Phosphorus heterocycles three-membered rings

About 20 structural studies on diphosphiranes have been reviewed in Koenig s recent accounts on three-membered ring compounds published in CHEC-II(1996) <1996CHEC-II(1)469> and in Phosphorus Carbon Heterocyclic Chemistry the... 

Keywords. Three-membered rings, Phosphorus heterocycles, Cations, Radicals, Valence isomers... 

For reviews on related topics, see a) Bourissou D, Bertrand G (1999) Acc Chem Res 32 561 b) Canac Y, Bourissou D, Gornitzka H, Schoeller WW, Baceiredo A, Bertrand G (1999) Phosphorus Sulfur Silicon 144-146 37 c) Etemad-Moghadam E, Koenig M (1996) Three-membered rings with two heteroatoms including phosphorus to bismuth. In Katritzky AR, Rees CW, Scriven EFV (eds) Comprehensive heterocyclic chemistry II. Pergamon, Oxford, vol 1 a, p 469... 

Since three-membered rings with two heteroatoms including phosphorus to bismuth were not treated in the first edition of Comprehensive Heterocyclic Chemistry (CHEC-I) we have attempted, in this review, to comprehensively cover the literature up until the middle of 1994, including several reviews already written on three-membered heterocycles (90CRV997,92CRV1839). Taking into account the various synthetic routes, and the different strategies for the stabilization of these strained three-membered heterocycles, we have attempted to divide them into several series. 

Diazaphospholes constitute the most widely investigated class of heterophospholes, the 67t-aromatic phosphorus heterocycles [1,2]. Diazaphospholes are unique in the manner that the five-membered ring incorporates one phosphorus atom. First diazaphosphole representative, i.e. 2//-[l,2,3 diazaphospholc was obtained as early as 1967 [3] and until 1980s the interest of organophosphorus chemists remained in the development of different synthetic routes and in investigating their varied reactivity due to the structural diversity within the class [4], On the basis of the relative positions of the three heteroatoms in the five-membered ring, six monocyclic diazaphosphole systems (A-F) are possible and all of them have been reported (Structure 1). 

Larger chalcogen-phosphorus heterocycles, although less common in the literature, are accessible via a variety of synthetic routes.2,83,84 For example, the cyclic trimer (SPR)3 (R = 2,4,6-tri-tert-butylphenyl) contains a puckered six-membered P3S3 ring and is produced in the reaction of a phosphinic chloride with lithium sulfide (Equation 73).98 Additionally (R P)3Se5 (R = 2,4-di-tert-butyl-6-wopropoxyphenyl), synthesised from the oxidation of a primary phosphine with three equivalents of elemental selenium (Equation 74), has... 

Electrochemical reduction of a,a -dibromoketones affords the unstable cyclo-propanone, which is in equlibrium with the dipolar intermediate 22. The cyclopro-panone hemiacetal is isolated in yields of 40 - 85 % from reaction in acetonitrile and methanol at -20 °C [99], The dipolar form can be trapped in a cycloaddition process with furan [100], Reaction with acetic acid leads to the a-acetoxy-ketone.[101]. Unstable three membered heterocyclic rings are intermediate in the reduction of sulphur and phosphorus linked dibromo compounds 23. In these reactions, the heteroatom is extruded leaving ci - and trans-stilbenes as the isolated products [102,103],... 

Chapter 2 by Laszlo Nyulaszi and Zoltan Benko deals with the chemistry and physical organic chemistry of aromatic phosphorus heterocycles and is divided into four subchapters dealing with three-, four-, five-, and six-membered rings, in which there may be more than one phosphorus atom. The chapter begins with a clear presentation of the electronic structure that phosphorus may achieve in molecules with CP bonds. For cyclopentadiene and phosphole eight aromaticity indices are collected. Almost all of them indicate that phosphole is more aromatic than cyclopentadiene. It is also shown that even small structural effects (substituent, bonding modes) can have a substantial impact on the chemistry of the reported systems. 

The compounds with five-membered rings containing three or four heteroatoms including at least one phosphorus were first systematically reviewed in Chapter 4.22 of CHEC-II(1996) by Schmidpeter <1996CHEC-II(4)771>. In CHEC(1984), only few examples of 2//4,2,3-diazaphospholcs were mentioned in the general chapter on phosphorus heterocycles <1984CHEC(1)493>. 

A short time ago numerous triorganyl-cyclotriphosphanes have become accessible as pure substances on various synthetic routes (2). Moreover, three-membered phosphorus heterocycles with carbon, silicon, germanium, arsenic, boron, and sulfur as hetero ring atoms could be synthesized (2). Recently, we have found that such compounds with elements of the fifth periodtare als stable enough fop existence. Through the reaction of K(Bu )P-P(Bu )K with Bu S C1 or Bu -SnC at -78°C, the diphosphastib rane ... 

High level ab initio calculations have been applied to estimate the ring strain energies of three-membered heterocycles, amongst other phosphorus three-membered heterocycles are included <2006JA4598>. 

The six-membered heterocyclic ring, cyclocarbophosphazene, [(NPCl2)2CCl] serves as the precursor for poly(carbophosphazene)s. The pentachlorocyclocarbophosphazene N3P2CCI5 can be considered as a hybrid of cyanuric chloride, N3C3CI3 and hexachlorocyclotriphosphazene, N3P3CI6. Thus, while cyanuric chloride has three carbons and the hexachlorocyclotriphosphazene has three phosphorus atoms, the cyclocarbophosphazene has one carbon and two phosphorus atoms (Fig. 5.3) [4, 5]. 

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