Synthesis of B-Organofunctional Borazine Derivatives - Advances in

25 Synthesis of B-Organofunctional Borazine Derivatives A Review of Recent Work at the Massachusetts Institute of Technology DIETMARSEYFERTH,HUBERT P. KOGLER, WALTER R. FREYER, MINORUTAKAMIZAWA,HIROSHI YAMAZAKI, and YASUHIKO SATO

Downloaded by MONASH UNIV on January 9, 2017 | http://pubs.acs.org Publication Date: January 1, 1964 | doi: 10.1021/ba-1964-0042.ch025

Department of Chemistry, Massachusetts Institute of Cambridge 39, Mass.

Technology,

Β-Tris-(trimethylsilylmethyl)-N-trimethylborazine was easily synthesized by the Grignard procedure, and was hydrolytically stable (through steric hindrance). Liquid borazines of the struc ture [(CH ) RSiCH BNCH ] were also pre pared. Silicon-substituted borazines also were accessible through the chloroplatinic acid-cata lyzed addition of Si-Η containing chlorosilanes and siloxanes to B-trivinyl-N -triphenylborazine. Other addition reactions of this vinylborazine were examined, and pure, crystalline products were obtained, with CBrCl , CBr , C H SH, (C H ) PH, HBr, (C H ) SnH, and (C H ) SnH. The condensation of silicon-containing dichloroboranes with aniline produced borazines. B o r azines containing S i - O - B and Si-B linkages are described. 3

2

2

3 3

3

6

5 2

6

5

3

4

6

2

H changed after it had been heated for 3 hours at 500° C. (3)],

5

5

3

examethylborazine is very stable thermally [it was recovered un but it is relatively unstable toward hydrolysis. It was one object of this r e search to prepare liquid borazines which would be stable hydrolytically as well as thermally. Another purpose was to prepare polymeric bor azines in which the polymerization process would occur through func tionality attached to boron atoms of the borazine ring. The combina tion of organosilicon chemistry with borazine chemistry through the synthesis of silicon-substituted borazines was used during the major part of this research as the approach which might yield results of value in terms of both objectives. The supposition that the major portion of hydrolytic attack on the borazine ring system would occur at the boron atom (as long as neutral or basic conditions were maintained) led to the logical suggestion that if one could prevent attack at the boron atom, one would have a hydro lytically stable borazine. The prevention of hydrolysis in readily hy drolyzable systems by sterically hindering approach of the attacking reagent is well known through many examples, and this approach was taken in this research. The highly branched trimethylsilylmethyl group therefore was of interest to us, since replacement of methyl groups attached to boron in hexamethylborazine by this group should introduce 259

Niedenzu; Boron-Nitrogen Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1964.

260

A D V A N C E S I N C H E M I S T R Y SERIES

tne desired steric factor. It had been shown by several groups of workers that -B-trihalo-iST-triorganoborazines reacted with Grignard reagents to give the corresponding £-triorganoborazines (2, 15). We used this procedure for the synthesis of £-tris-(trimethylsilylmethyl)N-trimethylborazine, a crystalline solid (9).

CI ί

CH -N

-N-CH

3

i

Cl-B

CH

3

+

η

R M g C 1

-N^

V

"

3

3

+

I

3

™eci

'3 B - R

R-B (CH ) SiCH

N-CH_

1

B-Cl ^ 3


3

2

CH„

2

This compound had the desired hydrolytic stability. It was stable toward air oxidation and toward hydrolysis under neutral or mildly alkaline conditions. It was not decomposed by alkaline hydrogen per oxide at room temperature, but was destroyed by hot, aqueous hydro chloric acid. This gain in hydrolytic stability was, unfortunately, ac companied by a loss in thermal stability. The trimethylsilylmethylsubstituted borazine showed appreciable stability only up to 400° C. and was completely decomposed when heated for 3 hours at 500° C. (8). Liquid borazines were obtained when the trimethylsilyl group's sym metry was disturbed. Thus the compounds [(CH ) (C H )SiCH BNCH ] and [ ( C H ) ( « - C H ) S i C H B N C H ] were high-boiling, viscous liquids. Of particular interest was the borazine derived from the reaction of the Grignard derivative of chloromethylpentamethyldisiloxane with Btrichloro-W-trimethylborazine, [(CH ) SiOSi(CH ) CH BNCH ] . A l though this point was not investigated, this borazine should be capable of being incorporated into polysiloxane systems through conventional siloxane redistribution-polymerization techniques. (CH ) SiCH MgCl did not appear to react with 5-trichloro-iNT-triphenylborazine, even under forcing conditions (7). Under comparable and even under much milder conditions C H M g B r reacts with this borazine to give B-tri methyl-N-triphenylborazine in good yield. Evidently steric interfer ence by adjacent phenyl groups attached to nitrogen can become i m portant when it is attempted to introduce highly branched substituents on boron. 3

