A compound of the formula I or I′, where the radicals R1 are each, independently of one another, a hydrogen atom or C1-C4-alkyl and R′1 is C1-C4-alkyl; X1 and X2 are each, independently of one another, a sec-phosphino group; R2 is (1) hydrogen, (2) R01R02R03Si—, (3) C1-C18-acyl substituted with halogen, hydroxyl, C1-C8-alkoxy or R04R05N—, or (4) R06—X01—C(O)—; R01, R02 and R03 are each, independently of one another, C1-C12-alkyl, unsubstituted or C1-C4-alkyl- or C1-C4-alkoxy-substituted C6-C10-aryl or C7-C12-aralkyl; R04 and R05 are each, independently of one another, hydrogen, C1-C12-alkyl, C3-C8-cycloalkyl, C6-C10-aryl or C7-C12-aralkyl, or R04 and R05 together are trimethylene, tetramethylene, pentamethylene or 3-oxapentylene; R06 is C1-C18-alkyl, unsubstituted or C1-C4-alkyl- or C1-C4-alkoxy-substituted C3-C8-cycloalkyl, C6-C10-aryl or C7-C12-aralkyl; X01 is —O— or —NH—; T is C-bonded C3-C20-heteroarylene; v is 0 or an integer from 1 to 4; X1 in the heteroring of the heteroarylene is bound in the ortho position relative to the T—C* bond; and * indicates a mixture of racemic or enantiomerically pure diastereomers or pure racemic or enantiomerically pure diastereomers.