Highly stereoselective formation of Cp*IrCl complexes of N,N-dimethylamino acids

Article abstract of DOI:10.1021/om9507890

N,N-Dimethylamino acids serve as (N, O)-chelating monoanionic ligands to the prochiral fragment Cp*IrCl. A series of four such complexes and one Rh analog all were formed with diastereoselectivity of >50:<1. The structure of the valine complex 5d (C¹⁷H₂₉ClIrNO₂) was analyzed at 298 K, from which a cis-arrangement of Cp* and valine side chain (i-Pr) was revealed. Solution NMR studies of 5d, aided by an unusual zero coupling between the two methine protons of the amino acid, showed that the structures in solution and the solid were very similar. The preference for the cis-oriented Cp* and amino acid side chain in 5 is attributed to a maximum number of gauche interactions in the metallacycle and its substituents, especially pronounced for N,N-disubstituted amino acids.

Full text of DOI:10.1021/om9507890

1230
Organometallics 1996, 15, 1230-1235
High ly Ster eoselective F or m a tion of Cp *Ir Cl Com p lexes
of N,N-Dim eth yla m in o Acid s
Douglas B. Grotjahn* and Camil J oubran
Department of Chemistry and Biochemistry, Box 871604, Arizona State University,
Tempe, Arizona 85287-1604
J ohn L. Hubbard†
Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322-0300
Received October 5, 1995X
N,N-Dimethylamino acids serve as (N, O)-chelating monoanionic ligands to the prochiral
fragment Cp*IrCl. A series of four such complexes and one Rh analog all were formed with
diastereoselectivity of g50:e1. The structure of the valine complex 5d (C17H29ClIrNO2) was
analyzed at 298 K, from which a cis-arrangement of Cp* and valine side chain (i-Pr) was
revealed. Solution NMR studies of 5d , aided by an unusual zero coupling between the two
methine protons of the amino acid, showed that the structures in solution and the solid
were very similar. The preference for the cis-oriented Cp* and amino acid side chain in 5
is attributed to a maximum number of gauche interactions in the metallacycle and its
substituents, especially pronounced for N,N-disubstituted amino acids.
Sch em e 1
In tr od u ction
Amino acid-metal coordination complexes1 are of
interest in biochemistry.2 Amino acids are sources of
chiral, enantiomerically pure organometallic catalysts.3
Some studies of organometallic amino acid complexes
have focused on the bonding of fragments to the amino
acid4 (usually as a chelate, but see ref 5), whereas others
have explored the asymmetric induction of the amino
acid on coordination of a prochiral metal fragment.6 For
example, interaction of amino acid salts 1 with [Cp*IrCl-
(µ-Cl)]2 (2)7 leads to 3 (Scheme 1), with essentially no
diastereomeric excess in the case of N-unsubstituted
amino acids and up to 84% de in the case of proline, an
N-monosubstituted acid.6a,b We wanted to expand our
investigations of the interactions of amino acids and
transition metals8 to include complexes similar to 3.
However, we initiated studies of the corresponding N,N-
dimethyl amino acids 4 (Scheme 2), with the idea that
the complexes would enjoy greater solubility in nonpolar
organic solvents, the methyl groups would be convenient
for monitoring reactions of 4 by NMR spectroscopy, and
potential proton transfer involving metal-complexed
NH2 moieties9 would be blocked. Here we report on the
completely stereoselective (g50:1) conversion of 4 to 5
and through X-ray crystallography and NMR spectros-
† X-ray crystal structure.
X Abstract published in Advance ACS Abstracts, J anuary 15, 1996.
(1) (a) Laurie, S. H. Amino Acids, Peptides and Proteins. In
Comprehensive Coordination Chemistry; Wilkinson, G., Ed.; Perga-
mon: Oxford, 1987, Vol. 2, pp 739-776. (b) Ioganson, A. A. Russ. Chem.
Rev. 1985, 54, 277-292. (c) Sigel, H.; Martin, R. B. Chem. Rev. 1982,
82, 385-426.
(2) Metalloproteins: Structural Aspects. Adv. Prot. Chem. 1991, 42.
Ibers, J . A.; Holm, R. H. Science 1980, 209, 223-235. For a leading
reference to organometallic complexes of other biomolecules, see: Chen,
H.; Maestre, M.; Fish, R. H. J . Am. Chem. Soc. 1995, 117, 3631-3632.
