Antimony ring systems

Examples of cyclophosphazenes with ring systems containing elements other than phosphorus or nitrogen continue to be reported. The linear phosphazene [Ph2(H2N)Pi N.i P(NH2)Ph2]+Cl is cyclized by antimony pentachloride to give the compound (35). This result contrasts with... 

The principles needed to design a polymer of low flammability are reasonably well understood and have been systematized by Van Krevelen (5). A number of methods have been found for modifying the structure of an inherently flammable polymer to make it respond better to conventional flame retardant systems. For example, extensive work by Pearce et al. at Polytechnic (38, 39) has demonstrated that incorporation of certain ring systems such as phthalide or fluorenone structures into a polymer can greatly increase char and thus flame resistance. Pearce, et al. also showed that increased char formation from polystyrene could be achieved by the introduction of chloromethyl groups on the aromatic rings, along with the addition of antimony oxide or zinc oxide to provide a latent Friedel-Crafts catalyst. 

In contrast to the cycloarsane chemistry, however, the NMR spectra of the n-alkylanti-mony ring systems reveal the presence of a second type of ring with equivalent substituents. As the intensities of these signals increase on dilution, these signals were attributed to tetramers instead of hexamers that should also have equivalent substituents. The chemical situation for n-alkyl antimony systems is described by equation 7. 

The search for antimony monocycles that are protected by bulky substituents has led to the syntheses of tetrastibetanes with bulky substituents like (MesSb) , ((Me3Si)2CHSb)4 and (MejCsSb) . The isopropylantimony system, however, is more closely related to the antimony rings with slim substituents. 

Six-Membered Heterocyclic Compounds with a Single Nitrogen Atom in the Ring to which are Fused Two or More Carbocyclic Ring Systems, and Six-Membered Ring Compounds where the Hetero-Atom is Phosphorus, Arsenic, Antimony or Bismuth. Alkaloids Containing a Six-Membered Heterocyclic Ring System... 

Whilst the majority of the work described in this section involves the synthesis of cyclic phosphorus compounds, most of the chemistry could equally apply to arsenic and antimony and to a lesser extent bismuth. Most of the compounds synthesized have more than one heteroatom in the ring system, and not infrequently there are a number of heteroatoms, several of them often bound to the bridging metalloid atom, M. The methods most sucessfully applied to the synthesis of the parent and monoheterobicyclic compounds have been those involving addition and cycloaddition reactions. 

Meerwein was the first to succeed in obtaining dioxolanylium ions of type 2, sufficiently stabilized as salts with non-polarizable anions that they could be isolated crystalline. The compounds can be prepared by splitting out of an anion from cyclic ortho esters or acetals wherein the required ring-system is already present. The ortho ester 1 reacts with antimony pentachloride or boron trifluoride, with splitting out of OR, to give 2. Acetals (3) from aldehydes can be converted, by hydride abstraction with triphenylmethyl or triethyl-oxonium fluorohorate, into salts (2) this reaction proceeds well only with acetals of the 1,3-dioxolane type (3) that have little steric hindrance. With acetals of the 1,3-dioxane type, formed from aldehydes, the reaction of hydride abstraction is not, as a rule, possible. In all such reactions, the anion involved is either SbClg or BF4 . 

This review covers 71-excessive five-membered ring systems containing one arsenic, antimony, or bismuth atom within the ring. Their benzo-, dibenzo-, unsaturated, partially unsaturated, and saturated analogues are also covered. These heteroatoms can have expanded valence shells and thus different coordination numbers (CN) are included as well. Although not strictly covered in this Chapter, mention is made of systems containing one of the above heteroatoms with another atom within the ring, but usually to show an important property of the system. 

While many different structural types are known for heterocycles containing arsenic, antimony, or bismuth, structural diversity comes from the expanded CN that are available with these heteroatoms. Throughout the Chapter, the letter E, as used in the ring, represents As, Sb, or Bi and its use is explained in the text. Scheme, or Equation. In general, few ring systems exist that contain ring bound heteroatom substituents, and as a result, hydroxy-oxo, amino-imino, and thiol-thioxo tautomerism is nearly nonexistent. 

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