Phospholes, Diels-Alder reaction

C. Reactions not involving P=0 or P=S Groups.—Enamine phosphine oxides (45) have been prepared by the addition of amines to 1-alkynyl-phosphine oxides, and the reactions of their anions with various electrophiles have been reported. - With ketones a Wittig-type reaction leads to the formation of a/3-unsaturated ketones, in 53—70% yield, while with epoxides cyclopropyl ketimines are formed. A Diels-Alder reaction of l-phenyl-A -phospholen-l-oxide (46) with 1,4-diacetoxybutadiene has been used in the preparation of l-phenyl-benzo[/>]phosphole (47), as... 

The fifth chapter l-(2,4,6-Trialkylphenyl)-lH-Phospholes with a Flattened P-Pyramid Synthesis and Reactivity presents the interesting chemistry of these compounds including electrophilic substitution and Diels-Alder reactions and sigmatropic rearrangements, making a variety of organophosphorus compounds accessible. 

The Diels-Alder reaction of phospholes was not studied extensively. It was known, however, that the cycloadditionoftheMatheyphosphole(46)withA-phenyhnaleimide afforded 7-phosphanorbomene 47 (Scheme 11) [50], Similar reactions with fuma-ronitrile or with another unit of phosphole led to products with the same anti configuration of the bridging P-moiety [51, 52],... 

Scheme 11 Diels-Alder-reaction of the Mathey-phosphole ...

Scheme 12 Diels-Alder reaction of l-(2,4,6-trialkylphenyl)phospholes...

MO calculations at the MP4SDTP/6-311G //MP2/6-311G level on the Diels-Alder reaction of ethylene with l-phosphabut-3-en-l-yne show that the reaction is endothermic compared with the all-carbon case, which is exothermic.249 Ab initio studies have been presented for hetero-Diels-Alder reactions between phosphorus-containing dienes and dienophiles.250 Ab initio calculations on the Diels-Alder reaction between 21 /-phosphole and phosphaethene indicate that the reaction is concerted and synchronous.251 The Diels-Alder reaction between 1,3,5-triphosphabenzene and phosphaacetylene to form tetraphosphabarrelene was studied at the MP4SDQ/6-... 

R1 and R2 are weak donors a radical mechanism dominates whereas, if they are strong donors, the carbenes 30 are formed by way of a zwitterionic intermediate26. In some cases such as 31 the fate of the carbene is uncertain and FVP at 600 °C gave dimethyl phthalate as the only isolable product27. Retro-Diels-Alder reaction of the phosphole dimers 32 at 400 °C gives mainly the phospholes 33 in over 90% yield but a minor product is the dihydrophosphindole 34, suggesting at least some extrusion of the bridge in the form of the phosphinidene R1 P 2X. Finally, it should be noted that the retro-Diels-Alder reaction... 

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... 

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). 

Hetero-Diels-Alder reactions have also been conducted with aldehydes (Scheme 10) <2003JOC2803>. The reaction is very sensitive to the substitution pattern on the aldehyde and on the phosphole. Reaction of 1-phenyl-3,4-dimethylphosphole with benzaldehyde or its ra-derivatives afforded the corresponding adducts 55 in quantitative yields (Scheme 10). An increase in the steric demands of the aldehydes or of the phosphole results in a dramatic decrease in the yield. For example, with 2,4,6-trimethylbenzaldehyde, no reaction takes place and only 30% conversion is observed with 2,6-dimethoxybenzaldehyde. Similarly, using the more sterically hindered 1-phenyl-2,3,4,5-tetraethylphosphole, no cycloadduct formation is observed with benzaldehyde. Note that with decanal, the yield reaches only 40%. The reaction affords a mixture of endo- and o o-phosphinites 55, the major component being the OT o-derivative. With /ra t-cinnamaldehyde, a mixture of adducts 56 and 57 was obtained (Scheme 10), showing that the 1-phosphadiene can react either via the C=0 or via the C=C moieties of a,/3-unsaturated aldehydes <2003JOC2803>. 

