ARTICLE pubs.acs.org/Organometallics
Reactivity of a Germene toward Terminal Alkynes: Competition between Cycloaddition, Ene-Addition, and CH-Insertion Laura C. Pavelka and Kim M. Baines* Department of Chemistry, University of Western Ontario, London, Ontario, N6A 5B7 Canada
bS Supporting Information ABSTRACT:
A variety of terminal alkynes were added to Mes2GedCHCH2t-Bu, a naturally polarized germene. Three different modes of reactivity were observed: cycloaddition to give germacyclobutenes, ene-addition to give vinylgermanes, and addition across the acetylenic CH bond to give germylacetylenes. Mechanisms for the formation of the products are proposed. The reactivity of the naturally polarized germene toward terminal alkynes is compared to that of the analogous silene.
’ INTRODUCTION The cycloaddition reactions of silenes have attracted the attention of the synthetic organic community.1 The regio- and stereospecificity of the reactions and the presence of the main group atom, which remains available for further functionalization of the primary product, are the key features that make the cycloaddition reactions of silenes appealing. Given the importance of the cycloaddition reactions of silenes, not only as a fundamental reaction in silicon chemistry2 but also in organic synthesis, our group has explored the cycloaddition reactions of alkynes and silenes in detail.3,4 The reactivity of silenes toward alkynes has been well-studied, especially for the relatively nonpolar Brook silenes, (Me3Si)2Sid C(R)(OSiMe3); typically, alkyne addition yields silacyclobutenes regioselectively and in high yield.2 Through the use of a molecular probe, cyclopropyl alkynes 1ac (Chart 1),5 the cycloaddition of alkynes to Brook silenes was determined to proceed via a biradical intermediate (Scheme 1).3 More recently, we examined the reactivity of the naturally polarized silene Mes2SidC(H)CH2t-Bu, 2,6 toward alkynes.4 When cyclopropyl alkynes 1ac, as well as a variety of other terminal alkynes, were added to neopentylsilene 2, the major products isolated were silylacetylenes 3ai, from insertion of the terminal CH bond of the alkyne across the SidC bond (Schemes 2, 3). Of the numerous terminal alkynes investigated, only phenylacetylene and ethoxyacetylene yielded cycloadducts upon addition to 2, silacyclobutenes 4 and 5, respectively, in r 2011 American Chemical Society
Chart 1. Alkynyl Mechanistic Probes
addition to the corresponding silylacetylenes 3d and 3g (Scheme 3). A minor amount of vinylsilane 6, from formal ene-addition, was also observed in the reaction between silene 2 and phenylacetylene (Scheme 3). Since 2 did not react with the CtC triple bond of cyclopropyl alkynes 1ac, no information could be discerned regarding the mechanism of alkyne addition using this approach. Given the preference for insertion into the CH bond of terminal alkynes, silene 2 does not appear to be a general substrate for the synthesis of organic ring systems. The preference for CH-insertion over cycloaddition was attributed to the polarity of silene 2 and provided an interesting example of how the polarity of the SidC bond can directly influence the type of products formed in a given reaction. Germenes are inherently less polar than silenes.7 Thus, the use of a naturally polarized germene, such as Mes2GedC(H)CH2tBu, 7,8 may lead to alkyne cycloadducts in higher yields Received: January 19, 2011 Published: March 24, 2011 2261
dx.doi.org/10.1021/om200048p | Organometallics 2011, 30, 2261–2271
Organometallics Scheme 1. Mechanism of Alkyne Addition to Brook Silenes
ARTICLE
Scheme 3. Addition of Simple Alkynes to Silene 2
Scheme 2. Addition of Alkynyl Probes to Silene 2
compared to the analogous silene. Surprisingly, there are no previous reports on the addition of alkynes to germenes.9,10 Herein, we report on the reactivity of neopentylgermene 7 toward terminal alkynes and compare the results with those obtained from neopentylsilene 2.
