Reaction of hindered organoboranes

• Main Author: Zhao, J.
• Published: Swansea University 1994
• Subjects: 547.05
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.636732

Organoboron chemistry, including the synthesis and some applications of organoboron compounds, especially hindered arylboranes, which relate to the background of this research work, is briefly reviewed in the first two chapters. Chapter 3 introduces successful methods for the preparation of two new hindered monoarylboranes, ArBH<SUB>2</SUB>, tripylborane and mesitylborane. It includes two approaches, one being the redistribution of Ar<SUB>2</SUB>BH and BH<SUB>3</SUB>-THF and the other <i>via</i> dimethoxyarylboranes. Mesitylborane, MesBH<SUB>2</SUB>, is the first monoorganoborane which exists in crystalline form. Chapter 4 introduces a stability study of ArBH<SUB>2</SUB>. By means of hydrolysis-gas titration and methanolysis-GC analysis, it was shown that TripBH<SUB>2</SUB> and MesBH<SUB>2</SUB> are far more stable than thexylborane. The hydroboration properties of TripBH<SUB>2</SUB> and MesBH<SUB>2</SUB> are described in Chapter 5. Tripylborane and mesitylborane hydroborate two equivalents of terminal alkenes or cyclohexene to give the corresponding ArBR<SUB>2</SUB>. Oxidation gives high yields of alcohols. Tripylborane and mesitylborane possess sequential hydroboration properties which allow the preparation of ArBR<SUP>1</SUP>R<SUP>2</SUP> (R<SUP>1</SUP>,R<SUP>2=</SUP> primary alkyl or cyclohexyl). Chapter 6, describes the cyanoborate process on ArBR<SUP>1</SUP>R<SUP>2</SUP>, which allows the formation of ketones R<SUP>1</SUP>R<SUP>2</SUP>CO. No aryl group migration is found. Chapter 6 also introduces the hydroboration of dienes with mesitylborane and subsequent cyanoborate process. When (+)-limonene was used, low yields of ketones were obtained. When 4-vinylcyclohexene was used, bicyclo[3.2.2]nonan-2-one was produced in 48% yield. Novel methods of synthesis of mixed triorganoboranes from ArBH<SUB>2</SUB> are introduced in Chapter 7. The methods involve the preparation of ArBR<SUP>1</SUP>R<SUP>2</SUP>, then aryl group replacement, for example by organolithium compounds, e.g. Bu<SUP>n</SUP>Li to form Bu<SUP>n</SUP>R<SUP>1</SUP>R<SUP>2</SUP>B. The aryl group in ArBR<SUP>1</SUP>R<SUP>2</SUP> can also be replaced by bromination. Subsequent reaction with a Grignard reagent, R<SUP>3</SUP>MgBr, yields R<SUP>1</SUP>R<SUP>2</SUP>R<SUP>3</SUP>B. The preparation and some reactions of mesitylisopinocampheylborane, Ipc(Mes)BH, are described in Chapter 8.