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# 化学代写|有机化学代写organic chemistry代考|Vocabulary of Stereochemistry and Stereoselective Synthesis II: Topicity, Asymmetric Synthesis

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

When all these results are analyzed accurately, it is seen that addition reactions to the $\mathrm{C}=\mathrm{X}$ double bond

• of achiral compounds (i.e., to $\mathrm{C}=\mathrm{X}$ double bonds with homotopic or enantiotopic faces) cannot take place enantioselectively; they always give racemic mixtures (e.g., hydroboration in Figure 3.19);
• of chiral compounds (i.e., to $\mathrm{C}=\mathrm{X}$ double bonds with diastereotopic faces) can take place diastereoselectively with achiral reagents. In this case we have substrate control of stereoselectivity (e.g., hydroborations in Figures $3.20$ and 3.21); or
• of achiral compounds (i.e., to $\mathrm{C}=\mathrm{X}$ double bonds with homotopic or enantiotopic faces) with chiral reagents can lead to enantiomerically enriched or enantiomerically pure compounds.

An enantioselective addition of the latter type (here) or, in general, the successful conversion of achiral starting materials into enantiomerically enriched or enantiomerically pure products is referred to as asymmetric synthesis (for examples, see Sections 3.4.2, 3.4.6, 8.4, 8.5.2, 14.4.7).

## 化学代写|有机化学代写organic chemistry代考|Asymmetric Hydroboration of Achiral Olefins

The conclusion drawn from Section 3.4.1 for the hydroborations to be discussed here is this: an addition reaction of an enantiomerically pure chiral reagent to a $\mathrm{C}=\mathrm{X}$ double bond with enantiotopic faces can take place via two transition states that are diastereomorphic and thus generally differing from one another in energy. In agreement with this statement, there are diastereoselective additions of enantiomerically pure mono- or dialkylboranes to $\mathrm{C}=\mathrm{C}$ double bonds that possess enantiotopic faces. Consequently, when one subsequently oxidizes all $\mathrm{C}-\mathrm{B}$ bonds to $\mathrm{C}-\mathrm{OH}$ bonds, one has realized an enantioselective hydration of the respective olefin.

An especially efficient reagent of this type is the boron-containing five-membered ring compound shown in Figure 3.24. Since this reagent is quite difficult to synthesize, it has not been used much in asymmetric synthesis. Nonetheless, this reagent will be presented here simply because it is particularly easy to see which face of a $\mathrm{C}=\mathrm{C}$ double bond it will attack.

In the structure shown in Figure $3.24$ the top side attack of this borane on the $\mathrm{C}=\mathrm{C}$ double bond of 1-methylcyclohexene prevails kinetically over the bottom side attack. This is because only the top side attack of the boranes avoids steric interactions between the methyl substituents on the borane and the six-membered ring. In other words, the reagent determines the face to which it adds. We thus have reagent control of stereoselectivity. As a result, the mixture of the diastereomeric trialkylboranes $\mathbf{C}$ and $\mathbf{D}$, both of which are pure enantiomers, is produced with $d s=97.8: 2.2$. After the normal $\mathrm{NaOH} / \mathrm{H}{2} \mathrm{O}{2}$ treatment, they give a $97.8: 2.2$ mixture of the enantiomeric trans-2-methylcyclohexanols. The $1 R, 2 R$ alcohol will therefore have an $e e$ value of $97.8 \%-$ $2.2 \%=95.6 \%$.

The $S, S$ enantiomer of this alcohol is obtained with the same $e e$ value of $95.6 \%$ when the enantiomer of the borane shown in Figure $3.24$ is used for the hydroboration of 1methylcyclohexene. The first problem we ran into in the introduction to Section $3.4$ is thereby solved!

## 化学代写|有机化学代写ORGANIC CHEMISTRY代考|Thought Experiment I on the Hydroboration of Chiral Olefins with Chiral Boranes: Mutual Kinetic Resolution

During the addition of a racemic chiral dialkylborane to a racemic chiral olefin a maximum of four diastereomeric racemic trialkylboranes can be produced. Figure $3.25$ il-lustrates this using the example of the hydroboration of 3 -ethyl-1-methylcyclohexene with the cyclic borane from Figure $3.24$. This hydroboration, however, was never carried out experimentally. This should not prevent us from considering what would happen if it were carried out.

