Methyl-1,3-butadienes, structure

The behavior described above has been verified by experiment and calculation on numerous substituted dienes and dienophiles. For example Fig. 10.13 shows results for 2°-D isotope effects on Diels-Alder reactions of 2-methyl-butadiene with cyano-ethylene and 1,1-dicyano-ethylene. The calculated and experimental isotope effects are in quantitative agreement with each other and with the results on (butadiene + ethylene). In each case the excellent agreement between calculated and observed isotope effects validates the concerted mechanism and establishes the structure of the transition state as that shown at the bottom center of Fig. 10.11 and the left side of Fig. 10.12a. 

Dehydration of (2-hydroxymethyl-1,3-butadiene)iron complexes or (hydroxymethyltrimethylenemethane)iron complexes with fluorosulfonic acid/liquid sulfur dioxide generates the corresponding (cross-conjugated dienyljiron cations (194) (equation 72). The H and NMR spectral data for these cations favor an " -TMM-methyl cationic structure (282) over an ) " -isoprenyl cationic stmcture (283). These cations react with water or alcohols to afford butadiene products via nucleophilic attack at C-5. As indicated earlier (Section 6.1.1), the cross-conjugated dienyl cations are believed to be intermediates in the substitution of (193) with weak carbon nucleophiles (Scheme 53). In these cases, nucleophihc attack occurs on C-4 to give predominantly TMM products. ... 

Synonyms 2-Methyl-l,3-butadiene 2-Methyl-butadiene )S-Methylbivinyl Isopentadiene Chemical/Pharmaceutical/Other Class Reactive branched diene Chemical Formula CsHg Chemical Structure ... 

Identify all the possible isomers (structural and stereoisomers) resulting from the polymerization of 2-methyl butadiene. 

Figure 5.27 illustrates the crystal structure of P-tra i -l,4-poly(2-methyl butadiene), also known as traw -polyisoprene or gutta-percha. The a-crystal is more stable, but not fully identified. The -CHj-group is attached to position 2 in the helix 4 1/1, drawn in the sketch of Fig. 5.27. The methyl group is in this case on the carbon atom that has undergone a right-handed rotation out of the planar zig-zag by about...

It turns out that they are chemically related to each other, as you can imagine from the chemical structures shown in Fig. 12.1. They are called terpenes and terpenoids. They can be regarded as derivatives from a five-carbon compound called isoprene (2-methyl butadiene). Two isoprene molecules combine to form mono-terpene (ten-carbon compound). The fragrant oils mentioned above are all the derivatives of mono-terpene. Terpenes are derived from a common metabolic intermediate of glucose, acetyl-CoA (coenzyme A). By the way, a tri-terpene (which three terpene molecules combine to form) called squalene leads to the formation of steroids, and if you connect a large number of isoprene in a linear fashion, you will get natural rubber (Chap. 5). 

In the case of polymerization of substituted dienes such as 2-methyl-butadiene (isoprene), the structure of the resulting product may be more complicated because it is possible to create polymers with structure of type 1.2 and 3.4, as well as 1, 4 cis and 1, 4 trans. Moreover, structures head to tail and head to head t3 e may be formed. The results of the analysis of S5mthetic polyisoprene have shown that, in general dominating is the head to tail arrangement and the 1, 4 trans configuration. The natural rubber is 1,4 cz5-polyisoprene, whereas gutta-percha and balata have 1, 4 trans structure. 

Cyclic polyenes are expected to show similar behavior thus, cyclopentadiene has El I of 9.00 e.v. rather similar to the methyl-butadienes and lower than that of butadiene itself. Cycloheptatriene, however, has El / = 8.55 e.v., somewhat higher than that of hex-atriene even allowing for the difference between UV and El determinations. The value is intermediate between that which would normally be expected for a cycloheptatriene structure ( 7.89 e.v. (47)] and that of toluene, El / = 9.23 e.v. (46), and suggests that tropylidene or tropylidene cation actually has r-bonding between the 2- and 7-positions, VI. 

The principal components of the cut are butene-1, butene-2, isobutylene and butadiene-1,3. Methyl, ethyl, and vinyl acetylenes, butane and butadiene-1,2 are present in small quantities. Butadiene is recovered from the C4 fraction by extraction with cuprous ammonium acetate (CAA) solution, or by extractive distillation with aqueous acetonitrile (ACN). The former process is a liquid-liquid separation, and the latter a vapor-liquid separation. Both take advantage of differences in structure and reactivity of the various C4 components to bring about the desired separation. 

FIGURE 8.16 The structure of isoprene (2-methyl-l,3-butadiene) and the structure of head-to-tail and tail-to-tail linkages. Isoprene itself can be formed by distillation of natural rubber, a linear head-to-tail polymer of isoprene units. 

