Electron withdrawing substituents on equatorial and apical phosphines have opposite effects on the regioselectivity of rhodium catalyzed hydroformylation

Article abstract of DOI:10.1021/ja9719440

The electronic effects of electron withdrawing aryl substituents on equatorial and apical diphosphines were investigated. Chelating diphosphines designed to coordinate in diequatorial or in apical-equatorial positions were synthesized, and their effects on the regioselectivity of rhodium catalyzed 1-hexene hydroformylation were observed, Only diequatorial coordination was observed for 2,2'-bis[(diphenylphosphino)methyl]-1,1'-biphenyl (BISBI) complexes (BISBI)Ir(CO)₂H (8) and [BISBI-(3,5-CF₃)]Ir(CO)₂H (10), and only apical-equatorial coordination was seen for 1,2-bis(diphenylphosphino)ethane (DIPHOS) complexes (DIPHOS)Ir(CO)₂H (14) and [DIPHOS-(3,5-CF₃)]Ir(CO)₂H (15). For the trans-1,2-bis[(diphenylphosphino)methyl]cyclopropane (T-BDCP) complexes, a mixture of diequatorial and apical-equatorial complexes was seen. For (T-BDCP)Ir(CO)₂H (12), 12-ae was favored over 12-ee by 63:37, but for [T-BDCP-(3,5-CF₃)]Ir(CO)₂H (13) the conformational preference was reversed and a 10:90 ratio of 13-ae:13-ee was seen. The electron withdrawing groups in the equatorial positions of BISBI-(3,5-CF₃) (1) and T-BDCP-(3,5-CF₃) (2) led to an increase in n-aldehyde regioselectivity in rhodium catalyzed hydroformylation. However, electron withdrawing aryl substituents in the apical positions of DIPHOS-(3,5-CF₃) (3) led to a decrease in n-aldehyde regioselectivity in rhodium catalyzed hydroformylation.

Full text of DOI:10.1021/ja9719440

J. Am. Chem. Soc. 1997, 119, 11817-11825
11817
Electron Withdrawing Substituents on Equatorial and Apical
Phosphines Have Opposite Effects on the Regioselectivity of
Rhodium Catalyzed Hydroformylation
Charles P. Casey,* Evelyn Lin Paulsen, Eckart W. Beuttenmueller, Bernd R. Proft,
Lori M. Petrovich, Brock A. Matter, and Douglas R. Powell
Contribution from the Department of Chemistry, UniVersity of Wisconsin,
Madison, Wisconsin 53706
ReceiVed June 12, 1997X
Abstract: The electronic effects of electron withdrawing aryl substituents on equatorial and apical diphosphines
were investigated. Chelating diphosphines designed to coordinate in diequatorial or in apical-equatorial positions
were synthesized, and their effects on the regioselectivity of rhodium catalyzed 1-hexene hydroformylation were
observed. Only diequatorial coordination was observed for 2,2′-bis[(diphenylphosphino)methyl]-1,1′-biphenyl (BISBI)
complexes (BISBI)Ir(CO)2H (8) and [BISBI-(3,5-CF3)]Ir(CO)2H (10), and only apical-equatorial coordination was
seen for 1,2-bis(diphenylphosphino)ethane (DIPHOS) complexes (DIPHOS)Ir(CO)2H (14) and [DIPHOS-(3,5-CF3)]-
Ir(CO)2H (15). For the trans-1,2-bis[(diphenylphosphino)methyl]cyclopropane (T-BDCP) complexes, a mixture of
diequatorial and apical-equatorial complexes was seen. For (T-BDCP)Ir(CO)2H (12), 12-ae was favored over 12-
ee by 63:37, but for [T-BDCP-(3,5-CF3)]Ir(CO)2H (13) the conformational preference was reversed and a 10:90
ratio of 13-ae:13-ee was seen. The electron withdrawing groups in the equatorial positions of BISBI-(3,5-CF3) (1)
and T-BDCP-(3,5-CF3) (2) led to an increase in n-aldehyde regioselectivity in rhodium catalyzed hydroformylation.
However, electron withdrawing aryl substituents in the apical positions of DIPHOS-(3,5-CF3) (3) led to a decrease
in n-aldehyde regioselectivity in rhodium catalyzed hydroformylation.
The hydroformylation reaction1 employing very efficient
homogeneous rhodium-phosphine catalysts, first developed by
Wilkinson2 and first implemented by Union Carbide,3 is one of
the most important industrial uses of homogeneous catalysis.
These catalysts are important because they are highly regio-
selective for the desired linear (n) aldehyde over branched (i)
aldehyde and require lower temperatures and pressures than
earlier cobalt-based catalysts. Control of the regiochemistry of
the hydroformylation of terminal alkenes remains a focus of
industrial research. Recent success in achieving high n:i ratios
of aldehydes include Texas Eastman’s use of the 2,2′-bis-
[(diphenylphosphino)methyl]-1,1′-biphenyl (BISBI) ligand4 and
Union Carbide’s development of diphosphite chelates.5 Stanley
has developed a unique binuclear rhodium catalyst that gives
high rates and selectivity.6 van Leeuwen has reported a variety
of diphosphite and diphosphine ligands that give high n:i
X Abstract published in AdVance ACS Abstracts, November 15, 1997.
(1) (a) Parshall, G. W. Homogeneous Catalysis: The Applications and
Chemistry of Catalysis by Soluble Transition Metal Complexes; Wiley: New
York, 1980. (b) Tkatchenko, I. In ComprehensiVe Organometallic Chem-
istry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon: Oxford,
1982; Vol. 8, pp 101-223. (c) Tolman, C. A.; Faller, J. W. In Homogeneous
Catalysis with Metal Phosphine Complexes; Pignolet, L. H., Ed.; Plenum:
New York, 1983; pp 81-109. (d) Pino, P.; Piacenti, F.; Bianchi, M. In
Organic Syntheses Via Metal Carbonyls; Wender, I., Pino, P., Eds.; Wiley
Interscience: New York, 1977; Vol. 2, pp 136-197. (e) Consiglio, G.;
Pino, P. Top. Curr. Chem. 1982, 105, 77. (f) Beller, M.; Cornils, B.;
Frohning, C. D.; Kohlpaintner, C. W. J. Mol. Catal. A 1995, 104, 17-85.
(2) (a) Evans, D.; Osborn, J. A.; Wilkinson, G. J. Chem. Soc. A 1968,
3133. (b) Yagupsky, G.; Brown, C. K.; Wilkinson, G. J. Chem. Soc. A
1970, 1392. (c) Brown, C. K.; Wilkinson, G. J. Chem. Soc. A 1970, 2753.
(3) Pruett, R. L. Ann. N. Y. Acad. Sci. 1977, 295, 239.
regioselectivity.7 Buchwald8 and Wink9 have obtained high
regioselectivities in hydroformylation of functionalized alkenes
using rhodium diphosphite systems. While extensive screening
of a wide variety of phosphine and phosphite ligands has been
reported, no detailed understanding of how phosphines control
regioselectivity or enantioselectivity has emerged.
In Wilkinson’s dissociative hydroformylation mechanism
(Scheme 1), aldehyde regioselectivity is determined in the
hydride addition step that converts a five-coordinate H(alkene)-
Rh(CO)L2 into either a primary or secondary four-coordinate
(alkyl)Rh(CO)L2. To understand and control regioselectivity,
it is important to know the detailed structure of the key five-
coordinate H(alkene)Rh(CO)L2 intermediate. Two monodentate
phosphine ligands might occupy two equatorial or one equatorial
and one apical site in a trigonal bipyramidal intermediate. For
L ) PPh3, Brown’s NMR studies showed an 85:15 diequatorial:
apical-equatorial mixture of isomers of (PPh3)2Rh(CO)2H
which are in rapid equilibrium at room temperature.10
Several years ago we set out to test the hypothesis that very
different regioselectivities might be obtained from diequatorial
(5) (a) Billig, E.; Abatjoglou, A. G.; Bryant, D. R. (to Union Carbide)
U.S. Patent 4,748,261 1988; 4,769,498 1988. (b) For related work with
phosphine-phosphite chelates, see: Nozaki, K.; Sakai, N.; Nanno, T.;
Higashijima, T.; Mano, S.; Horiuchi, T.; Takaya, H. J. Am. Chem. Soc.
1997, 119, 4413.
(6) Broussard, M. E.; Juma, B.; Train, S. G.; Peng, W.-J.; Laneman, S.
A.; Stanley, G. G. Science 1993, 260, 1784.
(7) (a) van Rooy, A.; Kamer, P. C. J.; van Leeuwen, P. W. N. M.;
Goubitz, K.; Fraanje, J.; Veldman, N.; Spek, A. L. Organometallics 1996,
15, 835. (b) Buisman, G. J. H.; Martin, M. E.; Vos, E. J.; Klootwijk, A.;
Kamer, P. C. J.; van Leeuwen, P. W. N. M. Tetrahedron: Asymmetry 1995,
6, 719. (c) Kranenburg, M.; van der Burgt, Y. E. M.; Kamer, P. C. J.; van
Leeuwen, P. W. N. M.; Goubitz, K.; Fraanje, J. Organometallics 1995, 14,
3081.
