Dune Acquisition Corp II

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE 14A

INFORMATION REQUIRED IN PROXY STATEMENT

SCHEDULE 14A INFORMATION

PROXY STATEMENT PURSUANT TO SECTION 14(A) OF

THE SECURITIES EXCHANGE ACT OF 1934

Check the appropriate box:

DUNE ACQUISITION CORPORATION II

(Name of Registrant as Specified In Its Charter)

(Name of Person(s) Filing Proxy Statement, if other than the Registrant)

Payment of Filing Fee (Check the appropriate box):

LETTER TO SHAREHOLDERS OF DUNE ACQUISITION CORPORATION II

Dear Dune Acquisition Corporation II Shareholder:

You are cordially invited to attend an extraordinary general meeting of Dune Acquisition Corporation II, a Cayman Islands exempted company (the“Company,” “we,” “us” or “our”), which will be held on April 21, 2026, at 11:00 a.m., New York Time (the “Extraordinary General Meeting”), at the offices of Reed Smith LLP, our legal counsel, at 599 Lexington Ave, New York, NY 10022 and virtually via live webcast athttps://www.cstproxy.com/duneacqii/2026. You can also listen to, but not attend, the Extraordinary General Meeting via teleconference* using the following dial-in information:

Telephone access (listen-only):

Within the U.S. and Canada: +1 800-450-7155 (toll-free)

Outside of the U.S. and Canada: +1 857-999-9155 (standard rates apply)

Conference ID: 6904241#

• Shareholders participating via the listen-only method will not be considered present for the Extraordinary General Meeting and will not be counted towards quorum.

The Extraordinary General Meeting may be held at such other date, time and place to which such meeting may be adjourned, to consider and vote on the proposals set forth herein.

The attached Notice of the Extraordinary General Meeting and proxy statement/information statement (“proxy statement”) describe the business the Company will conduct at the Extraordinary General Meeting and provide information about the Company that you should consider when you vote your shares. As set forth in the attached proxy statement, the Extraordinary General Meeting will be held for the purpose of considering and voting on the following proposals:

• Proposal No. 1 — Name Change Proposal— To approve, as special resolutions, the change of the name of the Company from “Dune Acquisition Corporation II” to “Collective Acquisition Corp.” and an amendment to the Company’s Amended and Restated Memorandum and Articles of Association (as may be amended from time to time, the “Articles”) in the form set forth in Annex A to the accompanying proxy statement, to reflect the change of name of the Company (the “Name Change” and such proposal, the “Name Change Proposal”); and

• Proposal No. 2 — Adjournment Proposal— To approve, as an ordinary resolution, the adjournment of the Extraordinary General Meeting to a later date or dates, or indefinitely, if necessary or convenient, to permit further solicitation and vote of proxies if, based upon the tabulated vote at the time of the Extraordinary General Meeting, there are not sufficient votes to approve the Name Change Proposal or if the Company’s board of directors (the “Board”) determines that additional time is necessary to effectuate the Name Change (the “Adjournment Proposal”).

Each of the Name Change Proposal and the Adjournment Proposal is more fully described in the accompanying proxy statement. Please take the time to carefully read each of the proposals in the accompanying proxy statement before you vote.

THE BOARD OF DIRECTORS UNANIMOUSLY RECOMMENDS A VOTE “FOR” THE NAME CHANGE PROPOSAL, AND, IF PRESENTED, “FOR” THE ADJOURNMENT PROPOSAL.

The purpose of the Name Change Proposal is to enable the Company to change its name from “Dune Acquisition Corporation II” to “Collective Acquisition Corp.” As previously disclosed in our Current Report on Form 8-K filed with the Securities and Exchange Commission on February 5, 2026, on January 30, 2026, Collective Acquisition Sponsor LLC, a Delaware limited liability company (the “New Sponsor”), the Company, Dune Acquisition Holdings II LLC, a Delaware limited liability company (the “Old Sponsor”), Carter Glatt, as the managing member of the Old Sponsor (the “Sponsor Member”), certain members of the Sponsor Member named as signatories thereto and certain other institutional investors signatories thereto entered into a Purchase and Sponsor Handover Agreement pursuant to which

New Sponsor has agreed to purchase from the Old Sponsor, an aggregate of (i) 4,475,000 Class B ordinary shares, $0.0001 par value per share and (ii) 1,000,000 private placement warrants of the Company, for an aggregate purchase price of $2,000,000 paid in cash at closing (the “Transaction”). The Transaction resulted in a change in ownership and control of the Company.