3

2

4

9

2

3

2

2

5

2

3

3

3

3

3

3

2

2

3

3

3

3

2

3

An attempt was made to link such silicon-substituted borazines into larger units through difunctional organic linking units. Thus, the par tially substituted borazine, jB-chloro-5-bis-(trimethylsilylmethyl)-iVtrimethylborazine was prepared by the reaction of 2?-trichloro-iV-trimethylborazine with a deficiency of ( C H ) S i C H M g C l The reaction of this borazine with the di-Grignard reagent from 1,4-dichlorobutane gave the carbon-bridged borazine, I (8). Compound I was not stable toward hydrolysis. Apparently a de crease in the steric shielding of just one of the three boron atoms of the three boron atoms of the borazine ring provides a site for hydro lytic attack, and the borazine system is destroyed. The bridged bisborazine also was thermally much less stable than £-tris-(trimethylsilylmethyl) -iV-trimethylborazine. 3

3

2


25

SEYFORTH ET AL

B-Organofunctional

Borazines

261

ϊ 2*< 3>3 Η

ΟΗ

CH-N

N-CH

(CH ) SiCH -B 3

3

B-CH Si(CH )

x

2

3

2

3

3


I An alternate approach to the preparation of silicon-substituted bor azines involved the hydrosilylation of J5-trivinyl-iV-triphenylborazine. This unsaturated borazine was prepared by Pellon et al. (4, 5), and its polymerization and copolymerization behavior was studied by these authors. Our work showed that trichlorosilane did not add to [CH = C H B N C H ] in the presence of radical initiators such as benzoyl per oxide. However, Si-Η-containing chlorosilanes and siloxanes added to the olefinic double bonds of this borazine in good yield in the pre sence of chloroplatinic acid (11). 2

e

3

5

3

Si-H

+

[CH =CHBNC H ] 2

6

5

2

3

S _

[

SiC^C^BNC^]

3

The direction of addition as shown was established in the case of d i methylchlorosilane by conversion of the [(CH ) ClSiCH C H B N C H ] produced to the completely methylated compound, [(CH ) SiCH CH2B N C H ] , by the Grignard procedure and oxidation of the latter with alkaline hydrogen peroxide. Only β-trimethylsilylethanol, ( C H ) S i C H C H O H , was formed, and this established the absence of any (CH ) SiCH(CH ) groups, since these would have resulted in formation of the other isomeric alcohol, (CH ) SiCH(CH )OH. Of particular i n terest was that pentamethyldisiloxane and s^m-heptamethyltrisiloxane also added readily to jB-trivinyl-iST-triphenylborazine in the presence of chloroplatinic acid to give [(CH ) SiOSi(CH ) CH CH BNC H ] and [ {(CH ) SiO} S i ( C H ) C H C H B N C H ] 3 , respectively. Here again we have siloxanes capable of being introduced into polysiloxane systems through standard equilibration techniques. Furthermore, these reactions indicate that higher polysiloxanylethylborazines should be accessible via similar addition of Si-H-terminated polysiloxanes of the type (CH ) SiO[(CH ) SiO] SiO(CH ) H, although this point has not yet been investigated. The availability of [(CH ) ClSiCH2CH BNC H ] and [ C H a C ^ S i C ^ CH BNC H ] in principle also made possible the preparation of j3siloxanylethylborazines. However, neither of the two procedures tried, cohydrolysis with chlorosilanes and condensation with lithium t r i methylsilanolate, was wholly satisfactory. In the cohydrolysis with trimethylchlorosilane the [ ( C H ) ( O H ) S i C H C H B N C H ] and [ C H (OH) SiCH CH BNC H ] produced condensed at a much slower rate than trimethylsilanol, so only hexamethyldisiloxane was formed from the latter. It is believed that steric hindrance, probably due to other ring substituents, explains this rather difficultly effected condensation 3

