p-4h California Association of Chemistry Teachers
George 8. Kauffman Nobuyuki Sugisaka
I
Resohring Coordination Compounds by a
Fresno State College Fresno, California
Second Order Asymmetric Synthesis
and Ian K. Reid
Australian National Univers~ty Canberra
1
A previous article (1) described as an
inorganic laboratory experiment the resolution of an optically active complex cation ([Ni(o-phen)3I2+) with tartar emetic and the subsequent use of the Lantipode of this cation to resolve an optically active complex anion ( [ C O ( C ~ O ~ ) ~ ] ~I n- ) .the present experiment, the species resolved is the trisoxalatochromate(II1) ion, which possesses the double historical distinction of being both the lirst resolved complex anion and the first resolved complex that did not contain nitrogen (2). Sincethe [Cr(C20&13-ion racemizes rapidly in aqueous solution, more so than the [ C O ( C ~ O ~ ion ) ~ ](3), ~ - its resolution provides the student with an excellent introduction to the phenomenon of second order asymmetric transformation. By this technique (2, 4), it is possible to separate virtually all of a labile complex as a single antipode, which precipitates as the less soluble diastereoisomer. In contrast to the previous experiment, which employed low temperatures to keep racemization to a minimum, the present experiment involves warming the solution during diastereoisomer formation in order to encourage racemization. The Experimenl
The resolving agent, D-oxalatobis(ethy1enediamine)cobalt(II1) iodide is first converted to the acetate:
+
D-[CO(C,H~N~~C,OIII AgCAOI +
+
D-[CO(CZH~N~)~C~O~IGH~OO A d1
which is then warmed with a solution of rmemic potassium trisoxalatochromate(II1) in order to precipitate the ~,n-diastereoisomer (second order asymmetric transformation) :
The diastereoisomer is decomposed with potassium iodide, which also regenerates the resolving agent:
n-[Co(CzH~N&C~O~]n n-[Cr(CnO,)rI .6Hn0
+ 3x1
+
-
3 ~ D - I C O ( C ~ H ~ N ~ ) ~1 GO~III &I-[CI(CZO,)~] ,ZH20
+ 4HsO
The D-antipode of Ka[Cr(CzOa)r]is precipitated from the filtrate with ethanol. Since it racernizes so rapidly, its isolation challenges the student with a more exacting test of technique than that of the previously described K3[Co(Cz0J3] (1). An analogous procedure employing the L-form of the resolving agent is used to obtain the bantipode. The specific rotations obtained in this experiment ([orlo = *1900°) are higher than those reported previously by Jaeger (5) (+420°), Johnson and Mead (6b) (*1170°), and Dwyer and Sargeson (7) (+1640°). Procedure
Precipitation of the Diastereoismner. Five grams (0.0127 mole) of D - [ C ~ ( C & N ~ ) ~ C ~ O ~ ] ([a]Y I (8) = +720°) is converted to the acetate by shaking for 10 min with 2.11 g (0.0126 mole) of AgCzHaOzsuspended in 30 ml of hot water. The precipitated AgI is removed by filtration and washed with 10 ml of hot water. The filtrate and washings are added with stirring to a solution of 2.00 g (0.00409 mole) of racemic &[Cr(C20&].3H20 (9, 10) in 10 ml of water. The red-brown diastereoisomer begins to precipitate almost immediately. The suspension is quickly warmed to 50°C, maintained a t that temperature for 5 min, and allowed to cool slowly to 35%. The precipitate is collected by filtration, washed successively with ice water, 60% aq. ethanol, 95% ethanol, and acetone, and is then air-dried. The yield is about 3.7 g or 74% assuming complete racemization of the L-antipode to the D-antipode. A 0.02% aq. solution in a 1-dm tube ~ +1350°. gives ory = +0.27", from which [ a ] = Analysis: calculated for [Co(CzHsN2)&20a]3[Cr(C204)1].6Hz0: C, 23.52; H, 4.94; N, 13.72. Found: C, 23.70; H, 4.99; N, 13.74. Smaller second and third fractions of diastereoisomer may be obtained by allowing the filtrate to stand a t room temperature for 24 hr and 48 hr respectively. Volume 4 1, Number 8, August 1964
/
461
Since the diastereoisonier raceinizes only slowly in the spectrophotoinetric polariineter described by solution, all three fractions may be collected and ICirschner, et al. (19). The antipodes will be found to combined (4.2 g) before proceeding with the following exhibit anomalous rotatory dispersion (5, 14-16), section. Acknowledgment Isolation of K3 D-[C~(C~O~)?] .2Hz0. Since the optical antipodes racemize rapidly in solution, all operations We grateful!^ acknowledge the assistance of Dr. should he carried out as quickly as possible using iced Alan A'I. Sargesou of the Australian National University solutions and iced apparatus. Students should work for helpful discussions and a generous supply of the in pairs. resolving agent. The diastereoisonler is suspended in 15 ml of water, and 4 ml of saturated K I solution is addcd gradually Literature Cited with constant stirring. The precipitated resolving G. B., A N D TAKAHASXI, L. T., TI115 JOITRNL (1) I~AUFFMAN, agent ( [ a ] " ~= +720°) is recovered (in yields as high 39, 481 (1962). as 3.54.0 g or 70-80%) by filtration, and is washed A., 8 8 7 . deut. chem. Ges., 45,3061 (1912). (2) WERNER, , A N D FRASER, R., J. Chem. Soe., 123, 2!)73 with a few ml of K I solution. I < ~ D - [ C ~ ( C ~ O ~ ) ~ ] . ~(3) F I ~TOn o x ~ s W., 119231. . . is immediatelg precipitated as bluish-mauve crystals UWYER,F. P., AND GYARFAS, E. C., J. Pme. lie!,. Soe., from the combined filtrate and washings by slow addi.V.S. Wales, 83, 263 (1949). tion of ethanol until precipitation appears almost JAEGER, F. M., Rec. lrav. chim., 38, 142, 243 (101!)). complete. The product is collected by filtration, (a) JOHNSON, C. H., Tram. Faradau Soc., 31,1612 (15135); JOHNSON SON, C. H., AND MEAD,A., Trans. Farndaj, Soe., mashed successive!y with 80% aq. ethanol, 95% ethanol, 31, 1621 (193.5); (c) BEESB,N. W. D., AN,) JO~ISSON, acetone, and then air-dried. The yield is 1.43 g or C. H.,T ~ a n sh. m d a y Soc., 31, 1832 (1835); (d) B a s ~ n ~ , 75% hased on I
