Hexadienes structure

Bicyclic azetines tend to be highly unstable species. The more strained the bicyclic system the greater the reactivity. Azetines of the 2-azabicyclo[2.2.0]hexadiene structure have been suggested as transient intermediates in photolytic conversion of pyridines (77JCS(P2)1148, 78JOC944) and of pyrimidines (81JCS(P1)943). 

Independently of the configuration of the phosphaalkene unit within the l,6-diphospha-l,5-hexadiene structure the four-centered transition state (seat configuration) for the rearrangement favors an E-configured CC double bond for the 1,2-diphosphacycloalkane. 

The Cope rearrangement is a term applied to conversion of a 1,5-hexadiene derivative into an isomer that also has the 1,5-hexadiene structure a bond is formed between C-l and C-6, the bond between C-3 and C-4 is broken, and the double bonds alter their positions, e.g. ... 

We have also recently shown that Bergman (Scheme 8.7) and related reactions (not shown) [84, 85] of polyunsaturated hydrocarbons, with a 1,3,5-hexatriene skeleton form a branch inside a larger Cope reaction family characterized by a common 1,5-hexadiene structural unit. The examination of this whole family of reactions allowed us to derive a very simple rule for involvement of transient biradicals in Cope-like reactions of hydrocarbons A non-concerted reaction takes place when biradical intermediates are stabilized either by aUyl or aromatic resonance [84, 85]. 

Step through the sequence of stmctures depicting Cope rearrangement of 1,5-hexadiene. Plot energy (vertical axis) vs. the length of either the carbon-carbon bond being formed or that being broken (horizontal axis). Locate the transition state. Measure all CC bond distances at the transition state, and draw a structural formula for it... 

Perkin pointed out that open chain compounds, which are analogous in structure to a terpene, show a certain similarity in behaviour thus the addition of an ethyl group to 2-methyl 1 5-hexadiene by converting it into 2-methyl 3-ethyl 1 5-hexadiene changes the unpleasant acrid odour into a pleasant one reminding of lemon and peppermint. 

What product(s) would you expect from the reaction of 1,4-hexadiene with NBS What is the structure of the most stable radical intermediate ... 

A Dimethyl butynedioate undergoes a Diels-Alder reaction with (2 ,4 )-hexadiene. Show the structure and stereochemistry of the product. 

Bicyclo[2.2.0]hexadienes and prismanes are valence isomers of benzenes. These compiounds actually have the structures that were proposed for benzenes in the nineteenth century. Prismanes have the Ladenburg formula, and bicyclo[2.2.0]-hexadienes have the Dewar formula. Because of this bicyclo[2.2.0]hexadiene is often called Dewar benzene. On page 32 it was mentioned that Dewar formulas are canonical forms (though not very important) of benzenes. Yet, they also exist as separate compounds in which the positions of the nuclei are different from those of benzenes. 

Bis(diamino)alanes (R2N)2A1H were used for the hydroalumination of terminal and internal alkenes [18, 19]. TiCb and CpjTiCb are suitable catalysts for these reactions, whereas CpjZrCb exhibits low catalytic activity. The hydroaluminations are carried out in benzene or THF soluhon at elevated temperatures (60°C). Internal linear cis- and trans-alkenes are converted into n-alkylalanes via an isomerization process. Cycloalkenes give only moderate yields tri- and tetrasubstituted double bonds are inert. Hydroaluminahon of conjugated dienes like butadiene and 1,3-hexa-diene proceeds with only poor selechvity. The structure of the hydroaluminahon product of 1,5-hexadiene depends on the solvent used. While in benzene cyclization is observed, the reaction carried out in THF yields linear products (Scheme 2-10). 

Figure 7.22 (a) 50A x 50A STM image showing the structure formed by 1,4-cyclo-hexadiene on Pt(l 1 1) at 1 x 10 5Torr and 300 K. Lines in the [1 1 0]-type directions of the underlying platinum lattice are drawn. The surface species form hexagonal units in domains containing a few unit cells and in... 

The poly(5-fnethyl-l, 4-hexadiene) fiber pattern (Figure 6) gave an identity period of 6.3 A, indicating a 3 isotactic helix structure. The X-ray diffraction pattern was not very sharp, which may be due to the difficulty of the side chain with a double bond to fit in a crystalline lattice. The crystallinity was determined to be 15% using the Hermans and Weidinger method (27). A Chloroform-soluble fraction free from catalyst residues showed no improvement in the sharpness of the X-ray diffraction pattern. These data show that the configuration of the 1,2-polymerization units in the homopolymer of 5-methyl-1,4-hexadiene is isotactic. 

