Terminal dienes

ADMET polymerization has also been applied to 1,5-hexadiene, and polybutadiene (PBD) exclusively in the 1,4 mode was obtained [49]. Unlike PBD produced by ROMP, this ADMET polymer has a trans content of 75% [52]. With [W]l and [Mo]2,theAfn of these polymers was approximately 8.0 X lO gmol , witha poly-dispersity near 2.0. Attempts to polymerize 1,5-hexadiene with [Ru]l, however, resulted in oligomers of approximately 1.0 x lO gmoD, in addition to cyclics and unreacted monomer, even after extended reaction times [53]. This decrease in activity was attributed to stable intramolecular ii-complexation of the distal olefin of the 4-penten-l-ylidene complex to the metal center. Such coordination could obstruct bimolecular coordination of another diene to the metal, and thereby prevent further polymerization. The absence of this effect with Schrock s catalysts was explained in part by the steric congestion around the metal center of those catalysts and the lack of a labile ligand. A number of hydrocarbon dienes have been utilized in ADMET polymerizations [54, 55]. 

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Another metathesis polymerization procedure uses terminal dienes such as hexa-1,5-diene (16) (acyclic diene metathesis (ADMET)). Here again, the escape of the gaseous reaction product, i.e. ethylene, ensures the irreversible progress of the reaction ... 

Acid catalyzed intramolecular Diels-Alder reactions in lithium perchlorate-diethyl ether acid promoted migration of terminal dienes prior to [4 + 2] cycioaddition in conformationally restricted substrates [101]... 

The allylation reaction can be adapted to the synthesis of terminal dienes by using l-bromo-3-iodopropene and stannous chloride. The elimination step is a reductive elimination of the type discussed in Section 5.8. Excess stannous chloride acts as the reducing agent. 

The synthesis of the C(17)-C(24) segment also began with a diastereoselective boron enolate aldol addition. The adduct was protected and converted to an aldehyde in sequence H. The terminal diene unit was installed using a y-silylallyl chromium reagent, which generates a (3-hydroxysilane. Peterson elimination using KH then gave the Z-diene. 

A simple two-step protocol for the generation of a terminal diene is to add allyl magnesium bromide to an aldehyde or a ketone and subsequent acid or base catalysed dehydration (equation 34)72. Cheng and coworkers used this sequence for the synthesis of some indole natural products (equation 35)72a. Regiospecific dienones can be prepared by 1,2-addition of vinyllithium to a,/l-unsaturated carbonyl compounds and oxidative rearrangement of the resulting dienols with pyridinium dichromate (equation 36)73. 

Crombie and Rainbow reported synthesis of terminal dienes of high. E-selectivity via samarium diiodide mediated scission of 2-vinyl-3-chlorotetrahydrofuran (equation 175) or the corresponding pyran derivatives296. Interestingly, both cis- and frans-2-substituted 3-chlorotetrahydrofurans give the same diene, indicating the involvement of identical intermediates formed by electron transfer from samarium diiodide. 

Terminal dienes are the preferred monomers in the ADMET reaction, due to both entropic and steric considerations, which will be discussed in greater detail... 

To complete the range of geometric isomers of terminal and non-terminal dienes and trienes available, systems nominally derived from inaccessible (Z)-alkenylzirconocenes are desirable. Fortunately, insertion of the various carbenoids discussed above into mono- or bis(alkynyl) zirconocenes 64 and 65 affords dienyne products 66 [38], which are readily reduced to the desired ( ,Z,2)-trienes (Scheme 3.15) [45—47]. Insertion of the f5-alkynyl carbenoid 62 allows a convenient access to (Z)-enediynes 67. 

Terminal dienes such as butadiene, isoprene and 2,3-dimethylbutadienereactregiospeci-fically with I(Py)2BF4, in the presence of a nucleophile, to give 1,2-iodofunctionalization (equation 58)87. In contrast, internal dienes such as (Z, )-2,4-hexadiene and 1,3-cyclooctadiene yield the 1,4-addition products under similar conditions (equation 59). 

Substitution at both terminal diene-synthon positions is allowed only if the substituent is a primary atom or a triply bonded functional group (such as a cyano group). 

Attempted recychng and reuse of the heterogeneous catalyst (10) led to dramatic losses of activity. Following quantitative RCM of diethyl diallyhnalonate and simple filtration, the recovered resin was notably less active (<40% conversion) in a second run and the reaction mixture was dark in color. Such comphcations were attributed to the instability [14, 21] of Ru methylene (11) (Scheme 11.2) that propagates during the RCM of terminal diene substrates. In an effort to offset this solution-phase decomposition, the use of alkene additives was investigated. The... 

The allylation reaction can be adapted to the synthesis of terminal dienes by using... 

Scheme 4 Ring-closing metathesis of terminal dienes...

Presumably, the complex IX is expected to generate the same intermediate RuCl2(=CH2)(PCy3)2 as the Grubbs catalysts after the first cycle of a terminal diene RCM reaction. 