The Company proposes to effect the Name Change in connection with the Transaction.

The purpose of the Adjournment Proposal is to allow the chairman to adjourn the Extraordinary General Meeting, if necessary or convenient, in order to provide additional time to solicit votes in favor of the Name Change Proposal or if we otherwise determine that additional time is necessary to effectuate any Name Change.

You are not being asked to vote on the any initial business combination (the “Initial Business Combination”) at this time. You will retain the right to vote on the Initial Business Combination when it is submitted to the Public Shareholders (as defined below) (provided that you are a shareholder on March 12, 2026 (the “Record Date”) for a meeting to consider such proposed Initial Business Combination) and the right to redeem your Class A Shares (as defined below) initially issued as part of the units in the Company’s IPO (the “Public Shares” and the holders of Public Shares, the “Public Shareholders”) for a pro rata portion of the trust account in the event the Initial Business Combination (or any other initial business combination) is approved and consummated or in the event the Company has not consummated the Initial Business Combination or another initial business combination by the liquidation date of the Company.

The approval of the Name Change Proposal requires a special resolution under the Companies Act (Revised) of the Cayman Islands (the “Companies Act”), being a resolution passed by a majority of not less than two thirds (2/3) of the votes cast by the holders of the issued and outstanding Class A ordinary shares, par value $0.0001 per share (“Class A Shares”) and Class B ordinary shares, par value $0.0001 per share (“Class B Shares”, and together with the Class A Shares, the “Ordinary Shares”), voting together as a single class, as, being entitled to do so, vote in person (including virtually) or by proxy at the Extraordinary General Meeting or any adjournment thereof.

Approval of the Adjournment Proposal requires an ordinary resolution under the Companies Act, being a resolution passed by a simple majority of the votes cast by the holders of the issued and outstanding Ordinary Shares, voting together as a single class, as, being entitled to do so, vote in person (including virtually) or by proxy at the Extraordinary General Meeting or any adjournment thereof.

The Board has fixed March 12, 2026, the Record Date, as the date for determining the Company’s shareholders entitled to receive notice of and vote at the Extraordinary General Meeting and any adjournment thereof. Only holders of record of Ordinary Shares on that date are entitled to have their votes counted at the Extraordinary General Meeting or any adjournment thereof.

After careful consideration of all relevant factors, the Board has determined that the Name Change Proposal and the Adjournment Proposal are in the best interests of the Company and its shareholders, and unanimously recommends that you vote or give instruction to vote “FOR” such proposals.

Under the Articles, no other business may be transacted at the Extraordinary General Meeting.

Enclosed is the proxy statement containing detailed information about the Extraordinary General Meeting, the Name Change Proposal and the Adjournment Proposal. Whether or not you plan to attend the Extraordinary General Meeting, the Company urges you to read this material carefully and give instructions to vote your shares.

By Order of the Board of Directors of Dune Acquisition Corporation II

/s/ Carter Glatt

Your vote is very important. Whether or not you plan to attend the Extraordinary General Meeting, please provide your instructions to vote your shares as soon as possible, and no later than 11:59 p.m., New York Time, on April 20, 2026, by following the instructions in the accompanying proxy statement to make sure that your shares are represented at the Extraordinary General Meeting. The approval of the Name Change Proposal requires a special resolution under the Companies Act, being a resolution passed by a majority of not less than two thirds (2/3) of the votes cast by the holders of the issued and outstanding Ordinary Shares, as, being entitled to do so, voting together as a single class, vote in person or by proxy at the Extraordinary General Meeting or any adjournment thereof. The approval of the Adjournment Proposal requires an ordinary resolution under the Companies Act, being a resolution passed by a simple majority of the votes cast by the holders of the Ordinary Shares, as, being entitled to do so, vote in person or by proxy at the Extraordinary General Meeting. Accordingly, if you fail to vote by proxy or to vote yourself at the Extraordinary General Meeting, your shares will not be counted in connection with the determination of whether a valid quorum is established, and, if a valid quorum is otherwise established, such failure to vote will have no effect on the outcome of any vote on the Name Change Proposal or Adjournment Proposal. If you hold your shares in “street name” through a bank, broker or other nominee, you will need to follow the instructions provided to you by your bank, broker or other nominee to ensure that your shares are represented and voted at the Extraordinary General Meeting.

