Phosphanes carbon compounds

In this lecture some new routes to phosphorus-carbon compounds with P-C multiple bonds, found in connection with our investigations on reactions of tertiary phosphanes with chlorinated carbon compounds, such as tetrachloromethane, hexachloroethane, phosgene, and isocyanide dichlorides are reported. Furthermore some stereochemical problems concerning this type of compound will be discussed. 

The method involving reductive C5HS removal using alkali metals can also be applied to other i)5-cyclopentadienyl transition metal compounds. In addition, other ligands such as phosphanes, carbon monoxide or dinitrogen can be used instead of olefins. 

Carbodiphosphoranes C(PR3)2 and related ligands CL2 which are also termed double ylides differ from the other carbon ligands which are discussed in this volume in the number of lone electron pairs at the carbon donor atom. Ylides, carbenes, allenylidenes, and cumulenylidenes have one lone electron pair but carbodiphosphoranes have two lone electron pairs with a and n symmetry. The bonding situation in a carbodiphosphorane (a special class of CL2 compounds, abbreviated as CDP) is best described in terms of donor-acceptor bonding between the phosphane ligands which serve as a donor and a naked carbon atom in an... 

The theoretical section introduced divalent carbon(O) compounds as molecules CL2 where the ligand L is a ct donor. In principle this task can be fulfilled by various neutral group 15 compounds (N2, nitriles, amines, phosphanes, arsanes, etc.), neutral group 16 compounds (sulfides, selenides, etc.) as well as by divalent C(II) with a free pair of electrons at a carbon atom, such as isonitriles, NHCs, carbenes, CO, ylides, etc. The neutral and isolable compound C2PPh3 [14,15] may also serve as a donor L to stabilize a carbon atom. 

In the group of Izod, the tris(phosphane oxide) 19 was 1,2-dilithiated by the reaction with two equivalents of w-butyllithium in THF at room temperature (Scheme 7). The similarity of the structural formula of compound 20 (Lewis formula) to 1,2-dilithium compounds found by Sekiguchi and coworkers (see Section n. E), where two lithium centres are bridging a C2 unit, is not maintained in the solid state. The X-ray structural analysis reveals a centrosymmetric dimer containing no carbon-lithium contacts (Figure 8). 

Attempts to prepare the A -benzylimine of hexafluoroacelone by treatment of the latter with (benzylimino)triphenyl-/. ,-phosphane give. /V-benzylidene-1,1,1,3,3,3-hexafluoropropan-2-amine [(CF,)2CHN = CHPh] in 56% yield, which results from complete isomerization of /V-benzyl-2,2,2-trifluoro-l-(trifluoromethyl)ethanimine [(CF3)2C = NCH,Ph] under the reaction conditions.12 Taking this into account, the reactivity of substituents R at the imine carbon of compounds 14 follows the order perfluoroalkyl > > aryl > > hydrogen > alkyl benzyl.14 11 as described below, alkoxycarbonyl as the R group in structure 14 (x-oxo ester derivatives) activates as effectively as perfluoroalkyl. 

The majority of the publications in the field of oxidative addition of HFA to tertiary phosphanes deal with the formation and properties of l,3,2-A -dioxaphospholanes 57 (60,61). These compounds are generally stable when the a carbon atoms do not bear hydrogen atoms. Heterocycles with a hydrogen atoms are stable up to about 70°C at this temperature the isomeric phosphetanes are formed. The syntheses of some sterically hindered phosphoranes require drastic conditions. 

Reaction of the bromotrifluoromethane/tris(diethylamino)phosphane system with benzalde-hyde leads to the formation of the phosphonium salt 19, which decomposes at 140 C, affording the bromophenylethanc 20 and not the alcohol. Due to the ease of formation of a phosphorus compound with a P = 0 bond, in this case the intermediate carbenium ion reacts with the bromide anion, giving rise to the final reaction product. The reaction is general in character pcrfluoroalkyl bromides with different carbon-chain lengths (from 2 to 6) have been used as reactants. 

The reaction of triphenyl-/ -phosphanes with carbonyl compounds, the Wittig olcfmation reaction, involves addition to the carbonyl carbon in a reaction closely related to the aldol condensation. Triphenyl-2 -phosphanes, derived from alkyltriphenylphosphorus salts, react with 1,1,1-trifluoroacetonc or other trifluoromethyl ketones " - to give products 29. 

An interesting class of compounds is the motif displayed in Fig. 5.19, where two phosphorus atoms sit in the 1 and 3 posihons of a cyclobutane system, with the two carbon atoms each formally possessing an unpaired electron. Nonetheless, the molecules are diamagnetic and show phosphorus resonances that are characterishc of phosphanes rather than phosphinidenes. 

Theoretical calculations show that the diradical form is usuaUy more stable than the equally possible /l -phosphane species seen at the far right of Fig. 5.19. Structures with a carbon carbon single bond forming a bicyclic compound and aromatic 6. -electron system involving both phosphorus lone pairs are predicted to be even less stable. 

The common link between the two areas of research mentioned above (carbon cages and polycyclic phosphanes) is provided by the phosphorus-carbon cage compounds — the topic of the present chapter. In accord with the philosophy of this book, we will concentrate mainly on the results from our own laboratories and outline their position in the wider perspectives... 

The aldol reaction between ynones 81, as source of nucleophile, and aromatic aldehydes have been successfully performed using catalyst 79c, giving as the main product anti-%2 with good diastereo- and enantioselectivities (Scheme 4.26). The best results were obtained for the less bulky ynone (81a, R =Me) [148]. The instability of compounds 82 led to their transformation to the corresponding 3-oxotetrahydrofuranone derivative by addition of an alkoxy moiety at the a-position of the triple carbon-carbon bond, catalyzed by phosphane compounds. 

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