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18 Chiral Allyl and Crotylsilanes: Me Ph H N Si N Cl H 704725 Me N Si O Cl 706671 For a complete list of allylation reagents available from Aldrich Chemistry, visit Aldrich.com/allylations Selective Metalations using i-PrMgCl·LiCl and s-BuMgCl·LiCl While halogen-metal exchange reactions are among the most common methods for preparing reactive organometallic reagents, Li-halogen exchange reactions typically require low temperatures and off er limited compatibility with other functionalities. On the other hand, Mg-halogen exchange reactions require higher temperatures and are often prone to elimination side reactions. To address these issues, Knochel and coworkers have found that the use of salt additives increase both the rate and the effi ciency of this Mg-halogen exchange reaction. The most eff ective reagents are generated with R-MgCl (R = i-Pr, s-Butyl) and 1.0 equiv of LiCl. The increased reactivity of these aptly named TurboGrignards may be due to the breakup of polymeric aggregates known to exist in classical solutions of Grignard reagents. TurboGrignards allow for the conversion of a variety of functionalized and highly sensitive substrates, including those containing CO2R, CN, OMe, and halogen moieties, to their corresponding functionalized organometallic derivatives. While rate enhancements are observed with TurboGrignards, this increased reactivity does not have a negative impact on the overall scope of the reaction, permitting transformations to occur in the presence of a broad range of functional groups (Table 7). 1 Aldrich.com Br Br Me Ph H N Si N Cl H 705098 Me N Si O Cl 719056 Br Br H N Si N Cl H 733075 Br CH3 Br H N Si N Cl H 733199 TO ORDER: Contact your local Sigma-Aldrich offi ce (see back cover), or visit Aldrich.com/chemicalsynthesis. Br CH3 Br Entry 1 2 i-PrMgCl LiCl Br 656984 (TurboGrignard) FG THF, -15 °C to 25 °C or heteroaryl halide FG = CO2R', CN, OMe, halogen FG MgCl LiCl Reagent Electrophile Product i-PrO O O O MgCl LiCl PhCHO Br N MgCl LiCl Allyl Bromide N MgCl LiCl PhCHO 3 S aThe halogen-metal exchange was conducted in THF/DMPU. bGrignard was transmetalated with CuCN 2LiCl before addition of E. Br E FG N OH N Ph S E Ph 80 a Allyl Isolated Yield Table 7: Aryl/heteroaryl Grignard reagents prepared using i-PrMgCl·LiCl and reactions with electrophiles. Advantages of TurboGrignards: • Increased functional group compatibility • Mild reaction conditions • Minimal side reactions • Allows for the large-scale production of reactive Grignard reagents References: (1) Krasovskiy, A.; Knochel, P. Angew. Chem. Int. Ed. 2004, 43, 3333. (2) P. Knochel, E P 1582 524 A1. TurboGrignard Reagents from Aldrich: H3C CH3 MgCl . LiCl 656984 For more information on these new reagents, visit Aldrich.com/metalations Sold in collaboration with H3C CH3 MgCl . LiCl 703486 93 b 87
Organotrifl uoroborates as Coupling Partners in Suzuki-Miyaura Reactions Suzuki-Miyaura cross-coupling reactions are some of the most common methods for the formation of C–C bonds in organic chemistry. The use of organotrifl uoroborate salts as boronic acid surrogates has lead to signifi - cant advancement in the fi eld of Suzuki-Miyaura reactions. Trifl uoroborates exhibit excellent functional group tolerance and stability towards 1, 2 common reagents, in turn leading to a truly versatile class of reagents. Our platform of trifl uoroborate salts is continually growing, with new product introductions occurring regularly. Benefi ts of Organotrifl uoroborates: • Stable tetracoordinate species Less prone to protodeboronation • Air-and moisture-stable • Potassium Vinyltrifl uoroborate as a Versatile Dianion Precursor Molander and co-workers have developed a powerful strategy for the production of a unique 1,2-dianion equivalent using potassium vinyltrifl uoroborate. 1 This useful organotrifl uoroborate undergoes selective hydroboration with 9-BBN to generate (3), a 1,2-dianion equivalent that can then undergo a variety of selective transformations (Scheme 6). 9-BBN BF3K Potassium vinyltrifluoroborate 655228 B 3 BF3K Scheme 6: Hydroborated intermediate (3) as a 1,2-dianion equivalent. Ready to scale up? For competitive quotes on larger quantities or custom synthesis, contact your local Sigma-Aldrich offi ce, or visit safcglobal.com. Organometallics This versatile 1,2-dianion equivalent undergoes sequential Suzuki-Miyaura cross-coupling with a range of organic electrophiles, including aryl-, heteroaryl-, and alkenyl halides (Table 8). BF 3K 1.) 9-BBN 2.) Pd(OAc)2, DavePhos, KF, R' Br 3.) RuPhos, K2CO3,toluene/H2O R'' Br R'-Br R''-Br yield % OMe MeO Br OMe MeO Br OMe MeO Br H O Me O Cl Me Me Br Br OMe N Cl S O Br Br N N N H Br OMe MeO MeO MeO Table 8: Sequential Suzuki-Miyaura cross-couplings to build molecular complexity from potassium vinyltrifl uoroborate. Me H O OMe OMe Me OMe Me O N N S R' OMe N O H N OMe References: Molander. G. A.; Sandrock, D. L. Org. Lett. 2009, 11, 2369. Alkenyl Potassium Trifl uoroborates from Aldrich: BF3K 655228 H3C BF3K CH3 720682 Br 684937 BF3K CH3 H3C BF3K CH3 720933 H3CO H3C 683590 BF3K CH3 BF3K 720747 R'' CH 3 82 84 74 60 80 BF3K 723916 19
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