The Catalytic Alkylation of Aromatic Imines by Wilkinson's Complex: The Domino Reaction of Hydroacylation andortho-Alkylation

Abstract

Transition metal catalyzed C À H bond activation and the subsequent coupling of the organic fragment to an olefin is a promising area in which to find a convenient method for the construction of a carbon skeleton. [1] We have studied CÀH bond activation through the hydroacylation of olefins using aldehydes, alcohols, and aldimines. [2, 3] Unexpectedly during our experiments on transimination-assisted hydroacylation with aldimines, [3] ortho-alkylation was observed. Alkylation of aromatic ketones at the ortho position in a ruthenium(ii)catalyzed reaction has been reported by Murai and coworkers, this is an outstanding example of sp 2 -CH/olefin coupling and a decisive breakthrough in efficiency and selectivity. [4] However, while the reaction shows a high efficiency for vinyl silane or vinyl siloxane, it exhibits limitations for other olefins, for example low reactivity for 1-alkenes bearing allylic protons, probably because of facile double bond isomerization, and no reactivity for internal olefins and a,w-dienes. [5] Herein, we report an efficient orthoalkylation of aromatic imines with various olefins by using Wilkinsons complex ([Rh(PPh 3 ) 3 Cl] (3)) and hydroacylation. This ortho-alkylation is chelation-assisted and shows generality as well as regioselectivity, and high efficiency.Treatment of the aldimine 1 a [Eq.(1)] with tert-butylethylene (2 a) at 170 8C for 6 h with 3 (2 mol % based upon 1 a) H N CH 2 Ph tC 4 H 9 O tC 4 H 9 tC 4 H 9 O tC 4 H 9 a) [Rh(PPh 3 ) 3 Cl] (3), 2 mol%; 2-amino-3-picoline (4), 10 mol% 170 °C, 6 h b) H + /H 2 O 5a 6a + 5 % 90 % (1) + 1a 2aand 2-amino-3-picoline (4; 10 mol % based upon 1 a) as a cocatalytic system gave, after hydrolysis, compound 5 a in 90 % yield along with a small amount of 6 a (5 %). Compound 6 a is a hydroacylated product of 1 a and is formed by a transimination reaction, while 5 a is both a hydroacylated and an ortho-alkylated product. Of the various aldimines employed 1 a, prepared from benzylamine and benzaldehyde, showed the best reactivity for this simultaneous hydroacylation and ortho-alkylation. [3] Compound 1 a did not react with 2 a without the cocatalyst 4 [Eq.(2)], [2] whereas, the ketimine 1 b (which is the benzylimine of acetophenone) was ortho-alkylated by 2 a in the 1a 3 without 4 + 2a no reaction (2) presence of 3 alone, to give 7 a in 97 % yield [Eq. (3)]. These results show that the rhodium(i)-catalyzed ortho-alkylation takes place in ketimines, not in aldimines and that there is no ortho-alkylation without hydroacylation. CH 3 N CH 2 Ph CH 3 O tC 4 H 9 a) 2 mol% 3, toluene 150 o C, 2 h + b) H + /H 2 O 2a 97 % 1b 7a(3)Ketimine 1 b was very reactive in the ortho-alkylation reaction with 3; thus, various olefins were tested in reactions with this ketimine 1 b and 3 as catalyst (Table 1). In contrast to When the emission spectra of the CPFs in methanol (10 % sodium phosphate buffer) excited at 370 nm were detected, the emission of the fluorescein acceptor could be observed in all CPFs. The hydrophobic interaction between the fluorophores wo...