Main Group Chemistry Flashcards
How do we understand structure in solution? What techniques can we use?
Multinuclear NMR spectroscopy
What is an issue of using NMR spectroscopy of isotopes of elements with low abundances and how can we fix it?
Low natural abundance will give weak signals in NMR, difficult to detect.
Can overcome this by enriching to make abundance in sample high
What is the main issue when investigating a structure in solution?
Speciation, ligand exchange
(equilibria between different aggregation states in solution)
How can we prevent fluxionality
Record spectra a low temperatures to prevent fast exchange/ rotation. Get defined peaks rather than merging into 1
Although a grignard is formulated as RMgX there is in fact an eqm between a mixture of species what is this eqm called?
Shlenck
Factors that influence the position of equilibrium for RMgX?
Temp
Concentration of RMgX
Natures of R and X
Solvent
Primary tool for investigating the Schlenck equilibrium
NMR 25Mg 1H 13C
What additives can be added to reactions involving s-block organometallics to have profound effect on reaction
Using another s-block element for example LiCl
How can highly functionalised grignard reagents be formed?
R-X + Grignard + 1 eq. of LiCl
Enhances reactivity and functional group tolerence and produces turbo grignard reagents that would be impossible to produce from Mg metal directly
What is DOSY NMR?
Diffusion ordered NMR spectroscopy - can be used to identify different components in solution and give some indication of relative sizes (by analysis of diffusion coefficients)
Often possible to determine the MW of specific species in solution
What are pre-inverse crowns?
Mixtures of s-block organometallic bases that are highly reactive and selective reagents for deprotonations
used to prepare inverse crowns by deprotonation of substrates.
Comparing reactivity of the KMg pre-inverse crown for deprotonation of naphthalene to other metallating agents
Li-base - able to lithiate naphthalene but regioselectivity and yield is poor
Grignard - No direct metallation reported
Na - different ring structure produced, more reactive than K equivalent
What is a FLP
Frustrated Lewis Pair
When some chemical features of the lewis acid or lewis base prevent or disfavour formation of an adduct
Typical chemical features that disfavour Lewis adduct formation
Bulky substituents on LA or LB - prevent close approach and adduct formation
Non flexible linker between LA and LB - cant reach each other to form adduct
How do FLPs reversibly activate and eliminate H2?
Form a (+) phosphonium with a protic hydrogen and a (-) Borate with a hydritic hydrogen
How can we know that the FLP doesnt have a LA-LB adduct?
11B and 31P NMR
Dont see 11B NMR split by P; I=1/2
What makes FLPs in H2 activation and elimination so ground breaking?
Potential to deliver new processes that utilise H2 without the cost/ scare resource issues of using rare, expensive heavy metals
How can we probe the mechanism for H2 activation?
Isotopic labelling studies using H/D or D2
Studies suggested at a high temp get H/D exchange
Intermolecular reaction - not unimolecular
How can we utilise FLP systems?
Catalytic hydrogenation without a T.M
Limitations of FLP based catalytic hydrogenation of imines
doesn’t hydrogenate less hindered amines
Cause catalyst poisoning by coordination of N lone pairs onto B on FLP
Main points of Paper 2 (FLPs)
Catalytic reduction of substrates such as imines
FLPs heterolytically cleave H2 to give protic hydrogen and hydritic hydrogen
Reactions monitored by: DTF studies, Isolation of intermediates, isotopic labelling studies
Internal alkynes are tolerated but not terminal alkynes or alkenes - deactivate catalyst
Excellent Cis alkene selectivity w no evidence of over reduction
DFT studies show relatively small barrier for hydroboration step which is reversible, other barriers similar so intermed. not seen
What are the challenges with main group systems for redox catalysis
Readily react with small molecules via OA. However these species do not undergo the reverse process (RE)
Because strong reducing agents (ofter alkali metals) used to form low O.S in first place.
RE is thermodynamically unfavourable
Correct choice of element for redox catalysis
Phosphorus - stable in both +3 and +5 oxidation state
Bismuth - relatively stable in +3 and +1 oxidation state
Why were constrained geometries thought to be important?
P or Bi trapped in a strained geometry increases reactivity.
Bond angle increased towards planarity reduces energy of px LUMO
Changing the relative energy of the frontier orbitals means more reactive
What are the series of stoichiometric steps for main group redox catalysis?
And what kind of substrate is required to do these reactions?
Oxidative addition
Ligand metathesis
Reductive elimination
Electron poor substrate - Nu attack at arene is a key step
Key points of Paper 3 (catalysis with simple phosphines)
Non redox Hydrodefluorination (HDF) system based on a fluorosilicate salt
Reaction monitored with: 19F NMR with internal standard for quantification, control reactions without catalyst, mechanistic studies (NMR, isolation of intermediates and testing reactivity, DFT)
Geometrically constrained phosphines not necessary - simple trialkylphosphines
Electron poor fluoroarenes/ Heteroarenes good substrates
HDF at para position of pentafluoropyridine
Aminodefluorination possible
NMR showed phosphonium intermediates in slower HDF reactions
Hydride transfer to P requires formation of fluorosilicate anions in the system.