(8) Cuny, G. D.; Buchwald S. L. J. Am. Chem. Soc. 1993, 115, 2066.
(9) Kwok, T. J.; Wink, D. J. Organometallics 1993, 12, 1954.
(10) Brown, J. W.; Kent, A. G. J. Chem. Soc., Perkin Trans. II 1987,
1597.
(4) (a) Devon, T. J.; Phillips, G. W.; Puckette, T. A.; Stavinoha, J. L.;
Vanderbilt, J. J. (to Texas Eastman) U.S. Patent 4,694,109, 1987; Chem.
Abstr. 1988, 108, 7890. For closely related systems, see: (b) Devon, T. J.;
Phillips, G. W.; Puckette, T. A.; Stavinoha, J. L.; Vanderbilt, J. J. (to Texas
Eastman) U.S. Patent 5,332,846, 1994; Chem. Abstr. 1994, 121, 280, 879.
(c) Bahrmann, H.; Lappe, P.; Herrmann, W. A.; Albanese, G. P.;
Manetsberger, R. B. (to Hoechst AG) DE Patent 4,333,307, 1995; Chem
Abstr. 1995, 123, 112,408.
S0002-7863(97)01944-6 CCC: $14.00 © 1997 American Chemical Society

11818 J. Am. Chem. Soc., Vol. 119, No. 49, 1997
Casey et al.
Scheme 1
Scheme 2
selectivity. In particular, we looked for a special role for flexible
wide bite angle diphosphines in promoting formation of linear
products.
Molecular mechanics calculations were employed to explore
whether selectivity arose from steric differences between the
transition states leading to the primary and secondary alkyl-
rhodium intermediates.17 The steric energy difference between
the transition state models leading to (BISBI)Rh(n-propyl)(CO)
and (BISBI)Rh(i-propyl)(CO) was calculated to be 1.9 kcal
mol-1 favoring the n-propyl transition state. This energy
difference corresponds to a 25:1 partitioning between n- and
isopropyl rhodium intermediates, in reasonable agreement with
the 66:1 n:i aldehyde selectivity observed in hydroformylation
with BISBI which corresponds to 2.5 kcal mol-1. However,
the calculated steric energy difference between the transition
state models leading to (DIPHOS)Rh(n-propyl)(CO) and
(DIPHOS)Rh(isopropyl)(CO) was even larger, 2.1 kcal mol-1
also favoring the n-propyl transition state. This energy differ-
ence corresponds to 35:1 n:i selectivity, a value much higher
than the observed DIPHOS selectivity of 2.6:1, which requires
an energy difference of only 0.6 kcal mol-1. These estimates
of steric effects incorrectly suggested that DIPHOS should be
more selective than BISBI, which is in fact the opposite of the
trend observed.
Since steric arguments failed to give a satisfactory explanation
for the much higher n:i aldehyde ratios observed from diequa-
torial chelates like BISBI compared with apical-equatorial
chelates like DIPHOS, we must consider whether the electronic
differences between diequatorial chelates and apical-equatorial
chelates are responsible for the differences in regioselectivity.
Because the electronic interaction between two apical ligands
or between two equatorial ligands is stronger than the interaction
between apical and equatorial ligands, electronic differences can
be expected between diequatorial chelated complexes and
apical-equatorial chelated complexes. For example, back
bonding from rhodium to the alkene ligand in the equatorial
plane would be expected to be stronger for the BISBI complex
with two strong donor phosphines in the equatorial plane than
for the DIPHOS complex with only a single donor phosphine
in the equatorial plane. Also, the apical hydride of the BISBI
complex is trans to a CO ligand and would be expected to be
more acidic than the hydride of the DIPHOS complex which is
trans to a phosphine.
diphosphine rhodium complexes and from apical-equatorial
diphosphine rhodium complexes (Scheme 2). In the course of
testing this hypothesis, we carried out molecular mechanics
calculations on a number of diphosphine chelates to determine
their “natural bite angle” and the flexibility of the bite angle.11
Chelates with wide natural bite angles near 120° were chosen
to achieve preferential diequatorial coordination, and chelates
with narrow bite angles near 90° were chosen to achieve apical-
equatorial coordination. A strong correlation was found between
regioselectivity for n-aldehyde formation and natural bite
angle.12 Chelates like BISBI with wide natural bite angles gave
a much higher % n-aldehyde (ân ) 113°, n:i ) 66) than
diphosphines like 1,2-bis(diphenylphosphino)ethane (DIPHOS)
with narrow bite angles (ân ) 85°, n:i ) 2.6).13,14 X-ray
structures of (BISBI)Rh(PPh3)(CO)H and (BISBI)Ir(CO)2H
indicated that BISBI is coordinated diequatorially.12 DIPHOS
is coordinated in an apical-equatorial manner in five-coordinate
(DIPHOS)Fe(CO)315 as well as in (DIPHOS)Ir(CO)2H.16 Deu-
terioformylation studies established that the predominant prod-
ucts were derived from irreversible addition of a rhodium
hydride to a complexed alkene to give a rhodium alkyl
intermediate that is committed to aldehyde formation.17
The correlation between regioselectivity and chelation mode
suggests that diequatorial diphosphines like BISBI and apical-
equatorial chelating diphosphines like DIPHOS have either
significantly different steric or electronic properties. We first
considered the steric differences between diequatorial and
apical-equatorial chelates as a possible explanation for regio-
(11) Casey, C. P.; Whiteker, G. T. Isr. J. Chem. 1990, 30, 299.
(12) Casey, C. P.; Whiteker, G. T.; Melville, M. G.; Petrovich, L. M.;
Gavney, J. A., Jr.; Powell, D. R. J. Am. Chem. Soc. 1992, 114, 5535.
(13) van Leeuwen also observed an increase in linear aldehyde regiose-
lectivity with increasing bite angle.7c In a series of bidentate chelating
phosphines containing xanthene-like ligand backbones, the bite angle was
varied from 102° to 131°, with minimal change in the steric or electronic
properties of the ligand. The resulting n:i ratio increased from 6.7 to 80.5
in the hydroformylation of 1-octene at 80 °C.
(14) Earlier12 we reported a 2.1:1 n:i ratio for DIPHOS, which is the
ratio seen early in the reaction. For consistency, we are now reporting ratios
obtained after 120-250 turnovers (20-50% conversion) for all ligands.
(15) Battaglia, L. P.; Delledonne, D.; Nardelli, M.; Pelizzi, C.; Predieri,
G.; Chiusoli, G. P. J. Organomet. Chem. 1987, 330, 101.
(16) (a) Fisher, B. J.; Eisenberg, R. Organometallics 1983, 2, 764. (b)
Fisher, B. J.; Eisenberg, R. Inorg. Chem. 1984, 23, 3216.
(17) Casey, C. P.; Petrovich, L. M. J. Am. Chem. Soc., 1995, 117, 6007.
To investigate electronic effects of equatorial and apical
phosphines, we have synthesized analogs of our chelating
diphosphines having electron withdrawing substituents on the
aryl rings and measured the resulting changes in hydroformy-
lation regioselectivity. Here we report that the introduction of

Electronic Effects of Electron Withdrawing Aryl Substitutents
J. Am. Chem. Soc., Vol. 119, No. 49, 1997 11819
electron withdrawing substituents on the aryl rings of the
diequatorial chelate BISBI-(3,5-CF3) (1) leads to an increase
in linear aldehyde selectivity as well as rate. In contrast,
introduction of electron withdrawing substituents on the aryl
rings of the apical-equatorial chelate DIPHOS-(3,5-CF3) (3)
resulted in a decrease in linear aldehyde selectivity when
compared with the phenyl-substituted DIPHOS.
Scheme 4
by addition of vinylmagnesium bromide to [3,5-(CF3)2C6H3]2-
PCl. The KO-t-Bu catalyzed coupling of 4 and 7 led to the
isolation of 1,2-bis{bis[3,5-di(trifluoromethyl)phenyl]phosphino}-
ethane (3) in 52% yield. The 31P NMR resonance of 3 at δ
-10.7 was shifted to higher frequency by 2 ppm compared with
the parent DIPHOS.
Synthesis of Iridium Complexes as Models for Catalytic
Species. The regioselectivity of aldehyde formation is deter-
mined by the partitioning of a chelating diphosphine rhodium
hydride alkene complex between linear and branched alkyl
rhodium species (Scheme 1). These intermediates have never
been directly observed due to rapid conversion to acyl rhodium
species. Possible models for these intermediates would be
chelating diphosphine rhodium dicarbonyl hydrides, but in many
cases such complexes are unstable relative to loss of hydrogen
and dimer formation. For example, attempts to synthesize (T-
BDCP)Rh(CO)2H led to [(T-BDCP)Rh]2(µ-CO)2.12 We have
employed related iridium dicarbonyl hydride complexes to probe
the chelation geometry of the five-coordinate metal center of
the catalytically important rhodium hydride alkene complexes.
Iridium and rhodium have similar coordination geometries, and
iridium bond lengths are only 1-2% longer than related rhodium
bond lengths.
