Lehrstuhl für Organische Chemie II, Ruhr-Universität Bochum, 44781 Bochum (Germany) http://www.rub.de/aksander.
Angew Chem Int Ed Engl. 2014 May 12;53(20):5122-5. doi: 10.1002/anie.201400176. Epub 2014 Apr 2.
Spin specificity is one of the most important properties of carbenes in their reactions. Alcohols are typically used to probe the reactive spin states of carbenes: O-H insertions are assumed to be characteristic of singlet states, whereas C-H insertions are typical for the triplets. Surprisingly, the experiments presented here suggest that the spin ground state of diphenylcarbene 1 switches from triplet to singlet if the carbene is allowed to interact with methanol. Carbene 1 and methanol form a strongly hydrogen-bonded singlet ground state complex that was synthesized in low-temperature matrices and characterized by IR spectroscopy. This methanol complex is only metastable, and even at 3 K slowly rearranges to form the product of O-H insertion through quantum chemical tunneling. Thus, the ground state triplet (in the gas phase) carbene 1 forms exclusively the products expected from a singlet carbene. Whereas the assumption of spin specific reactions of carbenes is correct, the spin state itself can be changed by solvent interactions, and therefore widely accepted conclusions drawn from earlier experiments have to be revisited.
旋光特异性是卡宾在反应中最重要的性质之一。醇通常用于探测卡宾的反应活性自旋态：O-H 插入被认为是单重态的特征，而 C-H 插入则是三重态的典型特征。令人惊讶的是，这里提出的实验表明，如果允许二苯卡宾 1 与甲醇相互作用，其自旋基态会从三重态转变为单重态。卡宾 1 和甲醇形成了一个强氢键的单重基态配合物，该配合物在低温基质中合成，并通过红外光谱进行了表征。这个甲醇配合物只是亚稳的，即使在 3 K 下也会缓慢重排，通过量子化学隧穿形成 O-H 插入的产物。因此，气相中的基态三重态卡宾 1 只会形成预期的单重态卡宾产物。虽然卡宾的自旋特异性反应的假设是正确的，但自旋态本身可以通过溶剂相互作用而改变，因此必须重新审视早期实验得出的广泛接受的结论。
