Elimination reactions, bond-order

Having established structural and electronic analogies between metal oxides and alkoxides of molybdenum and tungsten, the key remaining feature to be examined is the reactivity patterns of the metal-alkoxides. Metal-metal bonds provide both a source and a returning place for electrons in oxidative-addition and reductive elimination reactions. Stepwise transformations of M-M bond order, from 3 to 4 (37,38), 3 to 2 and 1 (39) have now been documented. The alkoxides M2(0R)6 (MiM) are coordinatively unsaturated, as is evident from their facile reversible reactions with donor ligands, eq. 1, and are readily oxidized in addition reactions of the type shown in equations 2 (39) and 3 (39). 

Direct reaction of iron pentacarbonyl with trimethylsilyl isocyanide ( C=N—SiMe3) at 65°-75° yields an air-sensitive substitution product Me3Si—N=C Fe(CO)4 in 93% yield, with elimination of carbon monoxide (152). It was shown by infrared spectroscopy (38) that complex formation lowers the N=C bond order for Me3Si—N=C , whereas it raises the N=C bond order for Me3C—N=C , presumably as a result of interaction between dv orbitals of silicon with the metal d orbitals. 

As for the alkyl group, there are two important structural effects to notice. First, at least one C-H bond adjacent (/3) to the leaving group is required. Second, the ease of E2 elimination follows the order tertiary R > secondary R > primary R. Unlike SN2 reactions, which are not observed for tertiary alkyl compounds because of steric hindrance to the approach of the nucleophile to carbon, the related E2 reaction usually occurs readily with tertiary RX compounds. The reason is that little or no steric hindrance is likely... 

The opposite of an addition to a double bond is a 1,2-elimination reaction. In solution, where the reaction is promoted by solvent or by base, the most common eliminations (and those to which we shall limit our discussion) are those that involve loss of HX, although loss of X2 from 1,2-dihalides and similar reactions are also well known. The mechanisms of eliminations of HX are of three main types (1) The Ex (elimination, first-order), shown in Equation 7.22, which is the reverse of the AdE2 reaction and which has the same first, and rate-determining,... 

Then the differences in rate caused by the electronic effect of the substituent are correlated by the Hammett equation log(kz/kH) = poz, where kz is the rate constant obtained for a compound with a particular meta or para substituent, ku is the rate constant for the unsubstituted phenyl group, and crz is the substituent constant for each substituent used. The proportionality constant p relates the substituent constant (electron donating or wididrawing) and the substituent s effect on rate. It gives information about the type and extent of charge development in the activated complex. It is determined by plotting log(kz/kQ) versus ov for a series of substituents. The slope of the linear plot is p and is termed the reaction constant. For example, the reaction shown above is an elimination reaction in which a proton and the nosy late group are eliminated and a C-N n bond is formed in their place. The reaction is second order overall, first order in substrate, and first order in base. The rate constants were measured for several substituted compounds ... 

A fully concerted mechanism for reaction 299 has been eliminated as inconsistent with 14C and 15N KIEs and also with the observed inverse solvent D2O effect. The reaction path for the deamination of AMP has been formulated613 as a stepwise conversion involving the formation of tetrahedral intermediate 515 characterized by full-bonded hydroxyl and amino groups (equation 300). The TS for slow formation of 515, resulting from the attack of the hydroxyl from enzyme zinc-activated water at the C(6), is characterized by the C(6) OH bond order of 0.8 0.1 (late TS) and fully bonded NH2, that is by the nearly complete conversion to sp3 at C(6), and by nearly complete protonation of Nq), 516, The protonation of NH2 (in 515) and departure of NH3 (with TS 517) take place in the subsequent rapid steps as shown in equation 300, Zinc hydroxide is formed prior to attack514 at C(6). Enzymatic degradation of [6-14C]AMP has been carried out to prove the position of the radiolabel in 513 (equation 301). No radioactivity in the allantoin... 

We form an amide linkage between the adipoyl chloride and the amine with the elimination of HC1. The polymer is called nylon 6-6 because there are six carbon atoms in the acyl chloride and six carbon atoms in the diamine. Other nylons, such as nylon 10-6, are made of sebacoyl chloride (a 10-carbon atom containing acyl chloride) and hexamethylene diamine (a six carbon atom containing diamine). We use an acyl chloride rather than a carboxylic acid to form the amide bond because the former is more reactive. NaOH is added to the polymerization reaction in order to neutralize the HC1 that is released every time an amide bond is formed. 

The mechanisms of the reductive eliminations in Scheme 5 were studied [49,83], and potential pathways for these reactions are shown in Scheme 6. The reductive eliminations from the monomeric diarylamido aryl complex 20 illustrate two important points in the elimination reactions. First, these reactions were first order, demonstrating that the actual C-N bond formation occurred from a monomeric complex. Second, the observed rate constant for the elimination reaction contained two terms (Eq. (49)). One of these terms was inverse first order in PPh3 concentration, and the other was zero order in PPh3. These results were consistent with two competing mechanisms, Path B and Path C in Scheme 6, occurring simultaneously. One of these mechanisms involves initial, reversible phosphine dissociation followed by C-N bond formation in the resulting 14-electron, three-coordinate intermediate. The second mechanism involves reductive elimination from a 16-electron four-coordinate intermediate, presumably after trans-to-cis isomerization. 

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