Results
Synthesis of Diphosphines with Electron Withdrawing
Substituents. The reaction of the potassium phosphide derived
from [3,5-(CF3)2C6H3]2PH (4) with the appropriate dihalide or
ditosylate was used to synthesize diphosphines having electron
withdrawing aryl substituents. 4 was prepared by reduction of
[3,5-(CF3)2C6H3]2PCl18 with LiAlH4 and subsequently converted
to the phosphide by treatment with KH.
Addition of the phosphide derived from 4 to 2,2′-bis-
(bromomethyl)-1,1′-biphenyl (5) produced BISBI-(3,5-CF3) (1)
in 54% yield (Scheme 3). The 31P NMR resonance of 1
BISBI Complexes. The X-ray crystal structure of (BISBI)-
Ir(CO)2H (8) shows a trigonal bipyramidal geometry with the
chelating diphosphine occupying diequatorial sites with a bite
angle of 117.9°.12 In the IR spectrum of 8, bands were observed
at 2074 cm-1 (w, νIrH), 1985 (s, sym νCO), and 1932 (s, asym
Scheme 3
ν
CO) cm-1. At -125 °C, two 31P{1H} NMR doublets were seen
for the diastereotopic phosphines at δ 10.6 (JPP ) 107 Hz) and
-0.7 (JPP ) 107 Hz). A Berry pseudorotation process
interchanges the environments of the phosphines and causes
coalescence of the phosphorus resonances at higher temperature.
At room temperature, a singlet at δ 4.3 is seen. In the 1H NMR
spectrum at -125 °C, the Ir-H appears as a triplet coupled
equally to two phosphorus atoms at δ -12.15 (t, JHP ) 20 Hz).
Evidently, the diastereotopic phosphines have identical coupling
constants to the cis-hydride.
Initially, we attempted to prepare the BISBI-(3,5-CF3) analog
of iridium dicarbonyl complex 8 by first preparing [BISBI-(3,5-
CF3)]Ir(PPh3)(CO)H (9) and then converting it to the dicarbonyl
complex [BISBI-(3,5-CF3)]Ir(CO)2H (10). The reaction of
BISBI-(3,5-CF3) (1) with (PPh3)3Ir(CO)H in THF produced 9
in 80% yield. The X-ray structure of 9 indicates a trigonal
bipyramidal structure with the chelating ligand occupying
equatorial sites with a P-Ir-P angle of 118.20 (5)° (Figure 1).
The carbonyl ligand occupies an apical position with a Ph3P-
Ir-CO angle of 94.6°, indicating that the Ir is distorted slightly
toward the carbonyl ligand. Although the hydride was not
located, we believe it occupies an apical site trans to CO. The
structure of 9 is very similar to that of the related rhodium
complex (BISBI)Rh(PPh3)(CO)H12 and supports the hypothesis
that iridium complexes are excellent structural models for
rhodium complexes (Table 1).
appeared at δ -8.5, shifted 2 ppm to higher frequency than
BISBI. Similarly, the reaction of the potassium phosphide
derived from 4 with the cyclopropane ditosylate 6 gave the
corresponding trans-cyclopropane diaryldiphosphine T-BDCP-
(3,5-CF3) (2) in 36% yield. The 31P NMR resonance of 2 at δ
-14.6 was also shifted to higher frequency by 5 ppm compared
with the diphenyldiphosphine analog T-BDCP (trans-1,2-bis-
[(diphenylphosphino)methyl]cyclopropane).
1,2-Bis(diarylphosphino)ethane derivatives were synthesized
by base catalyzed addition of the secondary aryl phosphine 4
to a functionalized diarylvinylphosphine (Scheme 4).19-21 The
vinyl phosphine [3,5-(CF3)2C6H3]2PCHdCH2 (7) was prepared
(18) Casalnuovo, A. L.; RajanBabu, T. V.; Ayers, T. A.; Warren, T. H.
J. Am. Chem. Soc. 1994, 116, 9869.
(19) (a) Clark, P. W.; Mulraney, B. J. J. Organomet. Chem. 1981, 217,
51. (b) Chatt, J.; Hussain, W.; Leigh, G. J.; Ali, H. M.; Pickett, C. J.; Rankin,
D. A. J. Chem. Soc., Dalton Trans. 1985, 1131.
(20) The o-tolyl and (o-trifluoromethyl)phenyl substituted DIPHOS
derivatives were synthesized by the same method. See Supporting Informa-
tion.
The presence of a hydride ligand in 9 was established by 1H
NMR spectroscopy, which showed a triplet of doublets at δ
(21) This synthesis route was designed in order to also have access to
unsymmetrically substituted 1,2-bis(diarylphosphino)ethane derivatives (Ar2-
PCH2CH2PAr′2) via the use of differently substituted secondary phosphines
and diarylvinyl phosphines.

11820 J. Am. Chem. Soc., Vol. 119, No. 49, 1997
Casey et al.
Table 1. Comparison of Bond Lengths (Å) and Angles (deg) of [BISBI-(3,5-CF3)]Ir(PPh3)(CO)H (9), (BISBI)Rh(PPh3)(CO)H‚CH2Cl2,12 and
12
(BISBI)Ir(CO)2H‚1/2O(CHMe2)2
[BISBI-(3,5-CF3)]Ir(PPh3)(CO)H (9)
(BISBI)Rh(PPh3)(CO)H
Bond Lengths (Å)
(BISBI)Ir(CO)2H
Ir-P(1)
2.277 (1)
2.270 (1)
2.319 (1)
1.906 (6)
Rh-P(1)
2.285 (1)
2.318 (1)
2.318 (1)
1.895 (5)
Ir-P(1)
Ir-P(2)
2.306 (3)
2.300 (3)
Ir-P(2)
Rh-P(2)
Ir-P(3)
Ir-C(65)O
Rh-P(3)
Rh-C(57)O
Ir-C(1CO)
Ir-C(2CO)
1.89 (1)
1.90 (1)
Bond Angles (deg)
P(1)-Ir-P(2)
P(1)-Ir-P(3)
P(2)-Ir-P(3)
P(1)-Ir-C(65)
P(2)-Ir-C(65)
P(3)-Ir-C(65)
118.20 (5)
117.18 (5)
120.19 (5)
100.2 (2)
96.4 (2)
P(1)-Rh-P(2)
P(1)-Rh-P(3)
P(2)-Rh-P(3)
P(1)-Rh-C(57)
P(2)-Rh-C(57)
P(3)-Rh-C(57)
124.8 (1)
P(1)-Ir-P(2)
117.9 (1)
122.6 (1)
108.0 (1)
95.3 (1)
93.3 (1)
103.1 (1)
P(1)-Ir-C(2CO)
P(2)-Ir-C(2CO)
P(1)-Ir-C(1CO)
P(2)-Ir-C(1CO)
93.4 (4)
98.3 (4)
125.8 (4)
112.2 (4)
94.6 (2)
T-BDCP Complexes. The 107° calculated natural bite angle
of the T-BDCP ligand lies between the ideal 90° angle of an
apical-equatorial chelate and the ideal 120° angle of a
diequatorial chelate. The only X-ray crystal structure of a
T-BDCP chelate is for (T-BDCP)Fe(CO)3, which has a P-Fe-P
bite angle of 123.9°.22 While this wide angle is close to the
expected 120° bite angle for diequatorial chelation, it does not
imply that T-BDCP is a selective diequatorial ligand since
nonchelated (R3P)2Fe(CO)3 compounds show an electronic
preference for a wide P-Fe-P bite angles. For example,
(Ph3P)2Fe(CO)3 has a P-Fe-P angle of 173°.23 (BISBI)Fe-
(CO)3 has an unusually large bite angle of 152° compared with
other BISBI complexes.22 Clearly, it is important to determine
experimentally whether T-BDCP coordinates to iridium as a
apical-equatorial chelate or a diequatorial chelate or a mixture
of chelates.
Figure 1. X-ray crystal structure of [BISBI-(3,5-CF3)](PPh3)Ir(CO)H
(9).
-11.52 (td, JHP ) 25, 20 Hz). The relatively small H-P
coupling constants are consistent with a structure containing
an apical hydride coupled to three cis equatorial phosphines.
The 31P NMR spectrum of 9 consisted of a complex ABC line
pattern with more than eight lines between δ 12 and 16.
The reaction of T-BDCP with (PPh3)3Ir(CO)H in CH2Cl2
The BISBI dicarbonyl iridium hydride complex 8 was formed
by bubbling CO through a solution of (BISBI)Ir(PPh3)(CO)H.12
However, when CO was bubbled through a solution of 9,
attempts to isolate the desired dicarbonyl compound failed,
resulting instead in reisolation of starting material. Therefore
the following alternative synthesis was devised.
produced (T-BDCP)Ir(PPh3)(CO)H (11) as a bright yellow solid
1
in 80% yield. In the H NMR spectrum of 11 in CD2Cl2, the
iridium hydride appeared at δ -11.51 (ddd, JHP ) 25.5, 21.5,
19.5 Hz). The 31P{1H} NMR spectrum of 11 in CD2Cl2
consisted of an AMX pattern due to complexed PPh3 and the
two diastereotopic phosphines of the T-BDCP ligand (δ 15.3,
11.0, 10.2; JPP ) 133, 126, 126 Hz). These coupling patterns
are consistent with a trigonal bipyramidal structure with three
equatorial phosphines. Apparently, the relatively bulky phos-
phine ligands prefer to chelate equatorially to minimize steric
interactions.
