Ferrocene-Based Heteronuclear Bidentate Lewis Acids via Highly Selectiveortho-Borylation of 1,1‘-Bis(trimethylstannyl)ferrocene

A new type of ferrocene-based heteronuclear bidentate Lewis acid with both Lewis acidic centers attached to the same cyclopentadienyl (Cp) ring is reported. Borylation of 1,1'-bis(trimethylstannyl)ferrocene (1) with BCl3 in hexanes at -78 degreesC occurs with high selectivity at the a-position to give 1-(Me3Sn)-2-(Cl2B)fc (2a-Cl; fc = ferrocenediyl) as the major product in ca. 87% spectroscopic yield. Only minor amounts of the two other isomers, the 1-stannyl-3-boryl- (2b-Cl) and the 1-stannyl-1'-borylferrocene (2c-Cl), are observed. The reaction rate and selectivity strongly depend on the steric and electronic properties of the electrophile. With the bulkier electrophile C6F5BCl2 larger amounts of the 1,3-product, 1-(Me3Sn)-3-(C6F5ClB)fc (2b-Pf-, ca. 40%), form in addition to the 1,2-isomer 1-(Me3Sn)-2-(C6F5ClB)fc (2a-Pf; ca. 60%). Use of the weaker electrophile PhBCl2 results in a significantly lower reaction rate. Treatment of 1 with excess borane in hexanes yields 1-(ClMe2Sn)-2-(Cl2B)fc (3a-Cl) and 1-(ClMe2Sn)-2-(ClPhB)fc (3a-Ph) as a result of preferred cleavage of an Sn-Me over the Sn-Cp bond in 2a-Cl and 2a-Ph. Facile Sn-Me bond cleavage also occurs during thermal treatment of 2a-Cl (50 degreesC) and 2a-Ph (80 degreesC), resulting in the rearranged species 1-(ClMe2Sn)-2-(ClMbB)fc (4a-Cl) and 1-(ClMe2Sn)-2-(PhMeB)fc (4a-Ph). The heteronuclear bidentate Lewis acids 3a-Cl, 3a-Ph, 4a-Cl, and 4a-Ph were fully characterized by H-1, C-13, B-11, and Sn-119 NMR spectroscopy, mass spectrometry, and elemental analysis. Single-crystal X-ray diffraction analyses of 4a-Cl and 4a-Ph reveal a dip angle between the Cp plane and the Cp-B vector of 13.8(2)degrees and 12.2(3)degrees, respectively, indicating a significant iron-boron interaction. The tin centers display a distorted trigonal bipyramidal arrangement as a result of short contacts to the boron-bound chloride (4a-Cl) and phenyl (4a-Ph) substituent, respectively. The 119Sn NMR signals are upfield-shifted by 19-35 ppm relative to FcSnMe(2)Cl, suggesting a similar interaction to be present in solution. The presence of two Lewis acidic substituents on ferrocene is further reflected in a bathochromic shift and increase in the intensity of the visible absorption bands relative to ferrocene.