Conjugated diene complexes cycloaddition

Danishefsky et al. were probably the first to observe that lanthanide complexes can catalyze the cycloaddition reaction of aldehydes with activated dienes [24]. The reaction of benzaldehyde la with activated conjugated dienes such as 2d was found to be catalyzed by Eu(hfc)3 16 giving up to 58% ee (Scheme 4.16). The ee of the cycloaddition products for other substrates was in the range 20-40% with 1 mol% loading of 16. Catalyst 16 has also been used for diastereoselective cycloaddition reactions using chiral 0-menthoxy-activated dienes derived from (-)-menthol, giving up to 84% de [24b,c] it has also been used for the synthesis of optically pure saccharides. 

The major developments of catalytic enantioselective cycloaddition reactions of carbonyl compounds with conjugated dienes have been presented. A variety of chiral catalysts is available for the different types of carbonyl compound. For unactivated aldehydes chiral catalysts such as BINOL-aluminum(III), BINOL-tita-nium(IV), acyloxylborane(III), and tridentate Schiff base chromium(III) complexes can catalyze highly diastereo- and enantioselective cycloaddition reactions. The mechanism of these reactions can be a stepwise pathway via a Mukaiyama aldol intermediate or a concerted mechanism. For a-dicarbonyl compounds, which can coordinate to the chiral catalyst in a bidentate fashion, the chiral BOX-copper(II)... 

Conjugated dienes can be dimerized or trimerized at their 1,4 positions (formally, [4 4- 4] and [4 4-4 4-4] cycloadditions) by treatment with certain complexes or other transition metal compounds. " Thus butadiene gives 1,5-cyclooctadiene and 1,5,9-cyclododecatriene. " The relative amount of each product can be controlled by use of the proper catalyst. For example, Ni P(OC6H4—o-Ph)3 gives predominant dimerization, while Ni(cyclooctadiene)2 gives mostly trimerization. The products arise, not by direct 1,4 to 1,4 attack, but by stepwise mechanisms involving metal-alkene complexes. " ... 

The behavior of complexes of nitroalkenes (42) with LA toward conjugated dienes is yet another factor underlying the role of these complexes. Conjugated nitroalkenes (42) are considered as active dienophiles in classical Diels-Alder reactions (104, 105). On the contrary, in the presence of SnCl4, nitroalkenes (42) react with cyclopentadiene and 1,3-cyclohexadiene exclusively at one double bond (103). Therefore, it is highly probable that the 42 + 43 cycloaddition proceeds by a nonconcerted mechanism in the presence of LA (see Scheme 3.40). 

When tricarbonyl(diglyme)molybdenum(0) (53) (78) is heated with the hydrocarbons 50a-50d and 50t in n-hexane, the analogous molybdenum complexes of the chromium species 42 are readily formed (70) [Eq. (30)]. These results prove the second explanation to be the correct one as to why tricarbonyl-/ 6-l,3,5-cycloheptatrienemolybdenum shows no [4 + 6] cycloaddition with conjugated dienes. 

Since the initial report in 1962 by Merten and Muller of the cycloaddition of an -acyl imine with a conjugated diene,a number of examples of this type of reaction have appeared. In general, IV-acyl im> ines are highly reactive, unstable species which are rarely isolated, but rather are generated in situ from stable precursors. Depending upon the method of formation of the particular dienophile and the reaction conditions, a neutral A -acyl imine or a protonated (or Lewis acid complexed) IV-acyl immonium ion may be involved in the Diels-Alder reaction. 

Arene oxides show the characteristic reactions of epoxides (isomerization to ketones, reductions to alcohols, nucleophilic additions, deoxygenations) and olefins or conjugated dienes (catalytic hydrogenation, photochemical isomerization, cycloaddition, epoxidation, metal complexation). Where a spontaneous, rapid equilibration between the arene oxide and oxepin forms exists, reactivity typical of a conjugated triene is also found. 

The use of transition metal complexes as catalysts allows 1,4-cycloadditions to be involved as the major pathway in several cases when conjugated dienes are reacted with norbornadiene. No normal homo-Diels-Alder reaction was observed by reaction of the latter with buta-1,3-diene in the presence of an iron complex catalyst, the main product obtained was such a 1,4-adduct 2f the same adduct 2 was obtained in good yield and selectivity when a catalyst formed from cobalt(II) chloride, diethylaluminum chloride and bis(l,2-diphenylphos-phinojethane was used. ... 

Finally, the intramolecular cycloaddition methodology also provides access to heterocyclic systems when methylenecyclopropanes with heteroatom-containing side chains are employed. However, as shown above, very complex structural requirements are deary operable. For example, methyl 4-[(l-methyl-2-methylenecyclopropyl)methoxy]but-2-ynoate only yields a complex product mixture mainly containing conjugated dienes (arising from a ring cleavage/ -elimination sequence) upon attempted palladium(0)-catalyzed cycloisomerization. ... 

Cycloaddition. A process leading to formation of a 1,3,6-cyclooctatriene system from a conjugated diene and two alkynes is useful. A cationic Rh(I) complex fulfills the catalytic purpose. ... 

Diene complexes can also be decomplexed by treatment with strong Lewis acids (Scheme 10.10). CO insertion occurs, leading to formation of a cyclopentenone 10.33. This reaction is, formally, a cycloaddition of carbon monoxide with a diene, followed by migration of the resulting alkene into conjugation. 

The Diels-Alder cycloaddition is an important reaction class involving the reaction of unsaturated carbonyl compounds with conjugated dienes. The carbon atoms at the 1- and 4-positions of the conjugated diene system become attached to the doubly bonded carbons of the unsaturated carbonyl compound to form a ring stiucture. Diels-Alder chemistry is used in the synthesis of a variety of complex organic compounds of commercial interest, including insecticides, fragrances, plasticizers, and dyes (92). These bimolecular reactions have been used... 

Treatment of [Cp Ru(Ti -C4H6)(X)] (X = Cl, Br) with excess buta-l,3-dicne in the presence of silver trifluoromethanesulfonate (AgOTf) followed by carbon monoxide afforded OS the cycloocta-1,5-diene complex [Cp Ru(Ti -COD)(CO)]+[(OTf)], by means of a (4 + 4] cycloaddition reaction. Similar dimerisation, both stoichiometric and catalytic, was observed with other conjugated dienes. 

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