Editor's Page Cite This: Organometallics 2018, 37, 4311−4312
pubs.acs.org/Organometallics
Organometallic Complexes of Electrophilic Elements for Selective Synthesis
Downloaded via TULANE UNIV on December 14, 2018 at 08:43:58 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
D
alkene hydroamination, respectively (DOI: 10.1021/acs. organomet.8b00510). The breadth of contemporary hydroelementation chemistry is also emphasized by the observation that a potassium derivative of dimesitylphosphaneoxide is a more than capable catalyst for the intermolecular hydrophosphorylation of alkynes. If you consider the s-block metals to be among the more uninteresting elements of the Periodic Table, then we recommend that you read and be inspired by the latest work of Westerhausen’s group, which persuasively underlines the relatively untapped potential of group 1 species in catalysis (DOI: 10.1021/acs.organomet.8b00368). As shown by the Clot and Schafer collaboration, computational mechanistic insight provides critical guidance for the development of new hydroaminoalkylation catalysts (DOI: 10.1021/acs.organomet. 8b00674). The polarity of heteroelement bonding characteristic of these electropositive metals also allows them to mediate reactions that either eliminate or heterolytically activate the H−H bond of dihydrogen, often without any need for adjustment of the metal’s formal oxidation state. In the first regard, Waterman and Cibuzar report that the heterodehydrocoupling of silanes and amines may be achieved with catalyst loadings as low as 0.8% (DOI: 10.1021/acs.organomet. 8b00372). The lanthanum amide [La{N(SiMe3)2}3THF2] significantly expands the usefulness of 4f-elements in such catalysis. In the opposing sense, Beller and co-workers outline the utility of molybdenum derivatives of a PNP pincer ligand to effect the hydrogenation of carbonyl derivatives (DOI: 10. 1021/acs.organomet.8b00410), whereas in reactivity that is again largely driven by the polarity of the M−C bonding, Sadow and co-workers devise heterobimetallic neodymium/ aluminum species that facilitate the first organometalliccatalyzed route to alkynylaluminums (DOI: 10.1021/acs. organomet.8b00374). Low-valent electropositive metals also have a rich history in reductive coupling of C−C and C−E bonds, with low-valent early transition metals in particular presenting challenges and opportunities. Continuing in this tradition, Beweries and coworkers report on the Cp*2MII (M = Ti, Zr) mediated intermolecular heterocoupling of nitriles, yielding 1-metalla2,5-diazacyclopentadienes (DOI: 10.1021/acs.organomet. 8b00421). These types of nitrile couplings have received attention as a potential route to nitrogen-rich heterocycles, which play an important role in organic electronic materials. Continuing the theme of C−N bond reactivity with group 4, the Norton group has synthesized several new zirconium iminoacyl complexes via insertion of isocyanides into Zr−CH3 bonds in a twist on classical zirconium acyl chemistry (DOI:
iscussion of the chemistry of the more electropositive elements on the Periodic Table (alkali metals, alkaline earths, early transition metals, rare-earth metals, and the heavier members of the boron group) often focuses on a common refrain: Complexes of these metals are too reactive or completely unreactive, and their chemistry is well-established or well-understood. These perspectives may arise from the rich tradition of stoichiometric organometallic chemistry that has been pursued for decades. Consequently, these elements are often overlooked and neglected when considering the development of new, selective reactions. However, important contributions from the 1980s and 1990s that established impressive stoichiometric C−C and C− E bond-forming reactions (E = O, S, N, Si, B) continue to provide inspiration for an array of advances, including recent opportunities in catalysis. As such, these elements exhibit beautiful and unique reactivity in synthesis. This rich tradition has evolved to include a broad spectrum of transformations and applications. A common thread among these electropositive elements from across the Periodic Table includes the feature that reactivity is largely driven by the polarity of the bonding. This Organometallics special issue brings together some of the leaders in electropositive element chemistry and highlights state-of-the-art applications in selective synthesis across a variety of research topics. A motivating factor in exploring electrophilic metal reactivity has been its application to the catalytic hydrofunctionalization of C−C multiple bonds. The importance of alkene hydroamination and hydroaminoalkylation as a primary exemplar of these capabilities is underscored by the contribution of Hannedouche and Schulz (DOI: 10.1021/acs.organomet. 8b00431). Their succinct review turns the spotlight on recent progress in both the intra- and intermolecular variants of these atom-efficient transformations when catalyzed by elements drawn from groups 3, 4, and 5. They illustrate how chemists have overcome challenges with enantioselectivity and with evolving catalyst development beyond widely employed metallocene species. These latter themes are also ably demonstrated by the latest research led by both Odom (DOI: 10.1021/acs.organomet. 8b00313) and by Doye (DOI: 10.1021/acs.organomet. 8b00332), who extend their interests in titanium(IV)-catalyzed alkyne hydroamination/aminoalkylation to the development of heterogenized silica-supported media and homogeneous indenyl-amido species, respectively. While the former contribution points the way toward more practical recyclable systems, the latter emphasizes the intense attention that has been drawn to the use of noncyclopentadienyl ligand sets in the development of this chemistry. In a similar vein, Hultzsch and co-workers highlight the levels of sophistication that may now be applied to these transformations through their report of the use of chiral triorganosilyl-substituted binaphtholate complexes of yttrium and lutetium to induce enantiomeric excesses of up to 96 and 66% for intra- and intermolecular © 2018 American Chemical Society
Special Issue: Organometallic Complexes of Electropositive Elements for Selective Synthesis Published: December 10, 2018 4311
DOI: 10.1021/acs.organomet.8b00849 Organometallics 2018, 37, 4311−4312
Organometallics
Editor's Page
