Covalent carbene addition

Ismaili, H., F.o. Lagugnei-Labarthet, and M.S. Workentin, Covalently assembled gold nanoparticle-carbon nanotube hybrids via a photoinitiated carbene addition reaction. Chemistry of Materials, 2011. 23(6) p. 1519-1525. 

The gaseous dichlorocarbene radical cation reacted with alkyl halides via a fast electrophilic addition to form a covalently bonded intermediate (CI2C—X—R)+ in a Fourier transform ion cyclotron resonance mass spectrometer. This intermediate fragments either homolytically or heterolytically to produce net halogen atom or halogen ion transfer product. Addition of carbonyls to the carbene ion is followed by homolytic cleavage of the C-O bond to yield a new carbene radical cation. 

Both the long C-C bond distance (1.50 A) and the very short Pt—C distances (2.0 A) indicate the strong interaction between the adsorbed molecule and the three platinum surface atoms. The covalent Pt—C distance would be 2.2 A. The shorter metal-carbon distances indicate multiple metal-carbon bonding that may be carbene or carbyne-like. Compounds with these types of bonds exhibit high reactivity in metathesis and in other addition reactions The carbon-carbon single bond distance indicates that the molecule is stretched as much as possible without breaking of this chemical bond. 

The stereochemistry of these cycloadditions is so specific that Skell used it as a diagnostic test for distinguishing between singlet and triplet carbenes. According to Skell, the addition of singlet carbene to an olefin occurs in a concerted manner and is therefore stereospecific. However, in the case of triplet carbene, both the unpaired electrons cannot form a new covalent bond because of their parallel spins. Therefore, in the latter case the reaction will take place in two steps. In the first step a triplet diradical is formed, which undergoes spin inversion and then ring closure. For this the radical has to wait for the appropriate... 

Carbenes are neutral reactive species that can form two bonds, either by insertion into a single covalent bond, or by addition to both ends of a double bond to give a cyclopropane ring. 

Olah emphasizes that the division of cations into classical and nonclassical is frequently arbitrary, since in many cations there is an intenn liate range of delocalization ( partial carbonium-ion character ) as in the 2-methylnorbomyl ion. The author does not want to name classical ions carbonium because it is restricted to highest valeiK state carbocations this requirement is nwt by penta-and tetracoordinate carbocations but not trivalent ones. On the other hand, while in the formation of other onium ions the atom of the donor (nitrogen, oxygen etc.) increases its covalence by one unit upon addition of the acceptor (electrophile), in the formation of a classical ion the covalence of the carbon atom decreases from 4 to 3. As for the name carbenium ion, in the author s opinion it reflects the logical relationships between the carbene and the carbeiunm ion, between the alkene and the carbenium ion ... 

Another important class of covalent catalysis is carried ont by chiral iV-heterocyclic carbenes [15] that react with carbonyl componnds forming chiral acyl anion eqniva-lents whose reaction with the corresponding electrophile complete the catalytic cycle (F, Fig. 2.2). Finally, it is worthy to mention the ability of certain bifnnctional organo-catalysts to perform simultaneous activation of the nncleophile and the electrophile. The nse of bifunctional organocatalysis has been shown to be very snccessfnl in conjngate additions. 

Arnold and co-workers conducted a range of work with tethered (bidentate) NHCs, in which the NHC was anchored to the metal centre covalently as well as via the carbene. This imparted additional stability to the complex, allowing the isolation of species such as lanthanide-NHC complexes 108 and 109 (Figure 2.6). However, it has been shown that the NHC-metal bond is often still quite labile and therefore this might have implications for catalysis. This lability can be used for interesting chemistry such as the functionalisation of silanes and the capture of CO2 and CS2. ... 

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