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Pinane-Based Versatile “Allyl”boranes † Outline 1. Introduction 2. Allylboration: Characteristics 3. Preparation of Allylboranes 4. Asymmetric Allylboration 5. Asymmetric Dialkylallylboranes 5.1. B-Methallyldiisopinocampheylborane 5.2. 3,3-Dimethylallyldiisopinocampheylborane 5.3. [(Z)-3-Alkoxyallyl]diisopinocampheylborane 5.4. B-[3-((Diisopropylamino)dimethylsilyl)allyl]diisopinocampheylborane 5.5. [(E)-3-(2,6-Dioxaborolyl)allyl]diisopinocampheylborane 5.6. [(E)-3-(Diphenylimino)allyl]diisopinocampheylborane 5.7. B-(E)- and (Z)-Crotyldiisopinocampheylborane 5.8. B-2’-Isoprenyldiisopinocampheylborane 5.9. Higher Crotylboranes 5.10. γ-Chloroallyldiisopinocampheylborane 5.11. B-2-Cyclohexen-1-yldiisopinocampheylborane 5.12. Other Chiral Dialkylallylboranes 6. Applications of Allyl- and Crotylboration 6.1. Allylboration–Ring-Closing- Metathesis Reactions for the Synthesis of α-Pyrones 6.2. HMG-CoA Reductase Inhibitor Analogs 7. Conclusion and Prospects 8. Acknowledgments 9. References and Notes 1. Introduction The discovery of hydroboration by Brown and Subba Rao and the utilization of the resulting organoboranes as reactants or reagents marked the beginning of a new era in organic chemistry. 1 In the two decades that followed, Brown and others explored the new “continent” of organoboranes for functional group transformations and new carbon–carbon bond-forming reactions. 2 These developments laid the groundwork for several of the asymmetric synthetic methods that emerged during the past two decades. 3-5 The pioneering report by Brown and Zweifel on the asymmetric hydroboration of (Z)-2-butene in high enantiomeric excess (ee) with α-pinene-derived diisopinocampheylborane (Ipc2BH) is the first successful nonenzymatic asymmetric synthesis. 6 Ipc2BH was the forerunner for a series of pinanebased versatile reagents (PVR) for asymmetric reduction, allyl- and crotylboration, ring opening of epoxides, homologation, enolboration, and other reactions (Figure 1). Several accounts of the chemistry of these reagents have appeared in the literature. 3,7 This review will focus on the “allyl”borating reagents derived from α-pinene. 8 2. Allylboration: Characteristics Almost four decades ago, Mikhailov and Bubnov pointed out that triallylborane, in marked contrast to the saturated trialkylboranes, undergoes a fast addition to carbonyl groups (eq 1). 9 Mikhailov and Pozdnev later showed that such reactions occur with allylic rearrangement. 10 Mikhailov, Bubnov, and their coworkers studied this reaction in detail and included several other electrophilic carbons as substrates. Thus, imine derivatives, vinyl ethers, and even acetylenes were subjected to allylboration (eq 2). 11 The homoallylic alcohol products contain hydroxyl and alkene moieties, which can be transformed into other functional groups (Figure 2). It is possible to conceive of several substituted allylborating agents to achieve the synthesis of different types of homoallylic alcohols (eq 3). Hoffmann, Brown, Roush, and others have made significant contributions by systematically studying this aspect of “allyl”boration. 7,8 The stereoelectronics of the substituents at the α carbon (substituents 1 and 2) controls the stereochemistry of the olefins in the P. Veeraraghavan Ramachandran Herbert C. Brown Center for Borane Research Department of Chemistry Purdue University West Lafayette, IN 47907, USA E-mail: chandran@purdue.edu products. Selective formation of E and Z olefinic products has been achieved. 8b,12 Hoffmann and Zeiss showed that the reaction of (E)- or (Z)-butenylboronates (crotylboronates) with aldehydes results in the formation of anti or syn β-methyl homoallylic alcohols, respectively. 13 It is crucial to control the allylic rearragements of crotylboranes (eq 4) in order to achieve maximum diastereoselectivity. Crotyldialkylboranes tend to rearrange more than crotylboronic acid esters. Conducting the reactions at low temperatures is essential to stopping the rearrangement and achieving high diastereoselectivity. 13,14 Due to the similarity in their reaction pathways (involving allylic rearrangement), alkoxyallylboration (eq 5), crotylboration (eq 6), allenylboration (eq 7), and propargylboration (eq 8) can all be broadly classified under “allyl”boration. For convenience, the term “higher crotyl”boranes is used for those reactions involving homologs of 2-butenylboranes (eq 6). The chemistry of boron enolates (enolboration, eq 9) is also very similar to allylboration. However, enolborinates do not undergo allylic rearrangement due to a strong VOL. 35, NO. 1 • 2002 23
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