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Counterion-mediated hydrogen-bonding effects: mechanistic study of gold(I)-catalyzed enantioselective hydroamination of allenes.
| Content Provider | Semantic Scholar |
|---|---|
| Author | Park, Sung-Woo Park, Sae Rom Lee, Sungyul Kang, Eun Joo |
| Copyright Year | 2011 |
| Abstract | The use of transition metals as unsaturated C C bond activators toward nucleophilic attack continues to grow exponentially for efficient and atom-economic organic transformations. The majority of nucleophilic additions proceed through the outer-sphere mechanism, that is, anti addition to the metal-coordinated C C multiple bond complexes. However, few alternative inner-sphere mechanisms have been proposed, in which the coordination of the nucleophile to the metal is followed by insertion of a C C multiple bond into the M Nu bond, such as the Ir, Ln, Pd, and Au-catalyzed hydrofunctionalization reactions. While the detailed variation of the inner-sphere mechanism highly depends on the electronic and redox character of transition metals, more importantly, these reactions are distinguished by syn-stereochemical pathways from the outer-sphere mechanism. While intensive stereochemical investigations into various kinds of gold(I) catalysis suggested the anti-addition mechanism, our specific interest began from the significant enantioselectivity in the gold(I)-catalyzed transformations of allenes. Despite a number of reports on transition metal catalyzed addition of heteroatom nucleophiles to allenes, the asymmetric variants are reported exclusively with the gold catalyst. This fact stimulates ideas for the perfect role of a gold catalyst in the challenging control of stereoselectivities, especially when substituted allenes are used as substrates. Enantioselective gold(I) catalysis is not restricted to classic p-activation processes. The efficient gold catalyst has been selected as the form of chiral bis ACHTUNGTRENNUNG(gold)-phosphine complexes, which could lead to formation of aurophilic Au Au interactions. Another intriguing phenomenon is the pronounced counterion effect that points to the importance of nonbonding interactions between the auxiliary (AuX) group and the reactive gold center. Herein we report a new type of the counterion-directed syn-addition pathway in gold-catalyzed hydroamination reactions. In parallel, we have found that the nonbonding interaction between the nucleophile and gold in the pre-reaction complex is the origin of the enantioselectivity of complex allene substrates. To obtain insight into the possible origins of enantioselectivity in the gold-catalyzed hydroamination reactions, we have carried out both experiments and quantum chemical studies, employing the density functional theory method B3LYP with the 6-31G basis set and the effective core potential for Au (Hay–Wadt VDZ), as implemented in the Gaussian 09 set program. Stationary structures are confirmed by ascertaining that all the harmonic frequencies are real. The structure of the transition state is obtained by verifying that one and only one of the harmonic frequencies is imaginary, and also by carrying out the intrinsic reaction coordinate (IRC) analysis along the reaction pathway. Zero point energies (ZPE) are taken into account, and default criteria are used for all optimizations. Based on the previous observations in the gold-catalyzed enantioselective hydroamination (Scheme 1), the enantioselectivity was concluded to be strongly affected by the remaining counterion coordinated to gold, and the para-nitrobenzoate (OPNB) counterion proved to be an ideal one to transfer the chiral information of the 2,2’-bis(diphenylphosphino)-1,1’-binaphthyl (binap) ligand to the relatively distant reaction center. Furthermore, this counterion effect in goldcatalyzed hydroamination was developed and applied by using a chiral 2,2’-dihydroxy-1,1’-binaphthyl (binol)-derived phosphate anion, thus rendering the chiral counterion-mediated transition metal catalysis powerful. To check the role of the counterion in the catalytically active species, we [a] J. H. Kim, S.-W. Park, S. R. Park, Prof. S. Lee, Prof. E. J. Kang Department of Applied Chemistry Kyung Hee University Yongin-si, Gyeonggi-do, 446-701 (Korea) Fax: (+82) 31-202-7337 E-mail : sylee@khu.ac.kr ejkang24@khu.ac.kr [] These authors contributed equally to this work. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201100135. |
| File Format | PDF HTM / HTML |
| DOI | 10.1002/asia.201100135 |
| PubMed reference number | 21735554 |
| Journal | Medline |
| Volume Number | 6 |
| Issue Number | 8 |
| Alternate Webpage(s) | http://orgchem.khu.ac.kr/pu/15%20CAJ_11_1982.pdf |
| Alternate Webpage(s) | https://doi.org/10.1002/asia.201100135 |
| Journal | Chemistry, an Asian journal |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