(3) (a) Brunner, H. Top. Stereochem. 1988, 18, 129-247. (b) Chiral
Catalysis; Asymmetric Synthesis, Vol. 5; Academic: Orlando, FL, 1984.
(c) Asymmetric Catalysis in Organic Synthesis; Noyori, R., Ed.; Wiley:
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VCH: New York, 1993.
(4) (a) Dowerah, D.; Singh, M. M. J . Ind. Chem. Soc. 1980, 57, 368-
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Dowerah, D.; Singh, M. M. Transition Met. Chem. 1976, 1, 294-5. (d)
Shinoda, S.; Inoue, N.; Takita, K.; Saito, Y. Inorg. Chim. Acta 1982,
65, L21-L23. (e) Sheldrick, W. S.; Heeb, S. J . Organomet. Chem. 1989,
377, 357-366. (f) Sheldrick, W. S.; Exner, R. Inorg. Chim Acta 1990,
175, 261-268. (g) Lippmann, E.; Kra¨mer, R.; Beck, W. J . Organomet.
Chem. 1994, 466, 167-174. (h) Bergs, R.; Su¨nkel, K.; Beck, W. Chem.
Ber. 1993, 126, 2429-2432. (i) Darensbourg, D. J .; Atnip, E. V.;
Klausmeyer, K. K.; Reibenspies, J . H. Inorg. Chem. 1994, 33, 5230-
5237. (j) Severin, K.; Su¨nkel, K.; Beck, W. Chem. Ber. 1994, 127, 615-
620.
(6) (a) Kra¨mer, R.; Polborn, K.; Wanjek, H.; Zahn, I.; Beck, W. Chem.
Ber. 1990, 123, 767-778. (b) Carmona, D.; Mendoza, A.; Lahoz, F. J .;
Oro, L. A.; Lamata, M. P.; San J ose, E. J . Organom. Chem. 1990, 396,
C17-C21. (c) Dersnah, D. F.; Baird, M. C. J . Organomet. Chem. 1977,
127, C55-C58. (d) Kra¨mer, R.; Polborn, K.; Robl, C.; Beck, W. Inorg.
Chim. Acta 1992, 198-200, 415-420. (e) Carmona, D.; Lahoz, F. J .;
Atencio, R.; Oro. L. A.; Lamata, M. P.; San J ose´, E. Tetrahedron:
Asymmetry 1993, 1425-1428. (f) Sheldrick, W. S.; Hauck, E.; Korn, S.
J . Organomet. Chem. 1994, 467, 283-292. (g) Sheldrick, W. S.; Brandt,
K. Inorg. Chim. Acta 1994, 217, 51-59. (h) Werner, H.; Daniel, T.;
Nu¨rnberg, O.; Knaup, W.; Meyer, U. J . Organomet. Chem. 1993, 445,
229-235. (i) Sheldrick, W. S.; Gleichmann, A. J . Organomet. Chem.
1994, 470, 183-187. (j) Sheldrick, W. S.; Exner, R. Inorg. Chim. Acta
1989, 166, 213-219.
(7) White, C.; Yates, A.; Maitlis, P. M. Inorg. Synth. 1992, 29, 228-
234.
(8) Grotjahn, D. B.; Groy, T. L. J . Am. Chem. Soc. 1994, 116, 6969-
6970; Organometallics 1995, 14, 3669-3682.
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(5) Cp2Ti(amino acid)2 complexes: Klapo¨tke, T. M.; Ko¨pf, H.; Torni-
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0276-7333/96/2315-1230$12.00/0 © 1996 American Chemical Society

Cp*IrCl Complexes
Organometallics, Vol. 15, No. 4, 1996 1231
Ta ble 1. Isola ted Yield , Color , IR Da ta , a n d Elem en ta l An a lyses of 5
calcd (%)
found (%)
H
a
compd
R
yield (%)
color
IR νCO (cm-1
)
molecular formula
C
H
N
C
N
5a
CH3
Ph
Ph (Rh)
PhCH2
i-Pr
98
96
99
81b
96
yellow
yellow
orange
yellow
yellow
1652
1661
1648
C15H25ClIrNO2
37.61
44.39
53.17
45.44
42.84
5.26
5.03
6.02
5.27
5.47
2.92
2.59
3.10
2.52
2.17
37.83
44.56
52.84
45.48
42.58
5.22
5.12
6.04
5.31
5.48
2.90
2.53
3.16
2.46
1.97
5b
5b′
5c
C
C
20H27ClIrNO2
20H27ClRhNO2
C21H29ClIrNO2
17H29ClIrNO2
5d
1656
C
a
b
In KBr. Yield of recrystallized product from CH2Cl2-Et2O. The crude yield was 100%.
