Communications to the Editor
4401
Table 1. Hydrogenation of Itaconic and a-Methylene Glutaric Acid Derivatives with the [Rh( 1,5-COD)(DiPAMP)]+BF4- Catalyst PrecursorQ X ee (0.4~)
Substrate
2 ee
Relative Rateb
(n.oozg)
(0.4%)
778 R
9
,COOH
2- CH>=C
38'6 R C
\
CHzCOOH COO-Na+
2
CHI=C 'CH2COO-xa+
7 8 4 Rd
50
88% Re
100
COOCII 3
$ CHi=C
\
CH:COOCIl COOH
5
55%
CHi=C \
xi
55Z R
2
oxygen free and properly oriented for chelation. Consequently, 6 is hydrogenated rapidly with a resultant good 88% ee. The proposed preference of the half-esters ( 5 and 6) to H bond intramolecularly in contrast to the intermolecular H bonding for itaconic acid is reasonable considering that (a) intermolecular association results in two H bonds per molecule for itaconic acid, while intramolecular H bonding would result in only one per molecule; (b) the half-esters achieve one H bond per molecule regardless of whether they are intra- or intermolecularly H bonded. Finally, for the itaconic acid series, the half-amide 7 reduces rapidly and with a high enantioselectivity of 90%. Since an amide group has a preference for a trans conformation6 (A), 7 cannot form a dimeric or polymeric species similar to itaconic acid. Consequently, the amide can exist to some extent as an H-bonded species (cf. 12) leaving the 4-carbonyl function available for chelation.
CH :COOCH
,COOCH3'
p
CHz=C \cH;cOOH
100
882 Rf
4
COOH
2, CH>=C
90% Rl'
'
100
CH: CONHCH p
COO-Na+
$2 &!
CHz=C \CH2 CH 2 COO-Naf
/cOOcl's CHI=C \CHz
10:i
0.6
2.88j
10
CH2 COOCHl
(I All hydrogenations were carried out under 50-psig hydrogen prcssure at 50 "C. Substrates 2 and 7 were hydrogenated in ethanol, 4,5, and 6 were hydrogenated in methanol, and 3 was reduced in 50% aqueous ethanol. Additional experimental data may be found in ref I f . In comparison, the common substrate (2)-a-acetamidocinnamic acid reduces with a relative rate of 59.Ir ' [a]*'D +17.OIo (c 4.41, EtOH): R. Rossi et ai., Go::. Chirri. Itol.. 98, I391 ( 1968). Converted into the diacid to determine ee. [ a ] * ' D +6.1 1 " (neat): refercnce in c. ./ Converted into the diester to determine ee. p Prepared by heating itaconic anhydride with 4-methoxybenzyl alcohol to get the half-ester. This was treated with diazomethane and then heated with trifluoroacelicacid in toluene togive6. [ c ~ ] ~ " ~ + 1 5(c . 31.01, " EtOH); determined by repeated crystallization of the 90% ee niaterial from EtOH. ' Converted into the diacid to determine ee. [.]"D +21.3" (c 2.0, EtOH): E. Berner and R. Leonardsen, Jtrsrus Lirhigs A m . Cheni., 538, 1 (1939). J [(U]'('D +24.46" (neat): reference in ('
I.
