& Alcohol Esterification Efficient and Simple Approaches Towards Direct Oxidative Esterification of Alcohols Ritwika Ray, Rahul Dev Jana, Mayukh Bhadra, Debabrata Maiti,* and Goutam Kumar Lahiri* [a] Abstract: The present article describes novel oxidative pro- tocols for direct esterification of alcohols. The protocols in- volve successful demonstrations of both “cross” and “self” esterification of a wide variety of alcohols. The cross-esterifi- cation proceeds under a simple transition-metal-free condi- tion, containing catalytic amounts of TEMPO (2,2,6,6-tetra- methyl-1-piperidinyloxy)/TBAB (tetra-n-butylammonium bro- mide) in combination with oxone (potassium peroxo mono- sulfate) as the oxidant, whereas the self-esterification is ach- ieved through simple induction of Fe(OAc) 2 /dipic (dipic = 2,6-pyridinedicarboxylic acid) as the active catalyst under an identical oxidizing environment. Introduction Ester groups are ubiquitous in various natural products, agro- chemicals, and in commonly used synthetic scaffolds. They constitute the backbone of several drugs as well as function as the building blocks of various polymers, such as polyethylene terephthalate (PET) (Figure 1). The characteristic odors of esters have made them useful constituents for artificial fragrances and flavoring agents of numerous products. [1] Traditional esterification strategies involving reactions of car- boxylic acids or their derivatives with alcohols [2] and carbonyla- tion of aryl halides [3] lack eco-friendliness. [4] Though one-pot oxidative esterification of aldehydes with alcohols has gained considerable attention in the recent past, [5] a more convenient and straightforward approach would be the direct esterifica- tion of alcohols primarily due to their easy availability and cost-effectiveness. [6–15] In this context, the initial approach by Milstein [6a–c] followed by Beller [6d] of acceptorless dehydrogena- tion of primary alcohols to esters by Ru-pincer complexes are commendable. [7] Precious metals such as Ru [8] and Rh [9] -cata- lyzed esterification of primary alcohols in the presence of hy- drogen acceptors [10] were also reported. A few Ir complexes were also shown to catalyze this type of reaction. [11] Palladium- catalyzed esterification of primary alcohols in the presence of O 2 as the terminal oxidant was reported by Lei [12] and Beller. [13] However, these methods made use of either an expensive Ag additive [12a] or a combination of an expensive additive and a ligand. [13] Very recently, Beller also demonstrated esterifica- tion of alcohols using reusable heterogeneous Co 3 O 4 -N@C cat- alysts. [14] However, development of more sustainable reaction protocols under homogeneous conditions demand further ex- ploration. [15] In this regard TEMPO-catalyzed oxidation of alcohols to alde- hydes and ketones in the presence of either NaNO 2 or oxone as the oxidant was reported by Hu [16a] and Bolm (Scheme 1). [16b] Furthermore, Borhan [16c] reported oxidation of aldehydes to esters and acids using oxone as the sole oxidant (Scheme 1). [17] These studies have been the incentive for the present article, in which we demonstrate a catalytic system under transition- metal-free conditions for the direct esterification of a wide vari- ety of alcohols in the presence of oxone and catalytic amounts of TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) [18–20] and TBAB Figure 1. Ester moieties in commercially available drugs and polymers. Scheme 1. Previous reports on metal free oxidation of alcohols and alde- hydes. [a] R. Ray, R. D. Jana, M. Bhadra, Prof. D. Maiti, Prof. G. K. Lahiri Department of Chemistry Indian Institute of Technology Bombay Powai, Mumbai, 400 076 (India) E-mail : dmaiti@chem.iitb.ac.in lahiri@chem.iitb.ac.in Supporting information for this article is available on the WWW under http ://dx.doi.org/10.1002/chem.201403786. Chem. Eur. J. 2014, 20, 15618 – 15624 # 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 15618 Full Paper DOI: 10.1002/chem.201403786