Dienes from alkynes

The stoichiometric head-to-head oxidative coupling of alkynes with CpRuBr(COD) affords a metallacyclic biscarbene complex [22], This process has been used to initiate catalytic formation of the RCH=CH-CH=C(Y)R backbone, to produce Junctional dienes from alkynes by addition of H-Y. The complex [Cp RuCl(COD)[ successfully catalyzes this new chemical transformation, involving the combination of two molecules of alkynes and one molecule of carboxylic acid to afford functional conjugated dienes (Scheme 4) [23]. 

Scheme 4. Functional dienes from alkyne/carboxylic acid coupling.

Pig. 22. Formation of dienes from alkynes and diazo compounds catalyzed by Cp Ru (COD)Cl. 

Cis-olefins or cis./rjns-dienes can be obtained from alkynes in similar reaction sequences. The alkyne is first hydroborated and then treated with alkaline iodine. If the other substituents on boron are alkyl groups, a cis-olefin is formed (G. Zweifel, 1967). If they are cir-alkenyls, a cis, trans-diene results. The reactions are thought to be iodine-assisted migrations of the cis-alkenyl group followed by (rans-deiodoboronation (G. Zweifel, 1968). Trans, trans-dienes are made from haloalkynes and alkynes. These compounds are added one after the other to thexylborane. The alkenyl(l-haloalkenyl)thexylboranes are converted with sodium methoxide into trans, trans-dienes (E. Negishi, 1973). The thexyl group does not migrate. 

Other sources say that, in general, nitrogen oxides give rise to relatively slow reactions with alkenes, 1,2-dienes and alkynes, which lead to an ignition of the medium only from 30°C onwards. For 1,3-dienes, the same sources say that the interaction between these reagents should be carried out between -180 and -100°C and becomes dangerous between -35 and -15°C. 

AUylzirconium complexes are conveniently obtained by the regio- and stereoselective hydrozirconation of allene [127-133] and can be, for example, used subsequently for the MAO-catalyzed allylzirconation of alkynes to prepare enyne derivatives [132]. Alternatively, the preparation of (E)-l,3-dienes from aldehydes and the appropriate allylstannane zirconocene derivative (R = SnBu,) is accomplished (Scheme 8-17) [131], Note that addition of [Cp2Zr(H)Cl[n (1) on the aUenyl reagent with the... 

Scheme 11. 1,3-Diene synthesis from alkyne and ethylene...

Diels-Alder cycloalkene from diene + alkene/alkyne... 

In general, sulfur-substituted allenes are accessible starting from alkyne precursors by a variety of transformations such as isomerization, rearrangement or addition reactions. The standard method for the synthesis of donor-substituted allenes is again the base-induced isomerization of alkynes. This very first method was applied for the preparation of achiral [11, 162, 163] and chiral [164] S-functionalized 1,2-dienes (Scheme 8.78). 

Blechert reported a skillful method of cross-enyne metathesis. Solid-supported alkyne 139 is reacted with alkene in the presence of Ic to give 140. For cleavage of 1,3-diene from solid-supported product 140 having an allyl acetate moiety, palladium-catalyzed allylic substitution is used. Thus, 140 is treated with Pd(PPh3)4 in the presence of methyl malonate to afford three-component coupling product 141 in good yield ... 

An excellent method for the synthesis of 1,3-diene from polymer-supported alkyne and olefin (Scheme 16) has also been reported. Reaction of polymer-supported alkyne 44 and alkene 47a in the presence of Ic gives polymer-supported 1,3-diene 45a, which is treated with a palladium catalyst in the presence of a nucleophile for cleavage from the polymer to give 46a in 66% yield. In a similar manner, 46b, 46c, and 46d are obtained from polymer-supported alkyne 44 in high yields. These results indicate that various kinds of nucleophiles are introduced at the diene allylic position corresponding to the propargylic position in 44 (Scheme 16). 

