Vitamin B12 Models with Macrocyclic Ligands - Advances in

10 Vitamin B Models with Macrocyclic Ligands 12

Downloaded by PENNSYLVANIA STATE UNIV on June 8, 2012 | http://pubs.acs.org Publication Date: December 10, 1980 | doi: 10.1021/ba-1980-0191.ch010

YUKITO MURAKAMI Department of Organic Synthesis, Faculty of Engineering, Kyushu University, Fukuoka 812, Japan

Bisdehydrocorrin (BDHC) and corrinoid complexes are comparable in the electronic nature of the nuclear cobalt. Consequently, the double bonds at the periphery of the A and D rings of BDHC are not in conjugation with the interior double bonds. The steric effect is more pronounced for Co(I)(BDHC) than for vitamin B when the reaction with alkyl halides is carried out, and is attributed to the 1,3-diaxial-type interaction between angular methyl groups placed at the C(1) and C(19) positions of BDHC and an approaching alkyl ligand. The photolysis of the methylated and ethylated Co(III)(BDHC) complexes results in the normal homolytic Co—C cleavage under anaerobic conditions. On the other hand, the Co—C bond in the isopropyl derivative undergoes heterolytic cleavage to yield the isopropyl anion and Co(III)(BDHC). The Co(BDHC) complex can be used as a catalyst for selective hydrogenation of primary alkyl halides using sodium hydroborate as the stoichiometric reducing agent. 12s

A

lthough the cobalt corrinoids have been studied extensively in . the last two decades (i), the significance of corrin as an equatorial ligand is not well understood. To characterize coenzyme B as an organocobalt derivative, a search for model cobalt complexes that can form a C o - C bond axial to a planar equatorial ligand has been stimulated. Studies on model systems (2-13), particularly on the cobaloxime derivatives (2-7), characterized their respective chemistry, but it is still not easy to establish a general correlation between the structure of an equatorial ligand and the properties of cobalt complex 1 2

0-8412-0514-0/80/33-191-179$05.00/0 © 1980 American Chemical Society

In Biomimetic Chemistry; Dolphin, D., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1980.

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regarding the redox behavior of the nuclear cobalt, the reactivity of the Co(I) nucleophile, and the nature of the C o - C bond. To determine the structure-reactivity correlation, a set of model compounds should be chosen carefully so that the alteration in structure can be manipulated with minimal (ideally one) structural parameters. In this respect, we have studied the cobalt complexes of the modified corrins, 8,12diethyl-l,2,3,7,13,17,18,19-octamethyl-AD-bisdehydrocorrin (BDHC) (14) and its tetradehydro analogue ( T D H C ) (14, 15). Co(II) ( T D H C ) and Co(III) (CN) (BDHC) were first prepared by Johnson et al., and characterization of the latter complex by electronic and N M R spectroscopy as well as formation and photolysis of the methylated derivative was described briefly (14). Both B D H C and T D H C have additional double bonds at the peripheral positions that would cause electronic perturbations of the interior conjugation system of the corrinoid. Another important structural aspect of B D H C and T D H C is that these modified corrins possess angular methyl groups at the C(l) and C(19) positions, while the corrinoid has only one at C(l). Thus, an axial ligand in the Co(BDHC) complex, regardless of its location at the upper or the lower side of the macrocycle, may be subjected to a steric interaction with one of the angular methyls. Consequently, the differences that might be found in properties among the B D H C , T D H C , and corrinoid complexes can be interpreted in a more straightforward manner. Co(II) (BDHC) perchlorate was prepared by hydrogenation of the corresponding Co(II) ( T D H C ) perchlorate and purified by preparative thin-layer chromatography (TLC) on silica gel (16). The general electronic structure and the coordination and reaction behaviors of the bisdehydrocorrin complexes are discussed here in reference to those of the corrinoid and tetradehydrocorrin complexes.


2

Co(corrinoid)

Co(BDHC)

Co(TDHC)

Electronic Properties of the Macrocyclic Chromophores The ^-conjugation effects in corrin, bisdehydrocorrin, and tetradehydrocorrin rings are reflected in the electronic spectra of their


10.

