Isoxazole can isomerize to oxazole under light irradiation. The mechanism of this process is as follows:

<MultiModal>![Isoxazole C1=CC=NO1 is first converted to $\bf{X1}$ under light irradiation, then to $\bf{X2}$, then to $\bf{X3}$, and finally to oxazole C1=CN=CO1](/media/uploads/e9dedbd8-ff75-4f29-8e0f-fb6abfc19a3d.png)</MultiModal>

Where $\bf{X2}$ contains a high-strain ring system.

The following statements are made:

1. $\bf{X1}$ does not have a conjugated system.

2. $\bf{X2}$ does not have a conjugated system.

3. $\bf{X3}$ has a charge-separated structure.

4. The carbon atom bonded to the nitrogen atom in oxazole is carbon atoms 3 and 4 in isoxazole.

Which of the following options contains the most correct statements?
异恶唑在光照条件下可异构为恶唑，这一过程机理如下：

<MultiModal>![异恶唑C1=CC=NO1在光照下首先转化为$\bf{X1}$，随后转化为$\bf{X2}$，接着转化为$\bf{X3}$，最后转化为恶唑C1=CN=CO1](/media/uploads/e9dedbd8-ff75-4f29-8e0f-fb6abfc19a3d.png)</MultiModal>

其中$\bf{X2}$含有高张力的环系

有以下说法：

1.$\bf{X1}$中不具有共轭体系

2.$\bf{X2}$中不具有共轭体系

3.$\bf{X3}$中具有电荷分离的结构

4.恶唑中氮原子所连的碳原子为异恶唑中的3号和4号碳原子

以下选项中含有最多正确说法的是：
1
2
3
4
1,2
1,3
1,4
2,3
2,4
1,2,3
1,2,4
2,3,4
1
2
3
4
1,2
1,3
1,4
2,3
2,4
1,2,3
1,2,4
2,3,4
Under illumination conditions, isoxazole molecules absorb energy and transition to an excited state. The weakest bond in the molecule is the $\mathrm{N-O}$ bond, which undergoes homolytic cleavage, leading to ring opening to yield $\bf{X1}$: [N]=C/C=C\[O], where the nitrogen and oxygen atoms each carry a single electron; or [N]/C=C/C=O, which contains a nitrene. Both of these structures have conjugated systems.

<MultiModal>![The left side shows the structure of the diradical, [N]=C/C=C\[O], where the nitrogen and oxygen atoms each carry a single electron; the right side shows the structure of the nitrene, [N]/C=C/C=O](/media/uploads/06c32751-f9c7-4ae7-8de5-1bb79cd65e83.png)</MultiModal>

<Checkpoint>[All possible structures of $\bf{X1}$ have conjugated systems, statement 1 is incorrect](1)</Checkpoint>

$\bf{X1}$ is unstable and will rapidly undergo intramolecular cyclization. The nitrogen atom on the nitrene will attack the other carbon atom of the double bond to which it is attached, forming a three-membered ring. This three-membered ring intermediate is $\bf{X2}$: O=CC1C=N1, which does not have a conjugated system.

<Checkpoint>[The structure of $\bf{X2}$ is O=CC1C=N1, which does not have a conjugated system, statement 2 is correct](1)</Checkpoint>

The strained three-membered ring intermediate $\bf{X2}$ will open again to release strain. This time, the weakest $\mathrm{C-C}$ single bond on the ring breaks, and the lone pair of electrons on the nitrogen atom participates in the formation of a new double bond, generating $\bf{X3}$: C#[N+]/C=C\[O-], which has a charge-separated structure.

<Checkpoint>[The structure of $\bf{X3}$ is C#[N+]/C=C\[O-], which has a charge-separated structure, statement 3 is correct](1)</Checkpoint>

During the isomerization process, the nitrogen atom forms a bond with the original carbon atom at position 4 when $\bf{X2}$ is formed, and the bonding atoms do not change again during the subsequent transformation. Therefore, the nitrogen atom in oxazole forms a bond with the carbon atoms at positions 3 and 4 in the original isoxazole.

<Checkpoint>[The nitrogen atom forms a bond with the original carbon atom at position 4 when $\bf{X2}$ is formed, and the nitrogen atom in oxazole forms a bond with the carbon atoms at positions 3 and 4 in the original isoxazole, statement 4 is correct](1)</Checkpoint>

Select option L
在光照条件下，异恶唑分子吸收能量跃迁到激发态。分子中最弱的键是$\mathrm{N-O}$键，它会发生均裂，导致开环得到$\bf{X1}$：[N]=C/C=C\[O]，其中氮原子与氧原子分别带有一个单电子；或[N]/C=C/C=O，其中含有氮宾。这两种结构均具有共轭体系。

<MultiModal>![左侧展示出双自由基的结构，[N]=C/C=C\[O]，其中氮原子与氧原子分别带有一个单电子；右侧展示出氮宾的结构，[N]/C=C/C=O](/media/uploads/06c32751-f9c7-4ae7-8de5-1bb79cd65e83.png)</MultiModal>

<Checkpoint>[$\bf{X1}$的可能结构均具有共轭体系，说法1错误](1)</Checkpoint>

$\bf{X1}$是不稳定的，会迅速发生分子内环化。氮宾上的氮原子会进攻其所连接的双键的另一个碳原子，形成一个三元环。这个三元环中间体是$\bf{X2}$：O=CC1C=N1，不具有共轭体系。

<Checkpoint>[$\bf{X2}$的结构为O=CC1C=N1，不具有共轭体系，说法2正确](1)</Checkpoint>

张力的三元环中间体$\bf{X2}$会再次开环以释放张力。这次是环上最弱的$\mathrm{C-C}$单键断裂，同时氮原子上的孤对电子参与形成新的双键，生成$\bf{X3}$：C#[N+]/C=C\[O-]，具有电荷分离的结构

<Checkpoint>[$\bf{X3}$的结构为C#[N+]/C=C\[O-]，具有电荷分离的结构，说法3正确](1)</Checkpoint>

在异构过程中，形成$\bf{X2}$时氮原子与原4号位碳原子成键，随后转化中成键原子未再发生改变，因此恶唑中氮原子与原异恶唑中3、4号位碳原子成键。

<Checkpoint>[形成$\bf{X2}$时氮原子与原4号位碳原子成键，恶唑中氮原子与原异恶唑中3、4号位碳原子成键，说法4正确](1)</Checkpoint>

选择选项L