Radical Fragment Coupling Route to Geminal Bis(boronates

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Cite This: Org. Lett. 2018, 20, 5304−5308

Radical Fragment Coupling Route to Geminal Bis(boronates) Qi Huang and Samir Z. Zard* Laboratoire de Synthèse Organique, CNRS UMR 7652 Ecole Polytechnique, Palaiseau, 91128 Cedex, France

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ABSTRACT: The radical addition of dithiocarbonates to 1,1bis(boronyl)-3-butene and related alkenes occurs without complications from fragmentation or hydrogen atom abstraction and delivers a vast array of highly functional geminal bis(boronates). The ability to assemble geminal bis(boronates) bearing polar functional groups not readily obtained through existing methods is particularly noteworthy. This approach also opens up access to geminal bis(boronyl) cyclopropanes and geminal bis(boronyl) tetrahydroquinolines.

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transition-metal-catalyzed diborylation of C−H bonds, especially of the benzylic type.9 As for their reactions, the most significant is their metalfree10 and transition-metal-catalyzed coupling with aliphatic and vinylic and aromatic halides and triflates (paths f and g).11 Importantly, incorporation of chiral ligands enables enantioselective couplings. Note, furthermore, that in the case of path g, highly useful allylic boronates are produced. Aldol addition and Wittig-type condensation with ketones and aldehydes have also been described (path h). 12 Morken has shown that deborylative alkylation can be accomplished according to path i.13 Finally, the reaction with pyridine and quinoline Noxides results in the loss of both boron groups to give the alkylated, reduced heteroarene (path j).14 Most, if not all of the geminal bis(boronates) described in the open literature are relatively simple, scarcely functionalized structures, reflecting the limitations inherent in the methods used for their preparation. A reaction allowing access to more densely decorated geminal bis(boronates) should therefore have a significant impact on the development of the field. The xanthate-exchange process we have developed over the years has proven to be one of the very few synthetic tools that allow the creation of carbon−carbon bonds starting with unactivated alkenes in an intermolecular setting, with good tolerance for a broad range of functional groups.15 It was therefore tempting to examine the possibility of adding a xanthate 3 to readily available allyl bis(boronate) 2, a process that would generate adducts 5 via intermediate radical 4 (Scheme 2).16 We had two apprehensions from the outset. The first is the lability of the remaining hydrogen (highlighted in red) toward abstraction by radical R• or by undecyl radicals produced by thermolysis of the dilauroyl peroxide (DLP) initiator. The ensuing radical 6 would be expected to be highly stabilized, as it is flanked by two boronate groups, each providing one empty

he chemistry of geminal bis(boronates) 1 has witnessed a remarkable development in recent times.1 These species correspond to air-stable, easy to handle 1,1-organodimetallic reagents and, in this respect, can participate in multiple carbon−carbon bond-forming processes. Routes for their synthesis and their more important reaction pathways are summarized in Scheme 1 (the reagents are omitted for clarity). Scheme 1. Synthesis and Reactions of Geminal Bis(boronates)

Perhaps the simplest approach is by alkylation of the anion generated from the parent methylene bis(boronate) or a monosubstituted derivative thereof, as initially described by Matteson (path a).2 The reaction of B2pin2 with gemdihalides,3 ketones,4 diazo compounds, or their synthetic precursors (paths b and c) has also been reported by several groups.5 Diboration of alkynes is the oldest established route and builds on early work by Zweifel and Brown.6 It was later improved and extended to the monoboration of vinyl boronates (paths d and e).7 Other reactions (not shown) involve the diborylation of lithiated carbamates8 and the © 2018 American Chemical Society

Received: July 17, 2018 Published: August 24, 2018 5304

DOI: 10.1021/acs.orglett.8b02235 Org. Lett. 2018, 20, 5304−5308

Letter

Organic Letters Scheme 2. Radical-Based Route to Geminal Bis(boronates)

