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2-amino-1,3,2-dioxaborolanes and their properties
J. inors,nucl.Chem.,1968,Vol.30, pp. 395 to 399. PergamonPressLtd. Printedin Great Britain
2-AMINO-1,3,2-DIOXABOROLANES PROPERTIES
AND
THEIR
R. H. C R A G G The Chemical Laboratory, University of Kent at Canterbury (Received3 May 1967)
Abstract-The preparation and properties of some 2-amino- 1,3, 2-dioxaborolanes, CH2---O
I
~BNRR' /
CH2"--4) (R = R' = Me; R = H and R' = Me, Et, Pr', Bun, Bu ~, Bu t and Ph) are described and their association is discussed. Reaction of phenyl isocyanate with 2-dimethylamino-l,3,2-dioxaborolane is exocyclic showing that the relative migratory aptitude is R2N > OR.
THERE have been several publications concerning molecular association in dioxaborolanes [ 1-5] where the association is through the boron-oxygen bond as represented in Fig. 1. CH2---O
X
\/ B~
O~CHz
/ //\
CH2"---O
)B
X
I
O---CH2
Fig. I.
The first examples of association studies, on 2-amino and 2-dialkylamino-l,3,2dioxaborolanes are here reported, where the association is through the boronnitrogen bond as represented in Fig. 2. R
H
CH2---O
~
/
CH2--O
/ R
~
O--CH2
H
O---CH2
Fig. 2. 1. 2. 3. 4. 5.
A. Finch and P. J. Gardner, J. inorg, nucl. Chem. 25, 927 (1963). R.J. Brotherton and A. L. McCIoskey, J. org. Chem. 26, 1668 (1960). E.W. Abel, S. H. Dandegaonker, W. Gerrard and M. F. Lappert,J. chem. Soc. 4116 (1957). J. Dale, B. Hargitay and A. J. Hubert, J. chem. Soc. 9 ! 0, 922 and 931 ( 1961). M. F. Lappert, M. K. Majumdar and B. P. Tilley, J. chem. Soc. (.4) 1590 (1966). 395
396
R.H.
CRAGG
However on the evidence available structures of the type (1) are equally possible. Association studies of analogous 2-amino-1,3,2-benzodioxaboroles have recently been published[5] and dimerisation of linear non cyclic boron compounds is well known. RESULTS AND DISCUSSION This is the first of a series of papers which are dealing with the effect of (a) electronegativities and (b) steric factors on the dimerisation of aminoboranes. The studies being made are on compounds of the type CH2----X
\ /
B-NRR'
CH2----X where R = R' = alkyl and R = alkyl R' = H and X = O, S, N, P and As. The 2-amino-l,3,2-dioxaborolanes were prepared by the interaction of 2-diethylamino- 1,3,2-dioxaborolane and the corresponding amine (Equation (1)). CH2---O
CH2--O ,
B-NEt~ + RNH2
H,--O/
\
l
jB-NHR
+ EtcNH.
(1)
CH2---O
The utilisation of the above method was made necessary by the fact that the aminoborane, being polymeric, and amine hydrochloride are difficult to separate. However, the usual method of preparing aminoboranes is by the reaction of chloroborane and amine (Equation (2)) and this proved successful in the preparation of dialkylamino derivatives. CH2---O I CHr-O
\ /
CH2--O B-CI + 2R2NH
,
\ /
BNR2 + R2NH'HCI.
(2)
CH2--O
The characteristic data of the 2-amino-l,3,2-dioxaborolanes is given in Table 1. The molecular weights of some of the 2-amino and dialkylamino-l,3,2-dioxaborolanes were determined by two methods (a) mass spectrometric and (b) osmetric, and the results are given in Table 2. The majority of the amino derivatives were oligomeric solids and due to their insoluble nature molecular weight determinations could not be carded out. It was hoped that the degree of association could be obtained mass spectrometrically but in the two determinations made the most intense peaks were at the position for a monomer showing that at least in the gas phase the compounds were monomeric. However, there were less intense peaks of higher molecular weight whose pattern suggested the presence of more than one boron atom per molecule. The molecular weights in benzene,
2-amino-1,3,2-dioxaborolanesand theirproperties Table
397
1
Analyses Compound CH2--O
\
Yield m.p.