(T-BDCP)Ir(CO)2H (12) was synthesized in 66% yield by
reaction of (T-BDCP)Ir(PPh3)(CO)H (11) with CO. The low
temperature 1H and 31P NMR spectra of 12 in CD2Cl2 provided
evidence for a 3.3:1 ratio of isomers. In the 1H NMR spectrum
-74 °C, the metal hydride resonance of the major isomer
appeared as a doublet of doublets at δ -12.07 (dd, JHP ) 97,
-23 Hz).24 This isomer was assigned the apical-equatorial
chelate configuration 12-ae based on the observation of a large
coupling between an apical hydride and an apical phosphorus
[BISBI-(3,5-CF3)]Ir(CO)2H (10) was prepared from the
reaction of BISBI-(3,5-CF3) (1) and Ir(acac)(CO)2 in CD2Cl2
under 1 atm of 1:1 CO:H2. In the IR spectrum of 10 in CH2-
Cl2 the Ir-H stretch at (w, 2079 cm-1) and carbonyl stretches
(2005 (s), and 1960 (vs) cm-1) are all shifted to higher frequency
than in the unsubstituted BISBI complex 8. The electron
withdrawing CF3 groups in 9 make the phosphines poorer donors
to iridium and leads to a reduction in backbonding from iridium
to the CO ligands. At -84 °C, the 31P{1H} NMR spectrum
showed an AB quartet (δ 13.28 and 6.40, JPP ) 122 Hz) for
1
the diastereotopic phosphines. In the H NMR spectrum, the
iridium hydride appeared at δ -12.54 (dd, JHP ) 25, 15 Hz)
and showed inequivalent coupling to two phosphines. At room
temperature a fluxional process interchanged the environments
of the phosphines; a single 31P NMR resonance was observed
at δ 9.2, and the 1H NMR resonance of the iridium hydride (δ
-12.32, t, JHP ) 20 Hz) showed averaged coupling to two
phosphines. The room temperature JHP of 20 Hz is the average
of the two low temperature coupling constants.
(22) Casey, C. P.; Whiteker, G. T.; Campana, C. F.; Powell, D. R. Inorg.
Chem. 1990, 29, 3376.
(23) Glaser, R.; Yoo, Y.-H.; Chen, G. S.; Barnes, C. L. Organometallics
1994, 13, 2578.

Electronic Effects of Electron Withdrawing Aryl Substitutents
J. Am. Chem. Soc., Vol. 119, No. 49, 1997 11821
and a small coupling between the hydride and an equatorial
phosphorus. The 31P NMR spectrum of 12-ae showed two
different phosphorus doublets at δ 0.93 (d, JPP ) 24 Hz) and
-5.15 (d, JPP ) 24 Hz), tentatively assigned to the equatorial
and apical phosphines, respectively. The hydride resonance of
the minor isomer appeared at δ -11.35 (JHP ) -26, -15 Hz).
This isomer was assigned a diequatorial chelate configuration
12-ee based on the observation of two small couplings between
an apical hydride and diastereotopic equatorial phosphines. In
the 31P NMR spectrum of 12-ee, the two diastereotopic
equatorial phosphines had similar chemical shifts and showed
stronger coupling (δ 7.25 and 5.34, JPP ) 105 Hz). Coupling
constants between diequatorial phosphines are generally larger
than couplings between equatorial and apical phosphines. The
IR spectrum of the mixture of 12-ae and 12-ee in hexane
confirms the existence of two dicarbonyl species, showing
carbonyl bands at 1990(s), 1978(s), 1943(vs), and 1930(vs)
H-P coupling constants of -17 and -28 Hz suggest that the
diastereotopic phosphines are cis to the hydride, consistent with
a diequatorial chelation. The large P-P coupling constant is
observed when phosphines are in the diequatorial position of
these five-coordinate iridium complexes. The IR spectrum at
room temperature in CH2Cl2 showed an Ir-H stretch at 2064
cm-1 and broad carbonyl bands at 2001 (s) and 1955 (vs) cm-1
;
because the carbonyl bands are broad, it is not possible to tell
whether a second isomer is present. These IR frequencies are
shifted to higher frequencies indicating a reduction in back
bonding compared to the diphenyl substituted complex.
At room temperature, a rapid fluxional process interchanges
the environment of the phosphine ligands, and a single 31P-
{1H} NMR resonance is observed at δ 11.34. In the 1H NMR,
a triplet is seen at δ -12.03 (t, JHP ) 16.5 Hz). If the
diequatorial isomer 13-ee were the only species present, then
the coupling constant would have been -22.5 Hz, the average
of the coupling constants seen for 13-ee at -84 °C. We attribute
the smaller observed coupling to the presence of a small amount
of the apical-equatorial isomer 13-ae, which should have
coupling constants similar to those of 12-ae (JHP ) 97, -23
Hz). Using calculations similar to those for 12-ae:12-ee, the
observed coupling is consistent with a 10:90 mixture of 13-ae:
13-ee.
cm-1
.
At room temperature, the two isomers, 12-ae and 12-ee,
rapidly interconvert, and all phosphorous environments are
interchanged, presumably by a Berry pseudorotation mechanism.
In the room temperature 31P{1H} NMR, a singlet at δ 1.3 was
observed. In the room temperature 1H NMR spectrum, a single
hydride resonance was observed at δ -11.48 (t, J ) 16 Hz).
Assuming the coupling constants do not vary with temperature,
the ratio of 12-ae:12-ee at 25 °C can be calculated using the
coupling constants in the formula JRT ) øee(Jee)avg + (1-øee)-
The electron-withdrawing CF3 substituents on T-BDCP-(3,5-
CF3) shift the equilibrium between diequatorial and apical-
equatorial isomers toward the diequatorial isomer compared with
the unsubstituted T-BDCP complexes. Hoffmann’s molecular
orbital analysis of ligand electronic site preferences in five
coordinate d8 ML5 complexes27 suggested that σ donor ligands
should prefer the apical position while σ-acceptor ligands should
prefer the equatorial position. The electron-withdrawing CF3
substituents make T-BDCP-(3,5-CF3) a better σ-acceptor ligand
(and weaker σ-donor ligand); this explains the shift in the
equilibrium toward diequatorial coordination upon introduction
of the CF3 substituents.
(Jae)avg where JRT ) coupling at 25 °C ) 16 Hz; (Jee)avg
)
average coupling of 12-ee at -74 °C ) (-26 + -15) ÷ 2 )
-20.5 Hz; (Jae)avg ) average coupling of 12-ae at -74 °C )
(97 + -23) ÷ 2 ) 37 Hz; øee ) fraction of 12-ee at 25 °C;
and øae ) 1 - øee ) fraction of 12-ae at 25 °C. The solution
to the equation gives a 63:37 ratio of 12-ae:12-ee at room
temperature.25
[T-BDCP-(3,5-CF3)]Ir(CO)2H (13) was prepared from the
reaction of T-BDCP-(3,5-CF3) (2) and Ir(acac)(CO)2 in CD2-
Cl2 under 1 atm of CO/H2.26 In contrast to (T-BDCP)Ir(CO)2H
(12) where a 63:37 ratio of apical-equatorial isomer 12-ae:
diequatorial isomer 12-ee was observed, only the diequatorial
isomer of [T-BDCP-(3,5-CF3)]Ir(CO)2H, 13-ee, was observed
DIPHOS Complexes. The calculated natural bite angle of
DIPHOS is 84.5° and bite angles of 84 ( 4° were found in a
sampling of 20 X-ray structures from the Cambridge data base.
DIPHOS would therefore be expected to span apical and
equatorial sites in a trigonal bipyramid. (DIPHOS)Ir(CO)2H
(14) was synthesized by a procedure similar to that reported by
Eisenberg16 (see Supporting Information). The room temper-
1
by low temperature H and 31P NMR spectroscopy. None of
the apical-equatorial isomer was observed, and as little as 5%
would have been easily detected. In the 1H NMR spectrum of
13 in CD2Cl2 at -84 °C, the hydride resonance at δ -12.3
(dd, JHP ) -28, -17 Hz) showed two small phosphorus
couplings. In the 31P{1H} NMR spectrum at -84 °C, an AB
quartet was observed at δ 13.06 and 10.78 (JPP ) 116 Hz) with
large coupling between the equatorial phosphines. The small
1
ature H and 31P NMR spectra of 14 were previously reported
by Eisenberg; a hydride with equal coupling to two phosphines
was seen at δ -11.20 (t, JHP ) 40 Hz), and a single phosphorus
resonance was observed at δ 32.5.
