Phospholes, cycloaddition

UV irradiation. Indeed, thermal reaction of 1-phenyl-3,4-dimethylphosphole with (C5HloNH)Mo(CO)4 leads to 155 (M = Mo) and not to 154 (M = Mo, R = Ph). Complex 155 (M = Mo) converts into 154 (M = Mo, R = Ph) under UV irradiation. This route was confirmed by a photochemical reaction between 3,4-dimethyl-l-phenylphosphole and Mo(CO)6 when both 146 (M = Mo, R = Ph, R = R = H, R = R" = Me) and 155 (M = Mo) resulted (89IC4536). In excess phosphole, the product was 156. A similar chromium complex is known [82JCS(CC)667]. Complex 146 (M = Mo, R = Ph, r2 = R = H, R = R = Me) enters [4 -H 2] Diels-Alder cycloaddition with diphenylvinylphosphine to give 157. However, from the viewpoint of Woodward-Hoffmann rules and on the basis of the study of UV irradiation of 1,2,5-trimethylphosphole, it is highly probable that [2 - - 2] dimers are the initial products of dimerization, and [4 - - 2] dimers are the final results of thermally allowed intramolecular rearrangement of [2 - - 2] dimers. This hypothesis was confirmed by the data obtained from the reaction of 1-phenylphosphole with molybdenum hexacarbonyl under UV irradiation the head-to-tail structure of the complex 158. 

If the reaction temperature is raised to 430 K and the carbon monoxide pressure to 3 atm, coordination of the metal atom in the rearranged product occurs via the phosphorus site, as in 159 (M = Cr, Mo, W) [84JOM(263)55]. Along with this product (M = W) at 420 K, formation of the dimer of 5-phenyl-3,4-dimethyl-2//-phosphole, 160 (the a complex), is possible as a consequence of [4 - - 2] cycloaddition reactions. Chromium hexacarbonyl in turn forms phospholido-bridged TiyP)-coordinatedcomplex 161. At 420 K in excess 2,3-dimethylbutadiene, a transformation 162 163 takes place (82JA4484). 

Nelson J. H. Transition Metal-Promoted Intramolecular [4 + 2 Diels-Alder Cycloadditions of Phospholes With Dienophilic Ligands in Phosphorus-31 NMR Spectral Prop. Compd. Charact. Struct. Anal. 1994 203, Ed. Quin L. and Verkade J. G., Pb. VCH N.Y. 

Irradiation of 2,3-diphenyl-2//-azitine in the presence of Cgo fullerene leads to the formation of mono- and ohgo adducts (98,99). A monoadduct, l,9-(3,4-dihydro-2,5-diphenyl-2//-pytTolo)fullerene-60 was isolated and characterized. Mechanistic studies showed that under conditions of direct irradiation it was formed by a classic nitrile yhde cycloaddition but in the presence of 1,4-napthalenedicarbonitrile (DCA) it resulted from reaction of the radical cation intermediate 108. Cycloaddition reactions have also been carried out with diaza-phospholes and diazaarsoles (100) to give adducts of the type 189 (A=As,P) and with cyanogen to give 190 and with atyldiazocyanides where addition to both the azo moiety and the cyano group were observed (101). 

The cycloaddition of phospholes with alkynes to synthesize phosphorins comes under this classification. For phosphorins to form, the transient 2H-phosphole has to react with alkynes followed by elimination of a carbene. This type of synthetic approach is less common and is discussed in the earlier chapter <1996CHEC-II(5)639>. 

Phosphol-1-oxide, z.B. 1,2,5-Triphenyl- bzw. 7,2,3,4,5-Pentaphenyl-phosphol-1-oxid oder das in situ herstellbare 2-Phenyl-2H-2-benzophosphol-2-oxid, konnen als reaktive Diene mit geeigneten Dienophilen eine [4 + 2]-Cycloaddition eingehen. Beispielsweise... 

