Cleavage of the Carbon-Hydrogen Bond on Achiral and Chiral Transition Metal Complexes

Summary

Inter- and intramolecular activation of carbon—hydrogen bonds has been investigated by reduction of ruthenium complexes L₄ RuCl₂ containing tri- and tetradentate polyphosphines as ancillary ligands. The transient cis-unsaturated 16e fragments L₄Ru(0) (L₄ = P(CH₂ CH₂ CH₂ PMe₂ ) 3, “pp3 ”; N (CH₂ CH₂ PMe₂ ) 3, “np3 ”; N (CH₂CH₂PPh₂ ) 3, “np’3“; PMe₃ /MeSi (CH₂ PMe₂) 3, “sips”), so generated, exhibit a distinct selectivity with respect to insertion across aromatic C-H bonds; yet they also undergo competitive cyclometalation. Reduction of (np₃)RuCl₂ with Na (Hg) in refluxing benzene thus affords a 4:1 mixture of $[(\textup{Me}2\overline{\textup{PCH}2})2 \textup{NCH}_2\textup{CH}_2\textup{P}(\textup{Me})\textup{CH}_2]\textup{RuH}$ NCH₂ CH₂ P (Me) CH₂] RuH and (np3 ) RuH ( C₆H₅), but on decreasing the temperature the intermolecular insertion of (np3) Ru (0) begins to occur preferentially to the cyclometalation reaction. In their reactions with substituted arenes, the 16e equivalents (pp3) Ru (0) and (sip3)(Me₃ P) Ru (0) show a clear-cut preference for insertion across the unhindered meta- and para-C-H bonds. In no case is activation of the weak benzylic C-H bonds observed. In contrast, reduction of (pp₂) (Me3 P) RuCl₂ (“pp₂” = MeP-(CH₂ CH₂ CH₂ PMe₂) 2 ) in mesitylene produces, via (pp2)(Me3 P) Ru (0) and (pp2) Ru (0) as viable intermediates, the product of benzyl C-H bond cleavage, (pp2) (Me3 P) RuH (CH₂ C₆H₃Me₂ -3, 5 ), accompanied by (pp2) $(\textup{Me}_2\overline{\textup{PCH}_2})\textup{RuH}$. — The complexes L₄ RuH (C₆ H₅) (L₄ = pp3, np’3, and sip3/PMe3) all serve as catalysts for the dimerization of alk-1-ynes RC=CH (R = Ph, n-Bu, t-Bu), which proceeds to yield 1, 4-disubstitued 1-en-3-ynes, RCH=CHC≡CR, with varying E/Z. stereoselectivities. Reaction of (sip3)(Me3P) RuH (C₆ H₅) with 4-MeC₆H₄NCE (E = O, S) leads to insertion of the heteroallenes into the Ru-H bond, generating (sip3)(Me3 P) Ru [N (Tol)-C(0)H] C₆H₅ and (sip3 ) (Me3 P) Ru [SC(=NTol)H] C₆H₅ as the primary products. While the latter forms (sip3 ) $\overline{\textup{Ru}[\textup{N}(\textup{Tol})\textup{\textup{C}}(\textup{S})\textup{H}]}\textup{C}_6\textup{H}_5$ by loss of PMe3, the former eliminates Me3 P=NC₆H₄ Me-4 by reaction with another molecule of isocyanate, affording the acyl compound (sip3) Ru [N(Tol) C (0) H] C (0) C₆H₅ as the ultimate product. —The 14e fragments [(± or -)-trans-1, 2-C₅ H₈ (PCy₂ ) 2 ] Pt (0), generated from enantiomerically pure [ trans-1, 2-C₅H₈ (PCy2) 2 ] Pt (H)-CH₂CMe₃ by thermolysis, interact with atropisomeric biaryls to produce, with thus far poor optical yields, C-H-activated aryl hydrides of composition [trans-1, 2-C₅H₈ (PCy2) 2 ] PtH (biar): biarH = 2-Me3 CC₆H₄-C₆H₄CMe3-2’ or 2-MeC_{1 0}H₆-C_{1 0}H₆Me-2 ’.