Magnesium diene complexes

This novel electroreductive cyclocoupling corresponds to a 1,4-addition of a one-carbon unit to the 1,3-diene, and does not take place without using magnesium electrode. The first step in this coupling reaction is the cathodic reduction of 1,3-diene to an anion radical, and the second step is the formation of a Mg-diene complex, which thereafter reacts with the ester to yield the coupling product as shown in equation 23b. 

The intermediary formation of the Mg-diene complex is confirmed by a two-step reaction method, namely in the first step a solution of 1,3-diene is electrochemically reduced with magnesium electrode in the absence of the ester. After a sufficient amount of electricity is passed, the current is terminated and the ester is added to the solution. The fact that the coupling product is also formed by this two-step method strongly supports the formation of the intermediate Mg-diene complex. 

Yasuda, H., Kajihara, Y., Mashima, K., Nagasuna, K., Lee, K. and Nakamura, A. (1982) 1,3-Diene complexes of zirconium and hafnium prepared by the reaction of enediyl-magnesium with MCl2Cp2. A remarkable difference between the zirconium and hafnium analogues as revealed by H NMR and electronic spectra. Organometallics, 1, 388-396. 

The magnesium-1,3-diene complex (s-m-PhCH=CHCH=CHPh)Mg-(THF)3 (65) was synthesized from 1,4-diphenylbutadiene and activated... 

Group 4 transition aza-metal-diene complexes have received considerable attention because of their unique M-N and M-C bonding properties and their high reactivity toward a broad range of electrophiles and unsaturated hydrocarbons. Reduction of CpTiCl3 with magnesium in THF in the presence of the appropriate 1-aza-l,3-diene affords the 1-aza-l,3-diene titanium complexes CpTiCl[N(R)CH=C(Me)CH(Ph)] (Scheme 217). Spectroscopic data indicate that the aza-diene ligands adopt a cis-supine conformation in the case of the Buc derivative a solution equilibrium with the /)nw-disposition is observed. The chemical shifts of the terminal carbon atoms of the aza-diene... 

Mono-diene complexes of zirconocene and hafnocene have been prepared by two methods [129-131 ], viz the photochemical reaction of diphenylzirconocene in the presence of diene and the reaction of metallocene dichlorides with diene magnesium adduct. The structures and reactivity of s-cis-dicm complexes indicate that the metal-lacyclopentene (B) is the preferred canonical form. Complexes of the type Cp2Zr(j-/ra/25-1,3-diene), have been prepared they were the first examples of this mode of coordination (C). Insertion of unsaturated compounds into a diene coordinated to zirconocene results in regioselective C—C bond formation [132-136]. 

It should be noted that the (l,4-diphenyl-2-butene-l,4-diyl)barium complex has been prepared in our laboratories and exhibits higher reactivity than its magnesium-diene counterpart. 

Scheme 4.5 illustrates a route for the synthesis of spiro-8-lactones from the magnesium complex of l,2-bis(methylene)cyclohexane 1 [126]. Initially, treatment of the l,2-bis(methylene)cyclohexane-magnesium reagent 2 with an excess of ethylene oxide at -78°C resulted in the formation of the 1,2-adduct of 3 by the incorporation of one equivalent of epoxides with the diene complex. Significantly, the bis-organomagnesium reagent 2 reacted with only one mole... 

Hydrogen peroxide (10 mL of a 30% aqueous solution) is added to a stirred solution of sodium hydroxide (610 mg, 1.53 mmol) in methanol (15 mL) at 0 °C. The solution is used immediately. A solution of the diene complex (016 mg, 0.029 mmol) in methanol (1 mL) at 0 °C is treated with the freshly prepared sodium hydroxide-hydrogen peroxide solution vide supra) (0.5 mL). After 5 min at 0 °C the reaction mixture is poured into diethyl ether-water (10 mL, 1 1) and the layers are separated. The aqueous layer is extracted with diethyl ether (3x5 mL) and the combined organic extracts are washed with brine (5 mL), dried (magnesium sulfate), and then concentrated in vacuo. Purification by flash column chromatography (Si02, diethyl ether-petrol ether 1 9) affords the diene as a yellow oil [a] , -62.9 (c 0.17, CH2CI2) 11 mg (94%). 

The proposed mechanism for Fe-catalyzed 1,4-hydroboration is shown in Scheme 28. The FeCl2 is initially reduced by magnesium and then the 1,3-diene coordinates to the iron center (I II). The oxidative addition of the B-D bond of pinacolborane-tfi to II yields the iron hydride complex III. This species III undergoes a migratory insertion of the coordinated 1,3-diene into either the Fe-B bond to produce 7i-allyl hydride complex IV or the Fe-D bond to produce 7i-allyl boryl complex V. The ti-c rearrangement takes place (IV VI, V VII). Subsequently, reductive elimination to give the C-D bond from VI or to give the C-B bond from VII yields the deuterated hydroboration product and reinstalls an intermediate II to complete the catalytic cycle. However, up to date it has not been possible to confirm which pathway is correct. 

The proposed catalytic cycle is shown in Scheme 31. Hence, FeCl2 is reduced by magnesium and subsequently coordinates both to the 1,3-diene and a-olefin (I III). The oxidative coupling of the coordinated 1,3-diene and a-olefin yields the allyl alkyl iron(II) complex IV. Subsequently, the 7i-a rearrangement takes place (IV V). The syn-p-hydride elimination (Hz) gives the hydride complex VI from which the C-Hz bond in the 1,4-addition product is formed via reductive elimination with regeneration of the active species II to complete the catalytic cycle. Deuteration experiments support this mechanistic scenario (Scheme 32). 

If the alkenes and acetylenes that are subjected to the reaction mediated by 1 have a leaving group at an appropriate position, as already described in Eq. 9.16, the resulting titanacycles undergo an elimination (path A) as shown in Eq. 9.58 [36], As the resulting vinyltitaniums can be trapped by electrophiles such as aldehydes, this reaction can be viewed as an alternative to stoichiometric metallo-ene reactions via allylic lithium, magnesium, or zinc complexes (path B). Preparations of optically active N-heterocycles [103], which enabled the synthesis of (—)-a-kainic acid (Eq. 9.59) [104,105], of cross-conjugated trienes useful for the diene-transmissive Diels—Alder reaction [106], and of exocyclic bis(allene)s and cyclobutene derivatives [107] have all been reported based on this method. 

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