Reactivity dicoordinated

This chapter will be limited to the preparation and properties of compounds (2) with one or two P—C bonds, since only these compounds contain functional groups of phosphorus in the sense of the Patai series. Thus, important classes of tervalent phosphorus acid derivatives with three electronegative groups, e.g. phosphites (3), tris(dialkylamino)phos-phines (4) and phosphoramidites (5), will only be included for illustration of a reaction or property which is common to tervalent phosphorus acid derivatives but has not been sufficiently studied for compounds with a P—C bond. The chapter will cover the highly reactive, dicoordinated derivatives 6, but not diphosphines 1 (X = PR2) or diphosphenes 6 (X = PR). 

Ethanolysis of the a-ethynyl-substituted vinyl triflates 93-95 proceeds via the intermediacy of the cumulenic dicoordinated cations 96-98 (equation 17) and suggests the following stability order for the carbocations 98 > 96 > 9745. The same relative reactivity order was found for the triflates 99-10145. 

The scope and mechanism of ionic hydrogenation of iminium cations have been investigated for a CpRuH catalyst bearing a chelating diphosphine.64 The mechanism involves three steps hydride transfer (from the catalyst) to form an amine, coordination of H2 to the resulting ruthenium cation, followed by proton transfer from the dicoordinated H2 to the amine. The cationic intermediate [e.g. CpRu(dppm)( 72-H2)+] can be used to hydrogenate enamines provided that the latter are more basic than the product amine. The relative reactivity of C=C and C=N bonds in a, ft -unsaturated iminium cations has also been investigated. 

Aryl cations are highly reactive intermediates with two possible electronic configurations, the singlet and triplet states. The former is a localized cation with a vacant a orbital at the dicoordinated carbon atom, whereas the latter has a diradical character with single occupancy of the g orbital and the charge being delocalized in the... 

Sn NMR spectra of this compound indicate that it is less associated in solution, being present as a mixture of a symmetrical dimer and a trimer in chloroform-d and of the dimer, the trimer, and a tetramer in toluene-dg-41 The populated dimer is probably the 3,3-dimer with O-2 dicoordinate. Thus, these products could be the result of trapping of the more populated dimer in solution by reaction of reactive electrophiles with the more nucleophilic oxygen atom. Alternatively, the intermediate formed initially may rearrange during workup, as demonstrated for esterification of acyclic diols (see earlier).53... 

At the (100) surface of MgO, the ions are pentacoordinated this high coordination decreases the reactivity. In this surface, the oxygen dianions are less reactive than those at the protruded ridges of the TiO2(110) surface that are dicoordinated. MgO is less basic than Ti02-... 

The 177-1,2-azaphospholes are colorless to yellow solids or oils and are stable at ambient temperature <20008417, 1998ICA(270)273, 2003ZNB44>. Most of the reactions reported so far involve the dicoordinated phosphorus atom, which represents the reactive center of the molecule. l,3-Di(fet/-butyl)-5-phenyl-177-l,2-diazaphosphole reacts under mild conditions with alkyl triflates CF38O3R (R = Me, Et) to afford the P-alkylated 1,2-azaphospholium compounds la and lb in quantitative yields. With an equimolar amount of iron nonacarbonyl, the Tj -complex 2 was selectively formed in 63% yield (Scheme 1) <2002EJ01664>. 

Another important aspect of these structures is related to the geometries of the dimers or terminal units of oligomers of stannylene acetals. ° The tin atoms adopt a distorted trigonal bipyramidal geometry with the alkyl groups in equatorial orientations. The tricoordinate oxygens are equatorial to one tin atom, but apical to the other. The dicoordinate oxygen atoms are apical (see Scheme 5.1.7) and this may contribute to their reactivity. ... 

In any dimer, it seems likely that the dicoordinate oxygen atoms are much more reactive than the tricoordinate oxygen atoms for three reasons. They are sterically less hindered because they are flanked by one tin atom rather than two, each tin atom bearing two butyl or other alkyl groups that are orthogonal to the Sn—O plane. They are probably more nucleophilic because they are not involved in an additional dative bond to a second tin atom. The dicoordinate tin atoms are apical in the trigonal bipyramidal geometry, which also conveys reactivity. ... 

Charge densities calculated by MNDO for indolizine and its 2-phospha derivative (11) and 1-phospha derivative (12) show the rr-electron excess in the five-membered ring to increase in this order. This result suggests that the phosphorus atom in (11) will be less likely to act as a donor than in (12) <91CB475>. As a dicoordinate phosphorus atom lowers the 7r-electron density at the adjacent carbon atoms, 2-phosphaindolizines are less reactive towards electrophiles in position 1 and 3 than indolizines. 

Z. Rappoport, in Reactive Intermediates, Vol. 3, R. A. Abramovitch, ed., Plenum Press, New York, 1985, Chap. 7 Y. Apeloig and T. Muller, in Dicoordinated Carbocations, Z. Rappoport and P. J. Stang, eds., John Wiley Sons, New York, 1997, Chap. 2. 

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