1.3- diene-magnesium

This facile one-pot transformation of a 1,3-diene-magnesium intermediate provides... 

Based upon the bis-nucleophilicity of 1,3-diene-magnesium intermediates, reactions of these intermediates with bis-electrophiles can lead to spiro or fused bicyclic molecules, depending upon the regioselectivity of the cyclization. It has been shown that treatment of magnesium complexes of l,2-bis(methylene) cyclohexane with l, -dibromoalkanes results in overall 1,2-cyclizations of the original dienes, affording spirocarbocycles in good to excellent yields [115]. 

With the aid of Rieke magnesium, fused carbocyclic enols can be effordessly synthesized from 1,3-diene-magnesium complexes by reacting with carboxylic esters [116]. By controlling the reaction temperature, p,y-unsaturated ketones can also be produced. 

Synthesis of Spiro-y-Lactones and Spiro-6-Lactones from 1,3-Diene-Magnesium Reagents... 

In most cases that were reported in the literature so far, the (s-cis-ri4-diene)metallocene complexes (and their s-trans- counterparts), were synthesized by treatment of the respective bent metallocene dichloride precursor with a suitable conjugated-diene-magnesium reagent. However, in a few cases alternative entries and procedures have been developed and described. None of these has found such a widespread application as the original procedure, but some interesting chemistry has evolved among those methods. 

It was recently discovered that Rieke magnesium reacts with 1,2-dimethylenecyclo-alkanes in THF at ambient temperature to afford the corresponding magnesium complexes in high yields. Significantly, treatment of these diene-magnesium reagents with bis-electrophiles provides a unique approach to commonly encountered spirocyclic systems [13]. 

The structure of (2,3-dimethyl-28butene-l,4-diyl)magnesium has not yet been determined. The most likely structures for aliphatic diene-magnesium complexes are five-membered metallocycles or organometallic oligomers. 

Table 3 Synthesis of Fused Carbocycles, j5,y-Unsaturated Ketones and 3-Cyclopentenols from Diene-Magnesium Reagents...

Table 5 Synthesis of Spiro <>-Lactoncs from Conjugated Diene-Magnesium Reagents. Epoxides, and COi...

Table 6 Synthesis of Alcohols from Conjugated Diene Magnesium Reagents and Epoxides, Followed by Acidic Hydrolysis...

Table 7 Synthesis of y-Lactams from Conjugated Diene-Magnesium Reagents, Imines, and CO2...

Diene-magnesium complexes are used in organic synthesis, including coupling with a,(u-dibromoalkanes, to form carbocycles, as illustrated in reaction (c) . 

If zrtrp, (70) [22] Other examples are also described, including hafnium analogues. A reaction of a diene, magnesium and zirconocene dichloride is thought to proceed via zirconocene rather than an organomagnesium intermediate [23]... 

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]. 

Bis-electrophiles were added to the THF solution of the diene-magnesium reagent at -78 C. The reaction mixture was then stirred at -78°C for 1 h prior to warming to the specified temperature. A, reflux (15h) B, room temperature (30min) C, reflux (lOh) D, room temperature (10 h) E, -30°C F, room temperature (30 h). 

Table 4.7 Reactions of diene-magnesium reagents with carboxylic esters formation of cyclopentenols. ...

We have developed a direct synthetic method for the one-pot synthesis of lactones, spirolactones, di-spirolactones, tertiary alcohols, and even 1,2-diols from the corresponding conjugated diene-magnesium reagents mediated by Rieke magnesium [122], The overall lactonization procedure can be considered... 

Table 4.10 Reactions of conjugated diene-magnesium reagents with epoxides followed by carbon dioxide.

The epoxide was added to the diene-magnesium complex at -78°C, and the reaction mixture was stirred at -78°C for 30min and then gradually warmed to 0°C followed by the bubbling of CO2. 

Itoh and coworkers developed a new route to y-lactams by the ruthenium-catalyzed cyclization of A -allyltrichloroacetamides [132]. Also, Stork and Mah have reported on the radical cyclization ofAT-protected haloacetamides to yield AT-protected lactams. The protecting groups can then be easily removed under a variety of conditions [133]. This efficient radical cyclization route to c -fused pyrrolidones and piperidones is interesting because of the widespread occurrence of related systems in natural products [134]. We have developed a direct method for the one-pot synthesis of y-lactams and secondary amines utilizing conjugated diene-magnesium complexes [135]. 

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