Phosphaalkenes cycloaddition

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>. 

These phosphaalkenes are extremely reactive, they undergo facile [2 + 4] cycloaddition reactions (equation 39) or reactions with protic acids... 

Cycloaddition phosphathioketenes, 33 332 reactions, in preparation of fluoroalicyclic derivatives, 15 327-328 reactions, of iminoboranes, 31 159-165 [2-1-2] Cycloaddition phosphaalkenes, 33 271-272 phosphathioketenes, 33 334 [2-t-l] Cycloaddition, phosphaalkenes, 33 269-271... 

Diazomethylene)phosphoranes 33 (Scheme 8.10), which represent another type of diazocumulenes (12) are easily obtained by the oxidative ylidation of the corresponding phosphanyl(trimethylsilyl)diazomethane with CCI4. The increased stability of these compounds as compared with diazocumulenes (R2C=C=N2) is probably due to the ylidic character of the P=C bond. These diazo compounds exhibit the expected dipolar reactivity toward electron-deficient alkenes, alkynes, phosphaalkenes, and heterocumulenes (12). Thus, 33 reacts with TCNE to form A -pyrazoline 35 (60). Furthermore, 33 could be converted into the phosphonio-borate-substituted diazo compound 34, which underwent subsequent cycloaddition with electron-deficient alkenes (e.g., 34 36) (61). 

Phosphaalkenes that possess a A, a -phosphorus atom can be isolated when appropriately substituted (151). These systems exhibit a much more expressed dipolarophilic than dienophihc reactivity, probably as a consequence of the polarity of the P=C bond. The [3 + 2] cycloaddition of diazo compounds with phosphaalkenes 87 leads to 4,5-dihydro-3//-l,2,4-diazaphospholes 88 (Scheme 8.21) that are not always isolated. Quite often, ehmination of molecular nitrogen occurs during the cycloaddition at or below 20 °C. In other cases, N2 extrusion is achieved at... 

The cycloadduct obtained from ethyl diazoacetate and the cyclic phosphaalkene 9-ferf-butyl-1,3-diphenyl- 10-phospha-1,3-etheno- 17/-benzopyran-4(37/)-one underwent spontaneous [3-1-2] cycloreversion and produced ethyl 5-tert-butyl-1,2,4-diazaphosphole-3-carboxylate (163). Still another transformation was found for P-trimethylsilyl-substituted diazaphospholes system 94, which suffered dediazonia-tion under the cycloaddition conditions and yielded phosphaalkene 95 (162) (Scheme 8.22). It was proposed that N2 extrusion and SiMe3 migration occur in concert. On the other hand, the cycloaddition products derived from phosphaalkene 93 and 2,2-dimethyl-1-diazopropane or diazo(trimethylsilyl)methane simply underwent tautomerization to the corresponding A -phosphapyrazoline (162) (94, R = f-Bu H shift R = SiMe3 SiMe3 shift). 

In certain cases, 4,5-dihydro-1,2,3-diazaphospholes rather than 3,5-dihydro-1,2,4-diazaphospholes are formed from the [3-1-2] cycloaddition reaction of diazo compounds with phosphaalkenes. This regiochemistry was encountered in the reaction of (mesityl)P=CPh2 with diazodiphenylmethane and was attributed to steric factors (164). Electronic factors may explain the orientation found in the... 

Similar to their reaction with phosphaalkenes, l-diazo-2-(oxoalkyl)silanes 29 react with various heterophospholes by [3 + 2] cycloaddition of the diazocumulene system 30 (which is in equilibrium with 29) across the P=C bond. With 2-acyl-1,2,3-diazaphospholes 119 (R = Ac, Bz no reaction with R = Me, Ph up to 60 °C), the expected cycloaddition products 120 (Scheme 8.27) could be isolated (186). Elimination of N2 from these bicyclic A -pyrazolines occurred upon heating at 100 °C and furnished the tricyclic systems 122 when SiRs was a trialkylsilyl group. Apparently, the thermolysis of 120 generates the 5-aIkenylidene-l,2,5-diazaphosphole 121 (by N2 extrusion) as well as diazaphosphole 119 (by a [3 + 2] cycloreversion process), which recombine in an intermolecular cycloaddition to furnish 122. When SiRa = SiPhaf-Bu, a formal intramolecular [3 + 2] cycloaddition of the C=P=C unit with an aromatic C=C bond occurs and the polycyclic compound 123 is obtained (187). 