Scheme 3. Examples of Geminal Bis(boronates)

p-orbital. One boronate group stabilizes an adjacent radical by at least 6−7 kcal/mol, according to theoretical calculations.17 We hoped nevertheless that this hydrogen would be inaccessible because of steric shielding by the two bulky Bpin groups. The other issue was related to the possibility of βfragmentation to generate alkene 7 and radical 8, a species also stabilized by the two Bpin groups. While homolysis by βscission of the carbon−carbon bonds is relatively rare, it can occur when the resulting radical is particularly stabilized. We have indeed exploited the rupture of a carbon−carbon bond and extrusion of a cumyl radical 12 to form ketones and aldehydes by the reaction of alkene 9 with xanthate 10 to give ketone 11, as shown in the lower part of Scheme 2.18 In the event, we were fortunate that none of our concerns materialized. The addition of the radicals to the terminal olefin proved faster than abstraction of the potentially labile hydrogen, and transfer of the xanthate group proved faster than the fragmentation. This lack of untoward competition from possible side reactions opened up a practical, highly convergent, and remarkably versatile access to a large variety of geminal bis(boronates), as conveyed by the examples displayed in Scheme 3. Nitriles (13 and 23), esters (14 and 16), malonates (15), imides (18), Weinreb amide (20), organofluorine motifs (21 and 22), tertiary alkyl group (25), heteroaromatics (29 and 37), and most importantly, ketones (27, 28, 30, 32, 33, and 35) are all accessible. Particularly significant in terms of functional group tolerance is cortisone-derived bis(boronate) 35. Of the nearly 400 geminal bis(boronates) of general structure 1 found in the REAXYS data bank, including boronates other than pinacolato, only one contained an unprotected ketone (a rather inert tert-butyl ketone) that was already present in the starting material.19 The few other examples of geminal bis(boronates) bearing an aldehyde or a ketone required starting from the protected form (mostly acetals or ketals) or introducing the carbonyl at a later stage, for example, by oxidation of an alcohol, which is itself initially protected. In our case, while it is usually advisable to mask an aldehyde close to the carbon bearing the xanthate group (e.g., 23), there is no need at all to protect a ketone. This opens a straigthforward, convergent entry into a vast array of geminal bis(boronates) containing ketones and many other functionalities.

One of the weaknesses of radical methods in general is the poor stereocontrol in open-chain structures. In cases where diastereoisomers are produced, little selectivity was observed, not unexpectedly. In these products, the xanthate was reductively removed using tris(trimethylsilyl)silane/AIBN in refluxing toluene and cyclohexane (1:1) to test the reduction procedure in the case of 19 and to simplify spectroscopic characterization for others (17, 24, 31, and 36).20 A more constructive use of the xanthate group is in the formation of a second carbon−carbon bond, for example, through cyclization onto an aromatic or heteroaromatic ring. This is illustrated by examples 34 and 39, whereby a tetrahydrobenzothiophene and an azaindane are produced, respectively, in moderate yield.21 In the case of azaindane 39, it is remarkable that the bis(boronate) moiety resisted the presence of trifluoroacetic acid (TFA, 1.2 equiv) needed to activate the pyridine ring toward the radical addition. To obtain tricyclic indole derivative 40, the intermolecular addition and cyclization were carried out in the same pot, without isolation of the intermediate adduct.22 The presence of a naked aldehyde in this example is noteworthy. There is another way to use the xanthate group, which results in a modular construction of even more heavily functionalized geminal bis(boronates). It consists of perform5305

DOI: 10.1021/acs.orglett.8b02235 Org. Lett. 2018, 20, 5304−5308

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Organic Letters ing the addition of a xanthate to an alkene prior to addition to bis(boronate) 2. In these sequential additions, it is necessary to respect certain rules, namely the need for the radical derived from the starting xanthate to be more stable than the radical arising from the addition step, neglecting in a first approximation the possibly important role of polar effects. Three such examples are displayed in Scheme 4. In the first,

Scheme 5. Geminal Bis(boronates) by Bidirectional Ketone Extension

Scheme 4. Modular Routes to Geminal Bis(boronates)

