Configurational Stability and Reactions of a-Acyloxy-Substituted a-Methylbenzyllithium CompoundsChiral carbanions with heteroatom-containing substituents have been investigated intensively during the last few years"]. Their configurational stability has attracted much attention since Still's discovery of non-racemic a-oxy-substituted organolithium compounds [']. Dipole-stabilized aoxycarbanions of the general structure 1 have been prepared for synthetic and mechanistic A common structural feature of compounds l a -l c is the intramolecular complexation of a lithium center by a C=O or P=O dipole whereby a five-membered chelate ring is formed. Carbamoyloxy-substituted carbanions 1 a have been introduced by Hoppe et al. and have proven very valuable due to their highly enantioselective preparation and configurational stability at -78°C for R' = H and R2 = a l k~l [~] . An X-ray crystal structure of a chiral, carbamoyloxy-substituted allyllithium proves that the tetracoordinated lithium is part of a five-membered chelate ringLS]. They react readily with a variety of electrophiles.Phosphoryloxy-substituted carbanions 1 b are obtained by deprotonation of benzyl phosphates with nBuLi and LDA. Carbanions stabilized by a P=O instead of a C=O dipole are reactive intermediates in the phosphate-phosphonate rearrangement, and the reverse process, the phosphonate-phosphate rearrangement (Scheme 1)i6]. This isomerisation may proceed in either direction, depending on the reaction conditions, and affords either 1 -hydroxyphosphonates 4 or phosphates 2. In an a-phosphoryloxy-alkyllitihium compound the carbanion attacks the electrophilic phosphorus to form a pentacoordinated species which gives the lithium salt 3 of a-hydroxyphosphonate 4, corresponding to an intramolecular acylation. This rearrangement is prevented at -78°C in the case of carbamates l a by a reduced electrophilicity of the C=O group and steric shielding by bulky substituents (R = iPr or R2N is a cyclic a,a'-tetrasubstituted amine). A rearrangement analogous to the phosphate-phosphonate rearrangement has been mentioned in the literature to occur at higher temperatures for carbanions l a , but has not been investigated more closely [7], except for benzoates[*l. Carbanions l a and l b with R' = Ph and R2 = Me are configurationally stable, but only l b rearranges whereby the configuration at the benzylic centre is retained during the short lifetime of the carbanion prior to the i~omerisation [~].Carbanions of type l c with a shielded carbonyl group, such as 2,4,6-triisopropylbenzoate esters describd by Beak et al., are more similar to carbanions of type l b than to those of the type la[1o]. Their configurational stability has not yet been investigated.

Transition metal dichalcogenide monolayers are highly interesting for potential valleytronic applications due to the coupling of spin and valley degrees of freedom and valley-selective excitonic transitions. However, ultrafast recombination of excitons in these materials poses a natural limit for applications, so that a transfer of polarization to resident carriers is highly advantageous. Here, we study the low-temperature spin-valley dynamics in nominally undoped and n-doped MoSe 2 monolayers using time-resolved Kerr rotation. In the ndoped MoSe 2 , we find a long-lived component of the Kerr signal which we attribute to the spin polarization of resident carriers. This component is absent in the nominally undoped MoSe 2 . The long-lived spin polarization is stable under applied in-plane magnetic fields. Spatially resolved measurements allow us to determine an upper boundary for the electron spin diffusion constant in MoSe 2 .Transition metal dichalcogenide (TMDC) monolayers have a peculiar band structure 1,2 , in which spin and valley degrees of freedom are coupled, and the optical selection rules allow for valley-selective generation of excitons 3 . An excitonic valley polarization can be read out optically via the circular polarization degree of the emitted photoluminescence (PL), and initial studies using continuous-wave excitation revealed a large steadystate polarization 4,5 , which was shown to be stable against depolarization in large in-plane magnetic fields 6 . While many early studies of valley physics focused on the naturally abundant MoS 2 , synthetic TMDC crystals quickly garnered scientific attention due to their spectrally narrow PL emission 7 , and the large tuning range of conduction-and valence-band spin splitting 8-10 afforded by the different combinations (or alloys 11,12 ) of the constituent elements Mo, W, S, Se, and Te. Time-resolved studies of exciton recombination and valley dynamics showed that ultrafast radiative recombination 13-17 is partially responsible for the large valley polarization observed in experiments, as the exciton lifetime limits the time window for valley dephasing. However, the ultrafast excitonic recombination also severely limits potential applications of the valley degree of freedom in novel devices, so that a transfer of valley polarization to resident carriers, which was observed in several TMDCs 18,19 , is highly advantageous. Among the TMDC family, MoSe 2 is characterized by a comparatively low optically induced valley polarization degree, which is only observable at all under near-resonant excitation conditions 20-22 and can be increased by modifying the recombination dynamics using coupling to photonic cavities 23 . To investigate the anomalously low valley polarization in MoSe 2 , timeresolved studies of valley dynamics are needed.Here, we directly compare the low-temperature spinvalley dynamics in nominally undoped and n-doped MoSe 2 monolayers using time-resolved Kerr rotation (TRKR). We find a long-lived component of the Kerr a) Electronic mail: tobias.ko...