Our earlier statements on substrate and reagent control of stereoselectivity during hydroborations are incorporated in Figure 3.25. Because of the obvious analogies between the old and the new reactions, the following can be predicted about the product distribution shown:

• As the main product we expect the racemic trialkylborane $\mathbf{E}$; the substrate and the reagent controls work together to promote its formation.
• The racemic trialkylborane $\mathbf{H}$ should be produced in trace quantities only. Its formation is disfavored by both the reagent and the substrate control.
• As minor products we expect the racemic trialkylboranes $\mathbf{F}$ and/or $\mathbf{G} ; \mathbf{F}$ is favored by reagent and disfavored by substrate control of stereoselectivity, whereas for $\mathbf{G}$ it is exactly the opposite.

We thus summarize: the yield ratios of the conceivable hydroboration products $\mathbf{E}$, $\mathbf{F}, \mathbf{G}$, and H should be much:little:little:none. One enantiomer of E comes from the reaction of the $S$-olefin with the $S, S$-borane; the other enantiomer of $\mathbf{E}$ comes from the reaction of the $R$-olefin with the $R, R$-borane. Thus each enantiomer of the reagent has preferentially reacted with one enantiomer of the substrate. The diastereoselectivity of this reaction thus corresponds to a mutual kinetic resolution.

The condition for the occurrence of a mutual kinetic resolution is thus that considerable substrate control of stereoselectivity and considerable reagent control of stereoselectivity occur simultaneously.

From the diastereoselectivities in Figure $3.25$ one concludes the following for the rate constants: $k_{5}>k_{6}, k_{7}>k_{8}$ (which implies $k_{5} \gg k_{8}$ ). It is not known whether $k_{6}$ or $k_{7}$ is greater.

For the discussion in Sections $3.4 .4$ and $3.4 .5$, we will assume(!) that $k_{6}>k_{7}$; that is, the reagent control of stereoselectivity is more effective than the substrate control of stereoselectivity. The justification for this assumption is simply that it makes additional thought experiments possible. These are useful for explaining interesting phenomena associated with stereoselective synthesis, which are known from other reactions. Because the thought experiments are much easier to understand than many of the actual experiments, their presentation is given preference.

## 化学代写|有机化学代写ORGANIC CHEMISTRY代考|A CYCLOADDITION FORMING THREE-MEMBERED RINGS

• of achiral compounds (i.e., to $\mathrm{C}=\mathrm{X}$ double bonds with homotopic or enantiotopic faces) cannot take place enantioselectively; they always give racemic mixtures (e.g., hydroboration in Figure 3.19);
• of chiral compounds (i.e., to $\mathrm{C}=\mathrm{X}$ double bonds with diastereotopic faces) can take place diastereoselectively with achiral reagents. In this case we have substrate control of stereoselectivity (e.g., hydroborations in Figures $3.20$ and 3.21); or
• of achiral compounds (i.e., to $\mathrm{C}=\mathrm{X}$ double bonds with homotopic or enantiotopic faces) with chiral reagents can lead to enantiomerically enriched or enantiomerically pure compounds.

## 化学代写|有机化学代写ORGANIC CHEMISTRY代考|THOUGHT EXPERIMENT I ON THE HYDROBORATION OF CHIRAL OLEFINS WITH CHIRAL BORANES: MUTUAL KINETIC RESOLUTION

• 作为主要产品，我们期望外消旋三烷基硼烷和; 底物和试剂对照共同作用以促进其形成。
• 外消旋三烷基硼烷H应仅以微量生产。试剂和底物对照都不利于其形成。
• 作为次要产品，我们预计外消旋三烷基硼烷F和/或G;F受试剂的青睐而不受立体选择性的底物控制的影响，而对于G恰恰相反。

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