Reactions of Cobaloxime Anions and/or Hydrides with Enynes as a New, General Route to 1,3- and 1,2-Dienylcobaloxime Complexes

Abstract

A new method for the preparation of dienylcobaloxime complexes which involves reactions of cobaloxime anions and/or hydrides with enynes is reported. This new dienyl complex preparative method leads to cobalt-substituted 1,3-or 1,2-dienes depending on the enyne substitution pattern chosen. Subsequent Diels-Alder reactions of the 1,3-dienyl complexes and demetalation reactions of cobalt-substituted cycloadducts are also reported.

Preparation of (L)(N,N‘-ethylenebis(salicylideneaminato))- (1,3-butadien-2-yl)cobalt(III) Complexes and Their Utilization in Facile and Regioselective Diels−Alder Reactions

“…Other dienophiles with two electron-withdrawing groups such as diethyl and dimethyl maleate and fumarate required refluxing in THF for a minimum of 18 h as did monosubstituted dienophiles methyl vinyl ketone and methyl acrylate. The vinyl protons in all of the [4 + 2] cycloaddition adducts were located between 3.4 and 4.5 ppm (CDCl 3 ) analogous to the upfield shifts we noted for protons β to cobalt in the diene complexes ( 1a − c ) . To verify these unusually low vinyl proton chemical shifts, HETCOR and APT experiments were carried out on several [4 + 2] cycloaddition adducts.…”

“…The original color is restored when the complex is dried at 60 °C (1 mmHg) for 6−24 h. The complexes ( 1a − c ) may also be dried at 25 °C (1 mmHg) over P 2 O 5 for 6−48 h. The 1 H NMR resonances for the 1,3-dienyl protons are presented in Table . As noted in the glyoxime series, the protons on the #1 carbon, H d and H e (Figure ) (H d , H e assignments were made by analogy to cobaloxime dienyl complexes), are strongly shielded by the adjacent low-valent cobalt. However, the effect here is even more dramatic than in the glyoxime series, since the H d and H e protons for the pyridine ( 1b ) and DMAP ( 1c ) complexes are 1 ppm upfield from their corresponding glyoxime complex values.…”

“…Second, diethyl maleate reacted much more slowly than diethyl fumarate in parallel reactions. The diethyl maleate required 73 h in refluxing THF to completely react with 1a while diethyl fumarate reacted in 48 h. The observation that diethyl maleate underwent cycloaddition slower than diethyl fumarate is the opposite of the cycloaddition reactivity we had noted previously for cobaloxime dienyl complexes ( 4 ) with dimethyl fumarate and maleate 1b…”

“…Over the past 4 years, we have been preparing cobaloxime-substituted 1,3-dienes and examining the rates, regioselectivities, and stereoselectivities of their reactions with dienophiles in Diels−Alder reactions .…”

DIMETILGLIOXIMA: un reactivo químico con 100 años de historia

“…5), que pueden ser aprovechados con fines académicos o en el campo de la química fina. (Stokes et al, 1996).…”