Carbenes substituents

In most transition metal-catalyzed reactions, one of the carbene substituents is a carbonyl group, which further enhances the electrophilicity of the intermediate. There are two general mechanisms that can be considered for cyclopropane formation. One involves formation of a four-membered ring intermediate that incorporates the metal. The alternative represents an electrophilic attack giving a polar species that undergoes 1,3-bond formation. 

Experimental evidence shows that both the metal and Ca can be the sites of electrophilic attack. Electrophiles would be expected to add to Ca when this atom is most negatively charged and when the 77-bonding orbital is most heavily concentrated there. These criteria are met in complexes where the metal center is electron-rich and where the carbene substituents are not good 77-donors, e.g.,... 

Indeed, nucleophilic attack at Ca is the most widely observed single reaction of Fischer carbenes. Substituent substitution is favored when one of the substituents on the carbene carbon is a good leaving group, e.g., halide, alkoxide, etc. Aminolysis of complex 41 typifies this mode of reaction (17),... 

Rearrangement of the ruthenium (diaminocarbene) isocyanide complex 28 has been noted above. Migration of the carbene substituent group is thought to occur via an intramolecular cyclization reaction (57,58) ... 

The Os—CcarbPh angle of 139(1)° in 59 is substantially increased from the angle of 120° expected for an sp2 hybridized carbon atom. This distortion is probably a consequence of both steric interactions between the phenyl ring and other equatorial ligands and the minimal steric demands of the proton (the other carbene substituent). 

Interaction of an electrophilic carbene or carbenoid with R—S—R compounds often results in the formation of sulfonium ylides. If the carbene substituents are suited to effectively stabilize a negative charge, these ylides are likely to be isolable otherwiese, their intermediary occurence may become evident from products of further transformation. Ando 152 b) has given an informative review on sulfonium ylide chemistry, including their formation by photochemical or copper-catalyzed decomposition of diazocarbonyl compounds. More recent examples, including the generation and reactions of ylides obtained by metal-catalyzed decomposition of diazo compounds in the presence of thiophenes (Sect. 4.2), allyl sulfides and allyl dithioketals (Sect. 2.3.4) have already been presented. 

The reactivities of carbenes toward alkenes have been correlated with the inductive and resonance effects of the carbene substituents, log k — a Eat + fcEaR+ + c.m Analogous correlations cannot be obtained for the reaction rates of carbenes with alcohols, neither with the substituent parameters used by Moss,109 nor with related sets.110 In particular, the substituent parameters do not describe the strong, rate-enhancing effect of aryl groups. For a detailed analysis, see the discussion of proton affinities (Section V.A). 

In summary, independently of the second carbene substituent, phosphinocarbenes have a singlet ground state, with a small singlet-triplet gap. They have a planar geometry at phosphorus and a short phosphorus-carbon bond, indicating an interaction between the phosphorus lone pair and the carbene vacant orbital. In the case of silyl- and phosphoniophosphinocar-benes, there is an additional interaction between the carbene lone pair and lowlying [Pg.179]

It appears that the reactivity (as well as the stability) of phosphinocar-benes (R2P-C-X) is strongly dependent on the nature of the other carbene substituent (X), and therefore this section will be subdivided with respect... 

CH3)2C=C(CH3)2 to 3.3 X 10 M-h- for the addition of CH3OCCI to trans-pentene. This million-fold variation testifies to the great modulating power of carbenic substituents on carbenic reactivity. 

In cases where more than one carbon-carbon double bond is present in the molecule, the possibility of selective cyclopropanation of one of them arises. Regiocontrol in intermole-cular cyclopropanation of substituted dienes has been the subject of much investigation and considerable differences can occur, depending on the structure of the substrate, catalyst and the carbene substituents. With a 1-substituted terminal diene such as 120, cyclopropanation, in general, occurs at the less-substituted double bond (equation 108)22 26,3, 16U62. In this case, the nature of the catalyst and of the carbenoid precursors are less important in determining the regioselectivity. 

Dirhodium(II) tetrakis(carboxamides), constructed with chiral 2-pyrroli-done-5-carboxylate esters so that the two nitrogen donor atoms on each rhodium are in a cis arrangement, represent a new class of chiral catalysts with broad applicability to enantioselective metal carbene transformations. Enantiomeric excesses greater than 90% have been achieved in intramolecular cyclopropanation reactions of allyl diazoacetates. In intermolecular cyclopropanation reactions with monosubsti-tuted olefins, the cis-disubstituted cyclopropane is formed with a higher enantiomeric excess than the trans isomer, and for cyclopropenation of 1-alkynes extraordinary selectivity has been achieved. Carbon-hydro-gen insertion reactions of diazoacetate esters that result in substituted y-butyrolactones occur in high yield and with enantiomeric excess as high as 90% with the use of these catalysts. Their design affords stabilization of the intermediate metal carbene and orientation of the carbene substituents for selectivity enhancement. 

Replacing the alkoxy carbene substituent by a better electron-donating amino group stabilizes the metal carbonyl bond. As a result, CO insertion in vinyl carbene D is hampered instead, cyclopentannulation via the chromacyclohexadiene I leads to aminoindenes K, which are readily hydrolyzed to indanones L (Scheme 6) [20]. 

For coordinatively saturated carbene complexes ligated by strongly 7t-acidic co-ligands, with the metal in a comparatively low formal oxidation state, the carbene carbon typically shows electrophilic character. Accordingly, the less reactive examples of these typically bear one or two ju-dative carbene substituents (OR, NR0, SR). 

As in organic chemistry, diazoalkanes have found wide application as ultimate carbene sources. In contrast to the organic chemistry of diazoalkanes, however, the reactions do not proceed via free carbenes but rather via metal-mediated transformations of coordinated diazoalkanes. In some cases, diazoalkane complexes may be isolated (Figure 5.8). The parent diazoalkane, H2C=N2, has found somewhat less success in the synthesis of terminal methylene complexes LJM=CH2 however, this is primarily due to the lack of any kinetic (steric) or thermodynamic (71-dative) stabilization by carbene substituents. Thus methylene ligands... 

In contrast, Schrock carbenes are electron deficient [10 to 16 valence electrons (VE)] early transition metal complexes with the metal atom in a high oxidation state and carbene substituents that are limited to alkyl groups and hydrogen [131]. Their bonding situation can be described in terms of the interaction of a triplet carbene with a triplet metal fragment resnlting in a covalent double bond [132], Tantalum complexes like [(np)3Ta=CHBu ] and [Cp2(Me)Ta=CH2] are representative of Schrock carbenes. 

Tlie nucleophilic character of these carbenes has been demonstrated by Hammett -Taft correlations (p = +2 in competition experiments, with Ar- N=C=S/Ar-N =C=0 as substrates) (20). The syn-anti distribution of isomers in cycle pro pa nation reactions with carbenes is the result of steric and electronic effects. The dissipation of the charge by secondary interactions of the carbene substituents and the substrate are determining factors. As an example. CCIF reacts according to the orientation of the Cl group syn to the... 

This problem has been more recently revised by Ando [24]. He found a linear correlation by considering the sum of O/ effects of the carbene substituents, where carbalkoxycarbenes are less selective than CCI2 ... 

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