1.3- pentadiene, reaction with

The [l,5]-hydrogen shift in Z-l,3-pentadiene is an example of a narcissistic reaction, with the reactant and product being identical. The TS is therefore located exactly at half-way, and has a symmetry different from either the reactant or product. By suitable constraints on the geometry the TS may therefore be located by a minimization within a symmetry consti ained geometry. 

With this rule in mind the outcome of CM-reactions can often be predicted. In the synthesis of organotrifluoroborate 79 [143] the terminal double bond is a type 1 substrate, while the 1,1-disubstituted olefin can be considered type III. The reaction of 2-methyl-1,4-pentadiene 77 with type II cross-partner 78 furnishes 79 efficiently (only 2 mol% catalyst used) in good yields after two steps. 

Ordinary Grignard reagents react with a, -unsaturated carbonyl compounds and afford both 1,2-adduct and 1,4-adduct. However, methylsulfonyhnethylmagnesium bromide or p-tolylsulfonylmethylmagnesium bromide gave only 1,2-adducts in the reaction with conjugated carbonyl compounds such as crotonaldehyde, cinnamaldehyde, trans-4-phenyl-3-buten-2-one, benzalacetophenone and l,5-diphenyl-2,4-pentadien-l-one. 

It was also shown that a cahonic allylnickel complex [(q3-crotyl)Ni P(OEt)3 2]PF6 catalyzes this reaction without added acid and that ds- and trans-1,3-pentadienes react with morpholine to give a mixture of 1,2- and 1,4-addition products [178]. 

As expected, the azaquinone C=N double bond is strongly dienophilic. This was demonstrated by reactions with several 1,3-butadienes, cyclo-pentadiene, and cyclohexadiene (Scheme 7). These reactions, carried out... 

The simplest nonconjugated, acyclic diene is 1,4-pentadiene (1), with its enthalpy of formation of 105.6 kJmol-1. The obvious question is whether the two double bonds are truly independent. If they are, then the enthalpy of hydrogenation of one double bond as in (the identical) reactions 4a and 4b would be precisely one half of that of the hydrogenation of both as in reaction 5. 

Schafer and coworkers devised a useful synthetic procedure based on this reaction [26]. Thus, the Diels-Alder reaction of /i-silylacrylic acid with cyclo-pentadiene gave the adduct which was oxidized anodically with the elimination of the carboxyl and the silyl groups. Successful formation of norbonadiene indicates that /J-silylacrylic acid can be used an a synthon of acetylene in Diels-Alder reactions with dienes (Scheme 33). 

In a similar study, Park has shown125 that cyclic dienes other than cyclo-pentadiene will also undergo a [4 + 2] cycloaddition reaction with the (trimethylsilyl)alkynylcarbene complex (184.b). When 184.b was reacted with 1,3-cyclohexadiene in THF, the diene complex 191 was isolated along with the vinylketene complex 192, which was prone to hydrolysis during column chromatography as expected.123 When 192 was stirred with silica gel and water in hexane, an almost quantitative conversion to the aldehyde complex 193 was observed. 

Diels-Alder reactions with alkynes as dienophiles have been known for a long time. Iminoboranes, however, will more readily cyclodimerize than react with dienes, and even the cyclodimers are generally superior to dienes in the competition for excess iminoborane. Among many attempts, therefore, only two reactions with iminoboranes as dienophiles have been successful, and in both of them the diene is cyclo-pentadiene [Eq. (55)] (9, 14). 

Reaction of 3,3-disubstituted-l,4-pentadiene 92 with a primary amine under cyclohydrocarbonylation conditions yielded cyclopenta[. ]pyrrole 96 as the predominant product accompanied by a small amount of cyclopentanone 95 (Scheme 15). This unique reaction is proposed to proceed through a cascade hydrocarbonylation-carbonylation process. The first hydrocarbonylation of 92 and the subsequent carbocyclization formed cyclopentanoylmethyl-Rh complex 93. If 93 immediately reacts with molecular hydrogen, 2-methylcyclopentanone 95 is formed. However, if CO insertion takes place faster than the hydrogenolysis, cyclopentanoylacetyl-Rh complex 94 is generated, which undergoes the Paal-Knorr condensation with a primary amine to yield cyclopenta[. ]pyrrole 96. ... 

Problem 8.40 Give possible products, with lUPAC names, of the following addition reactions (a) 1,3-butadiene and 1,4-pentadiene, each with 1 and 2mol of HBr (b) 2-methyl-l,3-butadiene and 1,3-pentadiene with 1 mol of HI. 

Other reactions of dienes with metal atoms are only of a limited synthetic use. Dibenzylideneacetone (PhCH=CH—CO—CH=CHPh DBA) reacts with palladium vapor to afford Pd2(DBA)3, a complex in which the coordination is through the two C=C units and does not involve the C=0 (5, 92). Cobalt vapor undergoes an extremely complicated reaction with 1,4-pentadiene, producing pentenes, C5H6, and various polymers as well as the organometallic product, HCo( 1,3-pen tadiene)2, which involves isomerization from a nonconjugated to a conjugated diene (104, 110). 

Cyclocondensation of aldehydes with silyloxydienes (12, 312). The stereoselectivity of this reaction depends not only on the catalyst but also on the solvent as summarized in equation (I) for reactions with l-methoxy-2-methyl-3-trialkylsi-lyloxy-l,3-pentadienes la and lb.8... 

As with the Aratani catalysts, enantioselectivities for cyclopropane formation with 4 and 5 are responsive to the steric bulk of the diazo ester, are higher for the trans isomer than for the cis form, and are influenced by the absolute configuration of a chiral diazo ester (d- and 1-menthyl diazoacetate), although not to the same degree as reported for 2 in Tables 5.1 and 5.2. 1,3-Butadiene and 4-methyl- 1,3-pentadiene, whose higher reactivities for metal carbene addition result in higher product yields than do terminal alkenes, form cyclopropane products with 97% ee in reactions with d-men thy 1 diazoacetate (Eq. 5.8). Regiocontrol is complete, but diastereocontrol (trans cis selectivity) is only moderate. 

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