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General organotin chemistry
Chapter 1
General organotin chemistry
Organotin compounds are defined as containing at least one tin-carbon bond. Thefirstdescription dates back to 1849, when Frankland synthesized diethyltin diiodide, Et2SnI2. Few papers were published before the period 1910-1950, when interest gradually rose but the annual number of publications remained less than 15. The mid-century marks the real take-off of organotin chemistry, probably stimulated by the development of industrial applications. By the end of 1965 about 3000 papers had been published, and by 1980 some 1000 papers were appearing annually. The number of reports devoted to applications in organic synthesis has grown dramatically since 1970. While this book is not intended to be exhaustive, more than 350 references from 1983 and 1984 are quoted.
1.1 Nomenclature The more metallic character of tin compared with silicon in group IV has led to a nomenclature in which organotins are regarded as derivatives of the metal by using 'tin' as a suffix; for example, Bu4Sn, tetrabutyltin; Me3SnCl, trimethyltin chloride; Ph3SnOH, triphenyltin hydroxide; Bu3SnOSnBu3, bis(tributyltin) oxide; Bu2Sn(OMe)2, dibutyltin dimethoxide; Bu3SnH, tributyltin hydride. However, an older system is still often used, by analogy with organic or organosilicon chemistry. Here organotins are considered as deriving from stannanes Sn„H2„+2; for example, Me3SnSnMe3, hexamethyldistannane; Ph 3 SnH, triphenylstannane; (Bu 3 Sn) 2 NMe, N-methylhexabutyldistannazane; MeCH=CHCH(OEt)SnBu3, l-ethoxy-2-butenyltributylstannane. For complex molecules it is convenient to regard an organotin group as a substituent for naming purposes; for instance: 2-trimethylstannylpentane 3-tributylstannylcyclohexanone ethyl 3-tributylstannyl-(E)-2-butenoate 3
4
General organotin chemistry
1.2 Literature Numerous specialized books or review articles are available on organotin chemistry in general. Developments from the early days to 1960 have been reviewed1, while two monographs2'3 and a multi-volume work4 cover the field up to 1970. Another review5 deals with the period 1970-1980. Significant chapters on organotin chemistry are included in larger treatises6"9. Besides a compilation10 of preparative methods and physical constants from 1937 to 1964 and a section in a recent dictionary11 of organometallic compounds, the Gmelin Handbuch is covering organotins comprehensively and several volumes have already appeared . Organic synthesis is treated more particularly in review articles13"22, one of which emphasizes the progress made between 1970 and 1976, a period in which the use of tin in organic synthesis really began to grow16. In addition to these general references, information or reviews can be found dealing more particularly with compounds containing Sn-H 23 ' 24 , Sn-O 25 " 27 , Sn-N27"29, Sn-C30"32, Sn-S 26 ' 33 , Sn-alkali metal34 and Sn-halogen35 bonds. Finally, papers on organotin chemistry are regularly surveyed in two annual surveys published in Journal of Organometallic Chemistry Library (Organometallic Chemistry Reviews) by Elsevier Science Publishers (Amsterdam) and in the Specialist Reports published by the Royal Society of Chemistry (London).
1.3 Tin and its bonds Tin has a 5s25p2 electronic configuration. While organotin(II) is known, organotin(IV) is the usual form; with few exceptions, monomeric stannylenes R2Sn exist as short-lived intermediates which are readily transformed into tin(IV) oligomers (R2Sn)„. Quadrivalent organotins often present the tetrahedral sp3 hybridization. This is so for tetraorganotins, hexaorganoditins, organotin hydrides and most thiotin derivatives: Me 1 Sn
Me Me
4
Ph
\M e
\Sn / Ph
Ph
Ph
Sn # —Ph
\Ph
Bu 1
Bu »»HI Sn
Bu
/ \H
However, when tin bears more electronegative substituents its Lewis acidity increases and coordination with electron-rich sites leads to sp3d (trigonal bipyramid) or sp3d2 (octahedral) hybridization. Accordingly, acid-base complexes are obtained and the compounds may show intramolecular coordination or autoassociation in the solid state or in solution, leading to dimers or polymers: CL Me—Sn s * \ ^Me
Some physical data
o
NMe2
/\^OMe
2
Sii
r^Y'
Ph
'""//sn^O ^yj
^ ,
I ^ph
ci L ^ u ^ OMe
Br
OMe % I *Sn—OMe Bu t I MeO — S n * B u | ^ Bu OMe B
5
Me Me ; I I —CN—Sn — CN^Sn—-CN 4% 4% Me Me Me Me
\
The covalent radius of tin is 0.14nm and consequently bonds to tin are long; average values in nm are Sn-C 0.22, Sn-H 0.17, Sn-Cl 0.24, Sn-O 0.21, Sn-S 0.24, Sn-Sn 0.28. In spite of the differences in electronegativity the bonds are commonly considered as essentially covalent but easily polarizable: v 8+ -^Sn
8. Y
In consequence, organotins show little ionization in solution and most are poorly soluble in water. Long bonds are associated with low bond dissociation energies36 which facilitate homolytic reactions more readily than is the case for instance with organosilicon compounds. This is particularly true for Sn-H bonds in organotin hydrides and for tin-allyl bonds37. In addition, whereas bond dissociation energies show that silicon has more affinity towards oxygen than tin has, tin empirically shows much more affinity towards sulphur than does silicon, although no accurate value for the Sn-S dissociation energy appears to be available. This difference can be easily understood in terms of the HS AB principle, tin being a softer acid than silicon. Finally, the larger size of the tin atom means not only a lower thermodynamic stability but also a higher kinetic reactivity, related to the greater accessibility of the metal atom. However, this reactivity does not imply instability under ordinary conditions. Most compounds are easily manipulated in air, are insensitive to moisture and can be stored for long periods. Only a few linkages, in particular Sn-O bonds in alkoxides and Sn-N bonds in amines, necessitate the use of inert atmospheres.