2

2

2

3

6

5

e

3

5

3

2

3

3

2

3

3

3

3

3

3

3

3

3

3

2

3

3

3

6

5

2

2

2

n

3

2

2

2

3

e

3

3

2

2

2

e

5

5

2

2

e

5

3

3

3

2

3

2

2

e

5

2

2

2

e

5

3

3


3

3

A D V A N C E S I N C H E M I S T R Y SERIES

262

of borazinylsilanols. The cohydrolysis experiments led to the isola tion of [ ( C H ) ( O H ) S i C H C H B N C H ] in the case of the monochloro compound, and of a resinous solid in the case of the dichloro com pound. The reaction of the chlorosilyl-substituted borazines with lith ium trimethylsilanolate gave the expected trimethylsiloxy derivatives, but in poor yield: 3

2

2

2

e

5

3

6 (CH ) SiOLi + [ C H g a g S i C ^ C H g B N C g H ^ g 3

- 6 L i a

3

+


[ j i C H ^ O ^ a i C H ^ C ^ C H g B N C ^

3

A third route to silicon-substituted borazines involved the synthesis of a suitable silicon-substituted borane and the reaction of the latter with an amine to form the borazine ring. This several-step procedure proved to be unsatisfactory because of the low over-all yield. The procedure examined may be summarized by the following equations. B„H
3

S 1 C H = C H

CH,OH

2-Slel?*

3

S i

< 2 4> C

H

B H

2—~ (CH ) Si(C H )B(OMe) 3

PCI

ΟβΗΛ 6 5 2,

2 — (CH^SKCgiyBClg

i * 2 L _ ^ [(CH3) Si(C H )BNC H ] 3

2

4

6

5

3

2

4

2

( a y ^ c ^ B C N H C ^

3

A portion of the dimethyl trimethylsilylethylboronate was oxidized with alcoholic hydrogen peroxide. Gas chromatographic analysis of the alcohols produced showed that a mixture of 55% a -trimethylsilylçthanol and 45% β-trimethylsilylethanol was present. Hence the hydro boration product contained a 55 to 45 mixture of (CH ) SiCH(CH )B(OCH ) and ( C H ) S i C H C H B ( O C H ) . This is in agreement with previous results, which showed that when an excess of diborane source was used in the hydroboration of trimethylvinylsilane, the a-trimethylsilylethyl group was present in higher percentage in the product than the β-trimethylsilylethyl group (6). The directive effects in the hydro boration of trimethylvinylsilane have been discussed (14). Silicon-substituted borazines in which the Si - Ο - Β or S i - Β link ages were present were also examined. The reaction of sodium t r i methylsilanolate with Β-trichloro-iV^-trimethylborazine in ether solu tion gave [(CH ) SiOBNCH ] in 60% yield This compound, however, was hydrolytically unstable. The action of triphenylsilyllithium on B trichloro-i^-trimethylborazine in ether-tetrahydrofuran medium r e sulted in formation of B-tris-(triphenylsilyl)-iV-trimethylborazine, [ ( C H ) S i B N C H ] , in 55% yield. This is the first compound con taining a silicon-boron bond to be reported (1, 9). This compound also was hydrolytically unstable, the action of water causing complete dis ruption of the borazine ring: 3

3

2

3

3

e

5

3

3

3

2

2

3

3

3

3

2

3

3


3

25

SEYFORTH ET AL [(C H ) SiBNCH ] 6

5

3

3

B-Organofunctional

3

+ 9 H 0

263

Borazines

- 3 ( C g H ^ S i H + 3 B(OH)

