Benzyl Borane NHC Adducts: Beyond B-C Bond Scission.

Benzyl-substituted boronates and borates are widely employed as mild sources in radical or anionic transfer reactions of benzyl entities. In this process the B-C bond to the benzyl moiety is essentially ruptured. In contrast, reactions with retention of the B-C bond are poorly investigated although several other reactive sites in benzyl-boron systems are clearly inherent. In this respect, the novel reactivity of the representative borane adduct IiPr-BH2 Bn [IiPr=:C{N(iPr)CH}2 , Bn=CH2 C6 H5 ] is demonstrated. Dihalogenation of the BH2 entity is observed with BCl3 and BBr3 , whereas BI3 either affords IiPr-BHI2 or proceeds with borylation of the aromatic phenyl ring to give a hydride-bridged bisborylated species. The photochemical mono- and dihalogenation of the benzylic CH2 group was demonstrated with elemental bromine Br2 . The brominated product IiPr-BBr2 -CHBr-C6 H5 was borylated at the benzylic carbon atom in an umpolung event with BI3 to afford the zwitterion IiPr-BI-CH(BI3 )-C6 H5 .

N-Heterocyclic Carbene Adducts of Alkynyl Functionalized 1,3,2-Dithioborolanes.

Alkynyl functionalized boron compounds are versatile intermediates in the areas of medicinal chemistry, materials science, and optical materials. In particular, alkynyl boronate esters [R1-C≡C-B(OR2)2] are of interest since they provide reactivity at both the alkyne entity, with retention of the B-C bond or alkyne transfer to electrophilic substrates with scission of the latter. The boron atom is commonly well stabilized due to (i) the extraordinary strength of two B-O bonds, and (ii) the chelate effect exerted by a bifunctional alcohol. We reasoned that the replacement of a B-O for a B-S bond would lead to higher reactivity and post-functionalization in the resulting alkynyl boronate thioesters [R1-C≡C-B(S2X)]. Access to this poorly investigated class of compounds starts form chloro dithioborolane cyclo-Cl-B(S2C2H4) as a representative example. Whereas syntheses of three coordinate alkynyl boronate thioesters [R1-C≡C-B(S2X)] proved to be ineffective, the reactions of NHC-adducts (NHC = N-heterocyclic carbene) of cyclo-Cl-B(S2C2H4) afforded the alkyne substituted thioboronate esters in good yield. The products NHC-B(S2C2H4)(C≡C-R1) are remarkably stable towards water and air, which suggests their use as boron-based building blocks for applications akin to oxygen-based boronate esters.

Completing the series of boron-nucleophilic cyanoborates: boryl anions of type NHC-B(CN)2.

Since the first seminal report of boron-centred nucleophiles, the area of boryl anions has developed only sporadically and requires further systematisation. The boryl anions of type NHC-B(CN)2- (NHC = N-heterocyclic carbene) described herein complete a consistent series with the known anions cAAC-B(CN)2- [cAAC = cyclic(alkyl)(amino)carbene] and B(CN)32-. A novel approach towards NHC-stabilised cyanoboranes based on alkylthio-cyano exchange at boron is presented, and in contrast to other methods affords the products in better purity and yield. Reduction of suitable NHC-dicyanoboranes gave two unprecedented examples of NHC-B(CN)2- boryl anions. The latter were shown to react as boron-centred nucleophiles with facile formation of B-E bonds, where E = C, Si, Sn, P, Au. Bonding analysis by DFT calculations suggests a systematic variation of the energy of the boron-centred HOMO depending on the carbene, which in turn can control the nucleophilic character.