An experimental and theoretical investigation of unstable Fischer chromium carbene complexes
- Authors: Makanjee, Che Azad
- Date: 2013 , 2013-03-27
- Subjects: Chromium , Organolithium compounds , Carbenes (Methylene compounds) , Organometallic chemistry , Organometallic compounds , Organochromium compounds
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4284 , http://hdl.handle.net/10962/d1002953 , Chromium , Organolithium compounds , Carbenes (Methylene compounds) , Organometallic chemistry , Organometallic compounds , Organochromium compounds
- Description: This organometallic study involves the use organostannanes and organolithiums as precursors to chromium Fischer carbene complexes. Fischer carbenes are typically electrophilic and are stabilized by a single π-donor substituent, and contain low oxidation state metals (often but not always from Group 6). They are highly reactive and can give access to a range of biologically active compounds through cyclopropanations, insertions, coupling and photochemical reactions. Synthesis and characterization of three MOM-protected α-alkoxy organostannanes was successfully carried out via a nucleophilic addition of tributylstannyllithium to suitable aldehydes, and immediate protection of the alcohol with MOM. Two N-BOC protected α-amino organostannanes were successfully synthesized and characterized via α-lithiation and tin-lithium exchange in the presence of TMEDA. Tin-lithium transmetallation of the organostannanes allowed access to the organolithiums required for the synthesis of novel Fischer carbenes. Addition of chromium hexacarbonyl to the organolithiums formed the acylpentacarbonyl chromate salt which was alkylated with Meerwein salt, resulting in the Fischer carbene and a by-product, tetrabutyltin, which proved difficult to remove. Several Fischer carbenes were synthesized and characterized, some simple and known and some novel. In silico work explored the reaction coordinate of the [2+2] cycloaddition towards the formation of β-lactams, and the photoactivation cycle that precedes this process. Computational work also showed the effect of the ligand on the stability and reactivity of the carbene. It was found that in some cases the oxygen on the ligand could negatively influence the stability of the carbene (when compared to a simple methyl carbene). A link between bond orders and back donation in Fischer carbenes was explored in an attempt to theoretically predict the stability of a range of carbenes. , Microsoft� Office Word 2007
- Full Text:
- Date Issued: 2013
- Authors: Makanjee, Che Azad
- Date: 2013 , 2013-03-27
- Subjects: Chromium , Organolithium compounds , Carbenes (Methylene compounds) , Organometallic chemistry , Organometallic compounds , Organochromium compounds
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4284 , http://hdl.handle.net/10962/d1002953 , Chromium , Organolithium compounds , Carbenes (Methylene compounds) , Organometallic chemistry , Organometallic compounds , Organochromium compounds
- Description: This organometallic study involves the use organostannanes and organolithiums as precursors to chromium Fischer carbene complexes. Fischer carbenes are typically electrophilic and are stabilized by a single π-donor substituent, and contain low oxidation state metals (often but not always from Group 6). They are highly reactive and can give access to a range of biologically active compounds through cyclopropanations, insertions, coupling and photochemical reactions. Synthesis and characterization of three MOM-protected α-alkoxy organostannanes was successfully carried out via a nucleophilic addition of tributylstannyllithium to suitable aldehydes, and immediate protection of the alcohol with MOM. Two N-BOC protected α-amino organostannanes were successfully synthesized and characterized via α-lithiation and tin-lithium exchange in the presence of TMEDA. Tin-lithium transmetallation of the organostannanes allowed access to the organolithiums required for the synthesis of novel Fischer carbenes. Addition of chromium hexacarbonyl to the organolithiums formed the acylpentacarbonyl chromate salt which was alkylated with Meerwein salt, resulting in the Fischer carbene and a by-product, tetrabutyltin, which proved difficult to remove. Several Fischer carbenes were synthesized and characterized, some simple and known and some novel. In silico work explored the reaction coordinate of the [2+2] cycloaddition towards the formation of β-lactams, and the photoactivation cycle that precedes this process. Computational work also showed the effect of the ligand on the stability and reactivity of the carbene. It was found that in some cases the oxygen on the ligand could negatively influence the stability of the carbene (when compared to a simple methyl carbene). A link between bond orders and back donation in Fischer carbenes was explored in an attempt to theoretically predict the stability of a range of carbenes. , Microsoft� Office Word 2007
- Full Text:
- Date Issued: 2013
The synthesis of α-alkoxy and α-aminostannanes as precursors to Novel Chromium Fischer Carbenes
