Carbene complexes substituent effects

Nielsen, D.J., CaveU, K.J., Skelton, B.W. and White, A.H. (2006) Methyl-palladium(U) complexes of pyridine-bridged bis(nucleophilic heterocyclic carbene) ligands substituent effects on structure, stabUity, and catalytic performance. Inorg. Chim. Acta, 359, 1855-69. 

Decomposition of more complex diaziriries follows first order kinetics also. Chlorophenyl-carbene adds to cyclohexene to give a norcarane derivative. Substituent effects of m-Cl, m-NOa or m-Me groups, as well as solvent effects, are small. Chlorotrichloromethyldiazirine yields tetrachloroethylene chlorocyclooctyldiazirine also leads to an alkene 74CJC246). 

Similar correlations of orbital interactions with substituent effects were also found in additions of alkenes to substituted carbenes and of N2 to transition metal complexes (see Zollinger, 1983 b, 1990). 

These observations are compatible with the model for the carbene complex presented in Section II,A. Both metal and w-donor substituents compete to donate electron density to unfilled carbenepz orbitals, and with good 7r-donors such as nitrogen, the metal is less effective. In terms of resonance formalism, the resonance hybrid 39 makes a more significant contribution than 40 to the structure of the carbene ligands in these compounds. Similar conclusions are reached when the structures of Group 6, 7, and other Group 8 heteroatom-substituted carbene complexes are considered. 

In scrutinizing the various proposed reaction sequences in Eq. (26), one may classify the behavior of carbene complexes toward olefins according to four intimately related considerations (a) relative reactivities of various types of olefins (b) the polar nature of the metal-carbene bond (c) the option of prior coordination of olefin to the transition metal, or direct interaction with the carbene carbon and (d) steric factors, including effects arising from ligands on the transition metal as well as substituents on the olefinic and carbene carbons. Information related to these various influences is by no means exhaustive at this point. Consequently, some apparent contradictions exist which seem to cast doubt on the relevance of various model compound studies to conventional catalysis of the metathesis reaction, a process which unfortunately involves species which elude direct structural determination. 

The substituent effects on the alkene were investigated in the reaction of enyne 12 and chromium carbene complex 2c [8]. In the reaction of enyne -12a having a phenyl group on the alkene with Fischer chromium carbene complex 2c, metathesis product 13a was obtained as a main product along with cyclopropane 14 and cyclobutanone 15 (Eq.4). The reaction of Z-12a with 2c gave only... 

Also in macrocyclizations the conformation of the starting diene has an influence on the RCM/ADMET ratio. Substituents on the diene which form hydrogen bonds between each other or with the intermediate carbene complex, or external Lewis acids can have an important effect on the course of the reaction. Such effects, often subtle, might explain the variable results sometimes obtained in macrocyclizations [842], but make synthesis-planning and the formulation of general guidelines difficult. 

The fact that the intrinsic rate constants for nucleophilic addition to Fischer carbene complexes are relatively low, for example, much lower than for most reactions with comparable vinylic substrates or carboxylic esters,188 constitutes strong evidence for the presence of substantial transition state imbalances. However, there have only been a few studies of substituent effects that demonstrate the imbalance directly by showing a uc > p uc or by providing an estimate of its magnitude from the difference a uc - p uc. One such study is the reactions of 76-Cr-Z and 76-W-Z with HC CCII20 and C.F3CH20, 183 It yielded a Llc 0.59 and p ]uc< 0.46 for 76-Cr-Z, and a[juc 0.56 and 0 42 for 76-W-Z, i.e., a ue > p uc as expected. 

Supercritical carbon dioxide with a minute co-solvent addition is an effective medium for the 1,3-dipolar cycloaddition of azomethine ylides with DMAD to produce substituted pyrroles.67 The 1,3-dipolar cycloaddition of nitrile ylides [e.g. benzonitrile (4-nitrobenzylide) and 4-nitrobenzonitrile(benzylide)] with acrylamides provided a synthesis of 3,4-dihydro-2//-pyrroles with moderate to good yields.68 The Pt(II)-or Au(III)-catalysed 3 + 2-cycloaddition of the transition metal-containing azomethine ylide (63) with electron-rich alkenes provided a carbene complex (64), which yields tricyclic indoles (65) having a substituent at 3-position (Scheme 17).69 The 1,3-dipolar cycloadditions of azomethine ylides with aryl vinyl sulfones are catalysed by Cu(MeCN)4C104-Taniaphos with nearly complete exo- selectivity and enantioselec-tivities up to 85% ee.10 The 3 + 2-cycloaddition of benzol/>]thiophene 1,1-dioxide... 

In a related observation, Furstner and co-workers reported that while both the FMC and the MC2 lead to the same E Z alkene ratio in the case of 18-RCM, the use of the same conditions with 19 leads to product enriched selectively in either of the two isomers <02MI657> depending on catalyst selection (Scheme 9). Furstner notes that this "illustrates the subtle influence of remote substituents on the stereochemical outcome of RCM in the macrocyclic series." Studies concerning the effects of chelation of the reactivity of ruthenium carbene complexes were reported by Furstner and co-workers <02OM331>. Paquette and co-workers observed that the use of either the GMC or the MC2 led to different outcomes in the macrocyclization RCM of 1,2-amino-alcohol-templated ene-dienes <02HCA3033, 02MI615> hinting that catalyst selection is also an important consideration in those processes. 

A common method for the preparation of carbene complexes without heteroatom substituents is the reaction of a coordinatively unsaturated metal complex with a diazoalkane. This method is effective for the preparation of CpOsCl(P Pr3)(=CHPh) from CpOsCl(PTr3)2. The carbene in this complex is electrophilic and reacts with aUcyl and aryl lithium compounds and with Grignard reagents see Grignard Reagents) (Scheme 14). ... 

A comprehensive treatment of the benzannulation of Fischer carbene complexes with alkynes is not possible in this review, and thus instead the material presented here will hopefully serve to give the reader an overview of its scope and limitations. The first report of this reaction was in 1975 by Dotz in which he describes the formation of the naphthol chromium tricarbonyl complex (236) from the reaction of the phenyl chromium complex (la) with diphenylacetylene. In the intervening years over 100 papers have been published describing various aspects of this reaction.The reaction of the generic cartene complex (233 Scheme 34) with alkynes will serve to focus the organization of the scope and limitations of the benzaimulation reaction. The issues to be considered are (i) the regioselectivity with unsymmetri-cal alkynes (ii) possible mechanisms (iii) applications in natural product syntheses (iv) the effect of substitution on the aryl or alkenyl substituent of the carbene carbon (v) functionality on the alkyne (vi) effects of the solvent and the concentration of the alkyne (vii) tandem applications with other reactions of carbene complexes (viii) reactions where aromatization is blocked (cyclohexadienone annulation) (ix) annulation of aryl versus alkenyl carbene complexes (x) the effect of the ligands L on the metal (xi) the effect of the ancilliary substituent RX and (xii) reactions with —C X functionality. 

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