Studies of the stereochemistry and mechanism of the ene reaction using specifically deuterated pinenes

Abstract

The ene reactions of cis-and r/wis-[3-2H]-/3-pinenes with methyl phenylglyoxylate and benzyne have been examined. In each case, it has been established that the hydrogen atom of the methylene group at C3 in ßpinene which is trans to the gem-dimethyl bridge is involved, overwhelmingly, in this reaction. This remarkable degree of stereoselectivity is presented in support of earlier proposals (ref 3) that these ene reactions occur in a concerted fashion through a single, cyclic transition state. In concert with this view: (1) no difference was observed in the rate of reaction of /3-pinene with methyl phenylglyoxylate (at 140°) when polar and nonpolar solvents (e.g., nitrobenzene and oxylene) were used; and (2) no products resulting from the rearrangement of possible carbonium ion intermediates have been observed in ene reactions with /3-pinene. We believe these data cannot be rationalized in terms of a two-step mechanism involving a dipolar intermediate.

The Aryne Ene Reaction

“…A later study by Arnold and co-workers provided additional support for a concerted process (Scheme 3b ). 18 The reaction between benzyne and β-pinene was shown to proceed in a highly stereoselective manner, engaging the C-3 hydrogen that is trans to the gem -dimethyl bridge. For example, isotope-labelled trans -(3-D)-β-pinene 13 afforded the corresponding ene adduct 14 in 53% yield and with >95% of the deuterium label attached to the benzene ring.…”

“…A later study by Arnold and co-workers provided additional support for a concerted process (Scheme 3b). 18 The reaction between benzyne and -pinene was shown to proceed in a highly stereoselective manner, engaging the C-3 hydrogen Crews and Beard identified substrate conformation as the key factor in controlling the relative distribution of competing [4+2], [2+2], and ene pathways in reactions between benzyne and a range of cyclic polyenes 15 (Scheme 4). 19 The propensity to undergo ene and [4+2] cycloaddition reactions was found to be highly dependent upon the substrates being able to adopt conformations that enabled correct orbital alignment with benzyne; attributed to the concerted nature of the two processes.…”

The Prins and Carbonyl Ene Reactions

“…(62) is opposite to that obtained from diene (61) and glyoxylate (55) with the same three-dimensional orientation of groups. Ene adduct (62) has been converted to (-)specionin. 48 SnCl4-catalyzed reaction of (55) with an excess of a mixture of racemates (63) and (64) gives a 72% yield of an 8: 1 mixture of (65) and (66).…”

Lewis acid catalysis of the ene addition of chloral and bromal to olefins; stereochemical and mechanistic studies

“…(a) Non-hydrogen atoms ( x lo4) 360 ( 10) 1 980 (10) 2 570 (10) 3 780 ( 10) 3 650 (10) 2 900 ( 10) 5 100 (10) 2 890 (10) 2 870 ( 10) 2 144 (9) 2 085 (9) 2 716 (9) 3 380 (10) 3 491( 9) 1 313 (9) 3 950 ( 10) 2 173 (6) 3 506 (7) 728( 7) 1 267 (7) 3 707 (7) 4 609( 8 2 920(20) 2 330(30) 6 290(20) 2 520(20) 2 390(20) 3 620(20) 5 570(20) 6 450(20) 5 480(20) 3 540(20) 2 550(20) 8 580(20) 2 330(20) 3 510 (10) 690(20) 9 380 ( 10) 9 300 (10) 3 060 ( 10) 620(20) 7 I8(7) 4 505 (7) 32 I( 8) 6) 1919( 5) 1236( 5) 847( 7) 44( 7) 1 563( 7) 1442 (7) 3 403(6) 4 253(6) 4 309(5) 5 125(7) 5 872(5) 5 794( In the study of the effects of reaction time the product ratio was monitored at various times. For the catalysed reactions the first sample was normally taken after 1 min and the final sample after ca.…”

“…In contrast, the ene additions to unsymmetrical 1,ldialkylethylenes were found to be much less regio-and stereoselective. Reaction of chloral-A1C1, with (+)-limonene afforded the adducts ( 6) : (7) : (8) in a ratio 79 : 15 : 6. Since (6) is not converted into (8) on treatment with AlCl, under the reaction conditions, it appears that ( 8) is formed independently by way of a Friedel-Crafts type intermediate [e.g.…”