Letter pubs.acs.org/OrgLett
Iron-Catalyzed Hydroboration of Vinylcyclopropanes Chenhui Chen, Xuzhong Shen, Jianhui Chen, Xin Hong, and Zhan Lu* Department of Chemistry, Zhejiang University, Hangzhou 310027, China S Supporting Information *
ABSTRACT: An iron-catalyzed hydroboration of vinylcyclopropane with HBpin is first reported for the preparation of valuable homoallylic organoboronic esters. The iron catalysts enable efficient and regioselective C−C cleavage of vinylcyclopropanes, stereoselectively delivering E-alkenes with good stereospecific selectivity at an allylic position. This protocol exhibits mild conditions with good functional group tolerability. The chiral homoallylic organoboronic esters could be further converted into chiral polysubstituted tetrahydrofuran and tetrahydropyran.
A
vinylcyclopropane gave an anti-Markovnikov selective product with 97/3 rr.5a The hydroborations of substituted VCPs with BH3 have been reported to afford a mixture of regioisomers.5 Additionally, the alkenes could not survive in these transformations. These results decreased the utility of this transformation. So far, it is highly desirable for the development of new strategies for highly selective hydroboration of vinylcyclopropanes. As a continuation of our interest in earth-abundant transition-metal-catalyzed transformations,6 herein we describe an iron-catalyzed Markovnikov-type hydroboration of vinylcyclopropanes to afford homoallylic organoboronic esters via selective C−C bond cleavage (Scheme 1c). The 1-phenyl-2-(1′-phenyl)vinyl cyclopropane 1a was chosen as a model substrate. The reaction of 1a with HBpin using OPPA as a ligand and FeCl2 as a precatalyst did not afford hydroboration products (Table 1, entry 1).6f Using Pybox· FeCl2 or PDI·FeCl2 as precatalysts which used to be suitable for hydroboration of alkenes and NaHBEt3 as a reductant, the reaction did not occur (entries 2 and 3). The iron complex OIP·FeCl2 was found to be a good precatalyst for hydroboration of 1a, delivering the homoallylic organoboronic ester 2a in 90% yield with 12/1 E/Z via a C−C bond cleavage process4,7 (entry 4). This protocol was a good supplement for the synthesis of polysubstituted homoallylic organoboronic esters which were difficult to obtain using boromethylation of allylic compounds.8 Using bromide as a counterion, the reaction afforded 2a in 88% isolated yield with 15/1 E/Z (entry 5). The yield decreased when 1 equiv of 1a was used (entry 6). The reaction could be completed in 10 min (entries 7 and 8). The control experiments without NaBHEt3, or precatalyst, or ligand were carried out and did not afford the desired products (entries 9−11). Various reductants have also been screened, however, to afford traces of products (see Table S1). The standard conditions were identified as racemic
lkyl boronic acid derivatives are a very important class of organic compounds in chemistry.1 Due to their chemical stability and easy elaboration, organoboronic acid derivatives are considered to be useful synthetic building blocks for the preparation of intricate molecules. Thus, the development of new methodologies for the highly efficient and selective synthesis of diversified organoboronic acid derivatives is continuously desirable. Late transition-metal-catalyzed olefin hydroboration is one of the most direct, efficient, and atomeconomic transformations in organoboron chemistry.2 Recently, the catalysts involving earth-abundant transition metals3 such as iron, cobalt, nickel, copper, and manganese have emerged with high activity and selectivity for alkene hydroboration (Scheme 1a). Vinylcyclopropane (VCP) as a simple and useful synthon has been used for an impressive range of synthetic applications, particularly for the construction of all-carbon rings.4 Hydroboration of VCP has the potential to afford organoboronates containing cyclopropanes or alkenes available for further elaboration (Scheme 1b). The hydroboration of simple Scheme 1. Hydroboration of Vinylcyclopropanes
Received: August 29, 2017 Published: September 27, 2017 © 2017 American Chemical Society
5422
DOI: 10.1021/acs.orglett.7b02691 Org. Lett. 2017, 19, 5422−5425
Letter
Organic Letters Table 1. Condition Optimizationsa
entry
cat.
t (h)
yield (%)b
Z/Eb
1 2 3 4 5 6c 7 8 9 10d 11
OPPA + FeCl2 Pybox·FeCl2 PDI·FeCl2 DMiP OIP·FeCl2 DMiP OIP·FeBr2 DMiP OIP·FeBr2 DMiP OIP·FeBr2 DMiP OIP·FeBr2 FeBr2 DMiP OIP·FeBr2 −
3 3 3 3 3 3 0.5 0.17 3 3 3
