Reactions of Boron Amidinates with CO₂ and CO and Other Small Molecules

Abstract

Reaction of Piers' borane, HB(C(6)F(5))(2), with either tert-butyl or isopropyl carbodiimide cleanly affords the boron amidinates HC(RN)(2)B(C(6)F(5))(2) [R = iPr (1), tBu (2)]. These species undergo a variety of insertion reactions. For example, treatment of 1 with CO(2) or excess carbodiimide gives HC(iPrN)(2)(CO(2))B(C(6)F(5))(2) (3) or HC(iPrN)(2)C(iPrN)(2)B(C(6)F(5))(2) (4), respectively. Similarly, exposure of 1 or 2 to 1 atm CO gives HC(RN)(2)(CO)B(C(6)F(5))(2) [R = iPr (5), tBu (6)], while reaction of 1 with CNtBu gives HC(RN)(2)(CNtBu)B(C(6)F(5))(2) [R = iPr (7), tBu (8)]. Compounds 1 and 2 also react with benzaldehyde, resulting in the formation of HC(RN)(2)(PhHCO)B(C(6)F(5))(2) [R = iPr (9), tBu (10)]. Compound 1 also reacts with MeCN to give HC(iPrN)(2)(MeCN)B(C(6)F(5))(2) (11) and effects heterolytic C-H cleavage to afford HC(iPrN)(iPrNH)(PhCC)B(C(6)F(5))(2) (12). In contrast, the species PhC(iPrN)(2)B(C(6)F(5))(2) (14) derived from PhC(iPrN)(2)BCl(2) (13) failed to react with any of the above substrates. These data, in addition to the isolation of HC(iPrN)(2)(C(6)F(4))BF(C(6)F(5)) (15), the product of thermolysis of 1, provide further support for the notion that the transient "open-chain" form of these amidinates is present in solution.

Frustrated Lewis Pair Chemistry: Development and Perspectives

“…[86] A similar seemingly "invisible" FLP was implied in the reactions of boron imidinates with CO 2 and CO (see Section 4). [87] More recent reports have described similar situations. For example, the reaction of the enamine piperidinocyclopentene with HB(C 6 F 5 ) 2 .…”

“…For example, despite the fact that the B-amidinate, HC( i PrN) 2 B(C 6 F 5 ) 2 exists as a four-membered ring, it behaves as a hidden FLP and reacts with CO 2 to give HC( i PrN) 2 -(CO 2 )B(C 6 F 5 ) 2 (112), [87] presumably via a transient ringopened N/B FLP (Scheme 45).…”

“…[125] Some saturated vicinal P/B and N/B FLPs exhibit similar behavior toward carbon monoxide. For example, both the FLPs 93, 8 [126] and boron-imidinates HC(NR) 2 B(C 6 F 5 ) 2 [87] (R = i Pr, t Bu) reacted with CO by cooperative P/B addition to form the bridged carbonyl adducts 178-181, respectively (Scheme 60). These compounds were characterized by X-ray diffraction and spectroscopy (178-181: IR: ñ(CO) = 1791, 1714, 1713 cm À1 ).…”

Design principles in frustrated Lewis pair catalysis for the functionalization of carbon dioxide and heterocycles

“…Since the first report of a FLP CO2 adduct by Stephan and Erker in 2009 [21], several other FLPs have been reported to sequester CO2 [22][23][24][25][26][27][28][29][30][31][32][33][34]. Although the reduction of CO2 has been shown to be possible in some instances [22,[35][36], no FLP system was able to perform efficient and selective catalytic transformations until our original report that demonstrated the catalytic activity of 1-Ph2P-2-Bcat-C6H4 (7, cat = catechol) for the hydroboration of CO2 to methoxyboranes, species that can be readily hydrolysed to methanol [37].…”

Main Group Lewis Acids in Frustrated Lewis Pair Chemistry: Beyond Electrophilic Boranes

“…Similarly, oxidation of (C 6 H 4 (NMe)) 2 PPh to (C 6 H 4 (NMeH)) 2 PF 2 Ph and reaction with t BuLi gave (C 6 H 4 (NMe)) 2 PPh which captures two equivalents of CO 2 to give (C 6 H 4 NMe(CO 2 )) 2 PPh (Scheme 28). 94 These reactions are reminiscent of that of boron-amidinates, 95 suggesting that ring opening affords a transient amido-phosphonium FLP which captures the CO 2 , thus implicating P(V) as the Lewis acid.…”