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|Concepts of Elimination Reactions

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|Concepts of Elimination Reactions

有机化学代写

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

两个原子或原子团的反应X和是从化合物中去除被称为消除F一世G在r和4.1. 在许多淘汰赛中X和是以不会成为同一个分子的成分的方式去除。在其他消除中,它们相互连接,从而以一种分子的形式离开X−是或者X=是或作为ñ≡ñ. 原子或基团X和是可以绑定到C基体中的原子和/或杂原子。这些原子可以是sp3或者sp2杂交。

取决于原子或基团之间的距离X和是从基材中取出,它们的消除有一个独特的名称。如果X和是是双生的,它们的去除是一种-消除。如果它们是相邻的,它是一个b-消除。如果X和是彼此隔开n原子,它们的去除被称为1,n- 消除,即,1,3−,1,4-消除,等等F一世G在r和4.1.

第 4 章仅限于讨论最重要的消除,即烯烃形成b-淘汰赛。

在这本书中,一种- 消除仅与其应用相关的描述:一种- 消除$\boldsymbol{\alpha}$-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 $\alpha$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.

化学代写|有机化学代写ORGANIC CHEMISTRY代考|THE TERMS SYN- AND ANTI-ELIMINATION

在各种消除中,该机制意味着消除的原子或基团之间存在明确的立体关系X和是和由此产生的平面C=C双键F一世G在r和4.2. 例如,X和是可能会留在这个双键两侧的同一个半空间中。然后将它们的移除称为s是n- 或ciselimination。乙和C一种r和F在l在H和n在s一世nG吨H和s和C这nd吨和r米:C一世s−和l一世米一世n一种吨一世这nsC一种n一种ls这G一世在和吨r一种ns−这r$和$−这l和F一世ns!还有其他消除组X离开基板在一个半空间和组的方向是朝着另一半空间的方向离开,都在侧翼C=C产生的双键。这些是所谓的反消除或反消除。一种G一种一世n,b和C一种r和F在l一种b这在吨在s一世nG吨H和s和C这nd吨和r米:吨r一种ns−和l一世米一世n一种吨一世这ns一种ls这C一种nl和d吨这C一世s−这r$从$−这l和F一世ns!第三种可能性涉及消除组之间不需要明确的空间关系的消除X和是被去除和得到的双键的平面。

化学代写|有机化学代写ORGANIC CHEMISTRY代考|WHEN ARE STEREOGENIC SYN- AND ANTI-SELECTIVE ELIMINATIONS STEREOSELECTIVE?

一种消除,其中一个顺式、一个反式、一个和, 或从双键的产生可以称为立体消除。严格的同义和反选择性立体基因b消除X和是可能但不一定是立体选择性的。它们在底物中时必然是立体选择性的X和是绑定到立体中心。这种类型的立体选择性的例子由图4.2为了X≠R1≠R2和是≠R3≠R4. 另一方面,立体选择性不能保证X和是从具有该结构的基板上去除X−CR1R2−C是2−R3,也就是说,从其中的基板是不绑定到立体中心,但仅X是b和C一种在s和$R1≠R2≠X$.

除其他外,最后一种情况的特点是从以下种类的基材中消除:1实体X要去除的是卤素原子或通过杂原子连接到发展中的烯属碳原子的基团$\mathrm{X}$ 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

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