1090 Table I. Rate of Reduction of Cobalt(II1) Complexes by Chromium(II)a Complex I. 11. 111. IV. V. VI. VII. VIII. IX. X. a
Rate, M-l sec-l
Ref
Co(NH3)6F2+ 9X106 Co(NH3)6C12+ 2.6 X 106 C0(NH3)sOH2+ 1.7 X CO(NH~)~(OAC)~+ 3.5 x lo-' Co(NHs)a(OAc)z+ 15 50[HsO+] CO(NH~)~(HZO)(OAC)~+47 2.8[H30+]-1 CO(NH~)~(OOCCH~OH)~+ 3.1 C O ( ~ ~ ) ~ ( H O C H ~ C O O ) ~38+ Co(en)2(OCH2COO)+ 9.9 x 102 Co(en)2(SCH2C00)+ >2 x 105
+ +
b =
1.0 M , 25".
b b C
d e e
f g g
g
* Reference 9.
A. Zwickel and H. Taube, Reference 7b. Reference This work. h At 20", I.( = 1.20 M .
J. Amer. Chem. Soc., 81, 1288 (1959).
4a,c.
f
Reference 6.
Q
+
kS2
Co(en)2(SCH2COO)+ Cr(I1) ----f products
at present except that the steric component of the latter influence should contribute in the observed direction. Less dramatic effects than those we present have previously been reported for Co(II1) complexes with ligands containing uncoordinated thio ether functions. l 2 We are extending our studies to other reductants (particularly of the outer-sphere class), sulfur functions, and oxidizing centers. Acknowledgment. We acknowledge with thanks support for this research provided by the National Science Foundation (Grants GP-6848 and GP- 12223), the Petroleum Research Fund (Grant G-821), and the Public Health Service (National Institutes of Health Fellowship (GM 44691) to R. H. L. 1969-1970). We appreciate the support of the University of Florida for the stopped-flow spectrophotometer, an assistantship to R. H. L., and a summer research appointment to L. E. B. Our report has benefitted from comments by Professors Henry Taube and John Endicott and the referees.
This reaction is also characterized by 1 : 1 stoichiometry (spectrally determined) yielding a red chromium(II1) (12) E. S . Gould, J . Amer. Chem. Soc., 88, 2983 (1966). product which is indicative of a thiolate bridged reaction. Subsequent discussion will focus on the mechRobert H. Lane, Larry E. Bennett anistically comparable kp paths. Department of Chemistry, University of Florida G a i n e s M e , Florida 32601 Table I summarizes our observations and relevant Received January 16,1970 results from previous work on inner-sphere reductions. The glycollate complex appears unreactive relative to C O ( N H ~ ) ~ O H ~Steric + . hindrance of the alkoxide oxygen by the methylene group probably contributes subIntramolecular Oxidative Phenol Coupling. 11. stantially to this effect. A decreased stability of the A Biogenetic-Type Synthesis of ( *)-Maritidinel precursor complex' is one likely consequence of the hindrance. We are studying the C~(II)-CO(NH~)~-Sir: Radioactive tracer experiments have verified that (HOCH3)3+* reaction to further evaluate the influence most, and probably all, of the Amaryllidaceae alkaloids of alkoxide ligands. are biosynthesized by way of intramolecular oxidative The most significant reactivity influence evident in coupling of either 0-methylnorbelladine (1) or 0,N-dithe results is that conferred by the thiolate ligand. The methylnorbelladine (2). Although it has been recogmercaptoacetate complex is more reactive toward renized that execution of this scheme in the laboratory duction by Cr(l1) than its oxygen analog by over three would provide an exceedingly simple synthetic route to orders of magnitude. In spite of its potentially hinthese alkaloids, Barton and Kirby's4 synthesis of galdering methylene substituent, it approaches the highest anthamine (3) remains the only reported biogeneticreactivities observed for this class of reactants and may type synthesis of an Amaryllidaceae alkaloid ; these approach rate-limiting substitution on Cr(II).7!9 An important barrier to activation for the class of reactions under consideration is the apparent necessity, OH I impressively supported e~perimentally,'~ to stretch the cobalt-bridging ligand bond prior to electron transfer. A further influential factor may be the extent to which the ligand u orbital overlaps the cobalt eg orbital" although experimental evidence on this point is less convincing. The much greater reactivity which we have observed for the sulfur over the oxygen complex corre1,R-H 3 lates nicely with a diminished steric hindrance of the 2, R = CH, larger thiolate sulfur by the methylene group and a lower bond strength and greater covalency expected for the workers were able to effect the intramolecular orthocobalt-sulfur bond, The potential influences of the para coupling of 2 in 1.4% yield. We recently dethermodynamic driving forces? and precursor complex scribed a new method for carrying out intramolecular stabilities' on the rate difference are difficult to assess oxidative phenol coupling' and wish now to report its use in the biogenetic-type synthesis of ( j=))-maritidine (7) (a) N. Sutin, Accounts Chew. Res., 1, 225 (1968); (b) H. Taube and E. Could, ibid., 2, 321 (1969); (c) J. C. Patel, R. E. Ball, J. F. Endicott, and R. G. Hughes, Inorg. Chem., 9, 23 (1970). (8) R. B. Jordan, A. M. Sargeson, and H. Taube, ibid., 5 , 1091 (1966). (9) J. P. Candlin and J. Halpern, ibid., 4, 766 (1965). (10) (a) R. I
