formed in a 100:3 ratio. The relative ratio
of the two products was determined by integration of the
CHMe2 signals in the 1H NMR spectrum of the crude
reaction mixture; the CHMe2 and CH2 oxazoline reso-
nances, as well as C6H5 resonances, for the two species are
significantly overlapping. Unfortunately, the minor product
could not be isolated and fully characterized, so we tenta-
tively assign it as the minor diastereomer on the basis of its
three isopropyl methyne resonances that are indicative of a
C1-symmetric species. No other products were observed in
the crude reaction mixture.
The mechanism of oxidative addition and observed high
diastereoselectivity are likely related, but unfortunately our
attempts to determine a rate law for these reactions have
been hindered by insufficiently separated resonances in the
1H NMR spectra. Despite the lack of quantitative kinetic
data, several qualitative mechanistic observations should be
considered. First, no change in rate or selectivity is observed
when the reactions are performed in the dark. This selectivity
contrasts the reactions of Tp*RhL2 and CHCl3 described by
Jones and co-workers, where Tp*RhCl2 is the thermal
product and Tp*Rh(CHCl2)Cl(L) is the photochemical
product.30 PyboxRhCl3 is an important side product
(30%) in the reaction of pyboxRh(η2-C8H14) and chloro-
form, and this chlorinated product is attributed to one-
electron chemistry.16 Second, unlike the well-studied SN2-
type oxidative addition reactions of TpRh(CO)L and MeI,
where both CO and L ligands are incorporated into the
Rh(III) products,5 one of the carbonyl ligands dissociates
during the oxidative addition reactions of both 3 and 4.
Carbonyl substitution is also observed in the reaction of the
rhodium(I) compound 3 and P(OMe)3, and this transforma-
tion suggests that CO dissociation/substitution (through an
associative mechanism) could occur prior to rhodium oxida-
tion in the addition reactions. We surmise that CO is not
labile in the rhodium(III) complexes because the η1-allyl
product 7 does not decompose to ToMRh(η3-C3H5)Br.
In contrast to a mechanism involving prior association of
CHCl3, the SN2-type pathway was proposed for the reaction
of TpMe2ClRh(CO)2 and CHCl3 because Tp*Rh(CO)2 and
CHCl3 do not react under thermal conditions.6 We also
observe differences in rate of CHCl3 oxidative addition with
3 versus 4, suggesting that these reactions do not involve
radical species. Given the current mechanistic ambiguity and
the product’s unknown absolute stereochemistry, several
models that rationalize the high diastereoselectivity could
be proposed. Regardless of the pathway, however, the third
oxazoline must play a significant role, either to distinguish
the faces of a square-planar rhodium(I) center or to control
the configuration of a five-coordinate rhodium intermediate.
spectrum of ToPRh(CHCl2)Cl(CO) contained a doublet
2
resonance due to the CHCl2 group (7.11 ppm, JRhH
=
3.2 Hz). Doublets were also observed for the RhCHCl2
1
(65.7 ppm, JRhC = 28.0 Hz) and the rhodium carbonyl
1
(179.7 ppm, JRhC=59.5 Hz) in the 13C{1H} NMR spec-
trum. All three oxazoline groups are inequivalent due to the
compound’s overall C1-symmetry.
A NOESY experiment showed a cross-peak between two
of the isopropyl methine hydrogens (2.37 and 2.17 ppm) with
the CHCl2 hydrogen, and these were the only hydrogen
atoms for which through-space correlations with the dichloro-
hydrocarbyl ligand were observed. Thus, the two corresponding
chiral oxazolines of the ToP ligand were assigned as cis; a
1H-15N HMBC experiment allowed the detection of the cis-
oxazoline nitrogen resonances (-204.3 and -217.7 ppm). The
remaining nitrogen resonance (-194.2 ppm) corresponds to an
oxazoline group trans to the CHCl2. As in 8, through-bond
correlations between CHCl2 and oxazoline nitrogen were de-
tected; in this case, cross-peaks between the CHCl2 group and
one of the cis-oxazolines and the trans-oxazoline nitrogen were
observed. The through-metal coupling between cis ligands, in
both 8 and 9, is unexpected. Additionally, no clear trend relating
Conclusion
Reactions of tris(oxazolinyl)borato rhodium(I) com-
pounds and MeOTf proceed via N-oxazoline methylation,
as previously reported for our related oxazolinylborato
iridium(I) compounds. However, the oxidative addition

4114 Organometallics, Vol. 29, No. 18, 2010
Ho et al.
