E double bonds

The tendency for conjugate addition is pronounced and E double bonds are generated almost exclusively... 

The enzyme catalyzed reactions that lead to geraniol and farnesol (as their pyrophosphate esters) are mechanistically related to the acid catalyzed dimerization of alkenes discussed m Section 6 21 The reaction of an allylic pyrophosphate or a carbo cation with a source of rr electrons is a recurring theme m terpene biosynthesis and is invoked to explain the origin of more complicated structural types Consider for exam pie the formation of cyclic monoterpenes Neryl pyrophosphate formed by an enzyme catalyzed isomerization of the E double bond m geranyl pyrophosphate has the proper geometry to form a six membered ring via intramolecular attack of the double bond on the allylic pyrophosphate unit... 

As a class the aliphatic polyalkenamers have low values due to a combination of low chain stiffness and low interchain attraction. The presence of double bonds has the effect of increasing the flexibility of adjacent single bonds (see Chapter 4) and overall this leads to a reduction in. Thus in the sequence from polydecenamer down to polypentenamer an increase in the double bond concentration leads to a lowering of Tg. On the other hand the Tg of polybutenamer, i.e. poly butadiene, is somewhat higher than that of polypentenamer, presumably because the proportion of stiff links, i.e. double bonds, becomes sufficiently high to override the flexibilising effect on adjacent chains. Consequently the polypentenamers have the lowest Tg values known for hydrocarbon polymers (cis- -114°C, trans- -97°C). 

E double bond (Higher-priority groups are on opposite sides.)... 

A Lewis acid is involved in the reaction media when RCu BF3 or R3Al is used to cleave an acetal or ketal framework, and the resulting enol ether contains a E double bond. This is quite reasonable, since the overall reaction proceeds in an anti-SN2 manner. When a v>- -SN2 process is involved, the formation of products containing a Z double bond is observed60 (Table 2). The reaction of alkyllithium reagents with a./J-ethylcnic acetals and ketals proceeds in a. H7i-SN2 manner without assistance of Lewis acids, giving mainly the Z-products61-63. 

The exact enthalpy of polymerization for a particular monomer will depend on the steric and electronic effects imposed by the substituents attached to the E=E double bond. For olefins, resonance stabihzation of the double bond and increased strain in the polymer due to substituent interactions are the most important factors governing AHp For example, propylene has a calculated AH of -94.0 kJ moT, whereas the polymerization of the bulkier 2-methylpropene is less exothermic (-78.2 kJ moT ) [63]. Due to resonance effects, the experimentally determined AH of styrene (-72.8 kJ mol ) is less exothermic than that for propylene, while that for bulkier a-methylstyrene is even less favorable (-33.5 kJ moT ) [63]. In general, bulky 1,2-disubstituted olefins (i.e., PhHC= CHPh) are either very difficult or impossible to polymerize. 

Figure 6. Plots of rings C-H after energy minimization with the ring E double bond syn to the ring F double bond (top) and the ring E double bond anti to the ring F double bond (bottom). Oxygen atoms are cross hatched.

Scheme 6.15 gives some representative examples of the orthoester Claisen rearrangement. Entry 1 is an example of the standard conditions for the orthoester Claisen rearrangement using triethyl orthoacetate as the reactant. The allylic alcohol is heated in an excess of the orthoester (5.75 equivalents) with 5 mol % of propanoic acid. Ethanol is distilled from the reaction mixture. The E-double bond arises from the chair TS. 

Entry 4 involves nitrogen participation and formation of an iminium ion that is reduced by NaBH4. The reaction in Entry 5 creates an 11-methylenebicyclo[4.3.1]undecen-3-one structure found in a biologically active natural product. Note that this fragmentation creates a bridgehead double bond. Entry 6 involves construction of a portion of the taxol structure. The reaction in Entry 7 is stereospecific, leading to the E-double bond. 

Imido chalcogen halides of the type RNEC12 (E=S, Se, Te) provide an interesting illustration of the reluctance of the heavier chalcogens to form -N=E< double bonds. The sulfur derivatives RNSX2 (X=F, Cl) are stable, monomeric compounds. 

Monomeric compounds (x = 1) containing M=E double bonds remained unknown until Power and Roesky et al. reported in 2001 on the synthesis of the first monomeric iminoalane and -gallane by oxidative addition of an organoazide to a sterically encumbered low-valent A1(I) compound 4 This reaction type has been shown previously to yield heterocyclic compounds by use of sterically less hindered [Cp Al]4.145 Very recently, von Hanisch and Hampe synthesized the first GaAs compound featuring a Ga=As double bond5 (Scheme 19, Figs. 41 and 42). 

When one of the reacting partners in the Wittig-Horner reaction, either the phosphine oxide or the carbonyl compound, has a double bond, the product is a diene. The Wittig-Horner reaction was utilized by Smith and coworkers in the total synthesis of milbemycin (equation 98)170. They found that when sodium hexamethyldisilazide was employed as a base, the desired E-diene selectivity is high (85%). Some examples from the literature where the Wittig-Horner reaction has been utilized for the construction of E-double bonds present in dienes and polyenes are given in Table 19171. 

Figure 54 Structures of two complexes featuring Ga=E double bonding character.

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