Terminal dienes gave slightly different results. Both cyclic and linear enol silyl ethers were produced in the reaction of 1,5-hexadiene, whereas for 1,7-octadiene only the straight chain E and Z isomers were produced. In neither case is the branched product observed.106... 

Reactions of pcrfluoroalkadienes with antimony(V) fluoride are similar to those of per-fluoroalkenes. Terminal dienes 1 rearrange with antimony(V) fluoride at 0-20 C selectively to internal dienes 2, in which the terminal trifluoromethyl groups are predominantly in the configuration.3,37,38 Higher temperatures and a threefold excess of catalyst lead to cyclo-pentenes (see Section 5.3.3.1.). 

In cases where more than one carbon-carbon double bond is present in the molecule, the possibility of selective cyclopropanation of one of them arises. Regiocontrol in intermole-cular cyclopropanation of substituted dienes has been the subject of much investigation and considerable differences can occur, depending on the structure of the substrate, catalyst and the carbene substituents. With a 1-substituted terminal diene such as 120, cyclopropanation, in general, occurs at the less-substituted double bond (equation 108)22 26,3, 16U62. In this case, the nature of the catalyst and of the carbenoid precursors are less important in determining the regioselectivity. 

Intermolecular cyclopropanation of 2-substituted terminal diene 121 with rhodium or copper catalysts occurs preferentially at the more electron-rich double bond (equation 109)37162. With a palladium catalyst, considerable differences in regiocontrol can occur, depending on the substituent of the diene. In general, palladium catalysed cyclopropanation occurs preferentially at the less substituted double bond (equation 110). However, with a stronger electron-donating substituent present in the diene, e.g. as in 2-methoxy-l, 3-butadiene, the catalytic process results in exclusive cyclopropanation at the unsubstituted double bond (equation 110)162. 

The reaction proceeds well with unhindered secondary amines as both nucleophiles and bases. The yield of allylic amine formed depends upon how easily palladium hydride elimination occurs from the intermediate. In cases such as the phenylation of 2,4-pentadienoic acid, elimination is very facile and no allylic amines are formed with secondary amine nucleophiles, while phenylation of isoprene in the presence of piperidine gives 29% phenylated diene and 69% phenylated allylic amine (equation 30).84 Arylation occurs at the least-substituted and least-hindered terminal diene carbon and the amine attacks the least-hindered terminal ir-allyl carbon. If one of the terminal ir-allyl carbons is substituted with two methyl groups, however, then amine substitution takes place at this carbon. The reasons for this unexpected result are not clear but perhaps the intermediate reacts in a a- rather than a ir-form and the tertiary center is more accessible to the nucleophile. Primary amines have been used in this reaction also, but yields are only low to moderate.85 A cyclic version occurs with o-iodoaniline and isoprene.85... 

Only monosubstituted terminal alkenes, including some terminal dienes, have so far been successfully methylaluminated. Within this restriction, however, the reaction appears to be reasonably general, as shown in Table 4.1. Specifically, n-alkyl, isoalkyl, and secondary alkyl substituents can be accommodated, but tertially alkyl-substituted ethylenes, such as r-BuCH=CH2, fail to react under the same conditions. In contrast with styrene, allylbenzene reacts normally. 

The use of the Grubbs catalyst is largely restricted to terminal dienes. In these cases the by-product is ethylene. Since this is an equilibrium process, the ethylene diffuses out of solution, which helps drive the reaction to completion. Neverdieless the catalyst is highly tolerant of functional groups and generally gives good yields. 

The result of acid-catalyzed isomerisation of F-dienes depends on several factors structure of substrate, catalyst, and temperature. Action of SbF5 on terminal dienes under mild conditions causes a 1,3 fluorine shift occurring stereoselectively to give trans-, frans-, and cis-, trans- isomers of the corresponding internal dienes [160] ... 

A very useful cross-metathesis is the reaction involving ethylene, which is called ethenolysis. Reaction of ethylene with internal alkenes produces the more useful terminal alkenes. Two terminal alkenes 45 and 42 are formed from the unsymmetric alkene 6 and ethylene. The symmetric alkenes 11 are converted to single terminal alkenes 45. The terminal dienes 46 are formed by ethenolysis of the cyclic alkenes 43. 

The 12-membered lasiodiplodine ring 107 was prepared in 94% yield by slow addition of terminal diene 106 to a solution of the Ru catalyst. The equilibrium was shifted to completion by bubbling with argon to remove the ethylene [38]. 

The disaccharide fragment of tricolorin A 110, which is a 19-membered macrolactone, was synthesized by efficient RCM of terminal diene 108 to give 109 and its hydrogenation in 77% yield [39]. The presence of sugar groups as a polar relay substituent, its proper distance to the alkene groups, and low steric hindrance close to the double bonds are decisive parameters for the efficient RCM of 108. 

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