Borylations allylic acetates

A masked allylic boron unit can be revealed through a transition-metal-catalyzed borylation reaction. For example, a one-pot borylation/allylation tandem process based on the borylation of various ketone-containing allylic acetates has been developed. The intramolecular allylboration step is very slow in DMSO, which is the usual solvent for these borylations of allylic acetates (see Eq. 33). The use of a non-coordinating solvent like toluene is more suitable for the overall process provided that an arsine or phosphine ligand is added to stabilize the active Pd(0) species during the borylation reaction. With cyclic ketones such as 136, the intramolecular allylation provides cis-fused bicyclic products in agreement with the involvement of the usual chairlike transition structure, 137 (Eq. 102). 

The palladium-catalyzed reaction of diboron with allyl acetates or chlorides is a convenient alternative to transmetallation method for the synthesis of functionalized boronates. The reaction of diboron 119 with allyl acetates309-312 smoothly occurred without the assistance of a base, whereas the presence of AcOK was critical for the coupling with allyl chlorides313 (Equation (57)). The boron atom coupled with the less-hindered terminal carbon giving the thermally stable (if)-allyl boronates. Thus, the (E)- and (Z)-cinnamyl acetate, chloride (entries 1-4), and their secondary derivatives (entries 7 and 8), all afforded an (if)-cinnamyl boronate. The borylation of prenyl acetate was slow (entry 6), but the corresponding chloride (entry 5) and tertiary derivative (entry 9) worked well for the same purpose (Equation (58)).309,310... 

Ahiko, T.-a., Ishiyama, T., Miyaura, N. Asequence of palladium-catalyzed borylation of allyl acetates with bis(pinacolato)diboron and intramolecular allylboration for the cyclization of oxo-2-alkenyl acetates. Chem. Lett. 1997, 811-812. 

As a typical example of umpolung, domino borylation of the allylic acetate 315 with diborane to generate allylborane 316, and intramoleeular nucleophilie allyla-tion of ketone afforded the eyelie homoallylic aleohol 317 in 88% yield [116]. 

Palladium-catalyzed substitution of an allylic acetate such as 37 with diboronyl reagent 24 gives good yields of allylboronates in DMSO as solvent (Equation 21) [57]. This reaction is a steieoconvergent process the boron tends to add to the least substituted end of unsymmetrical allylic units, and both E and Z isomers of acetate 37 yield the E-isomer of 38. Another drawback to this process is that the product is often formed with variable yields of the allylic dimer (e.g., 39). Allylic halides were also employed as substrates with pinacolborane as the borylating agent and a platinum catalyst [58]. 

Recently, allylic acetates were borylated as the first step of an efficient one-pot procedure for the allylation of aldehydes [59]. Borylation of allylic acetates can also be performed in tandem with an intramolecular allylboration (Section 6.4.1.6). 

In the recent development of a one-pot borylation of allylic acetates (e.g., 37) followed by in situ addition to sidfonyl-imines, Szabo and co-workers observed that the palladium catalyst is required for the addition of the transient allylboronate 38 to the N-sulfonyl imine (Equation 46) [59]. The authors proposed the intermediacy of a bis-allylpalladium species to explain this result. These additions are highly regio- and stereoselective. 

Miyaura and co-workers have developed a one-pot borylation/allylation tandem process based on the borylation of various ketone-containing allylic acetates [139]. 

SCHEME 6.12 Nickel-catalyzed borylation of allylic acetates [24]. 

Ramachandran PV, Pratihar D, Biswas D, Srivastava A, Venkat Ram Reddy R. Novel functionalized trisubstituted aUylboronates via Hosomi—Miyaura borylation of functionalized allyl acetates. Org Lett. 2004 6 481-485. 

Since palladium complexes have proven to be efficient in activating diboron compounds throughout transmetalation, a wide range of applications have been considered in the last decade, such as palladium-catalyzed transformation of aUyhc alcohols to allylboronates, borylation of allylic hahdes or aUyhc acetates, and the P-boration of a,P-unsaturated carbonyl substrates.Interestingly, both palladium and nickel showed to be similarly efficient to activate B2pin2 and catalyze the addition to unsaturated substrates. Oshima postulated that Ni(0) species react with substrate a,P-unsaturated esters and amides to generate the r -coordinated complex, which activates the B2pin2 to favor the formation of T -coordinated... 

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