Catalytic Addition of Amine NH Bonds to Carbodiimides by Half‐Sandwich Rare‐Earth Metal Complexes: Efficient Synthesis of Substituted Guanidines through Amine Protonolysis of Rare‐Earth Metal Guanidinates

Reaction of [Ln(CH 2 SiMe 3 ) 3 (thf) 2 ] (Ln=Y, Yb, and Lu) with one equivalent of Me 2 Si(C 5 Me 4 H)NHR′ (R′=Ph, 2,4,6‐Me 3 C 6 H 2 , t Bu) affords straightforwardly the corresponding half‐sandwich rare‐earth metal alkyl complexes [{Me 2 Si(C 5 Me 4 )(NR′)}Ln(CH 2 SiMe 3 )(thf) n ] ( 1 : Ln = Y, R′ = Ph, n =2; 2 : Ln = Y, R′ = C 6 H 2 Me 3 ‐2,4,6, n =1; 3 : Ln = Y, R′ = t Bu, n =1; 4 : Ln = Yb, R′ = Ph, n =2; 5 : Ln = Lu, R′ = Ph, n =2) in high yields. These complexes, especially the yttrium complexes 1 – 3 , serve as excellent catalyst precursors for the catalytic addition of various primary and secondary amines to carbodiimides, efficiently yielding a series of guanidine derivatives with a wide range of substituents on the nitrogen atoms. Functional groups such as CN, CCH, and aromatic CX (X: F, Cl, Br, I) bonds can survive the catalytic reaction conditions. A primary amino group can be distinguished from a secondary one by the catalyst system, and therefore, the reaction of 1,2,3,4‐tetrahydro‐5‐aminoisoquinoline with i PrNCN i Pr can be achieved stepwise first at the primary amino group to selectively give the monoguanidine 38 , and then at the cyclic secondary amino unit to give the biguanidine 39 . Some key reaction intermediates or true catalyst species, such as the amido complexes [{Me 2 Si(C 5 Me 4 )(NPh)}Y(NEt 2 )(thf) 2 ] ( 40 ) and [{Me 2 Si(C 5 Me 4 )(NPh)}Y(NHC 6 H 4 Br‐4)(thf) 2 ] ( 42 ), and the guanidinate complexes [{Me 2 Si(C 5 Me 4 )(NPh)}Y{ i PrNC(NEt 2 )(N i Pr)}(thf)] ( 41 ) and [{Me 2 Si(C 5 Me 4 )(NPh)}Y{ i PrN}C(NC 6 H 4 Br‐4)(NH i Pr)}(thf)] ( 44 ) have been isolated and structurally characterized. Reactivity studies on these complexes suggest that the present catalytic formation of a guanidine compound proceeds mechanistically through nucleophilic addition of an amido species, formed by acid–base reaction between a rare‐earth metal alkyl bond and an amine NH bond, to a carbodiimide, followed by amine protonolysis of the resultant guanidinate species.