Reactions of (Trimethyltriazacyclononane)Rh(vinyl)3, −Rh(Z-propenyl)3, and −Rh(vinyl)2Me with Protic Acids. The Relative Migratory Aptitude of Methyl and Vinyl Groups to an (Ethylidene)Rh Alkylidene Carbon
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Abstract
Preparations of CnRh(CHCH2)3, CnRh(Z-CHCHMe)3, and CnRhMe(CHCH2)2 (Cn = 1,4,7-trimethyl-1,4,7-triazacyclononane) are reported. An X-ray crystal structure determination of CnRh(CHCH2)3 revealed C3 molecular symmetry with average lengths Rh−N, 2.25 Å; Rh−C, 2.00 Å; and CC, 1.31 Å. Protonations of these compounds were carried out with HOTf (triflic acid, HOSO2CF3), [H(Et2O)2][BAr4] (BAr4 = (-)B[3,5-(CF3)2C6H3]4), and HCl in organic solvents as well as with methanol as solvent and acid. CnRh(CHCH2)3 gave [CnRh(CHCH2)(η3-E-CH2CHCHMe)]X in isolated yields up to ca. 80%. Use of CD3OD as acid/solvent led to formation of [CnRh(CHCH2)(η3-CH2CHCHCH2D)]X (X = (-)OCD3 assumed) as the only isotopomeric product, suggesting [CnRh(CHCH2D)(CHCH2)2]+ as a probable intermediate. No Rh(CH−CH3) functional group could be detected by NMR at −60 °C or above. Triflic acid protonation of CnRh(Z-CHCHMe)3 generated [CnRh(Z-CHCHMe)(η3-E,E-MeCHCHCHEt)], establishing that product formation is by alkenyl migration and not via an electrocyclic ring closure within the [Rh(CHEt)(CHCHMe)]+ moiety. Protonation of CnRhMe(CHCH2)2 generated [CnRhMe(η3-CH2CHCHMe)]+ as the sole product (96% by NMR). Reaction of CnRh(CHCH2)2Me in CD3OD generated [CnRhMe(η3-CH2CHCHCH2D)]+ as the sole isotopomer. Thus, the migratory competition established between methyl and vinyl groups for migration to the carbene carbon in proposed intermediate [CnRh(CHMe)(CHCH2)Me]+ is dominated by the vinyl group.
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