Construction of 1,3-diene moieties from alkynes and ethylene is a unique methodology for the synthesis of 1,3-dienes. Mori used this strategy for the synthesis of anolignan A. Two methylene groups from ethylene are introduced onto the alkyne carbon of 109 using Ig to give 1,3-diene 110. From this compound, short-step synthesis of anolignan A is achieved (Scheme 41). ... 

RCM of dienes to cycloalkenes provides a useful method for the syntheses of carbo- and heterocycles and thus has been proved to be extremely effective in total synthesis of various natural products. Usually, however, mixtures of (E)- and (Z)-olefms result. In contrast, ring-closing alkyne metathesis provides a reliable route for synthesis of both (E)- and (Z)-macrocycloalkenes in a stereoslective manner taking advantage of stereoselective partial reduction of resulting cycloalkynes. A Lindlar reduction gives (Z)-cycloalkenes, whereas a hydroboration/ protonation sequence afford ( )-cycloalkenes (Equation (23)). Recently, Trost reported an alternative procedure for the synthesis of (E)-olefins from alkynes through hydrosilylation by a ruthenium catalyst. This procedure converts cycloalkyne 130, for example, to vinylsilane 131 and then to (E)-cycloalkene 132 in a stereoselective manner (Scheme 46)7 ... 

Benzofurans and dihydrobenzofurans have been prepared on polymeric supports by the palladium-mediated reaction of 2-iodophenols with dienes or alkynes (Entries 1 and 2, Table 15.9). This reaction is closely related to the synthesis of indoles from 2-iodoanilines, and probably proceeds via an intermediate palladacycle (Figure 15.3). Benzofuran and isobenzofuran derivatives have also been prepared on cross-linked polystyrene by intramolecular addition of aryl radicals to C=C double bonds and by intramolecular Heck reaction. 

The addition of H—X, where X = F, Cl, Br and I, to alkenes, dienes and alkynes has been extensively studied from both mechanistic1-3 and synthetic standpoints. While it is one of the earliest methods employed for the synthesis of organic halides, it can often lead to mixtures of products, and the direct conversion of the corresponding alcohols is more commonly used nowadays. The early literature abounds with examples of this reaction in which either mixtures of products are formed, or the products are not well characterized. Few major advances have been reported in more recent times which overcome the synthetic disadvantages of this direct process. It can nevertheless be the method of choice for the synthesis of certain substrates. 

The regiochemistry of the insertion results from a combination of factors which are still being sorted out. It is possible to diink of the carbon attached to palladium as electron rich, and it tends to attack die it system at die least elecd on rich position. Thus alkenes widi electron-withdrawing groups react faster dian alkenes widi electron-donating groups. It is quite paradoxical, however, that alkenes, dienes, and alkynes react much more readily dian carbonyl compounds, even diough the latter are much more elecd on deficient. 

Diels-Alder diene + alkene/alkyne decarboxylation lose C02 from a carboxylic acid hydroxylation add -OH nitration add -N02... 

Addition of hydrosilane to alkenes, dienes and alkynes is called hydrosilylation, or hydrosilation, and is a commercially important process for the production of many organosilicon compounds. As related reactions, silylformylation of alkynes is treated in Section 7.1.2, and the reduction of carbonyl compounds to alcohols by hydrosilylation is treated in Section 10.2. Compared with other hydrometallations discussed so far, hydrosilylation is sluggish and proceeds satisfactorily only in the presence of catalysts [214], Chloroplatinic acid is the most active catalyst and the hydrosilylation of alkenes catalysed by E PtCU is operated commercially [215]. Colloidal Pt is said to be an active catalytic species. Even the internal alkenes 558 can be hydrosilylated in the presence of a Pt catalyst with concomitant isomerization of the double bond from an internal to a terminal position to give terminal silylalkanes 559. The oxidative addition of hydrosilane to form R Si—Pt—H 560 is the first step of the hydrosilylation, and insertion of alkenes to the Pt—H bond gives 561, and the alkylsilane 562 is obtained by reductive elimination. 

---