Heteroatom-carbon, triple bonding

Heteroatom-Carbon Double Bonds , Heteroatom-Carbon Triple Bonds Heterocumulenes Conjugate Acceptors Triply Bonded and Allenic Conjugate Acceptors... 

Reactivity modes of the poly(pyrazolyl)borate alkylidyne complexes follow a number of recognised routes for transition metal complexes containing metal-carbon triple bonds, including ligand substitution or redox reactions at the transition metal centre, insertion of a molecule into the metal-carbon triple bond, and electrophilic or nucleophilic attack at the alkylidyne carbon, C. Cationic alkylidyne complexes generally react with nucleophiles at the alkylidyne carbon, whereas neutral alkylidyne complexes can react at either the metal centre or the alkylidyne carbon. Substantive work has been devoted to neutral and cationic alkylidyne complexes bearing heteroatom substituents. Differences between the chemistry of the various Tp complexes have previously been rationalised largely on the basis of steric effects. 

Dipolarophiles utilized in these cycloadditions leading to five-membered heterocycles contain either double or triple bonds between two carbon atoms, a carbon atom and a heteroatom, or two heteroatoms. These are shown in Scheme 9 listed in approximate order of decreasing activity from left to right. Small rings containing a double bond (either C=C or C=N) are also effective dipolarophiles, but these result in six- and seven-membered ring systems. 

There is only one type of compound in this category, substituted nitriles. It will be useful to divide the data sets for substituent effects at carbon-heteroatom triple bonds into two classes, substituted nitriles and heteroethynylene sets. 

Results of Correlations for Carbon-Heteroatom Triple-Bond Sets... 

Abstract The use of A-heterocyclic carbene (NHC) complexes as homogeneous catalysts in addition reactions across carbon-carbon double and triple bonds and carbon-heteroatom double bonds is described. The discussion is focused on the description of the catalytic systems, their current mechanistic understanding and occasionally the relevant organometallic chemistry. The reaction types covered include hydrogenation, transfer hydrogenation, hydrosilylation, hydroboration and diboration, hydroamination, hydrothiolation, hydration, hydroarylation, allylic substitution, addition, chloroesterification and chloroacylation. 

Step 1. ICs are defined as any carbons containing no double or triple bonds to heteroatoms. ICs isolate functional groups from each other according to the classical Hammett concept, so that the treatment of complex structures is simple. 

Cyclodditions to Carbon-Heteroatom Triple Bonds. Transient electrophilic carbenes are known to react with nitriles to give transient46 or even stable nitrile ylides 30.47 No reaction of transient nucleophilic carbenes with nitriles has been reported. 

In fact, this heuristic principle (HP-8), which refers to nucleophilic heteroatoms directly attached to sp carbon atoms, may be also applied to nucleophilic heteroatoms directly attached to sp olefinic carbon atoms. In such cases, however, rather than a "disconnection" we would have an "unri-elimination" which will afford the substituted heteroatom as the "nucleofuge" and a triple bond as the "electrofuge". 

Most of the compounds that you have encountered in this text so far have been fairly simple. In addition to any carbon-carbon double or triple bonds, they have contained only one functional group and could be named by using suffixes such as -ynol or -dienone. However, for a compound that contains more than one heteroatom functional group, only one of these functional groups can be designated in the suffix. For example, consider the following compound, which has both alcohol and ketone functional groups ... 

Metal-mediated intramolecular addition of silyl enolates to alkynes is also valuable for the synthesis of cyclic ketones. A stoichiometric amount of HgCl2 or EtAlCl2 effectively promotes the cycloalkenylation via anti-addition to alkynes (Equations (87) and (88)).319 320a The -addition mode can be explained by a metal coordination to the triple bond and subsequent attack of the enolate moiety from the opposite side to the metal. The resultant alkenylmetals can be used for carbon-carbon and carbon-heteroatom bond formation as well as protonation. 

Besides carbon-halogen bonds, Bu3SnH is used routinely for hydrogenolysis of various carbon-heteroatom bonds. The deoxygenation from alcohols has been performed by a Barton-McCombie reaction where the addition of tin hydride to the triple bond is suppressed (Equation (2)).29-31... 

The propargylic cations [Co2(/i,i/2,Tj3-RC2CR2)(CO)6]+ react as electrophiles with a variety of heteroatom- and carbon-centered nucleophiles to provide, following demetalation, propargylated products with complete regioselectivity. Complexation of the triple bond circumvents isomerization to allenic products. Reaction with asymmetrical ketones results in attack by the cation exclusively (>95%) at the more substituted a-carbon.72,74 (See Scheme 11.)... 

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