1.4 Some physical data Most organotin compounds of interest for organic synthesis are liquid or solid and soluble in the usual organic solvents. Only a few compounds show appreciable insolubility, like polymeric dialkyltin oxide, R2SnO, or, to a lesser extent, organotinfluorides,R3SnF. Physical states under normal conditions are indicated by the boiling and/or melting points in Table 1.1. Organotin compounds can be investigated by all the usual Chromatog raphie and spectroscopic techniques. An important point for mass spectrometry is that tin has 10 naturally occurring isotopes (Table 1.2). Proton NMR analysis can be assisted in some cases by coupling with the
6
General organotin chemistry
Table 1.1 Organotin compounds: boiling points and melting points9 Compound Me4Sn Bu4Sn Ph4Sn Me3SnCl Bu3SnCl Bu3SnF Ph3SnCl Bu2SnCl2 Bu3SnOMe (Bu3Sn)20
M.pJ°C
225 37-38
223-226.5
Ph3SnOH
120
Compound
77/760 145/10
PhC02SnBu3 Me3SnH Bu3SnH 28 Ph3SnH Bu6Sn2 Ph6Sn2 237 Me3SnNMe2 Bu3SnNMe2 Bu2Sn(NMe2)2 Bu2Sn(SMe)2 (Ph3Sn)2S 144-145
152-154/760 152-156/14
248-252 105 40-41
Bu2Sn
B.p./°C/mmHg
91-94/0.1 90/0.1 154-158/0.2
M.p./°C B.p.l°C/mmHg 166-168/1 59/760 76-81/0.7 168-170/0.5 158/0.02 126/760 86/0.1 72/0.05 81/0.1
Table 1.2 Naturally occurring isotopes of tin 117
Isotope
112
114
115
116
Abundance/% Nuclear spin
0.95 0
0.65 0
0.34 Vi
14.24 7.57 Vi 0
118
119
24.01 8.58 0 Vi
122
124
32.97 4.71 0 0
120
5.98 0
non-zero-spin tin isotopes, but the massive absorptions due to large alkyl groups bound to tin sometimes partly mask the information. The 9Sn FT NMR spectra, usually recorded under proton decoupling, are convenient for investigation of organotin mixtures .
