Letter pubs.acs.org/OrgLett
Differentially Substituted Phosphines via Decarbonylation of Acylphosphines Rongrong Yu,† Xingyu Chen,† Stephen F. Martin,‡ and Zhiqian Wang*,† †
State Key Laboratory of Chemical Resource Engineering, Department of Organic Chemistry, College of Science, Beijing University of Chemical Technology, Beijing 100029, China ‡ Department of Chemistry, University of Texas at Austin, Austin, Texas 78712, United States S Supporting Information *
ABSTRACT: A new route to phosphines was developed by a method that features a “pre-join and transform” process that proceeds via acylphosphine intermediates that may be readily prepared from carboxylic acids and disubstituted phosphines. The efficient decarbonylations of these acylphosphines using a nickel catalyst delivered the corresponding phosphines. This method shows that the carboxyl group can play a role similar to halides or triflates for introducing a substituted phosphorus atom on an aromatic ring.
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corresponding acylphosphine may be conveniently realized by a straightforward acylation.6 We reasoned that this intermediate acylphosphine might undergo a “transform” sequence of C−P bond activation, decarbonylation, and reductive elimination. Some precedent for the initial C−P bond activation step is found in the pioneering work of Garg and Houk, who used amides as novel substrates for various transition-metal-catalyzed coupling and ester-forming reactions via a “C−N” bond activation process.7 Other methods featuring a C−P bond activation process as a key step in new C−P or C−C bond formation have been recently disclosed.8 In order to assess the feasibility of the overall process set forth in Figure 1, we conducted an exploratory study in which the acylphosphine 1a was converted to the triarylphosphine 2a in the presence of several transition metal catalysts (Table 1). Use of 5 mol % of Pd(OAc)2 and Pd2(dba)3 afforded the desired product 2a in 10−20% yield (entries 1, 2). Varying the phosphine ligands on the catalyst afforded little improvement in yield (data not shown). When rhodium(I) catalysts that perform well in C−C and C−H activation reactions were used,9 no conversion of the substrate was observed (entries 3, 4). However, use of homogeneous nickel complexes10 led to significant improvements in yield. For example, when the acylphosphine 1a was heated in toluene at 140 °C in the presence of NiCl2(dppp) and NiCl2(dppf), the phosphine 2a was obtained in yields of 56% and 67%, respectively (entries 5, 6). Simply changing the solvent to xylenes and increasing the temperature to 150 °C offered further improvements, and 2a was isolated in 92% yield when NiCl2(dppp) was used as the catalyst. In optimization studies to probe the effects of the phosphine ligand, we found that use of dppp, dppf, dppe, and PPh3 uniformly provided good yields of 2a (entries 7−10). Even NiCl2·6H2O catalyzed the transformation of 1a into 2a in 80%
ransition-metal (TM)-catalyzed carbon−phosphorus coupling reactions have been widely used to prepare substituted phosphines,1 especially chiral phosphines, that are commonly used in a variety of important catalytic processes.2 In these reactions, the aryl halides (−X), triflates (−OTf), and their derivatives are typically employed as the electrophilic partners for carbon−phosphorus bond formation (Figure 1a).3 Although
Figure 1. “Pre-join and transform” process.
carboxylic acids have recently emerged as attractive starting materials for decarboxylative bond formation reactions,4 they have rarely been employed as a coupling partner in TM-catalyzed C−P bond forming reactions.5 It is thus notable that we envisioned a novel two-step approach for the facile synthesis of a variety of trivalent phosphines using a “pre-join and transform” pathway that employs carboxylic acids as the electrophilic coupling partner (Figure 1b). In this procedure, a disubstituted phosphine is first coupled with a carboxylic acid to form an acylphosphine, which is then subjected to a decarbonylative C−P bond activation and reductive elimination to afford a trivalent phosphine. We have previously shown that the “pre-join” step merging a carboxylic acid and a secondary phosphine to form the © XXXX American Chemical Society
Received: February 25, 2017
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DOI: 10.1021/acs.orglett.7b00579 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Table 1. Condition Optimization for Decarbonylation
entry
catalyst
solvent
temp (°C)
yield (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Pd(OAc)2 Pd2(dba)3 RhCl(PPh3)3 [RhCl(coe)2]2 NiCl2(dppp) NiCl2(dppf) NiCl2(dppp) NiCl2(dppf) NiCl2(dppe) NiCl2(PPh3)2 NiCl2·6H2O Ni(COD)2 NiCl2(dppp) NiCl2(dppp) NiCl2(dppp) NiCl2(dppp)
toluene toluene toluene toluene toluene toluene xylene xylene xylene xylene xylene xylene C6H5Cl DMF DMSO xylene
140 140 140 140 140 140 150 150 150 150 150 140 150 150 150 150 (air)
20 10 >5 >5 56 67 92 (83a) 84 58 88 80 91 69 6
