Alkene conjugated diene

In Grignard reactions, Mg(0) metal reacts with organic halides of. sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand. Pd(0) complexes react more easily with halides of carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C tr-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 /J-hydro-gen. At the same time, the Pd(0) 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. 

Cyanide-containing cobalt catalysts, particularly potassium pentacyanocobalta-te(II) K3[Co(CN)5], are used in the reduction of activated alkenes (conjugated dienes).26,31 [Co(CO)4]2 is best known as a hydroformylation catalyst, but hydrogenation is also possible under specific conditions. Phosphine-substituted analogs are more successful. 

CO)2Fe (THF) BFT A transition state model for the syn stereoselective cyclo-propanations of alkenes with diazoacetic ester by Rh-porphyrin catalysts has been proposed. Alkenes , conjugated dienes and enol ethers are stereoselectively cyclopropanated with Rh(II) -stabilized 1- (alkoxycarbonyl)vinyl carbenoids derived from the diazo precursors and Rh2(OAc)4 (equation 95). The Cu(acac)2-catalyzed reactions of Me3SiCH2COCHN2 with alkenes provide the expected adducts in good yields ". ... 

A more efficient and economic procedure for the preparation of / -acetamido nitro compounds from alkenes, conjugated dienes and silyl enol ethers is based on the electrochemical generation of nitronium tetrafluoroborate in acetonitrile solution by anodic oxidation of dinitrogen tetrox-ide, using a divided cell (Table 6, Method B)142-143. From 1,3-dienes, mixtures of regio- and diastereomers were obtained. 

Generation of stereo- and regio-defined alkenylmetals via hydrometallation or carbometallation of alkynes followed by cross-coupling (Scheme 1-11) is a synthetically attractive notion for preparing arylated alkenes, conjugated dienes, and conjugated enynes. Its feasibility was demonstrated in 1976 [14,15] in the prototypical examples shown in Schemes 1-4 and 1-5. In these processes, hydroalumination was employed for generating the required alkenylmetals. 

Like other alkenes, conjugated dienes undergo addition not only by electrophilic reagents but also by free radicals. In free-radical addition, conjugated dienes show two special features they undergo 1,4-addition as well as 1,2-addition, and they are much more reactive than ordinary alkenes. We can account for both features—orientation and reactivity—by examining the structure of the intermediate free radical. 

I he above scries of studies, w liiclt has Ivcu introduced hv earlier review articles 11.5-221. now ensures the general applicability of die hydioinagnesialioii method lo a variety of unsaiuratcd compounds having alkene. conjugated diene, and alkyne functions, which are discussed in the following sections. 

Over the past two decades, Pd- or Ni-catalyzed cross-coupling, especially Pd-catalyzed version, has become one of the most common methods (possibly the most common method) for highly selective synthesis of arylated alkenes, conjugated dienes, conjugated enynes (Sect, in.2.8), and other related aUcene derivatives, hi addition to Mg, Zn, Al, and Zr used since the 1970s, several other metals including Sn,[2 ] [27] and Cu[2 ] [ i have been extensively employed since around 1980. 

Like alkenes, conjugated dienes can be prepared by eiimination reactions. Draw a stepwise mechanism for the acid-catalyzed dehydration of 3-methyl-2-buten-1-oi [(CH3)2C = CHCH20H] to isoprene [CH2= C(CH3)CH = CH2]. 

---