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12


TT

Wavelength/ Figure 1. Electronic absorption spectra of ( ) (CN) Co(III)(BDHC), (—) (CN) Co(III)(cobinamide), and (-—) (CN) Co(III)(TDHC) in water (for the former two complexes) and in methanol (for the last one) at room temperature. The spectrum for the cobinamide complex is taken from Ref. 19. 2

2

2

dicyanocobalt complexes (see Figure 1). The overall spectral features of the B D H C complex resemble those of the corrinoid complex (14, 17), but are far different from those of the T D H C complex. The four absorption bands of equal spacing (1.28 x 10 c m ) observed in the visible region are attributed to a 7r-7r* transition along with vibrational fine structure. Similar spectral features were observed for the corrinoid complex (18, 19). Both B D H C and corrinoid complexes show three main absorption bands in the near-UV region, but the intensity ratio between the y-band (in 360-370-nm range) and the immediate higher energy band is reversed among these cobalt complexes. This may reflect degeneracy of 7r energy levels, which is characteristic of the corrinoid complex that is different from the B D H C complex. The a-, jS-, and y-bands for both corrinoid and B D H C complexes with various axial ligands (X and Y) are summarized in Table I. 3

-1

The separation of 7r-7r* energy levels in the B D H C complexes is comparable with that in the corrinoid complexes. The extent of electronic perturbation on the rr and 7r* levels by the axial ligands is nearly the same for the two complexes, judging from the extent of band shifting. The red shift is increased by the axial ligands as follows: (H 0, H 0 ) < ( H 0 , O H ) < ( C N - H 0) < (OH", O H ) < (CN" Py) < ( C N " , O H " ) < ( C N - , C N " ) . A molecular orbital treatment advanced for the elucidation of axial ligation effects on electronic spectra of the corrin complexes (19) may be applied in this case. The diamagnetic shielding effects provided by B D H C and corrinoid macrocycles are the same, based on N M R measurements of Co(HI)(n-C H )(BDHC) and the corresponding n-propylated cobalt corrinoid (17, 21) (Figure 2). 2

2

2

3

2

7



361 369

474 C 496 481 502 510 529

BDHC

—

520 545

490 490 505 496

Corrinoid

P 499 C -525 516 534 540 568

BDHC

d

c

b

° The corrinoid complex is cobinamide and those coordinated with cyano group or groups are cited from Ref. 20. Shoulder. or and j8-Bands are not well separated in 480-500 nm. Split into two bands, 340 and 355 nm.

2

—

349 351 d 354

352 350-360" 360-370" 362 •374 372 378

2

2

H 0 OHOHH 0 Py OHCN-

H 0 H 0 OHCN" CN" CN" CN-

2

Corrinoid

BDHC

Y

7

0

C H — O — O • 3

Methylcobalamin was photolyzed under anaerobic conditions to yield C H radicals and Co(II) species that rapidly recombined to form the original complex (38). The cyanide ion greatly enhanced the reaction under anaerobic conditions in alkaline media (pH 9.93, carbonate buffer) (see Table IX). The reaction gave (CN) Co(III)(BDHC) and no stable intermediate was detected spectrophotometrically since clear isosbestic points 3

2

Wavelength / nm

Figure 8. Spectral change for the aerobic photolysis of [(CH )Co(III)(BDHC)] in water containing 1.0% (v/v) methanol at pH 4.43 (acetate buffer): irradiated with a 200-W tungsten lamp from a distance of 60 cm; duration of photolysis, 0, 10, 20, 40, 85, 175, 355, 955, and 1285 sec (read from A to B). 3

+


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Table IX. Anaerobic Photolysis of CH —Co(III)(BDHC) in the Presence of Cyanide Ion at p H 9.93 (Carbonate Buffer) 3

[CN-](M)

Half Life (min)

0 1.2 x 10" 1.0 x 10" 0.8 x 10"

3 x 10 7.8 7.3 7.3

4 3


2

2

were observed. The rate enhancement by the cyanide ion was also observed under acidic conditions to give [(H 0)(CN)Co(III)(BDHC)] . Because the oxidation of Co(II)(BDHC) is much slower than the photolysis of the methylated complex under the same conditions, the photolysis must yield directly [(H 0)(CN)Co(III)(BDHC)] without forming Co(II)(BDHC). A plausible reaction scheme for the cyanidepromoted photolysis is given as follows. +

