Carbenes carbon-hydrogen

Scheme 15.4 Mechanism of metal carbene carbon-hydrogen insertion.

A Fischer-type dithiocarbene complex (447) which undergoes protonation at the carbene carbon was reported by Le Bozec et al (265). The reaction, which produces 448, proceeds via formation of a /rans-hydrido-carbene intermediate, isomerization to the cis isomer, and a metal to carbene carbon hydrogen shift. 

One of the most dramatic recent developments in metal carbene chemistry catalyzed by dirhodium(II) has been demonstration of the feasibility and usefulness of intermolecular carbon-hydrogen insertion reactions [38, 91]. These were made possible by recognition of the unusual reactivity and selectivity of aryl- and vinyldiazoacetates [12] and the high level of electronic control that is possible in their reactions. Some of the products that have been formed in these reactions, and their selectivities with catalysis by Rh2(S-DOSP)4, are reported in Scheme 10. 

Among typical carbon-carbon bond (C-C) formation reactions with carbenes, the cyclopropanation reaction with olefins has been well studied including its application to industrial processes. The second typical reaction of carbenes is the insertion reaction into the carbon-hydrogen bond (C-H) which seems to be a direct and efficient C-C bond forming reaction. However, its use for synthetic purpose has often been limited due to low selectivity of the reactions.3... 

A second process that has a central position in the analysis of the chemical properties of carbenes is their reaction with hydrocarbons. As is the case for alcohols, singlet and triplet carbenes react with hydrocarbons in distinctive ways. It has long been held that very electrophilic singlet carbenes can insert directly into carbon-hydrogen bonds (11) (Kirmse, 1971). On the other hand, triplet carbenes are believed to abstract hydrogen atoms to generate radicals that go on to combine and disproportionate in subsequent steps (12)... 

Elimination of trimethylchlorosilane and nitrogen occurs when the (phos-phino)(silyl)diazomethane la is reacted with para-toluenesulfinyl chloride at low temperature. The formation of the four-membered heterocycle 92, obtained in 87% yield, can be rationalized by a multiple-step mechanism involving the formation of the (phosphino)(sulfinyl)carbene 2v. The insertion of the (phosphoryl)(sulfenyl)carbene 91, resulting from a 1,3-oxygen shift from sulfur to phosphorus in 2v, into a carbon-hydrogen bond of a diisopropylamino group readily accounts for the formation of 92.84... 

A prerequisite for the a-elimination is the absence of (3-hydrogen atoms in the alkyl groups and this was successfully achieved by using the neopentyl substituents at the metal centre. The nature of the double bond between the metal and carbon was established by its bond length and the occurrence of stereoisomers [13], Typical feature of the Schrock carbenes is that they contain an electrophilic, high-valent metal atom and an electron rich carbene carbon atom. The reverse is true for the older, Fischer carbene compounds, such as the one mentioned, (OC)5W=CPh2. 

If, instead of electrocyclization, electrophilic attack of the closer upper edge of the phenyl group by the carbene carbon atom occurs, a zwitterionic intermediate might result, which upon 1,4-elimination of (COljW would yield a 1-methoxy-1,3-cyclopentadiene. Suprafacial hydrogen migration would finally lead to the formation of the observed diastereomer. 

Because hydrogen, alkyl, or aryl groups can compensate only to a limited extent the electron deficit of the carbene carbon atom, it is mainly the metal and its ligands which provide stabilization in this type of carbene complex. For this reason the reactivity of these compounds depends mainly on the nature and oxidation state of the metal and on the electronic properties of the remaining ligands. 

Fig. 6) can activate hydrogen under mild conditions [219]. In contrast to transition metals, that act as electrophiles towards hydrogen, the (alkyl)(amino)carbenes mainly behave as nucleophiles initially creating a hydride like species, which then attacks the positively polarized carbene carbon atom. 

Stang etal. (94JA93) have developed another alkynyliodonium salt mediated approach for the synthesis of y-lactams including bicyclic systems containing the pyrrole moiety. This method is based on the formation of 2-cyclopentenones 114 via intramolecular 1,5-carbon-hydrogen insertion reactions of [/3-(p-toluenesulfonyl)alkylidene]carbenes 113 derived from Michael addition of sodium p-toluenesulfinate to /3-ketoethynyl(phenyl) iodonium triflates 112 (Scheme 32). Replacing 112 by j8-amidoethynyl (phenyl)iodonium triflates 115-119 provides various y-lactams as outlined in Eqs. (26)-(30). 

Intramolecular carbon-hydrogen insertion reactions have well known to be elTectively promoted by dirhodium(ll) catalysts [19-23]. Insertion into the y-position to form five-membered ring compounds is virtually exclusive, and in competitive experiments the expected reactivity for electrophilic carbene insertion (3°>2° 1°) is observed [49], as is heteroatom activation [50]. A recent theoretical treatment [51] confirmed the mechanistic proposal (Scheme 15.4) that C-C and C-H bond formation with the carbene carbon proceeds in a concerted fashion as the ligated metal dissociates [52]. Chemoselectivity is dependent on the catalyst ligands [53]. 

A completely different approach was used to probe the reactivity of tert-butyl-carbene, one of Frey s original examples. Table 7.6 shows the varying products of thermal and photochemical decomposition of the diazo compound. It would appear that carbon-hydrogen insertion and carbon-carbon insertion are about equally facile in the carbene presumed to be formed in photolytic reactions. Even in 1964, this observation should have seemed strange (as it clearly did to... 

It is generally observed that the isotope effect for abstraction of hydrogen by a triplet carbene is 2-9, whereas insertion into a carbon-hydrogen bond by a singlet carbene proceeds with an isotope effect of 1-2. ... 

Like carbene insertions into carbon-hydrogen bonds, metal nitrene insertions occur in both intermolecular and intramolecular reactions.For intermole-cular reactions, a manganese(III) meio-tetrakis(pentafluorophenyl)porphyrm complex gives high product yields and turnovers up to 2600 amidations could be effected directly with amides using PhI(OAc)2 (Eq. 51). The most exciting development in intramolecular C—H reactions thus far has been the oxidative cychzation of sulfamate esters (e.g., Eq. 52), as well as carbamates (to oxazolidin-2-ones), ° and one can expect further developments that are of synthetic... 

The carbon-hydrogen insertion (C,H insertion) is one of the most striking reactions of carbenes and carbenoids. The reaction is interesting and very useful for the construction... 

Trimethylsilyl-2-oxetanone has been isolated in unstated yield following pyrolysis of methyl trimethylsilyldiazoacetate (equation 96). This appears to be an interesting example of carbene insertion in the proximate carbon-hydrogen bond of the methoxyl group... 

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