Our low temperature NMR studies establish that DIPHOS
chelates in an apical-equatorial fashion in 14. At -79 °C, the
1H NMR spectrum shows the Ir-H coupled to two different
phosphorus atoms (dd, JHP ) 92, -13 Hz), and the 31P{1H}
NMR spectrum shows two doublets (JPP ) 10 Hz) at δ 33.2
and 31.3. The 13C{1H} NMR spectrum at -79 °C exhibited a
doublet at δ 173.5 (JCP ) 24 Hz) for the equivalent equatorial
(24) These coupling constants must have opposite signs to explain the
average 16 Hz coupling seen for the fluxional mixture of isomers at room
temperature. Similar coupling constants were assigned to (R3P)2(CO)2IrH:
(a) Meakin, P.; Muetterties, E. L.; Jesson, J. P. J. Am. Chem. Soc. 1972,
94, 5271. (b) Yagupsky, G.; Wilkinson, G. J. Chem. Soc. A 1969, 725.
(25) Mathematical solution of this equation gives two ratios depending
on whether the observed coupling is 16 or -16 Hz. If the coupling is 16
Hz, 12-ae:12-ee ) 63:37 and if the coupling is -16 Hz, 12-ae:12-ee )
8:92. The later ratio would require that the major species at -74 and 25 °C
be different.
(26) Attempts to prepare 13 from [T-BDCP-(3,5-CF3)]Ir(CO)(PPh3)H
failed. See Supporting Information.
(27) Rossi, A. R.; Hoffmann, R. Inorg. Chem. 1975, 14, 365.

11822 J. Am. Chem. Soc., Vol. 119, No. 49, 1997
Casey et al.
Table 2. Regiochemistry of Hydroformylation of 1-Hexene with Rhodium Diphosphine Catalysts
diphosphine
BISBI
BISBI-(3,5-CF3) (1)
T-BDCP
T-BDCP-(3,5-CF3) (2)
DIPHOS
DIPHOS-(3,5-CF3) (3)
ee:aea
aldehyde n:ib
regiochemistry %n
turnover ratec
100:0
100:0
37:63
90:10
0:100
0:100
66.5 ( 0.5
123 ( 3
12.1 ( 0.4
17.7 ( 0.3
2.6 ( 0.1
1.3 ( 0.1
98.52 ( 0.02
99.20 ( 0.02
92.4 ( 0.2
94.7 ( 0.1
71.9 ( 0.3
56.9 ( 0.9
12.4 ( 0.7
61.9 ( 2.6
3.7 ( 0.1
13.7 ( 0.4
3.5 ( 0.1
4.3 ( 0.1
a Ratio of diequatorial:apical-equatorial chelates of (diphosphine)Ir(CO)2H at room temperature. b Mol heptanal:mol 2-methylhexanal. c Turnover
rate ) [mol aldehyde] × [mol Rh]-1 h-1
.
CO ligands. These data unequivocally establishes the apical-
equatorial coordination of DIPHOS ligand in 14.28
The fluxional process that exchanges the environment of the
DIPHOS phosphorus atoms of 14 at room temperature is
probably also a Berry pseudorotation.
temperature coupling constant of 44 Hz for 15 requires opposite
signs for the large and small JHP couplings seen at low
temperature.
Coalescence of the phosphorus resonances occurred at -88
°C. This allowed calculation of ∆Gq ) 8.6 kcal mol-1 for a
fluxional process that interconverts the environments of the
phosphine groups,30 presumably involving a series of Berry
pseudorotations.
Summary of Coordination Geometries. Only diequatorial
coordination is observed for BISBI complexes 8 and 10, and
only apical-equatorial coordination is seen for DIPHOS
complexes 14 and 15. For the T-BDCP complexes, a mixture
of diequatorial and apical-equatorial complexes is seen. For
(T-BDCP)Ir(CO)2H (12), a 63:37 ratio of 12-ae:12-ee was
estimated at room temperature. Electron withdrawing aryl
groups of [T-BDCP-(3,5-CF3)]Ir(CO)2H reversed the confor-
mational preference, and a 10:90 ratio of 13-ae:13-ee was
estimated at room temperature.
Catalytic Hydroformylation of 1-Hexene. Hydroformyla-
tion of 1-hexene was carried out in a benzene solution at 36 °C
under 6 atm of 1:1 H2:CO using 0.2 mol % 1:1 Rh:diphosphine
catalyst prepared from Rh(CO)2(acac) and the chelating diphos-
phine. The production of heptanal and 2-methylhexanal was
monitored by gas chromatography utilizing toluene as an internal
standard. The results of the studies are summarized in Table
2.31 The turnover rate and n:i ratios are reported after 120-
250 turnovers (20-50% conversion).
[DIPHOS-(3,5-CF3)]Ir(CO)2H (15) was synthesized in several
-
steps starting from NBu4+Ir(CO)2I2 (16).29 Reaction of
DIPHOS-(3,5-CF3) (3) with 16 followed by reduction with
NaBH4 in ethanol gave fac-[DIPHOS-(3,5-CF3)]Ir(CO)H3 (17)
in 57% yield. Photolysis of 17 under CO gave [DIPHOS-(3,5-
CF3)]Ir(CO)2H (15).
The wide bite angle diphosphine BISBI occupies diequatorial
sites in (diphosphine)Ir(CO)2H and (diphosphine)Rh(CO)2H12
complexes. Rhodium catalyzed hydroformylation with BISBI
gave high regioselectivity and fast rates for propene hydro-
formylation4 and for hexene hydroformylation12 (n:i ) 66.5:1,
turnover rate ) 12.4 [mol aldehyde] × [mol Rh]-1 h-1]. The
introduction of electron withdrawing groups on the aryl rings
in BISBI-(3,5-CF3) (1) led to a 2-fold increase in n:i regiose-
lectivity and 5-fold faster rates (Table 2).
The T-BDCP ligand has a narrower bite angle than BISBI
(ân ) 106.6° vs 112.6°), and a 37:63 mixture of 12ee:12-ae
was observed for (T-BDCP)Ir(CO)2H (12). Introduction of
electron withdrawing groups on the aryl rings in T-BDCP-(3,5-
CF3) (2) led to an increased amount of diequatorial isomer of
[T-BDCP-(3,5-CF3)]Ir(CO)2H (13) (13ee:13ae ) 90:10). Rhod-
ium catalyzed hydroformylation with T-BDCP-(3,5-CF3) (2) led
to an increase in n:i regioselectivity from 12.1 for T-BDCP to
17.7 for T-BDCP-(3,5-CF3) (2). As in the case of BISBI,
electron withdrawing substituents gave rise to increased n:i
selectivity and to faster rates.
Room temperature 1H NMR showed a triplet at -10.90 ppm
in the hydride region with JHP ) 44 Hz. As in the DIPHOS
complex above, an average value of P-H coupling constant of
44 Hz suggests the existence of some type of fluxional process
interconverting the apical and equatorial phosphorus atoms,
1
making them equivalent. In the H NMR spectrum of 15 at
-110 °C, the hydride resonance appeared as a doublet of
doublets at δ -10.97 (JHP ) 101, -14 Hz). Two phosphine
resonances at δ 39.0 and 37.5 were observed in the 31P{1H}
NMR spectrum of 15 at -96 °C. When all but the hydride
resonance was decoupled, a broad singlet at δ 39.15 and a broad
doublet at δ 37.4 (JHP ) 103 Hz) were observed. These
coupling patterns are consistent with a conformation having an
apical hydride with a large coupling to an apical phosphine and
a small coupling to an equatorial phosphine.
At room temperature, a fluxional process interchanges the
environments of the phosphines, and a single 31P{1H} NMR
resonance was observed at δ 39.8 and the hydride resonance
appeared as a triplet at δ 10.09 with JPH ) 44 Hz. Note that
this coupling constant is the average of the couplings at low
temperature and supports the contention that JHP is temperature
independent that was used in estimating the ratio of 12-ae:12-
ee and 13-ae:13-ee at room temperature. The average room
(28) The (chelate)(CO)2IrH complexes of the o-tolyl and (o-trifluoro-
methyl)phenyl substituted DIPHOS derivatives also showed exclusive
apical-equatorial coordination (see Supporting Information).
(29) Forster, D. Inorg. Nucl. Chem. Lett. 1969, 5, 433.
(30) Friebolin, H. Basic One- and Two-Dimensional NMR Spectroscopy;
VCH Publishers: New York, 1993; p 295.
(31) Hydroformylation results for o-tolyl and (o-trifluoromethyl)phenyl
substituted DIPHOS derivatives are given in Table 1 of Supporting
Information.

Electronic Effects of Electron Withdrawing Aryl Substitutents
J. Am. Chem. Soc., Vol. 119, No. 49, 1997 11823
The DIPHOS ligand has a narrow bite angle (ân ) 84.5°)
that leads to a strong preference for apical-equatorial coordina-
tion in (DIPHOS)Ir(CO)2H. Rhodium catalyzed hydroformy-
lation with DIPHOS gave relatively low n:i regioselectivity of
2.6:1. In contrast to the other two chelates, introduction of
electron withdrawing substituents on the aryl rings in DIPHOS-
(3,5-CF3) (3) led to a lower n:i regioselectiVity of 1.3:1 and to
only a small rate increase.
Figure 2. Effect of equatorial and apical electron withdrawing groups
on n:i selectivity.
Discussion
For PAr3, electron-withdrawing groups are expected to increase
the fraction of the diequatorial isomer that is expected to give
more n-aldehyde. In addition, the presence of electron with-
drawing groups in the diequatorial isomer should also lead to
enhanced selectivity for n-aldehyde formation.