chemistry of compounds 3 and 4 has not been observed with
ToMIrL2 or ToPIrL2 (L2 = (CO)2, η4-C8H12). Regarding this
contrast between reactive rhodium and inert iridium tris-
(oxazolinyl)borates in thermal oxidative addition reactions,
either higher nucleophilicity or increased carbonyl lability
for the rhodium(I) compounds may be important. Although
greater basicity of TpIr(CO)2 than TpRh(CO)2 is shown by
to rhodium(III), as well as a basis for comparison to Cp-, Tp-,
and their derivatives.
The different reaction pathways for MeOTf, allyl bromide,
and chloroform substrates with 3 are readily ascribed to the
electrophilicity of the RX reagent; in particular MeOTf and
2H-4,4-dimethyl-2-oxazoline react rapidly by N-methyla-
tion at room temperature, whereas chloroform and the
same 2H-oxazoline do not react at 60 °C after 12 h. A second
feature that distinguishes MeOTf from allyl bromide
and chloroform is the relative bonding capability of OTf-
versus Br- and Cl-, where halides are better coordinating
ligands for rhodium than triflate. The relative binding
capabilities of halides to rhodium(I) and hard electrophiles
(Meþ, Hþ, Liþ) to an oxazoline may also play an important
part in the synthesis of oxazolinylborato rhodium com-
pounds, such that Tl[ToM] and Tl[ToP] proved to be super-
ior ligand transfer agents. These examples provide strategies
for developing new oxidative addition reactions to better
understand the effect of ancillary ligands, substrates, and
reaction mechanism on stereoselectivity in organometallic
chemistry.
their reactions with HBF4 Et2O,13 comparisons of kinetic
3
parameters for oxidative addition reactions of rhodium
versus iridium pyrazolylborates have not been established.
Assessing the nucleophilicity of the pyrazolylborate and
oxazolinylborate group 9 compounds is complicated by
several factors, including multiple accessible configurations
(κ2-boat, κ2-chair, and κ3-coordination geometries) that
rapidly equilibrate and steric effects that are difficult to
quantify. For example, the relative rate of reaction for chiral
4 and CHCl3 is greater than the rate of reaction involving
achiral 3; only one configuration is observed by IR spectros-
copy for the former, whereas three sets of carbonyl bands are
observed for the latter, and it is not clear which conformer(s)
are directly involved. These structures also complicate the
analysis of the electron-donating properties of tris(oxazolinyl)-
borate and tris(pyrazolyl)borate ligands; however the
carbonyl stretching frequency of the configurationally rigid
rhodium(III) series provides the trend: (κ3-ToM)Rh(CHCl2)-
Cl(CO) (8; νCO: 2088 cm-1) > (κ3-ToP)Rh(CHCl2)Cl(CO)
Acknowledgment. We thank Prof. Robert Angelici for
helpful discussions and Dr. Bruce Fulton for help with
NMR experiments. The U.S. Department of Energy,
Office of Basic Energy Science (DE-AC02-07CH11358),
is acknowledged for generous financial support. A.D.S. is
an Alfred P. Sloan Fellow.
(9; νCO: 2092 cm-1)>TpMe2ClRh(CHCl2)Cl(CO) (νCO
:
2110 cm-1).6 This trend correlates with the relative energies of
carbonyl stretching frequencies for rhodium(I) dicarbonyls, but
the “relative electron richness” of the rhodium center does
not correlate with relative rates of reactions of [Rh] with
CHCl3. These data provide an estimate of the relative
electron-donating capabilities of [ToM]- and [ToP]- ligands
Supporting Information Available: Tabulated X-ray collec-
tion parameters and crystallographic information files (CIF).
This materials is available free of charge via the Internet at
http://pubs.acs.org.