B.D. Gupta

Organocobaloximes: cobalt-carbon bond stability and synthesis

Abstract Following the discovery that the naturally occurring vitamin B 12 coenzyme contained a stable CoC sigma bond and the further realization that porphyrin-like corrin was an important factor in the stabilization of the CoC bond, has led to the synthesis of a large number of organocobalt complexes. Inspite of the many known examples of such complexes in the literature, the synthesis of the CoC bond with new or modified structural features continues to be a fascinating field of study. Many synthetic routes have been reported and many new ones are frequently emerging. In this article, a complete analysis of the synthetic aspects of organocobalt compounds and organocobaloximes, in particular, is undertaken. The stability of the CoC bond, general methods of synthesis and experimental procedures are discussed.

A study of cis influence in alkyl cobaloximes

Abstract Thirty-nine alkylcobaloximes have been synthesized (many of them new) and characterized with three different dioxime ligands, dmgH, dpgH and chgH. The chgH cobaloximes have been synthesized for the first time from ClCo(chgH)2Py complexes. A rapid purification procedure using column chromatography affording analytically pure products has been established for all the three series of cobaloximes. Methanol has been found to be the best solvent for the study of cobalt-carbon charge transfer (Co-C CT) band as the alkylcobaloximes exhibit a prominent maxima in this solvent. For MeCo(dpgH)2Py the Co-C CT band is not resolved; a λmax value of 453.6 nm is proposed for it. The literature value of 473 nm is doubtful. The variation of λmax values with increasing alkyl chain length is surprisingly similar for all the three series of cobaloximes. 13C-Spectroscopy has revealed that the Pα experiences the most cis influence and Pβ the least. The cis influencing ability of the chgH ligands have been found to be negligible (as compared to dpgH) on the alkyl chain as well as on the Pβ and Pγ carbons. 1H-NMR studies indicate that the cis influence is felt most by the cobalt bound methylene protons followed by Pα and Pβ. Interestingly, the O-H-O resonance for the chgH cobaloximes appear ∼0.5 ppm upfield than the analogous dmgH complexes in all the thirteen alkylcobaloximes. The order of cis influencing ability has been found to be dpgH > chgH > dmgH by all the three spectroscopic techniques. This sequence is exactly the reverse order of the corresponding Co(I) nucleophilicities. C=N and N-O stretching frequencies in the IR studies for three series of cobaloximes follow the order dmgH > chgH > dpgH. The order can be explained if one invokes the electronic effect of the substituents on the dioximic moiety.

ORGANOCOBALOXIMES : SYNTHESIS, OXYGEN INSERTION AND KINETICS

Abstract A total of 27 organocobaloximes have been synthesised and characterized with three different dioxime ligands, dmgH, chgH and dpgH. Many of the chgH and all the dpgH complexes have been synthesised for the first time. The insertion of molecular oxygen into these organocobaloximes, RCoIII(L2)B [L=dmgH, chgH and dpgH) under thermal and photochemical conditions have been studied. Kinetic studies at ambient temperature under irradiation show that the rate of insertion depends upon the nature of R, L, B and the solvent. The rate follows the order dpgH>chgH>dmgH; Naphthyl>heteroaromaticmethyl>benzyl; piperidine>morpholine>γ-picoline>pyridine>2-bromopyridine>2-acetylpyridine; and the rates are faster in acetonitrile than in acetone and chloroform. In methanol, the benzylic cobaloximes exist as an equilibrium mixture of solvated penta-coordinated and hexa-coordinated species and in chloroform these exist as hexa-coordinated species.

Molecular oxygen insertion into organocobaloximes: Kinetics and decomposition studies

The insertion of molecular oxygen into the CoC bond in organocobaloximes under thermal and photochemical conditions has been studied. Kinetic studies have shown that the reaction is of first order with respect to both the complex and oxygen. The thermal decomposition of the organocobaloximes and their insertion products has been carried out in benzene and methanol and a mechanism is proposed for the decomposition.

Organobridged dicobaloximes: Synthesis and NMR correlation

Abstract Organobridged dicobaloximes of the type Py(L) 2 Co III —(CH 2 ) n —Co III (L) 2 Py [L = dmgH − and dpgH − , n = 3,4,5,6 and 10] are synthesized by the reaction of Co I (L) 2 Py with the corresponding alkyl dihalides. A mixture of haloalkylcobaloximes and the dicobaloxime is formed in each reaction. The purification procedure for the dicobaloxime is established. A comparison of ‘cis influence’ due to the equatorial ligands in the haloalkyl and the dicobaloximes by 1 H NMR reveals that the extent of the cis influence on the Co-CH 2 resonance is more in the case of the dicobaloxime but it is nearly same for the P α and the P γ resonances.

Molecular oxygen insertion into picolyl, thenyl, furyl and furfuryl cobaloximes under thermal and photochemical conditions and their decomposition studies

Abstract Some new picolyl, thenyl, furyl and furfuryl cobaloximes have been synthesized. The insertion of molecular oxygen into the CoC bond occurs readily under photochemical stimulation but there is no insertion under heating. The thermal decomposition of the inserted product in benzene and methanol forms aldehydes and alcohols.

Halogenation of thenyl and furyl cobaloximes

Abstract The reactions of thenyl and furyl cobaloximes with halogens (bromine and chlorine) in acetic acid and chloroform indicate that, CoC bond cleavage occurs in 2-thenyl and 2-furyl case whereas the ring substitution is faster than CoC band cleavage in 3-thenyl and 3-furyl cobaloximes.

Organocobaloximes with mixed-dioxime equatorial ligands: a one-pot synthesis

BenzylCo(dmgH)(dpgH)Py has been synthesized by the reaction of benzyl chloride with a mixture of Co I (dmgH)2Py and Co I (dpgH)2Py, generated in situ, by the NaBH4 reduction of ClCo(dmgH)2Py and ClCo(dpgH)2Py. A mixture of three products is formed. The first crystal structure of an organocobaloxime with a mixed-dioxime ligand is reported.

CoC bond cleavage in the reactions of alkyl, benzyl and heteroaromaticmethyl cobaloximes with arene sulfenyl chloride: Homolytic and heterolytic pathways

Abstract The reactions of arene sulfenyl chlorides, ArSCl, (Ar=Ph, C6Cl5, 2,4 (NO2)2C6H3) with organocobaloximes, RCo(dmgH)2Py, (R=alkyl, benzyl and heteroaromaticmethyl) were carried out under thermal and photochemical conditions. A variety of organic and organometallic products are formed depending upon the substrate cobaloxime. For 3-methoxybenzyl and heteroaromaticmethyl cobaloximes the results suggest that they represent a unique class of cobaloximes whereby both the aromatic ring as well as the CoC bond are highly activated towards attack by the arene sulfenyl chloride. Both homolytic as well as heterolytic pathways are operative.

Organocobaloximes in organic synthesis an unusual radical-dependent five-member cyclization

Visible light photolysis of organocobaloximes R′CC[CH 2 ] 3 -Co III (dmgH) 2 Py 1–3 (R′ = Ph, TMS, H) with radical trapping agents ArSO 2 Cl (Ar = Ph, 4-MeC 6 H 4 , 4-OMeC 6 H 4 , 4-BrC 6 H 4 , 4-ClC 6 H 4 ) and MeSO 2 Cl afford rearranged or non-rearranged organic products depending on the R′ group. 2-Phenylcyclopentenyl sulfones 4a–4f have been obtained exclusively in the case of cobaloxime 1 .