Hydride abstraction steric effects

An interesting stereochemical profile of this cyclization is that in the five-membered product structures, substituents PhS and OH groups are placed cis to each other, whereas in six-membered products the placement is trans (Table 6, entry 7). The cis selectivity in the five-membered ring systems is not affected by a and fi-substituents of the alkoxides (entries 2, 3 and 4), indicating that the steric effect is not the dominant factor. Instead, interaction between the oxido and carbene center composes a five- (or seven-) membered transition state 28, which allows the carbene to abstract the nearest quasi axial hydrogen as a hydride to produce a carbonyl intermediate 29, leading to the cyclization products 27 and 30 (Scheme 13, Eq. 1). Similarly, the stereoselective stepwise cyclization of cis- and rra s-2-(3,3-dithiopropyl)cyclohexanol to 2-phenylthio-... 

If the nucleophilic site (HOMO) involves a nonbonded pair of electrons (path a), a stable covalently bonded complex will form. If the HOMO is a a bond, direct reaction is unlikely unless the bond is high in energy and sterically exposed, as in a three-membered ring, but if the bond is to H, hydride abstraction may occur (path b, steps 1 and 2) or a hydride bridge may form (path 6, step 1). The last two possibilities are discussed further in Chapter 10. If the HOMO is a n bond, a n complex may result (path c, step 1), or, more commonly, donation of the n electrons results in the formation of a a bond at the end where the n electron density was higher, the other end becoming Lewis acidic in the process (path c, steps 1 and 2). The effects of substituents on olefin reactivity were discussed in Chapter 6. 

In complexes (16) and (26), the situation is rather different, because there is now steric hindrance from the diene terminal substituent. As a result, the electronically less favorable products are predominant. However, when steric effects at both methylene groups are balanced, as in complex (19), the hydride abstraction proceeds with electronic control to give (30 equation 13). 

As mentioned earlier, steric effects can be important in determining the outcome of the hydride abstraction reaction. This is particularly vexing in cases where an alkyl substituent is present at the sp carbon of the cyclohexadiene complex. For example, complexes such as (47 equation 19) are untouched by trityl cation, provided traces of acid are not present (these are formed by hydrolysis of the trityl tetra-fluoroborate due to atmospheric moisture, and will cause rearrangement of the diene complex). This is due to the fact that only the hydride trans to the Fe(CO)3 group can be removed, and the methyl substituent prevents close approach to this hydrogen. 

These results indicate that DMD directs its action on the negatively charged nitro group of the a -adducts to form intermediate cyclohexadienone and subsequent aromatization. Sensitivity of KMnO oxidation of the a -adducts to the steric effects at the addition site and high value of kinetic isotope effect (KIE) k lk 10 at -70°C [12b] suggests that the oxidation proceeds via direct interaction of the oxidant with the oxidized site, hence probably via abstraction of the hydride anion. 

The sterically hindered hydride [(lndenyl)2YH]2 [8] is an effective catalyst to accomplish homo- and co-dimerisation of a wide range of terminal olefins CH2=CHR, R=Ph, n-Bu, /-Pr, t-Bu, SiMcj, CH2Ph. The presence of substituents and functionalities in the monomer are usually tolerated. An induction period of ca. 30 min may be necessary to cleave the dimeric structure of the catalyst precursor the yields are nearly quantitative within two days. Co-dimerisation of a-olefins with styrene proceeds via initial insertion of the a-olefin into the Y-H bond, followed by a 2,1-insertion of styrene into the Y-C bond of the alkyl intermediate. Subsequent P-H abstraction leads to the releasing of the dimer (Scheme 16). 

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