1.5 Toxicity and hazards Today there is considerable industrial activity in organotin chemistry because of its large-scale applications in the fields of polymer stabilization, biocides and catalysts. Because of the toxicity of organotins, criteria and recommendations for occupational exposure to organotins have been published by the US Department of Health, Education and Welfare (National Institute for Occupational Safety and Health)39, and the World Health Organization has presented a preliminary report on the biological effects of organotins and related environmental problems40. It is clear that the disastrous poisoning episode in France in 1954, caused by a medicine containing ethyltins, has led to a general distrust of these chemicals41. In fact only a few organotin compounds are highly toxic, though not more so than a number of reagents currently used in synthesis. The others are of moderate, low or very low toxicity. In contrast to those of other heavy metals like mercury or lead, the inorganic compounds are not toxic. In the series R„SnY4_„ the highest biological activity occurs at n = 3 (triorganotin compounds). Diorganotin derivatives, R2SnY2, are less toxic than R3SnY, but R4Sn show enhanced delayed toxicity due to in vivo transformation into R3SnY. Monoorganotins, RSnY3, have only a very small activity. The Y moiety has little effect, but the nature of the organic group R is very important. The maximum mammalian toxicity occurs with
Toxicity and hazards
7
R = Me or Et. There is a marked decrease for larger groups such as Bu or Ph and a minimum activity is observed for even longer chains such as Oct (dioctyltin compounds are permitted for stabilizing polymers in contact with food). Some experimental LD50 values for oral administration to rats provide a more quantitative picture (Table 1.3), showing ranges of values observed in various investigations42. Table 1.3 Organotin compounds: LD50 for oral administration to rats Compound LD^mgkg'1
Compound LD^mgkg'1
Compound LD5(Jimgkg~1
Me3SnCl Et3SnCl Bu3SnCl (Bu3Sn)20 Ph3SnCl Oct3SnCl
Me2SnCl2 Et2SnCl2 Bu2SnCl2 Bu2SnO
74-237 66-94 112-219 487-520
MeSnCl3
575-1370
BuSnCl3
2200-2300
Oct2SnCl2
>4000
OctSnCl3
>4000
9-20 10 122-349 148-234 118-135 >4000
As demonstrated in this volume, the more numerous applications in organic synthesis involve butyltins and to a lesser extent methyl- and phenyl-tins, mainly in the R3SnY state. Because of their toxicity it is recommended that work with ethyl compounds should be avoided and methyltins should be used only when they provide special capabilities beyond the scope of butyltins. To limit laboratory hazards it is necessary to avoid contact with the reagents. Tributylin compounds, for instance, can induce temporary skin burns, with the danger of absorption through the skin. Inhalation of dust or vapours of the most volatile compounds must also be avoided. Helpfully, trimethyltin compounds have strong and unpleasant odours, acting as a warning. The hazards can be minimized, and in fact have been minimized in the authors' laboratory for ca. 25 years, by simple safety measures such as wearing gloves, using good fume hoods and cleaning used glassware first in alkali solutions. Accidental spillage of reagents is efficiently controlled by the use of an absorbent such as sawdust. In the environment, organotins progressively lose their organic groups, leading to less toxic species (Scheme 1.1). R 4 Sn-
R3Sn--
- R2Sn :
RSn<
Sn0 2
Scheme 1.1
However, specialized waste disposal is recommended.
General organotin chemistry
Organotin compounds are defined as containing at least one tin-carbon bond. Thefirstdescription dates back to 1849, when Frankland synthesized diethyltin diiodide, Et2SnI2. Few papers were published before the period 1910-1950, when interest gradually rose but the annual number of publications remained less than 15. The mid-century marks the real take-off of organotin chemistry, probably stimulated by the development of industrial applications. By the end of 1965 about 3000 papers had been published, and by 1980 some 1000 papers were appearing annually. The number of reports devoted to applications in organic synthesis has grown dramatically since 1970. While this book is not intended to be exhaustive, more than 350 references from 1983 and 1984 are quoted.
1.1 Nomenclature The more metallic character of tin compared with silicon in group IV has led to a nomenclature in which organotins are regarded as derivatives of the metal by using 'tin' as a suffix; for example, Bu4Sn, tetrabutyltin; Me3SnCl, trimethyltin chloride; Ph3SnOH, triphenyltin hydroxide; Bu3SnOSnBu3, bis(tributyltin) oxide; Bu2Sn(OMe)2, dibutyltin dimethoxide; Bu3SnH, tributyltin hydride. However, an older system is still often used, by analogy with organic or organosilicon chemistry. Here organotins are considered as deriving from stannanes Sn„H2„+2; for example, Me3SnSnMe3, hexamethyldistannane; Ph 3 SnH, triphenylstannane; (Bu 3 Sn) 2 NMe, N-methylhexabutyldistannazane; MeCH=CHCH(OEt)SnBu3, l-ethoxy-2-butenyltributylstannane. For complex molecules it is convenient to regard an organotin group as a substituent for naming purposes; for instance: 2-trimethylstannylpentane 3-tributylstannylcyclohexanone ethyl 3-tributylstannyl-(E)-2-butenoate 3
4
General organotin chemistry
1.2 Literature Numerous specialized books or review articles are available on organotin chemistry in general. Developments from the early days to 1960 have been reviewed1, while two monographs2'3 and a multi-volume work4 cover the field up to 1970. Another review5 deals with the period 1970-1980. Significant chapters on organotin chemistry are included in larger treatises6"9. Besides a compilation10 of preparative methods and physical constants from 1937 to 1964 and a section in a recent dictionary11 of organometallic compounds, the Gmelin Handbuch is covering organotins comprehensively and several volumes have already appeared . Organic synthesis is treated more particularly in review articles13"22, one of which emphasizes the progress made between 1970 and 1976, a period in which the use of tin in organic synthesis really began to grow16. In addition to these general references, information or reviews can be found dealing more particularly with compounds containing Sn-H 23 ' 24 , Sn-O 25 " 27 , Sn-N27"29, Sn-C30"32, Sn-S 26 ' 33 , Sn-alkali metal34 and Sn-halogen35 bonds. Finally, papers on organotin chemistry are regularly surveyed in two annual surveys published in Journal of Organometallic Chemistry Library (Organometallic Chemistry Reviews) by Elsevier Science Publishers (Amsterdam) and in the Specialist Reports published by the Royal Society of Chemistry (London).
