Stabilized ylids

In almost all compounds that have pn-dn bonds, the central atom is connected to four atoms or three atoms and an unshared pair and the bonding is approximately tetrahedral. The pn-dn bond, therefore, does not greatly change the geometry of the molecule in contrast to the normal tc bond, which changes an atom from tetrahedral to trigonal. Calculations show that nonstabilized phosphonium ylids have nonplanar ylidic carbon geometries, whereas stabilized ylids have planar ylidic carbons. ... 

For successful reactions of stabilized ylids with ketones, under high pressure, see Isaacs, N.S. El-Din, G.N. Tetrahedron Lett., 1987, 28, 2191. See also Dauben, W.G. Takasugi, J.J. Tetrahedron Lett., 1987, 28, 4377. 

Calculations on two Wittig reactants, alkylidenetriphenylphosphorane (a non-stabilized ylid) and its benzylidene analogue (a semi-stabilized one), have been used to identify the origin of the product selectivities for the two classes. A planar transition state gives a trani-oxaphosphetane intermediate, while a puckered one leads to cis-. These two transition states were favoured by the semi- and un-stabilized reactants, respectively. 

When stabilized ylids such as Ph3P=CHC02Et are used, the cyclization of the open chain product to afford a C-glycoside can be effected by a base catalyzed... 

The reaction of DMAD with the cyanocarbalkoxy pyridinium methylide 39 in various solvents has also been studied.64 In benzene the expected indolizine 40 was obtained. In dimethylformamide, the car-bethoxy methylide gave some elimination of CN instead of COOR. However, in acetonitrile no indolizine was formed at all. The 1 1 crystalline adducts that were isolated were formulated on the basis of chemical and spectroscopic evidence as the highly stabilized ylids (41), formed by an internal rearrangement (Scheme 8). 

DEAD exothermically in benzene or DMF to give initially 489, which has three contiguous saturated nitrogen atoms and undergoes N—N bond fission to give the highly stabilized ylid 490, which was isolated as the stable radical cation 491. Analogous reactions with 486 were carried out. 

Two moles of aromatic aldehyde react with ethyl diazoacetate to form diastereomeric 1,3-dioxolanes.124 The reaction is catalysed by dirhodium(II) species, and proceeds via a carbonyl ylid. Stereo-control can be achieved using a bulky diazo substrate, and electronic effects of aromatic substituents are important. Different reactions show evidence of either a metal-stabilized ylid, a free ylid, or competition between the two. 

Selective reactions. Wittig reactions of 1 with an aldehyde are possible in the presence of keto, ester, and amino groups.2 Wittig reagents react under normal conditions with acid chlorides. The stabilized ylid I reacts preferentially with the acid chloride group of 4-lormylhcnzoyl chloride (2) lo give 3 as the major product. ... 

In contrast, the nonstabilized ylids (C6H5)3P=CH2 and (C6H3)3P=C(CH3)2 and even the moderately stabilized ylid (C6H5)3P=CHC6H5 react preferentially with the foemyl group of 2, even under salt-free conditions.4 Inverse addition is desirable to avoid presence of excess ylid. 

The fourth type of photoirradiated cationic initiator is dialkyl-4-hydroxyphenyl-sulfonium salt 47> (Table 1). Photoexcitation of 4 gives rise to the formation of a resonance-stabilized ylid 6 and an acid HX. 

We can divide ylids into two types those with conjugating or anion-stabilizing substituents adjacent to the negative charge (such as carbonyl groups) and those without. We call the first sort stabilized ylids, because the negative charge is stabilized not only by the phosphorus atom but by the... 

Stabilized ylids, that is ylids whose anion is stabilized by further conjugation, usually within a carbonyl group, give E-alkenes on reaction with aldehydes. These ylids are also enolates and were discussed in Chapter 27. 

These stabilized ylids really are stable—this one, for example, can be recrystallized from water. This stability means though that they are not very reactive, and often it is better not to use the phos-phonium salt but a phosphonate instead. 

This time the arrow is solid, not dotted, because this reaction really happens. You will discover in the next chapter that furans can also be made from 1,4-diketones so this whole process is reversible. The example we are choosing has other features worth noting. The cheapest starting material containing a furan is furan-2-aldehyde or furfural , a by-product of breakfast cereal manufacture. Here it reacts in a typical Wittig process with a stabilized ylid. 

Why does the stabilized ylid prefer to react with the double bond In order to understand this, let s consider first the reaction of a simple, unstabilized ylid with an unsaturated ketone. The enone 1 has two electrophilic sites, but from Chapters 10 and 23, in which we discussed the regioselectivity of j attack of nucleophiles on Michael acceptors like this, you would expect that direct 1,2-attack on the i ketone is the faster reaction. This step is irreversible, and subsequent displacement of the sulfide i leaving group by the alkoxide produces an epoxide. It s unimportant whether a cyclopropane prod- uct would have been more stable ihe epoxide forms faster and is therefore the kinetic product. 

Sulfoxonium ylids react with unsaturated carbonyl compounds in the same way as the stabilized ylids that you have met already do—they form cyclopropanes rather than epoxides. The example below shows one consequence of this reactivity pattern-—by changing from a sulfonium to a sulfoxonium ylid, high yields of either eposjde or cyclopropane can be formed from an unsaturated carbonyl compound (this one is the terpene carvone). 

The photolysis of dialkylphenacylsulfonium salts and dialkyl-4-hydroxyphenyl-sulfonium salts is different from that of triphenylsulfonium salts. The latter compounds undergo irreversible photoinduced carbon-sulfur bond cleavage the former compounds, however, react by reversible photodissociation and form resonance-stabilized ylids as shown in Fig. 5. Because of the slow thermally induced reverse reaction, only small equilibrium concentrations of the ylid and acid arc present during irradiation and the concentration will rapidly decrease when photolysis has been terminated. Therefore, in contrast to triarylsulfonium salt initiation, no dark reaction will continue after the irradiation step. 

Wittig, with a stabilized ylid, gives the -enal (A). The second, with an unstabilized ylid, i Z-alkene, and the final structure is an ,Z-diene. 

The product must be formed by a Wittig reaction of the usual sort (probably E-selective as this 5 a stabilized ylid) followed by base-catalysed isomerization of the alkene into conjugation with thi... 

The second reaction is a simple Wittig process. It stereoselectively gives the cis (Z-) alkene as it is a non-stabilized ylid (pp. 814—18). The three-dimensional stereochemistry is entirely controlled by the starting material. If we want a single enantiomer of the product, we must start with a single enantiomer of the aldehyde-ester. Asymmetry must be introduced before this reaction sequence. 

The second compound could be made by a Wittig reaction with a stabilized ylid and the required diol ikdehyde derived from an epoxy-alcohol and hence from an allylic alcohol by Sharpless epoxidation. 

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