Phosphole oxides are generally unstable since they dimerize rapidly via Diels-Alder reactions. Sterically demanding aryl substituents at the P-atom can provide some stabilization of the phosphole oxide, but not enough to allow for their isolation <1996JOM7801, 1997JOM109>. Note that such sterically hindered phosphole oxides can also be trapped by A-phenylmaleimide to give [4-F2] cycloadducts (Scheme 23) <1997JOM109>. 

Complex 196 was obtained by a Diels-Alder reaction involving one coordinated phosphole ligand of 195 and dimethyl acetylenedicarboxylate (Scheme 60) <2002CEJ58>. Thermal decomposition of 196 at 80°C yielded phospholene complex 197 as the major product. 

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. 

Hetero Diels-Alder reactions involving phosphole sulfides <2007TL3349>. 

A theoretical study of the Diels-Alder reactions between 1,3-butadiene and, respectively, cyclopentadiene and 2H-phosphole, has revealed a remarkable similarity between the two reactions. Further studies of photocycloaddition reactions of phosphole moieties have also been reported. Transition metal complexes of phospholide anions continue to attract attention, and in particular the chemistry of phosphaferrocene systems remains a major interest . The past year has also seen significant activity in the chemistry of di- and tri-phospholes, related polyphospholide anions, and also heterodiphosphole systems. Routes have been developed to the diphosphonio-l,2-diphospholes 356,... 

The Diels-Alder reactions of two-coordinate metalloid compounds and their A derivatives have been one of the most useful applications of these compounds for the synthesis of unsaturated heterobicyclic compounds. The two-coordinate heterocycles have the potential to react as dieno-philes or as dienes. As dienophiles, reaction across the M=C bond gives the possibility of synthesizing a range of fused-ring compounds. Thus dimethyl 2//-phosphole reacts with a diene to give... 

The first successful Diels-Alder reaction of a phosphole was accomplished with the 3,4-dimethyl-1-phenyl derivative, which gave the crystalline cycloadduct (69) with iV-phenylmaleimide (50%) (Equation (6)) <81TL319>. The structure at phosphorus is of interest in this adduct the phenyl substituent is anti to the double bond, while in cycloadditions of the corresponding phosphole oxide with the same dienophile the structure (70) with syn phenyl is the exclusive product (Equation (7)) <91HAC359>. In reactions of both systems, only the endo fused product is formed these stereochemical points have been discussed fully <90rha39>. 

Another example of a Diels-Alder reaction giving the anti isomer is seen in the dimerization of phosphole (71) <85PS(25)ii7>. This occurred on heating in xylene solution at 120°C for 17 h. The dimer (72) was observed by P NMR spectroscopy it was not isolated but has been made indirectly by isomerization at P of the syn dimer (73), made from the oxide dimer (74) (Scheme 7). 

An important new development is the demonstration that Diels-Alder reactions with phospholes and fumaronitriles are greatly accelerated under pressure, and this has led to the synthesis of a number of new 7-phosphanorbornene derivatives. Thus, 3,4-dimethyl-1-phenylphosphole reacted with fumaronitrile at 30 C under 9 kbar for 24 h to give an adduct in 67% yield (Equation (8)) <89T7083>. The stereochemistry at phosphorus was confirmed by x-ray analysis. 

A fascinating aspect of phosphole chemistry is revealed in consideration of a different form of Diels-Alder cycloadditions. It is feasible to conduct reactions with a phosphole while it is coordinated to a metal center, and this has made possible the performance of several Diels-Alder reactions with dienophiles that similarly are held in a state of coordination or are bonded to... 

Similar intramolecular Diels-Alder reactions between phospholes and vinylphosphines were also effected with complexes of ruthenium <89IC4132, 92OM1840), palladium <85JA6396>, molybdenum <89IC4536>, platinum <89iC2i7>, and iron <92OM4069>, with varying levels of reactivity and diastereo-selectivity to various vinylphosphine derivatives. 

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