2

+ 3 CH NH

3

3

2

jB-Tris-(triphenylsilyl)-iNT-trimethylborazine is stable in dry air. Its bromination gave as an initial reaction cleavage of the Si - B bond, triphenylbromosilane being isolated. The addition of other reagents to £-trivinyl-N-triphenylborazine was also examined. Triphenyltin hydride added readily in refluxing toluene solution to give [(C H ) S n C H C H B N C H ] in 74% yield. The direction of addition was confirmed by oxidation of the product to ( C H ) S n C H C H OH, whose structure was shown to be as written by means of its nuclear magnetic resonance spectrum. Triethyltin hydride did not react with the vinylborazine under these conditions, but it added in the presence of a radical initiator, 2, 2 -azobisisobutyronitrile, to give [ ( C H ) S n C H C H B N C H ] in 84% yield (11). Diphenylphosphine reacted with [CH = CHBNC Hg] in the presence of ter£-butyl peroxide, forming crystalline [ ( C H ) P C H C H B N C H ] . This phosphorus-substituted borazine was converted to the phosphine sulfide, [(C H ) P ( S ) C H C H B N C H ] , and to the mercuric chloride adduct, [ ( C H ) 2 P ( H g C l ) C H C H B N C H ] ^ ^ . Radical-initiated addition of benzenethiol to the vinylboraziné~gave [C H SCH CH BNC H ] . In a similar manner smooth radical-initiated addition of bromotrichloromethane and carbon tetrabromide was effected. e

e

5

3

2

5

3

2

2

e

5

3

2


T

2

5

3

2

2

e

5

3

2

e

3

e

5

5

2

2

2

6

3

e

5

2

e

2

2

5

e

2

5

3

2

2

e

5

3

e

2

2

e

3 CBrXg

+

5

5

3

[CH =CHBNC H ] 2

6

5

*

3

[CXgC^CHBrBNC^]

3

X=C1 and Br

Noteworthy is the formation of stable 1 to 1 solvates of the compound where X = Br, with benzene and carbon tetrachloride (12). Benzoyl peroxide-initiated addition of hydrogen bromide t o £ - t r i vinyl-iNT-triphenylborazine in benzene solution gave [ B r C H C H B N C Ηδ] , whose structure was confirmed by its NMR spectrum. In d i ethyl ether, on the other hand, complete destruction of the borazine was observed on treatment of [CH =CHBNC H ] with gaseous hydro gen bromide, the only crystalline product isolated being aniline hydrobromide. This difference in behavior is very likely a reflection of the different states of HBr in these solvents, HBr being present essen tially in the molecular form in benzene and as the strongly acidic oxonium salt in ether. Attempted condensation of [ B r C H C H B N C H ] with Grignard reagents resulted in β-elimination. Thus the action of phenylmagnesium bromide on this borazine gave hexaphenylborazine rather than the hoped-for [ C H C H C H B N C H ] (12). 2

2

e

3

2

e

5

3

2

e

5

3 C H M Br + [ B r C H ^ B N C ^ ] 6

5

g

2

2

3

e

.

5

2

e

5

3

3

[C^BNC^]

3

+

+ 3

MgBr

2

During this research B-triethynyl-iST-triphenylborazine was pre pared for the first time, but its chemical reactions and general stabilNiedenzu; Boron-Nitrogen Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1964.

264

A D V A N C E S IN CHEMISTRY

SERIES

ity have not yet been investigated. The reaction of HCsCMgBr with Β trichloro-N-triphenylborazine in tetrahydrofuran gave this compound in about 50% yield (13). The Β -organofunctional borazines prepared during the course of this work are listed in Table I. Table I.

Β -Organofunctional Borazines M.P., °C.

Compound [(CH ) SiCH BNCH ] 3

3

2

3


2

2

5

2

3

3

[(CH ) (n-C H )&CH BNCH ] 3

2

4

9

2

3

3

[(CH ) SiOSi(CH ) CH BNCH ] 3

3

3

2

2

3

[(CH ) SiCH CH BNC H ] 3

3

2

2

6

5

2

2

2

5

[CH Cl SiCH CH BNCgH ] 3

2

2

2

5

3

3

[(CH ) (OH)SiCH CH BNC H ] 3

2

2

2

3

3

[(CH ) ClSiCH CH BNCgH ] 3

6

5

3

3

2

2

2

6

5

3

[(CH

3

2

3

2

Si(C H )BNC H ]

3)3

2

(C H 2

4

6

5

2

(9) (9)

175-6(0.55)

(9)

157-9

(W

164-6

(11)

170-2

(W (11)

5

(11) Oil

3

(11) (13)

3

a mixture of - C H C H

4

183-4(1.0) 213-4(0.9)

86-7

3

[{(CH ) SiO} Si(CH )CH CH BNCgH ]

Ref. (9)

137

3

[(CH ) SiOSi(CH ) CH CH BNC H ] 3

B.P., (Mm.)