The P=C bond of ), a -phosphoranes is also accessible to a 1,3-dipolar cycloaddition reaction. The formation of 97 (Scheme 8.23), of 3,5-dihydro-1,2,4-diazaphosphole sulfide 132 from an amino(methylene)thioxophosphorane (190) and of phosphirane imine 133 from an amino(imino)alkylidenephosphorane (191) (Scheme 8.29) illustrate the chemical behavior of phosphaalkenes containing a A, a -phosphoms atom. 

In contrast to the P=C bond of phosphaalkenes (Section 8.2.2), double bonds between phosphorus and a heteroatom have not been used much as dipolarophiles. Most of the studies reported so far were devoted to the reactivity of the (X )P=N bond of iminophosphanes. Amino(iminophosphanes) react with diazoalkanes to form 4,5-dihydro-3//-l,2,3,4-triazaphospholes or, by N2 loss from the latter, to imino(alkylidene)-X -phosphoranes (5,238). With P-halogeno-(arylimino)phos-phanes 174 and the appropriate diazo compounds, 3//-l,2,3,4-triazaphospholes 175 (167) and 176 (239) (Scheme 8.40) were obtained as the major products after cycloaddition and eliminative aromatization. 

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. 

The synthesis and structural study of the stable P-heterocylic carbene 49 and related structures (e.g., structures 48 and 52 see Figure 3) have attracted some recent research activity <2005AGE1700, 2002JA2506, 2006AGE2598, 2006AGE7447>. The synthesis of the stable P-heterocylic carbene 49 was accomplished in two steps (1) a formal [3+2] cycloaddition of the readily available phosphaalkene 123 with acetonitrile in the presence of silver triflate afforded salt 124, and (2) the isolated and recrystallized salt 124 was deprotonated by lithium hexamethyldisilazide in tetrahydrofuran (THF) to afford carbene 49 as relatively stable light-yellow crystals (Scheme 10) <2005AGE1700>. 

Cycloaddition reactions between phosphaalkenes or -alkynes and activated alkenes or alkynes are also an available synthetic route to the four-membered rings. These reactions afforded not only phosphorus-containing rings, such as phosphetanes, phosphetenes, and phosphetes, but also arsine-containing rings. 

Dehalogenation of P-fluorinated As-phosphaalkenes and cyclodimerization of the resulting intermediates is a useful route for the preparation of lAs,3As-diphosphetes 16 <1998ZFA1116>. Cycloaddition reactions of unsaturated organophosphorus compounds and their heavy atom analogues are the most frequently used preparative route to... 

Hydrostannylation of a P-chloro phosphaalkene and subsequent R3SnCl elimination is a low-yield route to specific lA3,3A3-diphosphetanes <1998HAC453>. Hydrostannylation of the P-alkynes 44 generates the related stannylphosphaalkenes which react with the starting phosphaalkynes in a [2+2] cycloaddition to form... 

In ////-prepared l-adamantyl-2-diphenyl-phosphaalkene dimerizes spontaneously and forms an 1,2-diadamantyl-lA3,2A3-diphosphetane <2006IC5225>. Repetitive cycloaddition reactions between trimethylsilylphosphaalkyne 44f and cyclopentadiene, in a molar ratio of 3 2, lead to formation of the polycyclic triphospha compound 91 <1999EJ0363>. [WICOATHF] triggers [2+2] cycloaddition reactions of Mes -arsaalkyne 92 by rr-complexation... 

---