Arsabenzene

Arsenic is slightly more electropositive than carbon, so arsabenzene should be a little more reactive than benzene, arising from the resultant +/ effect of the heteroatom. However, arsabenzene is reported to be considerably more reactive than benzene, especially at the a- and -y-posi-tions, due to the additional high polarizability of the arsenic p orbitals. 

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Arsenin and Its Derivatives. Arsenin (arsabenzene) [289-31-6], C H As, the arsenic analogue of pyridine, can be prepared by the treatment of l,4-dihydro-l,l-dibutylstaimabenzene with arsenic trichloride (118) ... 

The first such compound to be prepared was the deep-yellow unstable compound 9-arsa-anthracene but the thermally stable colourless arsabenzene (arsenin) can now conveniently be made by a general route from l,4-pentadiyne ... 

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. 

There are many organometallic compounds of arsenic, antimony, and bismuth known that constitute series having chemical properties that differ markedly. These compounds generally decrease in stability in the order As > Sb > Bi, which agrees with the increasing difference in size of the atoms and carbon atoms. Arsenic compounds include both aliphatic derivatives and heterocycles such as arsabenzene,... 

Some work has been performed with bis (arene)vanadium and bis(heteroarene) vanadium complexes [7-10]. As indicated for the selected complexes shown in Table 1, replacement of benzene by phos-phabenzene and arsabenzene lowers the reduction potential. This counterintuitive result has been explained in terms of a greater positive charge on the metal... 

Ashe, III, A. J. The Group 5 Heterobenzenes Arsabenzene, Stibabenzene and Bisiriabenzene. 105,... 

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. 

The stability of phosphabenzene and of arsabenzene in the absence of air and the isolation of the silicon-carbon and silicon-silicon double bonds might suggest that silabenzene, appropriately substituted, could be stable enough to be isolable. Indeed, calculations suggest that it would have a it-resonance stabilization energy about two-thirds that of benzene (78JA6499). 

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.

Reaction of benzyne with arsabenzene affords l,4-etheno-l,4-dihydro-l-arsanaphthalene (192) from which an etheno bridge can be removed with 3,6-di(2-pyridyl)-l,2,4,5-tetrazine (184) to afford, via the polycyclic intermediate (193), the arsanaphthalene (194) (79CC880). 

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. 

Although several comprehensive reviews have appeared on phosphabenzene5-8 i2-i4), Javier heterobenzenes 6,8,12,13 15) have been only modestly covered in the secondary literature. The present work attempts to review arsabenzene, stibabenzene and bismabenzene. Phosphabenzene and pyridine chemistry are only selectively treated for comparison. 

The synthesis of the first arsabenzene, 9-arsaanthracene, 6, was simultaneously communicated by Bickelhaupt16) and Jutzi17> in 1969. The elimination of hydrogen chloride from 9,10-dihydroaarsanthracene 7 afforded 6. These precursors are available from either reduction of the corresponding arsinic acids 816,18) or by the exchange reaction of dihydrostannaanthracenes 9 with arsenic trihalides 17). 

The successful conversion of 9 to 6 provided the original inspiration for the synthesis of the parent arsabenzene 2. The stannohydration of 1,4-pentadiyne 14 with dibutyl-tin dihydride furnished the necessary organotin precursor 1523). The exchange... 

On the other hand, this procedure has been used to prepare arsabenzenes bearing functional groups in the 2-position. Thus, 6-acetoxy-1,4-hexadiyne 20g gave 2-acetoxy-methylarsabenzene 23g, which on hydrolysis followed by oxidation gave 2-arsabenz-aldehyde 24. 

Similarly, 3-substituted-l,4-pentadiynes 25 are converted to 4-substituted arsabenzenes 27 27,28,29). 4-Acetoxyarsabenzene 27b prepared in this manner was easily hydrolyzed to very useful 4-arsaphenol 28 28). 

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