Carbene theoretical treatments

Lopez R, Sordo TL, Sordo JA, Gonzalez J (1993) J Org Chem 58 7036 Cossio F, Ugalde JM, Lopez X, Lecea B, Palomo C (1993) J Am Chem Soc 115 995 Cossio FP, Arrieta A, Lecea B, Ugalde JM (1994) J Am Chem Soc 116 2085 GerrietaA,Lecea B, Cossio FP (1998) J Org Chem 63 5869. For a theoretical treatment of the photoreaction of chromium carbene complexes with imines see Arrieta A, Cossio FP, Fernandez I, Gomez-Gallego M, Lecea B, Mancheno MJ, Sierra MA (2000) J Am Chem Soc 122 11509... 

Reactions involving free carbenes are very exothermic since two new theoretical treatment of the addition of singlet methylene to ethylene suggests that there is no activation barrier.168 The addition of carbenes to alkenes is an important method for synthesis of many types of cyclopropanes and several of the methods for carbene generation listed in Scheme 10.8 have been adapted for use in synthesis. Scheme 10.9, at the end of this section, gives a number of specific examples. 

Fischer-Type and Schrock-Type Carbene Complexes Theoretical Treatment 3... 

Fischer-Type and Schrock-T pe Carbene Complexes Theoretical Treatment... 

In contrast with non-acceptor-substituted carbene complexes, most of which are rather stable compounds, only few acceptor-substituted carbene complexes have been isolated [500,502,947,948]. In particular, acceptor-substituted carbene complexes relevant to organic synthesis (e.g. copper or rhodium acylcarbene complexes) are normally highly reactive and have remained elusive to spectroscopic characterization (for theoretical treatments, see Section 1.2). The inference that these intermediates are indeed carbene complexes is in part based on the observation that the modes of generation and the reactivity of these reactive species correlate well with those of less reactive carbene complexes. 

Experimental results [1361] and theoretical treatment [28] indicate that the cyclo-propanation of alkenes by electrophilic carbene complexes is a concerted process. Z-Olefins normally lead to the formation of the corresponding c7. -cyclopropanes, and -olefins yield fran -cyclopropanes. The relative configuration of the carbene-bound substituent and the substituents of the alkene in the final cyclopropane seems to be mainly determined by the steric bulk of these groups. In cyclopropanations of terminal alkenes with ethyl diazoacetate low diastereoselectivities are often observed [1024,1351]. These can be improved by increasing the steric demand of the substituents at the carbene or at the alkene [1033,1362]. High diastereoselectivities can, e.g., often be achieved with terf-butyl, neopentyl or 2,6-di(rerr-butyl)phenyl diazoacetate [1362] as carbene complex precursors (Figure 4.19). 

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]. 

The first aliphatic phosphenium cations involving a phosphorus-sulfur bond, e.g. (330) have been prepared and characterised. The stereochemistry, regioselectivity and mechanism of the insertion of the diisopropylamino (chloro) phosphenium cation into the phenylcyclopropane ring, to form phosphetanes, has been studied,Ligand-exchange processes involving mono- and bis-adducts of phosphenium ions have received a theoretical treatment. The coordination chemistry of the carbene adduct (331) of the... 

Interestingly theoretical treatment has established that the energetics of the titanacyclobutane is more stable than the olefin-carbene complex resulting of its rearrangement [108] and a titanacyclobutane is in fact postulated as a titanocene methylene precursor in reaction (43). 

Theoretical treatment proved this analogy, which has not been reported up to now. Density functional theory (DPT) calculations show that the energy of the LUMO decreases as the shape of the orbital spreads across the whole phenyl group of the carbene moiety. In contrast to the Hammett dependence, the calculated LUMO energies of the whole series correlate well with the measured reduction potentials (Fig. 48.7). 

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