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化学代写|有机化学代写organic chemistry代考|Concepts of Elimination Reactions

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化学代写|有机化学代写organic chemistry代考|The Concepts of a,b- and 1,n-Elimination

Reactions in which two atoms or atom groups $X$ and $Y$ are removed from a compound are referred to as eliminations (Figure 4.1). In many eliminations $X$ and $Y$ are removed in such a way that they do not become constituents of one and the same molecule. In other eliminations they become attached to one another such that they leave as a molecule of type $\mathrm{X}-\mathrm{Y}$ or $\mathrm{X}=\mathrm{Y}$ or as $\mathrm{N} \equiv \mathrm{N}$. The atoms or groups $\mathrm{X}$ and $\mathrm{Y}$ can be bound to $\mathrm{C}$ atoms and/or to heteroatoms in the substrate. These atoms can be $s p^{3}$ or $s p^{2}$ hybridized.

Depending on the distance between the atoms or groups $X$ and $Y$ removed from the substrate, their elimination has a distinct designation. If $X$ and $Y$ are geminal, their removal is an $\alpha$-elimination. If they are vicinal, it is a $\beta$-elimination. If $\mathrm{X}$ and $\mathrm{Y}$ are separated from each other by $n$ atoms, their removal is called $1, n$-elimination, that is, $1,3-, 1,4$-elimination, and so on (Figure 4.1).

Chapter 4 is limited to a discussion of the most important eliminations, which are the olefin-forming $\boldsymbol{\beta}$-eliminations.

In this book, $\boldsymbol{\alpha}$-eliminations are described only in connection with their applications: the $\alpha$-elimination of $\mathrm{HCl}$ from $\mathrm{CHCl}{3}+$ base (Figure 3.11) and the $\alpha$-elimination of $\mathrm{XZnI}$ from carbenoids $\mathrm{X}-\mathrm{CH}{2}-\mathrm{ZnI}$ (Figure $3.12$ ) as well as the $\alpha$-elimination of $\mathrm{N}{2}$ from diazomalonic ester (Figure 3.13) in cyclopropanations, the $\alpha$-elimination of LiBr from carbenoids $\mathrm{Br}-\mathrm{CR}^{1} \mathrm{R}^{2}$ – Li in thermal rearrangements (Figures 11.23, 11.29), the $\alpha$-elimination of $\mathrm{N}{2}$ from $\alpha$-diazoketones in the photochemical Wolff rearrangement (Section 11.3.2), and the related $\alpha$-elimination of $\mathrm{N}_{2}$ from azides in the Curtius degradation (Section 11.4.5).
$\boldsymbol{\beta}$-Eliminations in which at least one of the leaving groups is removed from a heteroatom are considered to be oxidations. Eliminations of this type are therefore not treated here but in the redox chapter (mainly in Section 14.3.1).
1,3-Eliminations are mentioned in the preparation of 1,3-dipoles such as diazoalkanes or $\alpha$-diazoketones (Section 12.5.3) and nitrile oxides (Section 12.5.4), in connection with the decomposition of primary ozonides to carbonyl oxides (Section 12.5.5) and the decomposition of phenylpentazole to phenyl azide (Section 12.5.6).

化学代写|有机化学代写organic chemistry代考|The Terms syn- and anti-Elimination

In various eliminations the mechanism implies a well-defined stereorelationship between the eliminated atoms or groups $X$ and $Y$ and the plane of the resulting $C=C$ double bond (Figure 4.2). For example, $\mathrm{X}$ and $\mathrm{Y}$ may leave into one and the same half-space flanking this double bond. Their removal is then called a $\boldsymbol{s y n}$ – or ciselimination. (Be careful when using the second term: cis-eliminations can also give trans- or $E$-olefins!) There are other eliminations where group $\mathrm{X}$ leaves the substrate in the direction of one half-space and group $Y$ leaves in the direction of the other halfspace, both flanking the $\mathrm{C}=\mathrm{C}$ double bond produced. These are so-called anti- or trans-eliminations. (Again, be careful about using the second term: trans-eliminations also can led to cis- or $Z$-olefins!) The third possibility concerns eliminations in which there is no need of an unambiguous spatial relationship between the groups $\mathrm{X}$ and $\mathrm{Y}$ to be removed and the plane of the resulting double bond.

化学代写|有机化学代写ORGANIC CHEMISTRY代考|When Are Stereogenic syn- and anti-Selective Eliminations Stereoselective?

An elimination in which a cis, a trans, an $E$, or a $Z$ double bond is produced can be called a stereogenic elimination. Strictly syn- and anti-selective stereogenic $\beta$ eliminations of $\mathrm{X}$ and $\mathrm{Y}$ may be, but need not be, stereoselective. They are necessarily stereoselective when in the substrate $\mathrm{X}$ and $\mathrm{Y}$ are bound to stereocenters. Examples of stereoselectivities of this type are provided by the eliminations in Figure $4.2$ for $\mathrm{X} \neq \mathrm{R}^{1} \neq \mathrm{R}^{2}$ and $\mathrm{Y} \neq \mathrm{R}^{3} \neq \mathrm{R}^{4}$. On the other hand, stereoselectivity is not guaranteed when $X$ and $Y$ are removed from substrates possessing the structure $\mathrm{X}-\mathrm{CR}^{1} \mathrm{R}^{2}-\mathrm{CY}_{2}-\mathrm{R}^{3}$, that is, from substrates in which $\mathrm{Y}$ is not bound to a stereocenter but only $X$ is (because $R^{1} \neq R^{2} \neq X$ ).

This last situation characterizes, among others, eliminations from the following kinds of substrates: (1) the entity $\mathrm{X}$ to be removed is either a halogen atom or a group linked via a heteroatom to the developing olefinic carbon atom (for the latter case, the abbreviation “Het” will be used for X from now on in Chapter 4), and (2) the entity $\mathrm{Y}$ to be removed is an $\mathrm{H}$ atom. Accordingly, the substrates now considered possess the structure $\mathrm{Het}-\mathrm{CR}^{1} \mathrm{R}^{2}-\mathrm{CH}{2}-\mathrm{R}^{3}$. If $\mathrm{R}^{1}$ is different from $\mathrm{R}^{2}$, $\mathrm{H} /$ Het-eliminations from such substrates are stereogenic and may be stereoselective (quite independent of the elimination mechanism). As an illustration, we want to discuss three representative examples: stereogenic eliminations from Het- $\mathrm{C}(\mathrm{Ph}) \mathrm{H}-$ $\mathrm{CH}{2}-\mathrm{R}$ (Figure 4.3), from Het- $\mathrm{C}(\mathrm{Ph}) \mathrm{Me}-\mathrm{CH}{2}-\mathrm{R}$ (Figure $\left.4.5\right)$, and from Het$\mathrm{C}(\mathrm{Et}) \mathrm{Me}-\mathrm{CH}{2}-\mathrm{R}$ (Figure 4.6).

化学代写|有机化学代写ORGANIC CHEMISTRY代考|THE CONCEPTS OF A,B- AND 1,N-ELIMINATION

$\boldsymbol{\beta}$-Eliminations in which at least one of the leaving groups is removed from a heteroatom are considered to be oxidations. Eliminations of this type are therefore not treated here but in the redox chapter (mainly in Section 14.3.1).
1,3-Eliminations are mentioned in the preparation of 1,3-dipoles such as diazoalkanes or $\alpha$与初级臭氧化物分解成羰基氧化物有关小号和C吨一世这n12.5.5以及苯基五唑分解成苯基叠氮化物小号和C吨一世这n12.5.6.

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