Arsabenzene, aromaticity

The heterobenzenes of the group 15 elements (1-5) comprise a series in which elements of an entire column of the periodic table have been incorporated into aromatic rings. The comparative study of this series has been extremely valuable for evaluating p-p rr-bonding between carbon and the heavier elements.1 However, the heterobenzene series has two important limitations. Only arsabenzene has a well developed aromatic chemistry. Moreover, stibabenzene and particularly bismabenzene are so labile that it has been difficult to obtain derivatives stable enough for study. 

One of the most obvious examples is strong deshielding of the a-protons in the series pyridine (8 8.29 ppm), phosphabenzene (8.61), arsabenzene (9.68), stibabenzene (10.94), and bismabenzene (13.25), although other data unambiguously point to a falling off of the aromaticity in this sequence. Here the contribution by crAringcurr is mostly obscured by local effects connecting with nonuniform distribution of the electron density and by the anisotropy of the heavier heteroatoms. 

There are a number of heteroatom six-membered aromatic rings which are analogous to benzene in that they can donate six electrons to a metal. These include phosphabenzene. borabenzene anion, bomzire. and arsabenzene shown in Fig. I5.42.uv... 

Fig. 15.42 Complexes ot pho.spfiabenzene. borabenzene, borazme. and arsabenzene. These six-membered rings arc analogous to henzene. i e.. thev are six-eicciron donors and are aromatic.

The arsabenzene ring system has been actively studied for just over a decade. Its bond delocalization, diamagnetic ring current and electronic structure demonstrate that arsabenzene has a high degree of aromatic character. While its lack of basicity strongly differentiate it from pyridine, arsabenzene has a rich organic chemistry quite similar to that of normal benzocyclic aromatics. 

Competition experiments with benzocyclic aromatics show that arsabenzene is considerably more reactive than benzene. Arsabenzene is acetylated at approximately the same rate as mesitylene, that is about 103 faster than benzene. Deuterium exchange takes place at a rate comparable to that of p-xylene, again about 103 faster than benzene. 

Keywords aromaticity, arsabenzene, boratabenzene, germabenzene, heterocycles, metallabenzene, phosphabenzene, pyridine, pyridinium, pyrylium, silabenzene, thiopyrylium... 

A striking difference to pyridine is the lack of basicity and nucleophilicity for phosphabenzene and arsabenzene, which are protonated at the a- and y-carbon atoms (this reaction is responsible for deuteration). Neither alkyl halides nor trialkyloxo-nium salts can alkylate phosphabenzene, therefore there will be no discussion of quaternary salts for these pnictogena-hetarenes. Whereas pyridine can be oxidized to the zwitterionic N-oxide, phosphabenzene affords non-aromatic -oxidation products 35 and 36 with tetracoordinated P(V) phosphorus atoms, similar to phosphin-oxides and phosphonic acids, respectively. 

The bottom line on monocyclic six-membered aromatic compounds is that, so far, only benzene, the azines with one through four nitrogen atoms, phosphabenzene and arsabenzene, pyrylium, azapyrylium and chalcogenopyrylium cations (with or... 

Benzene (CH)6, of course, is the most prototypal aromatic system. When one or more of the CH groups are replaced by other atom(s), a heterocyclic aromatic system is obtained. Well-known examples include pyridine N(CH)5 and pyrimidine N2(CH)4, while lesser known cases are phosphabenzene P(CH)5 and arsabenzene As(CH)s. There are also systems where the heteroatom is a heavy transition metal examples include L Os(CH)s and L Ir(CH)5. 

Strong nucleophiles such as organolithium or organomagnesium derivatives do not react with substituted or unsubstituted phosphabenzene or arsabenzene (Y = P or As) by nucleophilic substitution as in the case of pyridines, but by addition to the heteroatom forming intermediate anions. These anions can then be converted into non-aromatic compounds by reaction with water yielding 1-alkyl-1,2-dihydro-derivatives, or they can be alkylated by an alkyl halide with the same or a different alkyl group, when two products may result a l,2-dialkyl-l,2-dihydro-derivative, or a 2 -derivative (Figure 17). The former products are kinetically controlled, whereas the latter compounds are thermodynamically controlled. 

The syntheses, as well as physical and theoretical studies regarding the aromatic character of heterobenzenes, such as phosphabenzene " , arsabenzene and bismabenzene , have been extended also for stibabenzene . The benzenelike structure has been attributed to stibabenzene on the basis of experimental study and theoretical interpretation of the microwave spectra of SbCjH5, SbCjH5, ) -dideuterio SbC5H3D2 and 8-dideuterio SbC5H3D2 in the region 26.5-40.0GHz. 

Several heterocyclic derivatives that might exhibit aromatic properties have now been generated. Whether these systems that are formally analogous to benzene, cyclopentadienide ion, naphthalene, or anthracene but contain boron or an atom with a 3p or a 4p valence shell, are aromatic is uncertain. Ultraviolet and NMR spectral data that have been obtained for heteroaromatic species have been compared to analogous organic molecules. However, preliminary photoelectron spectral data obtained for arsabenzene (XXXVIIq) and arsaanthracene (XLIV) show that the... 

Since 1965 arsenic chemistry and especially the chemistry of heterocyclic arsenic compounds has become revitalized because of the synthesis of the first six-membered, cyclic conjugated, Hiickel-aromatic phosphabenzenes (2) and arsabenzenes (3) <71JA3293>. Then, 50 years after the salvarsan period, a second boom in arsenic chemistry, especially in heterocyclic, unsaturated arsenic compounds began. 

The lUPAC name for the six-membered, cyclic conjugated, Hiickel-aromatic arsabenzene (3) was for some time arsenin . Since 1983 the lUPAC nomenclature has been arsinin <83PAC409). If one wants to point out that the arsenic atom is tervalent, coordination number 2, it is written as a -arsinin. Derivatives of (3) with pentavalent arsenic, coordination number 4 (7) are defined as A -aTsinines Chemical Abstracts 1,1-dihydroarsenins). 

Thus the strong similarity of the UV spectra of borazarenes to those of the corresponding carbocyclic compounds is considered evidence for considerable aromatic character of the former63 64 (see also Section III,D, 10). The ultraviolet spectra of phosphabenzenes,65 arsabenzenes,66 and stibabenzenes66 also reflects the considerable delocalization in these rings (Sections III,D, 4 and 5). 

Arsabenzene (arsenin) is a colourless liquid which is rapidly attacked by air, turning bright red. Its photoelectron, u.v. and n.m.r. spectra all support an aromatic structure. In the 2,4,6-triphenyl derivative the ring is planar. The two As—C bonds are equal in length and so are the four C—C bonds. This indicates considerable electron delocalization. Nevertheless arsenins undergo 1,4-addition reactions with alkynes. 1-Arsanaphthalenes and 9-arsaanthracenes are also known. 

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