Cycloaddition reactions phosphaalkynes

Some cycloaddition reactions of 4 are summarized in Scheme 1. This shows that silylene 4 undergoes reactions with nitriles [14], phosphaalkynes [15], silyl azides [16], diazabutadienes [17], 2,2 -bipyridyl and its derivatives [18, 19], a-ketoimines [19], and pyridine-2-aldimines [19]. 

More recently, [2+3] cycloaddition reaction of the tri-te/t-butylphenylphosphaethyne (25) has been reinvestigated, when in spite of the steric encumbrance of extremely bulky Mes group, the use of trimethylsilylated diazomethane (24) makes its cycloaddition successful, which is followed by SiMe3/H migration yielding bulky [l,2,4]diazaphospholes [33], Phosphaalkyne 25 reacts with 24 in a regioselective manner to form intermediate cycloadduct 26, which undergoes facile aromatization... 

In the first structurally characterized complexes of type A the metal-phosphorus triple bonds are kinetically stabilized by bulky substituents at the amido ligands. Therefore, these compounds reveal exclusively end-on reactivity via the phosphorus lone pair. This reactivity pattern seems also valid for the solution stable alkoxide derivative [(C/0)3Mo=P], for which the reaction potential is under investigation [13]. In contrast, due to their lesser degree of kinetic stabilization by bulky substituents the short-lived alkoxide containing complexes [(R 0)3W=Pj (R =t-Bu (3c), Ph (3d)), generated by the metathesis reaction between the alkoxide-dimer and the phosphaalkyne (cf. Eq. 8), show additionally a high side-on reactivity towards the phos-phaalkynes of the reaction mixture. Thus, there occurs a formal cycloaddition reaction with the phosphaalkynes, and a subsequent 1,3-OR shift yields the formation of four-membered diphospha-metallo-cyclobutane derivatives 6(Eq. 8) [15,31, 37]. 

Apparently independently, Markl et al. (139) and Regitz and co-workers (140-142) discovered that 1,3-dipolar cycloaddition reactions of mtinchnones and phosphaalkenes or phosphaalkynes provide a direct synthesis of 1,3-azaphospholes (240) (Table 10.7). The intermediate cycloadducts cannot be isolated. The various phosphaalkynes were generated from phosphaalkenes or, in the case of methyli-dynephosphane (239, R" =H), by flash vacuum pyrolysis of either 239 (R" = f-Bu) or dichloromethylphosphine. 

TABLE 10.7. 1,3-DIPOLAR CYCLOADDITION REACTIONS OF MUNCHNONES AND PHOSPHAALKYNES... 

Members of the previously unknown class of heterocycles, 1,2,4-selenadiphospholes 33 and 69, have been prepared (Scheme 44). The thermal reaction of 1,2,3-selenadiazole 1 with phosphaalkynes 209 gave products 33 and 69 in 17% and 16% yields, respectively <1996PS99>. These compounds are proposed to form by a sequence of [3+2] cycloreversion and cycloaddition reactions (see also Section 6.12.5.9). 

A new synthetic approach to heterophospholes based on the [3+2] cycloaddition reactions of phosphaalkynes has been developed by Regitz and co-workers <1998S1305>. In specific examples, a mixture of tautomeric 1,2,4-diazaphospholes 56 was prepared by cycloaddition of phosphaalkyne 55 and diazo compounds, while 1,2,4-oxazaphospholes 57 were prepared similarly from phosphaalkyne 55 and appropriate nitrile oxides (Scheme 1) <1998S1305>. 

In contrast to cycloaddition reactions of phosphaalkenes, cycloaddition reactions between phosphaalkynes and other unsaturated systems are comparatively rare. Indeed, there are only a limited number of reports for monophos-phacyclobutadiene) complexes, which are obtained from the corresponding phosphaalkyne. Relatively recently, the reaction of phosphaalkynes with highly electron deficient alkynes was reported <19990M4838>. Treatment of a CF3C=CGF3-coordinated dimeric rhodium complex with phosphaalkynes in hexane at — 20°C followed by warming to room temperature afforded the red air- and moisture-stable phosphete complexes 60 in ca. 50% isolated yields. When phosphaalkynes are allowed to react with a kinetically stabilized cyclobutadiene, 2-Dewar-phosphinines, for example 93 (Equation 30), are obtained <1998S1305>. 

