Note pubs.acs.org/joc
Cite This: J. Org. Chem. 2019, 84, 463−471
Synthesis of Aryl Trimethylstannane via BF3·OEt2‑Mediated CrossCoupling of Hexaalkyl Distannane Reagent with Aryl Triazene at Room Temperature Shuai Mao,†,⊥ Zhengkai Chen,‡,⊥ Lu Wang,† Daulat Bikram Khadka,§ Minhang Xin,† Pengfei Li,*,∥ and San-Qi Zhang*,† †
Department of Medicinal Chemistry, School of Pharmacy, Xi’an Jiaotong University, Xi’an 710061, China Department of Chemistry, Zhejiang Sci-Tech University, Zhejiang, Hangzhou 310018, China ∥ Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an, Shaanxi 710054, China § Department of Medicinal Chemistry, College of Pharmacy, University of Michigan, Ann Arbor, Michigan 48109, United States
Downloaded via IOWA STATE UNIV on January 9, 2019 at 13:31:27 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: BF3·OEt2-mediated cross-coupling of (SnMe3)2 with aryl triazene offers a new strategy for the synthesis of aryl stannane. A variety of synthetically useful aryl trimethylstannanes were produced in moderate to good yields with this metal-free approach. One-pot sequential Stille cross-coupling with different aryl bromides provides a short entry to both symmetrical and unsymmetrical biaryl compounds.
C
available synthetic methods, the development of novel and practical methods to access these compounds is still highly desirable. Aryl triazenes are an important class of molecules that have drawn a lot of attention from organic chemists.8 Owing to the low cost and easy availability of aryl triazenes, C−N electrophiles have emerged as powerful alternatives to aryl halides as coupling partners in the cross-coupling arena. In the last two decades, aryltriazene has been used as an equivalent of aryldiazonium salt in the presence of a Lewis or Brønsted acid in nucleophilic substitution reactions and transition-metalcatalyzed coupling reactions.9 In particular, BF3·OEt2 has been proven to be an efficient reagent to activate aryltriazene in Pdcatalyzed Suzuki−Miyaura coupling reactions.10 Although the use of triazane as a coupling partner in a coupling reaction has increased, only aryl triazenes substituted with electrondonating groups can be subjected to a coupling reaction efficiently; aryl triazenes with the electron-withdrawing group usually give poor yields.10a,b In comparison, the Stille coupling reaction represents one of the most reliable methods in syntheses of complex molecules where great substrate compatibility, mild reaction conditions, and good yields have been the main features. Moreover, in the transition-metal-
onsidering the great importance of the well-known Stille coupling reaction in the formation of a C−C bond, the rapid assembly of organotin reagents has attracted considerable interest among organic chemists.1 Apart from the construction of the C−C bond, the organotin reagents can also be applied in other useful transformations.2 The conventional methods to access aryl stannane compounds mainly rely on the reaction of air- and moisture-sensitive aryl-metal reagents with trialkyl tin chloride, under harsh reaction conditions.3 In this aspect, transition-metal-catalyzed stannylation of aryl halides, which possesses tremendous versatility and functional group compatibility is a more reliable route.4 Remarkably, this methodology has been recently expanded to include a C−H bond activation strategy and significant progress has been achieved.5 However, transition-metal catalysts are expensive and sometimes result in metal contaminants in the final products. Thus, more attention was beginning to shift to the transition-metal-free process in organic synthesis. Recently, stannylation of arylamine via oxidative deamination by alkylnitrite has been reported.6 It provides a direct conversion of arylamine to aryl stannanes under metal-free conditions, which is an innovative development in aryl stannane synthesis. More recently, our group reported an efficient metal catalystfree approach to synthesize trimethylarylstannanes from aryl halides promoted by UV light.7 However, given the great importance of aryl stannanes and the limited number of © 2018 American Chemical Society
Received: October 29, 2018 Published: December 12, 2018 463
DOI: 10.1021/acs.joc.8b02766 J. Org. Chem. 2019, 84, 463−471
Note
The Journal of Organic Chemistry
(SnMe3)2 was preferred over (SnBu3)2, which resulted in only a small amount of aryl(tributyl)stannane (Table 1, entry 17). With the optimized reaction conditions in hand, the scope and limitations of the transformation were examined (Table 2). A wide variety of aryl triazenes bearing electron-
catalyzed cross-coupling reactions, aryltriazenes usually behave only as the electrophile instead of a nucleophile. Therefore, effective transformation of aryltriazenes to aryl stannanes could facilitate the targeted coupling reactions by overcoming the limitations associated with aryl triazenes.” In this context, we demonstrate a BF3·OEt2-mediated deaminostannylation of a series of functionalized aryl triazenes for the assembly of corresponding aryl stannane derivatives. This method may potentially have wide applications in combinatorial chemistry. We commenced the investigation by choosing aryl triazene 1a with (SnMe3)2 (1,1,1,2,2,2-hexamethyldistannane) as model substrates to optimize the reaction conditions. The reaction was conducted by the employment of BF3·OEt2 as an additive in EtOH at room temperature (rt) under air (Table 1,
Table 2. Scope of the Stannylation Reaction of Diverse (Hetero)aryl Triazenesa,b
Table 1. Optimization of Reaction Conditionsa
entry
additive (equiv)
solvent
temp (°C)
time (min)
yieldb (%)
1 2c 3 4 5 6 7 8 9 10 11 12 13d 14 15 16 17e
BF3·OEt2 (1.0) BF3·OEt2 (1.0) BF3·OEt2 (1.0) BF3·OEt2 (1.0) BF3·OEt2 (1.0) BF3·OEt2 (1.0) BF3·OEt2 (1.0) BF3·OEt2 (1.0) BF3·OEt2 (1.0) BF3·OEt2 (1.0) BF3·OEt2 (1.0) BF3·OEt2 (1.0) other acids (1.0) BF3·OEt2 (0.5) BF3·OEt2 (1.5) none BF3·OEt2 (1.0)
EtOH dioxane DME THF DMF DMSO CH2Cl2 toluene CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN
rt rt rt rt rt rt rt rt rt 40 0 −10 rt rt rt rt rt
60 60 60 60 60 60 60 60 30 10 90 180 60 60 30 30 30
34 trace 26 23 trace trace 30 19 67 59 68 65