2

2

C H - C o ( I I I ) ( B D H C ) ^ ± [ C H - + Co(II)(BDHQ] C H - + Co(II)(BDHC) - ^ C H " + Co(III)(BDHC) 3

3

3

3

CN

ha

The increased reduction ability of the Co(II) species by coordination with the cyanide ion seems to be responsible for the above reaction (39). The aerobic photolysis of the ethylated B D H C complex yielded [(H 0) Co(III)(BDHC)] under acidic conditions, while the anaerobic photolysis yielded Co(II)(BDHC) in the same manner as was observed for the methylated complex. O n the other hand, the isopropyl derivative of Co(III)(BDHC) gave [(H 0)Co(III)(BDHC)] under dark acidic conditions and photolysis conditions. The half lives for these reactions are listed in Table X. 2

2+

2

2+

2

Table X.

Alkyl Moiety CH CH CH (CH ) CH 3

3

2

3

a

2

Half Lives for Photolysis" of Alkylated B D H C Complexes

T (aerobic)(min) T (anaerobic)(min) m

ll2

1.7 2.4 0.5

- 3 x 10 7 0.5

2

Irradiated with a 200-W tungsten lamp from a distance of 60 cm.


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On the basis of these findings, photolysis of the methylated and ethylated complexes results in the normal homolytic C o — C cleavage under anaerobic conditions. The faster rate for anaerobic photolysis of the ethyl derivative relative to the methyl derivative would be attributed partly to olefin formation through the following reaction sequence. C H C H — C o ( I I I ) ( B D H C ) ^ ± C H C H - + Co(II)(BDHC) 3

2

3

2


I

> CH =CH 2

2

+iH

2

The isopropyl derivative does not seem to undergo normal homolytic cleavage of the C o — C bond since the photolysis rates are comparable under both aerobic and anaerobic conditions. Thus, the C o - C bond undergoes heterolytic cleavage to yield the isopropyl anion and Co(III)(BDHC); intrinsic heterolysis, or initial homolysis is followed by rapid electron transfer before the radical is trapped by oxygen molecule. Owing to the steric pressure of the B D H C skeleton and the angular methyl groups at the C(l) and C(19) positions against the bulky isopropyl group, the cobalt species and the alkyl anion diffuse out mutually after heterolysis. The mutual electronic interaction is not retained. The Fe(II)/Fe(III) redox potential for the electron transfer from (octaethylporphinato)iron(II) to the methyl radical (40) is reported to be approximately 0 V vs. S C E (41). Fe(II)(OEP) + C H 3

Fe(III)(OEP) + C)

From the electrochemical viewpoint, the metal complex system can reduce the methyl radical if its redox potential is at most 0 V vs. S C E . The Co(III)/Co(II) redox potential for the B D H C complex without any axial ligand is +0.5 V vs. S C E . Even if the nuclear cobalt(II) in the complex does not have a high reducing ability, the Co(II) complex can reduce the alkyl radical upon input of 12 kcal of energy (corresponding to the potential difference of 0.5 V) from other sources. The estimated steric energy at the transition state for the reaction between Co(I)(BDHC) and ( C H ) C H I is 8 kcal mol" , as described above. Thus, the strain energy release upon the C o - C bond cleavage for the isopropyl derivative must be over 8 kcal m o l . O n the basis of these energy estimations, it is reasonable to expect that the B D H C complex with a bulky axial ligand may undergo novel heterolysis of the C o - C bond. 3

1

2

-1


10.

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Reactivity of the Cobalt-Carbon


