B. Other Catalyti; Process by Transition-Metal Complexes IV. Cross-Coupling Reaction A. Coupling of 1 -Alkenylboron Derivatives: Synthesis of Conjugated Dienes 6. Coupling of Arylboron Derivatives: Synthesis of Biaryls C. Coupling of Alkylboron Derivatives D. Coupling with Triflates E. Synthesis of Vinylic Sulfides F. Coupling with lodoalkanes: Alkyl-Alkyl CouDlino G. Coupling with Other Organic Halides and Boron Reagents V. Head-to-Tail Coupling VI. Carbonylative Coupling VII. Alkoxycarbonylation and Dimerization VIII. Conclusion 2457 2458 2458

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Palladium-catalyzed cross-coupling reactions of organoboron compounds

Steric effects of phosphorus ligands in organometallic chemistry and homogeneous catalysis

A number of studies were conducted to demonstrate C-H activation for the construction of C-B bonds. Investigations revealed that the conversion of C-H bonds to C-B bonds was both thermodynamically and kinetically favorable. The reaction at a primary C-H bond of methane or a higher alkene B 2(OR)4 formed an alkylboronate ester R' -B(OR)2 and the accompanying borane H-B(OR2. The ester and the borane were formed on the basis of calculated bond energies for methylboronates and dioaborolanes. The rates of key steps along the reaction pathway for the conversion of a C-H bond in an alkane or arene to the C-B bond in an alkyl or arylboronate ester were favorable. These studies also highlighted the accessible barriers for C-H bond cleavage and B-C bond formation during the borylation of alkanes and arenes.

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C−H Activation for the Construction of C−B Bonds

Since the discovery of organic azides by Peter Griess more than 140 years ago, numerous syntheses of these energy-rich molecules have been developed. In more recent times in particular, completely new perspectives have been developed for their use in peptide chemistry, combinatorial chemistry, and heterocyclic synthesis. Organic azides have assumed an important position at the interface between chemistry, biology, medicine, and materials science. In this Review, the fundamental characteristics of azide chemistry and current developments are presented. The focus will be placed on cycloadditions (Huisgen reaction), aza ylide chemistry, and the synthesis of heterocycles. Further reactions such as the aza-Wittig reaction, the Sundberg rearrangement, the Staudinger ligation, the Boyer and Boyer-Aube rearrangements, the Curtius rearrangement, the Schmidt rearrangement, and the Hemetsberger rearrangement bear witness to the versatility of modern azide chemistry.

Organic Azides: An Exploding Diversity of a Unique Class of Compounds

Hydrogenation is a core technology in chemical synthesis. High rates and selectivities are attainable only by the coordination of structurally well-designed catalysts and suitable reaction conditions. The newly devised [RuCl(2)(phosphane)(2)(1,2-diamine)] complexes are excellent precatalysts for homogeneous hydrogenation of simple ketones which lack any functionality capable of interacting with the metal center. This catalyst system allows for the preferential reduction of a C=O function over a coexisting C=C linkage in a 2-propanol solution containing an alkaline base. The hydrogenation tolerates many substituents including F, Cl, Br, I, CF(3), OCH(3), OCH(2)C(6)H(5), COOCH(CH(3))(2), NO(2), NH(2), and NRCOR as well as various electron-rich and -deficient heterocycles. Furthermore, stereoselectivity is easily controlled by the electronic and steric properties (bulkiness and chirality) of the ligands as well as the reaction conditions. Diastereoselectivities observed in the catalytic hydrogenation of cyclic and acyclic ketones with the standard triphenylphosphane/ethylenediamine combination compare well with the best conventional hydride reductions. The use of appropriate chiral diphosphanes, particularly BINAP compounds, and chiral diamines results in rapid and productive asymmetric hydrogenation of a range of aromatic and heteroaromatic ketones and gives a consistently high enantioselectivity. Certain amino and alkoxy ketones can be used as substrates. Cyclic and acyclic alpha,beta-unsaturated ketones can be converted into chiral allyl alcohols of high enantiomeric purity. Hydrogenation of configurationally labile ketones allows for the dynamic kinetic discrimination of diastereomers, epimers, and enantiomers. This new method shows promise in the practical synthesis of a wide variety of chiral alcohols from achiral and chiral ketone substrates. Its versatility is manifested by the asymmetric synthesis of some biologically significant chiral compounds. The high rate and carbonyl selectivity are based on nonclassical metal-ligand bifunctional catalysis involving an 18-electron amino ruthenium hydride complex and a 16-electron amido ruthenium species.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/1521-3773%2820010105%2940%3A1%3C40%3A%3AAID-ANIE40%3E3.0.CO%3B2-5