Wide bite angle diequatorial coordinating ligands like BISBI
give high n:i regioselectivity of aldehyde formation in rhodium
catalyzed hydroformylation, while apical-equatorial chelating
ligands like DIPHOS give much lower n:i regioselectivity. Since
our attempts to explain these differences in steric terms based
on molecular mechanics calculations of model transition states
failed to provide much insight, we initiated this study of
electronic effects. Minor perturbations in the electronic proper-
ties of the ligands were made by introducing electron withdraw-
ing groups on the aryl rings of the chelates.
The effect of electron withdrawing groups of equatorial
phosphines was studied by comparing the regioselectivity
obtained from BISBI and BISBI-(3,5-CF3) (1). The 2-fold
increase in regioselectivity achieved with electron withdrawing
substituents is similar to the increases seen for ferrocenyl
diphosphine chelates32 and for monodentate phosphines.33
Unruh and Christenson studied the regioselectivity of rhodium
catalyzed hydroformylation of 1-hexene using a series of 1,1′-
bis(diarylphosphino)ferrocene ligands, which coordinate diequa-
torially.34 They found a strong correlation between selectivity
for linear aldehyde formation and the electron withdrawing
ability of aryl substituents. Our results and Unruh’s both
demonstrate that electron withdrawing groups in the equatorial
position lead to an increase the n:i ratio. In less closely related
work, RajanBabu reported that placing electron withdrawing
substituents on arylphosphine chelates led to increased
enantioselectivity in the hydroformylation of 2-vinylnaphtha-
lenes.35
The T-BDCP complexes exist as a mixture of diequatorial
and apical-equatorial isomers. The addition of electron-
withdrawing groups in T-BDCP-(3,5-CF3) (2) shifted the
equilibrium toward the diequatorial isomer. This shift to
diequatorial coordination is consistent with Hoffmann’s mo-
lecular orbital calculations that show a preference for equatorial
coordination of σ-acceptor ligands.27 The increase in n:i
regioselectivity seen for T-BDCP-(3,5-CF3) (2) may be due to
two reinforcing effects. First, the increase in the amount of
diequatorial chelation would be expected to favor linear
aldehyde formation (high n:i for diequatorial BISBI). Second,
the introduction of electron withdrawing aryl substituents in the
diequatorial chelate should also enhance n:i selectivity as seen
for BISBI-(3,5-CF3) (1).
Mixtures of diequatorial and apical-equatorial isomers are
also observed for monodentate phosphines. For example, Brown
and Kent observed an 85:15 ratio of diequatorial and apical-
equatorial isomers of (PPh3)2Rh(CO)2H.10 In rhodium-catalyzed
hydroformylation with triarylphosphine ligands bearing electron-
withdrawing para substituents, Moser observed increased linear
aldehyde regioselectivity and faster rates.33 These trends can
be explained by the same rationale used to account for the
increased regioselectivity observed for T-BDCP-(3,5-CF3) (2).
The influence on regioselectivity of electron withdrawing aryl
groups on an apical phosphine cannot be measured directly and
must be extracted from the selectivity observed with apical-
equatorial chelating ligands. For the apical-equatorial bound
DIPHOS ligand, we observed a reduction in the n:i ratio upon
the addition of electron withdrawing groups. If electron
withdrawing groups on the equatorial bound phosphine of
DIPHOS-(3,5-CF3) (3) increase the n:i ratio as seen for BISBI-
(3,5-CF3) (1), then electron withdrawing groups in the apical
position of DIPHOS-(3,5-CF3) (3) must decrease the n:i
regioselectivity. Moreover, the negative influence of the apical
phosphine must exceed the positive influence of the equatorial
phosphine to obtain an overall decrease in the n:i ratio with
DIPHOS-(3,5-CF3) (3) (Figure 2). Our hypothesis then is that
an electron withdrawing phosphine in the equatorial position
increases the n:i ratio, while an electron withdrawing phosphine
in the apical position decreases the n:i ratio.
A risky prediction of this hypothesis is that a dissymmetric
DIPHOS derivative (Ar2PCH2CH2PAr′2) could lead to greater
n:i selectivity than either of the related symmetric DIPHOS
derivatives. A chelate designed to have an electron withdrawing
phosphine in the equatorial position and a electron-donating
phosphine in the apical position should give higher n:i regio-
selectivity than either of the parent symmetric DIPHOS
compounds. We are currently pursuing an experimental test
of this prediction.
Conclusion
Chelating diphosphines with electron withdrawing aryl sub-
stituents were synthesized and used in rhodium catalyzed
hydroformylation of 1-hexene. Minor changes in the electronic
character of phosphine ligands afforded significant changes in
aldehyde regioselectivity. Interestingly, electron withdrawing
groups in equatorial and apical positions had opposite effects
on n:i regioselectivity. Hydroformylation results with the
electron withdrawing diequatorial coordinated ligands BISBI-
(3,5-CF3) (1) and T-BDCP-(3,5-CF3) (2) showed an increase
in the n:i ratio, while apical-equatorial coordinated ligand
DIPHOS-(3,5-CF3) (3) showed a decrease in the n:i ratio. Thus,
electron withdrawing aryl substituents in the equatorial position
lead to higher n:i regioselectivity and in the apical position to
lower regioselectivity.
However, a disturbing inconsistency emerges if the macro-
scopic changes in the catalyst upon going from BISBI to
DIPHOS catalysts are considered. The macroscopic changes
in going from H(alkene)Rh(CO)(BISBI) to H(alkene)Rh(CO)-
(DIPHOS) involve replacement of an equatorial phosphine (one
equatorial BISBI arm) with a strongly electron accepting CO
ligand and replacement of an apical CO ligand by a strongly
electron donating phosphine ligand (apical DIPHOS arm)
(Figure 3). The second arm of each diphosphine, in the
(32) Unruh, J. D.; Christenson, J. R. J. Mol. Catal. 1982, 14, 19.
(33) Moser, W. R.; Papile, C. J.; Brannon, D. A.; Duwell, R. A.;
Weininger, S. J. J. Mol. Catal. 1987, 41, 271.
(34) Hughes, O. R.; Young, D. A. J. Am. Chem. Soc. 1981, 103, 6636.
(35) RajanBabu, T. V.; Ayers, T. A. Tetrahedron Lett. 1994, 35, 4295.

11824 J. Am. Chem. Soc., Vol. 119, No. 49, 1997
Casey et al.
Figure 3. Predicted electronic effect of BISBI to DIPHOS substitution on n:i regioselectivity.
to give an oil. Recrystallization from EtOH gave 1 (650 mg, 54%) as
a white solid, mp 133-138 °C. 1H NMR (CD2Cl2, 300 MHz) δ 7.9-
7.5 (m, aryl); 7.3-6.8 (m, biphenyl); 3.42, 3.26 (AB quartet, JAB ) 13
Hz, CHAHBP). 13C{1H} NMR (CD2Cl2, 75.4 MHz) (note diastereotopic
aryl groups) δ 141.0, 140.8, 140.7, 140.4 (all ipso C); 133.6 (br d, JCP
) 21 Hz, ortho-aryl(CF3)2); 132.8 (br d, JCP ) 18 Hz, ortho′-aryl-
(CF3)2); 132.3 (q, JCF ) 34 Hz, meta-aryl(CF3)2); 132.1 (q, JCF ) 33
Hz, meta′-aryl(CF3)2); 124.0 (sept, JCF ) 3 Hz para-aryl(CF3)2); 123.6
(sept, JCF ) 3 Hz para′-aryl(CF3)2); 133.9, 133.9, 133.8, 131.0, 130.3,
equatorial position, remains unchanged. Overall, the replace-
ment of BISBI by DIPHOS places a better acceptor ligand in
the equatorial position and a better donor ligand in the apical
position. By analogy to the effect of electron donor aryl
substituents, both of these changes should have resulted in higher
regioselectivity not in the observed lower regioselectivity seen
for DIPHOS.
Apparently, the major steric differences between diequatorial
chelates like BISBI and apical-equatorial chelates like DIPHOS
overwhelm any electron differences between the chelates.
Clearly, the regioselectivity of hydroformylation is governed
by a complex web of electronic and steric effects that have so
far defied unraveling.
130.2, 130.2, 128.6, 127.5 (other biphenyl and aryl); 123.5 (q, JCF
)
273 Hz, CF3); 34.4 (d, JCP ) 20 Hz, CH2P). 31P{1H} NMR (CD2Cl2,
121 MHz) δ -8.5 (s). EI MS: m/z (%) 1094 (10) [P+], 1075 (17) [P+
- F].
trans-1,2-Bis{[bis-{3,5-di(trifluoromethyl)phenyl}phosphino]-
methyl}cyclopropane, T-BDCP-(3,5-CF3) (2). Following a procedure
similar to that used for 1, trans-1,2-bis(tosyloxymethyl)cyclopropane42
(113 mg, 0.28 mmol) in THF (5 mL) was added to a phosphide solution
prepared from 4 (250 mg, 0.55 mmol) and KH (25 mg, 0.62 mmol) in
THF (4 mL) at -78 °C. Workup gave a clear oil that crystallized
upon addition of EtOH to give 4 (100 mg, 36%) as a white solid, mp
110-112 °C. 1H NMR (CD2Cl2, 300 MHz) δ 7.9-7.8 (m, aryl), 2.07
(m, CH2P), 0.54 (m, 2H, CHCH2P), 0.44 (m, cyclopropyl-CH2).
