Oxidative Addition of Chlorohydrocarbons to a Rhodium Tris(pyrazolyl)borate Complex

The reactive fragment [Tp′Rh­(PMe3)], generated from the thermal precursor Tp′Rh­(PMe3)­(Me)­H, is found to cleave the C–Cl bonds of chlorohydrocarbons under mild conditions. Reaction with chloromethane gives clean formation of an initial C–H activation product, which rearranges to form the C–Cl activation product at 30 °C. Reaction with dichloromethane or benzyl chloride gives a mixture of C–Cl activation products as well as products from chlorination. Reaction with chlorocyclohexane gives a mixture of intermediates from C–H activation, which react further upon heating to give a C–Cl cleavage product as well as the β-chloride elimination product Tp′Rh­(PMe3)­(Cl)H plus cyclohexene. Complete conversion from a C–H activation product to a C–Cl activation product was observed in the reaction with 1,2-dichloroethylene, where β-elimination is circumvented. Activation of 1-chlorobutane, 1,2-dichloroethane, or 1,4-dichlorobutane gives a mixture of C–Cl activation products as well as Tp′Rh­(PMe3)­(Cl)­H plus olefin. Similar to the case for activation of methylene chloride, C–Cl activation and hydride/chloride exchange was observed in the reaction with benzyl chloride, where C–H activation was not seen. The reaction with chlorobenzene gives isomeric species resulting from C–H activation, which react further to give the corresponding chloride derivatives upon heating. Reaction with pentachlorobenzene gives a cyclometalated product from C–H bond cleavage in the phosphine ligand. These reactions are compared and contrasted with related photoreactions with the [Tp′Rh­(CNneopentyl)] analogue, where C–H activation is solely observed in most cases. Mechanistic studies suggest the spectator ligand dependent reactivity relies greatly on the dissociation energy of the Tp′Rh–L bond.

本研究表明，由热前驱体Tp′Rh(PMe3)(Me)H生成的活性片段[Tp′Rh(PMe3)]，可在温和条件下断裂氯代烃的碳氯（C–Cl）键。与氯甲烷反应时，可干净地得到初始碳氢键（C–H）活化产物，该产物在30 ℃下会重排为碳氯键活化产物。与二氯甲烷或苄氯反应时，会生成碳氯键活化产物与氯化产物的混合物。与氯代环己烷反应时，可得到碳氢键活化的中间体混合物，经加热后可进一步反应生成碳氯键断裂产物，同时伴随β-氯消除产物Tp′Rh(PMe3)(Cl)H与环己烯的生成。在与1,2-二氯乙烯的反应中，可观测到碳氢键活化产物完全转化为碳氯键活化产物，该过程无β-消除反应发生。活化1-氯丁烷、1,2-二氯乙烷或1,4-二氯丁烷时，会得到碳氯键活化产物与Tp′Rh(PMe3)(Cl)H及烯烃的混合物。与二氯甲烷的反应情况类似，与苄氯的反应中也观测到了碳氯键活化及氢负离子/氯离子交换过程，未检测到碳氢键活化产物。与氯苯反应时，会生成碳氢键活化的异构物种，经加热后可进一步转化为对应的氯化衍生物。与五氯苯反应时，会在膦配体上发生碳氢键断裂，生成环金属化产物。将上述反应与类似的[Tp′Rh(CNneopentyl)]类似物的光化学反应进行对比后发现，后者在大多数情况下仅能观测到碳氢键活化。机理研究表明，依赖于旁观配体的反应活性在很大程度上取决于Tp′Rh–L键的解离能。