59-60

3

[(CH ) (C H )SiCH BNCH ] 3

°C.

2

2

and - C H - ) 132-146 CH 3

CH, (CH ) SiCH B " 3

3

^BCH Si(CH )

2

2

CH N 3

/

N

C H

3

3

105-8(0.03)

(8)

3

Β Cl

CH .St. (:CH ) SiCH B' 3

3

CH Ν 3

3

^BCH Si(CH ) 2

2

.NCH \β· CH CH 2

[(CH ) SiOBNCH ] 3

3

3

6

5

3

5

6

5

3

2

2

93-4

(8)

2

22-3

130(0.85)

248-51

3

[ C H ) SnCH CH BNCgH ] (

3

0

3

[(C H ) SiBNCgH ]

3

5

3

219-21


(9) (1,9) (11)

25

SEYFORTH ET AL

Table I.

B Organofunctional

B-Organofunctional Borazines M. P., °C.

Compound [(C H ) SnCH CH BNC H ] 2

5

3

2

2

6

5

[(C H ) PCH CH BNC H ] 6

5

2

5

2

2

2

6

2

5

2

3

6

5

3

(continued) B.P., °C.(Mm.)

Réf. (U)

160-2.5

(10)

217-9

(10)

267-9

(10)

3

134-5

(12)

165-6

(12)

[CCl CH CHBrBNC H ]

3

212-3

(12)

[CBr CH CHBrBNC H ]

3

[(C H ) P(HgCl )CH CH BNC H ] 6

5

2

2

2

2

[C H SCH CH BNC H ] 6

5

2

2

6

^rCH CH BNC H ] 2

3


265

87-8

3

[(C H ) P(S)CH CH BNC H ] 6

Borazines

2

6

5

2

5

3

6

5

6

5

3

241-1.5

(12)

[CH BrCHBrBNCgHg|

257-8

(12)

[HteCBNC H ]

264-5

(14)

3

2

6

2

6

Literature

5

3

5

Cited

(1) Cowley, Α. Η., Sisler, Η. Η., Ryschkewitsch, G. Ε., J. Am. Chem. Soc. 82, 501 (1960). (2) Mikhailov, Β. Μ., Uspekhi Khim. 1960, 972. (3) Newson, H. C . , English, W. D., McCloskey, A. L., Woods, L . G . , J. Am. Chem. Soc. 83, 4134 (1961). (4) Pellon, J. J., U. S. Patent 2,954,366 (1960). (5) Pellon, J. J., Deichert, W. G . , Thomas, W. M . , J. Polymer Sci. 55, 153 (1961). (6) Seyferth, D., J. Am. Chem. Soc. 81, 1844 (1959). (7) Seyferth, D., Freyer, W. R., unpublished work. (8) Seyferth, D., Freyer, W. R., Takamizawa, Μ., Inorg. Chem. 1, 710 (1962). (9) Seyferth, D., Kögler, H. P . , J. Inorg. Nucl. Chem. 15, 99 (1960). (10) Seyferth, D., Sato, Υ . , unpublished work. (11) Seyferth, D., Takamizawa, M . , Inorg. Chem. 2, 731 (1963). (12) Seyferth, D., Takamizawa, M . , J. Org. Chem. 28, 1142 (1963). (13) Seyferth, D., Yamazaki, H . , unpublished work. (14) Seyferth, D., Yamazaki, H . , Sato, Υ., Inorg. Chem. 2, 734 (1963). (15) Sheldon, J. C . , Quart. Revs. 14, 200 (1960). Received May 1, 1963. A review of work carried out in the senior author's laboratories at the Massachusetts Institute of Technology since September 1958 in the general area of borazine chemistry. Research supported by the U. S. A i r Force under Contracts A F 33(616)-5582, A F 33(616)-7124, and A F 33(657)-8532, monitored by Materials Cen tral, Aeronautical Systems Division, Wright-Patterson A i r Force Base, Ohio. This support is gratefully acknowledged.


Synthesis of B-Organofunctional Borazine Derivatives - Advances in

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