Carbenes reagent equivalents

First the normal Wittig reaction, and then a second mol of Wittig reagent must be behaving as a carbene equivalent, just as we hoped the sulphur yhd would. 

The use of an excess (3 equivalents) of the Simmons-Smith reagent gave exclusively the spiropentane derivatives 618 in 60-70% yield [163a], In contrast, not even trace amounts of these products were observed by using samar-ium-dihalomethane as the carbene source [4b],... 

The potential synthetic utility of titanium-based olefin metathesis and related reactions is evident from the extensive documentation outlined above. Titanium carbene complexes react with organic molecules possessing a carbon—carbon or carbon—oxygen double bond to produce, as metathesis products, a variety of acyclic and cyclic unsaturated compounds. Furthermore, the four-membered titanacydes formed by the reactions of the carbene complexes with alkynes or nitriles serve as useful reagents for the preparation of functionalized compounds. Since various types of titanium carbene complexes and their equivalents are now readily available, these reactions constitute convenient tools available to synthetic chemists. 

The addition of a particularly useful reagent, bearing two ester functionalities, is obtained by means of the Bingel-Hirsch cyclopropanation [29]. Although the mechanism may not be of carbene-type, the result is formally equivalent to a carbene addition. The presence of the ester groups renders this derivative susceptible for further modifications. 

We now turn to reagents which are important in context with the element/halogen exchange Compounds of the type 8 would be interesting reagents for nucleophilic halomethylation but are quite unstable due to carbene formation. Compound 9 would be an attractive synthetic equivalent for the synthons 8. As will be shown... 

Dialkyl sulphides are converted into trialkylsulphonium salts by treatment with an alkyl halide (the bromide or iodide is usually the reactant of choice). An important example of this group is trimethylsulphonium iodide, which is used as a methylene transfer reagent by virtue of its being converted in the presence of base into a sulphur ylide, which is a nucleophilic carbene equivalent. 

Parallel to Schrock s efforts were those of Grubbs and co-workers.28 They studied Tebbe s reagent and found that titanacyclobutanes derived from the reagent could be isolated. For example, they found that titanacycle 18 was in direct equilibrium with metal carbene 19 (equation 11,13). This system was not an active metathesis catalyst, but the work showed that both intermediates were important in the metathesis pathway. Further work found that reaction of Tebbe s reagent with norbomene resulted in metallacycle 20, which promoted ROMP of additional equivalents of norbomene and produced a polymer that was relatively monodisperse29 (equation 11.14).30... 

Three-membered rings with two heteroatoms are usually encountered only as reagents. Diazirines are nsefnl carbene precnrsors" - they are generally more stable than the equivalent isomeric diazo compounds, thongh they are sometimes explosive in the pure state. They can be prepared by oxidation of diaziridines that, in tnm, are available via the condensation of a ketone or aldehyde with ammonia and chloramine. Chloro-diazirines, from the reaction of amidines with hypochlorite, wiU undergo Sn2 or Sn2 displacement reactions."" ... 

This same disconnection of a carbon atom is also helpful for epoxides (11) without carbonyl substituents. The reagent should be a nucleophilic carbene equivalent and a sulphur ylid (12) is the answer.These can be made from the sulphide (13) by a similar process to phosphorus ylid synthesis (Chapter 15), though the reactions of the two ylids with carbonyl compounds are significantly different (there is a third type of reaction with the ylid CH2N2 in Chapter 31). 

All disconnections are the same on cyclopropane, requiring a carbene equivalent which will add to an unactivated double bond. Diazomethane will do this, but one of the best carbene sources is CH2I2 with a zinc-copper couple (the Simmons-Smith reaction ). This works particularly well on allylic alcohols (31), no doubt because of hydrogen bonding between the OH group and the reagent. The reaction is then totally stereoselective. 

Cyclopropanation. The reagent is an electrophilic carbene equivalent. Thus it behaves differently from dimethyloxosulfonium methylide toward 2-benzenesul-fonyl-1,3-butadiene. 

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