Carbenes electrophilic/nucleophilic compounds

Chameleonic features of carbenes can be further amplified by complexation with transition metals (Figure 5.42). In complexes with low valent/low oxidation state late transition metals (Fischer carbenes), carbenes display electrophilic properties, and often behave similarly to a carbonyl compound. Such carbenes also often have p-donor substituents, such as-OR or-NR, on the carbene carbon and x-acceptor ligands at the metal. In contrast, carbene complexes with high valent/high oxidation state early transition metals (Schrock carbenes) are nucleophilic. The ability of metal in the Schrock carbenes is further enhanced by donor ligands. 

Halogen exchange of F is usually with chloro compounds however, replacement of bromine has enabled fluorodiazirines to be obtained (83JA6513 86TL419). Diazirine (2) was previously obtained by a difficult route involving F2. The relative ease of access to (2) enables a carbene whose reactivity is intermediate between that of electrophilic ( CF2) and nucleophilic ( C(OMe)2) carbenes to be studied. 

The following chapter concerns another kind of low-valent organophosphorus compounds, namely phosphinidenes. Little is known about free phos-phinidenes in contrast to the corresponding transition metal complexes. Many new reagents have been generated exhibiting either electrophilic or nucleophilic properties. The reactivity of these carbene-like reagents is evaluated (K. hammer tsma). 

In view of the similarities between the bonding models for carbene and carbyne complexes it is not surprising that similar patterns of reactivity should be observed for these compounds. Thus nucleophilic and electrophilic additions to the metal-carbon triple bond are anticipated under appropriate circumstances, and both orbital and electrostatic considerations will be expected to play a role. 

The effect of metal basicity on the mode of reactivity of the metal-carbon bond in carbene complexes toward electrophilic and nucleophilic reagents was emphasized in Section II above. Reactivity studies of alkylidene ligands in d8 and d6 Ru, Os, and Ir complexes reinforce the notion that electrophilic additions to electron-rich compounds and nucleophilic additions to electron-deficient compounds are the expected patterns. Notable exceptions include addition of CO and CNR to the osmium methylene complex 47. These latter reactions can be interpreted in terms of non-innocent participation of the nitrosyl ligand. 

When the development of carbene-complex chemistry began in the mid seventies, two different patterns of reactivity emerged and led to a, maybe overemphasized, division of these compounds into (electrophilic) Fischer-type and (nucleophilic) Schrock-type carbene complexes (Figure 1.1). 

Today, however, carbene complexes covering a broad range of different reactivities have been prepared. Often it is no longer possible to predict whether a carbene complex will behave as an electrophile or as a nucleophile. Thus, a reactivity-based nomenclature would be difficult to apply consistently. For this reason in this book compounds with a carbon-metal double bond will be called carbene complexes or alkylidene complexes , terms not associated with any specific chemical behavior. 

This reactivity pattern is certainly unexpected. Why should low-valent complexes react as electrophiles and highly oxidized complexes be nucleophilic Numerous calculations on model compounds have provided possible explanations for the observed chemical behavior of both Fischer-type [3-8] and Schrock-type [9-17] carbene complexes. In simplified terms, a rationalization of the reactivity of carbene complexes could be as follows. The reactivity of non-heteroatom-stabilized carbene complexes is mainly frontier-orbital-controlled. The energies of the HOMO and LUMO of carbene complexes, which are critical for the reactivity of a given complex, are determined by the amount of orbital overlap and by the energy-difference between the empty carbene 2p orbital and a d orbital (of suitable symmetry) of the group L M. 

Transition metal complexes which react with diazoalkanes to yield carbene complexes can be catalysts for diazodecomposition (see Section 4.1). In addition to the requirements mentioned above (free coordination site, electrophi-licity), transition metal complexes can catalyze the decomposition of diazoalkanes if the corresponding carbene complexes are capable of transferring the carbene fragment to a substrate with simultaneous regeneration of the original complex. Metal carbonyls of chromium, iron, cobalt, nickel, molybdenum, and tungsten all catalyze the decomposition of diazomethane [493]. Other related catalysts are (CO)5W=C(OMe)Ph [509], [Cp(CO)2Fe(THF)][BF4] [510,511], and (CO)5Cr(COD) [52,512]. These compounds are sufficiently electrophilic to catalyze the decomposition of weakly nucleophilic, acceptor-substituted diazoalkanes. 

