Heteronuclear addition reactions

Heteronuclear addition reactions to alkynes usually produce the corresponding trans-alkene derivatives. However, in the case of strained cyclic alkynes, these would be even more heavily strained than the starting alkyne. Thus, in most reactions, cis-products are observed. The initial addition of the electrophile may, however, still proceed in an anh -manner, as was shown by Krebs et al. [1 b] for the reaction of a cycloheptyne derivative with trichloromethylsulfenyl chloride (Scheme 8-21). This reaction produced the first seven-membered rm s-cycloalkene derivative to be isolable at room temperature. 

Oxidative addition reactions have also been used to synthesize heteronuclear Group IB metal clusters. Clark and co-workers (54) first described this type of reaction in 1970 [Eq. (21)], and similar methods have subsequently been used to introduce one Au(PR3) unit into other neutral clusters [e.g., Eqs. (22)-(25)] (51,55-59). 

The pattern of adaptation of the orbital and energy structure of the alkyne molecule to the demands of the reaction of addition to a triple bond, shown in Fig. 6.3, is, in principle, retained also in addition reactions to heteronuclear triple bonds. This conclusion follows, for example, from the structure of the transition state V in the reaction of addition of hydrogen chloride to nitriles, which at the stage of the final product formation is characterized by the making of the bond C—Cl. The angle of nucleophilic attack is in the region of the transition strueture 126°. 

The addition of transition metal fragments ML (L = two-electron donor ligand) across formally unsaturated metal-metal or metal-carbon bonds is a well-developed synthetic route to heteronuclear clusters (1,2,11,12,27) and has received theoretical justification from Hoffmann s isolobal principle (46). The addition of a PtL2 fragment across an M=M double bond may be considered as analogous to the reaction of a carbene with an olefin, resulting in a cyclopropane. The use of isolobal analogies in the directed synthesis of heteronuclear clusters has been reviewed (11,12,27). 

As indicated previously, NMR may be used simply as an analytical technique for monitoring the decomposition of a reactant or formation of a product. In addition, NMR and ESR merit a special mention due to their importance in studying the dynamics of systems at equilibrium these so-called equilibrium methods do not alter the dynamic equilibrium of the chemical process under study. They have been used to study, for example, -transfer reactions, valence isomerisations, conformational interconversions, heteronuclear isotopic exchange processes (NMR) and electron-transfer reactions (ESR). These techniques can be applied to the study of fast or very fast reactions by analysis of spectral line broadening [16,39],... 

The addition of gold phosphine fragments to transition metal compounds is readily extended to include reactions of heteronuclear gold cluster compounds and has been used to build up clusters of increasing nuclearity, as the following examples illustrate ... 

Changes in cluster nuclearity have also been observed to accompany the addition of acid or base to heteronuclear gold cluster compounds and may result in aggregation, as in the reactions... 

A Ci-symmetric cis-1 bis-adduct with an additional bridge between the addends was obtained in the reaction between two molecules of ethyl propio-late and C6o in the presence of triphenylphosphane.318 Its structure (( )-162, Figure 1.37) was established with the help of the HMBC- (heteronuclear multiple bond correlation) NMR technique and includes stereogenic centers in the addend moiety as well as a noninherently chiral addition pattern. The latter can be related to the head-to-tail connectivity of the two propiolate units. 

A complex, multiply bridged 1,2,3,4-adduct of C60 (168) (Scheme 1.11) including a noninherently chiral addition pattern as well as a multitude of stereogenic centers in the addend moiety was obtained in a tandem reaction between the alkaloid scandine and Ceo-324 The sequence included a photoin-duced addition of the tertiary amine subunit of the alkaloid and a [2 + 2] cycloaddition of its vinyl group to the adjacent intrahexagonal formal double bond of the fullerene. The structural elucidation included 1H-1 H COSY-, HMQC- (heteronuclear multiple quantum coherence), HMBC-, and ROESY-(rotating frame Overhauser enhancement spectroscopy) NMR experiments and... 

The addition of peroxyacetic acid to the heteronuclear C=0 double bond of cyclohexanone i.e. the first step of the Baeyer-VilHger oxidation reaction) also exhibits a small solvent dependence [517],... 

Addition of anionic nucleophiles to alkenes and to heteronuclear double bond systems (C=0, C=S) also lies within the scope of this Section. Chloride and cyanide ions are effieient initiators of the polymerization and copolymerization of acrylonitrile in dipolar non-HBD solvents, as reported by Parker [6], Even some 1,3-dipolar cycloaddition reactions leading to heterocyclic compounds are often better carried out in dipolar non-HBD solvents in order to increase rates and yields [311], The rate of alkaline hydrolysis of ethyl and 4-nitrophenyl acetate in dimethyl sulfoxide/water mixtures increases with increasing dimethyl sulfoxide concentration due to the increased activity of the hydroxide ion. This is presumably caused by its reduced solvation in the dipolar non-HBD solvent [312, 313]. Dimethyl sulfoxide greatly accelerates the formation of oximes from carbonyl compounds and hydroxylamine, as shown for substituted 9-oxofluorenes [314]. Nucleophilic attack on carbon disulfide by cyanide ion is possible only in A,A-dimethylformamide [315]. The fluoride ion, dissolved as tetraalkylammo-nium fluoride in dipolar difluoromethane, even reacts with carbon dioxide to yield the fluorocarbonate ion, F-C02 [840]. 

A rational, and general, approach to complexes containing heteronuclear bonds between Ni or Pt and a second transition metal is by addition of a complex containing a metal-carbon or metal-metal multiple bond to a coordinatively unsaturated Ni(0) or Pt(0) species, thus producing a di- or tri-metallacyclopropane or -propene. These reactions proceed in good yield under mild conditions, and the products are quite predictable. Reactions (l)-(n) give some examples ... 

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