IJOCInternational Journal of Organic Chemistry2161-4687Scientific Research Publishing10.4236/ijoc.2016.64022IJOC-72851ArticlesBiomedical&Life Sciences Chemistry&Materials Science Pd-Catalyzed Microwave Irradiated Regioselective Aroylation Reaction of Crotyl- and Allyltrifluoroborates MohammadAl-Masum1SarahLegan1Kwei-YuLiu1Department of Chemistry, Tennessee State University, Nashville, TN, USA111120160604220232September 16, 2016Accepted: December 17, December 20, 2016© Copyright 2014 by authors and Scientific Research Publishing Inc. 2014This work is licensed under the Creative Commons Attribution International License (CC BY). http://creativecommons.org/licenses/by/4.0/

An interesting regioselectivity is observed when the mixture of potassium crotyltrifluoroborate ( 1a) and aroyl chlorides having electron-deficient and electron-rich groups is microwaved in the presence of palladium-catalyst. In the case of electron withdrawing group with phenyl ring of aroyl chlorides, isomerized α, β-unsaturated compound 3 is the product whereas electron donating group with phenyl ring of aroyl chlorides furnishes α-adduct 4. Similar aroylation reaction is also established for potassium allyltrifluoroborate ( 1b). In this case, regioselectivity is unaffected with changing electron-rich or electron-deficient groups in phenyl ring of the aroyl chlorides. Reactions proceed with, essentially in same rate, affording the corresponding aryl propenyl ketones (crotonophenones) 5 in good to high yields.

Direct Aroylation Regioselectivity Stereoselectivity Microwave1. Introduction

One of the major transformations in organic chemistry is carbon-carbon bond formation through allylation reaction. The most common source for this type of reaction is allyl-M species where M is transition metal, Mg, Sn, Si, B etc. [1] - [9] . Transition metal catalyzed cross-coupling reaction of allyl metal with aroyl chloride is rarely deliberated [9] . Organoboron reagent is extensively studied as a cross-coupling partner in Suzuki cross- coupling reaction. Because of the sensitivity and high reactivity of the allylboronic acid and allylboronic ester, they are essentially not used in cross-coupling reaction. Crotyl boronic acid and ester also show high reactivity. On the other hand, potassium allyltrifluoroborate and crotyltrifluoroborate are stable enough to use in reaction. The allyltrifluoroborate salt formed either from allylboronic acid or allylboronic ester is air- and water-stable effective organoboron reagent for cross-coupling chemistry. Similar process is used to synthesize crotyltrifluoroborate. The growing emphasis of their use in Michael reaction [10] - [14] , cross-coupling reaction [15] - [30] has interested us to explore the direct aroylation chemistry of potassium crotyl- and allyltrifluoroborates.

 2. Results and Discussion

We found a noticeable regioselectivity when the mixture of crotyltrifluoroborate and aroyl chlorides having electron-deficient and electron-rich groups was microwaved in the presence of palladium-catalyst (Scheme 1). PdCl2 (dtbpf) complex and its active use as a catalyst in various organic transformations involving potassium allyltrifluoroborate, potassium styryltrifluoroborates, and potassium aryltrifluoroborates in our laboratory inspire us to embrace cross-coupling chemistry in new reactions and methodology. It is well documented by X-ray structure that PdCl2 (dtbpf) has shown the largest P-Pd-P bite angle (104.22˚) for a ferrocenyl biphosphine ligand [31] - [34] compared to the other ferrocenyl biphosphine ligands such as PdCl2 (dippf) [θ = 103.59˚], PdCl2 (dCypf) [θ = 102.45˚], PdCl2 (dmpf) [θ = 99.3˚], PdCl2 (dppf) [θ = 97.98˚]. The high bite angle PdCl2 (dtbpf) shows may be the reason of high catalyst effect in organic trans- formations by cross-coupling reaction.

The cross-coupling reaction between crotyltrifluoroborate 1 and aroyl chlorides 2 with electron-deficient group in the phenyl rings are summarized in Table 1. When 4-trifluoromethoxy-benzoyl chloride (2a), 2-trifluoromethoxy-benzoyl chloride (2b), 2,4,5-trifluoro-benzoyl chloride (2c), cross-coupled with crotyltrifluoroborate 1a under microwave heating at 140˚C for 20 min all gave the isolated corresponding (E)- 2-methyl-1-aryl-but-2-en-1-ones 3a, 3b, and 3c respectively in good yields (Table 1).

Scheme 1. Regioselectivity.

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