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50 Years of Zweifel Olefination: a Transition-Metal-Free Coupling

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50 Years of Zweifel Olefination: a Transition-Metal-Free Coupling SYNTHESIS0039-78811437-210X Georg Thieme Verlag Stuttgart · New York 2017, 49, 3323–3336 short review 3323 en Syn thesis R. J. Armstrong, V. K. Aggarwal Short Review 50 Years of Zweifel Olefination: A Transition-Metal-Free Coupling Roly J. Armstrong 0000-0023-7590-61X Varinder K. Aggarwal* 0000-0030-3446-430 School of Chemistry, University of Bristol, Cantock’s Close, Bristol, BS8 1TS, UK [email protected] Dedicated to Professor Herbert Mayr on the occasion of his 70th birthday Received: 15.05.2017 Accepted after revision: 16.05.2017 Published online: 11.07.2017 DOI: 10.1055/s-0036-1589046; Art ID: ss-2017-z0328-sr License terms: Abstract The Zweifel olefination is a powerful method for the stereo- selective synthesis of alkenes. The reaction proceeds in the absence of a transition-metal catalyst, instead taking place by iodination of vinyl bo- ronate complexes. Pioneering studies into this reaction were reported in 1967 and this short review summarizes developments in the field over the past 50 years. An account of how the Zweifel olefination was modified to enable the coupling of robust and air-stable boronic esters Varinder K. Aggarwal (right) studied chemistry at Cambridge Univer- is presented followed by a summary of current state of the art develop- sity and received his Ph.D. in 1986 under the guidance of Dr Stuart ments in the field, including stereodivergent olefination and alkynyla- Warren. After postdoctoral studies (1986–1988) under Professor Gil- tion. Finally, selected applications of the Zweifel olefination in target- bert Stork, Columbia University, he returned to the UK as a Lecturer at oriented synthesis are reviewed. Bath University. In 1991, he moved to Sheffield University, where he 1 Introduction was promoted to Professor in 1997. In 2000, he moved to Bristol Uni- 2.1 Zweifel Olefination of Vinyl Boranes versity where he holds the Chair in Synthetic Chemistry. He was elected 2.2 Zweifel Olefination of Vinyl Borinic Esters Fellow of the Royal Society in 2012. 2.3 Extension to Boronic Esters Roly J. Armstrong (left) graduated with an MSci in Natural Sciences 3.1 Introduction of an Unsubstituted Vinyl Group from Pembroke College, Cambridge (2011) spending his final year 3.2 Coupling of α-Substituted Vinyl Partners working in the laboratory of Professor Steven Ley. He subsequently 3.3 Syn Elimination moved to Merton College, Oxford to carry out a DPhil under the super- 4 Zweifel Olefination in Natural Product Synthesis vision of Professor Martin Smith (2011–2015) working on asymmetric 5 Conclusions and Outlook counterion-directed catalysis. In October 2015, he joined the group of Professor Varinder Aggarwal at the University of Bristol as a postdoctor- Key words Zweifel olefination, coupling, boronic esters, alkenes, al research associate. transition-metal-free, enantiospecific 1 Introduction of secondary and tertiary (chiral) boronates remains prob- lematic.3 Furthermore, the high cost and toxicity of the pal- The stereocontrolled synthesis of alkenes is a topic that ladium complexes required to catalyze these processes also has attracted a great deal of attention owing to the preva- detract from the appeal of this methodology.4 lence of this motif in natural products, pharmaceutical The Zweifel olefination represents a powerful alterna- agents and materials.1 Of the many olefination methods tive to the Suzuki–Miyaura reaction, enabling the coupling that exist, the Suzuki–Miyaura coupling represents a highly of vinyl metals with enantioenriched secondary and tertia- convergent method to assemble alkenes (Scheme 1, a).2 ry boronic esters with complete enantiospecificity (Scheme However, although the coupling of vinyl halides with pri- 1, b).5 The reaction is mediated by iodine and base and pro- mary and sp2 boronates takes place effectively, the coupling ceeds with no requirement for a transition-metal catalyst. Georg Thieme Verlag Stuttgart · New York — Synthesis 2017, 49, 3323–3336 3324 Syn thesis R. J. Armstrong, V. K. Aggarwal Short Review This process is based upon pioneering studies reported in R' BH R' 2 I2, NaOH R' 1967 by Zweifel and co-workers on the iodination of vinyl R B R R' R THF THF/H2O boranes. This short review summarizes the key contribu- 12 tions made over the last 50 years that have enabled this anti transformation to evolve into an efficient and atom-eco- I2 iodination NaOH elimination nomical method for the coupling of boronic esters. Recent 1,2-metallate I OH OH I rearrangement B contributions to the field are described including the devel- B R' R R' R opment of Grignard-based vinylation, stereodivergent ole- R' R' fination and alkynylation processes. Finally, selected exam- 34 Me ples of Zweifel olefination in target-oriented synthesis are selected products: Cy Cy reviewed to highlight the utility of this methodology. Cy Cy Me Me 2a; 83 % yield 2b; 75 % yield 2c; 77 % yield 2d; 70 % yield (a) Suzuki–Miyaura Coupling 99:1 Z/E 92:8 Z/E 85:15 Z/E R1 X cat. Pd(0) R1 R3 + R3 B(pin) Scheme 2 Zweifel olefination: iodination of vinyl boranes R2 R2 (b) Zweifel Olefination with very high levels of E-selectivity. Chiral non-racemic 1 1 3 R M I2 R R Transition-Metal Free boranes could be transformed with complete stereospeci- 3 + R B(pin) Stereospecific ficity. 