448 research outputs found
Taming Ambident Triazole Anions: Regioselective Ion-Pairing Catalyzes Direct N-Alkylation with Atypical Regioselectivity
Controlling the regioselectivity
of ambident nucleophiles toward
alkylating agents is a fundamental problem in heterocyclic chemistry.
Unsubstituted triazoles are particularly challenging, often requiring
inefficient stepwise protectionâdeprotection strategies and
prefunctionalization protocols. Herein we report on the alkylation
of archetypal ambident 1,2,4-triazole, 1,2,3-triazole, and their anions,
analyzed by in situ <sup>1</sup>H/<sup>19</sup>F NMR, kinetic modeling,
diffusion-ordered NMR spectroscopy, X-ray crystallography, highly
correlated coupled-cluster computations [CCSDÂ(T)-F12, DF-LCCSDÂ(T)-F12,
DLPNO-CCSDÂ(T)], and Marcus theory. The resulting mechanistic insights
allow design of an organocatalytic methodology for ambident control
in the <i>direct</i> N-alkylation of unsubstituted triazole
anions. Amidinium and guanidinium receptors are shown to act as strongly
coordinating phase-transfer organocatalysts, shuttling triazolate
anions into solution. The intimate ion pairs formed in solution retain
the reactivity of liberated triazole anions but, by virtue of highly
regioselective ion pairing, exhibit alkylation selectivities that
are completely inverted (1,2,4-triazole) or substantially enhanced
(1,2,3-triazole) compared to the parent anions. The methodology allows
direct access to 4-alkyl-1,2,4-triazoles (<i>rr</i> up to
94:6) and 1-alkyl-1,2,3-triazoles (<i>rr</i> up to 99:1)
in one step. Regioselective ion pairing acts in effect as a noncovalent
in situ protection mechanism, a concept that may have broader application
in the control of ambident systems
Ultrafast Observation of a Photoredox Reaction Mechanism:Photoinitiation in Organocatalyzed Atom-Transfer Radical Polymerization
Photoredox
catalysis has driven a revolution in the field of organic
chemistry, but direct mechanistic insights into reactions of genuine
synthetic utility remain relatively scarce. Herein we report ultrafast
time-resolved spectroscopic observation of a bimolecular organocatalyzed
photoredox reaction, from catalyst photoexcitation through to photoinduced
electron transfer (PET) and intermediate formation, using transient
vibrational and electronic absorption spectroscopy with sub-picosecond
time resolution. Specifically, the photochemical dynamics of initiation
in organocatalyzed atom-transfer radical polymerization (O-ATRP) are
elucidated for two complementary photoredox organocatalysts (<i>N</i>,<i>N</i>-diaryl-5,10-dihydrophenazines). Following
photoexcitation, a dissociative bimolecular electron transfer is observed
from the first excited singlet state of both photocatalysts to methyl
2-bromopropionate in dichloromethane, toluene, and dimethylformamide.
The photocatalyst excited donor state, ground state, and radical cation
are tracked in real time alongside the debrominated radical fragment.
Our work challenges previously proposed mechanisms of initiation in
O-ATRP and indicates that PET from short-lived excited singlet states
can exert control of polymer molecular weight and dispersity by suppressing
the steady-state concentration of the reactive debrominated radical.
More broadly, we aim to demonstrate the potential of ultrafast absorption
spectroscopy to observe directly transient, open-shell intermediates
in mechanistic studies of photoredox catalysis
The Primordial Inflation Polarization Explorer (PIPER)
The Primordial Inflation Polarization Explorer (PIPER) is a balloon-borne
cosmic microwave background (CMB) polarimeter designed to search for evidence
of inflation by measuring the large-angular scale CMB polarization signal.
BICEP2 recently reported a detection of B-mode power corresponding to the
tensor-to-scalar ratio r = 0.2 on ~2 degree scales. If the BICEP2 signal is
caused by inflationary gravitational waves (IGWs), then there should be a
corresponding increase in B-mode power on angular scales larger than 18
degrees. PIPER is currently the only suborbital instrument capable of fully
testing and extending the BICEP2 results by measuring the B-mode power spectrum
on angular scales = ~0.6 deg to 90 deg, covering both the reionization
bump and recombination peak, with sensitivity to measure the tensor-to-scalar
ratio down to r = 0.007, and four frequency bands to distinguish foregrounds.
