The Transition Metal Coordination Chemistry of Hemilabile Ligands

ed from in the transition state. This answers the question of both the retention and the inversion of stereochemistry for the simple exchange reaction giving back I (which it should be noted is highly disfavored because k2 ≫ k−1). Retention of configuration would occur by first noting that the potassium ion would also be coordinated to the same face as the abstracted hydrogen atom because it was originally coordinated to the oxygen of the base. A molecule of deuterated alcohol can then approach this same face through coordination to the potassium. Rotation of these ligands around potassium would result in the deuterated alcohol now hydrogen bonded to the allylic anion (IV-top), which following collapse would return the chiral alkene I-d with the same configuration. On the other hand, inversion of stereochemistry would occur via the deuterated solvent approaching from the face of the allylic anion opposite to that hydrogen bonded, and exchange with the alcohol giving (IV-bottom). This could collapse and give chiral alkene I-d with inversion of configuration. The E/Z ratio of 60 for product III-h is also important because this value is extremely high as compared to the equilibrium value of 4.2 for the diastereoisomers, and compares to other systems as well. For example, the isomerization of allylbenzene into 1-propenylbenzene has an E/Z ratio of 44 when using potassium t-butoxide in DMSO. The reason for this higher-than-equilibrium ratio is explained in Scheme 3. Two “conformations” of the chiral alkene I are suitable for deprotonation and will lead to either the E or the Z allylic anions as shown. The rate of “conformational rotation” must be many orders of magnitude greater than the rates for Scheme 2. Proposed Base-Mediated Reaction Mechanism Scheme 3. Interactions Leading to Observed E/Z Ratios Chemical Reviews Review DOI: 10.1021/acs.chemrev.5b00052 Chem. Rev. XXXX, XXX, XXX−XXX E deprotonation (kE and kZ), and thus the E/Z ratio in the product must be related to the kE/kZ ratio. The transition state leading to the Z-allylic anion must thus be higher in energy than the one leading to the E-allylic anion, presumably due to the disfavored 1,3-allylic interactions, which would make charge stabilization through conjugation more difficult. Cram has reported this in more detail, including an analysis of collapse ratios (i.e., ratio of protonation of either carbon on the allylic anion), the stereochemical stability of allylic and vinyl anions, and the kinetic and thermodynamic stabilities of olefinic products formed by protonation of allylic anions. 2.2. Transition Metal-Mediated Mechanisms The mechanisms for the transition metal-mediated isomerizations have in some cases been very carefully determined, while in many they are assumed from one of two generally accepted pathways (Schemes 4 and 5). The first involves a discrete transition metal−hydride active catalyst and the formation of a transition metal−alkyl intermediate, while the second involves the formation and collapse of a η-allyl hydride complex. Each of the mechanisms described below bases its premise on the fact that each step is reversible, and thus the reactions are under thermodynamic control at equilibrium. 2.2.1. Alkyl Mechanism. For this mechanism to occur, the transition metal catalyst must have both an empty 2e− coordination site (e.g., via dissociation of a ligand) and a metal hydride bond that is typically generated in situ under the reaction conditions (see Scheme 4). The catalyst first coordinates to the π-electrons of the alkene followed by an insertion reaction to give either a primary or a secondary metal−alkyl intermediate. The primary metal−alkyl is generally formed faster for many catalysts, but is a mechanistic dead-end returning the starting material through β-hydride elimination and is thus a nonproductive pathway. The secondary metal− alkyl intermediate can however produce either the Eor the Z1-propenylbenzene on β-hydride elimination, which is clearly thermodynamically favored because of the conjugation to the benzene ring. 2.2.2. Allyl Mechanism. The allyl mechanism, however, requires a transition metal capable of having two vacant coordination sites, and, more specifically, no metal−hydride should be present or the alkyl mechanism will take place (see Scheme 5). The first step involves coordination of the πelectrons of the allylbenzene to one of the transition metal’s vacant sites. This is followed by an oxidative addition reaction giving an η-allyl metal−hydride complex, which can collapse to either the starting material or the rearranged and thus more stable alkene. The η-complex may also rearrange to the ηcomplex as part of the reaction pathway. The difference between these two mechanisms can be determined through deuterium labeling with 32 and crossover experiments (Scheme 6). The allyl mechanism is entirely intramolecular and involves an effective 1,3-hydride shift as the only mechanistic pathway. Thus, in