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Homologation and Alkylation of Boronic Esters and Boranes by 1,2-Metallate Rearrangement of Boron Ate Complexes', Chemical Record, Vol Edinburgh Research Explorer Homologation and Alkylation of Boronic Esters and Boranes by 1,2-Metallate Rearrangement of Boron Ate Complexes Citation for published version: Thomas, SP, French, RM, Jheengut, V & Aggarwal, VK 2009, 'Homologation and Alkylation of Boronic Esters and Boranes by 1,2-Metallate Rearrangement of Boron Ate Complexes', Chemical record, vol. 9, no. 1, pp. 24-39. https://doi.org/10.1002/tcr.20168 Digital Object Identifier (DOI): 10.1002/tcr.20168 Link: Link to publication record in Edinburgh Research Explorer Document Version: Publisher's PDF, also known as Version of record Published In: Chemical record Publisher Rights Statement: Copyright © 2009 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. All rights reserved. General rights Copyright for the publications made accessible via the Edinburgh Research Explorer is retained by the author(s) and / or other copyright owners and it is a condition of accessing these publications that users recognise and abide by the legal requirements associated with these rights. Take down policy The University of Edinburgh has made every reasonable effort to ensure that Edinburgh Research Explorer content complies with UK legislation. If you believe that the public display of this file breaches copyright please contact [email protected] providing details, and we will remove access to the work immediately and investigate your claim. Download date: 06. Oct. 2021 Homologation and Alkylation of Boronic THE CHEMICAL Esters and Boranes by 1,2-Metallate RECORD Rearrangement of Boron Ate Complexes STEPHEN P. THOMAS, ROSALIND M. FRENCH, VISHAL JHEENGUT, VARINDER K. AGGARWAL School of Chemistry, University of Bristol, Bristol, BS8 1TS, UK Received 6 August 2008; Accepted 1 December 2008 ABSTRACT: Organoboranes and boronic esters readily undergo nucleophilic addition, and if the nucleophile also bears an α-leaving group, 1,2-metallate rearrangement of the ate complex results. Through such a process a carbon chain can be extended, usually with high stereocontrol and this is the focus of this review. A chiral boronic ester (substrate control) can be used for stereocontrolled homologations with (dichloromethyl)lithium in the presence of ZnCl2. Subsequent alkylation by an organometallic reagent also occurs with high levels of stereocontrol. Chiral lithiated carbanions (reagent control) can also be used for the reaction sequence with achiral boronic esters and boranes. Aryl-stabilized sulfur ylide derived chiral carbanions can be homologated with a range of boranes including vinyl boranes in good yield and high diastereo- and enantioselectivity. Lithiated alkyl chlorides react with boronic esters, again with high stereocontrol, but both sets of reactions are limited in scope. Chiral lithiated carbamates show the greatest substrate scope and react with both boronic esters and boranes with excellent enantioselectivity. Furthermore, iterative homologation with chiral lithiated carbamates allows carbon chains to be “grown” with control over relative and absolute ste- reochemistry. The factors responsible for stereocontrol are discussed. © 2009 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 9: 24–39; 2009: Published online in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/tcr.20168 Key words: homologation; boronic ester; 1,2-metallate rearrangement; lithiated carbamates; sulfur ylides; chiral carbanions Introduction Organoboranes and boronic esters are very useful synthetic pheylborane by H. C. Brown in 1961 provided the fi rst non- intermediates as they can be converted into a broad range of enzymatic asymmetric synthesis that resulted in truly practical functional groups, often with complete stereospecifi city.1 These levels of enantioselection.2 In the early 1980s Matteson reported transformations are usually initiated by nucleophilic addition a complementary route to chiral boronic esters.3 In fact, the to the electrophilic boron atom followed by 1,2-migration. discoveries by Matteson that very high diastereoselectivities Among all the metals and semimetals, boron seems to possess could be achieved in: (i) reactions of (dichloromethyl)lithium a unique ability to orchestrate these processes cleanly and with with boronic esters, and (ii) reactions of Grignard reagents high stereochemical fi delity. Another attractive feature is that chiral boron reagents are easily accessible in high enantiopurity. Indeed, the hydroboration of alkenes using (−)-diisopinocam- ᭤ Correspondence to: Varinder K. Aggarwal; email: [email protected] The Chemical Record, Vol. 9, 24–39 (2009) 24 © 2009 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Homologation of Boronic Esters LiCHCl2 OR* -LiCl Cl OR* R2M Cl OR* 1 ∗ ∗ R B B B OR* M 1 -MCl OR* Substrate R1 OR* R R2 control R2 OR* 1,2-Metallate B rearragement ∗ M R1 OR* ∗ OR R1 LG LG OR 2 ∗ -MLG R B B OR M OR