Isomeric units from dienes

At low temperatures cyclopropenones and enamines or ketene acetals were shown to yield 2-azonia-bicyclo(3,l, 0)hex-3-enolates-3 (371, X=0), which can be isomerized thermally to penta-2,4-diene amides(372, X=0). At elevated temperatures the amides were found to be the principal products arising from C-N-insertion 237) (insertion of the cyclopropenone three-carbon unit into the C-N bond of the enamine). These were accompanied in some cases by 3-aminoenones 373 arising from C-C-insertion 237) (insertion of the cyclopropenone into the C-C double bond of the enamine) and a-amino cyclopentenones 375 formed by Stevens rearrangement of the ylide 369 and cyclopentenones 374 ( condensation 237)). 

The thermal isomerization of higher terminal alkynes also delivered some allene, from 1-hexyne and 1-heptyne, for example, some 1,2-diene was formed [30]. With an ,/l-unsaturated unit in the alkyne 9, a photochemical isomerization to 10 was successful but delivered only a low yield and 11 as a significant side-product [31]. These reactions tolerate different functional groups alcohols, ethers or, as in 12, tertiary amines and nitriles have been used (Scheme 1.5) [32, 33],... 

Other reactions of dienes with metal atoms are only of a limited synthetic use. Dibenzylideneacetone (PhCH=CH—CO—CH=CHPh DBA) reacts with palladium vapor to afford Pd2(DBA)3, a complex in which the coordination is through the two C=C units and does not involve the C=0 (5, 92). Cobalt vapor undergoes an extremely complicated reaction with 1,4-pentadiene, producing pentenes, C5H6, and various polymers as well as the organometallic product, HCo( 1,3-pen tadiene)2, which involves isomerization from a nonconjugated to a conjugated diene (104, 110). 

If a diene unit is located at an appropriate distance from the developing zwitterionic intermediate in such reactions (especially in good ionizing solvents), an intramolecular cycloaddition may intervene. This is what happens with the tetrahydrobenzo[c]thiophene 2,2-dioxide derivative (588), which leads to the bis-homobenzene (589) rapid valence isomerization of this gives the product (590) (770R(25)i). This process has been termed bis-homoconjugative rearrangement (Scheme 251). 

Tetraacetylenes such as 115 and 116 contain the 1,5-hexadiyne group as a bridging element. Since the base-catalyzed isomerization of this unit to hexa-l,3-dien-5-yne (6) constitutes the basic reaction of Sondheimer s annulene chemistry [75], it appeared attractive to attempt to apply this classic reaction of planar aromatic chemistry to a layered precursor and create three-dimensional relatives of Sondheimer s dehydroannulenes. Indeed, both 115 and 116 could be isomerized to their fully conjugated isomers 129 and 130, respectively, by treatment with potassium tert-butoxide in tert-butanol, the original Sondheimer conditions (Scheme 28). From the X-ray structure obtained for 130, it was concluded that both hydro-... 

The unhomogeneous composition of the products generated by the photochemical reaction is due to another mechanism. While the thermal isomerization of 1,5-dienes proceeds via a cyclic transition state in a synchronous sense, the photochemically induced transformation causes a reorientation of the allyl radicals generated from the educts. Warming up the reaction mixture to 100°C activates a complete transfer from 4c to 5c) of all isomers. This step may be explained by a radical CC bond split of the 1,2-diphenylethylene unit. Since the isomerization of the diastereomeric compound 4c to 5c is activated at much lower temperatures than for the Cope rearrangement (from 3c to 4c), it is clear that the thermal transfer exclusively forms the twofold changed product. 

Isomeric (s-cis- and (i-fra/w-V-conjugated diene)zirconocene and -haf-nocene complexes exhibit pronounced differences in their characteristic structural data as well as their spectroscopic features. These differences exceed by far the consequences expected to arise simply from the presence of conformational isomers of the 1,3-diene unit. While (f-rra/u-butadiene)-zirconocene (3a) shows a behavior similar to a transition metal olefin TT-complex, the (.r-cu-diene)ZrCp2 isomer 5a exhibits a pronounced alkylmetal character (23, 45). Typical features are best represented by a tr, 7T-type structure for 5 (55). However, the distinctly different bonding situation of the butadiene Tr-system/bent-metallocene linkage is not only reflected in differences in physical data between the dienemetallocene isomers 3 and 5, but also gives rise to markedly different chemical behavior. Three examples of this are discussed in this section the reactions of the 3/5 isomeric mbcture with carbon monoxide, ethylene, and organic carbonyl compounds. 

In the synthesis off 18]annulene, the 35 g. of crude cyclooctadecadiyne (4) obtained in the first step is dissolved in 800 ml. of benzene and heated to boiling on a water bath in a 2-1. round-bottomed flask fitted with a reflux condenser and a calcium chloride tube. A solution of potassium t-butoxide, prepared from 44 g. of potassium and I 1. of dry t-bulyl alcohol by boiling under reflux under nitrogen until the metal dissolved, is added and the mixture refluxed for 30 min. to isomerize three yne groups to diene units. The workup is tedious and includes chromatography. 

It follows from the existence of conformational scissors (Fig. 4) that in the polymerization of symmetric or quasisymmetric dienes in hydrocarbon media on an active center with a slightly polar carbon-metal bond the primary acts of monomer attachment lead to the cis-conformation of the end unit. This conclusion is in good agreement with the modern concepts of the formation mechanism of the cis-structure of anionic polydienes [70]. According to these concepts this is followed by either the attachment of the next monomer molecule or by the cis-trans isomerization of the end unit. The microstructure is fixed at the moment of the attachment of a new monomer unit to the active center, the configuration (cis- or trans-) of the end unit being retained in the polymer chain ... 

A diene contains two double bonds and during polymerisation only one of them opens, which leads to the formation of polymers of three distinctly different kinds. The three forms for polybutadiene, obtained from butadiene, H2C=CH—CH=CH2, are shown in fig. 4.6. The vinyl 1,2 form, for which the double bond is in the side group X, can have any of the types of tacticity discussed above. The two 1,4 types, for which the double bond is in the chain backbone, illustrate a form of configurational isomerism. Rotation around a double bond is not possible, so these two forms are distinct. In the cis form the two bonds that join the unit shown to the rest of the chain are on the same side of a line passing through the doubly bonded carbon atoms of the unit, whereas for the trans form they are on the opposite side of that line. 

The isomerization of the 2,3-benzo derivative at 80°C provided another pathway for isomerization. Here, various derivatives gave 2-1-2 dimers (or adducts with furan) of 3,4-benzobicyclo[3.2.0]cyclohepta-l(2),3-diene derivatives which apparently were formed via a homofulvene with a disrupted benzo unit which undergoes a 1,5-shift (or two 1,3-shifts) (Scheme 8.33). The stereochemistry depicted in the scheme was inferred from the stereochemistry of the dimers and the formation of the /iJ<9-methoxy homofulvene was as observed in the debenzo derivative above. 

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