trans-4,4-Dimethyl-2-pentene

• CAS: 690-08-4
• Catalog Number: ACM690084
• Category: Main Products
• Molecular Weight: 98.19
• Molecular Formula: C₇H₁₄
• Synonyms: 4,4-Dimethyl-2-pentene, (E)-4,4-Dimethyl-2-pentene, 2-Pentene, 4,4-dimethyl-, 2,2-DIMETHYLPENTENE, 4,4-Dimethyl-trans-2-pentene, trans-4,4-Dimethyl-2-pentene, 2-Pentene, 4,4-dimethyl-, (E)-, (E)-4,4-Dimethylpent-2-ene, NSC74143, (2-Chloroethyl)phosphonic dichloride, EINECS 211-714-9, NSC 74143, CID5326158, 2-Pentene, 4,4-dimethyl-, (E)- (8CI)(9CI), InChI=1/C7H14/c1-5-6-7(2,3)4/h5-6H,1-4H3/b6-5, 690-08-4, 50819-06-2
• IUPAC Name: (E)-4,4-dimethylpent-2-ene
• InChI Key: BIDIHFPLDRSAMB-AATRIKPKSA-N
• Boiling Point: 76.75ºC
• Melting Point: -115.235ºC
• Density: 0.6889
• Exact Mass: 98.10960
• H-Bond Acceptor: 0
• H-Bond Donor: 0

Case Study

trans-4,4-Dimethyl-2-pentene - When Steric Bulk Halts a Textbook Reaction at the Halfway Mark

Bravo, José A., et al. Revista Boliviana de Química 37.1 (2020): 46-63.

Hydroboration of alkenes is a cornerstone reaction in organic synthesis, typically proceeding through three sequential alkene additions to generate a trialkylborane intermediate subsequently oxidized to the corresponding alcohol. trans-4,4-Dimethyl-2-pentene serves as a prototypical sterically hindered internal olefin that challenges this norm: the severe congestion imparted by the tert-butyl substituent prevents the third hydroboration step, arresting the reaction at the dialkylborane stage. This behavioral anomaly provides an instructive case for how steric effects govern both regioselectivity and the degree of alkylation in electrophilic addition.
Experimental Protocol: The hydroboration of trans-4,4-dimethyl-2-pentene was conducted with diborane in ethereal solvents at ambient temperature, following the classic conditions of Brown and Zweifel. The olefin was allowed to react with borane until alkene uptake ceased. The resulting organoborane intermediate was oxidized with alkaline hydrogen peroxide, and alcohol products were analyzed to determine the regiochemical distribution of boron addition across the double bond.
Performance Evaluation: Unlike simple terminal alkenes such as 1-hexene, which achieve approximately 94% regioselectivity favoring the less substituted carbon and proceed cleanly to the trialkylborane, trans-4,4-dimethyl-2-pentene exhibits markedly reduced regioselectivity with addition partitioned nearly equally between the two olefinic carbons. Critically, the reaction stalls at the dialkylborane intermediate, as the steric demands of incorporating a third molecule of this highly hindered olefin exceed the accessible space around the boron center. This mirrors the behavior of trans-4-methyl-2-pentene but is more pronounced due to the greater bulk of the tert-butyl group. For comparison, the trisubstituted analogue 2,4,4-trimethyl-2-pentene displays the opposite extreme with 98% regioselectivity, illustrating how olefin substitution pattern, rather than absolute bulk, governs hydroboration outcome.