Butadienic groups

Chiral cyclopropanes. Carrie el al.b l have developed a highly enantioselective synthesis of cyclopropanes from the aldehyde 2, in which the butadiene group is protected as the iron tricarhonyl complex. The complex (2) is resolved by the method of Kelly and Van Rheenan (5, 289-290), and the two optical isomers arc then converted separately into a cyclopropanealdehyde (5a and 5b) as formulated. A sulfur ylide such as (CH3)2S=CHCOOCH3 can be used in place of diazomethane for cyclopropanation. Optical yields are > 90%,... 

Molecular orbital calculations by Hofmann (162) indicated that an if-allyl anion complex with a noncomplexed butadiene group is more stable than an -butadiene complex, in which the allyl anion portion of the seven-carbon ring is not bonded to the metal. We have now been able to confirm this result by an X-ray structure analysis (see Fig. 8) on [Ph4As][C7H7Fe(CO)3] (163). 

The earlier work on the (butadiene) Group 4 metallocenes and related systems was reviewed by Erker el al.6 and by Yasuda et al in 1985. In the meantime the (butadiene)metallocenes have found a widespread use, especially in organic and organometallic synthesis,8 and, more recently, in... 

SYNTHESES AND STRUCTURAL PROPERTIES OF (BUTADIENE) GROUP 4 METAL COMPLEXES... 

The (s-cA-butadiene) Group 4 metallocenes adopt a a2,7i-type structure. The actual strength of the n-bonding component and, hence, the metallacyclopentene character of the complexes depends very much on the substitution pattern of the diene ligand6 and it is also strongly influenced by the nature of the bent metallocene unit. These various influences were recently analyzed for some ansa-metallocene/ 1,3-diene combinations by means of computational chemistry,83,84 and the results were compared with the dynamic features (AGfw of the ring-flip inversion process, solid... 

Butadiene) Group 4 metal complexes and (allyl) complex systems derived thereof have also been suggested as reactive intermediates at various homogeneous Group 4 metal complex-catalyzed conjugated diene polymerization reactions.151... 

Figure 16-27 Two extreme formal representations of the bonding of a 1,3-butadiene group to a metal atom (a) implies that there are two more or less independent monoolefin metal interactions (b) depicts cr bonds to C-l and C-4 coupled with a monoolefin metal interaction to C-2 and C-3.

The reaction of butadiene iron tricarbonyl, Fe(CO)3(C4H6), in liquid SO2 with BF3 leads to an interesting product containing an O-bonded RSO2 moiety A crystal structure analysis (Fig. 34) of the product showed the presence of allylic and O-sulfinate interactions with Fe, resulting from electrophilic attack of SO2 on the coordinated butadiene group. In addition, a BF3 group was found to be bound reversibly to the... 

Near-infrared spectroscopy has been used to determine as 1,4, trans-1,4, and 1,2 butadiene groups in these polymers [20]. 

Miller and co-workers [21] used near-infrared spectroscopy to determine the microstructure and composition of polybutadiene and styrene-butadiene copolymers. The procedure was capable of distinguishing between cis-1,4, trans-1,4, and 1,2 butadiene groups. Geyer [22] has given details of a Bruker Spectrospin P/ID. 28 used for the identification of plastics using mid-infrared spectroscopy. 

Fig. 107. Perturbation of the frontier orbitals of aromatic and quinoid FT by one butadiene group, resulting in the formation of PITN. Reproduced and adapted by permission of the American Institute of Physics from Y. S. Lee and M. Kertesz, / Chem. Phys. 88, 2609 (1988). Copyright 1988, American Institute of Physics.

For the quinoid ground state, however, the value of the bandgap will increase. This phenomenon is interpreted in Figures 107 and 108 by treating the butadiene groups as successive perturbations to tbe frontier orbitals (HOMO and LUMO) of polythiophene. 

For the aromatic structure of polythiophene, the successive butadiene groups stabilize the LUMO and destabilize the HOMO, which results in a decrease of the bandgap. For the quinoid struc-tme of polythiophene, the addition of successive butadiene groups stabilizes the HOMO and destabilizes the LUMO. As a result, the bandgap increases steadify with an increase in the number of rings in the system, until a level crossing occurs that tempers the further increase of the gap. 

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