Phosphaalkynes cycloaddition

Treatment of the vinylcarbene chromium complex 97 with /-BuC=P affords the dihydrophosphetylketene complex 98 (Scheme 26). This transformation is believed to proceed via r -phosphaalkyne carbene-, phosphaalkenylcar-bene-, and phosphaalkenylketene complexes as intermediates. An intramolecular [2+2] cycloaddition completes the reaction sequence.53 Different carbene/carbon monoxide/phosphaalkyne cycloaddition products (e.g., 1,3-oxaphospholes, phosphaphenanthrenes) are obtained depending on substitution at the carbene ligand (vide infra). 

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... 

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. 

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]. 

Turning from iminophosphanes to alkylidenophosphanes (phospha-alkenes), the orientation of the [2 + 2]-cycloaddition is inverted, as far as phosphorus is concerned only one example has been worked out (product VIII) 19). The phosphaalkyne iBuC=P does not react with the iminoborane BuB=NtBu, which instead trimerizes (IS). An exotic [2 + 2]-cycloaddition is observed when the very reactive titanaethene... 

Bis- and tris(trimethylsilyl-diazomethyl)phosphines undergo a double and triple regiospecific cycloaddition with a phosphaalkyne <88TL925>. 

Phosphaalkynes of the type RC=P, featuring a three-valent phosphorus atom with coordination number 1 (A, a -P), represent novel organophosphorus compounds. Their chemistry has been extensively investigated since 1981, when the first synthesis of a kinetically stabihzed phosphaalkyne (t-BuC=P) was reported (283). Several reviews on the cycloaddition chemistry of these compounds with diazo compounds have been published (284—286). 

When phosphaalkynes are exposed to bis- and tris(diazo) compounds, bis- or tris(l,2,4-diazaphosphol-5-yl) compounds are formed that may be further converted into a variety of novel heterocyclic systems. For example, bis- and tris[diazo(tri-methylsilyl)methyl]phosphanes 237 and 240 afforded bis- and tris(diazaphospho-lyl)phosphanes 238 and 241 after cycloaddition with terf-butylphosphaacetylene followed by a subsequent 1,5-silyl shift (Scheme 8.56) (300). Reaction with electrophilic halides at the Wsilyl functions allows the introduction of a heteroatom bridge between the diazaphosphole ring leading to polycyclic ring systems such as 239 and 242. 

The reaction of the electron-rich phosphaalkyne 243 with monosubstituted diazo compounds (R CH= N2, R = CF3, COjMe) is remarkable because it furnishes a mixture of the regioisomeric cycloaddition products (IH-1,2,4- and IH-1,2,3-diazaphospholes) where the normal regioisomer predominates (301). When... 

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... 

Regitz and co-workers (143) found that 2,3,4-tri-tert-butylazete reacts with isomiinchnones to give relatively labile cycloadducts. This group (153) has employed the cycloaddition of isomiinchnones 256 with phosphaalkynes 257 to prepare 1,3-oxaphospholes 258 (Scheme 10.35). This sequence is clearly the method of choice for the synthesis of the relatively little investigated 1,3-oxaphosp-holes. The presumed bicyclic intermediates could not be detected by NMR. 

Stannoles bearing dialkylboryl groups in the 3-position (145) react with phosphaalkynes R4C=P (R4 = t-Bu, CH2Bu-t) by [4 + 2] cycloaddition (isomeric adducts were not detected) and elimination of stannylene to give phosphabenzenes (146 and 147) in high yield205. 

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). 

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