Terminal double bond formation

During the polyesterification reactions, especially at higher temperatures, it is possible to have some undesired side reactions take place, such as formation of terminal double bonds, formation of aldehydes, formation of polyenes which lead to coloured polyesters and loss of functionality. 

When this stereoelectronic requirement is combined with a calculation of the steric and angle strain imposed on the transition state, as determined by MM-type calculations, preferences for the exo versus endo modes of cyclization are predicted to be as summarized in Table 12.3. The observed results show the expected qualitative trend. The observed preferences for ring formation are 5 > 6, 6 > 7, and 8 > 7, in agreement with the calculated preferences. The relationship only holds for terminal double bonds. An additional alkyl substituent at either end of the double bond reduces the relative reactivity as a result of a steric effect. 

Chemoselectivity plays an important role in the benzannulation reaction as five-membered rings such as indene or furan derivatives are potential side products. The branching point is again the rf-vinylcarbene complex D intermediate which maybe formed either as a (Z)- or an ( )-metallatriene the (E)-configuration is required for the cyclisation with the terminal double bond. (Z)-Metallatriene D, however, leads to the formation of furan derivatives H (Scheme 8). Studies on the formation of (E)- and (Z)-isomers discussing stereoelectronic effects have been undertaken by Wulff [17]. 

Another elegant use of nonadienoate is the synthesis of a pheromone called brevicomin (148) (132). The ester was converted to 1,6-nonadiene (149). The terminal double bond was selectively converted to glycol via epoxide. The oxidation with PdCI2 produced brevicomin directly by intramolecular oxidative acetal formation. 

Again, rhodium-complexes, although in a completely different process, catalyzed the formation of indolizidine derivatives through the hydroformylation of pyrroles bearing a terminal double bond. The intermediate aldehyde reacted to afford the final product 74 (Scheme 23) <2004TA1821>. 

The reaction of the oxazolidinone with styrene-dg led to the selective formation of (Z)-27. Thus, styrene adds to the terminal double bond from the more congested face syn to the N-tosyl group. 

The possibility of Jt-allylpalladium complex formation through carbopalladation is excluded from the observation that no four- and/or six-membered rings are produced. The reaction apparently proceeds via an alternative pathway which involves a sequence of Jt-coordination of PhPdl to an allenic terminal double bond, oxypallada-tion and ensuing reductive elimination (Scheme 16.9). 

Again, the exclusive formation of six-membered rings indicates that the cyclization takes place by the electrophilic attack of a cationic center, generated from the enol ester moiety to the olefinic double bond. The eventually conceivable oxidation of the terminal double bond seems to be negligible under the reaction conditions since the halve-wave oxidation potentials E1/2 of enol acetates are + 1.44 to - - 2.09 V vs. SCE in acetonitrile while those of 1-alkenes are + 2.70 to -1- 2.90 V vs. Ag/0.01 N AgC104 in acetonitrile and the cyclization reactions are carried out at anodic potentials of mainly 1.8 to 2.0 V vs. SCE. 

In the next example, the strategy for the formation of fused cyclic ethers utilized the formation of an intermediate a-heterosubstituted carbon radical, generated by the reaction of (TMS)3Si radical with A-(ethoxycarbonyl)-l, 3-thiazolidine derivative [38]. This intermediate gives intramolecular C — C bond formation in the presence of proximate 1,2-disubstituted double bonds (Reaction 7.27). However, when terminal double bonds are used, the hydrosily-lation of the double bond by (TMS)3SiH can compete with the reduction and prevent forming the desired C—C bond formation. 

Product of epoxidation of the ring double bond 57%, product of epoxidation of the terminal double bond 4%, 3% diol formation. P 7% of enone. 

The first report on the construction of a SA multilayer appeared in 1983 [193]. A terminally bifunctional surfactant, 15-hexadecenyltrichlorosilane, was the initial building block for the formation of a SA monolayer. The trichloro-silane functionality reacted, as did OTS, with both of the hydroxyl groups on the substrate surface and with adsorbed water to form a network of Si -O-Si bonds in the SA monolayer. Conversion of the terminal double bonds to hydroxyl group functionality allowed the chemisorption of a new layer of surfactants to produce a SA bilayer (Fig. 20) [193]. The process could be repeated to form subsequent multilayers. 

Since the formation of the terminal double bonds is kinetically favored, short reactions times favor high amounts of vinylidene moieties. 

Secondary alcohol 11 is first protected as a silyl ether with TBS chloride, after which the terminal double bond is ozonized. The resulting methyl ketone is subsequently converted stereoselectively with a Homer-Wadswonh-Emnions reaction21 into olefin 13. This reaction sequence leads to tram selectivity in the formation of the terminal double bond in 13. 

The intermediate a-heterosubstituted carbon radicals generated by reaction of (TMS)3Si radical with 1,3-dithiane28, or A-(ethoxycarbonyl)- , 3-thiazolidinc76 derivatives, can participate in consecutive intramolecular C—C bond formation reactions in the presence of proximate 1,2-disubstituted double bonds (equation 51). In the presence of terminal double bonds or in an attempted intermolecular addition of the intermediate... 

