Organocatalytic versus Iron‐Assisted β‐Boration of Electron‐Deficient Olefins

Abstract

In the light of current debates surrounding sustainable and green chemistry, iron has now become an attractive alternative to the most commonly used expensive noble metals in a number of homogenous catalytic reactions, as it is abundant, inexpensive, less toxic, and environmentally more acceptable. [1] Despite the intense interest in the homogenous catalytic applications of iron complexes, the only known examples of iron-mediated C À B bond formation are the 1,4-hydroboration of 1,3-dienes to obtain linear (E)-gdisubstituted allylboranes, [2] and the C À H activation/borylation of furans and thiophenes catalyzed by a half-sandwich iron N-heterocyclic carbene complex. [3] Consequently, we became interested in exploring the benefits of iron in the bboration of electron-deficient olefins, [4] comparing it with the known catalytic systems of copper [5] and nickel [6] complexes, and in particular with the most recently developed organocatalytic systems. [7] In this study, we focused on the bboration of a,b-unsaturated esters and imines, which are highly versatile reactions to access polyfunctionalized molecules.We started our study by exploring whether the addition of iron salts has any positive influence on the b-boration of a,b-unsaturated esters and imines with diboron reagents. Therefore, we selected bis(pinacolato)diboron (B 2 pin 2 ) as the boron source, methanol as the proton source, ethylcrotonate 1 a, and (E)-1-phenyl-N-(4-phenylbutan-2-ylidene)methanamine 1 b as the substrates for the model reactions (Scheme 1).

Enantioselective formal hydration of α,β-unsaturated acceptors: asymmetric conjugate addition of silicon and boron nucleophiles

“…The same team of authors also showed that iron(II) salts serve as Lewis acids in the (racemic) organocatalysed conjugate borylation. 90…”

Catalytic methodologies for the β-boration of conjugated electron deficient alkenes

“…Assuming the organocatalytic variant proceeded through this sort of mechanism, Fernández et al examined the influence of Lewis acidic iron salt additives, as a means of activating 71 the Michael acceptor towards conjugate addition. 51 Scheme 15 Mechanism for the copper catalysed β-boration of α,β-unsaturated species as supported by Marder et al's DFT calculations. 69 Scheme 16 Spectroscopic evidence for the proposed organocatalytic route as described by Fernández et al 48 This journal is © The Royal Society of Chemistry 2012…”

“…Building on their previous work, Fernández and Gulyás et al explored their newly devised non-metal catalysed route to the β-boration of α,β-unsaturated species, and examined the role of iron as an additive as a means of assisting this process. 51 This case will be discussed later (section 1.5), as it provides mechanistic insight to the process of boron conjugate addition.…”

Copper-catalyzed hydroboration of arylalkenes at room temperature

“…To our delight, most substituted alkyl bromides were well tolerated under the optimized conditions, leading to the desired products in moderate to good yields. Many substituents could be tolerated, including fluoro, bromo, carboxylic, and acetic groups (entries [1][2][3][4][5][6][7][8]. Similarly, the boronic acid group of 4-vinylphenylboronic acid was fine in the transformation (entry 9).…”