期刊
ORGANOMETALLICS
卷 42, 期 19, 页码 2813-2825出版社
AMER CHEMICAL SOC
DOI: 10.1021/acs.organomet.3c00282
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This study reports the synthesis and characterization of novel Zn(II)-based organocations and their application in catalytic reactions. The results demonstrate that these cations exhibit good performance in catalyzing hydrosilylation reactions.
The present study details the synthesis and characterization of novel Zn(II)-based organocations of the type [IPr*-Zn-R](+) (IPr* = 1,3-bis[2,6-bis(diphenylmethyl)-4-methylphenyl]-1,3-dihydro-2H-imidazol-2-ylidene; R = alkyl, aryl) and their use in styrene, alkyne, and carbonyl hydrosilylation catalysis. The neutral IPr* adducts [IPr*-ZnR2] (1, R = Me; 2, R = Et; 3, R = Ph) were prepared by the reaction of IPr* with an equimolar amount of ZnR2 and isolated in good yields. Despite the severe steric hindrance of IPr*, compounds 1-3 are robust in solution, reflecting the bulky-yet-flexible nature of carbene IPr*. Adducts 1 and 2 can be readily ionized by [Ph3C][B(C6F5)(4)] to produce two-coordinate Zn(II) cations [IPr*-ZnMe](+) ([4](+)) and [IPr*-ZnEt](+) ([5](+)), both isolated in high yields (>85%) as [B(C6F5)(4)](-) salts. Interestingly, the more Lewis acidic cation [IPr*-ZnC6F5](+) ([6](+)), prepared by reaction of [4][B(C6F5)(4)] with a B(C6F5)(3)/HSiEt3 mixture, is further stabilized through pi arene interactions with Zn(II), indicating that IPr* may provide steric and electronic stabilization to Zn(II). The latter certainly explains the improved hydrolytic stability of the salt [6][B(C6F5)(4)]. Zn cations of the [IPr*-ZnR](+) series are less Lewis acidic than their [IPr-ZnR](+), yet they display a distinct reactivity in hydrosilylation catalysis. Thus, cation [6](+) catalyzes at room-temperature styrene and alkyne hydrosilylation with HSiEt3 as the silane source. Remarkably, it is also a highly effective ketone/aldehyde hydrosilylation catalysis for a rather broad silane and ketone scope and performs much better than [IPr-ZnR](+) systems. The density functional theory (DFT)-estimated mechanism for the hydrosilylation of benzophenone by [6](+) suggests that Si-H activation by the cationic Zn(II) center is required for the catalysis to proceed. Overall, the improved hydrolytic stability and straightforward synthesis of a well-defined Zn-based Lewis acid such as [6][B(C6F5)(4)] may promote its further use in various Lewis-acid-mediated transformations.
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