C addition

Osmium-mediated c/5-addition of nitrogen and oxygen to olefins. Regioselectiv-ity may be controlled by ligand. Nitrogen sources (X-NClNa) include ... 

Such cycloadditions are dependent on the interactions of the azepine HOMO and the diene LUMO. Theoretical consideration of these orbitals reveals that bonding overlap is favourable for C-6—C-7 and C-4—C-5 additions and that, on the basis of secondary orbital interactions, the endo product is favored. Experimentally, however, it is found that additions are periselective and C-4—C-5 addition predominates in the cycloaddition of 1//-azepines with cyclopentadienones, isobenzofurans, tetra- and hexa-chlorocyclopentadienes, 1,2,4,5-tetrazines, a-pyrones and 3,4-diazacyclopentadienones (8lH(15)1569). 

The use of dienecarboxylic acids (cf. Section 8.3.1.1, under Intramolecular 1,4-diacyloxylation ) under the conditions for haloacyloxylation in acetone resulted in a highly stereoselective chlorolactonization [Eqs.(44) and (45)] [67]. The reaction proceeds >98% as a 1,4-c/5 addition and involves the same intermediate lactone jr-allyl complex as in the intramolecular diacyloxylation. 

Carboalumination. The treatment of terminal alkynes and symmetrically disubstitut-ed alkynes with trimethylaluminum in the presence of a catalytic amount of Cp2ZrCl2 [bis(cyclopentadienyl)zirconium dichloride] results in a regioselective c/5-addition of the Me-Al moiety to the triple bond to furnish P-methyl-substsituted alkenylalanes. ° ... 

If the bromination is conducted in an alcohol, trapping of the intermediate by C-5 addition of the alcohol, then alcoholysis of C-2-bromide, produces 2,5-dialkoxy-2,5-dihydrofurans, as mixtures of cis- and trans-isomers ° hydrogenation of these species affords 2,5-dialkoxy-tetrahydrofurans, extremely useful as... 

The stabilised anion of tosylmethyl isocyanide (TosMIC) (or of benzotriazol-1-ylmethyl isocyanide - BetMIC ) adds in Michael fashion to unsaturated ketones and esters, with subsequent closure onto isocyanide carbon generating the ring. Proton transfer, then elimination of toluenesulfmate generates a 3-//-pyrrole which tautomerises to the aromatic system which is unsubstituted at both C-2 and C-5. Addition of the TosMIC anion to unsaturated nitro compounds gives rise to 2,5-unsubstituted-3-nitropyrroles. ... 

Interesting selectivity effects were found for various dienes and alkynes, the reduction of both of which could be stopped at the monoene stage. Almost exclusive c/5-addition of H2 to the alkyne was observed. Ketones can be reduced, although this is not a result of the cationic character of the system, since the catalysts can be deprotonated with NEt3 to give neutral species that also effect this reduction equally well. This illustrates a possible ambiguity in studying cationic catalysts if dissociation of a positive species, usually H", can occur. Fortunately, in this particular case, each system can be separately prepared and studied. 

If the bromination is conducted in an alcoholic solvent, trapping of intermediate by C-5 addition of methanol, then methanolysis of C-2-bromide, produces... 

Organocopper compounds make c/5-addition to a simple acetylene to afford the alkenylcopper compounds and further they react with organic halides to afford cis alkenes by a coupling reaction as shown in eq. (22.34) [82-85]. 

In the biosynthesis of nicotine 6.71) in Nicotiana species it has been found [45, 46] that both ornithine 6.60) and putrescine 6.63) are again involved in pyrrolidine-ring formation. For this alkaloid, however, incorporation of the amino acid is through at least one symmetrical intermediate, logically putrescine 6.63), because [2- " C]ornithine gave nicotine 6.71) with label equally spread over C-2 and C-5 additional results were obtained with [ N]ornithines. This symmetrical pathway is supported by experiments with C02, although a few results indicate an unsymmetrical route [47, 48]. 

The stereochemistry of both 2 -f 2 and Z - 4 cycloadditions has been investigated. For dichloroethylenes, the 2 + 2 addition is not stereospecific for dienes, however, the 4 + 2 addition is a stereospectfic c/5 addition. Benzyne thus shows behavior parallel to ground state ethylenesonundergoing concerted 2 + 4 cycloaddition, but nonconcerted 2 + 2 additions... 

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