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In 2016, Bera, Maiti, and coworker achieved the first Pd(II)‐catalyzed meta‐C–H olefination of phosphonates at room temperature using the 2‐cyanophenol template (Scheme 2.62) [54]. At elevated temperature, the reaction could be extended to meta‐C–H hydroxylation or acetoxylation by varying the acetoxylating agent PhI(OCOR)₂. Notably, sequential di‐meta‐olefination would afford tri‐alkenylated arenes that could be used in organic electronics as well as optoelectronics.

Scheme 2.62 meta‐C–H functionalizations of phosphonates.

Source: Modified from Bera et al. [54].

Later in 2017, Maiti and coworkers achieved the meta‐C–H alkylation of benzylphosphonates by using a pyrimidine‐based template using allyl alcohols, leading to the formation of β‐aryl aldehydes and ketones (Scheme 2.63) [39]. Notably, the generality of this meta‐alkylation had been demonstrated in phenethylsulfonyl ester as well as phenethyl carbonyl scaffolds as discussed earlier. Moreover, meta‐C–H cyanation of benzylphosphonates was also achieved by using the same pyrimidine‐based template, albeit only two examples were disclosed [41].

Scheme 2.63 meta‐C–H alkylation of phosphonates.

Source: Modified from Bag et al. [39].

Phosphonates are useful synthons in the synthetic chemistry, since they could be transformed readily to the versatile alkenyl product by well‐established Horner–Wadsworth–Emmons reactions. And due to the importance of deuterium‐labeled compounds for pharmaceutical industry and reaction mechanistic studies, efficient direct deuteration methods would be very valuable. Recently, Yu, Dai, and coworkers developed palladium‐catalyzed meta‐selective C–H deuteration of benzylphosphonates by using a pyridine‐based directing template (Scheme 2.64) [19]. Notably, this method could be generally used for other substrates such as benzylsulfonates and benzyl alcohols via the practical ester linkage.

Scheme 2.64 meta‐C–H deuteration of phosphonates.

Source: Modified from Xu et al. [19].