Alkadienes preparation

Our discussion of chemical reactions of alkadienes will be limited to those of conju gated dienes The reactions of isolated dienes are essentially the same as those of individual alkenes The reactions of cumulated dienes are—like their preparation— so specialized that their treatment is better suited to an advanced course m organic chemistry... 

Butadiene is an industrial chemical and is prepared by dehydrogena tion of butane Elimination reactions such as dehydration and dehydro halogenation are common routes to alkadienes... 

The scope of haloaliphatic additions to alkenes has been extended to the preparation of 1,2-bis(perfluoroalkyl)iodoethanes by the addition of perfluoroalkyl iodides tofluoroalkyl alkenes [73] (equation 12) or to alkadienes [14] (equation 13)... 

Finally, it has been reported that allylboranes 4 and 5 may be prepared by the hydroboration of 4-trimethylsilyl-2,3-alkadienes by using either 9-borabicyclo[3.3.1]nonane or dicyclo-... 

Abstract This chapter deals with the methods for the syntheses of 2,5-dihydro-l,2-oxaphosphole-2-oxide derivatives, and some recent results of their biological activity testing. The electrophilic addition to 1,2-alkadiene- and alkatrienephospho-nate derivatives is one of the easiest and fruitful synthetic strategies for obtaining these compounds in preparative amounts. 

The submitted methods for preparation of target compounds based on the electrophilic addition reactions of 1,2-alkadiene- and alkatrienephosphonates show that they are very promising from synthetic point of view. 

Alkadiene- and alkatrienephosphonates are available from acetylene-allene rearrangement of acetylene phosphites which could easily be prepared from the reaction of carbonyl compounds and 1-alkynes. 

Diorgano-l-plumbacycloalkanes or -alkadienes, like 1,1-diethylplumbole, are prepared by the following reactions383,384 ... 

The procedure described here Is an example of a general method for preparing conjugated alkadienes by the palladium-catalyzed reaction of 1-alkenylboranes or boronates with vinylic halides. Hydroboratlon of l-a1kynes with catecholborane Is a standard method for obtaining (E)-l-alkenylboronates (1). Several different types of alkenylboranes and boronates (2-4) are now available as reagents for the cross-coupling reaction with vinyl halides. 

The alkadienals could be formed from the autoxidation of PUFA and may contribute desirable arenas to freshly prepared foods (19). Further degradation of alkadienals often increased undesirable flavors. Josephson and Lindsay demonstrated that 2,4-decadienal could produce 2-octenal and ethanal (20) and 2,6-nonadienal could produce 4-heptenal and ethanal (21) via retro-aldol condensation mechanisms. Hsieh et al. (22) reported that iscmers of various alkadienals and alkatrienals gave green, greasy and oxidized fish oil odors in crude menhaden fish oil. 

All reactions of Table 9.1 were performed in pyridine, usually at room temperature by addition of water, methanol, acetic acid, etc. it was possible to prepare several functionalized compounds from alkenes and alkadienes. 

In addition to the head-to-head cross-coupling reactions, it has been discovered that the reaction of phenyl or 1-alkenyl iodides with (E)-l-alkenyl-l,3,2-benzo-dioxaboroles produces the head-to-tail cross-coupling products, 2-phenyl-1-alkenes or 2-alkyl-1,3-alkadienes, respectively (Eq. 107) The reaction is profoundly affected by catalytic quantities of palladium compounds (Pd black prepared by reduction of Pd(OAc)j with diborane is especially effective) in the presence of tri-ethylamine. 

As for many other nucleophiles, the nitrite anion undergoes addition to the iodonium ion generated by the reaction of alkenes and 1,3-alkadienes with electrophilic iodine reagents. Two procedures have been described bis(pyridine)iodine(I) tetrafluoroborate136,137 [prepared from mcrcury(II) oxide and tctrafluoroboric acid supported on silica gel and pyridine on dichloromethane] and copper(II) tetrafluoroborate [prepared from copper(II) oxide and te-trafluoroboric acid] and iodine138 139. trans Addition would be expected for all products from mechanistic considerations, however, only the cyclohexene adduct 1 has been shown to have trans configuration ( H-NMR spectroscopy)139. 

For a long time" , it has been known that there is rearrangement of 1-alkynes to 2-alkynes under basic conditions (for instance by alcoholic potassium hydroxide or powdered potassium hydroxide) at 175 C. It has been shown that in fact an equilibrium mixture is obtained . It contains isomeric compounds, mainly the starting 1-alkyne and isomeric 1,2-alkadiene and 2-aIkyne, but the latter, more stable from thermodynamic considerations, is predominant. Therefore the method could be used to prepare 2-alkynes from corresponding 1-alkynes. The reverse reaction is possible disubstituted acetylenes can be converted to sodium derivatives of 1-alkynes by sodium or sodamide . 1-Alkynes are recovered by hydrolysis. We have tried to apply both reactions to prepare acetylenes deuterated in defined positions with high isotopic purity. 

Martin, M G G, Horton, D, Preparative synthesis of C-(a-D-glucopyranosyl)-alkenes and -alkadienes Diels-Alder reaction, Carbohydr. Res., 191, 223 - 229, 1989. 

Titanium—acetylene complexes react with allylic or propargylic halides or acetates through regioselective titanacycle formation and subsequent fl-elimination [36,37]. The reaction therefore provides a convenient method of preparing 1,4-alkadienes, including those bearing functional groups as exemplified in Eq. 9.16 [38]. 

For obtaining rearranged conjugated trienes 52a-e and 53a-e on a preparative scale, the reaction mixture from the synthesis in 1 -(2, 6, 6 -trimelhyl-1 -cyclohexen-l -yl)-3-(phenylsulfinyl)-1,2-alkadienes 51a-c (200-300 mg) (both diastereomers of the vinylallene sulfoxides) after Et20 workup is allowed to stand in Et20 (15-20 mL) at r.t. for > lOh. The residue after removal of solvent is subjected to 1IPLC purification (Whatman Partisil M 9 10/50 column, 10-20% ElOAc/Skellysolve B) to afford the two conjugated trienes. From 51a yield 63% ratio (53a/52a) 81 19 from 51 b yield 91 % ratio (53b/52b) 92 8 from 51c yield 75% ratio (53c/52c) 92 8 from 51 d yield 80% ratio (53d/52d) 94 6 from 51e yield 58% ratio (53d/52d) >98 2,... 

---