Phosphaalkynes synthesis

A potentially useful synthesis of 177-1,2-azaphospholes by the reaction of alkynes with l-aza-2-phospha-4-vanada-2-cyclobutenes generated from R1N=VC13 and phosphaalkynes may be considered as an example of cyclic carbovanadation 4 (Scheme 50). 

To prevent the latter mentioned subsequent reactions, the bulky phos-phaalkyne Ph C P as well as tungsten alkoxides of reduced size as, e.g., [W2(ONp)6] were employed in these three-component reactions with no significant success [15]. The crucial steps for the side-product free synthesis of the phosphido complexes 18 are the introduction of a phosphaalkyne possessing a moderate steric bulkiness, which lies between those of f-BuC=P and Ph C P, and resulting from the P-NMR studies (cf. Eq. 5), a reaction temperature mode allowing the complete metathesis reaction to take place at very low temperatures over a long period of time until all the phosphaalkyne has been converted into the metathesis products (about 12 h) only then is the reaction mixture allowed to reach room temperature. We found that MesC=P meets these steric requirements, and the three-component-reaction between MesC=P, [W2(Of-Bu)6] and [M(CO)5(thf)] (M = Cr, W), carried out at -78 °C and warmed up to ambient temperature within 15 h, succeeded in the synthesis and isolation of the phosphido complex 18a,b (Scheme 2) [15]. Furthermore, if t-BuC=P is incorporated into these reactions, the steric requirements of the alkoxide dimer has to be slightly increased. Thus, f-BuC=P reacts with [W2(OPh )6] and [M(CO)5(thf)] (M=Cr, W) under the... 

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). 

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. 

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. 

The synthesis of alkali metal 1,4,2-diphosphastibolides parallels that of the 1,4,2-diphosphaarsolides 18 and 19. It is however regiospecific and no 1,2,4-isomer is formed. For the synthesis, a DME solution of lithium bis(trimethylsi-lyl)antimonide 31 (M = Li) is treated with 3equiv of the phosphaalkene 29. In the course of the reaction, the phosphaalkene 29 is converted to the phosphaalkyne 30 via the base-catalyzed elimination of hexamethyl disiloxane (Scheme 7). Alternatively, the phosphaalkyne 30 can be used directly in place of the phosphaalkene. After addition of TMEDA or 12-crown-4, the lithium 1,4,2-diphosphastibolide 22 (M = Li(TMEDA)2) or Li(12-crown-4)2 is isolated <1997JOM291>. 

Methylidynephosphine (HC=P), the parent member of this class of compounds, is isoelectronic with acetylene. It stands at the very beginning of the history of phosphaalkyne chemistry 5 two decades passed before the successful synthesis of the kinetically stabilized compound 9 (R = i-Bu, Scheme 2)19,20 that is employed most frequently for studies on this class of compounds. Figure 8.2 shows a survey of the general reactions of this highly reactive triple bond system. 

Phosphaalkynes 9 are obtained almost exclusively by P-elimination of hexa-methyldisiloxane from appropriately substituted phosphaalkenes 8 (for their synthesis, see Protocol 3). The original elimination from 8 (R = f-Bu)31 performed in solution at room temperature in the presence of sodium hydroxide was optimized (solid NaOH, temperatures between 160 and 180°C, vacuum distillation techniques) and also generalized. In particular cases, aluminium trichloride in dichloromethane has proved to be a useful reagent for the elimination.32... 

An excellent review has discussed the synthesis of organophosphorus cages employing phosphaalkynes as precursors.68 In order to avoid unnecessary duplication of coverage, the results here reported are restricted to transition-metal-assisted transformations. An intramolecular [2+4] cycloaddition between an 7 -phosphaalkyne and an V-cyclopentadienylli-gand to yield compound 134 is observed during the reaction of complex 114 with lithium cyclopentadienide, as shown in Eq. (29).56c... 

Tetrahydrophosphinines, synthesis with phosphaalkenes, 22-24 1,3,5,7-Tetraphosphabarralene, synthesis with phosphaalkynes, 55 Tetraphosphacubanes, synthesis with phosphaalkynes, 55, 57-58, 61 Tetraphosphanobornadiene, synthesis with phosphaalkynes, 59 l-Thia-2,4-diphosphole, synthesis with phosphaalkynes, 45... 

Phosphaallenes can be synthesized in the same way as 1,3-diaryl-substituted allenes following an aluminium-catalyzed propargyl rearrangement. Using sodium hydroxide-activated aluminium oxide (125), the synthesis is suitable on an enlarged scale without any detectable by-products [Eq. (63)]. A similar proton migration within a coordinated phosphaalkyne was reported recently (126). 

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>. 

P-Heterocycles, synthesis from phosphaalkynes 88AG(E)1484 90-CRV191. 

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