the distance between them are different. Finally, addition to vinyl tri-tert-butoxysilane affords the silicon−boron hybrid 64.25 The xanthate in the adducts is a stepping stone to numerous other functional groups. One particularly useful transformation is its exchange for a bromide by peroxide-mediated reaction with ethyl 2-bromoisobutyrate,26 as shown by the conversion of adduct 15 into bromide 65 (Scheme 6). This now allows Scheme 6. Synthesis of Cyclopropyl Geminal Bis(boronates) addition of cyanomethyl xanthate 42 to N-vinyl phthalimide 41 furnishes a new xanthate 43,23 which in turn adds efficiently to alkene 2 to give, after reductive dexanthylation, bis(boronate) 45 containing both a nitrile and a phthalimido group. In the same manner, chloropyridine bis(boronate) 50 and malonyl bis(boronate) 55 bearing, respectively, a maleimide and a protected vicinal diamino motif, were readily prepared. An alternative route to complex geminal bis(boronates) is illustrated by the transformations in Scheme 5. Thus, double addition of bis(xanthate) 56 furnishes symmetrical tetraboronate 57. Unsymmetrical derivatives can be obtained by starting with chloroacetonyl xanthate 58.24 A first addition to alkene 2 leads to adduct 59, and substitution of the chlorine with the xanthate salt provides key reagent 60 in high overall yield. Even though two xanthates are present in this molecule, only the xanthate vicinal to the ketone undergoes the radical addition to an alkene because it gives rise to a radical stabilized by the carbonyl group. The distal xanthate is the parent of an unstabilized secondary radical which is much more difficult to generate. This regioselective second addition is illustrated by the formation of masked amine 61 by reaction with Nallylphthalimide. The addition to allylBpin and vinylB(MIDA) furnishes, respectively, adducts 62 and 63 containing three boron atoms, where both the substituents on boron atoms and

the synthesis of cyclopropane 66 by mere exposure to potassium carbonate. Interestingly, treatment of this compound with the much stronger LTMP base and quenching with allyl bromide causes the formation of another cyclopropane 67 via anion 68. Cyclopropane 67 can be obtained directly from bromide 65 in 41% yield by the action of LTMP followed by allyl bromide. Geminal bis(boronyl) cyclopropanes are rather uncommon but highly useful structures, since the two boronate groups can be sequentially subjected to two different transition-metal-catalyzed coupling reactions.27 Finally, we were curious to see if addition to a more substituted geminal bis(boronate) alkene trap could be accomplished without the feared fragmentation discussed in Scheme 2 (4 → 7) taking place. We therefore prepared bis(boronate) 69 and subjected it to the action of xanthate 70 5306

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Organic Letters

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in the presence of lauroyl peroxide (Scheme 7). We noted that a stoichiometric amount of peroxide was required to complete

Letter

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Scheme 7. Synthesis of Tetrahydroquinolinyl Geminal Bis(boronates)

ORCID

Samir Z. Zard: 0000-0002-5456-910X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Ecole Polytechnique for a scholarship to Q.H. and Mrs. Sophie Bourcier and Mr. Vincent Jactel (both at Ecole Polytechnique) for HRMS measurements.

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DEDICATION This article is dedicated with respect and admiration to Professor Donald S. Matteson (Washington State University) and to the memory of Professor Gérard Cahiez (Institut de Recherche de Chimie Paris).

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the reaction but were pleasantly surprised to find that the product was geminal bis(boronyl) tetrahydroquinoline 72. The cyclization of intermediate radical 71 is clearly particularly favored, presumably by the Thorpe−Ingold effect exerted by the geminal bis(boronate) substituent. Indeed, it took place without the need for activation by TFA, in contrast to the case of azaindane 39 above (Scheme 3). Furthermore, neither the normal xanthate adduct 73 nor the unwanted fragmentation leading to stabilized radical 74 and ethyl 4-pentenoate 75 were observed. Two other analogues, 76 and 77, were prepared in the same manner. Geminal bis(boronyl) tetrahydroquinolines 72, 76, and 77 belong to a novel class of organoboron compounds. The majority of the geminal bis(boronates) described herein would be very difficult, if not impossible, to obtain by either ionic or organometallic methods or even by other radical reactions. The cheapness of the reagents, the simplicity of the experimental procedure, and easy scalability (adducts 15 and 18 were prepared on ≥2 g scale) should not obscure the sophistication of the unique mechanism by which this process operates. The xanthate group not only increases the lifetime of the intermediate radicals but also regulates their absolute and relative concentration in the medium. These features are unique to this process and essential for success; they explain the very broad range of substrates that can be used. The ability to associate so many different functional groups with the geminal bis(boronate) motif should allow a more thorough exploration of their chemistry and synthetic potential.

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ASSOCIATED CONTENT

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b02235. Experimental procedures, full spectroscopic data, and copies of 1H and 13C NMR for all new compounds (PDF) 5307

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