a crossover experiment, such as that in Scheme 6, the deuterium should (a) only be found at the 1and 3-positions of the allylic system, that is, 34, and (b) not be incorporated into the second nondeuterated substrate. In the case of the alkyl mechanism, products similar to those of the allyl mechanism may be detected in addition to (a) the nondeuterated substrate showing some deuterium incorporation and (b) the loss of deuterium and incorporation of hydrogen on the deuterated substrate. Furthermore, deuterium incorporation at the 2-position of the allylic system, 35, is also expected because the initial metal−hydride insertion reaction may have poor regioselectivity as already explained in Scheme 2. Of course it should be noted that these two general mechanisms are just that, and the specific reaction pathway for different transition metal catalysts will depend heavily on the transition metal, ligand, solvent, and substrate combinations. More detailed mechanistic data can be obtained from kinetic studies supplemented by state-of-the-art measurements, for example, nanosecond time-resolved IR, NMR, and DFT calculations. These studies have invariably revealed Scheme 4. Alkyl Mechanism M = transition metal; [L]n = bound ligand(s); [L]0 = dissociating ligand or vacant 2e− site. Scheme 5. Allyl Mechanism M = transition metal; [L]n = bound ligand(s); [L]0 = dissociating ligand or vacant 2e− site. Chemical Reviews Review DOI: 10.1021/acs.chemrev.5b00052 Chem. Rev. XXXX, XXX, XXX−XXX F mechanisms more complicated than those presented above, but at the same time generally holding true to them. One major mechanistic departure has been that proposed by Harvey and Lloyd-Jones in which they suggest a binuclear palladium complex being involved in the E/Z isomerization of alkenes. However, it should be noted that this is only applicable in some cases. More important is that the specific mechanisms themselves answer questions related to E/Z selectivity in these transition metal-mediated isomerizations. Simplistically, the reactions can be considered to be under thermodynamic control, and therefore the E/Z ratios will favor the E-isomer. The π-allyl mechanism has been linked, in a general sense, with higher E/Z ratios, but can by no means be used as proof for a particular mechanism. This is because the E/Z selectivity in the reaction is strongly governed not only by the thermodynamic stability of the E isomer (and intermediates leading to the E-isomer), but can be shifted through ligand and or kinetic control to high ratios of either the E or the Z isomers. The sections below detailing the specific examples in the literature will highlight these selectivities. 3. ISOMERIZATION METHODS − GENERAL Perusal of the literature quickly confirms that 2-propenylaryl isomerization reactions have been promoted by mainly two classes of methods, viz., the application of bases (section 4) or transition metal complexes (section 5). In this Review, these sections will thus be discussed separately, in addition to a final section describing miscellaneous allylaryl isomerizations (section 6). In each section, substrates of particular interest will be highlighted, with particular care being taken to convey important experimental data such as yields, the cis/trans ratios obtained, and any other relevant information. 4. BASE-MEDIATED ISOMERIZATIONS In general, base-initiated reactions require at least a stoichiometric amount of base to accomplish the isomerization of an allylbenzene. This method has seen much application in the past literature, and some general information has been collated in book chapters. 4.1. Hydroxide/Alkoxide Ion-Mediated Isomerizations This particular method involves the use of fairly harsh reaction conditions in that it generally comprises heating the substrate in a protic solution (ethanol, methanol, or n-butanol) of sodium or potassium hydroxide (giving rise to a mixture of the hydroxide and corresponding alkoxide under equilibrium conditions). Other examples include the use of hydroxide in DMSO. Of interest is that the KOH/ethylene glycolmediated isomerization of eugenol 5 has actually been incorporated into the curriculum of a teaching laboratory to demonstrate how the kinetics of a reaction can be studied using NMR spectroscopic, GC, and HPLC laboratory techniques. It should also be mentioned here that the importance of the potassium t-butoxide system means that a separate subsection will be dedicated to this method (see section 4.2). In the first section, examples of where hydroxide-mediated isomerization has been applied will be highlighted. This will be followed by methods in which the isomerization approach has been modified, albeit with additives (phase transfer), microwave heating, or different solvent systems. 4.1.1. Hydroxide/Alkoxide Ions in Alcohol (Methanol, Ethanol, n-Butanol) or without Solvent. It should be noted that the use of sodium ethoxide has been rigorously studied, showing that the isomerization of para-substi

Isomerization of allylbenzenes.

