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## 化学代写|有机化学代写organic chemistry代考|A Cycloaddition Forming Three-Membered Rings

Cycloadditions are ring-forming addition reactions in which the product, the so-called cycloadduct, possesses an empirical formula that corresponds to the sum of the empirical formulas of the starting material and the reagent. All one-step cycloadditions take place with cis selectivity. Three-, four-, five-, or six-membered but not larger rings can be produced by cycloadditions to monoolefins (Figure 3.10).

You will become familiar with selected cycloadditions that lead to four-, five-, or sixmembered rings in Chapter 12. Two more cycloadditions, which are also oxidations, will be examined in Chapter 14, which deals with oxidations and reductions: the ozonolysis reaction can be found in Section 14.3.2 (as well as in Section 12.5.5) and the cisvic dihydroxylation with $\mathrm{OsO}_{4}$ can be found in Section 14.3.2. Here we discuss only the addition of dichlorocarbene to olefins as an example of a cis addition of the cycloaddition type (Figure 3.11).

Dichlorocarbene cannot be isolated, but it can be produced in the presence of an olefin and then reacted with it immediately. The best dichlorocarbene precursor is the anion $\mathrm{Cl}{3} \mathrm{C}^{-}$, which easily eliminates a chloride ion. This anion is obtained from $\mathrm{Cl}{3} \mathrm{CH}$ and fairly strong bases. The $\mathrm{OH}^{-}$ion is sufficiently basic for effecting this deprotonation provided that it comes into contact with the $\mathrm{Cl}{3} \mathrm{CH}$ molecules. Consequently, when chloroform is stirred with potassium hydroxide solution, there is only moderate conversion into the anion $\mathrm{Cl}{3} \mathrm{C}^{-}$. This is because $\mathrm{Cl}{3} \mathrm{CH}$ and aqueous $\mathrm{KOH}$ are not miscible so that $\mathrm{KOH}$ cannot efficiently migrate from the aqueous phase into the $\mathrm{Cl}{3} \mathrm{CH}$. The solvation of the $\mathrm{K}^{+}$and $\mathrm{OH}^{-}$ions in the organic phase would be far too low.
The transfer of only the $\mathrm{OH}^{-}$ions into the chloroform, however, succeeds very well in the presence of a phase transfer catalyst. The most frequently used phase transfer catalysts are tetraalkylammonium chlorides or tetraalkylammonium hydrogensulfates. With a large excess of $\mathrm{OH}^{-}$ions, they give the corresponding tetraalkylammonium hydroxides in an equilibrium reaction. These dissolve in chloroform because the cation is so large that it requires practically no solvation. Via the Coulomb attraction, these cations pull the nonsolvatable $\mathrm{OH}^{-}$ions with them into the chloroform phase.

## 化学代写|有机化学代写organic chemistry代考|Additions to C“C Double Bonds That Are Related to Cycloadditions and Form Three-Membered Rings, Too

In contrast to the dichlorocyclopropanations from Section 3.3.1, the reactions discussed in this section are not cycloadditions in a strict sense. The reason is that the empirical formula of the addition products presented here is not equal to the sum of the empirical formulas of the reaction partners. Accordingly, the reaction products here are not cycloadducts in the strict sense.

The addition of the Simmons-Smith reagent to olefins leads in a single step to chlorine-free cyclopropanes (Figure 3.12). This addition also runs stereoselectively and stereospecifically. A Simmons-Smith reagent can be produced from diiodomethane and $\mathrm{Zn} / \mathrm{Cu}$ couple, which in turn is produced from zinc dust and catalytic amounts of $\mathrm{CuSO}{4}, \mathrm{CuCl}{2}$, or $\mathrm{Cu}(\mathrm{OAc}){2}$. This Simmons-Smith reagent is usually assigned the structural formula $\mathrm{I}-\mathrm{CH}{2}-\mathrm{ZnI}$. This species, however, is subject to an equilibrium similar to the one that is known as the Schlenk equilibrium for Grignard compounds (cf. Figure 8.1):
$$2 \mathrm{I}-\mathrm{CH}{2}-\mathrm{ZnI} \rightleftharpoons \mathrm{I}-\mathrm{CH}{2}-\mathrm{Zn}-\mathrm{CH}{2}-\mathrm{I}+\mathrm{ZnI}{2}$$
In the Simmons-Smith reaction, as formulated in Figure 3.12, it has seemingly been clarified that the attacking species is $\mathrm{I}-\mathrm{CH}{2}-\mathrm{ZnI}$ rather than $\mathrm{I}-\mathrm{CH}{2}-\mathrm{Zn}-\mathrm{CH}{2}-\mathrm{I}$, which is in equilibrium with it in small amounts. A reagent that has properties quite similar to those of the actual Simmons-Smith reagent (but is advantageously produced in a homogeneous reaction) results from a halogen/metal exchange between diiodomethane and $\mathrm{ZnEt}{2}$. It can be written as $\mathrm{I}-\mathrm{CH}_{2}-\mathrm{ZnEt}$.

