Ylenes = Ylides

Phosphorus atoms in ylides largely retain the tetrahedral ligand geometry of the phosphonium cation precursor, but the bond to the ylene-ylide carbon atom is shortened, indicating an increase in bond order. 

More recent developments are based on the finding, that the d-orbitals of silicon, sulfur, phosphorus and certain transition metals may also stabilize a negative charge on a carbon atom. This is probably caused by a partial transfer of electron density from the carbanion into empty low-energy d-orbitals of the hetero atom ( backbonding ) or by the formation of ylides , in which a positively charged onium centre is adjacent to the carbanion and stabilization occurs by ylene formation. 

NMR spectroscopic studies f111,13C, and 31P) are consistent with the dipolar ylide structure and suggest only a minor contribution from the ylene structure.234 Theoretical calculations support this view.235 The phosphonium ylides react with carbonyl compounds to give olefins and the phosphine oxide. 

Phosphonium ylides, which can be written in the two familiar canonical forms, are available with a wide variety of substituents both at the phosphorus and at the carbon atoms (Scheme 30). In gold complexes, without any exceptions, they function as two-electron donors, as proposed by the dipolar form to give discrete Au-C cr-bonds (771, monohapto). No side-on, 7r-coordination (t 2), as might be expected out of the ylene form, has been observed to date. 

The bonding characteristics of phosphorus ylides is a matter of continuos discussion in the literature. This bonding is most frequently referred to as ylide, ° while sometimes the ylene description is also used (Scheme 2). Some characteristics of the X -P=C... 

Fig. 5 Schematic representation of the ylene/allene form vs double ylide structure of CL2 compounds which is used for the formulas of the experimentally observed compounds...

Heterocyclic phosphorus ylides are a rather diverse and little known class of compounds. A variety of such structures are now known and in some cases these are of considerable synthetic value. In this chapter we have attempted to review all heterocyclic compounds containing one or more exocyclic phosphorus ylide functions, i.e. of general structure 1. It should be noted that in many cases these exist predominantly in the phosphonium ylide (P+—C ) form but for simplicity they are represented in the ylene form 1. Cyclic ylides 2 and 3 in which the phosphorus atom is within the ring are not included. 

The structural analyses in this chapter present a fairly unihed description of the bonding in phosphonium ylides. The bonding in these compounds can best be described by the ylide resonance form A with a small contribution from the ylene form B. The evidence for this conclusion is summarized below. 

EPR studies of two radical cations of ylides show little delocalization of the radical on to phosphorus, discounting the ylene structure. 

Schier and Schmidbaur93 performed a clever experiment that addressed part of this question does the orientation of the carbanion relative to the phosphorus atom play any role Scheme 2 shows two syntheses of ylides involving cyclopropyl substituents. In the first reaction, since the pKa of cyclopropane is considerably below that of propane, the expected product is the cyclopropylide. However, the isopropylide is the only recovered product. The second reaction also demonstrates the avoidance of the cyclopropylide product. The cyclopropylide possesses a very pyramidal carbanion that is directed away from phosphorus, allowing for minimal orbital overlap. The isopropylide is much less pyramidal and phosphorus can better assist in stabilizing the carbanion. While this stabilization does not require explicit orbital overlap (the electrostatic interaction of the carbanion with the onium is expected to be smaller in the cyclopropylide since it is directed away from P), it does suggest that some orbital interactions are involved. Hence, although the ylene contribution is small, it is unlikely that the ylene contribution is nil. 

Thermal treatment of carbethoxymethylene- or/j-bromobenzoylmeth-ylene triphenylarsorane (25) with dimethyl acetylenedicarboxylate gave rise to stable ylides (97). 

The bonding in certain tricoordinate phosphorus compounds (ct , 5-P) is also termed hypervalent (DA Scheme 5). In these structures the lone pair of the planar phosphorus is effectively involved in the delocalization (IIB Scheme 5). Bis-meth-ylene-phosphorane (and its N, O, and S analogues) are typical examples for such bonding (Scheme 5). Like in the case of the ylides, no -orbital participation is needed to describe the bonding in these compounds. Rather, the delocalized 3c-4e model can again be used [33], which also accounts for the positive charge at the central phosphorus atom, in accordance with the ylidic resonance structure (IIC Scheme 5). 

Scheme 3 Ylide and ylene bonding description with cri.X3- ...

The ylide-ylene carbon atom is found to be the center of a trigonal-planar arrangement of ligands. This planar configuration may be part of a it system, thus giving rise to restriction in bond rotation and to the existence of geometrical isomers. 

Dipole moments of ylides, although measured only with some of the more complicated species (31), indicate a high degree of charge separation in the ylidic function, ranging between 5 and 9 D. These results strongly favor the zwitterionic (ylide) over the double-bond (ylene) formula. 

In this article the author has been using the traditional ylide-ylene nomenclature as well as the modern phosphorane-arsorane-stiborane formalism. No attempt has been made to discriminate between one or the other of these modalities because it is felt that both offer a sufficiently clear description of at least those species that are derived from the main Group Vb elements. (Fortunately no Nli,s or BiR5 had to be given names.). However, when this procedure was followed with the vanadium, niobium, and tantalum compounds, an interesting problem arose because the authors... 

It should be remembered that the heteroatom under scrutiny belongs to the second full period of the periodic table of the elements. Thus, the double bond of an ylene form is not a p, p%, but a d%, p% double bond. Though a d%, p% double bond is not very stable, the ylene resonance should make some contribution to the electron distribution in the ylides of Figure 11.1. One reason is that this resonance form must does not suffer from charge separation, as does the ylidic resonance form. 

Synthesen fiber Stickstoff-ylide und Phosphor-ylene. G. Wittig, Experientia XII, 41 (1956). 

The ionic representation of the ylides in Figure 9.1 shows only one of two conceivable resonance forms of such species. In contrast to the N atom in the center of the N ylides, the P or S atoms in the centers of the P and S ylides (Figure 9.1) may exceed their valence electron octets and share a fifth electron pair. For P and S ylides one can therefore also write a resonance form with a C=P or C=S double bond, respectively these are resonance forms free of formal charges (Figure 9.1). For the sulfoxonium ylide there is a second resonance form in which the S atom exceeds its valence electron octet however, this does contain formal charges. Resonance forms of ylides in which the heteroatom exceeds its valence electron octet are called ylene resonance forms. The ene part of the designation ylene refers to the double bond between the heteroatom and the deprotonated alkyl group. 

Nevertheless, it is doubtful as to whether the negative charge on the carbanionic C atom of P and S ylides is indeed stabilized by partial double bond formation to the heteroatom, as is expressed by the ylene resonance form. Probably, the formal charge at the carbanionic center of these ylides is rather stabilized to a considerable extent by two other effects ... 

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