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Hydroboration, a Brief Historical Review Through REVISTA BOLIVIANA DE QUÍMICA Received 09 11 2019 37(1); Jan./Apr. 2020 Accepted 04 19 2020 ISSN 0250-5460 Rev. Bol. Quim. Paper edition Published 04 30 2020; DOI:10.34098/2078-3949.37.1.7 ISSN 2078-3949 Rev. boliv. quim. Electronic edition José A. Bravo et José L. Vila RBQ Vol. 37, No.1, pp. 46-63, 2020 Received 09 11 2019 HYDROBORATION, A BRIEF Accepted 04 19 2020 Published 04 30 2020 HISTORICAL REVIEW THROUGH Vol. 37, No.1, pp. 46-63, Ene./Abr.2020 MECHANISTIC VIEWS, PART I: Revista Boliviana de Química ALKYL- AND ARYL-SUBSTITUTED 37(1), 46-63, Jan./Apr. 2020 OLEFINS, AS ADDITION- Bolivian Journal of Chemistry SUBSTRATES; THE ORGANIC DOI: 10.34098/2078-3949.37.1.7 CHEMISTRY NOTEBOOK, Nº 15 HIDROBORACIÓN, UNA BREVE REVISIÓN HISTÓRICA A TRAVÉS DE VISTAS MECANICISTAS, PARTE I: OLEFINAS ALIFÁTICAS Y AROMÁTICAS COMO SUSTRATOS DE ADICIÓN; EL CUADERNO DE QUÍMICA ORGÁNICA, Nº 15 Short review Peer-reviewed José A. Bravo1,*, José L. Vila2 1Natural Product Laboratory, Phytochemistry, Chemical Sciences Department, Instituto de Investigaciones Químicas IIQ, Facultad de Ciencias Puras y Naturales FCPN, Universidad Mayor de San Andres UMSA, P.O. Box 303, Calle Andrés Bello s/n, Ciudad Universitaria Cota Cota, phone +59122792238, La Paz, Bolivia, [email protected], [email protected], www.umsa.bo 2Natural Product Laboratory, Synthesis and Hemisynthesis, Chemical Sciences Department, Instituto de Investigaciones Químicas IIQ, Facultad de Ciencias Puras y Naturales FCPN, Universidad Mayor de San Andres UMSA, P.O. Box 303, Calle Andrés Bello s/n, Ciudad Universitaria Cota Cota, phone +59122772269, La Paz, Bolivia, [email protected], [email protected], www.umsa.bo Keywords: Organic Chemistry, Addition reaction, Multiple bonds, Organoboranes, Hydroboration, Mechanisms of Reactions. Palabras clave: Química orgánica, Reacción de adición, Enlaces múltiples, Organoboranos, Hidroboración, Mecanismos de reacción. ABSTRACT The Organic Chemistry Notebook Series, a Didactical Approach, is the series designed with educational purposes in the organic synthesis field. With the present paper we add to a total of fourteen contributions so far in the series. This series of studies is designed to help students when getting started in the synthesis subject. The method of learning includes many fully and explicitly designed reactions step by step. The best manner to understand a synthesis is by means of graphical views, in this case, the ones proposed by the authors of this series, and, when in most of the cases they are accompanied by illustrative comments that describe the graphical mechanistic proposals and add some criteria deduced from the different mechanistic steps. During the past 14 chapters we have taken a series of reactions compiled by W. Carruthers in ‘Some modern methods of organic synthesis’, and we have proposed didactical and mechanistic views for them. Now we are boarding another important compiling source in the synthesis studies: ‘Advanced Organic Chemistry, Part B: Reaction and Synthesis’ by Francis A. Carey and Downloadable from: Revista Boliviana 46 de Química. Volumen 37 Nº1. Año 2020 http://www.bolivianchemistryjournal.org, http://www.scribd.com/bolivianjournalofchemistry REVISTA BOLIVIANA DE QUÍMICA Received 09 11 2019 37(1); Jan./Apr. 2020 Accepted 04 19 2020 ISSN 0250-5460 Rev. Bol. Quim. Paper edition Published 04 30 2020; DOI:10.34098/2078-3949.37.1.7 ISSN 2078-3949 Rev. boliv. quim. Electronic edition José A. Bravo et José L. Vila RBQ Vol. 37, No.1, pp. 46-63, 2020 Richard J. Sundberg. This theme is included in the chapter “Electrophilic additions to carbon-carbon multiple bonds”, and references therein. *Corresponding author: [email protected], [email protected] RESUMEN La Serie de Cuadernos de Química Orgánica, un Enfoque Didáctico, es la serie diseñada con fines educativos en el campo de la síntesis orgánica. Con el presente trabajo agregamos un total de quince contribuciones hasta ahora en la serie. Esta serie de estudios está diseñada para ayudar a los estudiantes cuando se inician en el tema de síntesis. El método de aprendizaje incluye muchas reacciones diseñadas de forma completa y explícita paso a paso. La mejor manera de comprender una síntesis es mediante vistas gráficas, en este caso, las propuestas por los autores de esta serie y, cuando en la mayoría de los casos, van acompañadas de comentarios ilustrativos que describen las propuestas gráficas mecanicistas y agregan algunos criterios deducidos de los diferentes pasos mecanicistas. Durante los 14 capítulos anteriores, hemos tomado una serie de reacciones compiladas por W. Carruthers en "Algunos métodos modernos de síntesis orgánica", y hemos propuesto puntos de vista didácticos y mecanicistas. Ahora abordamos otra fuente importante en los estudios de síntesis: "Química orgánica avanzada, Parte B: Reacción y síntesis" de Francis A. Carey y Richard J. Sundberg". Este tema se incluye en el capítulo "Adiciones electrofílicas a los enlaces múltiples carbono-carbono" en el texto mencionado y referencias incluidas. INTRODUCTION Due to lack of knowledge of classical mechanisms, students experiment emptiness when comprehending a determined synthesis found in the literature. Since a mechanistic proposal is naturally mandatory for a rational explanation of products emerging from a synthesis, we offer the present series theming on mechanistic approaches on several published syntheses. As academics we are committed with the didactics and we have designed a series of articles exposing mechanistic theoretical proposals, articles have a character of short review. The present contribution: Hydroboration, a brief historical review through mechanistic views, part I: alkyl- and aryl-substituted olefins, as addition-substrates, The Organic Chemistry Notebook, Nº 15, that refers to regioselectivity considerations, is the fifteenth study in the series: “The Organic Chemistry Notebook Series, a Didactical Approach” [1-14]. REVIEW OF REACTIONS, MECHANISTIC THEORETICAL PROPOSALS, DISCUSSION Hydroboration of olefins Hydroboration is a good method in many synthesis reactions employing electron-rich compounds as substrate, namely alkenes and alkynes [15]. Boron as borane BH3 is a Lewis acid with a free orbital suitable for rooming an electron pair. Borane is available as a dimer, B2H6, with the boron atoms bridged by two of the six hydrogens between them [15]. Due to its Lewis acid nature (Figure 1), boron interacts with Lewis bases such as oxygen, + - + - + - nitrogen or sulphur in ethers, tertiary amines and sulfides, R2O - BH3, R3N - BH3, R2S - BH3 [15], see boron hybridation´s changement when borane overlaps with Lewis bases (Figure 2). 2 p H sp2 sp2 B H 2 H sp Figure 1. Boron electronic structure, borane structure Downloadable from: Revista Boliviana 47 de Química. Volumen 37 Nº1. Año 2020 http://www.bolivianchemistryjournal.org, http://www.scribd.com/bolivianjournalofchemistry REVISTA BOLIVIANA DE QUÍMICA Received 09 11 2019 37(1); Jan./Apr. 2020 Accepted 04 19 2020 ISSN 0250-5460 Rev. Bol. Quim. Paper edition Published 04 30 2020; DOI:10.34098/2078-3949.37.1.7 ISSN 2078-3949 Rev. boliv. quim. Electronic edition José A. Bravo et José L. Vila RBQ Vol. 37, No.1, pp. 46-63, 2020 H sp3 H sp3 sp3 B sp3 H Figure 2. Boron hybridation´s changement when borane overlaps with Lewis bases Using THF as solvent or DMS, the addition of borane over most alkenes is immediate [15], a reaction known as hydroboration. Hydroboration is stereospecific and very regioselective [15]. Steric and electronic effects are involved in the addition of borane and they favor the attack over the carbon the less substituted [15]. Since borane reacts with alkenes until completion of the hydride substitution in BH3 and their replacement by the adding on three alkene molecules to give a trialkyl borane, the less substituted carbon is chosen by boron in order to avoid steric hindrance when adding the second and the third alkenic residue, e.g. the addition of BH3 to 2-methyl-1-propene, where atom 1 is preferred over atom 2 for the new B-C bond [15], see Figure 3. This behavior gives this reaction its regioselective quality [15]. The minor option of addition of the boron atom to the carbon with the most steric hindrance represents severe nonbonded repulsions, see Figure 4. 3 3 3 3 2 CH3 CH CH 2 CH3 2 3 2 3 CH CH3 H CH CH3 3 H 3 2 1 + H 1 H 1 H H 1 3 H 1 H H H H H B- B H B B 2-methyl-1-propene H H H H HH B cis addition H H monoalkylborane borane H T.S. 3 3 3 CH CH 2 2 3 2 3 CH CH3 H CH CH3 3 3 2 + H H 1 H 1 1 3 H 1 H H H H B- B H B B 2-methyl-1-propene monoalkylborane H H R HR R dialkylborane 3 3 3 CH CH 2 2 3 2 3 CH3 CH3 H CH3 CH3 + H H 2 1 H 1 1 3 H 1 H H H R B- B R B B 2-methyl-1-propene dialkylborane H R R RR R trialkylborane B Nonbonded repulsions reduced trialkylborane Figure 3. Hydroboration of 2-methyl-1-propene, boron’s addition over the carbon with the less steric hindrance. Nonbonded repulsions reduced [15]. Theoretical mechanistic proposals by the authors Downloadable from: Revista Boliviana 48 de Química. Volumen 37 Nº1. Año 2020 http://www.bolivianchemistryjournal.org, http://www.scribd.com/bolivianjournalofchemistry REVISTA BOLIVIANA DE QUÍMICA Received 09 11 2019 37(1); Jan./Apr. 2020 Accepted 04 19 2020 ISSN 0250-5460 Rev. Bol. Quim. Paper edition Published 04 30 2020; DOI:10.34098/2078-3949.37.1.7 ISSN 2078-3949 Rev. boliv. quim. Electronic edition José A. Bravo et José L. Vila RBQ Vol. 37, No.1, pp. 46-63, 2020 3 3 3 3 CH 2 CH3 CH CH3 CH3 2 3 3 2 CH3 2 CH3 CH3 H 3 1 H H 1 H 1 B- 1 2 H H + H H B H H 1 borane B H H H H B B H H H H HH H H 2-methyl-1-propene cis addition T.S.
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