Functionalization of closo‐Borates via Iodonium Zwitterions

Abstract

Asimple method for the functionalization of closoborates [closo-B 10 H 10 ] 2À (1), [closo-1-CB 9 H 10 ] À (2), [closo-B 12 H 12 ] 2À (3), [closo-1-CB 11 H 12 ] À (4), and [3,3'-Co(1,2-C 2 B 9 H 11 ) 2 ] À (5)i sd escribed. Treatment of the anions and their derivatives with ArI(OAc) 2 gave aryliodonium zwitterions,w hichw ere sufficiently stable for chromatographic purification. The reactions of these zwitterions with nucleophiles provided facile access to pyridinium, sulfonium, thiol, carbonitrile,a cetoxy, and amino derivatives.T he synthetic results are augmented by mechanistic considerations. Scheme 1. closo-Borates used in this study.S pheres represent aC À H fragment, and all other vertices are BÀHf ragments.Scheme 2. Functionalization of closo-borates. X = B(n=2), X = C (n = 1).

Regioselective Functionalization of the [closo-1-CB9H10]− Anion through Iodonium Zwitterions

“…Recently, we described a simple and efficient pathway to zwitterionic aryliodonium closo-borates, including anions 1a and 2a, and demonstrated their facile transformation to a range of derivatives through a nucleophilic displacement of the iodoarene (Figure 3). 30 We also found that ArI(OAc) 2 exhibits the highest regioselectivity in electrophilic substitution of the anions and, in reactions with anion 2a, gives rise to an ∼1:2 ratio of the 10-vs 6-substituted products, [closo-1-CB 9 H 9 -10-IPh] (5 [10]a) vs [closo-1-CB 9 H 9 -6-IPh] (5 [6]a). This suggested an opportunity to access a broad range of 10substituted derivatives through a direct electrophilic substitution of the cluster, followed by reactions with nucleophiles.…”

“…During the reaction, the 5 [6]b isomer was converted to the 6-acetonitrilium zwitterion [closo-1-CB 9 H 8 -1-COOH-6-NCMe] (7 [6]b), which, upon acidic hydrolysis, gave amino acid [closo-1-CB 9 H 8 -1-COOH-6-NH 3 ] (8 [6]b) in 54% overall yield. 30 This method was used to isolate the isomerically pure 5 [10]a in an overall unoptimized yield of 22%, based on the parent anion 2a (Table 1) or ∼9%, based on B 10 H 14 . 32 The regioisomeric iodonium zwitterions also demonstrate significant differences in reactivity toward strong nucleophiles in weakly nucleophilic solvents, which can be used for selective transformations of one isomer into the desired derivative in the presence of another.…”

“…Similar to that observed with the isomeric mixtures of iodonium zwitterions, reactions of 5 [10] can be conducted either with neat nucleophiles or with strong nucleophiles in MeCN solutions. The former is particularly effective for weakly nucleophilic reagents, such as MeCN, thian, 30 and Me 2 NCHS. For instance, reactions of 5 [10]a with pyridine and Me 2 NCHS at 45 and 80 °C, respectively, gave the expected derivatives 9[10]a and 12 [10]a, respectively, as the only products isolated in yields of ∼90% (see Scheme 3).…”

“…Preliminary experiments indicated that, unlike isomers of other clusters (including 1a), isomers 5 [10]a and 5 [6]a could not be separated by chromatography, but they exhibited a substantial difference in thermal stability, allowing for selective reactions. 30 Indeed, this was demonstrated for a mixture of isomers [closo-1-CB 9 H 8 -1-COOH-6-IPh] (5 [6]b) and [closo-1-CB 9 H 8 -1-COOH-10-IPh] (5 [10]b) with warm MeCN, in which the former selectively reacted, while the pure antipodal analogue 5 [10]b was isolated in an overall yield of 25%, based on the starting acid [closo-1-CB 9 H 9 -1-COOH] − (2b). 30 The subsequent reaction of 5 [10]b with 4-heptyloxypyridine gave [closo-1-CB 9 H 8 -1-COOH-10-(NC 5 H 4 OC 7 H 15 -4)] (6 [10]b) in six steps and an overall yield of ∼9%, based on B 10 H 14 , which is a shorter and more-efficient (by a factor of >4) sequence than that reported previously.…”

“…30 Indeed, this was demonstrated for a mixture of isomers [closo-1-CB 9 H 8 -1-COOH-6-IPh] (5 [6]b) and [closo-1-CB 9 H 8 -1-COOH-10-IPh] (5 [10]b) with warm MeCN, in which the former selectively reacted, while the pure antipodal analogue 5 [10]b was isolated in an overall yield of 25%, based on the starting acid [closo-1-CB 9 H 9 -1-COOH] − (2b). 30 The subsequent reaction of 5 [10]b with 4-heptyloxypyridine gave [closo-1-CB 9 H 8 -1-COOH-10-(NC 5 H 4 OC 7 H 15 -4)] (6 [10]b) in six steps and an overall yield of ∼9%, based on B 10 H 14 , which is a shorter and more-efficient (by a factor of >4) sequence than that reported previously. 29,31 This result suggested that the iodonium zwitterion method is promising for regioselective functionalization of the [closo-1-CB 9 H 10 ] − (2a) and significant expansion of the types of available substituents.…”