Synthesis and Characterization of R₂PNP(ⁱBuNCH₂CH₂)₃N:  A New Bulky Electron-Rich Phosphine for Efficient Pd-Assisted Suzuki−Miyaura Cross-Coupling Reactions

Abstract

Pro-azaphosphatrane 1a [P(iBuNCH2CH2)3N] reacts with iodine under mild conditions to give [IP(iBuNCH2CH2)3N]I in excellent yield, which on subsequent reaction with ammonia followed by deprotonation with KOtBu provided HN=P(iBuNCH2CH2)3N (3a) in quantitative yield. Reaction of 3a with R'2PCl afforded sterically bulky electron-rich phosphines of the type R'2PN=P(iBuNCH2CH2)3N (4) [R'=Ph (4a), iPr (4b), tBu (4c)]. The Pd(OAc)2/4c catalyst system was particularly efficient for the coupling of arylboronic acids with aryl bromides as well as aryl chlorides to give biaryls in excellent yields.

Suzuki cross-coupling catalyzed by palladium (II) complexes bearing 1-aryl-3,4,5,6-tetrahydropyrimidine ligands

“…From Table 3, it can be seen that as expected, the effect of the substituent on the para-position of the arylhalides is mainly depending on the electronegativity of the substituent, electronwithdrawing substituents leading to relatively high yields in the cross-coupling reaction (Table 3, entries 4, 5, 6, 8, 9, 10), while electron-donating substituents resulting in relatively low yields (Table 3, entries 12,13,16,17). While effect of the substituent on the ortho-position of the arylbromide is mainly depending on the size of the substituent, bulky ortho-substituted arylbromide afforded biaryls in relatively low yields, and the yields decreased according to the size increment of the substituent (Table 3, entries 7, 15,19).…”

Highly Efficient Ligands for the Palladium‐Assisted Double N‐Arylation of Primary Amines for One‐Sep Construction of Carbazoles

“…Previous work from our group has established that the commercially available proazaphosphatrane 8 [11] bearing isobutyl groups on the PN 3 nitrogens and its derivative (t-Bu) 2 P À N=P(i-BuNCH 2 CH 2 ) 3 N) 9a [12] Scheme 1. Pd-catalyzed double N-arlyation of primary amines with 2,2'-dihalobiphenyls.…”

“…(Scheme 1, both first synthesized in our laboratories) are highly effective ligands for several palladium-catalyzed cross-coupling reactions of aryl bromides, iodides and chlorides, such as Suzuki [12,13] , Stille [14] , a-arylation [15] and Buchwald-Hartwig amination reactions. [16] We have suggested that the unusually high activity of 8 and 9a as supporting ligands in Pd-catalyzed coupling reactions is due primarily to their steric bulk and the uniquely electron-rich nature of their trivalent phosphorus atoms stemming from the geometrically favorable arrangement of the three sp 2 nitrogens contributing their unhybridized p electron lone pair density to the phosphorus and the possibly temporary N!P transannulation donating the bridgehead nitrogens lone pair to the phosphorus in the proazaphosphatrane framework in each of these molecules.…”

“…[16] We have suggested that the unusually high activity of 8 and 9a as supporting ligands in Pd-catalyzed coupling reactions is due primarily to their steric bulk and the uniquely electron-rich nature of their trivalent phosphorus atoms stemming from the geometrically favorable arrangement of the three sp 2 nitrogens contributing their unhybridized p electron lone pair density to the phosphorus and the possibly temporary N!P transannulation donating the bridgehead nitrogens lone pair to the phosphorus in the proazaphosphatrane framework in each of these molecules. [12,17] We envisioned the possibility that the steric bulk and electron-richness of 8 and 9 might also function well in facilitating Pd-catalyzed double N-arylation of primary amines with 2,2'-dihalobiphenyls for a onepot synthesis of carbazoles (Scheme 1). Thus we first examined the reaction of 2,2'-dibromobiphenyl 5a with aniline 6a for screening purposes (Table 1).…”

“…Even the sterically encumbered 2,6-diisopropylaniline 6g successfully coupled with 5a to afford impressive yields (92-96%) of 7g with both catalyst systems (entries 9 and 10). Anilines with F or Cl functional groups also produced carbazoles in excellent yields (entries [12][13][14][15][16]. Generally, the use of ligand 8 required lower catalyst loadings and shorter reaction times than 9a for the reaction to proceed to completion.…”