DSpace Community: Department of Chemistry

Synthetic and catalytic studies of Group 11 N-heterocyclic carbene complexes

Sat, 29 Oct 2005 22:58:59 GMT

Title: Synthetic and catalytic studies of Group 11 N-heterocyclic carbene complexes

Authors: Laitar, David S. (David Stephen)

Abstract: This thesis presents the synthesis, structure and reactivity of two-coordinate Group 11 metal complexes, supported by N-heterocyclic carbene (NHC) ligands. The NHC ligand was found to stabilize monomeric, terminal fluoride complexes of copper, silver and gold, the last representing the first isolable gold(I) fluoride. These complexes were shown to be reactive synthons for new metal complexes. The ability of the NHC ligand to support unusual coordination environments for these metals inspired us to explore the chemistry of copper(I) bonded to various main group elements, leading to the development of new catalytic reactions. The first well-characterized copper(I) boryl complex was synthesized and shown to react with a variety of unsaturated organic substrates. This complex reacts rapidly with CO2 to form CO and a copper(I) borate complex. The boryl complex may be regenerated by treatment of the borate complex with the bis(pinacolato)diboron, (pin)B-B(pin), giving the stable byproduct (pin)B-O-B(pin). The use of a copper(I) alkoxide precatalyst and stoichiometric diboron reagent results in catalytic reduction of CO2, with high turnover numbers (1000 per Cu) and frequencies (100 per Cu in 1 hour) depending on supporting ligand and reaction conditions.; (cont.) Carbon dioxide also inserts into the Cu-Si bond of a copper silyl complex. The resulting complex evolves CO to give a copper siloxide complex. Mesitaldehyde inserts cleanly and selectively into the Cu-B bond of (NHC)CuB(pin), to form a B-O and a copper-carbon bond. This complex reacts with bis(pinacolato)diboron to regenerate (NHC)CuB(pin) and produce an aldehyde diboration product, in which a diboron reagent has been added across the C=O bond of mesitaldehyde. A copper boryl complex with a smaller NHC supporting ligand proved to be a much more effective diboration catalyst and a wide range of aldehydes react cleanly with bis(pinacolato)diboron. The insertion of alkenes into an (NHC)copper(I) boryl affords isolable -boroalkyl complexes in high yields; competition experiments using substituted styrenes show that electron-donating substituents slow the reaction. Although the insertion products are stable at ambient temperature, a P-hydride elimination/reinsertion sequence affords a rearranged a-boroalkyl complex on heating.

Description: Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2006.; Vita.; Includes bibliographical references.

Biochemical characterization of the E. coli Very Short Patch Repair pathway and its coordination with methyltransferase repair of 0⁶-methylguanine

Sat, 29 Oct 2005 22:58:59 GMT

Title: Biochemical characterization of the E. coli Very Short Patch Repair pathway and its coordination with methyltransferase repair of 0⁶-methylguanine

Authors: Rye, Peter Thomas

Abstract: The E. coli Very Short Patch Repair (VSPR) system corrects T:G mismatches that arise through Dcm-mediated methylation and subsequent deamination of the underlined cytosine residue in the palindromic sequence 5'-CCWGG-3' (W is an adenine or thymine). Vsr initiates VSPR by producing a single stranded nick on the 5' side of the mismatched T. The MutS and MutL mismatch recognition proteins stimulate this activity, as cells lacking either of these proteins display diminished VSPR. Genetic studies also indicate that Pol I is responsible for removing and replacing a short tract of nucleotides downstream of the incision site and that DNA Ligase seals the nick to complete the repair event. However, until now, biochemical investigation of the repair steps downstream of Vsr incision have been lacking. Herein, we describe two novel in vitro assays used to probe the biochemical events of VSPR. The first was used to verify the reconstitution of VSPR using purified E. coli Vsr, Pol I, and DNA Ligase enzymes, while the second was used to measure the distribution of VSPR patch sizes in whole cell extracts.; (cont.) By monitoring the loss of radiosignal from a series of substrates that contained the label at prescribed distances downstream of the T:G mismatch, we were able to determine that VSPR patches are distributed around 2 to 4 deoxynucleotides in length. Interestingly, under certain reaction conditions, the addition of DNA Ligase improved the efficiency of repair initiation by Vsr, suggesting that VSPR may be optimal in the context of a multi-protein complex. Lastly, we investigated the effect of VSPR proteins on methyltransferase (MTase) repair of O6-methylguanine (6mG). MTase repair of O6mG opposite T results in a G:T mismatch that must be further processed to yield the native G:C base pairing. The G:T mismatch is therefore an intersection of the two pathways and led us to hypothesize that MTase and VSPR proteins might interact. Indeed, cells lacking the functions of MutS, MutL, or Vsr proteins displayed decreased MTase repair in vivo, revealing a previously unknown interaction. The cooperation between proteins of these two repair systems may shed light on the biological significance of the VSPR system.

