Phospholans and Phospholens

Phospholans and Phospholens.—Details have appeared of the preparation and properties of homocubylphosphoranes, e.g. (24), and related compounds. An improved synthesis of (24) is as shown. A full account has been given of the dynamic n.m.r. of the spirophosphoranes (25) in which the naphthyl 

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Pietrusiewicz and Stankevic utilized the Birch-reduction to prepare cyclohexadienyl-phospholane and phospholene oxides. As can be seen, in some cases the hetero ring was also reduced (Scheme 53). ... 

In marked contrast to tri-covalent nitrogen, tri-covalent phosphoms has a significant barrier to pyramidal inversion, as discussed in section 10.2. In C-substituted phospholanes and the 4-membered phosphetanes as well, cis,trans isomers are stable and separated easily. This possibility was not recognized until it was demonstrated in 1965 that 1,2-dimethyl derivatives 10.12 and 10.13 of dihydrophospholes, which are known as 3-phospholenes, could be separated by fractional distillation or gas chromatography. ... 

Shang G, Zhang X (2008) Phospholes, phospholenes, phospholanes and phosphinanes. In BOrner A (ed) Phosphorus ligands in asymmetric catalysis synthesis and applications. Wiley-VCH, Weinheim, pp 135-177... 

Using the new preparative method, combined with reactions at the allylic position in the phospholene 1,2,3,4-tetrasubstituted-phospholanes, a series of the tetrofuranose-type phospha sugars can be prepared (Scheme 28 and Tables 3 and 4) [37,38]. 

Allylic oxidation of 2-phospholenes affording 4-oxophospholanes 126 as the new starting materials for pentofuranose and Cl-homologation of 2,3-epoxy-phospholanes to prepare pentofuranose-typep/zosp/za sugars are also reported (Scheme 42 and Table 12) [55-57]. ORTEP drawing of 126a is shown in Fig. 19. 

The stereochemistry of the addition has been recognized on the basis of structural features and chemical behavior of the complexes 9, produced in selected reactions of j with 1-ethoxy-1-oxo-2-phospholene 8. Neutralization of these complexes gave exclusively cis-3-substituted phospholane oxides J 0 while alkylation resulted in stereoselective formation of cis-3, cis- -disubstituted phospholane oxides JJ. The progress of the latter reaction was strongly dependent on the size of the alkyl groups involved. 

A great number of dihydrophospholes, namely 2- and 3-phospholenes, and tetrahydrophospholes (phospholanes) are known. These compounds have found significant utility as building blocks for the preparation of chiral ligands. 

The chemistry of reduced phospholes has been extensively developed since the two dihydro isomers, namely 2- and 3-phospholene, together with phospholanes are easily accessible and generally stable derivatives. 

The complexes HCo(CO)3L, where L is a variety of phospholenes, phospholes, and a phospholane, were compared under industrial conditions for the hydroformylation of 1-octene. It was found that 3,4-dimethyl-phospholenes and phospholes give comparably high yields of linear 1-nonanol and that both classes of ligands gave more efficient and more selective catalysts than the best phosphine, 9-phenyl-9-phosphabicyclononane. It was argued that the phospholes are strongly bound to cobalt, greatly increase the hyd-ridic character of the Co-H bond, and favor the formation of HCo (CO)2L (olefin) in preference to HCo (CO)3 (olefin) under the reaction conditions. 

Historically, phospholanium salts have been prepared by quaternization in high yield of a selected phospholane with an alkyl halide. Although numerous methods exist for the preparation of the desired phospholanes (recently reviewed ), most of these have major drawbacks, including (1) very critical conditions, such as reaction time, dilution effects, and temperature (2) expensive and/or difficult to manipulate reagents and (3) the necessity of a multistep synthetic sequence giving overall low yields of phospholane. A general example of the latter would be the reaction of a substituted dihalophosphine with a 1,3-diene and hydrolysis to the phospholene oxide, which then can be catalytically reduced to the phospholane oxide and subsequently converted to the phospholane. ... 

Syntheses in the phospholan series include the oxides (6), (7), and (8). Of these, l-methyl-3-phospholanone 1-oxide (6) has been studied in considerable detail. It is a keto-enol mixture, and the reactions outlined here reflect the reactivity of the hydrogens at C-2. The isomeric oxides (7) and (8) were prepared by photochemical methods. The A -phospholen 1-oxide (8) undergoes an unusual photochemical reduction in alcoholic solution (see ref. 41). 

The 1,1-dihalides are rarely isolated but instead are hydrolysed to the 1-oxides,40-45 the position of the double bond being apparently unaltered thereby.39 The 1-sulfides can be obtained from (1) or (2) by means of H2S,46 and the phospholenes themselves by reduction with Mg or LiAlH4.47 Hydrogenation of the phospholene 1-oxides over Raney nickel leads to the saturated phospholane 1-oxides.48... 

The rich chemistry of phospholes, phospholenes, and phospholanes is described in several reviews. 

Phospholene oxides from the McCormack reaction also stand as useful precursors of phos-pholanes as noted, the double bond can be easily hydrogenated, and then the oxygen removed from phosphorus with silanes, etc. (Schemes 56 and 57 Equations (51) to (53)). Phospholanes can be prepared by other cyclization methods, however, some of which are outlined below. 

The esterification of cyclic phosphinic acids such as l-hydroxy-3-phospholene oxides (7 and 8), 1-hydroxy-phospholane oxides (11 and 13), and l-hydroxy-l,2,3,4,5,6-hexahydrophosphinine oxide (15) was carried out in the presence of c. 15-fold excess of the alcohols in a closed vessel to afford the phosphinates (9,10,12,14, and 16) in acceptable to excellent (44-95%) yields [33-37]. The method elaborated seems to be of general value. It was foxmd that the esterification of phosphinic acids is thermoneutral and kinetically controlled, and also represented as an irreversible process [35]. 

After the esterifications and thioesterifications, the direct amidation of cyclic phosphinic acids was also attempted. The interaction of l-hydroxy-3-phospholene oxide (7) and 1-hydroxy-phospholane oxides (11 and 13) with primary amines under MW conditions took place in conversions of ca. 33% (Scheme 5). The amides (18-20) were isolated with 23-29% yields [37,39]. 

G. Keglevich, P. Bagi, A. SzoUosy, T. Kortvelyesi, P. Pongracz, L. KoUar, L. Drahos, Platinum(ll) complexes incorporating racemic and optically active l-alkyl-3-phospholenes and 1-propyl-phospholane P-Hgands synthesis, stereostructure, NMR properties and catalytic activity, J. Organomet. Chem. 696 (2011) 3557-3563. 

E. Jablonkai, L. Drahos, Z. Drzazga, K.M. Pietrusiewicz, G. Keglevich, 3-P(0)< Fimctionalized phospholane 1-oxides by the Michael reaction of l-phenyl-2-phospholene 1-oxide and dialkyl phosphites, H-phosphinates or diphenylphosphine oxide. Heteroatom Chem. 23 (2012) 539-544. 

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