Several unique chiral 3,4-dihydro-2H-pyrrole-2-thiones were made readily available by carrying out, in each case, a chiral-Mg(OTf)2/N,N'-dioxide-complex-promoted formal [2+1+2] cycloaddition in the presence of tetraethylenediamine. Control experiments revealed that in situ-generated ammonium thiocyanate was crucial for maintaining high enantioselectivity through its inhibition of the HNCS-induced racemization of the products.