Butadienes monosubstituted

The combination of cis-trans isomerism with iso-syndio and erythro-threo dispositions gives complex stractures as exemplified by the 1,4 polymers of 1-or 4-monosubstituted butadienes, such as 1,3-pentadiene (72, 73), and 2,4-pentadienoic acid (74, 75) and of 1,4-disubstituted butadienes, for example, sorbic acid (76). This last example is described in 32-35 (Scheme 6, rotated Fischer projection). Due to the presence of three elements of stereoisomerism for each monomer unit (two tertiary carbons and the double bond) these polymers have been classed as tritactic. Ignoring optical antipodes, eight stereoregular 1,4 structures are possible, four cis-tactic and four trans-tactic. In each series (cis, trans) we have two diisotactic and two disyndiotactic polymers characterized by the terms erythro and threo in accordance with the preceding explanation. It should be noted that here the erythro-threo relationship refers to adjacent substituents that belong to two successive monomer units. 

For example, polymers from monoalkyloxiranes and thiiranes, 1- (or 4)-monosubstituted butadienes, ot-substituted- 3-propiolactones, ot-amino acids, and so on. 

Several highly enantioselective Diels-Alder reactions are known for which the di-enophile does not fit any of the above classes. Corey and coworkers applied the chiral aluminum reagent 36 with a C2-symmetric stilbenediamine moiety (videsu-pra) to the Diels-Alder reaction of maleimides as dienophiles [54] (Scheme 1.68). In most asymmetric Diels-Alder reactions the reactants are usually relatively simple dienes such as cyclopentadiene or monosubstituted butadienes, and unsym-... 

Polymer stereochemistry, sometimes referred to as tacticity, is not the only source of variation in polymer configuration. For the monosubstituted butadiene isoprene, the structures shown in Figure 3.2 are possible. 

Stereoisomerism of polymers derived from 1,3-pentadiene and other unsymmetric terminally monosubstituted butadienes (CH2 = CH CH = CHR) is more complex. Stereoregular polymers can be formed from these monomers via their... 

The reaction of [Fe(CO)3(NO)] with butadiene in the presence of methyl iodide followed by treatment with pyridine (100) has produced the complex (72) of the monosubstituted butadiene. 

Because a monosubstituted alkene has a AT/Ohyc rog of approximately -126 kj/mol, we might expect that a compound with two monosubstituted double bonds would have a Af/0hyjrog approximately twice that value, or -252 kj/mol. Nonconjugated dienes, such as 1,4-pentadiene (AH°hydrog = —253 kj/mol), meet this expectation, but the conjugated diene 1,3-butadiene (AT/°hydr0g = -236 kj/mol) does not. 1,3-Butadiene is approximately 16 kj/mol (3.8 kcal/mol) more stable than expected. 

Table 1.3 Regioselectivity of Diels-Alder reactions of disubstituted 1,3-butadienes with monosubstituted ethenes (RCH=CH2)...

With trisubstituted benzoquinones and use of the cationic oxazaborolidinium catalyst B, 2-[tra-(isopropyl)silyloxy]-l,3-butadiene reacts at the monosubstituted quinone double bond. The reactions exhibit high regioselectivity and more than 95% e.e. With 2-mono- and 2,3-disubstimted quinones, reaction occurs at the unsubstituted double bond. The regiochemistry is directed by coordination to the catalyst at the more basic carbonyl oxygen. 

The situation is exactly analogous to the polymerization of monosubstituted alkenes the various polymer structures would be those in Fig. 8-1 with R = — CH=CH2. With chloroprene and isoprene, the possibilities are enlarged since the two double bonds are substituted differently. Polymerizations through the 1,2- and 3,4-double bonds do not yield the same product as they would in 1,3-butadiene polymerization. There are, therefore, a total of six structures possible—corresponding to isotactic, syndiotactic, and atactic structures for both 1,2- and... 

Selective modification of polyols such as ethylene glycol, 1,3-propylene glycol, or glycerol with butadiene (1) has been studied [7-10]. The monosubstituted compounds are preferred due to their potential applications as surfactants, PVC plasticizers, or even in cosmetics. The telomerization of 1 with ethylene glycol yields a complex mixture including linear and branched mono- and ditelomers, as well as 1,3,7-octatriene and vinyl cyclohexene (Fig. 2) [11]. 

