Durham E-Theses Studies on the organic chemistry of boron Livingstone, J.G. How to cite: Livingstone, J.G. (1961) Studies on the organic chemistry of boron, Durham theses, Durham University. Available at Durham E-Theses Online: http://etheses.dur.ac.uk/9111/ Use policy The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that: • a full bibliographic reference is made to the original source • a link is made to the metadata record in Durham E-Theses • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders. Please consult the full Durham E-Theses policy for further details. Academic Support Oce, Durham University, University Oce, Old Elvet, Durham DH1 3HP e-mail: [email protected] Tel: +44 0191 334 6107 http://etheses.dur.ac.uk (11 ft u 4 SIAO rsi ^> V 3^NVJ-i3>Vc)V3 Nl 39IMVH3 0 6> CM 5 li 70 7D 0 0> 6 ID 8 79 730 Uoi 0 STUDIES ON THE ORGANIC CHEMISTRY OF BORON BY J. Gr« LIVINGSTONE A dissertation submitted for the Degree of Doctor of Philosophy in the Durham Colleges of the University of Durham. 1961 CONTENTS Page Acknowledgments (1) Memorandum (II) Summary (Ill) FART I Introduction Page Organor-Derivatives of Boric Acid I Trialkyl- and Triaryl Boranes 8 Mixed Trialkyl and Aryl-Alkylboranes 40 Cyclic Boranes 43 Boronic Acids and Esters 46 Borinic Acids and Esters 54 Boroxines 62 Alkyl- and Aryl- Dihaloboranes 65 Dialkyl. and Diary1 Haloboranes 72 PART II Preparation of Reagents Page Chlorodiphenylborane 78 Chlorodi-p-tolylborane 79 Chlorodi-p-bromphenylborane 79 Pluorodimesitylborane - 80 Dimethylaminoborp^^feuLoride 80 (22SEPIW ) Page Diphenylphosphine 80 Di-m-tolylphosphine 81 Tri-m-tolylphosphine 81 Triphenylarsine 82 Diphenylarsine 82 Phenylphosphine 83 PART III Experimental Results Aminoboranes Page Aminodiphenylborane 85 Aminodimesitylborane 85 Dimethylarain odiphenylborane 86 DiTn.fttliyla.Triiriodi-p-tolyl'bora.jie 86 Dimethylaminodi-p-bromophenylborane 86 Diphenylaminodiphenylborane 88 Di-p-tolylamino:diphenylborane 89 Diphenylaminodi-p-tolylborane at 89 AMINO &I - o - tOLYLbo»ANe. Phosphinoboranes Diphenyl(diethylphosphino)borane 90 Di-p-broraphenyl(diethylphosphino)borane 90 Diphenyl(diphenylphosphino)borane 91 Chlorodiphenyl(diphenylphosphino) borane 92 Triethylammoniura chlorodiphenyl(di• phenylphosphino) borane 92 Diphenyl(di-m-tolylphosphino)borane Di-m-tolylphosphinodi-p-tolylborane Di-p-bromphenyl(di-m-tolylphosphino) borane Phenyl(bis-diphenylphosphino)borane Dichlorophenyl(phenylphosphino)borane Phenyl(phenylphosphino)borane Di~m-tolylphosphino( dimesityl)borane Di-m-to-lylphosphino( bis-biphenyl)boran Bis-diphenyl(phenylphosphino)borane Arsinoboranes Mphenylarsinodiphenylborane Diphenyiarsino(d i-p-1 oly1)b orane Diphenylarsino(di-p-bromphenyl)borane Methods of Analysis Boron Phosphorus Dipole Moment Measurements Theory of Measurements Evaluation of Results Physical Measurements Experimental Discussion Appendix Introduction to Borinic Acids Page Experimental 139 Discussion 140 Introduction to white phosphorus 141 Reaction with Phenyllithium 142 Reaction with Methyllithium 144 Reaction with Boron Trichloride 144 Discussion 145 Tables References Page Experimental 139 Discussion 140 Introduction to white phosphorus 141 Reaction with Phenyllithium 142 Reaction with Methyllithium 144 Reaction with Boron Trichloride 144 Discussion 145 Tables References (i) ACKNOWLEDGEMENTS The author wishes to express his most sincere thanks to Professor G. E. Coates, M.A., D.Sc, F.R.I.C., under whose direction this research was carried out, for his constant encouragement and extremely valuable advice, and to Dr. G. Kohnstam and Dr. P. Glockling for their comments. The author also takes this opportunity to express, his thanks to the Senate of Durham University at which laboratories this work was carried out, and to the Department of Scientific and Industrial Research for the award of a grant. (ii) MEMORANDUM The work included in this thesis was carried out at the Durham Colleges Science Laboratories in the University of Durham between September 1958 and September 1961, and has not been submitted for any other Degree. Part of this work has already been the subject of a publication in the Journal of the Chemical Society. All the work described is the original work of the author, except that acknowledged by reference. (iii) SUMMARY Several aminodiarylboranes (Ar2B NR2) have been prepared by the methods: Ar2BCl + LiNR2 = Ar^B^NRg + LiCl (l) Ar^BCl + R2NH + Bt^N = Ar2B NR? + Et^NHCl .... (2) 2ArMgBr + C12B NR? = Ar2B NR2 + MgCl2 + MgBr2 . (3) Aminodiphenylborane (R = H, method 2) is dimeric and non- polar in benzene solution. Experiments with molecular models indicate that larger groups