Dienes with hydrogen halides

The reduction of acyl halides with hydrogen to form aldehydes using Pd catalyst is well known as the Rosenmund reduction[756]. Some acyl chlorides give decarbonyiation products rather than aldehydes under Rosenmund conditions. The diene 890 was obtained by decarbonyiation in an attempted Rosenmund reduction of acetyloleanolic acid chloride (889)[757], Rosenmund reduction of sterically hindered acyl chlorides such as diphenyl- and tnpheny-lacetyl chloride (891) gives the decarbonylated products 892[758],... 

With electrophiles such as hydrogen halides, perfluoropropadiene affords products with the central carbon atom of the allene moiety being protonated [57]. Although HX are normally considered as electrophiles, these reactions with tetrafluoropropa-diene may be nucleophilic in nature [57]. 

The addition of a hydrogen halide, such as HBr, is an important addition reaction for alkenes often seen in Organic Chemistry 1. However, conjugated dienes may behave differently. An example is the reaction of HBr with 1,3-butadiene as illustrated in Figure 4-11. 

In the absence of nucleophiles, the elimination of /3-hydrogen from tr-allyl-palladium intermediates to give the (E, )-l-aryl-l,3-dienes 267 and 268 takes place[173,180]. l-Aryl-l,3,5-hexatriene 269 reacts with an aryl bromide to give the 1,6-diaryl-1,3,5-hexatriene 270. The best results are obtained with aryl halides having EWGs in the para-position[181]. 

In Grignard reactions, Mg(0) metal reacts with organic halides of sp carbons (alkyl halides) more easily than halides of sp2 carbons (aryl and alkenyl halides). On the other hand, Pd(0) complexes react more easily with halides of sp2 carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C rr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes, conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /(-hydrogen. At the same time, the PdfO) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg. Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

Secondary and tertiary propargyl alcohols are directly converted to halo-allenes on reaction with concentrated aqueous hydrogen halides in the presence of the corresponding cuprous halide [60,72-73]. (See Table VII.) Better yields are obtained with hydrogen bromide. Hydrogen chloride yields chloroallene, propargyl chloride, and the chloro- 1,3-diene isomers (Eq. 60). 

Further 1,4-eliminations of hydrogen halide from gfem-dihalogenocyclopropanes carrying a substituent with an available 1 -hydrogen atom have been recorded, and the dehalogenation of (224) provides a new route to 2-fIuorobuta-1,3-dienes which is more efficient than the previously employed 1,4-dehydrochlorination. ... 

In the presence of soluble complexes of metals such as Ni and Pd, alkenyl halides undergo carbon-carbon bond formation with alkenes to produce dienes. In this process, called the Heck reaction, a molecule of hydrogen halide is liberated. 

The stereospecificity of the formation of 2fl-nonafluoropent-2-ene when the method employed undoubtedly involves a nucleophilic mechanism [equation (3)], coupled with the resistance shown by perfluoropenta-1,2-diene towards attack by hydrogen halides except for hydrogen fluoride, expected to be the most powerful proton donor, has led to the suggestion" that the... 

A number of r-enyl tricarbonyl halides may be prepared by standard routes and in all cases the jjn-isomers are the more stable [18,76,77, 77a, 82]. The anti-isomers may be prepared by the protonation of diene iron tricarbonyls with acids such as HBF4 whose anions are virtually nonco-ordinating. With hydrogen chloride the dienes give the jji -isomers, as shown below. It is thought the HCl undergoes cir-addition to the diene and the intermediate inverts forming the jjm-allyl product [76]. 

A remarkable ring closure of polyenic acetals can be performed in a highly stereospecific manner with SnCl4 to form polycyclic products with a multiplicity of asym. centers. 3-Alkyl-4-halogeno-tetrahydropyrans can be obtained by a novel reaction of olefins with paraformaldehyde and hydrogen halides . 1,3-Diphenyl-naphtho[2,3-c]furan, the first known stable 2,3-naphthoquinonoid, is an extremely reactive diene component in Diels-Alder syntheses . A modified Pechmann coumarin synthesis uses enamine derivatives of yS-ketoacids without catalyst. ... 

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