Sch em e 2
analytically pure yellow or yellow-orange solid in g95%
yields (see Table 1). The Rh analog of 2 was used to
make 5b′, an orange solid. Chelation of the amino acid
to the metal was immediately evidenced by the appear-
1
ance of two three-proton singlets in H NMR spectra of
5, ascribed to diastereotopic methyl protons; if the amino
acid nitrogen were not bound to the metal, rapid
inversion at N would lead to observation of a six-proton
1
singlet for the methyl groups. Further, both H and 13C
NMR spectral data for 5 (Tables 2 and 3) revealed the
presence of resonances attributable to a single complex,
whereas on the basis of literature precedent6a,b two were
expected. From cursory inspection of NMR data and
molecular models it was unclear which diastereomer
should predominate, and the assumption that 5 had the
same relative configuration as the major diastereomer
3a previously observed seemed unwarranted.
Thus, single crystals of the valine derivative 5d were
grown by vapor diffusion of hexane into a toluene
solution of the complex. The results of X-ray diffraction
of 5d are presented in Figure 1 and Tables 4-6. The
molecular structure in Figure 1 verifies the composition
of the product and the chelation of the amino acid. The
absolute configuration of the complex is confirmed by
refinement of the η parameter to a value of +1.1(1).12
The geometry about the metal can be described as that
of a three-legged piano stool,13 with the two angles Cl-
Ir-O(1) and Cl-Ir-N [88.1(3) and 87.7(3)°, respectively]
being just under the ideal value of 90 and the N-Ir-
O(1) angle being even more acute [76.4(4)°]. Very
similar values for these and other metrical parameters
were observed in proline-derived complexes 3.6a,b The
greatest metrical difference between reported structures
of 3 and that of 5a appears to be that the Ir-N distance
in 5d [2.158(11) Å] is slightly larger than the highest
value reported for 3 [2.140(8) Å].6b This could be a
result of the greater bulk of N,N-dimethyl substituents
at N compared to the single N-alkyl group in proline.
Whereas 3 was obtained as a mixture of diastereo-
mers 3a and 3b, 5d is formed as a single diastereomer,
with the largest metallacycle substituents, i-Pr and Cp*,
cis to each other, as in 3a . Both the i-Pr and Cp* groups
occupy pseudoequatorial positions. Insight into the
conformational and sterochemical preference for the
systems 3 and 5 is gained by examining the relative
orientations of the metallacycle substituents. The C(8)-
C(7) bond to the CH(CH3)2 group is gauche to both of
the N-CH3 bonds, as indicated by torsional angles
C(8)-C(7)-N-C(12) ) +65.9° and C(8)-C(7)-N-C(11)
) -54.2°, respectively. Similarly, the vector from Ir to
the centroid of the Cp* ring (C*) is gauche to both the
N-CH3 bonds, as revealed by the torsional angles C*-
Ir-N-C12 ) -72.3° and C*-Ir-N-C11 ) 49.6°,
copy provide some insight into the origins of the high
selectivity for 5.
Resu lts
F or m a tion a n d Str u ctu r e of 5 in th e Solid a n d
in Solu tion . The requisite N,N-dimethylamino acids
4 were made from 1 by known reductive amination
methods.10 Enantiomeric purity of 4b was confirmed
by DCC/HOBT-mediated coupling with (S)-R-methyl-
benzylamine, leading to 7 (88%),11 none of the diaste-
reomer being detected in the crude mixture.
Stirring 4, 2, and anhydrous K2CO3 (molar ratio 1.00:
0.50:1.00) in acetonitrile for 12-36 h, followed by rotary
evaporation, addition of CH2Cl2, filtration of the result-
ing mixture, and concentration, generally left 5 as
(10) Bowman, R. E.; Staoud, H. H. J . Chem. Soc. 1950, 1342-1345.
(11) The enantiomer has been reported: Takeuchi, S.; Ohgo, Y. Bull.
Chem. Soc. J pn. 1981, 54, 2136-2141.
(12) Rogers, D. Acta Crystallogr., Sect. A 1981, A37, 734-741.
(13) Isobe, K.; Bailey, P. M.; Maitlis, P. M. J . Chem. Soc., Dalton
Trans. 1981, 2003-2008.