## 化学代写|有机化学代写ORGANIC CHEMISTRY代考|cis-Hydration of Olefins via the Hydroboration/Oxidation/Hydrolysis Reaction Sequence

In monoborane $\left(\mathrm{BH}{3}\right)$, monoalkylboranes $\mathrm{RBH}{2}$, or dialkylboranes $\mathrm{R}{2} \mathrm{BH}$ there is only an electron sextet at the boron atom. In comparison to the more stable electron octet, the boron atom thus lacks two valence electrons. It “obtains” them by bonding with a suitable electron pair donor. When no better donor is available, the bonding electron pair of the B-H bond of a second borane molecule acts as the donor so that a “twoelectron, three-center bond” is produced. Under these conditions, boranes are consequently present as dimers: ” $\mathrm{BH}{3}$,” for example, as $\mathrm{B}{2} \mathrm{H}{6}$. Still, small fractions of the monomers appear as minor components in the dissociation equilibrium of the dimer: $\mathrm{B}{2} \mathrm{H}{6}$, for example, thus contains some $\mathrm{BH}_{3}$.

The Lewis bases $\mathrm{Me}{2} \mathrm{~S}$ or THF are better electron pair donors than the boranes themselves. Therefore, they convert dimeric boranes into $\mathrm{Me}{2} \mathrm{~S}$ or THF complexes of the monomers such as $\mathrm{Me}{2} \mathrm{~S} \cdots \mathrm{BH}{3}$ or THF $\cdots \mathrm{BH}_{3}$. This type of complex also dissociates to a small extent and thus contains a small equilibrium concentration of free monomeric boranes.

## 化学代写|有机化学代写ORGANIC CHEMISTRY代考|ADDITIONS TO C“C DOUBLE BONDS THAT ARE RELATED TO CYCLOADDITIONS AND FORM THREE-MEMBERED RINGS, TOO

$$2 \mathrm{I}-\mathrm{CH}{2}-\mathrm{ZnI} \rightleftharpoons \mathrm{I}-\mathrm{CH}{2}-\mathrm{Zn}-\mathrm{CH}{2}-\mathrm{I}+\mathrm{ZnI}{2}$$

## Matlab代写

MATLAB 是一种用于技术计算的高性能语言。它将计算、可视化和编程集成在一个易于使用的环境中，其中问题和解决方案以熟悉的数学符号表示。典型用途包括：数学和计算算法开发建模、仿真和原型制作数据分析、探索和可视化科学和工程图形应用程序开发，包括图形用户界面构建MATLAB 是一个交互式系统，其基本数据元素是一个不需要维度的数组。这使您可以解决许多技术计算问题，尤其是那些具有矩阵和向量公式的问题，而只需用 C 或 Fortran 等标量非交互式语言编写程序所需的时间的一小部分。MATLAB 名称代表矩阵实验室。MATLAB 最初的编写目的是提供对由 LINPACK 和 EISPACK 项目开发的矩阵软件的轻松访问，这两个项目共同代表了矩阵计算软件的最新技术。MATLAB 经过多年的发展，得到了许多用户的投入。在大学环境中，它是数学、工程和科学入门和高级课程的标准教学工具。在工业领域，MATLAB 是高效研究、开发和分析的首选工具。MATLAB 具有一系列称为工具箱的特定于应用程序的解决方案。对于大多数 MATLAB 用户来说非常重要，工具箱允许您学习应用专业技术。工具箱是 MATLAB 函数（M 文件）的综合集合，可扩展 MATLAB 环境以解决特定类别的问题。可用工具箱的领域包括信号处理、控制系统、神经网络、模糊逻辑、小波、仿真等。