- Authors: Meyer, Annalene
- Date: 2012
- Subjects: Alkoxides , Organometallic compounds , Carbenes (Methylene compounds) , Chromium , Molybdenum , Tungsten , Organolithium compounds
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4371 , http://hdl.handle.net/10962/d1005036 , Alkoxides , Organometallic compounds , Carbenes (Methylene compounds) , Chromium , Molybdenum , Tungsten , Organolithium compounds
- Description: The present study involves the use of main group organometallics: organostannanes and organolithiums as precursors to chromium Fischer carbene complexes. Fischer carbenes are well stabilized by the π‐donor substituents such as alkoxy and amino groups and low oxidation state metals such as Group 6 (Chromium, Molybdenum or Tungsten). Carbenes are an important intermediate in the synthesis of a range of compounds through cyclopropanations, insertions, coupling and photochemical reactions. Synthesis and successful characterisation of three α‐alkoxystannanes was achieved via nucleophilic addition of tributylstannyllithium to the respective aldehydes, followed by an immediate MOM protection of the resulting alcohol. Six α‐aminostanannes were synthesised, consisting of N‐BOC, N‐acetyl and N‐ethyl derivatives of pyrrolidine and piperidine, via α‐lithiation and subsequent tinlithium transmetallation in the presence of TMEDA. The ¹³C NMR analysis highlighted an interesting phenomenon of tin‐carbon coupling that revealed unique structural information of both types of stannanes. DFT analysis was completed on the series of stannanes; a predicted frequency analysis was obtained which complemented the experimental Infra‐red data in elucidation of the compounds. The α‐alkoxy and α‐aminostannanes provided stable precursors to the organolithiums required for the synthesis of the novel Fischer chromium carbenes. The organolithiums were obtained via tinlithium exchange at low temperatures, followed by the addition of chromium hexacarbonyl to form the acylpentacarbonyl‐chromate salt. Alkylation of this intermediate using a Meerwein salt, Me₃OBF₄, gave rise to the novel Fischer chromium carbene complexes. Fischer chromium carbenes derived from the two isomeric butyl and isobutyl stannanes and the two N‐ethyl α‐aminostannanes were successfully synthesised. The difficulty encountered in the purification of the Fischer carbene complexes hindered the full characterisation, due to the presence of a by‐product, tetrabutyltin.
- Full Text:
- Date Issued: 2012
- Authors: Meyer, Annalene
- Date: 2012
- Subjects: Alkoxides , Organometallic compounds , Carbenes (Methylene compounds) , Chromium , Molybdenum , Tungsten , Organolithium compounds
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4371 , http://hdl.handle.net/10962/d1005036 , Alkoxides , Organometallic compounds , Carbenes (Methylene compounds) , Chromium , Molybdenum , Tungsten , Organolithium compounds
- Description: The present study involves the use of main group organometallics: organostannanes and organolithiums as precursors to chromium Fischer carbene complexes. Fischer carbenes are well stabilized by the π‐donor substituents such as alkoxy and amino groups and low oxidation state metals such as Group 6 (Chromium, Molybdenum or Tungsten). Carbenes are an important intermediate in the synthesis of a range of compounds through cyclopropanations, insertions, coupling and photochemical reactions. Synthesis and successful characterisation of three α‐alkoxystannanes was achieved via nucleophilic addition of tributylstannyllithium to the respective aldehydes, followed by an immediate MOM protection of the resulting alcohol. Six α‐aminostanannes were synthesised, consisting of N‐BOC, N‐acetyl and N‐ethyl derivatives of pyrrolidine and piperidine, via α‐lithiation and subsequent tinlithium transmetallation in the presence of TMEDA. The ¹³C NMR analysis highlighted an interesting phenomenon of tin‐carbon coupling that revealed unique structural information of both types of stannanes. DFT analysis was completed on the series of stannanes; a predicted frequency analysis was obtained which complemented the experimental Infra‐red data in elucidation of the compounds. The α‐alkoxy and α‐aminostannanes provided stable precursors to the organolithiums required for the synthesis of the novel Fischer chromium carbenes. The organolithiums were obtained via tinlithium exchange at low temperatures, followed by the addition of chromium hexacarbonyl to form the acylpentacarbonyl‐chromate salt. Alkylation of this intermediate using a Meerwein salt, Me₃OBF₄, gave rise to the novel Fischer chromium carbene complexes. Fischer chromium carbenes derived from the two isomeric butyl and isobutyl stannanes and the two N‐ethyl α‐aminostannanes were successfully synthesised. The difficulty encountered in the purification of the Fischer carbene complexes hindered the full characterisation, due to the presence of a by‐product, tetrabutyltin.
- Full Text:
- Date Issued: 2012
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