84115-21-9Relevant academic research and scientific papers
Electrofugal Fragmentation of Alkylcobalmin Derivatives Using Cob(I)alamin and Heptamethyl Cob(I)yrinate as Catalysts
Fischli, Albert
, p. 1167 - 1190 (2007/10/02)
The cob(I)alamin- (1(I) and the heptamethyl cob(I)yrinate- (2(I)) catalyzed transformation of an epoxide to the corresponding saturated hydrocarbon 3 -> 4 -> 5 is examined (see Schemes 1 and 3-5).Under the reaction conditions, the epoxyalkyl acetate 3 is opened by the catalysts with formation of appropriate (β-hydroxyalkyl)-corrinoid derivatives (13, 14, 17, 18, see Schemes 12 and 14).Triggered by a transfer of electrons to the Co-corrin-? system, the Co,C-bond of the intermediates is broken, generating the alkenyl acetate 4 (cf.Schemes 12 and 14) following an electrofugal fragmentation (cf.Schemes 2 and 12).The double bond of 4 is also attacked by the catalysts, leading to the corresponding alkylcorrinoids (15, 19, see Schemes 12 and 14) which is turn are reduced by electrons from metallic zinc, the electron source in the system, inducing a reductive cleavage of the Co,C-bond with production of the saturated monoacetate 5 (see Schemes 2, 5 and 12).In the cascade of steps involved, the transfer of electrons to the intermediate alkylcorrinoids (13-15, 17-19, see Schemes 12 and 14) is shown to be rate-limiting.Comparing the two catalytic species 1(I) and 2(I), it is shown that the ribonucleotide loop protects intermediate alkylcobalmins to some extent from an attack by electrons.The protection function of the ribonucleotide side-chain is shown to be present in alkylcobalamins existing in the base-on form (cf.Chap. 4 and see Scheme 14).
Hydroboration. 57. Hydroboration with 9-Borabicyclononane of Alkenes Containing Representative Functional Groups
Brown, Herbert C.,Chen, Jackson C.
, p. 3978 - 3988 (2007/10/02)
The hydroboration of alkenes containing representative functional groups was examined with 9-borabicyclononane (9-BBN) in order to extend the hydroboration reaction for the preparation of functionally substituted organoboranes.Terminal alkenes containing a remote functional group are hydroborated with a remarkable regioselectivity (>=98percent terminal), producing the corresponding stable organoboranes. 9-BBN hydroborates the allylic derivatives so as to place boron essentially on the terminal carbon atom (>=97percent).The directive effect is further enhanced (>=99percent) in the case of β-methylallyl derivatives.The hydroboration of crotyl derivatives attaches boron predominantly at the 2-position, followed by an elimination-rehydroboration sequence.However, crotyl alcohol can be protected against elimination as the tert-butyl or tetrahydropyranyl ethers.The hydroboration-oxidation of ethyl crotonate involves a series of elimination, hydroboration, and condensation processes.In the vinyl, crotyl, and isobutenyl systems, the mesomeric effect of the substituent favors the placement of boron at the β-position, while the inductive effect favors the α-position, with the former effect predominating in most cases.Acyclic β-substituted organoboranes undergo rapid elimination.Nonpolar solvents and lower reaction temperatures decrease the rate of elimination.However, those derived from cyclic vinyl derivatives are relatively stable under neutral conditions, undergoing facile elimination in the presence of a base.