attached to the nitrogen would cause substantial steric interference even with the hydrogen atoms in ortho-positions on the aryl groups. Similarly, aminodiraesitylborane is monomeric. All the other aminodiarylboranes prepared are raonomeric in nitrobenzene solution, though often slightly associated in benzene particularly in the more concentrated solutions. The monomeric aminodiarylboranes have low dipole moments whose direction is reversed on passing from Ph2B*NMe2 to Ph2BNPh2. This reversal of polarity could be due to the electron repelling effect of a methyl group, whereas a phenyl group can act as an electron acceptor in such a system. All the observed moments are consistent with only a small or zero net polarity for the B-N bond. (iv) The rapid and quantitative hydrolysis, by cold water, of the aminoboranes was the basis of a method sometimes used for their analysis, and more important for a most con• venient method for the preparation of diarylborinic acids. Some twelve acids have been prepared as the monoethanolamine esters (yields 51-93$) by hydrolysis, nf the diarylarainor- borane obtained from the reaction of a slight excess of tbe appropriate Grignarrt reagent with dichlorodiphenylaminoborane. This would appear to be the most convenient preparative method for diarylborinic acids. Phosphinodiarylboranes are monomeric but unlike the amino nompounds they are quite stable to boiling dilute acids and alkalies* However, quantitative oxidation by aqueous alcoholic hydrogen peroxide to boric acid the phosphinic acid and the phenol has sometimes been used as a method for their analysis. Their dipole moments are generally greater than those of the corresponding amino-compounds, and the phosphino- group is at the negative end of the dipole. This surprising result suggests that phosphorus acts as a % donor (to boron) only weakly if at all in these compounds, possibly owing to an unfavourable relation between the "sizes" of boron 2p- and phosphorus 3p- orbitals. (v) Formula I accounts in a qualitative way not only for the observed dipole moments, but also for the lack of chemical reactivity, since both boron and phosphorus are co-ordinatively saturated (or partly so). Further r— the indicated charge separation between boron and phosphorus should result in B-P stretching force constants greater than would be expected for B-P single bonds, since the B-P bond would amount to a o-bond possibly with a small Ti-component but with some electrostatic attraction in addition. In this sense the B-P bond could perhaps be described as a "serai-polar double bond", thus accounting for the rather high frequencies (1400 - 1500 cm-1) associated with the stretching of the B-P bond. The reason for the phosphinodiarylboranes being monomeric, when the series R^B'PR^ (R = H, alkyl) form trimers, tetramers, or polymers, thus lies in the boron atom in the diarylborane series being already coordinatively saturated (or nearly so) not by TC-bonding with phosphorus but by ir-bonding with the aryl groups. The arsino-derivatives are similar to the phosphino- (vi) compounds but they are less polar. Perhaps the most sign• ificant difference is that oxidation with aqueous alcoholic hydrogen peroxide affords boric acid, arsenic acid and the appropriate phenols e.g. Ph2B AsAr2 + H,^ — 2PhOH + 2ArOH.+ H^BO^ + I^AsO^ The smaller dipole moments of the arsinoboranes is probably due to a smaller degree of electron transfer from arsenic to the aryl groups bound to it. It now seems evident that though the o'-donor character of nitrogen, phosphorus, and arsenic towards boron diminishes in that order the rc-donor character diminishes even more rapidly. The phosphinoboranes Ph B (PPh2)2, Ph P (BPh2)2, (Ph B PPh) and (Ph B PPh) have also been prepared. They are all more sensitive to oxidation than the phosphinodiarylboran.es men• tioned earlier. PART I INTRODUCT ION 1. A) ORGANO-DERIVATIVES OF BORIC ACID 1. PREPARATION OF TRIALKYL- & TRIARYL BORATES The best known and most stable organo derivatives of boric acid are the trialkyl and triaryl borates (RO)^B; unsymmetrical esters ROB(OR')2 are known but they tend to disproportionate to the symmetrical esters. Indeed disproportionation is a common and sometimes troublesome feature of organo-boron chemistry. Lappert"1" in his review describes several methods of p preparation of the borates. Ebelmen and Bouquet , the first workers to mention orthoborates, prepared trimethyl, triethyl, and triamyl borates by heating the appropriate alcohol with boron trichloride in a sealed tube, but attempts to prepare triallyl and tribenzyl borates were unsuccessful. Michaelis and Hillringhaus^ prepared triaryl