Incorporation of the heteroatom

Five-membered ring systems can be obtained from hetero-l,3-dienes on reaction with oxiranes and thiiranes. To avoid competition from a possible 1,4-addition, the nucleophilic attack of the terminal heteroatom of the diene has to be sterically or electronically hindered by incorporation of the heteroatom into... 

Polythiophene (PT) 20 can be regarded as a hybrid structure of cis-polyacetylene and sulfur, where the incorporation of the heteroatom leads to a much higher environmental stability of PT compared with that of PA [131], The outstanding physical and bulk properties make PT a desirable material for device applications like sensors, energy storage, solar cells, LEDs, NLO devices, and more [132], Due to the large number of possible technical applications, much effort has been made to tailor the properties of PT by synthesis. 

The barriers to inversion at the three-coordinate heteroatom increases markedly with atomic number (ca. 35, 40, 45, and 60 kcal mol for P, As, Sb, and Bi respectively) and is generally sufficiently slow to allow the synthesis of optically active compounds [Pg.865]

In the present paper, we review the main results obtained in the preparation of transition metal-containing molecular sieves. In particular, the attention will be focused on the synthesis procedures and the physico-chemical evidence supporting the effective framework incorporation of the heteroatom. 

The assessment of true incorporation of the heteroatom in a zeolite framework assumes paramount importance. In fact, the physico-chemical and catalytic properties of the materials strictly depend on the location of heteroatoms in framework sites. This is particularly true for titanium it is well known that a dramatic difference in catalytic performance exists between a well synthesized TS-1 sample and one containing extra-framework titanium species. 

By adopting a hydrothermal digestion of vanadium-alumino-sihca glasses in basic solutions at 120 °C, Dubanska claimed the possibiHty to synthesize a number of V-molecular sieves including NU-15, ANA, LTA,KF, SOD [113]. However, no conclusive evidence supporting true incorporation of the heteroatoms was given. 

Modification of reaction schemes leading to furans, thiophenes and pyrroles by incorporation of a heteroatom into the substrate provides ready access to imidazoles, thiazoles, oxazoles, dithioles, oxathioles, etc. Incorporation of two heteroatoms into the substrates... 

This section deals mainly with nitrogen-containing, eight-membered heterocycles that are able to form a fully conjugated n-electron system, including both 8 n ( pyrrole type ) and iOn ( pyridine type ) systems, i.e. the free lone pair electrons of the heteroatom incorporated, or not incorporated, in conjugation. 

Scheme 3.3-7. Incorporation of groupl4 heteroatoms into the decaborane(14) framework.

Evans and co-workers simultaneously developed of the rhodium-catalyzed [4-+2 + 2] car-bocychzation reaction through the identification of a novel mode of reactivity for the five-membered metallacycle intermediate [14, 37]. Evans proposed that 1,3 dienes should be amenable to addition to this intermediate, which would provide a new reaction pathway. The tethered enyne 80 (Scheme 12.9), was expected to afford the metallacycle I, which, upon sequential incorporation of a butadiene and reductive ehmination, would afford the desired 5,8-fused product 81. Additionally it was rationahzed that this transformation should be highly diastereoselective, as outlined in Scheme 12.10. Treatment of the heteroatom-tethered enyne I under standard carbocyclization (Pauson-... 

DFT calculations have been used to follow the formation of iminium ions from secondary amines and acrolein.56 Energy barriers in the process can be lowered by incorporation of a heteroatom (N or O) in the a-position of the amine, or an electron-withdrawing group (carbonyl or thiocarbonyl) in the -position. 

A second possibility, mentioned above, is the incorporation of the metal as heteroatom into covalent nomnetal rings, to give for example, metallacyclophosphazenes, metallacyclosiloxanes, metallacyclosilazoxanes, metal-laborazines, and so on. Many such rings are known (for a review, see 184). 

Various polyheteroaromatic ladder structures were obtained, for example polymers that consist of phenoxazine- (Z -0-), phenothiazine (Z -S-) or quinoxaline- (Z -NH-) subunits with different patterns of the heteroatoms incorporated [6]. 

Carbene reactions giving rise to seven-membered rings incorporating a nitrogen atom generally occur with the participation of the heteroatom. 

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