Dienes magnesium complexes

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. 

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

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. 

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]. 

S. Akutagawa and S. Otsuka, J. Amer. Chem. Soc., 1976, 98, 7420 the formula for isogeraniol lacks the 6,7-double bond and the diene minor product is named incorrectly. No experimental details are given for the hydrolysis of this myrcene-magnesium complex but the results contradict those reported by Cookson et al. who have extensively investigated its reactions with carbonyl compounds, epoxides, carbon dioxide, and acetonitrile. ... 

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. 

Mixed cyclopentadienyl-diene titanium complexes, Cp TiX(diene)(X = Cl, Br, I), have been prepared in 30-60% yield by the stoichiometric reaction of CpTiXs with (2-butene-l,4-diyl)magnesium derivatives or by the reduction of CpTiXs with RMgX (R = i-Pr, f-Bu, Et X = Cl, Br, I) in the presence of conjugated dienes, as shown in Scheme 4. The butadiene, 1,3-pentadiene, and 1,4-diphenylbutadiene complexes of Cp TiX exhibit a unique prone (endo) conformation (13), while the isoprene, 2,3-dimethylbutadiene, and 2,3-diphenylbutadiene complexes prefer the supine (exo) conformation (14). Reduction of Cp TiX(diene) with RMgX or Mg gives a low-valent species, which catalyzes a highly selective (>99%) tail-to-head linear dimerization of isoprene and 2,3-dunethylbutadiene. " ... 

Larger substituents at the 2-position of the diene were also demonstrated to increase selectivity. As shown in Scheme 4, the magnesium complex of myrcene reacted with trimethylsilyl chloride, followed by cyclohexanone, to yield the two compounds shown in... 

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]. 

Although 1,2-dimethylenecyclopentane and 1,2-dimethylenecycloheptane dienes have been used in combination with Rieke magnesium, most work has concentrated on the 1,2-dimethylenecyclohexane-magnesium complex. All three of these cyclic dienes have been synthesized by established procedures [14]. 

When this approach is applied to the magnesium complexes of acyclic 1,3-dienes, it provides an easy route to both spiro y-lactones and y-lactones. Scheme 5 gives an outline for the synthesis of spiro y-lactones from (2,3-dimethyl-2-butene-l,4-diyl)magnesium 18. 

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... 

Magnesium complexes of 1,3-dienes have been used to form carbocycies and w-bro-moalkenes. l,4-Diphenyl-2-butene-l,4-diylmagnesium (70) was prepared by reacting ( , )-l,4-diphenyl-l,3-butadiene with magnesium freshly generated by reducing anhydrous magnesium chloride with lithium in THF (equation 37). While 70 reacted with... 

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]. 

The myrcene-magnesium complex, formed by reaction of 7-methyl-3-methylene-octa-1,6-diene with magnesium in the presence of ferric chloride and ethyl bromide, undergoes a general reaction with aliphatic esters whose products include cyclo-pentenols via a 1,4-addition, as shown in Scheme 3. 

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]. 

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... 

A solution of -butyllithium (19.57 mL, 2.5 M in tetrahydrofuran, 48.9 mmol) is added at 0 °C to a solution of diisopropylamine (6.85 mL, 48.9 mmol) in tetrahydrofuran (100 mL). After 10 min, the reaction is cooled to -78 °C followed by dropwise addition of tricarbonyl(methyl 3,5-hexadienoate)iron (12.39 g, 46.6 mmol) in tetrahydrofuran (5 mL). The reaction mixture is kept at this temperature for 20 min. After the addition of methyl iodide (10 mL, 22.7 g, 159.9 mmol), the mixture is allowed to warm to room temperature over 1 h and quenched with a saturated solution of ammonium chloride. The aqueous layer is extracted with ethyl acetate (3 x 50 mL). The combined organic layers are washed with 1 N phosphoric acid (2 x 40 mL) and saturated aqueous sodium hydrogen carbonate (1 X 40 mL), then dried with magnesium sulfate and concentrated. The resulting oil is purified by chromatography (hexane/ethyl acetate, 19 1) to afford the a-methylated (r -diene)iron complex as a yellow solid mp 43-44 °C Rf = 0.53 (hexane/ethyl acetate, 9 1) dr 94 4 9.46 g (72%). ... 

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). 

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