1232 Organometallics, Vol. 15, No. 4, 1996
Ta ble 2. 1H NMR (δ, p p m ) Da ta for 5a
Grotjahn et al.
compd
solvent
CDCl3
C5(CH3)5
N(CH3)2
CHR
R
5a
1.55 (s, 15H)
2.92 (s, 3H)
2.93 (s, 3H)
2.91 (s, 3H)
2.92 (s, 3H)
2.28 (s, 3H)
2.46 (s, 3H)
2.84 (s, 3H)
2.85 (s, 3H)
2.815 (s, 3H)
2.841 (s, 3H)
2.66 (s, 3H)
2.67 (s, 3H)
2.663 (s, 3H)
2.669 (s, 3H)
2.76 (s, 3H)
3.10 (s, 3H)
4.18
(q, J ) 6.8, 1H)
3.97
(q, J ) 7.2, 1H)
4.44
1.08
(d, J ) 6.8, 3H)
1.02
(d, J ) 7.2, 3H)
0.96
(d, J ) 7.0, 3H)
7.2-7.33 (m, 5H)
CD3CN
C6D6
1.54 (s, 15H)
1.08 (s, 15H)
1.59 (s, 15H)
1.57 (s, 15H)
1.60 (s, 15H)
1.60 (s, 15H)
1.53 (s, 15H)
(q, J ) 7.0, 1H)
5.29 (s, 1H)
5b
5b′
5c
CDCl3
CD3CN
CDCl3
CD3CN
CDCl3
5.05 (s, 1H)
4.96 (s, 1H)
4.75 (s, 1H)
7.19-7.26 (m, 2H)
7.32-7.42 (m, 3H)
7.12-7.21 (m, 2H)
7.21-7.30 (m, 3H)
7.14-7.20 (m, 2H)
7.29-7.37 (m, 3H)
4.57
2.75 (dd, J ) 7.9, 15.4, 1H)
3.39 (dd, J ) 5.3, 15.4, 1H)
7.08-7.17 (m, 1H)
(dd, J ) 5.3, 7.9, 1H)
7.18-7.28 (m, 4H)
CD3CN
1.53 (s, 15H)
2.97 (s, 3H)
3.04 (s, 3H)
4.23
2.81 (dd, J ) 3.7, 14.2, 1H)
2.93 (dd, J ) 8.0, 14.2, 1H)
7.13-7.21 (m, 1H)
(dd, J ) 3.7, 8.0, 1H)
7.23-7.30 (m, 4H)
5d
CDCl3
1.54 (s, 15H)
1.53 (s, 15H)
3.00 (s, 3H)
3.07 (s, 3H)
3.84 (s, 1H)
3.65 (s, 1H)
1.12 (d, J ) 6.7, 3H)
1.15 (d, J ) 7.2, 3H)
1.96 (septet, J ) 7, 1H)
1.06 (d, J ) 6.7, 3H)
1.09 (d, J ) 7.4, 3H)
1.93 (septet, J ) 7, 1H)
CD3CN
2.98 (s, 3H)
3.07 (s, 3H)
a
Multiplicity and coupling constants (Hz) in parentheses. Values referenced to residual CHCl3 (7.24), CHD2CN (1.93), or C6HD5 (7.15
ppm). Spectra were measured at ambient probe temperature at 300 MHz.
Ta ble 3. 13C NMR (δ, p p m ) Da ta for 5a
compd
solvent
CDCl3
C5(CH3)5
84.35
C5(CH3)5
N(CH3)2
CHR
CO2
R
5a
8.62
48.97
49.18
49.92
50.16
49.23
50.73
65.89
182.58
b
CD3CN
85.61
84.48
9.09
8.68
67.13
74.82
182.95
179.75
9.52
c
5b
5b′
5c
CDCl3
127.99
128.83
130.48
133.53
129.02
129.94
132.92
134.91
127.82
128.55
130.62
133.33
128.07
129.66
133.07
134.70
31.76
139.82
126.41
128.78
128.98
31.41
142.07
127.42
129.78
130.60
19.22
CD3CN
CDCl3
CD3CN
CDCl3
85.85
93.20
9.16
8.71
9.21
8.62
50.40
51.90
76.22
75.47
76.89
71.52
180.52
176.81
177.55
181.20
47.79
48.20
(d, J ) 8.5)
94.68
(d, J ) 9.2)
48.88
49.26
84.28
49.59
50.50
CD3CN
85.72
9.09
50.17
50.47
73.20
182.02
5d
CDCl3
84.20
85.52
8.90
9.07
49.69
51.63
73.92
75.31
179.40
180.82
23.37
23.75
19.42
23.86
CD3CN
50.38
52.46
24.29
a
Singlets unless otherwise specified; coupling constants in Hz. Values referenced to CDCl3 at 77.00 ppm or CD3CN at 1.30 ppm.