Reagent R1 R2 control LG = leaving group Scheme 1. Complementary routes to homochiral organoboranes involving 1,2-metallate rearrangement. with (α-haloalkyl)boronic esters opened up a whole new fi eld Homologation–Alkylation of Chiral Boronic in organoboron chemistry.4 Esters: Substrate Control In Matteson’s approach, a chiral auxiliary is embedded in the diol moiety of the boronic ester and subsequent transfor- Reaction of an enantiopure boronic ester 1 with mations are controlled by its architecture (substrate control; (dichloromethyl)lithium forms (dichloromethyl)borate Scheme 1). An alternative approach uses a chiral reagent to complex 2. In the presence of zinc chloride, borate 2 construct a related chiral ate complex that subsequently evolves rearranges via transition structure (TS) 3 leading to a single into the chiral boronic ester following 1,2-metallate rearrange- α-chloroboronic ester 5 with a high diastereomeric purity, ment (reagent control; Scheme 1). These two approaches are often >99 : 1 dr (Scheme 2). It has been proposed that in discussed in this review. the preferred TS 3 zinc chloride promotes and directs the ᭤ Stephen P. Thomas was born in Toronto, Canada, in 1981 and moved to the United Kingdom in 1991. He received his MChem from Cardiff University (2003) under the supervision of Dr Nicholas Tomkinson and then moved to Cambridge University for his Ph.D. (2007) working with Dr Stuart Warren. After postdoctoral work with Professor Andreas Pfaltz (2007–2008) at the University of Basel, Switzerland, he returned to a lectureship at the University of Bristol. ᭿ ᭤ Rosalind M. French was born in Taplow, England. She received her M.Sc degree from Oxford University in 2006 under the supervision of Dr. Veronique Gouverneur. In 2006 she commenced her Ph.D. studies in the group of Prof. Varinder K. Aggarwal at Bristol University, and is currently working on the lithiation/borylation reactions of secondary benzyl carbamates. ᭿ © 2009 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. 25 THE CHEMICAL RECORD Cl Cl Cl Cl ZnCl2 R R H Zn Zn O R H LiCHCl2 O R ZnCl2 O O Cl O 1 Cl O Cl O R Cl Cl R Cl R R B B B B B O THF, − °C r.t. O O R 1 1 H 1 Cl R R R R R R1 Cl H 1 2 3 3 4 Favoured Disfavoured X Cl R Cl MX2 R Cl O H H M R1 O R 1 O R O B R 1 O R X B O R B O Cl B 1 O B R 2 R R 2 R O R R 2 H O R R R R2 R1 5 6 7 7 8 Scheme 2. Matteson homologation–alkylation sequence. migration of R1 by complexation to the less hindered oxygen chloride (which becomes more nucleophilic in the TS) and the atom of the boronic ester while simultaneously assisting the C-H bond (which becomes more electrophilic in the TS as the departure of a chloride ion.3c It is also believed that this TS is chloride leaves).5 No such interaction can occur in the migra- further stabilized by an interaction between the chloride of zinc tion of the other diastereotopic chloride that is consequently ᭤ Dr. Vishal Jheengut was born in Nouvelle France, Mauritius, in 1977. He received his B.Sc in chemistry at the University of Mauritius in 2001 and Ph.D. in synthetic organic chemistry in 2007 at the University of Saskatchewan, Canada, under the supervision of Professor Dale E. Ward. During his Ph.D., he worked on the development of new aldol strategies using thiopyran scaffolds and the synthesis of polypropionates. He joined the laboratory of Professor Varinder K. Aggarwal at the University of Bristol, UK in 2007 as postdoctoral researcher where he is currently working on the synthesis of all carbon quaternary centres using chiral carbenoids and their application to natural products. ᭿ ᭤ Varinder K. Aggarwal was born in Kalianpur in North India in 1961 and emigrated to the United Kingdom in 1963. He received his B.A. (1983) and Ph.D. (1986) from Cambridge University, the latter under the guidance of Dr Stuart Warren. He was subsequently awarded a Harkness fellowship to carry out postdoctoral work with Professor Gilbert Stork at Columbia University, NY (1986–1988). He returned to a lectureship at Bath University and in 1991 moved to Sheffi eld University, where in 1997 he was promoted to Professor of Organic Chemistry. In 2000, he moved to the University of Bristol to take up the Chair of Synthetic Chemistry. He is the recipient of the AstraZeneca Award, Pfi zer Award, GlaxoWelcome Award, Novartis Lectureship, RSC Hickinbottom Fellowship, Nuffi eld Fellowship, RSC Corday Morgan Medal and the Leibigs Lectureship, Liebigs Lecturship (Germany), 1999/2000 (inaugural), RSC Green Chemistry Award 2004, Zeneca Senior Academic Award 2004, RSC Reaction Mechanism Award 2004, Royal Society Wolfson Merit Award 2006, RSC/GDCh-Alexander Todd-Hans Krebs Lectureship 2007, RSC Tilden Lecturer awarded 2007, and an EPSRC Senior Research Fellowship 2007. ᭿ 26 © 2009 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Homologation of Boronic Esters disfavored (TS 4). Indeed, Midland has calculated that there Cl O R O R O R −1 6 LiCHCl2 RLi is a 12.6 kcal mol difference between the two TSs.
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