1.3 Tin and its bonds Tin has a 5s25p2 electronic configuration. While organotin(II) is known, organotin(IV) is the usual form; with few exceptions, monomeric stannylenes R2Sn exist as short-lived intermediates which are readily transformed into tin(IV) oligomers (R2Sn)„. Quadrivalent organotins often present the tetrahedral sp3 hybridization. This is so for tetraorganotins, hexaorganoditins, organotin hydrides and most thiotin derivatives: Me 1 Sn
Me Me
4
Ph
\M e
\Sn / Ph
Ph
Ph
Sn # —Ph
\Ph
Bu 1
Bu »»HI Sn
Bu
/ \H
However, when tin bears more electronegative substituents its Lewis acidity increases and coordination with electron-rich sites leads to sp3d (trigonal bipyramid) or sp3d2 (octahedral) hybridization. Accordingly, acid-base complexes are obtained and the compounds may show intramolecular coordination or autoassociation in the solid state or in solution, leading to dimers or polymers: CL Me—Sn s * \ ^Me
Some physical data
o
NMe2
/\^OMe
2
Sii
r^Y'
Ph
'""//sn^O ^yj
^ ,
I ^ph
ci L ^ u ^ OMe
Br
OMe % I *Sn—OMe Bu t I MeO — S n * B u | ^ Bu OMe B
5
Me Me ; I I —CN—Sn — CN^Sn—-CN 4% 4% Me Me Me Me
\
The covalent radius of tin is 0.14nm and consequently bonds to tin are long; average values in nm are Sn-C 0.22, Sn-H 0.17, Sn-Cl 0.24, Sn-O 0.21, Sn-S 0.24, Sn-Sn 0.28. In spite of the differences in electronegativity the bonds are commonly considered as essentially covalent but easily polarizable: v 8+ -^Sn
8. Y
In consequence, organotins show little ionization in solution and most are poorly soluble in water. Long bonds are associated with low bond dissociation energies36 which facilitate homolytic reactions more readily than is the case for instance with organosilicon compounds. This is particularly true for Sn-H bonds in organotin hydrides and for tin-allyl bonds37. In addition, whereas bond dissociation energies show that silicon has more affinity towards oxygen than tin has, tin empirically shows much more affinity towards sulphur than does silicon, although no accurate value for the Sn-S dissociation energy appears to be available. This difference can be easily understood in terms of the HS AB principle, tin being a softer acid than silicon. Finally, the larger size of the tin atom means not only a lower thermodynamic stability but also a higher kinetic reactivity, related to the greater accessibility of the metal atom. However, this reactivity does not imply instability under ordinary conditions. Most compounds are easily manipulated in air, are insensitive to moisture and can be stored for long periods. Only a few linkages, in particular Sn-O bonds in alkoxides and Sn-N bonds in amines, necessitate the use of inert atmospheres.