2 NaOMe 2 R R Enantiospecific R' Scheme 1 Olefination of boronic esters R'2BH BrCN, CH2Cl2 B R R R R' R' THF then workup 2.1 Zweifel Olefination of Vinyl Boranes 56with aq. NaOH syn BrCN bromination In 1967, Zweifel and co-workers reported that vinyl elimination CN boranes 1, obtained by hydroboration of the corresponding CN Br Br B R' B R' alkynes, could be treated with sodium hydroxide and R R' 1,2-metallate rearrangement R R' iodine, resulting in the formation of alkene products 2 7 8 6 (Scheme 2). Intriguingly, although the intermediate vinyl selected products: Me Me nBu nBu boranes were formed with high E-selectivity, after addition nBu Cy of iodine, Z-alkenes were produced. A reaction with a dia- Me stereomerically pure secondary borane afforded the cou- 6a; 69 % yield 6b; 68 % yield 6c; 75 % yield Me 96:4 E/Z 93:7 E/Z 98:2 E/Z pled product, 2d, as a single anti diastereoisomer, indicating that the process proceeds with retention of configuration.7 Scheme 3 Synthesis of E-alkenes using cyanogen bromide Mechanistically, this reaction is thought to proceed by acti- vation of the π bond with iodine along with complexation A related syn elimination process was reported by Levy of sodium hydroxide to form a zwitterionic iodonium inter- and co-workers (Scheme 4).11 In this case, a vinyl lithium mediate 3. This species is poised to undergo a stereospecific reagent was prepared by lithium–halogen exchange and 1,2-metalate rearrangement resulting in the formation of a then combined with a symmetrical trialkylborane resulting β-iodoborinic acid 4. In the presence of sodium hydroxide, in formation of boronate complex 9. Treatment of this inter- this intermediate then undergoes anti elimination to afford mediate with iodine resulted in stereospecific iodination to the resulting Z-alkene product.8 produce β-iodoborane 10. The enhanced electrophilicity of Because vinyl borane intermediates could only be ac- this species (compared to β-iodoborinic acids such as 4) en- cessed by hydroboration of alkynes, the iodine-mediated abled a syn elimination to occur, generating the corre- Zweifel coupling was initially limited to the synthesis of Z- sponding trisubstituted alkene 11 with high levels of ste- alkenes.9 However, Zweifel and co-workers subsequently reocontrol. Although the substrate scope of the process is reported an elegant strategy for the complementary syn- wide, the method was limited to the use of symmetrical tri- thesis of E-alkenes (Scheme 3).10 This transformation was alkyl boranes. achieved by reacting dialkyl vinyl borane 5 with cyanogen Brown and co-workers demonstrated that the Zweifel bromide under base-free conditions. Following stereospe- olefination can also be applied to the synthesis of alkynes cific bromination, a boranecarbonitrile intermediate 8, was (Scheme 5).12 In this case, monosubstituted alkynes were formed, a species that was sufficiently electrophilic to un- deprotonated to form lithium acetylides, which were react- dergo syn elimination. A variety of boranes underwent this ed with trialkylboranes to form alkynylboronate complexes transformation, forming alkenes 6a–c in high yields and 12. Addition of iodine triggered a 1,2-metallate rearrange- ment to generate β-iodoboranes 13, which spontaneously Georg Thieme Verlag Stuttgart · New York — Synthesis 2017, 49, 3323–3336 3325 Syn thesis R. J. Armstrong, V. K. Aggarwal Short Review R R2 i. nBuLi, THF 2 2 R R R I2, THF I B R R I B R3 3 2 R R R R syn R3 ii. R3B Li then workup R3 R elimination 9 with NaOH/H2O2 10 11 selected products: Et Me nPr Et Et Et Et Et nHex Et Et nPr Ph 11a 11b; 80 % yield 11c; 87 % yield 11d; 38 % yield 66 % yield >95:5 Z/E >95:5 E/Z >97:3 E/Z Scheme 4 Olefination of symmetrical trialkylboranes underwent elimination to form alkyne products. This pro- ful when the borane is challenging to access or expensive. cess represents a convenient alternative to the alkylation of One solution to this problem would be to employ a mixed lithium acetylides with alkyl halides and has been success- borane in which one (or two) of the boron-bound groups fully employed in total synthesis.13 demonstrates a low migratory aptitude (e.g., thexyl).14 However, in practice, determining which group will migrate 2.2 Zweifel Olefination of Vinyl Borinic Esters has proved to be non-trivial and highly substrate-depen- dent.
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