Important Notice Regarding the Availability of Proxy Materials for the Extraordinary General Meeting to be held on April 21, 2026: The notice of meeting and the accompanying proxy statement are available atwww.sec.gov and athttps://www.cstproxy.com/duneacqii/2026.

NOTICE OF EXTRAORDINARY GENERAL MEETING OF SHAREHOLDERS

OF DUNE ACQUISITION CORPORATION II

TO BE HELD ON APRIL 21, 2026

To the Shareholders of Dune Acquisition Corporation II:

NOTICE IS HEREBY GIVEN that an extraordinary general meeting (the “Extraordinary General Meeting”) of the shareholders of Dune Acquisition Corporation II, a Cayman Islands exempted company (the“Company,” “we,” “us” or “our”), will be held on April 21, 2026, at 11:00 a.m., New York Time, at the offices of Reed Smith LLP, our legal counsel, at 599 Lexington Ave, New York, NY 10022 and virtually via live webcastat https://www.cstproxy.com/duneacqii/2026. You can also listen to, but not attend, the Extraordinary General Meeting via teleconference* using the following dial-in information:

Telephone access (listen-only):

Within the U.S. and Canada: 1 800-450-7155 (toll-free)

Outside of the U.S. and Canada: 1 857-999-9155 (standard rates apply)

Conference ID: 6904241#

• Shareholders participating via the listen-only method will not be considered present for the Extraordinary General Meeting and will not be counted towards quorum.

The Extraordinary General Meeting may be held at such other time, or on such other date and at such other place to which the meeting may be postponed or adjourned. You are cordially invited to attend the Extraordinary General Meeting for the purpose of considering and voting on the following proposals (unless the Company determines that it is not necessary to hold the Extraordinary General Meeting as described in the accompanying proxy statement/information statement (“proxy statement”)), more fully described below in the accompanying proxy statement, which is dated March 27, 2026 and is first being mailed to shareholders on or about that date:

• Proposal No. 1 — Name Change Proposal— To approve, as special resolutions, the change of the name of the Company from “Dune Acquisition Corporation II” to “Collective Acquisition Corp.” and an amendment to the Company’s current Amended and Restated Memorandum and Articles of Association and Articles of Association (as may be amended from time to time) in the form set forth in Annex A to the accompanying proxy statement, to reflect the change the name of the Company (the “Name Change” and such proposal, the “Name Change Proposal”).

The text of the special resolutions are as follows:

“RESOLVED, as a special resolution, that:

(a) subject to the proposed new name conforming with section 30 of the Companies Act (Revised) of the Cayman Islands, the Company change its name from Dune Acquisition Corporation II to Collective Acquisition Corp. with immediate effect (the “Name Change”); and

(b) immediately following the Name Change, the Company adopt the Amended and Restated Memorandum and Articles of Association (the “Articles”) in the form set forth in Annex A hereto in substitution for and to the exclusion of, the Company’s existing amended and restated memorandum and articles of association, to reflect the Name Change.”

• Proposal No. 2 — Adjournment Proposal— To approve, as an ordinary resolution, the adjournment of the Extraordinary General Meeting to a later date or dates, or indefinitely, if necessary or convenient, to permit further solicitation and vote of proxies if, based upon the tabulated vote at the time of the Extraordinary General Meeting, there are not sufficient votes to approve the Name Change Proposal or if the Company’s Board of Directors (the “Board”) determines that additional time is necessary to effectuate the Name Change. This proposal is referred to as the “Adjournment Proposal”.

