School of Chemistry, Monash University, Wellington Road, Clayton VIC 3800, Australia.
J Phys Chem B. 2010 Dec 16;114(49):16517-27. doi: 10.1021/jp108550z. Epub 2010 Nov 18.
In this work the nucleophilic addition of water and methanol to the dicyanonitrosomethanide anion (dcnm, C(CN)(2)(NO)) in the absence of the usual transition metal promoters was investigated. Experimentally it was shown that a quantitative conversion of the dcnm anion to carbamoylcyanonitrosomethanide (ccnm, C(CN)(CONH(2))(NO)) by the addition of 1 equiv of water to a nitrile group is complete in 48 h at 100 °C, or in 1.5 h at 150 °C when the reaction is conducted in a microwave reactor. Attempts to add a second equivalent of water to the anion failed with thermal degradation of the anion occurring at 200 °C. Ab initio calculations show that the reaction proceeds via three distinct transition states: (1) the transfer of a proton from a water molecule to the nitrile group, (2) the subsequent attack of the generated hydroxide anion on the carbon atom of the nitrile group, and (3) a rapid proton transfer to form a carbamoyl group. The attacking water molecule is shown to be a stronger proton donor when modeled as part of a hydrogen-bonded three water molecule chain, leading to a significant reduction in the reaction barrier. Only the anti-ccnm anion is formed in the reaction. There is a high-energy barrier to the formation of the syn isomer by the rotation of the nitroso group. While the syn isomer of ccnm is shown to be the more thermodynamically stable conformation, examination of the HOMO-1 molecular orbital that arises during the second transition state of the reaction indicates the addition of the hydroxide anion to the carbon atom is forbidden due to orbital symmetry, with a similar effect responsible for the failure of a second equivalent of water to add to the ccnm anion. Under analogous reaction conditions the addition of 1 equiv of methanol to dcnm to form cyano(imino(methoxy)methyl)nitrosomethanide (cmnm, C(CN)(C(OMe)NH)(NO)) failed, although ab initio calculations initially indicated the reaction should proceed more readily than the addition of water. When the energy required to break the hydrogen-bonded cyclic hexamers in methanol is taken into consideration, the energy barrier to the first transition step is greatly increased. The addition of a second equivalent of methanol to cmnm is unlikely to occur even in the presence of a transition metal as the resultant anion would be marginally thermodynamically unstable.
在这项工作中，研究了在没有常见过渡金属促进剂的情况下，水和甲醇对二氰亚硝代甲酰胺阴离子（dcnm，[C(CN)(2)(NO)]-）的亲核加成。实验表明，在 100°C 下，向一个腈基中加入 1 当量的水可使 dcnm 阴离子完全转化为氨甲酰氰亚硝代甲酰胺（ccnm，[C(CN)(CONH2)(NO)]-），反应在 48 小时内完成，而在 150°C 下微波反应器中反应 1.5 小时即可完成。尝试向阴离子中添加第二当量的水时，由于阴离子在 200°C 时发生热降解，反应失败。从头算计算表明，反应通过三个不同的过渡态进行：（1）质子从水分子转移到腈基，（2）生成的氢氧根阴离子随后攻击腈基上的碳原子，（3）快速质子转移形成氨甲酰基。当将水分子建模为氢键三水分子链的一部分时，表明其为更强的质子供体，从而显著降低了反应势垒。反应中仅形成反式 ccnm 阴离子。硝基肟基团旋转形成顺式异构体的能垒很高。虽然顺式 ccnm 异构体的热力学稳定性更高，但在反应的第二个过渡态中出现的 HOMO-1 分子轨道的研究表明，由于轨道对称性，氢氧根阴离子加成到碳原子上是被禁止的，类似的效应导致第二当量的水无法添加到 ccnm 阴离子中。在类似的反应条件下，向 dcnm 中加入 1 当量的甲醇形成氰基（亚氨基（甲氧基）甲基）亚硝代甲酰胺（cmnm，[C(CN)(C(OMe)NH)(NO)]-），尽管从头算计算最初表明反应会比加水更容易进行，但反应未能发生。当考虑到甲醇中氢键环状六聚体断裂所需的能量时，第一个过渡步骤的能垒大大增加。即使存在过渡金属，向 cmnm 中加入第二当量的甲醇也不太可能发生，因为生成的阴离子在热力学上仅略微不稳定。