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Vitamin B Models Bond

The B D H C complex with the dodecyl group at its axial site was obtained by the reaction of Co(I)(BDHC) with 1-dodecyl bromide. After decomposition of excess sodium hydroborate with acetic acid, the solution of the alkylated B D H C complex in aqueous micellar hexadecyltrimethylammonium bromide (CTAB) was irradiated with a 200-W tungsten lamp under nitrogen. The main reaction product was dodecane, which was recovered in a 50% yield. Since the methyl radical does not abstract a hydrogen atom from water (42, 43), dodecane is not formed appreciably through the radical mechanism. In addition, the methyl radical formed by the photolysis of methylcobaloxime does not incorporate the methyl hydrogen of the peripheral methyl groups in cobaloxime (44). Thus, the alkyl radical, if formed, does not seem to take up a hydrogen atom from the peripheral alkyl substituents of B D H C . Alkylated cobaloximes yield the corresponding dimeric species of alkyl radicals by photolysis under acidic conditions. But the B D H C complex with a hexyl or benzyl group at its axial site does not yield the corresponding dimeric species by photolysis (dodecane and bibenzyl, respectively). Consequently, the hydrogenation product must be obtained through the formation of a carbanion by heterolytic cleavage of the C o - C bond, followed by its protonation. Based on the kinetic data on the alkylation of Co(I)(BDHC) (Table VII), the B D H C complex can be used as a catalyst for selective hydrogenation of primary alkyl halides using sodium hydroborate as the stoichiometric reducing reagent (see Scheme 1). In fact, when 1- dodecyl bromide was used as a substrate in aqueous micellar C T A B , dodecane was obtained in 500% yield on the basis of the B D H C complex. In the competitive hydrogenation of 1-dodecyl bromide and 2- octyl bromide, the latter was not hydrogenated appreciably. l-Bromo-3-bromomethylcyclohexane, which has both primary and secondary bromo groups, was hydrogenated to yield 1bromo-3-methylcyclohexane and bromomethylcyclohexane at a 10: 1 ratio. The selectivity of the primary bromo group should be noted. Hydrogenation of the bromo compounds is not generally a facile reacScheme 1. Co(II/III)(BDHC)

N a B

% Co(I)(BDHC) - ^ U R - C O ( I I )(BDHC) NaBH,

hv

H.O

Co(III)(BDHC) + R H


198

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tion. The hydrogenation described in this chapter prevails over other methods in the following respects. (1) The hydrogenation (or reduction) can be performed in an aqueous medium. (2) The reaction can be carried out selectively with the primary alkyl halides. The results of this investigation may provide a useful criterion for the development of vitamin B models. The cobalt complex of B D H C bears a close resemblance to vitamin B in electronic structure, but the methyl groups at the C(l) and C(19) positions of the former complex exercise a pronounced steric effect on the axial ligand, which gives the C o — C a reactivity quite different from that of the corrinoid complex. The novel heterolysis of the C o - C bond involved in the B D H C complex with a bulky axial ligand can be applied to the selective hydrogenation of primary alkyl halides. To simulate various isomerization reactions observed for the vitamin B system, the reaction intermediate and the cobalt species formed by the C o - C bond cleavage need to be retained in mutual proximity without diffusing so that intimate electronic interaction between the two species can be promoted. 1 2


1 2

1 2

Acknowledgment The research I have reviewed here is the result of the experimental and intellectual efforts of my students and of collaborators whose names appeared in the references. Particularly, I am grateful to my associate Yasuhiro Aoyama, who has guided the more quantitative aspects of this work. Literature Cited 1. Pratt, J. M. "Inorganic Chemistry of Vitamin B "; Academic: London, 1972. 2. Schrauzer, G. N. Acc. Chem. Res. 1968, 1, 97. 3. Costa, G. Coord. Chem. Rev. 1972, 8, 63. 4. Biggetto, A.; Costa, C.; Mestroni, C.; Pellizer, G.; Puxeddu, A.; Reisenhofer, E.; Stefani, L.; Tauzher, G. Inorg. Chim. Acta Rev. 1970,4, 41. 5. Dodd, D.; Johnson, M. D. Organomet. Chem. Rev. 1973, 52, 1. 6. Schrauzer, G. N. Angew. Chem. 1976, 88, 465. 7. Esperson, J. H.; Martin, A. H. J. Am. Chem. Soc. 1977, 99, 5953. 8. Ochiai, E.; Long, K. M.; Sperati, C. R.; Busch, D. H. J. Am. Chem. Soc. 1969,91,3201. 9. Farmery, K.; Busch, D. H. Inorg. Chem.1972,11,2901. 10. Mok, C. Y.; Endicott, J. F. J. Am. Chem. Soc. 1977, 99, 1276. 11. Mok, C. Y.; Endicott, J. F. J. Am. Chem. Soc. 1978, 100, 123. 12. Schaefer, W. P.; Waltzman, R.; Huie, B. T. J. Am. Chem. Soc. 1978, 100, 5063. 13. Elroi, H.; Meyerstein, D. J. Am. Chem. Soc. 1978, 100, 5540. 14. Dolphin, D.; Harris, R. L. N.; Huppatz, J. L.; Johnson, A. W.; Kay, I. T. J. Chem. Soc. 1966, C, 30. 15. Murakami, Y.; Aoyama, Y. Bull. Chem. Soc. Jpn. 1976, 49, 683. 12