species In this conformation, 5 cannot form a chelate with rhodium and thus hydrogenates via a less efficient monodentate complex. Half-ester 6 can assume only the intramolecular H-bonded structure 11 which would still leave the 4-carbonyl 0
/CH3
O,
c= o.,
Two a-methyleneglutaric acid derivatives 13 and 14, chosen because no H-bonding complications can exist, were hydrogenated in order to probe the geometric requirements for chelate formation. Both were found to hydrogenate slowly and with low optical inductions. Apparently a suitable bidentate complex does not form when the ring is seven membered (counting both carbons of the coordinating olefin). Now, concerning the questions raised a t the beginning of this communication, we propose that (a) itaconic acid is a poor substrate with most rhodium(1) bisphosphine catalyst systems because H bonding prevents it from forming a bidentate complex in the transition state (in the absence of such influences it hydrogenates efficiently, being geometrically and electronically suitable); (b) bisphosphine 8 successfully reduces 2 because the amide moiety in the ligand can act to break up H bonding in the coordinated substrate so as to allow chelation. CPK models show that the ligand's amide oxygen can be properly oriented for interaction with the 1 -carbonyl hydrogen.3b The basic conclusion seems to be that the major limitation to efficient asymmetric hydrogenation with rhodium( I ) chiral bisphosphines is the ability of the substrate to form a bidentate complex in the transition state. This study has contributed to an understanding of the requirements necessary for chelation. The demands for chelation appear to be stringent, as demonstrated by the facts that perturbations as small as an unfavorable H-bonded conformation or chelate ring extention by one CH? (i.e., going from itaconic to wmethyleneglutaric acid derivatives) destroys the ring-forming properties. Still, there may well be new rhodium( I)-based chiral ligand-substrate combinations found that give efficient asymmetric reduction. Their design will most likely be facilitated, however, by considering all the known effects, including those presented in this communication.
References and Notes
II
N
(1) (a) W. S. Knowles, M. J. Sabacky, and B. D. Vineyard, J. Chem. SOC.,Chem. Commun., 10 (1972). (b) T. P. Dang and H. B. Kagan. Chem. Commun., 481 (1971). (c) H. B. KaganandT. P. Dang, J. Am. Chem. SOC.,94, 6429(1972), (d) W. S.Knowles, M. J. Sabacky, B. D. Vineyard, and D. J. Weinkauff, ibid., 97, 2567 (1975).(e) G. Gelbard and H. B. Kagan. Tetrahedron, 32,233 (1976). (f) B. D. Vineyard, W. S. Knowles, M. J. Sabacky. G. L. Bachman, and D. J. Weinkauff, J. Am. Chem. Soc., 99, 5946 (1977). (9) H. B. Kagan, N. Langlois, and T. P. Dang, J. Organomet. Chem., 90,353 (1975). (h) M. D. Fryzuk and B. Bosnich, J. Am. Chem. Soc., 99, 6262 (1977); 100, 5491 (1978). (i) M. Tanaka and I. Ogata. J. Chem. SOC., Chem. Commun., 735 (1975). (j) M. Fiorini, G. M. Giongo, F. Marcati, and W. Marconi, J. Mol. Catal., 451 (1975-1976). ( k ) Y . Kawabata, M. Tanaka, and I. Ogata, Chem. Lett., 1213 (1976). (I) T. Hayashi. T. Mise, S.Mitachi, K. Yamamoto. and M. Kumada, Tetrahedron Lett., 1133 (1976). (m) P. A. Violet, U S . Patent 3 949 000 (1976). (n) K. Achiwa, J. Am. Chem. Soc., 98, 8265 (1976). ( 2 ) (a) J. Halpern, Trans. Am. Crystallogr. Assoc.. 59, (March 20, 1978); (b) J.
4408
(3) (4) (5)
(6)
Journal of the American Chemical Society
M. Brown and P. A. Chaloner, J. Chem. SOC., Chem. Commun., 321 (1978). (a) I. Ojirna, T. Kogure, and K. Achiwa. Chem. Lett., 567 (1978); (b)I. Ojima and T. Kogure. ibid., 1145 (1978); (c) K. Achiwa, Tetrahedron Lett., 1475 (1978). R . L. Harlow and C. E. Pfluger, Acta Crystallogr., Sect. 6, 29, 2965 (1973). We would like to point out that cases of intramolecular H bonding in molecules similar to 10 are known. See, for example. (a) G. C. Pimentel and A. L. McClellan, "The Hydrogen Bond", W. H. Freeman, San Francisco, Calif., 1960, pp 169-171; (b) V. F. Bystrov. S. L. Portnova, V. I. Tsetlin, V. T. Ivanov, and Yu. A. Ovchinnikov, Tetrahedron, 25, 493 (1969). M. B. Robin, F. A. Bovey, and H.Basch in "The Chemistry of Amides", J. Zabicky, Ed., Wiley, New York, 1970, p 19.
