Carbon-sodium bond, transition

Rate Constants of the Transitions Between the Three Forms of the Carbon-Sodium-Bond... 

Under conditions where reactions follow second-order kinetics, dehydro-brominalion of ordinary isopropyl bromide by sodium ethoxide takes place seven times as fast as that of the labeled compound, (CD3)2CHBr. An isotope effect of this size, we have seen (Sec. 11.15), reveals the breaking of a carbon-hydrogen bond in the transition state of the rate-determining step. 

In the reduction of dienes and polyenes, combinations of a metal hydride and transition metal halides can also be used. Sodium borohydride and cobalt(n) halides were applied in the selective reduction of unsatnrated carbon-carbon double bonds . LiAlIU, in the presence of Zr -, Ti" - or -halides, is a selective reducing agent of dienes . The following reactions were carried out with sodium borohydride and iodine (equation 28) . 

At the other extreme of elimination mechanisms is a concerted process. In an E2 reaction (here illustrated by the reaction of 2-bromobutane with sodium ethoxide) proton transfer to the base, formation of the carbon-carbon double bond, and ejection of the bromide ion occur simultaneously all bond-breaking and bond-forming steps are concerted. Because the base removes a j8-hydrogen at the same time that the C— Br bond is broken to form a halide ion, the transition state has considerable double-bond character (Figure 9.7). 

Displacements by thiophenoxide ion have an interesting possibility - the nucleophile can attack one electron at a time, transferring an electron to produce a thiophenoxy radical while reductively cleaving the electrophile to form an alkyl radical. Then the two radicals, in a solvent cage, can couple (Fig. 1.23). In an exploration of this process, called the SET mechanism, we used thiophenoxide with the sodium salt of p-carboxybenzyl iodide, and with the corresponding mesylate. We saw that there was a large acceleration by added ethanol in the iodide case, but not with the mesylate. We proposed that in the iodide displacement this reflected the conversion of thiophenoxide ion, with its delocalized charge, into the much more hydrophobic thiophenoxy radical at the transition state. Other evidence as well supported the SET mechanism. The carbon-iodine bond is more easily reductively cleaved than is the carbon-mesylate bond. 

The synthesis of key intermediate 6 begins with the asymmetric synthesis of the lactol subunit, intermediate 8 (see Scheme 3). Alkylation of the sodium enolate derived from carboximide 21 with allyl iodide furnishes intermediate 26 as a crystalline solid in 82 % yield and in >99 % diastereomeric purity after recrystallization. Guided by transition state allylic strain conformational control elements5d (see Scheme 4), the action of sodium bis(trimethylsilyl)amide on 21 affords chelated (Z)-enolate 25. Chelation of the type illustrated in 25 prevents rotation about the nitrogen-carbon bond and renders... 

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