Ylide formation, and hence X-H bond insertion, generally proceeds faster than C-H bond insertion or cyclopropanation [1176], 1,2-C-H insertion can, however, compete efficiently with X-H bond insertion [1177]. One problem occasionally encountered in transition metal-catalyzed X-H bond insertion is the deactivation of the (electrophilic) catalyst L M by the substrate RXH. The formation of the intermediate carbene complex requires nucleophilic addition of a carbene precursor (e.g. a diazocarbonyl compound) to the complex Lj,M. Other nucleophiles present in the reaction mixture can compete efficiently with the carbene precursor, or even lead to stable, catalytically inactive adducts L M-XR. For this reason carbene X-H bond insertion with substrates which might form a stable complex with the catalyst (e.g. amines, imidazole derivatives, thiols) often require larger amounts of catalyst and high reaction temperatures. 

Thermolysis of 78a in methanol yielded compounds 86 (R = OMe and R = H) in a 1 1 ratio (77JA633). In this case, the nucleophilic substitution process becomes more important because of the increased electrophilicity of the carbene. 

Carbonyl ylides can be viewed as an adduct between a carbonyl group and a carbene and, in fact, some ylides have been prepared this way (see above). The application of carbonyl ylides to the synthesis of complex natural products has been greatly advanced by the finding that stabilized carbenoids can be generated by the decomposition of ot-diazocarbonyl compounds with copper and rhodium complexes. The metallocarbenoids formed by this method are highly electrophilic on carbon and readily add nucleophiles such as the oxygen of many carbonyl derivatives to form carbonyl ylides. This type of reaction is in fact quite old with the first report being the addition of diazomalonate and benzaldehyde (33,34). 

Other reactions for which a discussion of their structure-reactivity behavior in terms of the PNS has provided valuable insights include nucleophilic addition and substitution reactions on electrophilic alkenes, vinylic compounds, and Fischer carbene complexes reactions involving carbocations and some radical reactions. 

The reaction of Ru(TMP) with ethyl diazoacetate yielded a carbene complex, e.g. Ru(CHC02Et)(TMP) [316], An excess of the diazo compound led to catalytic formation of cis- and trans diethyl maleate in an unexpected ratio of 15 1. The nucleophilic ethyl diazoacetate is proposed to attack the electrophilic carbene complex and produce an intermediate betaine-line species which eliminates both Ru(TMP) and N2 to form the maleates. Similar reactions were observed with Os(TTP) complexes [313a], These reactions are reminiscent of the above-mentioned lability of a putative methyleneruthenium porphyrin. 

Arasabenzene, with chromium, 5, 339 Arcyriacyanin A, via Heck couplings, 11, 320 Arduengo-type carbenes with titanium(IV), 4, 366 with vanadium, 5, 10 (Arene(chromium carbonyls analytical applications, 5, 261 benzyl cation stabilization, 5, 245 biomedical applications, 5, 260 chiral, as asymmetric catalysis ligands, 5, 241 chromatographic separation, 5, 239 cine and tele nucleophilic substitutions, 5, 236 kinetic and mechanistic studies, 5, 257 liquid crystalline behaviour, 5, 262 lithiations and electrophile reactions, 5, 236 as main polymer chain unit, 5, 251 mass spectroscopic studies, 5, 256 miscellaneous compounds, 5, 258 NMR studies, 5, 255 palladium coupling, 5, 239 polymer-bound complexes, 5, 250 spectroscopic studies, 5, 256 X-ray data analysis, 5, 257... 

Carbene complexes of transition metals [2,21,225-236] are typical representatives of compounds with a double metal-carbon bond. They are seen as derivatives of a two-covalent carbon in their singlet state [226,232,236]. As a rule, the carbene ligand is an effective a-donor and a comparatively weak n-acceptor. Formation of a cr-bond M — C takes place via transference of a nonbonding electronic pair with a nucleophilic a-orbital of the carbenic carbon to the metal atom. Simultaneously, it is also possible to form a 7t-bond as a result of the interaction of symmetrically appropriate metallic d-AO with a vacant electrophilic /7-orbital of the carbene [236,237], This situation is a key factor that determines the polarization of most of the carbene complexes according to type 145 (Fig. 2.6). 

As mentioned earlier, the carbene ligand in our complexes shows nucleophilic character with respect to the metal fragment. Therefore, we decided to combine it with an electrophilic carbene. For this purpose we treated pentacarbonyl[methoxy(phenyl)carbene]chromium(0) with phenyl(tri-chloromethyl) mercury (85). Compounds of this kind have been studied intensively by Seyferth et al. (86) and are known as a source of dihalogeno-carbenes. The carbene complex reacted with the carbenoid compound at... 

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