A chiral Mt -O73-butenyl) species will give, depending on its structure and thus on the orientation of the incoming monomer, a new Mt ( -butenyl) species of the same chirality as the previous one (and hence an isotactic diad) or of the opposite chirality (and hence a syndiotactic diad) it is obvious that the tacticity may concern only 1,2-polymers of non-substituted or substituted butadiene and 1,4-polymers of terminally monosubstituted and symmetrically disubstituted butadiene. The mode of the formation of the butenyl group of the same or opposite chirality with respect to the preceding butenyl group is shown, for 1,3-butadiene insertion, in Figure 5.4 [7],... 

The stereospecific insertion of 2-monosubstituted alkenyl carbenoids was successfully employed in the preparation of 1-alkyl-1-zircono-dienes. The Z and E carbenoids of 1-chloro-l-lithio-l,3-butadiene (69 and 70, respectively) are generated in situ fromE- andZ-l,4-dichloro-2-butene [53] (Scheme 25). Inversion of configuration at the carbenoid carbon during the 1,2-metalate rearrangement stereospecifically yields terminal dienyl zirconocenes 71 and 72 [54] (Scheme 25). As the carbenoid-derived double bond is formed in 9 1=Z E for 69 and >20 1=E Z isomeric mixtures for 70, the metalated dienes 71 and 72 are expected to be formed with the same isomeric ratio. Carbon-carbon bond formation was achieved by palladium-catalyzed cross-coupling with allyl or vinyl halides to give the functionalized products with >95 5 stereopurity [55-57]. 

Hexaphenyl Monosubstituted eihylenes, disubstituted butadienes, maleic anhydride, maleimide, dehydrobenzene 320, 321... 

Alkenyllithium compounds can also be prepared by metaUation of alkenes, particularly when alkenyl hydrogens are rendered acidic by an a-substituent (equation 22). Transmetallation of alkenyl stannanes with organohthium reagents gives alkenyUithium compounds with retention of alkene stereochemistry (equation 23). Tin lithium fransmet-allation has been used to prepare 1,4-dihthio-l,3-butadiene. Monosubstituted alkenylhthium compounds RHC=CHLi, can also be prepared from the corresponding diorganotel-luride, RHC=CHTeBu, by reaction with butylhthium in... 

The most general synthetic route to benzene oxides-oxepins is that initially developed by Vogel for 1. 1,4-cyclohexadienes (readily available from [2+4] cycloaddition of alkynes and butadienes, lithium-ammonia reduction of arenes, or dehydration of cyclohexenols) were converted to dibromoepoxides, the immediate precursors of benzene oxides. Modifications of this route have been used to prepare Ic and Id. Treatment of the monosubstituted arene oxide 43 (Figure 3) with (Et)4NF or thermal isomerization of 3-oxaquadricyclane provide additional synthetic routes to la. Similarly, the thermal (or photochemical) isomerization of the monoepoxide of Dewar benzene yielded la. ... 

Cyclobutadiene-iron tricarbonyl is prepared through reaction of S,4-dichlorocydolmtene and diiron enneacarbonyl. In an analogous manner, one can prepare 1,2-diphenyl- 1,2,3,4-tetramethyl- and benzocyclobutadiene-iron tri-carbonyl complexes. Cyclobutadiene-iron tricarbonyl is aromatic" in the sense that it undergoes facile attack by electrophilic reagents to produce monosubstituted cydo-butadiene-iron tricarbonyl complexes. Functional groups in the substituents display many of their normal chemical reactions which can be used to prepare further types of substituted cyclobutadiene-iron tricarbonyl complexes. 

Confirmation of this unexpected stability comes from data on the partial hydrogenation of 1,3-butadiene to yield 1-butene. The amount of energy released is -110 kJ/mol, some 16 kJ/mol less than that for the isolated monosubstituted double bond in 1-butene. 

Reactions of 3-methylthio-4-trimethylsilyl-1,2-butadiene with electron-poor monosubstituted and disubstituted alkenes were promoted by a catalytic amount of ethylaluminum dichloride, affording the corresponding methylenecyclobutanes with high selectivities and with yields ranging from 37% for methyl crotonate to 97% for methacrylonitrile. ... 

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