(%)
Found
Required
N
B
N
B
B-NHEt
86-8 °
88
12.2
9-4
12.6
9.8
B-NHPr'
liquid
76
I 1.1
7.9
10.9
8.3
~B_NHBun
66-9 °
82
9.3
7.4
9.7
7.6
~'B_NHBu l
73-5 °
84
9.3
7.9
9"7
7"6
~B_NHBut
liquid
78
9.9
7.2
9.7
7-6
\B-NHPh
90-2 °
85
8-8
7-0
8-6
6-7
/ CH2--O CH2--O
\ /
CH2--O CH2--O
I
/
CH2---O CHz---O
/ CH2--O CH2--O
I
/
CH2---O CH2~
/
CH2---O
determined osmometrically, varied in such a way that it could only be concluded that isolated molecular weight determinations in solution of compounds of this type are very misleading. B "NMR determinations were unsatisfactory as it was found that the chemical shifts, [wrt (MeO)aB], were all about 7 ppm. From this it would appear that the effect of substituents on the nitrogen, because of the nitrogen quadrepole moment, have very little effect on the boron atom and all that is measured, in fact, is the chemical shift of the ring. The i.r. spectra of all t' e compounds were recorded. The region between 1 5 5 0 - 1 0 0 0 c m -1 was very complex and it is therefore very difficult to make unequivocal assignments for B - O and B - N stretching modes. However, the B - N stretching frequencies are tentatively assigned in the region 1530-1545 cm -1 and the B-O stretching frequencies in the region 1 3 0 2 - 1 2 3 0 c m -1. This has been interpreted[6] to mean that the B - N bond order is higher than normal and the 6. J. A. Blau, W. Gerrard, M. F. Lappert, B. A.
Mountfieldand H. Pyszora,J. chem. Soc. 380 (1960).
398
R. H. CRAGG Table 2 Molecular weight Compound
Association (%)
Found
Calculated
460 115"
114"8
4
309
142.8
2.2
101"
100"8
1
341
128.8
2-7
300
142.8
2.1
CH2---O
CH~-O
1
CHc-O ~B-NEh / CHf--O CHf--O ~'B-NHM, / CH2---O CH2--O I ~B-NHPr' / CH2---O
CHe--O I
~B_NHBut / CH~--O
*These two determinations were carried out mass spectrometrically.
B - O bond order lower than normal which shows that the contributions from a structure of the type (a) are far greater than that from a structure (b) CH2~O
\- ÷/ ~N / \
CH2---O
(a)
+ CH2---O
/
B---N
\
CH2--O
(b)
Aminoboration and alkoxyboration of isocyanates is well known[7]. Phenyl isocyanate and 2-dimethylamino-l,3,2-dioxaborolane react in a 1 : 1 molar ratio to give N-phenyl-N'-dimethylureido-l,3,2-dioxaborolane, excess isocyanate giving the same product. The structure was confirmed by analysis and i.r. spectrum which showed that the dioxaborolane ring was intact and that the carbonyl stretching frequency was in the same position as for analogous ureidoboranes. This indicates that the reaction was exocyclic i.e. R M A R2N > OR. 7. R. H. Cragg, M. F. Lappert and B. P. Tilley, J. chem. Soc. 2108 (1964).
2-amino- 1,3,2-dioxaborolanes and their properties
399
EXPERIMENTAL
General procedures Diethyl ether, light petroleum (b.p. 40-60°) and benzene were stored over sodium wire and distilled before use. Infra-red spectra were recorded as liquids or mulls using a Perkin-Elmer 227 spectrophotometer. Molecular weights were determined using a Mecrolab osmometer and MS9 mass spectrometer. In the synthesis of all 2-alkyl- and 2-dialkylamino-l,3,2-dioxaborolanes two methods were used, for each of which one example is given. The remainder of the data appears in Table 1.