Spectra were measured at ambient probe temperature at 75.5 MHz. Not seen. May be isochronous with C5(CH3)5 resonance. c CDCl3
b
data measured on racemic sample.
respectively. Finally, in 5d the chloride ligand avoids
eclipsing either N-CH3 bond: the C(11) methyl is trans
to Cl, whereas the cis C(12) methyl is gauche to Cl
[torsional angle Cl-Ir-N-C(12) ) 65.0°]. Our working

Cp*IrCl Complexes
Organometallics, Vol. 15, No. 4, 1996 1233
Ta ble 5. Atom ic Coor d in a tes (×104) a n d
Equ iva len t Isotr op ic Disp la cem en t Coefficien ts
(Å3 × 103)
a
x
y
z
Ueq
Ir
Cl
-1739(1)
123(4)
-7793(1)
-8277(1)
-8867(3)
42(1)
65(1)
63(6)
58(5)
67(5)
80(7)
85(8)
98(9)
118(11)
131(13)
153(14)
147(12)
48(4)
51(3)
70(4)
53(5)
50(4)
-8588(3)
C(1)
C(2)
C(3)
C(4)
C(5)
C(21)
C(22)
C(23)
C(24)
C(25)
N
O(1)
O(2)
C(6)
C(7)
C(8)
C(9)
C(10)
C(11)
C(12)
-2433(15)
-2965(14)
-3630(12)
-3489(19)
-2786(19)
-1703(19)
-2984(22)
-4446(17)
-4054(19)
-2433(21)
-606(10)
-1498(9)
-199(12)
-538(14)
208(12)
-7375(14)
-6723(12)
-7331(15)
-8420(17)
-8453(12)
-6997(19)
-5580(15)
-6919(25)
-9364(18)
-9357(17)
-6771(8)
-9682(11)
-8991(11)
-8308(13)
-8617(15)
-9445(17)
-10531(11)
-9039(16)
-7496(15)
-8103(22)
-10043(18)
-7410(8)
F igu r e 1. Molecular structure of 5d , shown with 30%
thermal ellipsoids. Hydrogen atoms other than that shown
(assumed position of H(7a)) are omitted for clarity.
-8614(6)
-8937(7)
-6989(7)
-5781(7)
-8419(10)
-7454(10)
-6912(13)
-7430(20)
-6813(15)
-6142(12)
-6072(13)
-6470(10)
-6791(10)
-6005(12)
-5934(17)
-5005(11)
-6760(13)
-8004(12)
Ta ble 4. Cr ysta llogr a p h ic Da ta for 5d
empirical formula
color; habit
C17H29NO2ClIr
yellow blocks
968(14)
2210(19)
398(22)
64(6)
117(11)
98(8)
71(6)
77(6)
cryst size (mm)
cryst system
0.4 × 0.5 × 0.6
orthorhombic
-1420(15)
space group
P212121
156(17)
cell consts (Å)
a ) 10.905(2)
a
Ueq
)
1/3∑i∑jUijai*aj*aiaj.