10.1021/acs.organomet.8b00677). The Beckhaus group provides two reports on the exciting application of tuck-in type η5:η1-pentafulvene titanium complexes, which can be directly converted to titanium imidos (DOI: 10.1021/acs.organomet. 8b00264) or hydrazidos (DOI: 10.1021/acs.organomet. 8b00343). Easy preparation of these moieties allows ready access to a large library of stoichiometric reactions, ranging from cycloadditions to N−N bond cleavage. Moving to the main group elements, Crimmin and coworkers report a series of main group metal-bridge ruthenium hydride complexes, demonstrating subtle effects of the main group metal on the degree of activation of a Ru−N2 moiety that are connected to the electronegativity, and thus ionic character, of the main group metal (DOI: 10.1021/acs. organomet.8b00340). Finally, the Berben team demonstrates the reversible protonation of an ONO pincer ligand on aluminum, reminiscent of classic pincer chemistry more commonly seen on late transition metals (DOI: 10.1021/acs. organomet.8b00628). The Editors of this Organometallics Special Issue want to thank all of the authors for their excellent contributions and for providing us with inspiring and surprising transformations that can be realized with organometallic complexes of these “wellestablished and -understood” electropositive elements. We also thank the reviewer community for its valuable commentary and suggestions that have enhanced the presentation of this work. We find it notable that this Special Issue features complexes from across the Periodic Table, with examples from the s, p, d, and f blocks. This assembly of papers highlights the diverse organometallic complexes and reactivity that are possible to leverage the commonality of polar bonding regimes. Reports that feature supported catalysts, benchtop-friendly catalysts, catalytic redox chemistry, photocatalysis, polymerization, hydrofunctionalization, solution-phase chemistry, structural work, and computational investigations are all included. If you are seeking inspiration for emerging opportunities in developing new, selective, organometallic transformations, be they stoichiometric or catalytic, then we are pleased to present this collection that illustrates the creative contributions that are being realized by building upon the ground-breaking work from decades past. We anticipate that these papers will serve to remind all organometallic chemists of the overlooked potential of electropositive elements in controlled and selective organic and organometallic synthesis.
10.1021/acs.organomet.8b00690). Meanwhile, Tonks and coworkers have shown that Cp2TiII can both make and break C− N bonds when reacting with 2H-azirines (DOI: 10.1021/acs. organomet.8b00522). Translating low-valent electropositive chemistry to in situ protocols is important to bridge the gap between the synthetic inorganic and organic communities: Not everyone has a glovebox! Taking up this challenge, the Okamoto group reports on new in situ-generated low-valent titanium catalysts for [2 + 2+2] alkyne cyclotrimerization, along with a surprising proposal for the role of TMSCl (DOI: 10.1021/acs.organomet. 8b00678), while the Tonks, Tsurugi, and Mashima teams collaborate on designing benchtop-compatible in situ titanium catalysts for [2 + 2+1] pyrrole synthesis (DOI: 10.1021/acs. organomet.8b00474). Electropositive metals can mediate a range of transformations with impressive chemoselective, regioselective, or enantioselective outcomes. For example, Harman and coworkers reveal that a resolved, chiral molybdenum α-pinene derivative shows remarkable configurational stability to efficiently facilitate the synthesis of enantioenriched fluorinated building blocks by catalytic dearomatization (DOI: 10.1021/ acs.organomet.8b00027). The Hill group further demonstrates how identical magnesium Bpin derivatives can display divergent reactivity to access either C−B or C−C bondforming reactions in nucleophilic borylation reactions with different carbonyl electrophiles (DOI: 10.1021/acs.organomet. 8b00408). Continuing with the theme of remarkably selective reactivity, Wright and co-workers show that phosphorus heterocycles can be efficiently assembled in one pot from primary phosphines using a lithium−alkyl−antimony superbase (DOI: 10.1021/acs.organomet.8b00480). Meanwhile, the use of n-butyllithium by the Harder group leads to divergent reactivity with saturated versus unsaturated N-heterocyclic olefin (NHO) derivatives, such that interesting new ligands can be prepared by the controlled deprotonation of the heterocyclic backbone of the uniquely saturated NHOs (DOI: 10.1021/acs.organomet.8b00572). Roesky and co-workers extend chiral selectivity to the ring-opening polymerization of rac-lactide by using enantiopure borohydride complexes of rare-earth metals. Their work shows that scandium promotes enhanced chiral induction for this important transformation (DOI: 10.1021/acs.organomet.8b00172). Another emerging opportunity in early transition metal catalysis is elaborated by the Milsmann team, which highlights the importance of ligand design in zirconium photocatalysis for C−C bond formation (DOI: 10.1021/acs.organomet.8b00388). These contributions illustrate advances in realizing controlled and selective polymeric and small-molecule synthesis using electropositive metals. Meanwhile, the selective synthesis of new and interesting organometallic structures remains a driver of innovation. Accessing metal−element multiple bonds with lanthanides is a long-standing challenge in targeting reactive species for new chemistry. In this case, Schelter and co-workers highlight the use of silyl group transfer strategies to develop cerium imido bonding motifs (DOI: 10.1021/acs.organomet.8b00366). In bioinspired chemistry, molybdenum complexes from the Okuda group have been used in oxygen-atom transfer reactivity (DOI: 10.1021/acs.organomet.8b00386). The Veige group shows how a molybdenum-alkylidyne is involved in side reactivity that interferes with polymerization chemistry (DOI:
■
Laurel Schafer* Michael Hill Ian Tonks
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Laurel Schafer: 0000-0003-0354-2377 Michael Hill: 0000-0001-9784-9649 Ian Tonks: 0000-0001-8451-8875 Notes
Views expressed in this editorial are those of the authors and not necessarily the views of the ACS.
4312
DOI: 10.1021/acs.organomet.8b00849 Organometallics 2018, 37, 4311−4312