Further confirmation of the structure and tacticity of poly/5-methyl-l,4-hexadiene)was obtained from X-ray diffraction and u-NMR data of its hydrogenated polymer (Scheme 2). The hydrogenated polymer sample showed a highly crystalline pattern (Figure 7), with diffraction spots that were well defined. This pattern was identical to that of isotactic poly(5-methyl-l-hexene) as reported in the literature (26) (measured identity period, 6.2 A lit., 6.33 A). 

We showed (7) earlier that copolymers of higher a-olefins, particularly 1-hexene, with 5-methyl-1,4-hexadiene can be sulfur-cured readily and that they contain unsaturation approximating the level of the methylhexadiene charged. In view of this and the excellent durability (8) during flexing exhibited by vulcanizates of such copolymers, we were interested in determining the copolymer structure and the reactivity ratios of 1-hexene and 5-methyl-l,4-hexadiene during copolymerization. 

The trans/cis ratio of hexadiene is also increased by addition of aluminum cocatalysts containing —OR or —NR groups. It is proposed that this is due to selective blocking of a coordination site on the nickel by the formation of an intermediate such as 31, and this too would favor the formation of the transoid structure. 

In the literature there are many reports of the formation of active catalyst for the 1 1 codimerization or synthesis of 1,4-hexadiene employing a large variety of Co or Fe salts, in conjunction with different kinds of ligands and organometallic cocatalysts. There must have been many structures, all of which are active for the codimerization reaction to one degree or another. The scope of the catalyst compositions claimed to be active as the codimerization catalysts is shown in Table XV (69-82). As with the nickel catalyst system discussed earlier, the preferred Co or Fe catalyst system requires the presence of phosphine ligands and an alkylaluminum cocatalyst. The catalytic property can be optimized by structural control of these two components. 

The Co system is more reactive as well as much more selective than the Ni and Rh catalyst systems (Table XVII). The best systems allow almost 100% conversion with almost 100% yield of c -l,4-hexadiene. The best of the Ni and Rh systems known so far are still far from such amazing selectivity. The tremendous difference between the Ni system and the Co or Fe system must be linked to the difference in the nature of the coordination structures of the complexes, i.e., hexacoordinated (octahedral complexes) in the case of Co and Fe and tetra- or penta-coordinated (square planar or square pyramidal) complexes in the case of Ni. The larger number of coordination sites allows the Co and Fe complex to utilize chelating phosphines which are more effective than monodentate phosphines for controlling the selectivity discussed here. These same ligands are poison for the Ni (and Rh) catalyst system, as shown earlier. 

The next homolog, 1,5-hexadiene (1,5-HD), is of special chemical interest because the molecule is capable of undergoing the so-called Cope rearrangement. A GED study of 1,5-HD was also recently reported6. Because of the increased conformational complexity of this molecule compared to that of 1,4-PD, the structural details of the various con-formers could not be resolved and only averaged structure parameters were determined from the gas phase. Molecules in the solid state are frozen, mostly in only one conformation, which may but must not represent the conformational ground state. Therefore, conformational isomerization is usually not discussed with X-ray structures presented in the literature. 

Cyclopolymerization of Nonconjugated Dienes. Cyclopolymerization is an addition polymerization that leads to introduction of cyclic structures into the main chain of the polymer. Nonconjugated dienes are the most deeply studied monomers for cyclopolymerization and for cyclocopolymerizations with alkene monomers 66 In general, (substituted and unsubstituted) dienes with double bonds that are linked by less than two or more than four atoms cannot undergo efficient cyclization and result in crosslinked materials.12 In fact, efficient cyclopolymerization processes have been described, for instance, for a,oo-dienes like 1,5-hexadiene, 2-methyl-l,5-hexadiene, 1,6-heptadiene, and 1,7-octadiene,67 73 which lead to formation of homopolymers and copolymers containing methylene-1,3-cycloalkane units. 

Figure 11.17 Structure of 1,4-hexadiene, showing how the systematic name of an organic compound can be derived from its structure (and vice versa).

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