Wang Jin, Burdzinski Gotard, Kubicki Jacek, Platz Matthew S
Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio, 43210, USA.
J Am Chem Soc. 2008 Aug 20;130(33):11195-209. doi: 10.1021/ja803096p. Epub 2008 Jul 26.
The photochemistry of a p-biphenylyl diazo ester (BpCN2CO2CH3) and diazo ketone (BpCN2COCH3) were studied by ultrafast time-resolved UV-vis and IR spectroscopies. The excited states of these diazo compounds were detected and found to decay with lifetimes of less than 300 fs. The diazo ester produces singlet carbene with greater quantum efficiency than the ketone analogue due to competing Wolff rearrangement (WR) in the excited state of the diazo ketone. Carbene BpCCO2CH3 has a singlet-triplet gap that is close to zero in cyclohexane, but the triplet is the ground state. The two spin states are in rapid equilibrium in this solvent relative to reaction with cyclohexane. There is (for a carbene) a slow rate of singlet to triplet intersystem crossing (isc) in this solvent because the orthogonal singlet must rotate to a higher energy orientation prior to isc. In acetonitrile and in dichloromethane BpCCO2CH3 has a singlet ground state. Ketocarbene BpCCOCH3 has a singlet ground state in cyclohexane, in dichloromethane, and in acetonitrile and decays by WR to form a ketene detected by ultrafast IR spectroscopy in these solvents. Ketocarbenes have more stable singlet states, relative to carbene esters, because of the superior conjugation of the filled hybrid orbital of the carbene with the pi system of the carbonyl group, the same factor that makes methyl ketones more acidic than the analogous esters. The rate of WR of BpCCOCH3 is faster in cyclohexane than in dichloromethane and acetonitrile because of intimate solute-solvent interactions between the empty p orbital of the carbene and nonbonding electron pairs of heteroatoms of the solvent. These interactions stabilize the carbene and retard the rate of WR.
通过超快时间分辨紫外可见光谱和红外光谱研究了对二苯基重氮酯（BpCN2CO2CH3）和重氮酮（BpCN2COCH3）的光化学性质。检测到这些重氮化合物的激发态，并发现其寿命小于300飞秒。由于重氮酮激发态中的沃尔夫重排（WR）竞争，重氮酯产生单线态卡宾的量子效率比重氮酮类似物更高。卡宾BpCCO2CH3在环己烷中的单线态-三线态能隙接近零，但三线态是基态。相对于与环己烷的反应，这两种自旋态在该溶剂中处于快速平衡。在该溶剂中（对于卡宾而言）单线态到三线态的系间窜越（isc）速率较慢，因为正交单线态在isc之前必须旋转到更高能量的取向。在乙腈和二氯甲烷中，BpCCO2CH3具有单线态基态。酮卡宾BpCCOCH3在环己烷、二氯甲烷和乙腈中具有单线态基态，并通过WR衰变形成在这些溶剂中通过超快红外光谱检测到的烯酮。由于卡宾的填充杂化轨道与羰基的π体系具有更好的共轭作用，酮卡宾相对于卡宾酯具有更稳定的单线态，这也是甲基酮比类似酯更酸性的相同因素。由于卡宾的空p轨道与溶剂杂原子的非键电子对之间存在紧密的溶质-溶剂相互作用，BpCCOCH3在环己烷中的WR速率比在二氯甲烷和乙腈中更快。这些相互作用使卡宾稳定并减缓WR速率。