Both experimental [7] and theoretical [8] investigations have shown that the anti complexes of acrolein and boranes are the most stable and the transition states were located only for these four anti complexes. The most stable transition-state structure was calculated (RHF/3-21G) to be NC, while XT is the least stable of the four located. The activation energy has been calculated to be 21.6 kcal mol for the catalyzed reaction, which is substantially above the experimental value of 10.4 1.9 kcal mol for the AlCl3-catalyzed addition of methyl acrylate to butadiene [4a]. The transition-state structure NC is shown in Fig. 8.5. 

Isoprene (2-methyl 1,3-butadiene) is the second most important conjugated diolefin after butadiene. Most isoprene production is used for the manufacture of cis-polyisoprene, which has a similar structure to natural rubber. It is also used as a copolymer in butyl rubber formulations. 

Regardless of Llieu apparent structural differences, all terpenoids are related. According to a formalism called the isoprene rule, they can be thought of as arising from head-to-tail joining of 5-carbon isoprene units (2-methyl-1,3-butadiene). Carbon 1 is the head of the isoprene unit, and carbon 4 is the tail. For example, myreene contains two isoprene units joined head to tail, forming an 8-carbon chain with two 1-carbon branches. a-Pinene similarly contains two isoprene units assembled into a more complex cyclic structure, and humulene contains three isoprene units. See if you can identify the isoprene units in a -pinene and humulene. 

Unlike polyethylene and other simple aikene polymers, natural rubber is a polymer of a diene, isoprene (2-methyl-l,3-butadiene). The polymerization takes place by addition of isoprene monomer units to the growing chain, leading to formation of a polymer that still contains double bonds spaced regularly at four-carbon intervals. As the following structure shows, these double bonds have Z stereochemistry ... 

Reaction of isoprene (2-methyl-l,3-butadiene) with ethyl propenoate gives a mixture of two Diels-Alder adducts. Show the structure of each, and explain whv a mixture is formed. 

Problem 31.6 Draw the structure of an alternating segment of butyl rubber, a copolymer of iso-prene (2-methyl-],3-butadiene) and isobutylene (2-methylpropene) prepared using a cationic initiator. 

At least 90 percent of free-radical-polymerized 2,3-dimethylbutadiene consists of 1,4 units according to ozone degradation experiments. Successive substitution of the methyl groups on carbons 2 and 3 of butadiene is seen to increase the proportion of 1,4 units formed. In polychloroprene no less than 97 percent of the structure consists of 1,4 Cl... 

Fig. 6.7. Transition structures for the reaction between 1,3-butadiene and the methyl acrylate—BF3 complex calculated at die ab initio HF/6-31G level. Relative energies are in kcal/mol. Adapted from Tetrahedron, 53, 6057 (1997), by permission of Elsevier.

Complexes 17-19 can be written in one valence structure as a, /3-unsaturated carbonyl compounds in which the carbonyl oxygen atom is coordinated to a BF2(OR) Lewis acid. The C=C double bonds of such organic systems are activated toward certain reactions, like Diels-Alder additions, and complexes 17-19 show similar chemistry. Complexes 17 and 18 undergo Diels-Alder additions with isoprene, 2,3-dimethyl-1,3-butadiene, tram-2-methyl-l,3-pentadiene, and cyclopentadiene to give Diels-Alder products 20-23 as shown in Scheme 1 for complex 17 (32). Compounds 20-23 are prepared in crude product yields of 75-98% and are isolated as analytically pure solids in yields of 16-66%. The X-ray structure of the isoprene product 20 has been determined and the ORTEP diagram (shown in Fig. 3) reveals the regiochemistry of the Diels-Alder addition. The C-14=C-15 double bond distance is 1.327(4) A, and the... 

The addition of 2,3-dimethyl-1,3-butadiene to 17 gives only one structural isomer, 21. The pseudo-first order half-life of this Diels-Alder addition reaction is 47.5 minutes at 22.2°C in neat diene solution. The Ea of this reaction is estimated to be 9.2 kcal/mol. This reaction rate is 50 times faster than the rate of addition of this diene to methyl methacrylate (33). 

Takemura and Shida54 prepared the allene radical ion by /-radiolysis of halocarbon solid solution of allene at low temperatures and showed that the radical cation has a lower D2 structure than the precursor with a skew angle of 30-40°. Kubonzo and coworkers55 56 produced by /-radiolysis in a low-temperature halocarbon matrix several derivatives of the allene radical cation, i.e. the radical cations of 1,2-butadiene, 3-methyl-1,2-butadiene,... 

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