Cycloaddition reaction of phosphaalkyne 42 and bis(2-/-butyl-4,5,6-trimethylphenyl)germylene 43 leads to germadiphosphacyclobutene 24 in 59% yield (Equation 5) <2001CC215, 2002JOM(646)39>. 

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... 

Various bicyclic phosphaalkenes 21-24 were synthesized from oxirane 118. Acceptor-substituted oxiranes underwent a ring opening under thermal stress to give the unstable carbonyl ylides. It was shown that upon heating 2,3-diphenylindenone oxide 118 underwent conversion into ylide 119, which can react with various phosphaalkynes in [3+2] cycloaddition reaction to give 21-24 <2000T6259>. 

The availability of phosphaalkenes and phosphaalkynes has led to a further route for the synthesis of phosphiranes and phosphirenes by the formal addition of carbenes or carbenoides to P-C multiple bonds. An example already depicted in Scheme 6 involved in the [2+1] cycloaddition reaction of a stable phosphinotrimethylsilylcarbene to tert-butylphosphaalkyne <1995JA10785, 1999CEJ274>. A carbenoid was also used in the synthesis of an unusual phosphirene from a siloxy-substituted phosphaalkene (Equation 30) <1997JOM(529)127>. 

This route involves the cycloaddition of phosphaalkynes RC=P or phosphaalkenes R(TMS)C=PC1, which lose TMSCl in the course of the reaction. Reaction partners are 1,3-dipoles such as nitrile ylides, or their precursors. 

In spite of their sterically demanding substituents, the phosphaalkynes chibit an enormous potential for cycloaddition reactions. Even though no intermediates can yet be detected in their thermal cyclooligomerization reactions, the obtained product palette allows the assumption of head-to-head and head-to-tail dimerizations, i.e. [2 + 2]-initiating reactions. 

Compound 19, which is obtained in almost quantitative yield, reacts differently from the simple germaethenes. Although these molecules undergo smooth cycloaddition reactions with phosphaalkynes or with dimethylbutadiene [10], the conjugated Ge-C double bonds do not react... 

MO studies have shown that the Diels-Alder reaction of substituted selenocar-bonyl compounds with buta-1,3-diene or 2-methoxybuta-1,3-diene proceeds through a concerted, asynchronous transition state. Q ,jS-Unsaturated seleno ketones and seleno aldehydes readily undergo 4 - - 2-cycloaddition with alkenes and 4 - - 2-dimerization. The reaction of phosphaacetylene (138) with buta-1,3-diene produces triphosphatri-cyclooctenes (139) through a sequence involving Diels-Alder, ene, and intramolecular 4 -I- 2-cycloadditions (Scheme 53). The 4 + 2-cycloaddition of phosphaalkynes with 5,8-bis(trimethylsilyl)cycloocta-l,3,6-triene (140) readily yields the tricyclodecadiene (142) via the bicyclic intermediate (141) (Scheme 54). "... 

With substrates featuring a more energetic, multiply unsaturated polarized bonding scheme, like carbodiimides or isocyanides, a [2+2] cycloaddition reaction to the C=Y bond subsequent to the preliminary nucleophilic attack of the benzyl ligand could be observed [68] (Scheme 17). The same cycloaddition was also observed with those substrates and the corresponding iodinated yttrium complex, which also reacted with tert-butyl phosphaalkyne [67]. However, no subsequent [2+2] cycloaddition was observed with pivalonitrile or adamantly azide on the benzylated complex. 

Regitz et al. described [4 + 2] cycloaddition reactions of phosphaalkynes with cyclopentadienones or pyrones under subsequent exclusion of CO and CO2, respectively. In this way, alkyl-substituted phosphinines, such as 4, are synthetically accessible (Scheme 6.3) [25, 26]. 

Similarly, the (phosphino)(silyl)carbene 2a reacts at -30°C with a slight excess of the tert-butylphosphaalkyne cleanly affording the 2-phosphino-2//-phosphirene 34.53 The reaction leading to 34 is strictly analogous to that observed on reacting the transient dichlorocarbene with the tert-butyl-phosphaalkyne, in which the 2//-phosphirene 36 was obtained.54 The three-membered heterocycle 34 appeared to be rather unstable and rearranged, quantitatively, to afford the lA5,2A3-diphosphete 35 after 3 h at room temperature.55 Once again, these results as a whole indicate that a concerted [1 + 2]-cycloaddition process is involved in the formation of the 2//-phosph-irene 34. 

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