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

Article abstract of DOI:10.1021/om980423y

Preparations of CnRh(CH=CH₂)₃, CnRh(Z-CH=CHMe)₃, and CnRhMe(CH=CH₂)₂ (Cn = 1,4,7-trimethyl-1,4,7-triazacyclononane) are reported. An X-ray crystal structure determination of CnRh(CH=CH₂)₃ revealed C₃ molecular symmetry with average lengths Rh-N, 2.25 ?; Rh-C, 2.00 ?; and C=C, 1.31 ?. 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(CH=CH₂)₃ gave [CnRh(CH=CH₂)(η³-E-CH₂CHCHMe)]X in isolated yields up to ca. 80%. Use of CD₃OD as acid/solvent led to formation of [CnRh(CH=CH₂)(η³-CH₂CHCHCH₂D)]X (X = ^(-)OCD₃ assumed) as the only isotopomeric product, suggesting [CnRh(=CHCH₂D)-(CH=CH₂)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-CH=CHMe)₃ generated [CnRh(Z-CH=CHMe)(η³-E,E-MeCHCHCHEt)], establishing that product formation is by alkenyl migration and not via an electrocyclic ring closure within the [Rh(=CHEt)(CH=CHMe)]⁺ moiety. Protonation of CnRhMe(CH=CH₂)₂ generated [CnRhMe(η³-CH₂CHCHMe)]⁺ as the sole product (96% by NMR). Reaction of CnRh(CH=CH₂)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)(CH=CH₂)Me]⁺ is dominated by the vinyl group.