58th Annual Report on Research 2013 Under Sponsorship of the ACS Petroleum Research Fund

Scheme 1

We have synthesized diazirines 1^3-5 and co-photolyzed each with phenylchlorodiazirine (6)⁶ at 350 nm at room temperature in pentane; 1a was also co-photolyzed with phenanthride 5-Cl,^7-9 at 300 nm.  Isolation, purification, and characterization of products corresponding to 3a with each of chlorocarbenes R'CCl and 2b + PhCCl (i.e., 4a-Ph, 4a-Cl, and 4b-Ph, respectively) have been completed.  As determined by GC, the photosylate of 1a + 6 contains 11% of 4a-Ph; 1b + 6 yields 6% of 4b-Ph.

Scheme 2

The formation of 4a surprised us.  We propose its origin via the mechanism shown in Scheme 2.  To investigate this path, we monitored the photolysis of 1a + 6 via ¹H NMR spectroscopy by following the disappearance of the diazirine proton of 1a and the appearance of the vinyl protons of E- and Z-4a-Ph.  Given the Platz group's report of a microsecond lifetime of 3a at room temperature,³ we did not expect to see 3a by NMR, but we did investigate the possibility of the formation (and subsequent disappearance) of adduct 7-Ph.  Careful exploration of the far upfield range (d -0.1 to +0.5 ppm) showed no appearance of the cyclopropyl proton of 7-Ph.

Based on the Platz group's observations of bimolecular trapping of adamantylcarbene (2b),³ we believed the formation of 4b-Ph to result from a mixed carbene “dimerization” of 2b + PhCCl.

Further mechanistic investigations have involved laser flash photolysis (LFP) kinetics measurements (collaboration with Professor Dasan Thamattoor, Colby College).  To obtain rate data for the photolysis of 1a + 6 at room temperature, we measured the rate of formation of the pyridine ylide¹⁰ of PhCCl at 460 nm at varying concentrations of 3a.  To vary the concentration of 3a, we measured k_obs at different [1a].  To connect [3a] with [1a], separately we measured A₃₂₅ of 3a at each concentration of 1a.  Using reported molar absorptivities of stable anti-Bredt olefins¹¹ as an estimate of e for 3a, we obtained [3a].  We found k_obs for the formation of the pyridine ylide of PhCCl in the presence of 3a (from 1a) to be independent of [3a]:  k_obs = 4-5 x 10⁷ M^-1s^-1.  We conclude that this reaction to form 4a-Ph is not competitive with other processes present in the reaction mixture to affect k_obs.

LFP was also used to determine the rate constant for the formation of adamantylethylenes 4b-Ph, which we believe is generated by the reaction of adamantylcarbene (2b) with PhCCl.  We measured k_obs of the decay of PhCCl at constant [1b] at room temperature in pentane:  1.6 x 10⁷ M^-1s^-1.  Partitioning k_obs according to the product distribution gives the bimolecular rate constant of 9.6 x 10⁵ M^-1s^-1 for the formation of 4b-Ph.  We believe this value to be the first reported rate constant for the reaction of two different carbenes.

This report covers the period 7/1/13-8/31/13.  During this time, five Barnard College undergraduates worked on this project, one of whom was funded directly by the PRF.  Four of these students continue at Barnard as juniors or sophomores.  One student (F. Dewan) graduated in May 2013; she is now in the Medical Science Preparatory Program at Drexel University.  F. Dewan presented a poster on this work at the April 2013 ACS National Meeting in New Orleans.  Manuscripts on chlorocarbene additions to each of adamantene and adamantylcarbene are currently in preparation and are projected to be submitted by December 2013.  The co-authors on these two manuscripts will include the five students above and five other Barnard students.  The projected publications will assist substantially in the PI's efforts towards promotion from associate to full professor.

References cited:

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