’ RESULTS Germene 78 was synthesized prior to each reaction and used in situ without purification. A colorless pentane solution of fluorovinylgermane 8 was cooled to 78 °C and then treated with t-BuLi. Upon warming to room temperature, the solution became pale yellow in color and a fine precipitate appeared (LiF), indicating the formation of germene. It was necessary to add slightly less than 1 equiv of t-BuLi to fluorovinylgermane 8 when forming 7 to prevent polymerization of the germene,11 and thus, germene 7 was always contaminated with residual fluorogermane 8 (∼515%). The pentane was removed and the residue was dissolved in C6D6 before the alkyne was added; the presence of 7 was confirmed by 1H NMR spectroscopy. The reactions were kept at room temperature for up to 6 days11 and monitored by 1H NMR spectroscopy. A mixture of germacyclobutene 9a, vinylgermane 10a, and germylacetylene 11a was produced after 23 days upon reaction of germene 7 with phenylacetylene (Scheme 4 and Table 1). Analogous products were obtained with 1-ethynyl-4-(trifluoromethyl)benzene (9b, 10b, and 11b; ∼24 h) and 4-ethynylanisole (9c, 10c, and 11c; 45 days) (Scheme 4 and Table 1). In contrast, germylacetylene 11d and small quantities of vinylgermane 10d were formed when germene 7 was allowed to react with trimethylsilylacetylene, and only a germylacetylene (11e) was formed upon reaction with tert-butylacetylene (Scheme 4 and Table 1). Vinylgermanes 10ac and germylacetylenes
11ae were isolated from residual fluorogermane 8 by chromatography; however, germylacetylenes 11ac could not be separated from the corresponding vinylgermanes 10ac. Germacyclobutenes 9ac decomposed upon prolonged exposure to the atmosphere or upon adsorption to silica gel and, thus, were not isolated. The addition of ethoxyacetylene to germene 7 yielded a complex mixture of compounds within a few minutes. Germacyclobutene 12 and vinylgermane 13 were identified as the major products by 1H NMR spectroscopy (Scheme 4). The ratio of germacyclobutene 12 to vinylgermane 13 was variable; 12 ranged from 5% to 38% of the product mixture. The relative amount of 13 increased as the concentration of 7 increased. Unlike aryl-substituted germacyclobutenes 9ac, 12 did not decompose to any significant degree upon adsorption to silica, but did decompose after prolonged exposure to the atmosphere. Despite many attempts, 12 and 13 could not be separated from each other by chromatography; however, after the eventual decomposition of 12, a sample of 13 was isolated. The product ratios (9:10:11) observed in the reactions between germene 7 and phenylacetylene, 1-ethynyl-4-(trifluoromethyl)benzene, 4-ethynylanisole, trimethylsilylacetylene, or tert-butylacetylene varied considerably depending on the reaction conditions (Table 1); the scale of the reaction, the order of addition, and the alkyne concentration all influenced the ratio. The concentration of germene 7 was kept constant in all reactions (0.240.26 M). Reaction of 7 with each of the aromatic alkynes gave a mixture containing germacyclobutene 9, vinylgermane 10, and germylacetylene 11. The relative amount of germylacetylene 11 varied the most of the three products, in the range 484% (11a; Table 1, entries 15), 1297% (11b; Table 1, entries 69), and 2762% (11c; Table 1, entries 1013) depending on the conditions. Furthermore, even when the reactions were carried out under identical conditions, the relative amount of 11 varied, at times, quite dramatically (Table 1, compare entry 3a to 3b). Interestingly, regardless of the percentage of 11ac, the ratio of germacyclobutene 9 to vinylgermane 10 remained fairly constant (9a:10a = 1:3, 9b:10b = 1:2, 9c:10c = 1:5), even as the absolute amounts fluctuated. Qualitatively, of the three aromatic alkynes investigated, 1-ethynyl-4-(trifluoromethyl)benzene reacted faster than phenylacetylene, which reacted faster than 4-ethynylanisole with germene 7 under identical conditions. Plots of the normalized 2262
dx.doi.org/10.1021/om200048p |Organometallics 2011, 30, 2261–2271
Organometallics
ARTICLE
Scheme 4. Reactions of Terminal Alkynes with Germene 7
Table 1. Product Ratios from the Reactions of Terminal Alkynes with Germene 7 entry 1
RCtCH Ph (a)
scalea (mmol 7)
alkyne concb (M)
orderc
product ratiod (9:10:11) 26:70:4
reaction time
adducte (%)
0.13
0.30
germene
3 days
73
2
0.13
0.30
alkyne
7:23:70
3 days
90
3a 3b
0.13 0.13
0.60 0.60
alkyne alkyne
11:27:62 18:48:34
2 days 2 days
95 90
4
0.26
0.30
germene
14:43:43
3 days
81
5
0.26
0.30
alkyne
2:14:84
2 days
>99
0.13
0.30
germene
28:60:12
24 h
90
7
0.13
0.30
alkyne
13:41:46
24 h
94
8
0.13
0.60
alkyne
25:59:16