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16. Murakami, Y.; Aoyama, Y.; Tokunaga, K.Inorg.Nucl.Chem. Lett. 1979, 15, 7. 17. Murakami, Y.; Aoyama, Y.; Nakanishi, S. Chem. Lett. 1977, 991. 18. Firth, R.A.;Hill, H. A. O.; Mann, B. E.; Pratt, J. M.; Thorp, R. G.; Williams, R. J. P. J. Chem. Soc.1968,A,2419. 19. Offenhartz, P. O.; Offenhartz, B. H.; Fung, M.M.J.Am. Chem. Soc. 1970, 92, 2966. 20. Firth, R. A.; Hill, H. A. O.; Pratt, J. M.; Thorp, R.G.;Williams, R. J.P.J. Chem. Soc. 1968, A, 2428. 21. Brodie, J. D.; Poe, M. Biochemistry1971,10,914. 22. Elson,C.M.;Hamilton, A.; Johnson, A.W.J.Chem.Soc.,Perkin Trans. 1 1973, 775. 23. Bayston, J. H.; Looney, F. D.; Pilbow, J.R.;Winfield, M. E. Biochemistry 1970, 9, 2164. 24. Bayston, J. H.; King, N. K.; Looney, F. D.; Winfield, M. E. J. Am. Chem. Soc. 1969, 91, 2775. 25. Lexa, D.; Saveant, J.M.J.Am. Chem. Soc. 1976, 98, 2652. 26. Lexa, D.; Saveant, J. M.; Zickler, J. J. Am. Chem. Soc. 1977, 99, 2786. 27. Hogenkamp, H. P. C.; Holmes, S. Biochemistry 1970, 9, 1886. 28. Pratt, J. M. "Inorganic Chemistry of Vitamin B "; Academic: London, 1972; Chapter 7. 29. Hayward, G. C.; Hill, H. A. O; Pratt, J. M.; Vanston, N. J.; Williams, R. J. P. J. Chem. Soc. 1965, 6485. 30. Hayward, G. C.; Hill, H. A. O.; Pratt, J. M.; Williams, R. J.P.J.Chem. Soc. 1971, A 196. 31. Stanienda, A.; Biebl, G. Z. Phys. Chem. (Frankfurt am Main) [N.S.]1967, 52, 254. 32. Felton, R. H.; Linschitz, H. J. Am. Chem. Soc. 1966, 88, 1113. 33. Conlon, M.; Johnson, A. W.; Overend, W. R.; Rajapaksa, D.; Elson, C. M. J. Chem.Soc.,Perkin Trans. 1 1973, 2281. 34. Hush, N. S.; Woolsey, I. S. J. Am. Chem. Soc. 1972, 94, 4107. 35. Schrauzer, G. N.; Deutsch, E. J. Am. Chem. Soc. 1969, 91, 3341. 36. Friedrich, W.; Messerschmidt, R. Z. Naturforsch. 1969, 24b, 465. 37. Pratt, J. M. "Inorganic Chemistry of Vitamin B "; Academic: London, 1972; Chapter 6. 38. Endicott, J. F.; Ferraudi, G. J. J. Am. Chem. Soc. 1977, 99, 243. 39. Schrauzer, G.N.;Sibert, J.W.;Windgassen, R.J.J.Am. Chem. Soc. 1968, 90, 6681. 40. Castro, C. E.; Robertson, C.; Davis, H. F. Bioorg. Chem. 1974, 3, 343. 41. Davis, D. G.; Bynum, L. M. Bioelectrochem. Bioenerg. 1975, 2, 184. 42. Gilbert, B.C.;Norman, R. O. C.; Placucci, G.; Sealy, R. C. J. Chem. Soc., Perkin Trans. 2 1975, 885. 43. Thomas, J.K.J.Phys. Chem. 1967, 71, 1919. 44. Golding, B.T.;Kemp, T. J.; Sellers, P. J.; Nocchi, E. J. Chem.Soc.,Dalton Trans. 1977, 1266. 45. Jencks, W. P.; Regenstein, J. "Handbook of Biochemistry and Molecular Biology"; Fasman, G. D., Ed.; CRC Press: Cleveland, 1976; Vol. 1. 12

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Vitamin B12 Models with Macrocyclic Ligands - Advances in

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