Amides of trivalent phosphorus acids as phosphorylating reagents for proton-donating nucleophiles.

Im Vergleich zur Chemie anderer wohlbekannter Elektronenmangelverbindungen wie der Carbene, Silylene und Nitrene ist die Chemie der Phosphinidene RP: auserst wenig erforscht. Ein kritisches Studium der Literatur zeigt, das als wahrscheinliche Ursachen sowohl eine schlechte Zuganglichkeit dieser Spezies als auch ein ihnen eigener Mangel an Reaktivitat anzunehmen sind. Die Ergebnisse von Rechnungen deuten darauf hin, das es moglich ist, durch Komplexierung mit Ubergangsmetall-Komplexfragmenten wie M(CO)5 (M Cr, Mo, W) die Elektrophilie der Phosphinidene zu erhohen und ihren Singulett-Zustand zu stabilisieren. Die entsprechenden Komplexe mit terminalen Phosphiniden-Liganden [RPM(CO)5] zeigen eine reichhaltige und vielseitige Chemie, die haufig der der Singulett-Carbene entspricht. Herstellen lassen sich kurzlebige Phosphiniden-Komplexe durch thermische Zersetzung von 7-Phosphanorbornadien-Komplexen; ihre Stabilisierung jedoch ist eine noch nicht bewaltigte Aufgabe.

Die Entwicklung einer carbenartigen Chemie von Phosphiniden‐Übergangsmetallkomplexen

This paper describes a structure–property study using two dithieno[3,2-b;2′,3′-d]phosphole building blocks for the generation of white light emission and the incorporation of these units in a single polystyrene material. The emission of one of the light-emitting organophosphorus building blocks can efficiently be switched from orange to green by simple protonation of the amino functional groups that are part of the π-conjugated scaffold. The resulting three components (blue, green, and orange) exhibit photophysical properties that allow for an efficient fluorescence resonance energy transfer (FRET) in the mixture/polymer and provide intense white fluorescence upon excitation of the blue component; the fluorescence is close to pure white in solution and similar to the emission of an incandescent light bulb in the thin film. The results nicely illustrate the intriguing features that can be obtained by exclusively using organophosphorus-based organic electronic materials.

Simple and Efficient Generation of White Light Emission From Organophosphorus Building Blocks

Aminophosphanes RP(NEt2)2 and R2P(NEt2), obtained from alkenyl Grignard compounds RMgX (R =—CH=CH2, —CH2CH=CH2, o(CH2=CH)C6H4—, o(CH2=CHCH2)C6,H4—) and aminochlorophosphanes, reacted smoothly with hydrogen halide to give alkenylhalogenophosphanes RPX2 and R2PX (X = Cl, Br). Properties and reactions of some of these compounds, e.g. with &8 H2O2, H2O and LiAIH4 re reported. On reaction of RPX2 with magnesium only alkenylcyclophosphanes (RP) n were obtained while heterocycles as possible products of an intramolecular reaction of intermediately formed phosphinidenes were not observed. Die aus Alkenylgrignardverbindungen RMgX (R = —CH=CH2, —CH2CH=CH2, o-(CH2=CH)C6H4—, und Aminochlorphosphanen erhaltenen Aminophosphane RP(NEt2)2 und R2 PNEt2 ergebent mit Halogenwassestoff glatt die Halogenphoshane RPX2 und R2 PX (X = Cl, Br). Eigenschaften und Reaktionen einiger dieser Phosphane mit S8, H2O2, H2O und LiAlH4 werden beschrieben. Bei der Umsetzung von RPX2 mit Magnesium bildeten sich ledighch die entsprec...

Zur Kenntnis Der Organophosphorverbindungen Xxiii1 üBer Alkenylsubstituierte Amino-Bzw. Halogenphosphane Und Versuche Zum Intramolekularen Phosphinidenabfang

Beatrix Kottwitz

Papers

Zur Kenntnis Der Organophosphorverbindungen Xxiii1 üBer Alkenylsubstituierte Amino-Bzw. Halogenphosphane Und Versuche Zum Intramolekularen Phosphinidenabfang

Alkenylphosphane RPX2 und R2PX (X = NEt2, CI): IHRE funktion als einzähnige liganden und potentiell hemilabile chelatbildner in metall(0)-komplexen