b ) 12.820(2)
c ) 13.751(2)
1922.4(4)
4
V (Å3)
Z
Ta ble 6. Bon d Len gth s (Å) a n d Selected An gles
(d eg) for 5d
fw
507.1
1.75
7.09
992
Ir-Cl
2.413(4)
C(3)-C(23)
C(4)-C(5)
C(4)-C(24)
C(5)-C(25)
N-C(7)
1.521(27)
1.374(31)
1.531(32)
1.471(30)
1.510(17)
1.496(20)
1.471(20)
1.293(18)
1.214(17)
1.544(19)
1.530(21)
1.511(26)
1.515(24)
D(calcd) (g/cm3)
Ir-C(1)
2.144(15)
2.153(15)
2.146(14)
2.123(20)
2.145(22)
2.158(11)
2.076(9)
1.392(22)
1.472(25)
1.494(24)
1.420(23)
1.467(24)
1.468(28)
abs coeff (mm-1
F(000)
diffractometer used
radiation
temp (K)
monochromator
2θ range (deg)
scan type
)
Ir-C(2)
Ir-C(3)
Siemens P4/Series II
Mo KR (λ ) 0.710 73 Å)
298
highly oriented graphite cryst
3-55
2θ-θ
variable 3.00 to 15.00°/min in ω
1.20° plus KR separation
stationary cryst and stationary
counter at beginning and end of scan,
each for 25.0% of tot scan time
2 measd every 100 reflcns
0 e h e 14; 0 e k e 16; 0 e l e 17
2551
2530 (Rint ) 1.7%)
1973 (F > 4 σ(F))
empirical, ψ scan
0.75/1.0
Ir-C(4)
Ir-C(5)
N-C(11)
Ir-N
N-C(12)
Ir-O(1)
C(1)-C(2)
C(1)-C(5)
C(1)-C(21)
C(2)-C(3)
C(2)-C(22)
C(3)-C(4)
O(1)-C(6)
O(2)-C(6)
C(6)-C(7)
C(7)-C(8)
C(8)-C(9)
C(8)-C(10)
scan speed
scan range (ω)
bkgd measment
std reflcns
Cl-Ir-N
87.7(3)
88.1(3)
76.4(4)
C(4)-C(5)-C(25)
Ir-N-C(7)
Ir-N-C(11)
129.3(18)
107.2(7)
108.5(9)
108.9(11)
112.8(9)
109.5(11)
109.8(11)
118.4(8)
124.8(13)
115.1(11)
index ranges
reflcns collcd
indepdt reflcns
obsd reflcns
Cl-Ir-O(1)
N-Ir-O(1)
C(2)-C(1)-C(5)
107.7(15) C(7)-N-C(11)
C(2)-C(1)-C(21) 124.1(17) Ir-N-C(12)
C(5)-C(1)-C(21) 128.1(17) C(7)-N-C(12)
abs corr
min/max transm
soln and refinement
soln method
refinement method
quant minimized
H atoms
weighting scheme
no. of params
final R indices for
obsd reflcns
final R indices for
indepdt reflcns
goodness-of-fit
η-param
C(1)-C(2)-C(3)
109.6(14) C(11)-N-C(12)
Siemens SHELXTLPLUS (VMS)
Patterson
C(1)-C(2)-C(22) 125.1(16) Ir-O(1)-C(6)
C(3)-C(2)-C(22) 124.8(16) O(1)-C(6)-O(2)
full-matrix least-squares
C(2)-C(3)-C(4)
106.0(15) O(1)-C(6)-C(7)
∑w(Fo -Fc)2
C(2)-C(3)-C(23) 126.5(18) O(20)-C(6)-C(7) 120.1(13)
riding model, fixed isotropic U
w-1 ) σ2(F) + 0.0012F2
200
C(4)-C(3)-C(23) 127.1(18) N-C(7)-C(6)
C(3)-C(4)-C(5) 109.2(17) N-C(7)-C(8)
108.4(11)
117.0(11)
116..6(12)
109.3(14)
117.2(14)
110.3(16)
C(3)-C(4)-C(24) 125.2(18) C(6)-C(7)-C(8)
C(5)-C(4)-C(24) 125.6(19) C(7)-C(8)-C(9)
R ) 0.0412, wR ) 0.0467
C(1)-C(5)-C(4)
107.5(16) C(7)-C(8)-C(10)
R ) 0.0774, wR ) 0.0878
C(1)-C(5)-C(25) 123.2(19) C(9)-C(8)-C(10)
0.95
1.1(1)
the N-CH3 bonds. It appears that conformational
changes in 6d would not allow the Cl and Cp* sub-
stituents to achieve gauche orientation to the N-CH3
bonds without engendering eclipsing interactions be-
tween the N-CH3 bonds and the C(8)-C(7) and C(7)-
H(7a) bonds.
The relationship of solid-state and solution structures
was probed using 5d , particularly because in H NMR
spectra of 5d a surprising lack of observable three-bond
coupling between the two methine protons was noted
(Table 2). Using assumed positions for the hydrogen
atoms in 5d , the torsional angle H(7a)-C(7)-C(8)-
largest and mean ∆/σ 0.001, 0.000
data-to-param ratio
9.9:1
largest diff peak (e/Å3) 0.75
hypothesis is that the presence of both N-alkyl groups
is responsible for the highly selective formation of 5
rather than 6. For example, inspection of molecular
models built on the basis of Figure 1a suggests that
inversion at Ir by exchange of Cp* and Cl ligands (6d ),
without moving the other atoms of the complex, would
place the Cp* and Cl ligands in eclipsing positions to
1

1234 Organometallics, Vol. 15, No. 4, 1996
Grotjahn et al.
has not been offered, nor has the effect of varying
substitution at the amino acid nitrogen been explored.
