Lewis acid–base adducts of group 13 elements: synthesis, structure and reactivity toward benzaldehyde

Abstract

Lewis acid-base adducts [LGa-M(C6F5)3] (M = B 1, Al 2, Ga 3) were prepared by the reaction of gallanediyl LGa {L = HC[C(Me)N(2,6-i-Pr2C6H3)]2} with the Lewis acids M(C6F5)3 (M = B, Al, Ga). Benzaldehyde reacts with [LGa-M(C6F5)3] (M = B 1, Al 2) at room temperature with the insertion and formation of [LGa(C6F5){CH(Ph)(OB(C6F5)2)}] (4) and the zwitterionic species [LGa(C6F5){CH(Ph)(OAl(C6F5)2)}] (5), respectively, which was found to decompose at 80 °C with the formation of {(C6F5)2Al(OCH2Ph)}2 (6). Any attempts to isolate the insertion complex of [LGa-Ga(C6F5)3] with benzaldehyde failed and only {(C6F5)2Ga(OCH2Ph)}2 (7) was isolated at elevated temperatures. 2-5 and 7 were structurally characterized by heteronuclear NMR spectroscopy and single crystal X-ray diffraction.

Understanding the Subtleties of Frustrated Lewis Pair Activation of Carbonyl Compounds by N‐Heterocyclic Carbene/Alkyl Gallium Pairings

“…As observed for compounds 3-6,t wo singlets appear in the 1 HNMR spectrum at 8.67 and 7.21 ppm for the imidazole hydrogen atoms, which is consistent with the lack of symmetry in the heterocyclicr ing. 13 CNMR spectrum showeda ni nformative resonance at 81.7 ppm belonging to the reduced C=O group, which resonates at 191.2 ppm in the starting material.…”

“…[30] This reactivity contrasts with that observedb yM itzel for the reaction of 4-ethynyl-2,6-lutidine with GaEt 3 ,i nw hich metallation of the ethynyl group is observed along with elimination of ethane. [10] Replacing ItBub yS ItBu furnished the analogous compound [SItBuH} + {Ph 2 C=C=NGaR 3 } À ] ( 13).…”

“…1 HNMR (298 K,[D 8 ]THF) d À0.69 (6 H, s, CH 2 SiMe 3 ), À0.02 (27 H,s,Si(CH 3 ) 3 ), 1.56 (18 H,s,C(CH 3 ) 3 ), 6.46 (2 H, m, p-CH), 6.92 (4 H, t, m-CH), 7.28 (4 H, t, o-CH), 7.59 (2 H, s, imidazole backbone CH), 8.68 ppm (1 H, s, C2-H). 13 C{ 1 H} NMR (298 K,[D 8 ]THF) d 2.7 (CH 2 SiMe 3 ), 3.1 (Si(CH 3 ) 3 ), 29.7 (C(CH 3 ) 3 ), 55.7 (Ph 2 C=C=N), 61.1 (C(CH 3 ) 3 ), 117.9 (Ar-CH), 121.4 (imidazole backbone CH), 123.1 (Ar-CH), 128.4 (Ar-CH), 131.9 (NCHN), 143.4 (Ph 2 C=C=N), 144.2 ppm (Ar-C ipso ).…”

“…[12a] Interesting recent work by Schultz has also uncovered the double agentr ole of Ga systems as they can reverse their role in FLP chemistry,a cting insteada saLB, being part of ag allanedyl framework, which facilitatesi nsertion of benzaldehyde when combinedw ith [M(C 6 F 5 ) 3 ]( M= Bo rA l) LAs. [13] Advancing rational design of elusive main-group abnormal NHC (aNHC) complexes, our first look at NHC-Ga alkyl complexess hed light on normal-to-abnormal NHC rearrangement mechanisms, [14] whichb ears resemblance to mechanisms proposedi nN HC/borane FLP systems. [4] In particular,i tw as found that sterici ncompatibility prevented trimethylsilylmethylgallium(III)( GaR 3 )a nd 1,3-bis(tert-butyl)imidazol-2-ylidene( I tBu) from forming as table normal adduct.…”

“…[14] Encouraged by these initial findings, here we present as ystematic studyp robingt he reactivity of theseN HC/Ga combinations towards carbonyl compounds, an area where other, mostly, but not limitedt o, PR 3 /B-basedF LP systems have shown considerable promise in synthesis and catalysis. [13,[15][16][17] Combining X-ray crystallography with spectroscopica nd theoretical investigations,n ew insights are provided into the intriguing chemoselectivities and mecha-nisms of these processes,whichintensifies the potential of gallium tris(alkyl) complexes for FLP activation.…”