Description: Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2006.; Vita.; Includes bibliographical references.

Synthesis of molybdenum olefin metatheses catalysts through protonation reactions

Sat, 29 Oct 2005 22:58:59 GMT

Title: Synthesis of molybdenum olefin metatheses catalysts through protonation reactions

Authors: Sinha, Amritanshu

Abstract: The attempted syntheses of molybdenum imido alkylidene complexes of the type Mo(NArc,)(CH-t-Bu)[Biphen] and Mo(N-2-CF3C6H4)(CHCMe2Ph)[Biphen] (Biphen2 = 3,3'-di-t-butyl-5,5',6,6'-tetramethyl- 1,1'-biphenyl-2,2'-diolate) from Mo(NArcl)(CH-t-Bu)(OTf)2(dme) and [Biphen]K2 have sporadically afforded mixtures containing the desired products along with the corresponding amido alkylidyne complexes, Mo(NHArcl)(C-t-Bu)[Biphen] and Mo(NH-2-CF3C6H4)(CCMe2Ph)[Biphen], respectively. The reaction of [Biphen]K2 with Mo(NArc,)(CH-t-Bu)(OTf)2(dme) and 10 equivalents of triethylamine reproducibly gave Mo(NHArc,)(C-t-Bu)[Biphen] in 40% yield. An X-ray crystal structure of a related complex, Mo(NHArc,)(CCMe2Ph)[S-Biphen] confirmed the proposed structure and also revealed that one ortho chloride approaches within 2.93 A of the metal approximately trans to the alkylidyne ligand. Attempts to prepare three other amido alkylidyne complexes in an analogous manner from Mo(NR")(CH-t-Bu)(OTf)2(dme) (NR" = N-2-CF3C6H4, N-2,6-i-Pr2C6H5, N-2,6-Me2C6H5) with [Biphen]K2 in the presence of 10-20 equivalents of triethylamine failed.; (cont.) Chapter 2 The reaction between Mo(NAr)(CH-t-Bu)(CH2-t-Bu)2 (Ar = 2,6-i-Pr2C6H3) and various alcohols (1-AdamantylOH, t-BuOH, ArOH, (CF3)2CHOH, (CF3)2MeCOH, (CF3)3COH, C6F5OH) in pentane or toluene yielded either complexes of the type Mo(NAr)(CH-t-Bu)(CH2-t-Bu)(OR) through direct addition of ROH across a Mo-C bond, or complexes of the type Mo(NAr)(CH2-t-Bu)3(OR) through direct addition of ROH across a Mo=C bond. The trineopentyl species appear to be formed when the alcohol has a relatively low pKa. The outcome also can depend upon whether the alcohol is employed neat, or in benzene, and mixtures are observed in some circumstances. The conversion of Mo(NAr)(CH2-t-Bu)3(OR) into Mo(NAr)(CH-t-Bu)(CH2-t-Bu)(OR) was shown to be unimolecular in several examples. Mo(NAr)(CH-t-Bu)(CH2-t-Bu)(OR) complexes have been found to be surprisingly active catalysts for various metathesis reactions. In contrast, M(NAr)(CH-t-Bu)(CH2-t-Bu)2 species are virtually inactive for metathesis. X-ray structures are reported for Mo(NAr)(CH2-t-Bu)3(OC6F5), Mo(NAr)(CH-t-Bu)(CH2-t-Bu)IOSi(O-t-Bu)3], [Mo(NAr)(CH-t-Bu)(CH2-t-Bu)(OC6F)12, and Mo(NAr)(CH-t-Bu)(CH2-t-Bu)(OC6F5)(PMe3).; (cont.) Chapter 3. Complexes of the type Mo(NR")(CHR')(N(R)3,5-C6H3Me2)2 (NR" = N-2,6-i-Pr2C6H, N-2,6-Me2C6Hs; R' = t-Bu, CMe2Ph; R' = i-Pr, t-Bu) and Mo(NR")(CHR')(NR2)2 (NR" = N-2,6-i-Pr2C6H,, N-2,6-Me2C6H5; R' = t-Bu, CMe2Ph; R = Me, Ph) can be isolated as orange-red solids in 30-35% yields or oils by reacting Mo(NR")(CHR')(OTf)2(dme) with LiN(R')(3,5-C6H3Me2)(ether) or with LiNR2. The synthesis of Mo(NR")(CHCMe2Ph)(NPh2)2 can be improved to 70-90% isolated yields when Mo(NR")(CHCMe2Ph)[OCMe(CF3)212 is used with LiNPh2(ether). Mo(NAr)(CHCMe2Ph)(NPh2)2 has been crystallographically characterized. Mo(NR")(CHR')(N(R)3,5-C6H3Me2)2 species reacted with t-BuOH and Me(CF3)2COH in benzene to give Mo(NR")(CHR')(OR)2 (OR = O-t-Bu, OCMe(CF3)2) in situ. However, no reactions of Mo(NR")(CHR')(N(R')3,5-C6H3Me2)2 were observed with enantiomerically pure diols such as [R-TRIP]H2 (3,3'-2,4,6-i-Pr3C6H2-binaphthol), [R-Ph]H2 (3,3'-C6H5-binaphthol), [rac-Mesitylbinap]H2 (3,3'-2,4,6-Me3C6H2-binaphthol) and [R-TMSbinapJH2 (3,3'-SiMe3-binaphthol).; (cont.) Bisamido complexes of the type Mo(NR")(CHR')(NPh2)2 were found to react with the aforementioned alcohols and diols to give Mo(NR")(CHR')(diolate) species in situ, which were further reacted in a catalytic fashion with two substrates to give the corresponding ring-closed products. Preliminary :results of the in situ catalysis demonstrated here compare fairly well with the analogous catalytic reactions reported with isolated catalysts. Appendix A. Mo(NAr)(CH-t-Bu)(CH2-t-Bu)(OC6F5) (Ar = 2,6-i-Pr2C6H3) can be reacted with 5-10 equivalents of trans-3-hexene to give a crystallographically characterized dimeric complex, [Mo(NAr)(CH2-t-Bu)(OC6F5)]2 that contains an unbridged Mo=Mo bond (2.410(8) A) in high yields. The above complex can also be prepared by treating Mo(NAr)(CH-t-Bu)(CH2-t-Bu)(OC6F5) with 0.5 equivalents of divinylbenzene. IMo(NAr)(CH2-t-Bu)(OC6F5)]2 will slowly catalyze the metathesis reactions of simple substrates, although less than 5% of the catalyst seems to be activated in such reactions.; (cont.) It was observed that catalytically active species for metathesis reactions can be generated by another Mo (d2) species, Mo(NArcl)(Biphen)(H2C=CH2)(ether) (NArc, = N-2,6-C12C6H3, Biphen2 = 3,3'-di-t-butyl-5,5',6,6'-tetramethyl-1,1'-biphenyl-2,2'-diolate) that could effect the ring-opening metathesis polymerization of norbornene. A mixture of Mo(NArcl)(Biphen)(H2C=CH2)(ether) and 20 equivalents of diallylether in benzene-d6 when treated with 10 equivalents of norbornene gives 54% conversion to dihydrofuran in 10 days.

