N-Ylids

The only other reaction with an aromatic substance is the C-H insertion into ferrocene [85], giving 41,which illustrates the highly electrophilic character of the phosphinidene complex. Other aromatic C-H insertions have been observed, but these likely occur by means of intermediate P,0- and P,N-ylids,such as the reaction of (0C)5W=PR withbenzophenone and azobenzene that give 42 and 43,respectively [56a, 86]. 

C-H insertion also occurs in the reactions with acetone and acetophenone, presumably through the rearrangement of transient OH-substituted phosphi-ranes [87]. C-C insertions occur for diketones to give 45 and have been postulated to occur via initial 1,2-addition to the conjugated enol 44 [87]. Diimines 46 also undergo C-C insertions [88]. Based on a theoretical evaluation, the products 47 are considered to result from a 2,3-sigmatropic rearrangement of initial formed P,N-ylids. 

Das ans Benzyl-dimethyl-(2-oxo-2-phenyl-ethyl)-ammonium-broniid durch Dehydrobro-mierung mittels Natronlauge erhaltliche N-Ylid kann, nach Isolierung in wasserfreier Form, thermisch in fast quantitativer Ausbeute zu 2-Dimethylamino-l,3-diphenyl-l-oxo-propan (X = H R = CgH,) umgelagert werden2. 

I. Zugravescu, M. Petrovanu, N-Ylid Chemistry, McGraw-Hill, New York (1976), Chpt 2 (review) J Heterocyclic Chem 18 63 (1981)... 

Trost, B. M. Melvin, L. S., Jr. In Sulfur Ylides-, Academic Press New York, 1975. Johnson, A. W. In Ylide Chemistry, Academic Press New York, 1966. Zugravescu, I. Petrovanu, M. In N-Ylid Chemistry, McGraw-Hill New York,... 

Zugravescu, I. Petrovanu, M. N-Ylid Chemistry, McGraw-Hill New York, 1976. 

Benzothiazolium N-ylids, trapping with acetylenic esters, 56, 131 Benzothiazol-2-one, 4-bromo-3-methyl-, bromine migration, 59, 276 Benzothieno[2,3-(fJpyrimidine-2,4-diones, 55, 199... 

Oxo-6-phenyl- E16c, 691 (aus Pyrazolidin-N-ylid) lH-2,4-Diazepin 5-Methoxy- E16c, 185 (4-Nj —isoquinolin/ H3C-OH)... 

The most convincing evidence concerning the relative stabilities of thiophene S,C-ylids, S,N-ylids, and the sulfoxides comes from ab initio restricted Hartree-Foch molecular orbital (MO) calculations, in which the molecular parameters of the ylid 18 and the S-methyl thiophenium salt 19 were compared (84CC859). The ylid (18) has never been isolated, nor would one intuitively expect it to enjoy any real stability. However, it was chosen to simplify the MO calculations. The stability of ylids such as 14 and 15 is almost certainly due to the ability of these systems to delocalize the formal negative charge over several atoms, and clearly this facility is not present in 18. Thus, 18 represents a highly localized ylidic structure. In principle, the salt 19 can be... 

Fig. 2. Wavefunctions corresponding to the highest occupied molecular orbitals (HOMOs) of thiophene(a), a thiophene S,C-ylid (b), thiophene l-oxide c), and a thiophene S,N-ylid (d). [Figure 2a-c reprinled from Bowles ei al. (84CC859).]...

D. Reactions of Thiophene S,N-Ylids 1. Sulfur-Nitrogen Bond Cleavage... 

Thiophene 1-oxide is unusually reactive both as a diene and as a dienophile in the Diels-Alder reaction and thiophene S,N-ylids apparently exhibit similar reactivity. When electron-rich alkenes, (e.g., acenaphthalene) are treated with 60 (R = C02Et) (Scheme 10), a rapid reaction ensues to yield the product derived from a [4 -f- 2]-cycloaddition followed by cheleotropic elimination of a thionitroso compound. This reaction appears to be fairly general for nonhindered alkenes, and even relatively unreactive systems, such as thiophene itself, give low yields of 61. This is an unusual reaction... 

Although phosphorus ylids are most commonly used to alkenylation reactions, nitrogen ylids can occasionally be used. As an example, the reaction of A-benzyl-A-phenylpiperidinium bromide with base generated a N-ylid, which reacted with benzaldehyde to form styrene. The stmcture has been determined for an intermediate in an aza-Wittig reaction. 

Trimethylsilyl-methyl)-l,3-benzothiazolium-trifluormethansulfonate ergeben mit Butindi-saure-dimethylester in Gegenwart von Casiumfluorid l-( 1,2-Dimethoxycarbonyl-ethe-nylthio)-2-(3,4-dimethoxycarbonyl-pyrrolo)-benzole intermediar wird durch Desily-lierung ein N-Ylid freigesetzt972. 

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