Preparation of 2-dimethylamino- 1,3,2-dioxaborolane Dimethylamine (19-2 g, 2.97 mols.) in light petroleum was added to 2-chloro-1,3,2-dioxaborolane (15.2 g, 1 tool.) in the same solvent at --78 °. The mixture was allowed to attain room temperature. A white precipitate dimethylamine hydrochloride (11-7 g, 100 per cent) was formed. This, when filtered off and freed of solvent, afforded 2-dimethylamino-l,3,2-dioxaborolane (10-3 g, 64 per cent) b.p. 45°/15 mm. (Found: N, 12-2; B, 9.1. Calculated for C4H1002NB N, 12-1; B, 9.3 percent.)
Preparation of 2-methylamino-1,3,2-dioxaborolane Methylamine (2.95 g, 1.93 mol.) was added to 2-diethylamino-l,3,2-dioxaborolane (6.3 g, 1 mol.) in petroleum spirit at --78 °. A precipitate was formed immediately and on removal of the solvent 2-methylamino-l,3,2-dioxaborolane (4.5g, 93 per cent) m.p. 90-1 °. (Found: N, 13.3; B, 10.6. CzH802NB requires N, 13.7; B, 10.6) was obtained.
Interaction of phenyl isocyanate and 2-dimethylamino-1,3,2-dioxaborolane Phenyl isocyanate (2"6g, 1 mol.) and 2-dimethylamino-l,3,2-dioxaborolane (2.75 g, 1.1 mole) were refluxed in benzene for 2 hr. The solvent was removed, leaving a viscous residue. After washing with a mixture of diethyl ether and petroleum spirit, the residue afforded N-phenyl-N'-dimethylureido1,3,2-dioxaborolane (4.1 g, 80 per cent) m.p. 92-5 °. (Found: N, 12.0; B, 4.6. C11H15N~O3B requires N, 12.0; B, 4.4 per cent.)
Acknowledgements-The initial stages of this work were carried out during the tenure of the Lord Adams Fellowship at the University of Newcastle-upon-Tyne.
2-AMINO-1,3,2-DIOXABOROLANES PROPERTIES
AND
THEIR
R. H. C R A G G The Chemical Laboratory, University of Kent at Canterbury (Received3 May 1967)
Abstract-The preparation and properties of some 2-amino- 1,3, 2-dioxaborolanes, CH2---O
I
~BNRR' /
CH2"--4) (R = R' = Me; R = H and R' = Me, Et, Pr', Bun, Bu ~, Bu t and Ph) are described and their association is discussed. Reaction of phenyl isocyanate with 2-dimethylamino-l,3,2-dioxaborolane is exocyclic showing that the relative migratory aptitude is R2N > OR.
THERE have been several publications concerning molecular association in dioxaborolanes [ 1-5] where the association is through the boron-oxygen bond as represented in Fig. 1. CH2---O
X
\/ B~
O~CHz
/ //\
CH2"---O
)B
X
I
O---CH2
Fig. I.
The first examples of association studies, on 2-amino and 2-dialkylamino-l,3,2dioxaborolanes are here reported, where the association is through the boronnitrogen bond as represented in Fig. 2. R
H
CH2---O
~
/
CH2--O
/ R
~
O--CH2
H
O---CH2
Fig. 2. 1. 2. 3. 4. 5.
A. Finch and P. J. Gardner, J. inorg, nucl. Chem. 25, 927 (1963). R.J. Brotherton and A. L. McCIoskey, J. org. Chem. 26, 1668 (1960). E.W. Abel, S. H. Dandegaonker, W. Gerrard and M. F. Lappert,J. chem. Soc. 4116 (1957). J. Dale, B. Hargitay and A. J. Hubert, J. chem. Soc. 9 ! 0, 922 and 931 ( 1961). M. F. Lappert, M. K. Majumdar and B. P. Tilley, J. chem. Soc. (.4) 1590 (1966). 395
396
R.H.
