Carbene complexes steric hindrance

The Pd-catalysed Heck reaction performed with thiourea as the Ugand exhibit good activities for some catalysts. As for carbene ligands [104], steric hindrance improves catalytic results. Thus, thioureas wearing bulky substituents afford the formation of air- and moisture-stable Pd complexes [105]. For example, the catalyst obtained with 2mol% Pd(dba)2 and Ar,M -dimesitylene-ethylene thiourea (Scheme 24) was still active even after 2 months in an air atmosphere. 

The fact that Schrock s proposed metallocyclobutanes decomposed to propylene derivatives rather than cyclopropanes was fortunate in that further information resulted regarding the stereochemistry of the olefin reaction with the carbene carbon, as now the /3-carbon from the metal-locycle precursor retained its identity. The reaction course was consistent with nucleophilic attack of the carbene carbon on the complexed olefin, despite potential steric hindrance from the bulky carbene. Decomposition via pathways f-h in Eq. (26) was clearly confirmed in studies utilizing deuterated olefins (67). 

But not only palladium(O) complexes can activate CO or O2, also palla-dium(II) complexes have been reported to be active in the presence of carbon monoxide or dioxygen as it was shown in the direct synthesis of polycarbonate from CO and phenol or bisphenol A [79,80]. The authors could confirm the positive influence of the NHC ligand comparing the activity and reactivity of the palladium-carbene complex with the corresponding PdBr2 catalyst. The molecular weights and yields of the polycarbonates improved with increasing steric hindrance of the substituents in the l,T-position of the car-bene complex. 

In much of the early work the monomers used were mixtures of isomers. For example, endo,anti-115 as prepared contains minor amounts of the ero.syw-isomer, a smaller amount of the endo,syn-isom T, but no exo,anti-isomer. Again, 185 is the main isomer (66%) in a mixture with seven others of which only one is present in a significant amount (33%). In general the ewrfo-isomers are less reactive than the exo-isomers, no doubt because of the greater degree of steric hindrance when the ewdo-isomer approaches the propagating complex. However, ewrfo-isomers that are unreactive with A- or B-type catalysts, as defined in Table 9, have sometimes been found to react slowly when placed in contact with a metal carbene initiator. For example, when 3.2 equiv of exo-190, containing some... 

In the enantioselective conjugate addition, the use of p,P-disubstituted enones to create quaternary chiral centers is more difficult because of the steric hindrance of the P-position [21]. Alexakis showed that Cu complexes with the chiral carbene ligand 49 are capable of catalyzing domino conjugate additions to the trisubstituted enone 50 in a one-pot process (Scheme 11.11). The magnesium enolates reacted poorly with alkyl halides, but their reactivity improved with use of an HMPA/THF... 

In 2006, Carmona et al. achieved the tautomerization of 2-substituted pyridines into the carbene tautomers mediated by the [Tp Ir(C6H5)2N2] (TpMe2 hydrotris(3,5-dimethylpyrazolyl)borate) fragment (Scheme 26). The steric hindrance provided by the substituents at the 2-position precluded the formation of the (K, N)-coordinated pyridine complexes while the formation of the NHCs was not affected by this issue, as the substituent in the 2-position is away from the coordinated carbon atom. 

Only a few examples have been obtained through the classical methodologies followed in group 6 metal chemistry. Most rf -Cs Fischer-type ruthenium and osmium carbenes arise from the nucleophilic additions of alcohol and amino groups at the electrophilic carbenic Ca-atom of both allenylidene and vinylidene complexes. The fate of the reaction depends on the electrophilicity as well as the steric hindrance around the Ca-atom, which can control its accessibility, especially for bulky nucleophiles. These features have been thoroughly discussed in a recent review. ... 

First synthetic attempts with compound 11 using catalyst [Ru]-VII [17] delivered the undesired bicyclic structure 12, which results from the initial attack of the ruthenium carbene complex at the terminal olefin via intermediate 13 (Scheme 11.5). To direct the initiation of the catalytic cycle into the other double bond, through intermediate 14, Honda et al. decided to introduce steric hindrance using a disub-stituted olefin in the alkyl chain. Their results showed, however, that the resulting compound was not reactive enough to undergo metathesis under the chosen reaction conditions. With the use of a more electron-rich allyl ether moiety such as in 15, it was possible to isolate the desired product in 74% yield and successfully complete the synthesis of securinine. Ruthenium complexes [Ru]-I and [Ru]-II were... 

Alkyl-, aryl- and alkenylalkoxycarbene complexes are suitable carbene transfer reagents for the cyclopropanation of a variety of alkenes [9]. Steric hindrance in the alkene caused either by the number or the size of substituents may be a limitation of the cyclopropanation reaction [9c] moreover, insertion of the carbene ligand into an olefmic C-H bond may be observed as a side reaction [9,10],... 

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