PIPER will accomplish this by mapping 85% of the sky in four frequency bands
(200, 270, 350, 600 GHz) over a series of 8 conventional balloon flights from
the northern and southern hemispheres. The instrument has background-limited
sensitivity provided by fully cryogenic (1.5 K) optics focusing the sky signal
onto four 32x40-pixel arrays of time-domain multiplexed Transition-Edge Sensor
(TES) bolometers held at 140 mK. Polarization sensitivity and systematic
control are provided by front-end Variable-delay Polarization Modulators
(VPMs), which rapidly modulate only the polarized sky signal at 3 Hz and allow
PIPER to instantaneously measure the full Stokes vector (I, Q, U, V) for each
pointing. We describe the PIPER instrument and progress towards its first
flight.Comment: 11 pages, 7 figures. To be published in Proceedings of SPIE Volume
9153. Presented at SPIE Astronomical Telescopes + Instrumentation 2014,
conference 915
Organizing degrowth: The ontological politics of enacting degrowth in OMS
As degrowth notions begin to gain traction within business schools and organization and management studies (OMS), this paper draws on Science and Technology Studies to interrogate the ontological politics of enacting degrowth in this relatively new context. We argue that the âdegrowth multipleâ is a boundary object which takes on different forms as it circulates among different epistemic communities and within their respective boundaries, institutional arrangements, practices, and agendas. We investigate this empirically to elucidate how degrowth is being enacted within the OMS epistemic apparatus, revealing three set of practices characterizing extant OMS-degrowth engagements: stabilizations, reconfigurations, and projections. These motivate a subsequent discussion of the ontological politics unfolding through degrowth performances in OMS, its transformations (t)herein, and degrowthâs wider enrollment within the OMS epistemic apparatus. We thus contribute a reflexive intervention to organizing degrowth such that it remains a politically actionable concept across multiple contexts, and avoids becoming uncritically black-boxed, fetishized, and/or diluted by diverging cross-boundary enactments
Social democracy, embeddedness and decommodification: On the conceptual innovations and intellectual affiliations of Karl Polanyi
Of the several debates that revolve around the work of the economic historian and political economist Karl Polanyi, one that continues to exercise minds concerns his analysis of, and political attitudes toward, post-war capitalism and the welfare state. Simplified a little, it is a debate with two sides. To borrow IvĂĄn SzelĂ©nyi's terms, one side constructs a âhardâ Karl Polanyi, the other a âsoftâ one. The former advocated a socialist mixed economy dominated by redistributive mechanisms. He was a radical socialist for whom the market should never be the dominant mechanism of economic coordination. His âsoftâ alter ego insisted that the market system remain essentially intact but be complemented by redistributive mechanisms. The âdouble movementâ â the central thesis of his âGreat Transformationâ â acts, in this reading, as a self-correcting mechanism that moderates the excesses of market fundamentalism; its author was positioned within the social-democratic mainstream for which the only realistic desirable goal is a regulated form of capitalism. In terms of textual evidence there is much to be said for both interpretations. In this article I suggest a different approach, one that focuses upon the meaning of Polanyi's concepts in relation to their socio-political and intellectual environment
The GISMO Two-millimeter Deep Field in GOODS-N
We present deep continuum observations using the GISMO camera at a wavelength of 2 mm centered on the Hubble Deep Field in the GOODS-N field. These are the first deep field observations ever obtained at this wavelength. The 1Ï sensitivity in the innermost ~4' of the 7' diameter map is ~135âÎŒJy beam^(â1), a factor of three higher in flux/beam sensitivity than the deepest available SCUBA 850âÎŒm observations, and almost a factor of four higher in flux/beam sensitivity than the combined MAMBO/AzTEC 1.2 mm observations of this region. Our source extraction algorithm identifies 12 sources directly, and another 3 through correlation with known sources at 1.2 mm and 850âÎŒm. Five of the directly detected GISMO sources have counterparts in the MAMBO/AzTEC catalog, and four of those also have SCUBA counterparts. HDF850.1, one of the first blank-field detected submillimeter galaxies, is now detected at 2 mm. The median redshift of all sources with counterparts of known redshifts is med(z) = 2.91±0.94. Statistically, the detections are most likely real for five of the seven 2 mm sources without shorter wavelength counterparts, while the probability for none of them being real is negligible
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