CRAGG
However on the evidence available structures of the type (1) are equally possible. Association studies of analogous 2-amino-1,3,2-benzodioxaboroles have recently been published[5] and dimerisation of linear non cyclic boron compounds is well known. RESULTS AND DISCUSSION This is the first of a series of papers which are dealing with the effect of (a) electronegativities and (b) steric factors on the dimerisation of aminoboranes. The studies being made are on compounds of the type CH2----X
\ /
B-NRR'
CH2----X where R = R' = alkyl and R = alkyl R' = H and X = O, S, N, P and As. The 2-amino-l,3,2-dioxaborolanes were prepared by the interaction of 2-diethylamino- 1,3,2-dioxaborolane and the corresponding amine (Equation (1)). CH2---O
CH2--O ,
B-NEt~ + RNH2
H,--O/
\
l
jB-NHR
+ EtcNH.
(1)
CH2---O
The utilisation of the above method was made necessary by the fact that the aminoborane, being polymeric, and amine hydrochloride are difficult to separate. However, the usual method of preparing aminoboranes is by the reaction of chloroborane and amine (Equation (2)) and this proved successful in the preparation of dialkylamino derivatives. CH2---O I CHr-O
\ /
CH2--O B-CI + 2R2NH
,
\ /
BNR2 + R2NH'HCI.
(2)
CH2--O
The characteristic data of the 2-amino-l,3,2-dioxaborolanes is given in Table 1. The molecular weights of some of the 2-amino and dialkylamino-l,3,2-dioxaborolanes were determined by two methods (a) mass spectrometric and (b) osmetric, and the results are given in Table 2. The majority of the amino derivatives were oligomeric solids and due to their insoluble nature molecular weight determinations could not be carded out. It was hoped that the degree of association could be obtained mass spectrometrically but in the two determinations made the most intense peaks were at the position for a monomer showing that at least in the gas phase the compounds were monomeric. However, there were less intense peaks of higher molecular weight whose pattern suggested the presence of more than one boron atom per molecule. The molecular weights in benzene,
2-amino-1,3,2-dioxaborolanesand theirproperties Table
397
1
Analyses Compound CH2--O
\
Yield m.p.
(%)
Found
Required
N
B
N
B
B-NHEt
86-8 °
88
12.2
9-4
12.6
9.8
B-NHPr'
liquid
76
I 1.1
7.9
10.9
8.3
~B_NHBun
66-9 °
82
9.3
7.4
9.7
7.6
~'B_NHBu l
73-5 °
84
9.3
7.9
9"7
7"6
~B_NHBut
liquid
78
9.9
7.2
9.7
7-6
\B-NHPh
90-2 °
85
8-8
7-0
8-6
6-7
/ CH2--O CH2--O
\ /
CH2--O CH2--O
I
/
CH2---O CHz---O
/ CH2--O CH2--O
I
/
CH2---O CH2~
/
CH2---O
determined osmometrically, varied in such a way that it could only be concluded that isolated molecular weight determinations in solution of compounds of this type are very misleading. B "NMR determinations were unsatisfactory as it was found that the chemical shifts, [wrt (MeO)aB], were all about 7 ppm. From this it would appear that the effect of substituents on the nitrogen, because of the nitrogen quadrepole moment, have very little effect on the boron atom and all that is measured, in fact, is the chemical shift of the ring. The i.r. spectra of all t' e compounds were recorded. The region between 1 5 5 0 - 1 0 0 0 c m -1 was very complex and it is therefore very difficult to make unequivocal assignments for B - O and B - N stretching modes. However, the B - N stretching frequencies are tentatively assigned in the region 1530-1545 cm -1 and the B-O stretching frequencies in the region 1 3 0 2 - 1 2 3 0 c m -1. This has been interpreted[6] to mean that the B - N bond order is higher than normal and the 6. J. A. Blau, W. Gerrard, M. F. Lappert, B. A.
Mountfieldand H. Pyszora,J. chem. Soc. 380 (1960).
