Synthetic photochemistry of organoboranes. Photocyclization of dicyclohexyl-3-methyl-trans-1,3-butadienylborane

Abstract

The structure of iodolactone 6 was further demonstrated by elimination of HI to form 9, which occurred in hot base10 when KI was added but not without KI addition. This implies that the only proton available for elimination is cis to iodine so that elimination is only effected after initial epimerization of iodide by iodide in an Sn2 displacement. The olefinic lactone 9 produced (bp 140°, 70 µ ) shows only one vinyl proton (t 4.33) in the nmr spectrum, consistent with the assigned structure 9.This synthetic sequence represents a readily available source of perhydroazulenes (25% overall yield to 5, 20% overall to 9) which is applicable to several conversions. Thus, A-bromosuccinimide on 9 affords a single secondary allylic bromide as a route to functionalizing the 5-ring. The typical three-carbon lactone appendages at C-7 in the guaianolide and pseudoguaianolide sesquiterpenes4 are virtually always trans to the C-l hydrogen and so can be stereospecifically introduced via cyclic intermediates from the (cis) carboxyl at C-10. One such procedure could be Claisen condensation of propionate ester with 8, followed by internal displacement of the epoxide by the ß-keto ester anion thus formed.11The perhydroazulenes shown here are presently under investigation for conversion to pseudoguaianolides and conformational analysis models for cycloheptanes.(10) The reaction required 96 hr at 115°in dimethylformamide with diazabicyclooctane (DABCO) as base (yield 91%); higher temperatures caused increasing reversion to acid 5b.(11) The epoxide 8 is very resistant to attack of external nucleophiles, presumably owing to the requisite axial position of the carboxyl at C-10 and the hindrance it offers to backside approach. We have not yet, however, tested the internal displacement.(12) National

Mechanistic Aspects of Bond Activation with Perfluoroarylboranes

“…Experimental and computational studies strongly suggest that H 2 forms a side-on adduct with these highly Lewis acidic boranes, which leads to cleavage of one of the internal B–C bonds of the somewhat strained and antiaromatic borole ring. The resulting cisoid -borylbutadiene compounds undergo rapid recyclization to yield the observed products, , a mechanism that nicely explains the observation of both cis - and trans -boracyclopentene products 3-H/F . These results show explicitly that highly Lewis acidic boron centers can engage in chemically consequential interaction with H 2 , as is invoked in the mechanism depicted in Scheme iii .…”

Dihydrogen Activation by Antiaromatic Pentaarylboroles

“…Photochemically generated cis-1,3-butadienylboranes similar to those depicted in Scheme 5 have been shown to rapidly cyclize to boracyclopent-3-enes. 35,36 That this reaction occurs so rapidly in the absence of a frustrated Lewis base partner has implications for the mechanism of H 2 splitting by FLPs. Kinetic, thermodynamic, and computational investigations that will address these issues in detail are underway; the greater solubility of unfluorinated pentaphenylborole 4 and the more forgiving time scale of its reaction with H 2 make it ideal for further study.…”

“…The details of the mechanism of the reaction are yet to be determined, but the presence of the trans isomers of 3 and 5 as the major isomers in solution suggests that the H 2 adducts under go B−C α bond cleavage followed by rapid cyclization to a mixture of boracyclopent-3-ene products (Scheme ). Photochemically generated cis -1,3-butadienylboranes similar to those depicted in Scheme have been shown to rapidly cyclize to boracyclopent-3-enes. , That this reaction occurs so rapidly in the absence of a frustrated Lewis base partner has implications for the mechanism of H 2 splitting by FLPs. Kinetic, thermodynamic, and computational investigations that will address these issues in detail are underway; the greater solubility of unfluorinated pentaphenylborole 4 and the more forgiving time scale of its reaction with H 2 make it ideal for further study.…”

Mechanistic Studies on the Metal-Free Activation of Dihydrogen by Antiaromatic Pentarylboroles

“…We thus sought to provide more evidence for the proposal through the separate synthesis of a 1-bora-2,4pentadiene complex analogous to cis-or trans-III. Early attempts by Zweifel and co-workers 30,31 to generate such boranes led directly and rapidly to 1-bora-3-cyclopentenes analogous to products 3 and 4 herein, and so we have only succeeded in generating III (Ar = C 6 H 5 ) in situ. This was accomplished by the synthesis of the borinic ester 5 via the protic ring-opening of 2 by treatment with 1 equiv of phenol (Scheme 3).…”

“…In our initial report, we speculated that H 2 binding to the boron atom of the borole was followed by ringopening to a 1-bora-pentadiene that then gave the observed cis and trans products via an electrocyclic ring closure and 1,2 hydride migration process. 30,31 Herein we present an experimental and computational study of this mechanism, using both the fully fluorinated system 1 and its unfluorinated pentaphenylborole 32,33 analogue 2.…”