Phosphaalkenes, formation

Because the main routes to phosphaalkene formation have already been published in diverse review articles (8-11), this report can be restricted to a short survey of the general principles. 

Equation 10. Phosphaalkene formation by silyl anion addition to a diphosphirenium salt. 

Another approach to a donor adduct of the methylene phosphenium cation is the addition of a phosphonium cation to the phosphaalkyne. The reaction of the protic cation [HPPhal + lCFaSOa] with CjoHuCP produced a white powder which was identified as the P-phosphonio-substituted phosphaalkene [74]. Alternatively to the elimination reaction the phosphaalkynes were protonated. C-protonation of adamantylphosphaacetylene and ferf-butylphosphaacetylene occurred in superacid media under formation of phosphavinyl cations. From these spirocyclic betaines by reaction of l-Ad-C=P (Ad = adamantyl) withB(OTf)3 a phosphavinyl cation could be detected [75]. 

The reactivity of the Cp (L)Ir=PMes (L=PPh3, CO) phosphinidene complexes is much less diverse than those with Zr. Only the formation of phosphaalkenes has been observed in the reaction with CH2I2 and CHI3 [102]. This reduced reactivity of the Ir complexes as compared to Zr complex 53 has been... 

Abstract Many similarities between the chemistry of carbon and phosphorus in low coordination numbers (i.e.,CN=l or 2) have been established. In particular, the parallel between the molecular chemistry of the P=C bond in phosphaalkenes and the C=C bond in olefins has attracted considerable attention. An emerging area in this field involves expanding the analogy between P=C and C=C bonds to polymer science. This review provides a background to this new area by describing the relevant synthetic methods for P=C bond formation and known phosphorus-carbon analogies in molecular chemistry. Recent advances in the addition polymerization of phosphaalkenes and the synthesis and properties of Tx-con-jugated poly(p-phenylenephosphaalkene)s will be described. 

Scheme 10. Formation of the phosphaalkenes 39 and 40 by reaction of the phosphasilene (15a) with mesitylisocyanide and 1,6-diisocyanohexane, respectively.

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. 

Flash vacuum pyrolysis of dichloroneopentylphosphine in the presence of magnesium gave a phosphaethene polymer. The formation of a phosphaethene polymer from phosphaalkene 55 had been noted elsewhere... 

Similar phosphetane ring formation by thermal rearrangement of spirodiphosphirane has been reported <1994IC596>. In this reaction, the phosphetane 100 with an exocyclic phosphaalkene moiety was obtained as a major product (70%) with 1,4-diphosphanorbornadiene 101 as a minor product (30%) (Equation 36). 

In the flash vacuum pyrolysis of dichloroneopentylphosphine, a phosphaethane polymer was obtained. The formation of a polymer from phosphaalkene 55 has been noted but was not characterized. These products probably result from phosphetane intermediate 57 (Scheme 4) <1997TL8417>. 

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... 

Subsequently, following the successful isolation of stable phosphaalkene-con-taining polymers 156, the use of the same substituent for the preparation of a related polymer featuring P=P moieties was attempted. It is known that dipho-sphenes M can be prepared by dimerization of transient phosphinidenes J generated by photolysis of phospha-Wittig reagents L (Scheme 4.43) [80e]. Photolysis at room temperature or thermolysis (neat, 250 °C, 2 min) of bifunctional compound 155 does indeed result in the formation of polymer 157 in near quantitative yield (Scheme 4.43) [80d], This soluble material was characterized by NMR spectroscopy and GPC analysis, which revealed a rather low molecular weight (Mn = 5900). The UV-Vis spectrum of 157 shows a jt-jt transition (435 nm) accompa-... 

The class of phosphaalkenes with isolated P=C double bonds was first synthes ized by Becker.33 His synthetic strategy starting from trimethylsilylphosphines and acyl chlorides is still the most versatile (Protocol 3). The principle is based on the easily achievable, 1,3-silatropic migration of a silyl group bonded to phosphorus to a doubly bonded element such as nitrogen, oxygen, or sulfur. The process is favoured energetically by the construction of the P=C double bond with concomitant formation of a very stable silicon-element bond. 