Phosphorigsaure-Derivate (Ester1-2, Halogenide4, Ester-halogenide6-8) unterliegen mit 1,3-Dienen einer 1,4-Cycloaddition und man erhalt die meist sehr instabilen und feuchtig-keitsempfindlichen 2,5-Dihydro-As-phosphole ... 

Phosphinidenes (R-P), short-lived, reactive intermediates that are isoelec-tronic with nitrenes and carbenes, are accessible by fragmentation of phos-phiranes, phosphol-3-enes, triazaphospholenes, and oxazaphospholenes and can be characterized by trapping reactions. The metal-complexed species [R-P-M, e.g., M = W(CO)5] are more stable (but also not isolable) and react more selectively. As a model example we describe the transformation of (3,4-dimethyl-l-phenylphosphole)(pentacarbonyl)tungsten 4 with dimethyl acetylenedicarboxylate to the 7-phosphanorbomadiene derivative 5.11 The (phosphinidene)(pentacarbonyl)tungsten complex 6 is generated from the latter by a [4+ 1]-cycloreversion and trapped with diphenylacetylene via a [2+1]-cycloaddition to furnish the triphenyl-2-phosphirene complex 7 (Scheme l).12... 

Outstanding properties are the transformation to 1H- or 2/f-phosphirenes after carbene addition (9->- i0),12b 21 22 [3 + 2]-cycloadditions of 1,3-dipoles leading to a wide variety of heteroatom-substituted phospholes (9 — ll)18 and Diels-Alder reactions (9 - 12) that make not only the phosphinines but also their valence isomers accessible.23,24 In ene reactions phosphaalkynes... 

A [3 + 2] cycloaddition of mesityl nitriloxide on 1-trimethylsilyl-2-trimethylsiloxi-2-adamantyl-l-phosphaethene gives the 1,2,4-oxaaza-phosphole [Eq. (20)]. Probably the dihydro compound is formed in the... 

It is important to recall that the reactivity pattern of phosphoies is very different from that of the related S, N, and O ring systems due to their limited aromatic character. For example, electrophilic substitution takes place only with a handful of phosphoies that have been specifically tailored via increasing the bulkiness of the P substituent (see Section 3.15.10.4, Scheme 83). In fact, electrophiles react at the phosphoms atom affording a panel of neutral and cationic CN 4 derivatives (Scheme 8). Phosphoies are also versatile synthons for the preparation of other heterocyclic systems via Diels-Alder reactions. The cycloaddition can involve the dienic moiety of the phosphole ring or can occur following a 1,5-shift of the P-substituent (Scheme 8). Finally, phosphoies can be transformed into phospholide ions, which are powerful nucleophiles that have found a variety of applications (Scheme 8). All these facets of phosphole reactivity are presented in this section. It should also be noted that CN 3 phosphoies exhibit a rich coordination chemistry toward transition metals (see Section 3.15.12.2). 

The same reactivity pattern, namely a 1,5-shift followed by cycloaddition, was observed with aromatic phospholes (see Section 3.15.2.1) such as l-(2,4,6-triisopropylphenyl)phosphole 27 (Scheme 11) <2005HAC104> and l-(2,4-di-ferZ-butyl-O-methylphenylphosphole 11 <2003HAC316>. 

The vast majority of bicyclic phosphole derivatives with norbornene- and norbornadiene-derived skeletons are prepared via classical [4+2] cycloaddition reactions of phospholes. While Diels-Alder reactions of phospholes with CN 4 (e.g., oxides and sulfides) are common through direct reaction with dienophiles, this type of reaction is comparatively rare for CN 3 phospholes (see Section 3.15.5.1.4). 

If similar Diels-Alder reactions are undertaken with equimolar mixtures of 1-arylphosphole oxides, four different [4+2] cycloadditions take place <2001HAC633>. Both the expected symmetric phosphole oxide dimers as well as crossed cycloadducts are obtained. 

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