Alkyl with mono-substituted alkynes

Cycloaddition of a nitrile oxide to an alkyne generates an aromatic isoxazole directly. Mono-alkyl or -aryl-substituted alkynes lead to 5-substituted isoxa-zoles with other mono-substituted alkynes mixtures are obtained. The use of tin- and boron-substituted alkynes produces the metal-substituted heterocycle which can then be used, for example, in coupling processes. 

For alkyl-substituted alkynes, there is a difference in stereochemistry between mono-and disubstituted derivatives. The former give syn addition whereas the latter react by anti addition. The disubstituted (internal) compounds are considerably ( 100 times) more reactive than the monosubstituted (terminal) ones. This result suggests that the transition state of the rate-determining step is stabilized by both of the alkyl substituents and points to a bridged intermediate. This would be consistent with the overall stereochemistry of the reaction for internal alkynes. 

The hydroboration with 1 1 stoichiometry reduces the percentage of monohydroboration, drastically. However, for comparison the hydroboration of alkynes with 1 1 alkyne-9-BBN under essentially neat condition provides better line shapes for the carbon a to boron and no interference from solvent absorptions. (Trimethylsilyl)acetylene (le) results in only P-mono- (P to TMS) and dihy-droboration products (Eq. 5.23), the behavior common to nonsilylated terminal acetylenes, but differing in the extent of mono- and dihydroboration. Moreover, it is found that this silylalkyne is less reactive toward 9-BBN as compared with its alkyl counterpart. In addition, the steric repulsions between the substituted boron atom, and the a-TMS groups are sufficiently large so as to prevent their site from competing with the alternative, unencumbered position. 

Cationic sandwich complexes of the type CpCo(arene) + were first prepared by hydride abstraction from cyclohexadi-enyl cations (Section 7.1). They are accessible in broader variation from the reaction of CpCoX half-sandwich complexes with arene in the presence of AICI3. Their electrochemical reductions to the corresponding 19-electron monocations and to 20-electron neutral complexes have been studied. The stability of electron-rich sandwich complexes increases with increasing alkyl substitution in either ring despite the more negative redox potential mass spectrometry studies of bond dissociation energies of (arene)Co+ complexes corroborate these results. However, neutral sandwich complexes are not very stable in the polar solvents necessary for the reduction of mono- or dications and have been isolated only from alkyne trimerization with CpCo precursors in nonpolar solvents (Section 5.1.4). 

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