Carbenes, coupling electrophilic

Photolytically generated carbene, as mentioned above, undergoes a variety of undiscriminated addition and insertion reactions and is therefore of limited synthetic utility. The discovery (3) of the generation of carbenes by the zinc-copper couple, however, makes carbene addition to double bonds synthetically useful. The iodo-methylzinc iodide complex is believed to function by electrophilic addition to the double bond in a three-center transition state giving essentially cis addition. Use of the... 

Stetter expanded Umpolung reactivity to include the addition of acyl anion equivalents to a,P-unsaturated acceptors to afford 1,4-dicarbonyls Eq. 5a [57-60]. Utilizing cyanide or thiazolylidene carbenes as catalysts, Stetter showed that a variety of aromatic and aliphatic aldehydes act as competent nucleophilic coupling partners with a wide range of a,p-unsaturated ketones, esters, and nitriles [61]. The ability to bring two different electrophilic partners... 

Arasabenzene, with chromium, 5, 339 Arcyriacyanin A, via Heck couplings, 11, 320 Arduengo-type carbenes with titanium(IV), 4, 366 with vanadium, 5, 10 (Arene(chromium carbonyls analytical applications, 5, 261 benzyl cation stabilization, 5, 245 biomedical applications, 5, 260 chiral, as asymmetric catalysis ligands, 5, 241 chromatographic separation, 5, 239 cine and tele nucleophilic substitutions, 5, 236 kinetic and mechanistic studies, 5, 257 liquid crystalline behaviour, 5, 262 lithiations and electrophile reactions, 5, 236 as main polymer chain unit, 5, 251 mass spectroscopic studies, 5, 256 miscellaneous compounds, 5, 258 NMR studies, 5, 255 palladium coupling, 5, 239 polymer-bound complexes, 5, 250 spectroscopic studies, 5, 256 X-ray data analysis, 5, 257... 

Addition of electrophilic carbenes to enamines usually does not proceed with good efficiency, very likely because of the disturbance by the Lewis basic nitrogen 15). If however the less basic enamide derivatives are used as olefins, high conversions to donor-acceptor cyclopropanes are possible. Thus cyclic carbamate 245, which itself originates from an oxycyclopropane, gives the bicyclic compound 246 almost quantitatively. Its cleavage with aqueous base provides lactone 247 that could be coupled with tryptophyl bromide to afford 248, a direct precursor of the alkaloid eburnamoni-ne 105>. 

Treatment of the yttrium(III) adduct 60 with potassium naphthalenide in dme-diethyl ether mixture results in deprotonation of the C4 carbon and migration to afford the abnormal carbene complex 63 (Fig. 13).72 The C2 binding carbon migrates from the yttrium(III) centre to the incorporated potassium(I) cation. The C4 carbanion forms a short bond with the yttrium(III) centre in the solid state (2.447(2) A) and exhibits a large jYc coupling constant of 62 Hz in solution. Complex 63 may be quenched with a variety of electrophiles. For example, reaction with Me3SiCl silylates the NHC backbone to afford 64. 

In the above cycloaddition reactions, carbene is generated in situ. A more convenient way is to use Simmons-Smith reagent ", which transfers methylene from methylene iodide and zinc-copper couple to a carbon-carbon double bond (Scheme 2.56). In the reaction in Scheme 2.56, free carbene is not generated. The intermediate is believed to be ICH2ZnI, which behaves as an electrophile known as carbenoid. 

Schrock carbene complexes may be identified by their alkyl substituents on the carbene carbon and an electropositive, early-transition metal. As shown in Figure 2, the Schrock complexes are considered a result of spin-coupling between the carbene triplet state and the two electrons on the metal [8]. The reactivity of the Schrock carbene carbon is nucleophilic [4]. The reaction involving bis(cyclopentadienyl)carbene titanium(IV) and the electrophile acyl chloride in Eq. (3) [9], for example, illustrates the nucleophilicity of... 

This chapter has shown the great control and selectivity of modern radical reactions, especially intramolecular radical coupling reactions. Carbenes, especially those derived from diazoalkenes, have also played a prominent role in organic synthesis. The chemical reactions illustrated in this chapter are excellent additions to the list of nucleophilic, pericyclic, and electrophilic reactions presented previously. This chapter concludes the methodology for generating carbon-carbon bonds. With the functional group reactions in the first part of the book, all the tools for pursuing a synthesis are at hand. 

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