Amino and Alkyl B-Substituted P-Stabilized Borenium Salts

Abstract

The ability of the phosphino-naphthyl moiety to stabilize amino- and alkylborenium cations has been studied. Surprisingly, the phosphine–aminochloroborane precursor 2 was found to exist in neutral open form (without P→B interaction) in benzene solution and in the solid state but to ionize spontaneously in chloroform to generate the P-stabilized borenium salt 3. Addition of gallium trichloride shifts the process forward and affords the corresponding tetrachlorogallate borenium salt 3′. The phosphine group of 2 remains available for external reactivity, as shown by the ready formation of the corresponding phosphine gold­(I) chloride complex 4. The P-stabilized cyclohexylborenium cation 6 has also been prepared by reacting the corresponding bromoborane 5 with gallium tribromide. Compound 6 is a very rare example of an alkyl-substituted borenium salt. The structures of 2, 3′, and 4–6 have been unambiguously ascertained by multinuclear NMR spectroscopy and X-ray crystallography.

Insights into Frustrated and Regular peri‐Substituted (Ace‐)Naphthylaminoboranes and (Ace‐)Naphthylphosphinoboranes

“…[31] Interestingly, the topological bond analysis revealed no principal difference between attractive and enforced B-P bonds in (ace-)naphthylphosphinoboranes. The new acenaphthylphosphinoboranes 13-16, bearing functionality at the B atom, might be of interest as starting materials for the synthesis of compound classes, such as P 2 B 2 heterocycles, [12,16] borenium ions, [18,20,21] and boron-centered radicals. [19] Experimental Section General: Reagents were obtained commercially (Sigma-Aldrich, Germany) and were used as received.…”

“…The balance between the forces is usually achieved by the judicious choice of the substituents, which contribute both via their electronic nature, as well as their steric bulk. In recent years, a number of aminoboranes and phosphinoboranes have been assembled in the peri positions of (ace‐)naphthyl scaffolds, which hold the Lewis acid and the Lewis base in close proximity. Besides their relevance for frustrated Lewis pair chemistry, some of the functionalized (ace‐)naphthylphosphinoboranes have been the starting point for the synthesis of interesting new compound classes, such as P 2 B 2 heterocycles,, borenium ions,, , and boron‐centered radicals .…”

“…In recent years, a number of aminoboranes [9][10][11] and phosphinoboranes [12][13][14][15][16][17][18][19][20][21] have been assembled in the peri positions of (ace-)naphthyl scaffolds, which hold the Lewis acid and the Lewis base in close proximity. Besides their relevance for frustrated Lewis pair chemistry, some of the functionalized (ace-)naphthylphosphinoboranes have been the starting point for the synthesis of interesting new compound classes, such as P 2 B 2 heterocycles, [12,16] borenium ions, [18,20,21] and boron-centered radicals. [19] A selection of these peri-substituted (ace-)naphthylaminoboranes (1)(2)(3)(4)(5)(6) and (ace-)naphthylphosphinoboranes (7)(8)(9)(10)(11)(12) are shown in Schemes 1 and 2.…”

Frustrated Lewis Pair based on a peri‐Substituted Biphenylene Scaffold

“…The exploitation of frustrated Lewis pairs (FLPs), for the capture and activation of small unreactive molecules, such as dihydrogen and carbon dioxide, marks a paradigm change in the field of catalysis. Single component FLPs (and related regular Lewis pairs) based on phosphinoboranes mounted on rigid peri ‐substituted aromatic scaffolds, such as naphthalene, acenaphthene, dimethylxanthene, and dibenzofurane (Scheme ), may offer two potential advantages over conventional two component FLP systems: (i) Firstly, the rigid scaffolds allow tuning of the donor acceptor distances, which for high catalytic activity needs to be in the range between approximately 3 Å and 5 Å . (ii) Secondly, the entropy penalty associated with the capture of small molecules should be considerably smaller in single component FLPs than in two component FLPs , .…”

Lewis pairing and frustration of group 13/15 elements geometrically enforced by (ace)naphthalene, biphenylene and (thio)xanthene backbones

“…Our synthetic strategy begins with classical formation of neutral P-B adducts starting from lithiation of the 1-iodo-8-phosphinonaphthalene followed by trapping with a dibromoborane to furnish bromoboranes 61-63 with varying donating ability of the P centre (Scheme 18). [43][44][45] Notably, derivative 63 is a rare example of a stable and isolable phosphine-borane with an alkyl substituent on B. The cyclohexyl group is a weaker donor compared to Mes and previously described aromatic substituents.…”