Description: Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2006.; Vita.; Includes bibliographical references.

Spectroscopic investigations of intersystem crossing and triplet state structure in acetylene

Sat, 29 Oct 2005 22:58:59 GMT

Title: Spectroscopic investigations of intersystem crossing and triplet state structure in acetylene

Authors: Thom, Ryan L. (Ryan Louis), 1978-

Abstract: A multispectral study employing both Ultraviolet Laser Induced Fluorescence (UV-LIF) and Surface Electron Ejection by Laser Excited Metastables (SEELEM) spectroscopies has yielded a complete characterization of singlet-triplet interactions relevant to intersystem crossing in the region of the 3v3 vibrational level of the A 1A, excited electronic state of acetylene. Intersystem crossing proceeds through a "doorway" state which has been plausibly assigned as a specific low-lying vibrational level of the third triplet electronic state, T3. The predominantly T1,2 dark states observed in the SEELEM spectrum show the remarkable property of spectral regularity, in the sense that they are able to be arranged into smooth, well-behaved "vibrational levels" as judged from reduced rotational term value plots. This fact, together with the observation of essentially all the expected triplet density of states, strongly suggest that these triplet states fall into the "strongly coupled," highly mixed regime.

Description: Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2006.; Vita.; Includes bibliographical references (p. 93-96).