Phosphorus five-membered rings

Chiral cyclic phosphines have useful properties as ligands in transition metal asymmetric catalytic systems. The most impressive example is the five-membered ring phosphorus (phospholane)-based chiral ligand DuPFIOS <2000ACR363>. 

Five-membered Ring Phosphorus Heterocycles Oxaphospholes and 164... 

Phosphorus heterocyclic compounds, 1, 493-538 five-membered ring systems, 1, 513-523 nomenclature, 1, 496 six-membered ring systems, 1, 497-513 Photoaromatization oxirenes from, 7, 125-126 Photobleaching chromenes in, 3, 880 Photochemical reactions heterocyclic compound synthesis from, 5, 159 reviews, 1, 56 heterocyclic compounds reviews, 1, 71, 72... 

Ti -Cyclopentadienyl(triphenylphosphine)cobalt reacts with phosphites and forms complexes of 1-alkoxyphosphole oxides 251 (R = Me, Et, Ph) through a step involving (ri -cyclopentadienyl)(phosphite)cobalt (80JA4363). (ri -Cp)Co(PF3)2 reacts with hexafluorobut-2-yne and 252 is formed, which hydrolyzes into 253 (X = OH) [73JCS(CC)583 75JCS(D)197]. The five-member ring has the envelope conformation, in which the carbon atoms are coplanar, and the phosphorus atom deviates from this plane in the direction opposite to the cobalt atom. The heterocycle is a four-electron donor relative to the metal center. 

Polymerization of 4-bromo-6,8-dioxabicyclo[3.2.1 ]octane 2 7 in dichloromethane solution at —78 °C with phosphorus pentafluoride as initiator gave a 60% yield of polymer having an inherent viscosity of 0.10 dl/g1. Although it is not described explicitly, the monomer used seems to be a mixture of the stereoisomers, 7 7a and 17b, in which the bromine atom is oriented trans and cis, respectively, to the five-membered ring of the bicyclic structure. Recently, the present authors found that pure 17b was very reluctant to polymerize under similar conditions. This is understandable in terms of a smaller enthalpy change from 17b to its polymer compared with that for 17a. In the monomeric states, 17b is less strained than 17a on account of the equatorial orientation of the bromine atom in the former, whereas in the polymeric states, the polymer from 17b is energetically less stable than that from 17a, because the former takes a conformation in which the bromine atom occupies the axial positioa Its flipped conformation would be even more unstable, because the stabilization by the anomeric effect is lost, in addition to the axial orientation of the methylene group. 

Hypervalent Structures with a Pentacoordinated Phosphorus and Two Five-Membered Rings... 

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). 

Examples of reactions involving replacement and cyclization are the long-known preparation of thiophenes (89) from 1,4-diketones, and the formation of l,2-dithiole-3-thione (90) from the salicylate ester analog (91).120 In the latter instance, oxidative cyclization with formation of an S—S bond has occurred this is a common feature of these reactions, particularly if such a link is needed to complete a five-membered ring. Another example of this aspect is afforded by the reaction of the propane-1,3-dione derivatives (92) which yield 3,5-diaryl-1,2-dithiolylium salts (93) when heated with phosphorus pentasulfide in carbon disulfide, followed by perchloric acid.121... 

As with five-membered heterocycles, phosphorus pentasulfide may replace a ring oxygen atom by sulfur, or it may effect replacement in an acyclic precursor and then bring about cyclization. Thus isochroman-l-one (99) is converted into thioisochroman-l-thione (100) under the same conditions a 3-arylisocoumarin( 101) produces a 3-ary 1-1-thioisocoumarin (102) only, with no replacement of the ring oxygen atom.122 A typical example of thiation, followed by cyclization, is the formation of 2-alkyl- or 2-aryl-3,l-benzo-thiazine-4-thiones (103) from /V-acylanthranilic acid esters (104) by treatment with phosphorus pentasulfide in boiling xylene.122... 

Selenium, and Phosphorus Atoms in the Five-Membered Ring 714... 

Ring Synthesis of Fused Pyridazines, Pyrimidines, and Pyrazines with Oxygen, Sulfur, Selenium, and Phosphorus Atoms in the Five-Membered Ring... 

Further examples of deoxygenation of epoxides by phosphine sulphides or selenides have appeared,41 as shown in Scheme 6 for simple epoxides (50). Incorporation of phosphorus into a five-membered ring appears to be responsible for the relatively rapid deoxygenations by the phosphole and A3-phospholen derivatives.41... 

Vacuum thermolysis (160°C) of the hemiaminal 11 generates the azaphos-phetane 65 in 85% yield.32 This product clearly results from the intramolecular insertion of the transient (amino) (phosphino)carbene 2h into the C-H bond of a diisopropylamino group bonded to phosphorus. Note that the four-membered heterocycle 65 is formed exclusively in spite of the ratio of six methyl-CH bonds to one methine-CH bond, and that only one of the two possible diastereomers is detected. The same regio- and diastereo-selectivity have already been observed with the di(phosphino)carbene 2g,74 but is in marked contrast to the exclusive formation of five-membered rings... 

Let us now direct our attention to the P=C bond in phosphaalkene ion-radicals. The literature contains data on two such anion-radicals in which a furan and a thiophene ring are bound to the carbon atom, and the 2,4,6-tri(tert-butyl)phenyl group is bound to the phosphorus atom. According to the ESR spectra of anion-radicals, an unpaired electron is delocalized on a n orbital built from the five-membered ring (furanyl or thienyl) and the P=C bond. The participation of the phosphaalkene moiety in this MO was estimated at about 60% and some moderate (but sufficient) transmission of the spin density occurs through the P=C bridge (Jouaiti et al. 1997). Scheme 1.6 depicts the structures under discussion. 

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