Umpolung of Methylenephosphonium Ions in Their Manganese Half‐Sandwich Complexes and Application to the Synthesis of Chiral Phosphorus‐Containing Ligand Scaffolds
TL;DR: The synthetic potential of this umpolung approach is illustrated herein by the preparation of novel chiral pincer-type phosphine-NHC-phosphine ligand architectures.
Abstract: Half-sandwich manganese methylenephosphonium complexes [Cp(CO)2Mn(η(2)-R2P=C(H)Ph)]BF4 were obtained in high yield through a straightforward reaction sequence involving a classical Fischer-type manganese complex and a secondary phosphine as key starting materials. The addition of various nucleophiles (Nu) to these species took place regioselectively at the double-bonded carbon center of the coordinated methylenephosphonium ligand R2P(+)=C(H)Ph to produce the corresponding chiral phosphine complexes [Cp(CO)2Mn(κ(1)-R2P-C(H)(Ph)Nu)], from which the phosphines were ultimately recovered as free entities upon simple irradiation with visible light. The synthetic potential of this umpolung approach is illustrated herein by the preparation of novel chiral pincer-type phosphine-NHC-phosphine ligand architectures.
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TL;DR: In this paper, the authors compare pincer ligands whose central donor group exhibits a sp3-hybridized atom with those based on boron and carbon, and demonstrate that in contrast to amine donor groups, ligand-stabilized Borylenes are not acting as a proton source and rather undergo a reversible redox-reaction with the coordinated metal centre.
55 citations
TL;DR: This is demonstrated and described by the recently developed FLP-formylborane chemistry, which has led to the formation of the unique (η2 -formyl Borane)FLP adducts and opened a way of preparing a genuine formylboranes compound, which shows an interesting follow-up chemistry.
Abstract: Frustrated Lewis pair chemistry has taken a steep development in the recent years. It offers possibilities of developing new variants of known reactions and of finding new chemical transformations. This is demonstrated and described by the recently developed FLP-formylborane chemistry, which has led to the formation of the unique (η2 -formylborane)FLP adducts and opened a way of preparing a genuine formylborane compound, which shows an interesting follow-up chemistry. FLPs have helped finding phosphorus analogues of the enamine Stork reaction and the Claisen reaction. These reactions lead to new organophosphorus compounds and they make new phosphane/borane systems available. P/B FLPs add to a variety of small main group element oxides. They undergo 1,2-addition reactions to CO2 , SO2 and other heterocumulenes and they feature unique 1,1-addition reactions to carbon monoxide, to isonitriles and even to nitric oxide (NO), the latter yielding examples of a new class of persistent nitroxide radicals, the FLPNO nitroxyls. Eventually, some remarkable radical reactions of FLPs and related compounds are briefly mentioned.
51 citations
TL;DR: The attachment of electron-rich PCP pincer ligands bis(2-(dialkylphosphino)phenyl)methane to rhodium via reactions with [(COE)2Rh(μ-X)]2 (X = Cl, OSO2CF3) through C–H bond activations is reported.
34 citations
TL;DR: The IR data for the PCcarbeneP complexes shows the 2,3-benzo[b]thiophene linked system to be the poorest overall donor, while the phenyl bridged ligands incorporating electron donating dialkyl amino groups para to the anchoring carbene are very strongly donating pincer arrays.
Abstract: The donor properties of five different PCcarbeneP ligands are assessed by evaluation of the CO stretching frequencies in iridium(I) and rhodium(I) carbonyl cations. The ligands feature dialkyl phosphine units (R = iPr or tBu) linked to the central benzylic carbon by either an ortho-phenylene bridge, or a 2,3-benzo[b]thiophene linker; in the former, substituent patterns on the phenyl linker are varied. The carbonyl complexes are synthesized from the (PCcarbeneP)M–Cl starting materials via abstraction of the chlorides in the presence of CO gas. In addition to the expected mono carbonyl cations, products with two carbonyl ligands are produced, and for the rhodium example, a novel product in which the second carbonyl ligand adds reversibly across the RhC bond to give an η2 ketene moiety was characterized. The IR data for the complexes shows the 2,3-benzo[b]thiophene linked system to be the poorest overall donor, while the phenyl bridged ligands incorporating electron donating dialkyl amino groups para to the anchoring carbene are very strongly donating pincer arrays.
31 citations
TL;DR: A series of P-NHC-type hybrid ligands containing both PR2 and N-heterocyclic carbene (NHC) donors on meta-bis-substituted phenylene backbones was accessed through a modular synthesis from a common precursor, and their coordination chemistry with coinage metals was explored and compared.
