Conjugate organometallic species

A new approach to a variety of a-branched alkynes 70 was achieved by a Knoevenagel type condensation of 4-unsubstituted isoxazolin-5-ones 66 with aldehydes or ketones 67, followed by conjugate addition of an organometallic species and nitrosative cleavage of the heterocyclic ring <02SL1257>. 

The (conjugated-diene)zirconocene and -hafnocene systems reviewed above are unique in that three isomeric organometallic species can be distinguished by physical and/or chemical means. Two of these, 3 and 5, represent stable isolable molecules, while the third, (T) -diene)MCp2 (4), functions as a chemically detectable intermediate connecting the (rf-diene)metallocene isomers (see Scheme 1). 

Covalent attachment has also been exploited for protein incorporation of non-native redox active cofactors. A photosensitive rhodium complex has been covalently attached to a cysteine near the heme of cytochrome c (67). The heme of these cytochrome c bioconjugates was photoreducible, which makes it possible for these artificial proteins to be potentially useful in electronic devices. The covalent anchoring, via a disulfide bond, of a redox active ferrocene cofactor has been demonstrated in the protein azurin (68). Not only did conjugation to the protein provide the cofactor with increased water stability and solubility, but it also provided, by means of mutagenesis, a means of tuning the reduction potential of the cofactor. The protein-aided transition of organometallic species into aqueous solution via increased solubility, stability and tuning are important benefits to the construction of artificial metalloproteins. 

Ferrocene is an attractive and convenient organometallic species to incorporate into conjugated polymers—it is stable, easy to functionalize, has reversible electrochemistry, and its derivatives (both monomeric and polymeric) can be easily characterized [e.g., by nuclear magnetic resonance (NMR) spectroscopy]. Ferrocene has been incorporated into the backbone and side chains of polythiophene 45 In 1999, both Higgins and Wolf independently reported the synthesis of polythiophenes with ferrocene in the backbone.46,47 Electropolymerization of ferrocene substituted... 

As mentioned before, the scientific results discussed in this contribution focus on monitoring of reactive organometallic species by ATR-IR sp>ectroscopy utilizing mid-IR fibre probes. While alkoxide sp>ecies, the conjugated base of an alcohol, are rather generally applied in synthetic chemistry (e.g. bases, nucleophiles and hj nds) organohthium compounds require further introduction. 

Phospholes are known to exhibit characteristic optical and electrochemical properties derived from the phosphorus-bridged 1,3-dienic jt system [34]. Particular interest has recently been paid to their Jt-conjugated derivatives, such as nonfused phospholes, dibenzo[f ,d] phospholes, benzo[f ] phospholes, benzo[c] phospholes, and related compounds [35]. There are several methods to synthesize phospholes [36]. The classical method for the synthesis of phospholes is the reaction between the nucleophilic substitution of a P-X bond with a stoichiometric amount of an organometallic species such as organolithium or organomagnesium reagents (Scheme 4.21) [37]. 

In Grignard reactions, Mg(0) metal reacts with organic halides of. sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand. Pd(0) complexes react more easily with halides of carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C tr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes. conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /J-hydro-gen. At the same time, the Pd(0) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg, Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

Thermodynamic data on the acidity of organometallic hydrides should help identify situations where apparent reactions of acidic transition-metal hydrides actually result from their conjugate bases. A case in which both species can react but give different products (as was pointed out by Prof. Espenson three years ago (18)) is the addition of hydridocobaloximes, HCo(dmgH)2B, to olefins with electron-withdrawing substituents... 

As well as organic chiral auxiliaries, organometallic fragments have found some application as chiral auxiliaries in conjugate addition reactions. Particularly noteworthy are chiral molybdenum allyl complexes [69], chiral iron complexes [70], and planar chiral arene chromium species [71]. 

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