The first 2/f-l-aza-2-phosphirene complexes 8a,b result from the action of amino-carbene complexes 5a,b with ClP=C(SiMe3)2 in the presence of triethylamine. A rationale for the products 8a,b invokes the initial formation of carbene complex-functionalized phosphaalkenes 6a,b, which undergo 1,3-hydrogen shifts and isomerization of the transient iminophosphanes 7a,b. Intermediates, however, are not detectable see Eq. (3).7... 

Reaction of Cp (CO)2FeP(SiMe3)2 with 3 ClP=C(SiMe3)2 in THF at 20°C leads to the formation of the red crystalline 3-methanediyl-l,3,5,6-tetraphosphabicyclo[3.1.0]hex-2-ene compound 93. It is assumed that the initially formed ferriodiphosphapropene 14d is converted to 2,3,4-triphos-phapentadiene I by means of a second molecule of the chlorophospha-alkene. Condensation of I with the third equivalent of phosphaalkene generates transient 2,4,5,6-tetraphosphahepta-l,3,6-triene H, which cyclizes to product 93 (Scheme 24). 50... 

The reaction of 8 with MejSiCl leads to the formation of E/Z isomers (1 1) of the phosphaalkene 9 (Eq. 7), thus affording a novel access to phosphaalkenes. 

With respect to the reaction mechanism a nucleophilic attack of the bis(trimethylsilyl)phosphanide anion at the sp -hybridized carbon of diethyl carbonate, followed by an elimination of one molecule of ethoxy trimethylsilane and formation of a still undetected phosphaalkene is supposed. Splitting off a second molecule ethoxytrimethylsilane the P-C double bond of the intermediate is converted into a triple bond thereafter (Eq. 10). 

Nb and Ta derivatives are hard acids and then-complexes with P- or As-donors are limited. Tertiary phosphines, especially PMes, have been widely used to stabilize low-valent derivatives. C-H activation reactions, promoted by the formation of thermodynamically stable Ta-H, Ta-C, and Ta=C bonds have resulted in metallacycles based on unusual anionic phosphorus donors. Nucleophilic Ta phosphinidene complexes could be stabilized by a tripodal tetradentate [NN3] amido ligand. The terminal PR ligand reacts smoothly with aldehydes, providing a general synthesis of phosphaalkenes RP=C(H)R and act thus as a phospha-Wittig reactant see Phosphorus Organophosphorus Chemistry). 

Hydrozirconation see Hydrozirconation) of phosphai-mines, phosphaalkenes, or bis(imino)phosphorane takes place with the formation of various three- or four-membered phosphazirconacycles. ... 

The 4,5-dihydro-37/-l,2,3-diazaphosphole 34 produced by a [3+2] cycloaddition reaction of diphenyldiazomethane with a phosphaalkene is unstable and decomposes by extrusion of Na and formation of a new P-C bond yielding the phosphirane 35 (Scheme 14) <2001ZFA1241>. 

Condensation of the chloromethylene phosphanes C1P=C(R)(TMS) (R = TMS, Ph) with dihydrophosphasilete 20 proceeded quantitatively with the formation of phosphaalkenes 21a and 21b as yellow oils. When heated to 180 °C for 2 h they underwent isomerization to the bicyclic diphosphiranes 22a and 22b (Scheme 7) <1999CEJ1581>. 

The same type of ring opening was observed when salt 91 was treated with lithium amides or mesityllithium in THE at -78 °C. Addition of lithium diisopropylamide to 91 cleanly regenerated phosphaalkene 90 in 90% yield. When lithium dicyclohexylamide was used, a 50 50 mixture of / -(dicyclohexylamino)phosphaalkene 156 and its isomer 157 was isolated. Monitoring this reaction by P NMR at -78 °C proved the initial formation of 156 with phosphaalkene 157 only appeared on warming to room temperature. Lastly, yellow, oily 7 -(mesityl)phosphaalkene 158 resulted from the combination of 91 and mesityllithium (70%) <1994JA6149> (Scheme 51). 

The bis(methylenephosphonium ylide) platinum complex (61) reacts with 3-butyn-l-ol to give the vinylphosphonium salt (62) and evidence is presented for intermediate formation of the carbene complex (63). Phosphoranylidenephosphine complexes (64) have been synthesized and shown to undergo "phospha-Wittig" reactions with aldehydes to give the phosphaalkene complexes (65) which can be isolated or trapped. ... 

Strong Evidence for an Unconventional 1,2-(C->P)-Silyl Migration Formation and Reactions of a P-Silyl Phosphaalkene Complex... 

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