Organocatalytic functionalisation through boron chemistry

Abstract

Diboron reagents can be activated both, from a metal or free-metal context, and consequently the addition of boryl units to unsaturated substrates proceeds sometimes with complementary selectivity. We highlight here the power of boron chemistry to functionalize molecules and provide new routes towards challenging compounds.

Diboron(4) Compounds: From Structural Curiosity to Synthetic Workhorse

“…The interest in Lewis base adducts of B 2 pin 2 has grown over the past few years, due to the proposed requirement of such compounds in the catalytic cycle of borylation reactions, in the presence or absence of a transition metal (vide infra). ,,− The formation of such compounds had been proposed by Miyaura and co-workers as early as 2000 as a means of enhancing the nucleophilicity to generate a boryl anion equivalent. Miyaura et al thus proposed that the in-situ formation of [B 2 pin 2 OAc] − aided in the formation of a Cu–boryl complex, required to facilitate the copper-catalyzed β-borylation of α,β-enones. , Furthermore, anionic adducts, such as [B 2 pin 2 OR] − , have been discussed and observed in computational and spectroscopic studies; ,,,− however, the full characterization of a series of anionic sp 2 –sp 3 Lewis base adducts of B 2 pin 2 , such as [K­(18-Crown-6)]­[B 2 pin 2 OMe] and [Me 4 N]­[B 2 pin 2 F], was only published recently by Marder et al (Figure ). , The reactivity of these isolated adducts with aryl electrophiles will be discussed later (section ).…”

“…An intriguing recent discovery by Fernández and co-workers is that diborations of various unsaturated substrates with B 2 pin 2 can take place without a transition-metal catalyst. − , An excess amount of alcohol and a catalytic amount of a base are present and serve to generate the catalytically active anionic species, [B 2 pin 2 OR] − , which is an alkoxide adduct of B 2 pin 2 (for further information see section ). Upon formation of this adduct, the boron–boron bond becomes strongly polarized and interacts with the substrate . It was proposed that the intermediate rearranges to the diboration product, while the alkoxide ion is abstracted from the product by protonation (Scheme ).…”

Scalable, Metal- and Additive-Free, Photoinduced Borylation of Haloarenes and Quaternary Arylammonium Salts

“…30 Although B 2 (OR) 4 generally behave as electrophiles, complexation with Lewis bases (e.g., fluoride, alkoxides, N-heterocyclic carbenes, 4-methylpyridine, and phosphines) 31 produces nucleophilic sp 2 -sp 3 Lewis base–diboron adducts that react with organic electrophiles in the absence of transition metal catalysts. 32 It is possible that the photoheterolytically-produced fluoride ion forms a transient nucleophilic sp 2 -sp 3 B 2 (OR) 4 F - complex that then reacts with the triplet cation. 33 Interestingly, B 2 pin 2 F - adduct was proposed as an intermediate in the borylation of arenediazonium salts, 13c and [NMe 4 ][B 2 pin 2 F] was subsequently shown to effect the diazonium borylation.…”

Theoretical study on homolytic B–B cleavages of diboron(4) compounds

“…3 In general, the synthetic reactions involving diboron(4) compounds such as B 2 X 4 , B 2 (NR 2 ) 4 , B 2 (alkyl) n , B 2 (OR) 4 , etc. mainly include borylation reactions, [4][5][6][7][8][9][10][11] diboration reactions, [12][13][14][15][16][17][18] borylative cyclization reactions, [19][20][21] borylative ring opening reactions, [22][23][24][25][26][27] boracarboxylation reactions, 28 hydroboration and carboboration reactions, 29-32 etc. For instance, Miyaura et al 33,34 developed the borylation reactions using dialkoxyboranes or diboron ( 39 reported the rhodium(I)-catalyzed diboration reaction of Estyryl boronate esters with B 2 cat 2 , in which the 1,1,1-triborylalkane is the dominant product.…”

“…Szabó et al 41 studied the borylative ring opening reaction of substituted vinylcyclopropanes and vinyl aziridines with tetrahydroxydiboron catalyzed by palladium. Besides the above exemplied reactions catalyzed by the transitional-metal catalyst, there are also many researches on the reactions involving diboron (4) compounds under the transition-metal-free condition, such as borylation reactions, 10,11 diboration reactions [16][17][18] and borylative ring opening reactions, [25][26][27] etc.…”