1.4 Some physical data Most organotin compounds of interest for organic synthesis are liquid or solid and soluble in the usual organic solvents. Only a few compounds show appreciable insolubility, like polymeric dialkyltin oxide, R2SnO, or, to a lesser extent, organotinfluorides,R3SnF. Physical states under normal conditions are indicated by the boiling and/or melting points in Table 1.1. Organotin compounds can be investigated by all the usual Chromatog raphie and spectroscopic techniques. An important point for mass spectrometry is that tin has 10 naturally occurring isotopes (Table 1.2). Proton NMR analysis can be assisted in some cases by coupling with the
6
General organotin chemistry
Table 1.1 Organotin compounds: boiling points and melting points9 Compound Me4Sn Bu4Sn Ph4Sn Me3SnCl Bu3SnCl Bu3SnF Ph3SnCl Bu2SnCl2 Bu3SnOMe (Bu3Sn)20
M.pJ°C
225 37-38
223-226.5
Ph3SnOH
120
Compound
77/760 145/10
PhC02SnBu3 Me3SnH Bu3SnH 28 Ph3SnH Bu6Sn2 Ph6Sn2 237 Me3SnNMe2 Bu3SnNMe2 Bu2Sn(NMe2)2 Bu2Sn(SMe)2 (Ph3Sn)2S 144-145
152-154/760 152-156/14
248-252 105 40-41
Bu2Sn
B.p./°C/mmHg
91-94/0.1 90/0.1 154-158/0.2
M.p./°C B.p.l°C/mmHg 166-168/1 59/760 76-81/0.7 168-170/0.5 158/0.02 126/760 86/0.1 72/0.05 81/0.1
Table 1.2 Naturally occurring isotopes of tin 117
Isotope
112
114
115
116
Abundance/% Nuclear spin
0.95 0
0.65 0
0.34 Vi
14.24 7.57 Vi 0
118
119
24.01 8.58 0 Vi
122
124
32.97 4.71 0 0
120
5.98 0
non-zero-spin tin isotopes, but the massive absorptions due to large alkyl groups bound to tin sometimes partly mask the information. The 9Sn FT NMR spectra, usually recorded under proton decoupling, are convenient for investigation of organotin mixtures .
1.5 Toxicity and hazards Today there is considerable industrial activity in organotin chemistry because of its large-scale applications in the fields of polymer stabilization, biocides and catalysts. Because of the toxicity of organotins, criteria and recommendations for occupational exposure to organotins have been published by the US Department of Health, Education and Welfare (National Institute for Occupational Safety and Health)39, and the World Health Organization has presented a preliminary report on the biological effects of organotins and related environmental problems40. It is clear that the disastrous poisoning episode in France in 1954, caused by a medicine containing ethyltins, has led to a general distrust of these chemicals41. In fact only a few organotin compounds are highly toxic, though not more so than a number of reagents currently used in synthesis. The others are of moderate, low or very low toxicity. In contrast to those of other heavy metals like mercury or lead, the inorganic compounds are not toxic. In the series R„SnY4_„ the highest biological activity occurs at n = 3 (triorganotin compounds). Diorganotin derivatives, R2SnY2, are less toxic than R3SnY, but R4Sn show enhanced delayed toxicity due to in vivo transformation into R3SnY. Monoorganotins, RSnY3, have only a very small activity. The Y moiety has little effect, but the nature of the organic group R is very important. The maximum mammalian toxicity occurs with
Toxicity and hazards
7
R = Me or Et. There is a marked decrease for larger groups such as Bu or Ph and a minimum activity is observed for even longer chains such as Oct (dioctyltin compounds are permitted for stabilizing polymers in contact with food). Some experimental LD50 values for oral administration to rats provide a more quantitative picture (Table 1.3), showing ranges of values observed in various investigations42. Table 1.3 Organotin compounds: LD50 for oral administration to rats Compound LD^mgkg'1
Compound LD^mgkg'1
Compound LD5(Jimgkg~1
Me3SnCl Et3SnCl Bu3SnCl (Bu3Sn)20 Ph3SnCl Oct3SnCl
Me2SnCl2 Et2SnCl2 Bu2SnCl2 Bu2SnO
74-237 66-94 112-219 487-520
MeSnCl3
575-1370
BuSnCl3
2200-2300
Oct2SnCl2
>4000
OctSnCl3
>4000
9-20 10 122-349 148-234 118-135 >4000
As demonstrated in this volume, the more numerous applications in organic synthesis involve butyltins and to a lesser extent methyl- and phenyl-tins, mainly in the R3SnY state. Because of their toxicity it is recommended that work with ethyl compounds should be avoided and methyltins should be used only when they provide special capabilities beyond the scope of butyltins. To limit laboratory hazards it is necessary to avoid contact with the reagents. Tributylin compounds, for instance, can induce temporary skin burns, with the danger of absorption through the skin. Inhalation of dust or vapours of the most volatile compounds must also be avoided. Helpfully, trimethyltin compounds have strong and unpleasant odours, acting as a warning. The hazards can be minimized, and in fact have been minimized in the authors' laboratory for ca. 25 years, by simple safety measures such as wearing gloves, using good fume hoods and cleaning used glassware first in alkali solutions. Accidental spillage of reagents is efficiently controlled by the use of an absorbent such as sawdust. In the environment, organotins progressively lose their organic groups, leading to less toxic species (Scheme 1.1). R 4 Sn-
R3Sn--
- R2Sn :
RSn<
Sn0 2
Scheme 1.1
However, specialized waste disposal is recommended.