Rearrangement of 1-Noradamantyl and 1-Adamantylcarbene to Bridgehead Alkenes:  Lifetimes of Two Bridgehead Carbenes in Solution

Abstract

B3LYP density functional calculations with the 6-31G* basis set predict that 3-noradamantylcarbene 7 rearranges to adamantene 9 and protoadmant-3-ene 10 over barriers of 0.35 and 7.84 kcal/mol, respectively, after zero-point energy correction. The same level of theory predicts that 1-adamantylcarbene 8 rearranges to homoadamantene 11 over a barrier of 6.06 kcal/mol after zero-point energy correction. To test these predictions, the photochemistry of 3-noradamantyldiazirine 5 and 1-adamantyldiazirine 6 was studied. Photolysis of 5 and 6 in cyclohexane produces the known dimers of the strained alkenes. 1-Adamantylcarbene 8 can be trapped in solution with cyclohexane and piperidine. 1-Adamantylcarbene 8 can be trapped with pyridine to form an ylide upon laser flash photolysis (LFP) of 6. 3-Noradamantylcarbene 7 cannot be intercepted with pyridine in LFP experiments. 3-Noradamantylcarbene 7 is not trapped with cyclohexane and is trapped with piperidine in only miniscule yield. The data indicates that 1-adamantylcarbene 8 is formed more efficiently from its diazirine precursor than is 3-noradamantylcarbene 7 and that the solution-phase lifetime of 1-adamantylcarbene 8 is at least 10 times longer than that of 3-noradamantylcarbene 7.

The Protoadamantane Radical Cation

“…Evaporation gave 220 mg of a mixture of 1/6-bromoadamantane/ 3-bromoadamantane in a ratio of 0.30:1.00:0.05 according to GC/ MS. 6-Protoadamantyl-and 3-protoadamantyl bromides were identified from standard samples prepared from 6-hydroxyprotoadamantane [50] and 3-hydroxyprotoadamantane, [51] respectively, by treatment with SOBr 2 in CCl 4 .…”

Experimental and Computational Mechanistic Investigation of Chlorocarbene Additions to Bridgehead Carbene–Anti-Bredt Systems: Noradamantylcarbene–Adamantene and Adamantylcarbene–Homoadamantene

“…18,19 They computed the singlet 2a ⇌ 3a equilibrium to greatly favor 3a by 32.4 kcal/mol (B3LYP/6-31G*) and a nominal 0.35 kcal/mol barrier for the conversion of singlet adamantylcarbene to adamantene. 18 The minimal barrier for conversion of 2a → 3a helped explain Platz and colleagues's matrix isolation and laser flash photolysis (LFP) studies of the photolysis of 1a. They did not observe 2a upon photolysis of 1a at 350 nm in an Ar matrix at 14 K, but they did see UV and IR features attributed to 3a: UV−vis λ max = 320 nm, IR υ = 1478 cm −1 .…”

“…They did not observe 2a upon photolysis of 1a at 350 nm in an Ar matrix at 14 K, but they did see UV and IR features attributed to 3a: UV−vis λ max = 320 nm, IR υ = 1478 cm −1 . 19 In benzene 19 or cyclohexane 18 solution, LFP of 1a produced an absorption assigned to 3a (λ max = 325 nm) that decayed over "many microseconds" via second-order processes. These processes were attributed to the formation of [2 + 2] dimers of 3a and protoadamantene (5).…”

“…18 They determined that the minimal structural change of adding one methylene unit in going from 1a to 1b resulted in a significant reactivity difference between the two systems. Similar to the noradamantylcarbene− adamantene equilibrium, the computed adamantylcarbene− homoadamantene equilibrium strongly favored homoadamantene: 3b was 45.9 kcal/mol more stable than singlet 2b (B3LYP/6-31G*).…”

“…The major (stable) products from these photolyses are adducts of 2b with cyclohexane solvent (11%) and [2 + 2] dimers of homoadamantene, 3b (28%). 18 From these experiments, they deduced the room-temperature lifetime of 2b to be 1.5 ns in cyclohexane and 2.5 ns in cyclohexane-d 12 . They also were able to trap 2b with piperidine: photolyses in cyclohexane containing increasing concentrations of piperidine resulted in increased 2b−piperidine adduct and reduced amounts of homoadamantene (3b) dimers and 2b−cyclohexane adducts.…”