31P{1H} NMR (CD2Cl2, 121 MHz) δ -14.6 (s). 13C{1H} NMR (CD2-
Cl2, 75.4 MHz) (note diastereotopic aryl groups) δ 141.3 (d, JCP ) 10
Hz, ipso), 141.1 (d, JCP ) 10 Hz, ipso′), 133.3 (d, JCP ) 18 Hz, ortho),
133.1 (d, JCP ) 18 Hz, ortho′),132.3 (q, JCF ) 31 Hz, meta, meta′),
123.7 (s, para, para′), 123.6 (q, JCF ) 272 Hz, CF3), 32.9 (d, JCP ) 13
Hz, CH2P), 16.8 (dd, JCP ) 14, 12 Hz, CH), 15.8 (t, JCP ) 10 Hz,
CH2). LSIMS calcd (obsd) for C37H20F24P2 982.5 (983.0).
[3,5-(CF3)2 C6H3]2P(CHdCH2) (7). Following a procedure similar
to that used for diphenylvinylphosphine,43 a THF solution of vinyl
magnesium bromide (70 mL, 1.0 M, 70 mmol) was added slowly to a
benzene solution of [3,5-(CF3)2C6H3]2PCl (21.0 g, 43 mmol). The
solution was refluxed for 20 h and hydrolyzed with 100 mL of 10%
aqueous NH4Cl. The organic layer was separated, and the aqueous
layer was extracted with ether and hot benzene. The combined organic
layers were washed with water, dried (MgSO4), and evaporated to give
a brownish oil. High vacuum distillation of the oil from a 100 °C oil
bath gave 7 as a white solid (9.7 g, 47%). 1H NMR (CD2Cl2, 300
MHz) δ 7.94 (m, para), 7.90 (d, JHP ) 6.7 Hz, ortho), 6.71 (ddd, JHH
) 18.1, 11.5 Hz, JHP ) 7.4 Hz, CH)CH2), 6.25 (ddd, JHP ) 38.2 Hz,
Experimental Section
General Methods. See Supporting Information. (3,5-(CF3)2C6H3)-
PCl,18 t-BDCP,36 NBu4[Ir(CO)2I2],29 (DIPHOS)Ir(CO)I,16 (Ph3P)3Ir-
(CO)H,37 Ir(CO)2(acac),38 and Rh(CO)2(acac)39 were prepared by
literature methods.
[3,5-(CF3)2C6H3]2PH (4). Following a procedure similar to that used
for diphenylphosphine,40 a solution of [3,5-(CF3)2C6H3]2PCl (17.5 g,
0.036 mol) in ether (200 mL) was slowly added to a suspension of
LiAlH4 (1.35 g, 0.036 mol) in ether (250 mL) at -78 °C. The pale
brown slurry was refluxed for 2 h, cooled to 0 °C, and hydrolyzed
with 10% aqueous NH4Cl. The fluorescent yellow-green organic layer
was separated, and the remaining white precipitate was extracted 5 times
with ether. The combined ether extracts were washed with water, dried
(Na2SO4), and evaporated to give 4 (12.6 g, 77%) as a pale yellow
air-sensitive solid. Sublimation at 60 °C under vacuum gave 4 as a
white solid, mp 69-71 °C. 1H NMR (CD2Cl2, 300 MHz) δ 7.97 (d,
JHP ) 6.5 Hz, ortho), 7.91 (s, para), 5.52 (d, JHP ) 225 Hz, PH).
13C{1H} NMR (CD2Cl2, 75 MHz) δ 137.1 (d, JCP ) 15.8 Hz, ipso),
134.4 (d, JCP ) 18.4 Hz, ortho), 132.5 (qd, JCF ) 33.7 Hz, JCP ) 5.7
Hz, meta), 123.8 (br d, JCP ) 3.8 Hz, para), 123.6 (q, JCF ) 273 Hz,
CF3). 31P{1H} NMR (CD2Cl2, 121 MHz) δ -40.2 (s). 19F{1H} NMR
(CD2Cl2, 282 MHz) δ -63.3 (s). IR (CCl4): 2320 (νP-H); 1359, 1167,
1137 (νCF); 1279 (δCdC-H) cm-1. HRMS Calcd (obsd) for C16H7F12P
458.0094 (458.0086).
2,2′-Bis{[bis-{3,5-di(trifluoromethyl)phenyl}phosphino]methyl}-
1,1′-biphenyl, BISBI-(3,5-CF3) (1). Following a procedure similar
to that used for (R,R)-trans-4,5-bis[[bis(2-trifluoromethylphenyl)phos-
phino]methyl]-2,2-dimethyl-1,3-dioxolane,41 2,2′-bis(bromomethyl)-
1,1′-biphenyl (370 mg, 1.09 mmol) in THF (10 mL) was added
dropwise to a purple phosphide solution prepared from 4 (250 mg,
0.55 mmol) and KH (25 mg, 0.62 mmol) in THF (10 mL) at -78 °C.
The reaction mixture was stirred overnight at room temperature.
Solvent was evaporated and the residue was dissolved in Et2O. The
ether solution was washed with water, dried (Na2SO4), and evaporated
JHH ) 11.5, 1.5 Hz, CHdCHH), 6.00 (ddd, JHH ) 18.1, 1.5 Hz, JHP
)
16.7 Hz, CHdCHH). 13C{1H} NMR (CD2Cl2, 75 MHz) δ 141.1 (d,
JCP ) 17 Hz, CH)CH2), 135.4 (d, JCP ) 31 Hz, ipso), 133.65 (d, JCP
) 13 Hz, CHdCH2), 133.4 (d, JCP ) 20 Hz, ortho), 132.7 (qd, JCF
)
33 Hz, JCP ) 6 Hz, meta), 123.9 (s, para), 123.8 (q, JCF ) 273 Hz,
CF3). 31P{1H} NMR (121 MHz, CD2Cl2) δ -9.5 (s). 19F{1H} NMR
(CD2Cl2, 282 MHz) δ -63.3 (s). IR (CCl4): 1616 (νCdC); 1354, 1187,
1146 (νCF); 1279 (δCdC-H) cm-1
.
1,2-Bis{bis[3,5-di(trifluoromethyl)phenyl]phosphino}ethane,
DIPHOS-(3,5-CF3) (3). Following a procedure similar to that used
(36) Aviron-Violet, P.; Colleuille, Y.; Varagnat, J. J. Mol. Cat. 1979, 5,
41.
(37) Wilkinson, G. Inorg. Synth. 1972, 13, 126.
(38) Roberto, D.; Cariati, E.; Psaro, R.; Ugo, R. Organometallics 1994,
13, 4227.
for 1-diphenylphosphino-2-bis(m-fluorophenyl)phosphinoethane,44
a
mixture of 4 (5.0 g, 10.8 mmol), 7 (5.2 g, 10.8 mmol), and KO-t-Bu
(0.2 g, 1.8 mmol) in THF (20 mL) was refluxed for 4 h. Solvent was
(39) Varshavskii, Y. S.; Cherkasova, T. G. Russ. J. Inorg. Chem. 1967,
12, 899.
(40) Taylor, R. C.; Kolodny, R. and Walters, D. B. Synth. Inorg. Met.-
Org. Chem. 1973, 3, 175.
(42) () Blomquist, A. T.; Longone, D. T. J. Am. Chem. Soc. 1959, 81,
2012.
(43) Berlin, K. D.; Butler, G. B. J. Org. Chem. 1961, 26, 2537.
(44) Kapoor, P. N.; Pathak, D. D.; Gaur, G. and Kutty, M. J. Organomet.
Chem. 1984, 276, 167.
(41) McKinstry, L.; Livinghouse, T. Tetrahedron 1994, 50, 6145.

Electronic Effects of Electron Withdrawing Aryl Substitutents
J. Am. Chem. Soc., Vol. 119, No. 49, 1997 11825
evaporated, and the resulting brown residue was recrystallized twice
from THF and then from CHCl3 to give 3 (5.3 g, 52%) as a colorless
solid, mp 178-182 °C. 1H NMR (CD2Cl2, 300 MHz) δ 7.91 (s, para),
7.78 (br s, ortho), 2.24 (virtual t, 10.5 Hz splitting, CH2CH2). 31P{1H}
NMR (CD2Cl2, 121 MHz) δ -10.7 (s). 19F{1H} NMR (CD2Cl2, 282
cell purged with CO, was photolyzed for 30 min at 36 °C in a Rayonet
photoreactor equipped with RPR 3000A bulbs with a maximum
emission at 300 nm. The solvent was evaporated to give 15 (55 mg,
67%) as a pale yellow solid. 1H NMR (CD2Cl2, 500 MHz, 25 °C) δ
8.2-7.5 (m, aryl), 2.6 (m, CH2P), -10.90 (t, JHP ) 44 Hz, IrH). 1H
NMR (CD2Cl2, 500 MHz, -88 °C) δ -10.97 (dd, JHP ) 101, -14
Hz). 31P{1H} NMR (CD2Cl2, 121 MHz, 25 °C) δ 39.8 (s). 31P{1H}
(CD2Cl2, 202 MHz, -96 °C): δ 39.0 (s), 37.5 (br s). 31P NMR{1H
decoupled, except for the hydride region} (CD2Cl2, 202 MHz, 25 °C):
δ 39.8 (JPH ) 43 Hz). 31P {1H decoupled, except for the hydride
MHz) δ -63.3 (s). EI MS: m/z (%) 942 (46) [M+], 923 (24) [M+
F], 914 (35) [M+ - C2H4].