Asymmetric Catalysis by Architectural and Functional Molecular Engineering: Practical Chemo‐ and Stereoselective Hydrogenation of Ketones

Electrophilic Substituent Constants

Organic syntheses via boranes

Publisher Summary This chapter discusses boranes in organic chemistry, diborane as a reducing agent, volatile compounds of uranium, the alkali metal hydride approach to diborane, and alkali metal borohydrides. The search for solvents led to the discovery of sodium borohydride's being an excellent reducing agent for aldehydes and ketones. The search for catalysts to enhance the reducing power of sodium borohydride led to an anomalous result in the reduction of ethyl oleate. Investigation of this anomalous result led to the discovery of hydroboration. Hydroboration made the organoboranes readily available for the first time. This led to a systematic exploration of the chemistry of the organoboranes and established the fact that they constitute the most versatile organometallic available currently. The chapter describes molecular addition compounds and steric strains, steric assistance in solvolysis, the nonclassical ion problem, selective reductions, hydroboration, and versatile organoboranes. On the theoretical side, study of the dissociation of addition compounds of amines with trimethylborane, boron trifluoride, and borane provides a new quantitative approach to steric strains. These studies quickly removed doubts as to the importance of steric effects in chemical behavior and led to the study of related carbon compounds and the establishment of steric effects as an important factor in their behavior. One phase of these studies involved steric assistance to ionization in highly branched tertiary halides and related derivatives.

Boranes in Organic Chemistry

A new technique for the conversion of olefins into organoboranes and related alcohols

