Pd-catalyzed allylative dearomatisation using Grignard reagents

Abstract

Dearomatisation of benzylic halides have been achieved using Grignard reagents resulting in good isolated yields in short reaction times.

Electrophilic Trapping of Semibenzenes

“…The only reported non-radical reaction is a sigmatropic rearrangement of propargyl substituted semibenzene to allenyl benzenes . Alternatively, it has also been shown that non-fully substituted semibenzenes quickly react in the presence of acids to yield the rearomatized compounds (by elimination of H + from the sp 3 carbon). ,, In the 1980s, an analogous rearomatization was reported by the group of Reutov (Scheme b), who found that semibenzenes would form the corresponding benzylic organometallic reagent upon reaction with transition-metal salts (mainly Hg, but also Sn, Ge, and Au), in a so-called aromative metalation. , …”

“…Semibenzenes (3-methylenecyclohexa-1,4-dienes) are a class of unstable compounds often overlooked by chemists since they are challenging to synthesize and work with. − However, in 2001, seminal work by Yamamoto et al provided a remarkable turnaround in this regard, showing that the synthesis of mono- and di-substituted allylated semibenzenes is rather straightforward via Pd-catalyzed dearomative allylation of benzylic halides . Various modified and improved procedures have been reported since, but synthetic applications of semibenzenes remain scarce to this day. The only examples were reported by the group of Yamaguchi (Scheme a), namely, a cyclopropanation and an oxidation of the external double bond of semibenzenes, leading to the corresponding alcohols or α,β-unsaturated carbonyls. , …”

Palladium-catalyzed indium-mediated reductive aromatic C–H allylation of N-benzylsulfonimides with allyl esters

“…2 This method complements traditional functionalization of aromatic rings, providing convenient access to complex cyclic nonaromatic and aromatic systems. In addition to benzyl chlorides, 1,3 benzyl phosphates, 4 benzyl carbonates, 5 benzylammonium salts, 6 and N -benzylsulfonimides 7 have later proved to serve as alternative benzyl electrophiles (Scheme 1). On the other hand, the dearomative allylation or the aromatic C–H allylation of benzyl electrophiles was successfully extended to allylsilanes, 3 c allylboronates, 3 d ,4 and allyl Grignard reagents.…”

Palladium-Catalyzed para-Selective Allylation of 1-(Cyanomethyl)arenes with Allyl Acetates

“…Meanwhile, the dearomative allylation of aromatic C–H bonds for benzyl electrophiles is well-studied . Benzyl halides, benzyl phosphates, , and benzyl ammonium analogs , undergo the dearomative para -allylation (Figure B). However, with the exception of one intramolecular iridium-catalyzed transformation (Figure C), these reactions require unsubstituted organometallic allyl nucleophiles and a leaving group at the benzylic position, thus featuring a limited scope and low atom economy.…”