Among Cp*IrCl and Cp*RhCl complexes,6a,b 5 is formed
with uniquely high diastereoselectivity, apparently a
result of N,N-dialkyl substitution, rather than the bulk
of the amino acid side chain. A Cp*CoCl complex to a
cyclic N-monoalkyl amino acid was reported to be
formed as a single diastereomer, also with cis-oriented
Cp* and amino acid side chain.6i The complete selectiv-
ity may be a result of shorter metal-ligand bond
lengths. On the other hand, the diastereomer of a
Cp*Ir(CCtBu) chelate complex with Cp* and side chain
trans was reported to be more stable.6e Related (ben-
zene)Os(PR3) complexes bearing bulky phosphines were
reported to give chelates with trans-disposed arene and
amino acid side chain,6h perhaps because the phosphine
is actually bulkier than the benzene ligand.
Nonetheless, it appears that a cis-arrangement of Cp*
and amino acid side chain is strongly preferred when
the amino acid nitrogen bears two alkyl substituents
and the remaining ligand on Ir (here, chloride) is
relatively small compared to Cp*. In our previous
work,8 similar preference was seen when smaller ligands
were added to Cp*Ir-amino acid complexes. These
design criteria should aid the development of stereose-
lective organometallic and organic reactions using amino
acid-metal complexes.
F igu r e 2. Proposed solution structure of 5d . Chemical
shifts (δ, ppm) are shown next to protons. nOe enhance-
ments (in parentheses) are in percent.
H(8a) was determined to be 87.6°, explaining the
absence of H-H coupling.14 A series of nOe measure-
ments15 showing surprisingly specific enhancements
also strongly suggested a solution conformation very
close to that in Figure 1, depicted as Figure 2. Irradia-
tion of the metallacycle methine H(7a) (84% saturation)
led to enhancement of the upfield N-methyl singlet
(3.6%), the i-Pr methine (1.8%), and the downfield i-Pr
methyl group (3.7%). These data suggested the assign-
ments shown in Figure 2. As in the remainder of this
discussion, enhancements which amounted to less than
0.3% per H are regarded as insignificant. Conversely,
irradiation of the upfield N-methyl singlet (δ 3.00 ppm)
produced enhancements of the resonances for the meth-
ine protons H(7a) and H(8a) but almost no change in
the i-Pr methyl signals and a slight enhancement of the
singlet for the Cp* protons. Significantly, saturation
of the the downfield N-methyl singlet (δ 3.07 ppm) led
to 6% enhancement of the upfield i-Pr doublet, identified
as the C(10) methyl, virtually no enhancement of the
downfield i-Pr doublet, and significant enhancements
of the H(8a) and Cp* singlets. Taken together, these
data clearly identify the resonances at δ 3.07 and 3.00
as belonging to the C(11) and C(12) methyl protons,
respectively. Remarkably, saturation of the Cp* singlet
led to a greater enhancement of the singlet at δ 3.07
than of the singlet at 3.00 (4.1 vs 2.6%, respectively),
consistent with the crystallographically determined C*-
C(11) and C*-C(12) distances of 4.07 and 4.29 Å,
respectively (Figure 2).
Exp er im en ta l Section
Gen er a l Meth od s. Although the compounds synthesized
in this work all appear to be air-stable, all reactions were
performed under nitrogen using Schlenk or glovebox tech-
niques. However, workup was normally conducted in air, and
solvent removal was conducted using a rotary evaporator.
Other general experimental conditions were detailed else-
where.8 Elemental analyses were performed at Arizona State
or at Atlantic Microlabs, Norcross, GA.
F or m a tion of Ch ela te Com p lexes 5. Rep r esen ta tive
P r oced u r e, Exem p lified by 5b. Solid 2 (106.6 mg, 0.1339
mmol), 4b (48.4 mg, 0.270 mmol, 2.02 mol/mol of 2), and K2-
CO3 (42.1 mg, 0.305 mmol, 2.28 mol/mol of 2) were combined
in a 50 mL round-bottomed flask with a stir bar. Acetonitrile
(26 mL) was added, the flask was capped with a septum, and
the resulting yellow cloudy mixture was deoxygenated by
bubbling nitrogen through it for 5 min using a syringe needle.
