Cations Carey & Sundberg, Part A Chapter 5, "Nucleophilic Substitution",

B.A., 1941, UCLA Ph.D. 1944, UCLA Instructor, Harvard, John D. Roberts was born in 1918, starting his career in He became Prof. at MIT and then Prof. at Caltech where he is still active. His work is centered on mechanisms of organic reactions. John D. Roberts graduated from the University of California at Los Angeles where he had received A. B. (hons) degree in 1941 and the Ph. D. degree in In he was a National Research Council Fellow and Instructor at Harvard. Later on, he went to MIT in 1946 as an Instructor. He had introduced the terms "nonclassical" carbocations and "benzyne" into organic chemistry. He had won numerous awards; he is a member of the National Academy of Sciences (1956) and the American Philosophical Society (1974). He received the Welch Award (1990, with W. E. Doering), the National Medal of Science (1990), and the ACS Arthur C. Cope Award (1994). Since 1939 his research has been concerned with the mechanisms of organic reactions and the chemistry of small-ring compounds. His current work involves applications of nuclear magnetic resonance spectroscopy to physical organic chemistry. Roberts made major research and pedagogic contributions to mechanistic organic chemistry. He pioneered the use of 14C and other isotopic labels to follow molecular rearrangements as, for example, in the complex and subtle solvolysis of cyclopropylcarbinyl systems. He introduced the terms "nonclassical" carbocations and "benzyne" into organic chemistry, and used isotopic labeling to establish the intermediacy of each. Roberts was early to recognize NMR's potential, and used 1H NMR to study nitrogen inversion, long-range spin-spin coupling and conformational isomerism, and later 13C and 15N NMR to study other reactions, including the active sites of certain enzymes. Roberts' superb short books on "Nuclear Magnetic Resonance" (1959), "Spin-Spin Splitting in High Resolution NMR" (1961) and "Notes on Molecular Orbital Calculations" (1961) did much to popularize and clarify these subjects for organic chemists. His highly successful text "Basic Principles of Organic Chemistry" (1964), written with Marjorie Caserio, introduced spectroscopy early to undergraduates. Roberts received many awards, including the Roger Adams (1967) and Priestley (1987) Medals. An excellent photographer, Roberts graciously supplied several of the photographs for the MSU collection. “One of the joys of being a professor is when an exceptional student comes along and wants to work with you”. J.D. Roberts, The Right Place at the Right Time. p. 63.

Carbocations

Cationic Systems

Carbocation Stability

Carbocation Generation

Carbocation Stability

Carbocations

Preparation of a vinyl cation no good nucleophiles prevent loss of H + stabilizing  -Si groups Müller T., Juhasz, M., Reed, C. A., Angew. Chem. Int. Ed., 2004, 43,  -Si stabilization (hyperconjugation)

NMR evidence Only one 29 Si signal Symmetric in solution (confirms ring closure) =C + is far downfield Si resonance is downfield No solvent effect C and 29 Si NMR chemical shifts

IR spectrum Typical Frequencies: C=C 1660 cm -1 C≡C 2200 cm -1 Exp. C=C cm -1 Calculated: 1956 cm -1 CB 11 H 6 Br C=C + B-H

Crystal Structure crystal packing Selected distances/angles C2 - C11: Å  C2-C11-C12: ° Si1 – C2: Å Si3 – C2: Å Selected distances/angles C2 - C11: Å  C2-C11-C12: ° Si1 – C2: Å Si3 – C2: Å Müller T., Juhasz, M., Reed, C. A., Angew. Chem. Int. Ed., 2004, 43,

Cyclopropyl Cations

Carbocations In Bridged Systems

Carbocation [1,2] Sigmatropic Rearrangements

The Prins Reaction

Tandem Prins-Pinacol Reaction

Overman’s Laurenyne Synthesis

Overman’s trans-Kumausyne Synthesis

Overman’s trans-Kumausyne Synthesis

The  -Silicon Effect

Reactions of Allylsilanes

Iminium Ions

N-Acyliminium Ion Rearrangements

Aza-Cope Manich Reactions

Terpenes

Isoprene : Nature's C 5 Building Block

Terpene Biosynthesis

Steriod and Squalene Oxide Cyclization

Steriod Biosynthesis; Squalene Oxide Cyclization