Abstract: A series of P-NHC-type hybrid ligands containing both PR2 and N-heterocyclic carbene (NHC) donors on meta-bis-substituted phenylene backbones, L(Cy), L(tBu) and L(Ph) (R = Cy, tBu, Ph, respectively), was accessed through a modular synthesis from a common precursor, and their coordination chemistry with coinage metals was explored and compared. Metallation of L(Ph)·n(HBr) (n = 1, 2) with Ag2O gave the pseudo-cubane [Ag4Br4(L(Ph))2], isostructural to [Ag4Br4(L(R))2] (R = Cy, tBu) (T. Simler, P. Braunstein and A. A. Danopoulos, Angew. Chem., Int. Ed., 2015, 54, 13691), whereas metallation of ·HBF4 (R = Ph, tBu) led to the dinuclear complexes [Ag2(L(R))2](BF4)2 which, in the solid state, feature heteroleptic Ag centres and a 'head-to-tail' (HT) arrangement of the bridging ligands. In solution, interconversion with the homoleptic 'head-to-head' (HH) isomers is facilitated by ligand fluxionality. 'Head-to-tail' [Cu2Br2(L(R))2] (R = Cy, tBu) dinuclear complexes were obtained from L(R)·HBr and [Cu5(Mes)5], Mes = 2,4,6-trimethylphenyl, which also feature bridging ligands and heteroleptic Cu centres. Although the various ligands L(R)l ed to structurally analogous complexes for R = Cy, tBu and Ph, the rates of dynamic processes occurring in solution are dependent on R, with faster rates for R = Ph. Transmetallation of both NHC and P donor groups from [Ag4Br4(L(tBu))2] to AuI by reaction with [AuCl(THT)] (THT = tetrahydrothiophene) led to L(tBu) transfer and to the dinuclear complex [Au2Cl2L(tBu)] with one L(tBu) ligand bridging the two Au centres. Except for the silver pseudo-cubanes, all other complexes do not exhibit metallophilic interactions.
25 citations
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TL;DR: The first applications of asymmetric organocatalytic cascade reactions to the total synthesis of natural products are presented, paving the way for a new and powerful strategy that can help to address these issues.
Abstract: The total synthesis of natural products and biologically active compounds, such as pharmaceuticals and agrochemicals, has reached an extraordinary level of sophistication. We are, however, still far away from the 'ideal synthesis' and the state of the art is still frequently hampered by lengthy protecting-group strategies and costly purification procedures derived from the step-by-step protocols. In recent years several new criteria have been brought forward to solve these problems and to improve total synthesis: atom, step and redox economy or protecting-group-free synthesis. Over the past decade the research area of organocatalysis has rapidly grown to become a third pillar of asymmetric catalysis standing next to metal and biocatalysis, thus paving the way for a new and powerful strategy that can help to address these issues - organocatalytic cascade reactions. In this Review we present the first applications of such asymmetric organocascade reactions to the total synthesis of natural products.
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TL;DR: Modern variants of the Mannich reaction that expand the potential of the classical intermolecular reaction significantly and enable efficient control of the regioselectivities and stereoselectivity are therefore the topic of intensive research.
Abstract: Important building blocks for the synthesis of drugs or natural products are found in Mannich bases and their derivatives Modern variants of the Mannich reaction that expand the potential of the classical intermolecular reaction significantly and enable efficient control of the regioselectivity and stereoselectivity are therefore the topic of intensive research Intramolecular reactions, in particular as part of domino reaction sequences, often afford astoundingly simple and elegant approaches to complex target compounds
967 citations
TL;DR: The key unifying feature of almost all molecules discussed in this review is that they are generally stabilized by the use of bulky substituents which block associative or various decomposition pathways.
Abstract: This review is essentially an update of one entitled “πBonding and The Lone Pair Effect in Multiple Bonds Between Heavier Main Group Elements” which was published more than 10 years ago in this journal.1 The coverage of that survey was focused on the synthesis, structure, and bonding of stable compounds2 of heavier main group elements that correspond to the skeletal drawings reproduced in Tables 1 and 2. A row of numbers is listed at the bottom of each column in these tables. This refers to the number of stable complexes from each class that are currently known. The numbers in parentheses refer to the number of stable species that were known at the time of the previous review. Clearly, many of the compound classes listed have undergone considerable expansion although some remain stubbornly rare. The most significant developments for each class will be discussed in detail under the respective sections. As will be seen, there are also a limited number of multiple bonded heavier main group species that do not fit neatly in the classifications in Tables 1 and 2. However, to keep the review to a manageable length, the limits and exclusions, which parallel those used earlier, are summarized as follows: (i) discussion is mainly confined to compounds where experimental data on stable, isolated species have been obtained, (ii) stable compounds having multiple bonding between heavier main group elements and transition metals are not generally discussed, (iii) compounds in which a multiple bonded heavier main group element is incorporated within a ring are generally not covered, and (iv) hypervalent main group compounds that may incorporate faux multiple bonding are generally excluded. Such compounds are distinguished from those in Tables 1 and 2 in that they apparently require the use of more than four valence bonding orbitals at one or more of the bonded atoms. The remainder of this review is organized in a similar manner to that of the previous one wherein the compounds to be discussed are classified according to those summarized in Tables 1 and 2. The key unifying feature of almost all molecules discussed in this review is that they are generally stabilized by the use of bulky substituents which block associative or various decomposition pathways.3 Since the previous review was published in 1999, several review articles that cover parts of the subject matter have appeared.4
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