The text of the ordinary resolution is as follows:

“RESOLVED, as an ordinary resolution that, the adjournment of the Extraordinary General Meeting to a later date or dates, or indefinitely, if necessary or convenient, to permit further solicitation of proxies if, based upon the tabulated vote at the time of the Extraordinary General Meeting, there are not sufficient votes to approve any of the Name Change Proposal or the Board determines that additional time is necessary to effectuate the Name Change be confirmed, adopted, approved and ratified in all respects.”

The purpose of the Name Change Proposal is to enable the Company to change its name from “Dune Acquisition Corporation II” to “Collective Acquisition Corp.” As previously disclosed in our Current Report on Form 8-K filed with the Securities and Exchange Commission on February 5, 2026, on January 30, 2026, Collective Acquisition Sponsor LLC, a Delaware limited liability company (the “New Sponsor”), the Company, Dune Acquisition Holdings II LLC, a Delaware limited liability company ( the “Old Sponsor”), Carter Glatt, as the managing member of the Old Sponsor (the “Sponsor Member”), certain members of the Sponsor Member named as signatories thereto and certain other institutional investors signatories thereto entered into a Purchase and Sponsor Handover Agreement pursuant to which New Sponsor has agreed to purchase from the Old Sponsor, an aggregate of (i) 4,475,000 Class B ordinary shares, $0.0001 par value per share and (ii) 1,000,000 private placement warrants of the Company, for an aggregate purchase price of $2,000,000 paid in cash at closing (the “Transaction”). The Transaction resulted in a change in ownership and control of the Company.

The Company proposes to effect the Name Change in connection with the Transaction.

The purpose of the Adjournment Proposal is to allow the chairman to adjourn the Extraordinary General Meeting, if necessary or convenient, in order to provide additional time to solicit votes in favor of the Name Change Proposal or if we otherwise determine that additional time is necessary to effectuate the Name Change.

You are not being asked to vote on any initial business combination (the “Initial Business Combination”) at this time. You will retain the right to vote on the Initial Business Combination when it is submitted to the public shareholders (the “Public Shareholders”) (provided that you are a shareholder on March 12, 2026 (the “Record Date”) for a meeting to consider such proposed Initial Business Combination) and the right to redeem your public shares (“Public Shares”) for a pro rata portion of the trust account in the event the Initial Business Combination is approved and completed or in the event the Company has not consummated the Initial Business Combination by the liquidation date of the Company.

The approval of the Name Change Proposal requires a special resolution under the Companies Act (As Revised) of the Cayman Islands (the “Companies Act”), being a resolution passed by a majority of not less than two thirds (2/3) of the votes cast by the holders of the issued and outstanding Class A ordinary shares, par value $0.0001 per share (“Class A Shares”) and Class B ordinary shares, par value $0.0001 per share (“Class B Shares”, and together with the Class A Shares, the “Ordinary Shares”), voting together as a single class, as, being entitled to do so, vote in person (including virtually) or by proxy at the Extraordinary General Meeting or any adjournment thereof.

Approval of the Adjournment Proposal requires an ordinary resolution under the Companies Act, being a resolution passed by a simple majority of the votes cast by the holders of the issued and outstanding Ordinary Shares, voting together as a single class, as, being entitled to do so, vote in person (including virtually) or by proxy at the Extraordinary General Meeting or any adjournment thereof.

Record holders of Ordinary Shares on March 12, 2026, the Record Date, are entitled to vote or have their votes cast at the Extraordinary General Meeting. On the Record Date, there were (i) 14,482,813 issued and outstanding Class A Shares, and (ii) 5,750,000 Class B Shares issued and outstanding.

This proxy statement contains important information about the Extraordinary General Meeting and the proposals to be considered. Whether or not you plan to attend the Extraordinary General Meeting, the Company urges you to read this material carefully and give instructions to vote your shares.

This proxy statement is dated March 27, 2026 and is first being mailed to shareholders on or about that date.

By Order of the Board of Directors of Dune Acquisition Corporation II.

/s/ Carter Glatt