text: Complexes of bisoxazolines and these species can be used to induce enantioselectivity into aldol and Mannich-type reactions.
full_question: Complexes of bisoxazolines and these species can be used to induce enantioselectivity into aldol and Mannich-type reactions. One method to test the "strength" of these species adds a triethylphosphine oxide probe followed by phosphorus-31 NMR analysis; that is the Gutmann-Beckett method. The addition of these substances to acetals can influence whether acetal substitution occurs by an SN1 or SN2 mechanism. These substances are needed to activate the carbonyl group in the Sakurai reaction. These compounds typically catalyze an electrophilic aromatic substitution reaction which alkylates or acylates an aromatic ring. Friedel-Crafts reactions can be catalyzed by aluminum chloride or boron trifluoride, both examples of these species. For 10 points, name these acids which accept pairs of electrons.
first_sentence: Complexes of bisoxazolines and these species can be used to induce enantioselectivity into aldol and Mannich-type reactions.
raw_answer: Lewis acid [prompt on "acid" before mention]