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Configurationally stable, dipole-stabilised benzyllithium compounds (R)-and (S)-1 b, prepared by deprotonation of the corresponding esters with sBuLi in toluene/diethyl ether ( 5 : 1 j , reacted with (-)-menthyldimethyltin bromide to afford the stannanes (-)-3 and (+)-4, respectively. A single-crystal X-ray structure analysis of compound (-)-3 proved that stannylation occurred with inversion of the configuration at the benzylic centre, assuming retention for the deprotonation step. Lithio-destannylation and deuteration with MeOD follow a retentive course.

We used spatially-and time-resolved Kerr rotation microscopy to show that in lateral wire-like structures, based on a modulation-doped GaAs-AlGaAs quantum well, an optically initialized spin polarization can be deterministically transferred to specific lateral positions, employing the persistent spin helix (PSH). To this end, we show that confinement in two directions leads to a strong enhancement of the effective decay time of spin polarization, which can be exploited to transfer spin polarization over relatively large lateral distances. This is demonstrated by the investigation of L-shaped wire-like lateral structures, where the legs are positioned in directions parallel and perpendicular to the wave vector of the PSH. Published by AIP Publishing.[http://dx.doi.org/10.1063/1.4966184]The key tool to manipulate the spin in semiconductorbased devices is spin-orbit interaction. 1 However, on the downside, the spin-orbit interaction leads to spin dephasing, which limits the performance of spintronics or spin-based information processing devices. In this respect, an important milestone has been the proposal of a SU(2) spin rotation symmetry in quantum wells, based on the zincblende-type semiconductors, by Bernevig et al. 2 For (001)-grown quantum wells, this outstanding situation occurs for balanced linear Rashba 3 and Dresselhaus 4 spin-orbit fields. Very appealingly, the SU(2) symmetry ideally protects the system against Dyakonov-Perel spin dephasing 5 and should support a helical mode, the so called persistent spin helix (PSH), which opens up interesting application possibilities. Even before the proposal of Bernevig et al., Schliemann et al. suggested a non-ballistic spin field-effect transistor, based on the same theoretical principles of balanced spin-orbit fields. 6 Since then, a wealth of experimental and theoretical investigations has appeared along these lines. Most prominent are the first experimental evidences of the existence of a PSH by transient spin grating spectroscopy, 7 direct mapping via time-and spatially-resolved Kerr microscopy, 8 and weak localization/antilocalization experiments. 9 Closely related, a spin Hall effect transistor was demonstrated by Wunderlich et al. 10 Subsequently, the PSH was investigated in, e.g., gated structures, 11 laterally confined structures, 12 structures with imbalanced spin-orbit interactions 13,14 or via inelastic light scattering. 15 Very recently, investigations in biased structures under charge transport were reported. 16,17 In this work, we used spatially-and time-resolved Kerr rotation microscopy to show that spin polarization can be deterministically transferred to specific lateral positions via the PSH. We started by experimentally verifying the PSH in the as-grown sample, a GaAs-AlGaAs single quantum well, via spatially-resolved Kerr microscopy, followed by the investigation of narrow channels with lateral widths down to 3 lm. These experiments have shown that the effective decay time of the spin polarization is prolonged by a factor of about three in nar...