The mixture was stirred for 17 h before it was concentrated
and the residue was taken up in CH2Cl2 (10 mL). The
resulting mixture was filtered through Celite, and the filter
cake was rinsed with several portions of CH2Cl2 (total, 50 mL),
until the filtrate was colorless. The combined yellow filtrates
were concentrated and the yellow oily residue was scratched
with Et2O (5 mL) to obtain a yellow powder and a nearly
colorless supernatant, which was removed using a pipet.
Storage of the yellow powder under vacuum left 5b (138.7 mg,
The configuration of the other members of the series
is assumed to be the same as that of 5d , which
corresponds to the major isomer 3a formed from N-
unsubstituted or -monosubstituted amino acids.6a,b In-
frared absorptions for the carboxylato carbonyl of 5 are
similar to those for 3 and fall in the general range for
monodentate carboxylato ligands in general.16
o
96%), mp 225-230 C (dec) (the melting point of 5 seemed to
depend on previous temperature and rate of heating and was
not used as an indication of purity).
Cr ysta l Str u ctu r e of 5d . A crystal of 5d was selected and
mounted vertically in a 0.5 mm X-ray capillary and centered
Discu ssion
Although a growing number of amino acid complexes
to prochiral metal fragments have been reported,6,8 few
have been formed as 5 with complete diastereoselectiv-
ity. An explanation of observed diastereoselectivities
on a Siemens P4 autodiffractometer controlled at 25 °C.
A
primitive orthorhombic cell was determined from 25 randomly
selected peaks with 2θ values between 15 and 30°. Data were
collected in the 2θ-θ mode. Reduction of the data revealed
the space group P212121. Solution and refinement of the
structure utilized the SHELXTL Plus package of programs
from Siemens on a Vaxstation 3100 computer. The absolute
configuration of the molecule was determined by the successful
refinement of the η parameter12 as a free variable, leading to
a value of +1.1(1) for the enantiomer shown in Figure 1.
(14) For example: Abraham, R. J .; Fisher, J .; Loftus, P. Introduction
to NMR Spectroscopy; Wiley: Chichester, U.K., 1988; Chapter 3, pp
34-59.
(15) Neuhaus, D.; Williamson, M. The Nuclear Overhauser Effect
in Structural and Conformational Analyses; VCH: New York, 1989;
Chapter 7.
(16) Nakamoto, K. Infrared and Raman Spectra of Inorganic and
Coordination Compounds, 4th ed.; Wiley: New York, 1986.

Cp*IrCl Complexes
Organometallics, Vol. 15, No. 4, 1996 1235
n Oe Ch a r a cter iza tion of 5d . The experiments were
performed at ambient probe temperature in CDCl3 at 400 Hz.
Significant enhancements (g0.3% per H) are listed.
grateful to Dr. Ron Nieman, the NSF (Grants CHE-88-
13109, MPE 2498, and BBS 88-04992), and ASU for
vital NMR instrumentation and instruction. J .L.H.
acknowledges the NSF (Grant CHE-90-02379) for par-
tial funding of the diffractometer.
resonance
T1 (s)
1.4
% sat.
enhancements (ppm, %)
3.84
3.07
84
89
3.00, 3.6; 1.96, 1.8; 1.15, 3.7
3.84, 0.6; 1.96, 6.0; 1.54, 8.7;
1.12, 6.0
0.5
Su p p or tin g In for m a tion Ava ila ble: Crystallographic
information on 5d , including tables on bond distances and
angles, anisotropic thermal parameters, and H positional and
thermal parameters (3 pages). This material is contained in
many libraries on microfiche, immediately follows this article
in the microfilm edition of this journal, can be ordered from
the ACS, and can be downloaded from the Internet; see any
current masthead page for ordering information and Internet
access instructions.
3.00
1.96
0.5
0.8
89
62
3.84, 6.1; 1.96, 7.2; 1.54, 5.1
3.84, 1.1; 3.07, 2.6; 3.00, 3.5;
1.15 and 1.12, 4.9
1.54
1.15, 1.12
1.7
0.01-0.04
82
59
3.07, 4.1; 3.00, 2.6
3.84, 3.3; 3.07, 3.2; 3.00, 0.6;
1.96, 5.0
Ack n ow led gm en t . We thank the ASU VP for
Research for partial support of this work and J ohnson
Matthey Aesar/Alfa for a loan of iridium salts. We are
OM9507890