1. That Fascinating Nonclassical Ion Problem.- 1.1. Introduction.- 1.2. Origins.- 1.3. The Nonclassical Ion Era.- 1.4. Steric Assistance.- 1.5. An Alternative Interpretation.- 1.6. The Rococo Period of Carbonium Ion Structures.- 1.7. Difficulties in Challenging an Accepted Theory.- 1.8. Further Difficulties-A "Soft" Theory.- 1.9. Still Further Difficulties-Selective Reviews.- 1.10. Conclusion.- Comments.- 2. Steric Assistance in Solvolytic Processes.- 2.1. Introduction.- 2.2. Steric Assistance in the Solvolysis of Highly Branched Alkyl Derivatives.- 2.3. Steric Assistance in the Relative Effects of Methyl and tert-Butyl Groups.- 2.4. Steric Effects in Norbornyl Derivatives.- 2.5. Steric Effects in Ring Systems.- 2.6. Conclusion.- Comments.- 3. ?-Participation-A Factor in Fast Rates?.- 3.1. Introduction.- 3.2. n-, ?- and ?-Participation.- 3.3. ?-Participation and Fast Rates.- 3.4. ?-Participation vs. Steric Assistance.- 3.5. Steric Assistance-Not ?-Participation.- 3.6. The Fast Rates of Exo-Norbornyl and Cyclopropylcarbinyl.- Comments.- 4. Carbon-Bridged Cations.- 4.1. Introduction.- 4.2. What Is a Nonclassical Ion?.- 4.3. ?-Bridging vs. Hyperconjugation.- 4.4. Static Classical, Equilibrating Classical, or ?-Bridged Cations.- 4.5. Stereochemistry and ?-Bridged Cations.- 4.6. ?-and Aryl-Bridged Cations.- 4.7. Conclusion.- Comments.- 5. The Cyclopropylcarbinyl Cation.- 5.1. Introduction.- 5.2. Exceptionally Fast Rates of Solvolysis.- 5.3. Rearrangements in the Solvolysis of Cyclopropylcarbinyl Derivatives.- 5.4. ?-Bridged Structures for the Cation.- 5.5. Stereochemical Characteristics.- 5.5. Conclusion.- Comments.- 6.The 2-Norbornyl Cation.- 6.1. Introduction.- 6.2. Nonclassical "Structures" for the 2-Norbornyl Cation.- 6.3. Basic Facts.- 6.4. Transition State or Intermediate.- 6.5. Equilibrating Cations.- 6.6. Are Exo Rates Unusually Fast?.- 6.7. Do the High Exo:Endo Rate Ratios Require ?-Bridged Cations?.- 6.8. Do the High Exo:Endo Product Ratios Require ?-Bridged Cations?.- 6.9. Theoretical and Empirical Solutions.- 6.10. Conclusion.- Comments.- 7. Stabilized 2-Norbornyl Cations.- 7.1. Introduction.- 7.2. The Gassman-Fentiman Approach.- 7.3. Exo:Endo Rate Ratios in 2-p-Anisyl-2-norbornyl.- 7.4. Exo:Endo Rate Ratios in 2-p-Anisyl-2-camphenilyl.- 7.5. Exo:Endo Product Ratios.- 7.6. Goering-Schewene Diagrams.- 7.7. Other 2-p-Anisyl Systems.- 7.8. Other Stabilized Systems.- 7.9. The Selectivity Principle and the 2-Norbornyl Problem.- 7.10. Conclusion.- Comments.- 8. Exo:Endo Rate Ratios as a Steric Phenomenon.- 8.1. Introduction.- 8.2. Steric Characteristics of the Norbornyl System.- 8.3. Steric Characteristics of the 7,7-Dimethylnorbornyl System.- 8.4. Steric Hindrance to Ionization.- 8.5. Misconceptions.- 8.6. Steric Hindrance to Ionization and the Foote-Schleyer Correlation.- 8.7. Steric Effects in U-Shaped Systems.- 8.8. Exo:Endo Rate and Product Ratios as a Steric Phenomenon.- 8.9. Conclusion.- Comments.- 9. Equilibrating Tertiary 2-Norbornyl Cations.- 9.1. Introduction.- 9.2. Theoretical Considerations.- 9.3. The 1,2-Di-p-anisyl-2-norbornyl Cation.- 9.4. The 1,2-Diphenyl-2-norbornyl Cation.- 9.5. The 1,2-Dimethyl-2-norbornyl System.- 9.6. Conclusion.- Comments.- 10. Effect of Increasing Electron Demand.- 10.1. Introduction.- 10.2. Basic Considerations.- 10.3. Electron Demand in ?-Systems.- 10.4. Electron Demand in Cyclopropylcarbinyl Systems.- 10.5. Electron Demand in 2-Norbornyl Systems.- 10.6. Increasing Electron Demand in Secondary 2-Norbornyl.- 10.7. Extrapolation of Data from the Tertiary to the Secondary Systems.- 10.8. Conclusion.- Comments.- 11. Substituent and Structural Effects in 2-Norbornyl.- 11.1. Introduction.- 11.2. Secondary vs. Tertiary 2-Norbornyl Cations.- 11.3. Substituents as a Probe for Charge Delocalization.- 11.4. The Search for Charge Delocalization at C6.- 11.5. The Search for Charge Delocalization at C2.- 11.6. The Search for Charge Delocalization at C1.- 11.7. Deuterium as a Substituent.- 11.8. Deactivating Substituents.- 11.9. Structural Modifications.- 11.10. Conclusion.- Comments.- 12. Capture of Unsymmetrical 2-Norbornyl Cations.- 12.1. Introduction.- 12.2. Deamination of 2-Norbornylamine.- 12.3. Attempted Trapping of Unsymmetrical Cations in Solvolytic Processes.- 12.4. Capture of Unsymmetrical Cations in Solvolytic Processes.- 12.5. Additions to 2,3-Dideuterionorbornene.- 12.6. Addition of Deuterium Chloride to Norbornene.- 12.7. Addition of Deuteriotrifluoroacetic Acid to Norbornene.- 12.8. Addition of Perdeuterioacetic Acid to Norbornene.- 12.9. Resume.- 12.10. Conclusions.- Comments.- 13. Equilibrating Cations under Stable Ion Conditions.- 13.1. Introduction.- 13.2. Equilibrating Cations.- 13.3. Applicability of Results to Solvolysis.- 13.4. 13C NMR Shifts as a Measure of Charge Delocalizations.- 13.5. Nonadditivity of 13C Shifts as a Basis for Nonclassical Structures.- 13.6. PMR Spectra.- 13.7. ESCA Spectra.- 13.8. Cation Stabilities.- 13.9. Conclusion.- Comments.- 14. New Concepts-New Systems.- 14.1. Introduction.- 14.2. The Hyperconjugative Model.- 14.3. The Exo-6-H Participation Model.- 14.4. The Search for a Stereoelectronic Contribution.- 14.5. New Proposed ?-Bridged Systems.- 14.6. Conclusion.- Comments.- 15. Final Comments.- 15.1. Introduction.- 15.2. The Search for ?-Participation and ?-Bridging.- 15.3. Does ?-Delocalization Exist?.- 15.4. Are There ?-Bridged Cations?.- 15.5. Position on ?-Bridged Cations.- 15.6. Conclusion.- Comments.- Epilog.

Herbert C. Brown

Papers

Electrophilic Substituent Constants

Organic syntheses via boranes

Boranes in Organic Chemistry

A new technique for the conversion of olefins into organoboranes and related alcohols

The nonclassical ion problem