Reduction of 1,3-dienes

The full reduction of 1,3-dienes with Et3SiH/TFA occurs in certain systems although the yields are only modest.231 For example, 1,3-cyclohexadiene gives a 65% yield of cyclohexyl trifluoroacetate, presumably by way of cyclohexene (Eq. 88).211 On the contrary, 1,4-cyclohexadiene fails to undergo reaction with 10 equivalents of triethylsilane and 20 equivalents of trifluoroacetic acid even after 24 hours at room temperature (Eq. 89). 

On the other hand, it has been found that the electrochemical reduction is a very unique and useful tool in synthetic organic chemistry when magnesium is used as the material of the electrode. The cathodic reduction of 1,3-dienes with magnesium electrode gives very unique products, i.e. 3-cyclopentenol derivatives when it is carried out in the presence of a carboxylic acid ester (equation 23)17. 

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. 

Reactions of t-Bu3Si(/-Pr3Si)SiBr2 (20) with activated magnesium or the reagents derived by the reduction of 1,3-dienes with magnesium gave rise to... 

Methyl (trifluoromethylsulfonyl)-methyl sulfone, 193 Tributyl(iodomethyl)tin, 314 Reduction of 1,3-dienes to monoenes Arene(tricarbonyl)chromium complexes, 19... 

Carbon-carbon double bonds conjugated to multiple bonds other than aromatic systems may also be reduced with metals. Again the nature of the reduction product is dependent on the availability of a proton donor in the reaction medium. In the absence of an excess of proton donors, dimerization of the initially formed anion radical is observed. Both the reduction of 1,3-dienes and trapping experi-... 

Ultrasound treatment of Raney Ni prior to modification results in a much faster hydrogenation rate and better enantiodifferentiation in the reduction of 1,3-dienes and... 

The 17-ethylene ketal of androsta-l,4-diene-3,17-dione is reduced to the 17-ethylene ketal of androst-4-en-3,17-dione in about 75% yield (66% if the product is recrystallized) under the conditions of Procedure 8a (section V). However, metal-ammonia reduction probably is no longer the method of choice for converting 1,4-dien-3-ones to 4-en-3-ones or for preparing 5-en-3-ones (from 4,6-dien-3-ones). The reduction of 1,4-dien-3-ones to 4-en-3-ones appears to be effected most conveniently by hydrogenation in the presence of triphenylphosphine rhodium halide catalysts. Steroidal 5-en-3-ones are best prepared by base catalyzed deconjugation of 4-en-3-ones. ... 

Rhodium-catalyzed reductive coupling of 1,3-dienes, 1,3-diynes, or 1,3-enynes and glyoxals, and (iV-sulfinyl)imi-... 

To date, the reductive cyclization of allenic alkenes remains undeveloped. However, the reductive cyclization of activated alkene partners in the form of 1,3-dienes and conjugated enones has been achieved using late transition metal catalysts. Indeed, the hydrosilylative dimerization of 1,3-dienes reported in 1969 appears to be the first reductive... 

It is proposed that the reaction proceeds through (i) oxidative addition of a silylstannane to Ni(0) generating (silyl)(stannyl)nickel(n) complex 25, (ii) insertion of 1,3-diene into the nickel-tin bond of 25 giving 7r-allylnickel intermediate 26, (iii) inter- or intramolecular allylation of aldehydic carbonyl group forming alkoxy(silyl)nickel intermediate 27, and (iv) reductive elimination releasing the coupling product (Scheme 69). 

Olefin formation, by reductive elimination of 3-hydroxysulfones, 72, 2 Olefins, hydroformylation of, 56, 1 Oligomerization of 1,3-dienes, 19, 2 Oligosaccharide synthesis on polymer support, 68, 2 Oppenauer oxidation, 6, 5 Organoboranes ... 

METHOXYCARBONYL-1,1,6-TRIMETHYL-1,4,4a,5,6,7,8,8a-OCTAHYDRO-2,3-BENZOPYRONE, an intramolecular Diels-Alder reaction is responsible for the diastereoselectivity. The stereoselective 1,4-functionalization of 1,3-dienes is exemplified by a two-step process leading to cis- and trans-1-ACETOXY-4-(DICARBOMETHOXYMETHYL)-2-CYCLOHEXENE. The effectiveness of a silyl hydride in providing a means for erythro-directed reduction of a p-keto amide is applied in a route to ERYTHRO-1 -(3-HYDROXY-2-METHYL-3-PHENYL-PROPANOYLJPIPERIDINE. This is followed by an asymmetric synthesis based on a chiral bicyclic lactam leading to (R)-4-ETHYL-4-ALLYL-2-CYCLOHEXEN-1-ONE. The stereoselectivity with which acetoxy migration can operate to an adjacent radical center is reflected in the one-step reaction that gives rise to 1,3,4,6-TETRA-O-ACETYL-2-DEOXY-a-D-GLUCOPYRANOSE. 

The formation of vinyl cations has also been observed [61]. The acyclic vinyl iodide (24) was irradiated at 254 nm in CH3OH. This afforded reduction, nucleophilic-trapping, and allene intermediate products (Scheme 18). In the presence of CH3OD the allylic ether had 72% incorporation of deuterium at the vinylic position. On irradiation in CH2C12 or pentane, reduction and 1,3-diene product were observed. 

To obviate 0-alkylation, the readily accessible l,5-dimethoxy-l,4-cyclohexa-diene surrogate derived from Birch reduction of 1,3-dimethoxybenzene can be used. Enol ether hydrolysis affords the alkylated 1,3-cyclohexadiones in excellent yields. " ... 

Ruthenium-catalyzed hydroacylation of 1,3-dienes with aromatic and heteroaromatic aldehydes occurs in relatively good yields to afford the corresponding fi, /-unsaturated ketones . Isoprene and benzaldehyde were treated with 4 mol% Ru(COD)(COT) (COD = 1,5-cyclooctadiene, COT = 1,3,5-cyclooctatriene) and 4 mol% PPhs under argon for 40 hours to give 54% 80 (equation 41). The key intermediate is an acyl- ) -(allyl)ruthenium complex which undergoes reductive elimination to give the corresponding... 

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