After successful completion of all rearrangement reactions, the incorporation of the different side chains of the tetraponerines was attempted by employing a cross metathesis reaction. However, the cross metathesis of 19 and 22 with allyltrimethylsilane in the presence of 10% [Ru-1] was unsuccessful due to the formation of a carbene with low reactivity. The use of Schrock s molybdenum catalyst26 [Mo] (Figure 7) also failed to show any conversion. The terminal double bonds of 19 and 22 were assumed to be too hindered for cross metathesis. An alternative route to incorporate the different alkyl chains of the tetraponerines was necessary (Scheme 8). 

The nickel-catalyzed hydrocyanation of butadiene is a two-step process (Figure 3.32). In the first step, HCN is added to butadiene in the presence of a nickel-tetrakis(phosphite) complex. This gives the desired linear product, 3-pente-nenitrile (3PN), and an unwanted branched by-product, 2-methyl-3-butenenitrile (2M3BN). The products are separated by distillation, and the 2M3BN is then isomerized to 3PN. In the second step, 3PN is isomerized to 4PN (using the same nickel catalyst), followed by anti-Markovnikov HCN addition to the terminal double bond. The second step is further complicated by the fact that there is another isomerization product, CH3CH2CH=CHCN or 2PN, which is thermodynamically more stable than 4PN. In fact, the equilibrium ratio of 3PN/2PN/4PN is only 20 78 1.6. Fortunately, the reaction kinetics favor the formation of 4PN [95],... 

C-l and C-6 carbon atoms in form (XLVb) would result in the formation of a terminal double bond complexed to the nickel atom and might be supposed to favor reaction through this isomer. 

The formation of these compounds may be explained by the following transformations. The weakly nucleophilic hexamethyldisilazane initially reacts at the carbon atom of the multiple bond, and fluoride ion elimination takes place in the new zwitterion, forming a terminal double bond. Further catalysis by the fluoride ion generates an active N-nucleophile intramolecular cyclization involving the latter leads to a four-membered heterocycle. 

The reaction occurs via the formation of the intermediate adduct 126 and a mixture of /Z-isomeric alkenes 127 (for R1 = R2 = R3 = R4 = H, the yield is 29 and 11%, respectively). The reaction product is formed by intramolecular cyclization of olefins initiated by bases (water, Na2C03, NaOH) in tetrahydrofuran (xylene). The authors did not give any interpretation of the reaction route. One can assume that the reaction proceeds by the following scheme. Under the action of bases, olefins undergo elimination of hydrogen fluoride and the formation of compound 128 containing a terminal double bond. 

Further addition of the fluoride anion at the internal multiple bond generates hetero-anion F, which is involved in the intramolecular cyclization affecting the active terminal double bond. Stabilization of carbanion G occurs by fluoride ion elimination and formation of compound 129 having a multiple bond with a mobile fluoride atom. 

The diboration of allenes afforded another series of allylboron compounds that each have a boryl group at the vinyl carbon. The addition had a strong tendency to occur at the internal double bond of terminal allenes such as 1,2-heptadiene (Equation (33)).237 However, steric hindrance in both allenes and phosphine ligands forced the addition toward the terminal double bond of dimethylallene (Equation (34)).237 On the other hand, addition to the terminal double bond occurred selectively for both monosubstituted and 1,1-disubstituted allenes when Pd(dba)2 was used in the presence of a co-catalyst (RI) such as I2, Arl, and iodoalkenes (Equation (35)).238 The role of co-catalyst was attributed to in situ formation of I-Bpin intermediate, which undergoes oxidative addition and insertion leading to 2-boryl-7r-allylpalladium intermediate. 

Among the haloboranes, dibromoborane-methylsulfide has proven to be an excellent hydroborating reagent. Its reaction with terminal and internal alkynes cleanly affords the corresponding vinylic dibromoboranes without the concomitant formation of 1,1-dibora derivatives. This reagent reacts only with internal triple bond in the presence of terminal double bond, and with disubstituted terminal double bond in the presence of monosubstituted one 36c). 

A major proportion of the glucosinolate hydrolysis products formed upon myrosinase cleavage in some plants are nitriles. In vitro, nitrile formation associated with myrosinase-catalyzed hydrolysis is enhanced at low pH (pH<3) and in the presence of ferrous ions. In vivo, protein factors in addition to myrosinase may be responsible for nitrile formation. If the glucosinolate side chain has a terminal double bond, the sulfur released from the thioglucosidic bond may be captured by the double bond and an epithionitrile is formed.9 This reaction takes place only in plants that possess a protein factor known as epithiospecifier protein (ESP). ESP activities have been identified in several species of the Brassicaceae and shown to influence the outcome of the myrosinase-catalvzed hydrolysis reaction although they have no hydrolytic activity by themselves.10 12 The mechanism by which ESPs promote epithionitrile formation is not known. 

Figure 7.6 Industrial use of (from the top) propylene dimerization, butadiene dimerization, butadiene trimer-ization, and butadiene plus ethylene codimerization. In EPDM rubber, the terminal double bond of 1,4-hexadiene takes part in polymer formation. The internal double bond is used during curing.

Oxiranes. Intramolecular substitution is a straightforward approach to stereoselective formation of oxiranes . A two-step protocol involving diastereoselective iodohy-dration of the terminal double bond in compound 17 followed by ring closure affords chiral terminal oxirane 18 in good overall yield (Scheme 17) <2003TA3557>. 

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