398
R. H. CRAGG Table 2 Molecular weight Compound
Association (%)
Found
Calculated
460 115"
114"8
4
309
142.8
2.2
101"
100"8
1
341
128.8
2-7
300
142.8
2.1
CH2---O
CH~-O
1
CHc-O ~B-NEh / CHf--O CHf--O ~'B-NHM, / CH2---O CH2--O I ~B-NHPr' / CH2---O
CHe--O I
~B_NHBut / CH~--O
*These two determinations were carried out mass spectrometrically.
B - O bond order lower than normal which shows that the contributions from a structure of the type (a) are far greater than that from a structure (b) CH2~O
\- ÷/ ~N / \
CH2---O
(a)
+ CH2---O
/
B---N
\
CH2--O
(b)
Aminoboration and alkoxyboration of isocyanates is well known[7]. Phenyl isocyanate and 2-dimethylamino-l,3,2-dioxaborolane react in a 1 : 1 molar ratio to give N-phenyl-N'-dimethylureido-l,3,2-dioxaborolane, excess isocyanate giving the same product. The structure was confirmed by analysis and i.r. spectrum which showed that the dioxaborolane ring was intact and that the carbonyl stretching frequency was in the same position as for analogous ureidoboranes. This indicates that the reaction was exocyclic i.e. R M A R2N > OR. 7. R. H. Cragg, M. F. Lappert and B. P. Tilley, J. chem. Soc. 2108 (1964).
2-amino- 1,3,2-dioxaborolanes and their properties
399
EXPERIMENTAL
General procedures Diethyl ether, light petroleum (b.p. 40-60°) and benzene were stored over sodium wire and distilled before use. Infra-red spectra were recorded as liquids or mulls using a Perkin-Elmer 227 spectrophotometer. Molecular weights were determined using a Mecrolab osmometer and MS9 mass spectrometer. In the synthesis of all 2-alkyl- and 2-dialkylamino-l,3,2-dioxaborolanes two methods were used, for each of which one example is given. The remainder of the data appears in Table 1.
Preparation of 2-dimethylamino- 1,3,2-dioxaborolane Dimethylamine (19-2 g, 2.97 mols.) in light petroleum was added to 2-chloro-1,3,2-dioxaborolane (15.2 g, 1 tool.) in the same solvent at --78 °. The mixture was allowed to attain room temperature. A white precipitate dimethylamine hydrochloride (11-7 g, 100 per cent) was formed. This, when filtered off and freed of solvent, afforded 2-dimethylamino-l,3,2-dioxaborolane (10-3 g, 64 per cent) b.p. 45°/15 mm. (Found: N, 12-2; B, 9.1. Calculated for C4H1002NB N, 12-1; B, 9.3 percent.)
Preparation of 2-methylamino-1,3,2-dioxaborolane Methylamine (2.95 g, 1.93 mol.) was added to 2-diethylamino-l,3,2-dioxaborolane (6.3 g, 1 mol.) in petroleum spirit at --78 °. A precipitate was formed immediately and on removal of the solvent 2-methylamino-l,3,2-dioxaborolane (4.5g, 93 per cent) m.p. 90-1 °. (Found: N, 13.3; B, 10.6. CzH802NB requires N, 13.7; B, 10.6) was obtained.
Interaction of phenyl isocyanate and 2-dimethylamino-1,3,2-dioxaborolane Phenyl isocyanate (2"6g, 1 mol.) and 2-dimethylamino-l,3,2-dioxaborolane (2.75 g, 1.1 mole) were refluxed in benzene for 2 hr. The solvent was removed, leaving a viscous residue. After washing with a mixture of diethyl ether and petroleum spirit, the residue afforded N-phenyl-N'-dimethylureido1,3,2-dioxaborolane (4.1 g, 80 per cent) m.p. 92-5 °. (Found: N, 12.0; B, 4.6. C11H15N~O3B requires N, 12.0; B, 4.4 per cent.)
Acknowledgements-The initial stages of this work were carried out during the tenure of the Lord Adams Fellowship at the University of Newcastle-upon-Tyne.