Remote carbenes

By convention, the carbenes displayed in Fig. 1.7 are normal NHCs as they are coordinated to the metal centre throngh the atom. By contrast, abnormal (also named non-classical or unnsnal) are those bound through the atom. Abnormal is also a term used for NHCs having a valence representation requiring additional charges. Remote is a term nsed to describe a carbene which does not have any heteroatom on the a-position to the carbenic carbon (Fig. 1.8) [57]. 

The acid was added to copper wool and rinsed down with DMSO. Within 20 s, the contents of the flask were ejected and the neck was distorted by intense heat [1], Adding the copper wool to a solution of the acid in the solvent gave an exothermic but controlled reaction [2], (Formation of a reactive carbene species by dehydrohalogenation seems a remote possibility.)... 

Substituent G has no drastic influence on the energy associated with equation 3 since G is remote from the terminal end of vinylidene. This is not the case for equation 4 where a substituent G can be on the carbene carbon (equation 5). 

Recently a new type of metal bonded IV-heterocycUc carbenes named remote Af-heterocyclic carbenes (rNHCs) has been described [138]. In contrast to normal NHCs, rNHCs possess no heteroatom adjacent to the carbene carbon atom. The less efficient stabilization of the carbene carbon atom has so far prevented the isolation of a free rNHC which consequently have to be stabilized by coordination to a suitable metal center. 

Alkylidenes (alkylidene carbenes) are valence isomers of alkynes. They have been prepared by alkyne pyrolysis, by homologation of ketones, and by generation of alkenyl anions bearing oc-leaving groups. Generated by any of these means, an alkylidene will insert intramolecularly into a remote C- H bond to form a new C—C bond and thus a cyclopentene. A concerted two-electron process, this reaction proceeds with retention of absolute configuration at the C - H site. 

In general, the simple thermal insertion of an unsubstituted alkyl carbene or carbcnoid into a remote C —H bond is not a useful synthetic method. Activation of the C —H bond by an a-alkoxy group, as with the cyclization of 5,5-bis(phenylthio)pentanol, makes this process more efficient31. 

These observations clearly support the naive idea that the para substituents exert almost no effect at remote carbenic center of brominated DPCs, at least in terms of steric congestion. Furthermore, electronic effects do not seem important enough to change the reactivity of brominated DPCs so significantly. For example there is rather small distribution in the Hammett and spin-delocalizing substituent constants. 

The addition of a carbene unit to the double bond of silyl-substituted dienes has been executed. It is noteworthy that the addition reaction occurs at the double bond more remote from the silyl substituent. Further reactions of these cyclopropyl products 188 with AgBF4... 

The zirconoxycarbene complexes undergo a variety of typical Fischer-carbene reactions. Typically, unsaturated nine-membered zirconoxycarbene complexes such as 103 are readily deprotonated in the a-position to the carbene carbon atom. The stereochemistry of the subsequent alkylation reaction is very efficiently controlled by the remote stereogenic center at C2, resulting in an effective 1,5-asymmetric induction113 (Scheme 34, Fig. 10). 

In a related observation, Furstner and co-workers reported that while both the FMC and the MC2 lead to the same E Z alkene ratio in the case of 18-RCM, the use of the same conditions with 19 leads to product enriched selectively in either of the two isomers <02MI657> depending on catalyst selection (Scheme 9). Furstner notes that this "illustrates the subtle influence of remote substituents on the stereochemical outcome of RCM in the macrocyclic series." Studies concerning the effects of chelation of the reactivity of ruthenium carbene complexes were reported by Furstner and co-workers <02OM331>. Paquette and co-workers observed that the use of either the GMC or the MC2 led to different outcomes in the macrocyclization RCM of 1,2-amino-alcohol-templated ene-dienes <02HCA3033, 02MI615> hinting that catalyst selection is also an important consideration in those processes. 

Note Chirality in NHC ligands is normally more remote than that in phosphane ligands, relative to the metal centre of an appropriate metal complex. The reason is the inherently achiral nitrogen atom adjacent to the carbene carbon atom. 

Note The chiral carbene ligand in Figure 5.9 has its asymmetric centres far removed from the carbene carbon atom (five bonds). This yields a transition metal carbene complex with remote chirality that cannot be expected to efficiently transfer the chiral information to a prochiral substrate resulting in low chiral resolution (% ee) of the product. 

The insertion reactions described above probably proceed via the free alkylidene carbenes. The analogous alkylidene carbenoids also insert efficiently into remote C—H bonds. Ochiai has demonstrated that such alkylidene carbenoids are conveniently generated from the corresponding iodinium tosylates. Depending on the substitution pattern employed, either [5 -1- 0] cyclization to give (49 equation 19), or [2 + 3] cyclization to give (52 equation 20), can be obtained. 

Stoichiometric use of transition-metal-carbene complexes in organic synthesis has been thoroughly reviewed.Various examples using carbene complexes containing cyclopropane subunits have been reported.Here, the cyclopropyl moiety is either attached directly to the carbene carbon or is placed in a more remote position. This section only discusses isolated carbene complexes. Related transition-metal-catalyzed conversions of diazo compounds containing cyclopropane subunits, which are interpreted to proceed via carbene intermediates (e.g. ref 130), are not discussed here. 

Several examples have appeared of diversion of the normal photochemistry of ortho-alkyl aromatic ketones due to apparent interception of the intermediate biradical by an unsaturated group present elsewhere in the molecule.Thus the vinyl aryl ketones (376) are converted to (377) in reasonable yields upon irradiation with ultra-violet light,while irradiation of the ortho-alkynyl acetophenone (378) in methanol yields diastereomers of (379)The latter reaction is thought to proceed by coupling of the initially formed ketyl radical onto the alkyne to give a carbene (380). The photochemistry of the jS-diketone (381) gives a mixture of the benzocyclobutenol (382) and the tetralone (383), and the proportions depend on the identity of the R-substituent in (381). The formation of (383) can be rationalised in terms of interception of the ketyl radical in the initially formed 1,4-biradical by the remote carbonyl group. 

It would appear, however, that this stepwise process is a photochemical artifact, and that the nitrile ylides do indeed react as true carbenes, undergoing concerted 1,1-cycloaddition to the remote double bond. Fischer and... 

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