-
[BISBI-(3,5-CF3)]Ir(CO)2H (10). A CD2Cl2 solution of Ir(CO)2-
(acac) (3.2 mg, 9.2 µmol) and 1 (10 mg, 9.2 µmol) under 1 atm of 1:1
CO:H2 was shaken in a NMR tube for 20 h to give a solution of 10. 1H
NMR (CD2Cl2, 300 MHz) δ 8.1-5.6 (m, aryl), 4.20 (dt, JHH ) 13.5
Hz, JHP ) 3.4 Hz, PCHH), 4.05 (dt, JHH ) 13.5, JHP ) 7.5 Hz, PCHH),
-12.32 (t, JHP ) 20 Hz, Ir-H). 31P{1H} (CD2Cl2, 121 MHz) δ 9.2 (s).
19F{1H} (CD2Cl2, 282 MHz) δ -63.39 (s), -63.43 (s). 1H NMR (CD2-
Cl2, 300 MHz, -84 °C) δ 8.0-7.7 (m, 3,5-(CF3)2-C6H3), 7.4-6.9 (m,
biphenyl), 6.0 (m, biphenyl), 4.25-3.8 (m, both CHAHBP), -12.54 (dd,
JHP ) 25, 15 Hz, IrH). 31P{1H} (CD2Cl2, 121 MHz, -84 °C) δ 13.28,
6.40 (ABq, JPP ) 122 Hz). IR (CH2Cl2): 2079 (w, νIr-H), 2005 (s,
region} (CD2Cl2, 202 MHz, -98 °C) δ 39.2 (br s), 37.4 (br d, JPH
103 Hz). 19F{1H} NMR (CD2Cl2, 282.2 MHz, 25 °C) δ -63.38 (s).
IR (CH2Cl2) 2002, 1954 cm-1
)
.
Catalytic Hydroformylation of 1-Hexene. Hydroformylation reac-
tions were performed as described earlier in a 90-mL Fischer-Porter
bottle equipped with a star-head magnetic stir bar.12 Rh(CO)2(acac)
(10 mg, 0.03875 mmol) and a chelating diphosphine (0.03875 mmol)
were dissolved in benzene (6.0 mL) under 70 psig of analyzed CO:H2
(50.02% CO, 49.98% H2) with toluene (0.20 mL, 1.9 mmol) added as
an internal GC standard. After 1 h of stirring at 34 ( 2 ° C, 1-hexene
(2.50 mL, 0.020 mmol) was added. Samples containing heptanal and
2-methylhexanal were analyzed by temperature-programmed gas chro-
matography on an HP5890A chromatography interfaced to a HP3390A
integrator using an HP-1 10 m × 0.53 mm methyl silicone gum capillary
column.
ν
CO), 1960 (vs, νCO) cm-1
(T-BDCP)Ir(CO)2H (12). A solution of (T-BDCP)Ir(PPh3)(CO)H
.
(11) (334 mg, 0.362 mmol) in 6 mL of CH2Cl2 was stirred under 1
atm of CO at 25 °C for 2 days. Addition of 20 mL of a CO-saturated
ethanol gave a precipitate which was immediately filtered and rinsed
with ethanol to yield 12 (165 mg, 66%) as a white solid. 1H NMR
(CD2Cl2, 500 MHz) δ 7.6-7.3 (m, aryl), 3.33 (m, CH2P), 1.64 (m,
CH), 0.64 (virtual t, 7 Hz splitting, CH2), -11.48 (t, JHP ) 16 Hz,
IrH). 1H NMR (CD2Cl2, 500 MHz, -74 °C) δ 7.6-7.3 (m, aryl), 3.33
(m, CH2P), 1.64 (m, CH), 0.64 (virtual t, 7 Hz splitting, CH2), -11.35
(dd, JHP ) 97, -23 Hz, IrH), -12.07 (dd, JHP ) -26, -15 Hz, IrH).
13C{1H} NMR (CD2Cl2) δ 183.5 (s, CO); 132.5 (t, JCP ) 6 Hz), 132.3
(t, JCP ) 6 Hz), 129.7 (d, JCP ) 17 Hz), 128.4 (m) (aryl); 38.8 (m,
CH2P); 19.3 (t, JCP ) 8 Hz, CH); 13.9 (t, JCP ) 15 Hz, CH2). 31P{1H}
(CD2Cl2) δ 1.3 (s). 31P{1H} (CD2Cl2, -74 °C) δ 7.3 (d, JPP ) 105
Hz), 5.3 (d, JPP ) 105 Hz), 0.9 (d, JPP ) 24 Hz), -5.2 (d, JPP ) 24
Acknowledgment. Financial support from the Department
of Energy, Office of Basic Energy Sciences, is gratefully
acknowledged. E.W.B. thanks the Fulbright commission for a
fellowship. B.R.P. thanks the Alexander von Humboldt Stiftung
for a von Lynene Fellowship. Grants from NSF (CHE-9105497)
and from the University of Wisconsin for the purchase of the
X-ray instruments and computers are acknowledged. We thank
Alpha Aesar for a loan of iridium compounds.
Hz). IR (CH2Cl2): 2064 (m, νIr-H), 1970 (s, νCO), 1915 (s, νCO) cm-1
IR (hexane) 1990 (s), 1978 (s), 1943 (vs), 1930 (vs) cm-1
.
.
Supporting Information Available: General experimental
methods and experimental details and characterization for 8, 9,
11, 14, 17, (2-CH3C6H4)2P(O)H, (2-CF3-C6H4)2P(O)H, (2-
CH3C6H4)2PCl, (2-CF3-C6H4)2PCl, (2-CH3C6H4)2P(CHdCH2),
(2-CF3-C6H4)2P(CHdCH2), (2-CH3C6H4)2PH, (2-CF3C6H4)2PH,
(2-CH3C6H4)2PCH2CH2P(2-CH3C6H4)2 (DIPHOS-(2-CH3)), (2-
CF3C6H4)2PCH2CH2P(2-CF3C6H4)2 (DIPHOS-(2-CF3)), [T-BD-
CP-(3,5-CF3)]Ir(PPh3)(CO)H, [DIPHOS-(3,5-CF3)]Ir(CO)I,
[DIPHOS-(2-CH3)]Ir(CO)I, [DIPHOS-(2-CF3)]Ir(CO)I, [DIPHOS-
(2-CH3)]Ir(CO)H3, [DIPHOS-(2-CF3)]Ir(CO)H3, and [DIPHOS-
(2-CH3)]Ir(CO)2H, and [DIPHOS-(2-CF3)]Ir(CO)2H and X-ray
crystallographic data for 9 (26 pages). See any current masthead
page for ordering and Internet access instructions.
[t-BDCP-3,5-(CF3)2]Ir(CO)2H (13). A CD2Cl2 solution of Ir(CO)2-
(acac) (4 mg, 11.5 µmol) and 2 (12 mg, 12.2 µmol) under 1 atm of 1:1
CO:H2 was shaken in a NMR tube for 20 h to give a solution of 13. 1H
NMR (CD2Cl2, 300 MHz) δ 8.2-7.8 (m, aryl), 3.6 (m, CH2P), 0.9 (m,
2H, CHCH2P), 0.6 (m, cyclopropyl-CH2), -12.03 (t, JHP ) 16 Hz,
IrH). 31P{1H} (CD2Cl2, 121 MHz) δ 11.34 (s). 19F{1H} (CD2Cl2, 282
MHz) δ -63.39 (s), -63.43 (s). 1H NMR (CD2Cl2, 300 MHz, -84
°C) δ 8.2-7.6 (m, aryl), 3.9 (m, CHHP), 3.4 (m, CHHP), 0.9 (m, 2H,
CHCH2P), 0.5 (m, cyclopropyl-CH2), -12.30 (dd, JHP ) 28, 17 Hz,
IrH). 31P{1H} (CD2Cl2, 121 MHz, -84 °C) δ 13.06, 10.78 (AB q, JPP
) 116 Hz). IR (CH2Cl2): 2064 (w, νIr-H), 2001 (s, νCO), 1955 (s, νCO
)
cm-1
.
[DIPHOS-(3,5-CF3)]Ir(CO)2H (15). Following a procedure similar
to that used for (DIPHOS)Ir(CO)2H,16 a benzene solution of [DIPHOS-
(3,5-CF3)]Ir(CO)H3 (17) (80 mg, 0.067 mmol) in a quartz photolysis
JA9719440