This thesis is focused on describing and predicting catalytic reactions. The major part of the work is based on density functional theory (DFT). In some cases where the size of the investigated system precluded the use of more accurate methods molecular dynamics was employed. In several cases the proposed mechanism was later tested in the laboratory. A few examples where the predictions were confirmed are:

• The formation of an acyl intermediate in the activation of a ruthenium catalyst used for racemizing alcohols. This intermediate was observed by both NMR and in situ FT-IR.
• The improvement of the substrate specificity and catalytic activity of Candida antarctica lipase A by modifying amino acids close to the active site.
• The improved specificity of Candida antarctica lipase B toward δ-substituted secondary alcohols by an enzyme variant where the alanine in position 281 was exchanged for a serine.

In other cases experimental results were complemented with a theoretical investigation, for example:

• The observed second order rate constant for a ruthenium based catalyst used for water oxidation was explained and a novel intramolecular mechanism based on a high valent ruthenium dimer was suggested.
• The effects of electron withdrawing/donating axial ligands on the performance of ruthenium catalyzed water oxidation were addressed.
• Mechanisms of H₂ activation by Lewis acid/Lewis base adducts were rationalized. One example of the predictive power of computational chemistry is the mechanism of hydrogen uptake by phosphanylboranes; the potential energy barrier for the transition state could be predicted within a few kcal/mol based on the orbital energies of the starting material.

Enzymes are attractive catalysts in organic synthesis since they are efficient, selective and environmentally friendly. A large number of enzyme-catalyzed transformations have been described in the literature. If no natural enzyme can carry out a desirable reaction, one possibility is to modify an existing enzyme by protein engineering and thereby obtain a catalyst with the desired properties. In this thesis, the development of enantioselective enzymes and their use in synthetic applications is described. 

In the first part of this thesis, enantioselective variants of Candida antarctica lipase A (CALA) towards α-substituted p-nitrophenyl esters were developed by directed evolution. A highly selective variant of CALA towards p-nitrophenyl 2-phenylpropanoate was developed by pairwise randomization of amino acid residues close to the active site. The E value of this variant was 276 compared to 3 for the wild type.

An approach where nine residues were altered simultaneously was used to discover another highly enantioselective CALA variant (E = 100) towards an ibuprofen ester. The sterical demands of this substrate made it necessary to vary several residues at the same time in order to reach a variant with improved properties.

In the second part of the thesis, a designed variant of Candida antarctica lipase B (CALB) was employed in kinetic resolution (KR) and dynamic kinetic resolution (DKR) of secondary alcohols. The designed CALB variant (W104A) accepts larger substrates compared to the wild type, and by the application of CALB W104A, the scope of these resolutions was extended.

First, a DKR of phenylalkanols was developed using CALB W104A. An enzymatic resolution was combined with in situ racemization of the substrate, to yield the products in up to 97% ee. Secondly, the KR of diarylmethanols with CALB W104A was developed. By the use of diarylmethanols with two different aryl groups, highly enantioselective transformations were achieved.