/
101:15
/ July 18, 1979
a 'AX. JAV
-0-CHT
.0(2 2
13 (NO3&
W. C . Christopfel,* B. D. Vineyard Monsanto Company, St. Louis, Missouri 63166 Received January 2, I979
Conformational Preference and Structural Similarity of the Lanthanide [2.2.1] Cryptates Sir: While ligand conformations preferred by d-transition metal chelates have received careful study,' no observations of this phenomenon have heretofore been observed for complex ions of f-transiton elements.* We now report a 180-MHz ' H N M R examination of the trivalent lanthanide [2.2. I ] cryptates, [Ln(2.2.1)]X3 (X = CI-, N03-, C104-), which reveals that these species either show strong conformationalpreference or a r e structurally rigid within the time frame of the N M R experiment and are, as well, isostructural over the entire ~ e r i e s . ~ T h e low-energy barriers known for conformational motion in five-membered chelate rings render the preference of ligand gauche conformations most likely,' but our N M R studies cannot distinguish between rigidity or preference of conformat ion. Several representative ' H N M R spectra of the title compounds are shown in Figure I . Of particular interest is that displayed by a CD3CN solution of [La(2.2.1)](N03)3 wherein signal assignments have been verified by decoupling methods. Geminal methylene protons N to bridgehead nitrogens on the two dioxygen strands may be seen to exhibit an A M doublet ( J A M = 13.5 Hz) which is vicinally coupled to the adjacent = P-CH2 group denoted as protons H x and H y ( J z x = 3.5, J M X = 9.45, J M Y = 6.15 Hz). Observation of a single A M X Y pattern for all four I Y - C H ~dioxygen fragments of the molecule implies either that both strands have the same rigid conformation, or that a single conformation is highly favored thermodynamically.1d,4The observed variance i n vicinal M(X,Y) coupling constants arises from dihedral angle differences as expected from the Karplus equation.5 The remaining triplet in the N - C H I spectral region is due to the monooxygen strand cu-CH2 coupling to symmetry equivalent P - C H l protons with J K L= 5.25 Hz. The 0 - C H , spectral region is complicated by extensive resonance overlaps, but the A and B branches of an AA'BB' pattern due to protons HE, HF, Ht ', and HF, ( J E F= 36.5 Hz) may be seen on either side of a complex pattern which is made up of the triplet of the P - C H l protons of the monooxygen ligand strands ( J K I .= 5.25 Hz) and the multiplet from the P-CH, protons of the dioxygen strands. T h e essential spectral features discussed above are also detected for the salts in D20, but with less chemical shift separation in the cu-CHr spectral region. Evidence for structural identity of lanthanide [2.2. I] cryptates is deduced from ' H N M R spectra of the paramagnetic ion species in DlO which show eight widely dispersed, equally intense resonances (sec Figure I ) which were assigned by considering resonance frequencies and line widths. Lanthanide ion induced chemical shifts obey an equation of the form 0002-7863/79/150l-4408$0l .OO/O
r
I E
I
4.5
35
Ppm
b
3
C
HK
'1
HI/H
1
leom
I
0
-iEppr
Figure 1. 180-MH7 ' H N W R ~peciraof\onieInnthanidc[2.2.1] cryptatcs: ( a ) [ L a ( ? . 2 . 1 ) ] ( N 0 3 ) 3 in CD:CU mlution. M q S i reference: ( b ) ( P r ( Z . Z . I ) ] ( Y O ~i n) ~D20: (c) [Er(2.?.I)](IC03)3in D20:r-BuODref-
crcncc i n D20.
where 6, is the chemical shift of the i t h proton measured from the diamagnetic resonance position of the La3+ complex, A, (S,) is the Fermi contact term, G, is a geometric factor dependent on the position of the proton relative to the ion, and D is a constant of different value for each lanthanide whose magnitude is determined by temperature and g value anisotropy.6 Relative proton resonance line widths in lanthanide complexes depend on the radial distance of the proton from the ion' as shown in eq 2. By using X-ray structural data, it is therefore possible to calculate metal-proton distances and expected radius and line width ratios. Since no detailed crystal data were available for any lanthanide cryptates, the recently published structureXof the
0 1979 American Chemical Society
